: C++ Annotations
            Version 13.04.01
: Frank B. Brokken
University of Groningen,
The Netherlands 

        Published at the University of Groningen 

        ISBN 90 367 0470 7
: 1994-2026


    This document is intended for knowledgeable users of C (or any other
language using a C-like grammar, like Python, Perl or Java)
who would like to know more about, or make the transition to, C++. This
document is the main textbook for Frank's C++ programming courses. Feel
free to contact Frank B. Brokken if you're interested in
organizing a local, on-site C++ course anywhere in the world.

    The C++ Annotations do not cover all aspects of C++, though. In
particular, C++'s basic grammar is not covered when equal to C's
grammar. Any basic book on C may be consulted to refresh that part of
C++'s grammar.

If you want a hard-copy version of the C++ Annotations: printable versions
are available in zip-archives containing files in postscript, pdf and other
formats at
    
        
    https://gitlab.com/fbb-git/cppannotations-zip
        

    Pages of files having names starting with cplusplus are in A4 paper
size, pages of files having names starting with cplusplusus are in the US
legal paper size. The C++ Annotations are also available as a Kindle book.

The latest version of the C++ Annotations in html-format can be
browsed at:
        

    https://fbb-git.gitlab.io/cppannotations/

        



    

Table of Contents

Chapter 1: Overview Of The Chapters
Chapter 2: Introduction
2.1: What's new in the C++ Annotations
2.2: C++'s history
2.2.1: History of the C++ Annotations
2.2.2: Compiling a C program using a C++ compiler
2.2.3: Compiling a C++ program
2.2.3.1: C++ under MS-Windows
2.2.3.2: Compiling a C++ source text
2.3: C++: advantages and claims
2.4: What is Object-Oriented Programming?
2.5: Differences between C and C++
2.5.1: The function `main'
2.5.2: End-of-line comment
2.5.3: Strict type checking
2.5.4: Function Overloading
2.5.5: Default function arguments
2.5.6: NULL-pointers vs. 0-pointers and nullptr
2.5.7: The `void' parameter list
2.5.8: The `#define __cplusplus'
2.5.9: Using standard C functions
2.5.10: Header files for both C and C++
2.5.11: Defining local variables
2.5.12: The keyword `typedef'
2.5.13: Functions as part of a struct
2.5.14: Evaluation order of operands
Chapter 3: A First Impression Of C++
3.1: Notable differences with C
3.1.1: Using the keyword `const'
3.1.2: Namespaces
3.1.3: The scope resolution operator ::
3.1.4: `cout', `cin', and `cerr'
3.2: Functions as part of structs
3.2.1: Data hiding: public, private and class
3.2.2: Structs in C vs. structs in C++
3.3: Several additions to C's grammar
3.3.1: References
3.3.2: Rvalue References
3.3.3: Lvalues, rvalues and more
3.3.4: Strongly typed enumerations
3.3.5: Initializer lists
3.3.5.1: Designated initialization
3.3.6: Initializers for bit-fields
3.3.7: Type inference using `auto'
3.3.7.1: Structured binding declarations
3.3.8: Defining types and `using' declarations
3.3.9: Range-based for-loops
3.3.10: Raw String Literals
3.3.11: Binary constants
3.3.12: Selection statements with initializers
3.3.13: Attributes
3.3.14: Three-way comparison (<=>)
3.4: New language-defined data types
3.4.1: The data type `bool'
3.4.2: The data type `wchar_t'
3.4.3: Unicode encoding
3.4.4: The data type `long long int'
3.4.5: The data type `size_t'
3.4.6: The data type `std::byte'
3.4.7: Digit separators
3.5: A new syntax for casts
3.5.1: The `static_cast'-operator
3.5.2: The `const_cast'-operator
3.5.3: The `reinterpret_cast'-operator
3.5.4: The `dynamic_cast'-operator
3.5.5: Casting 'shared_ptr' objects
3.6: Keywords and reserved names in C++
Chapter 4: Namespaces
4.1: Namespaces
4.1.1: Defining namespaces
4.1.1.1: Declaring entities in namespaces
4.1.1.2: A closed namespace
4.1.2: Referring to entities
4.1.2.1: The `using' directive
4.1.2.2: `Koenig lookup'
4.1.3: The standard namespace
4.1.4: Nesting namespaces and namespace aliasing
4.1.4.1: Defining entities outside of their namespaces
4.2: The std::chrono namespace (handling time)
4.2.1: Time resolutions: std::ratio
4.2.2: Amounts of time: std::chrono::duration
4.2.3: Clocks measuring time
4.2.4: Points in time: std::chrono::time_point
4.3: The std::filesystem namespace
4.3.1: The class 'error_code'
4.3.2: Types and permissions of file system elements:              file_status
4.3.2.1: The status of file system entries
4.3.3: Names of file system entries: path
4.3.3.1: Accessors, modifiers and operators
4.3.3.2: Free functions
4.3.4: Handling directories: directory_entry
4.3.4.1: Visiting directory entries: (recursive_)directory_iterator
4.3.5: Information about the space of file systems: space_info
4.3.6: File system exceptions: filesystem_error
Chapter 5: The `string' Data Type
5.1: Operations on strings
5.2: A std::string reference
5.2.1: Initializers
5.2.2: Iterators
5.2.3: Operators
5.2.4: Member functions
5.2.5: Conversion functions
5.3: std::string_view
Chapter 6: The IO-stream Library
6.1: Special header files
6.2: The foundation: the class `ios_base'
6.3: Interfacing `streambuf' objects: the class `ios'
6.3.1: Condition states
6.3.2: Formatting output and input
6.3.2.1: Format modifying member functions
6.3.2.2: Formatting flags
6.3.2.3: The std::quoted manipulator
6.4: Output
6.4.1: Basic output: the class `ostream'
6.4.1.1: Writing to `ostream' objects
6.4.1.2: `ostream' positioning
6.4.1.3: `ostream' flushing
6.4.2: Output to files: the class `ofstream'
6.4.2.1: Modes for opening stream objects
6.4.3: Output to memory: the class `ostringstream'
6.4.4: The `put_time' manipulator
6.5: Input
6.5.1: Basic input: the class `istream'
6.5.1.1: Reading from `istream' objects
6.5.1.2: `istream' positioning
6.5.2: Input from files: the class `ifstream'
6.5.3: Input from memory: the class `istringstream'
6.5.4: Copying streams
6.5.5: Coupling streams
6.6: Advanced topics
6.6.1: Moving streams
6.6.2: Redirecting streams
6.6.3: Reading AND Writing streams
Chapter 7: Classes
7.1: The constructor
7.1.1: A first application
7.1.2: Constructors: with and  without arguments
7.1.2.1: The order of construction
7.2: Ambiguity resolution
7.2.1: Types `Data' vs. `Data()'
7.2.2: Superfluous parentheses
7.2.3: Existing types
7.3: Objects inside objects: composition
7.3.1: Composition and (const) objects: (const) member initializers
7.3.2: Composition and reference objects: reference member initializers
7.4: Data member initializers
7.4.1: Delegating constructors
7.5: Uniform initialization
7.6: Defaulted and deleted class members
7.7: Const member functions and const objects
7.7.1: Anonymous objects
7.7.1.1: Subtleties with anonymous objects
7.8: The keyword `inline'
7.8.1: Defining members inline
7.8.2: When to use inline functions
7.8.2.1: A prelude: when NOT to use inline functions
7.8.3: Inline variables
7.9: Local classes: classes inside functions
7.10: The keyword `mutable'
7.11: Header file organization
7.11.1: Using namespaces in header files
7.12: Sizeof applied to class data members
Chapter 8: Static Data And Functions
8.1: Static data
8.1.1: Private static data
8.1.2: Public static data
8.1.3: Initializing static const data
8.1.4: Generalized constant expressions (constexpr)
8.1.4.1: Constant expression data
8.2: Static member functions
8.2.1: Calling conventions
Chapter 9: Classes And Memory Allocation
9.1: Operators `new' and `delete'
9.1.1: Allocating arrays
9.1.2: Deleting arrays
9.1.3: Enlarging arrays
9.1.4: Managing `raw' memory
9.1.5: The `placement new' operator
9.2: The destructor
9.2.1: Object pointers revisited
9.2.2: The function set_new_handler()
9.3: The assignment operator
9.3.1: Overloading the assignment operator
9.3.1.1: The member 'operator=()'
9.4: The `this' pointer
9.4.1: Sequential assignments and this
9.5: The copy constructor: initialization vs. assignment
9.6: Revising the assignment operator
9.6.1: Swapping
9.6.1.1: Fast swapping
9.7: Moving data
9.7.1: The move constructor (dynamic data)
9.7.2: The move constructor (composition)
9.7.3: Move-assignment
9.7.4: Revising the assignment operator (part II)
9.7.5: Moving and the destructor
9.7.6: Move-only classes
9.7.7: Default move constructors and assignment operators
9.7.8: Moving: implications for class design
9.8: Copy Elision and Return Value Optimization
9.9: Unrestricted Unions
9.9.1: Implementing the destructor
9.9.2: Embedding an unrestricted union in a surrounding class
9.9.3: Swapping unrestricted unions
9.9.4: Assignment
9.10: Aggregate Data Types
9.11: Conclusion
Chapter 10: Exceptions
10.1: Exception syntax
10.2: An example using exceptions
10.2.1: Anachronisms: `setjmp' and `longjmp'
10.2.2: Exceptions: the preferred alternative
10.3: Throwing exceptions
10.3.1: The empty `throw' statement
10.4: The try block
10.5: Catching exceptions
10.5.1: The default catcher
10.6: Functions unable to throw exceptions: the `noexcept' keyword
10.7: Iostreams and exceptions
10.8: Standard exceptions
10.8.1: Standard exceptions: to use or not to use?
10.9: System error, error_category, and error_condition
10.9.1: The class `std::error_category'
10.9.2: The class `std::error_condition'
10.9.3: The class system_error
10.9.4: Exception propagation: std::exception_ptr
10.10: Exception guarantees
10.10.1: The basic guarantee
10.10.2: The strong guarantee
10.10.3: The nothrow guarantee
10.11: Function try blocks
10.12: Exceptions in constructors
10.13: Exceptions in destructors
Chapter 11: More Operator Overloading
11.1: Overloading `operator[]()'
11.1.1: Multi-argument `operator[]()'
11.2: Overloading insertion and extraction operators
11.3: Conversion operators
11.4: An alternative implementation of the `byte' type
11.5: The keyword `explicit'
11.5.1: Explicit conversion operators
11.6: Overloading increment and decrement operators
11.7: Overloading binary operators
11.7.1: Member function reference bindings (& and &&)
11.7.2: The three-way comparison operator `<=>'
11.8: Overloading `operator new(size_t)'
11.9: Overloading `operator delete(void *)'
11.10: Operators `new[]' and `delete[]'
11.10.1: Overloading `new[]'
11.10.2: Overloading `delete[]'
11.10.3: The `operator delete(void *, size_t)' family
11.10.4: `new[]', `delete[]' and exceptions
11.11: Function Objects
11.11.1: Constructing manipulators
11.11.1.1: Manipulators requiring arguments
11.12: Lambda expressions
11.12.1: Lambda expressions: syntax
11.12.2: Using lambda expressions
11.13: The case of [io]fstream::open()
11.14: User-defined literals
11.15: Overloadable operators
Chapter 12: Abstract Containers
12.1: Notations used in this chapter
12.2: The `pair' container
12.3: Available Containers
12.3.1: The `array' container
12.3.2: The `vector' container
12.3.3: The `list' container
12.3.4: The `queue' container
12.3.5: The `priority_queue' container
12.3.6: The `deque' container
12.3.7: The `map' container
12.3.7.1: The `map' constructors
12.3.7.2: The `map' operators
12.3.7.3: The `map' public members
12.3.7.4: The `map': a simple example
12.3.8: The `multimap' container
12.3.9: The `set' container
12.3.10: The `multiset' container
12.3.11: The `stack' container
12.3.12: Intermezzo: hashing and unordered containers
12.3.13: The `unordered_set' container
12.3.13.1: The `unordered_set' constructors
12.3.13.2: The `unordered_set' public members
12.3.13.3: The `unordered_multiset' container
12.3.14: The `unordered_map' container (`hash table')
12.3.14.1: The `unordered_map' constructors
12.3.14.2: The `unordered_multimap' container
12.3.15: Heterogeneous lookup
12.4: The `complex' container
Chapter 13: Inheritance
13.1: Related types
13.1.1: Inheritance depth: desirable?
13.2: Access rights: public, private, protected
13.2.1: Public, protected and private derivation
13.2.2: Promoting access rights
13.3: The constructor of a derived class
13.3.1: Move construction
13.3.2: Move assignment
13.3.3: Inheriting constructors
13.3.4: Aggregate Initializations
13.4: The destructor of a derived class
13.5: Redefining member functions
13.6: Multiple inheritance
13.7: Conversions between base classes and derived classes
13.7.1: Conversions with object assignments
13.7.2: Conversions with pointer assignments
13.8: Using non-default constructors with new[]
Chapter 14: Polymorphism
14.1: Virtual functions
14.1.1: Constructors of polymorhic classes
14.2: Virtual destructors
14.3: Pure virtual functions
14.3.1: Implementing pure virtual functions
14.4: Explicit virtual overrides
14.5: Virtual functions and multiple inheritance
14.5.1: Ambiguity in multiple inheritance
14.5.2: Virtual base classes
14.5.3: When virtual derivation is not appropriate
14.6: Run-time type identification
14.6.1: The dynamic_cast operator
14.6.2: The `typeid' operator
14.7: Inheritance: when to use to achieve what?
14.8: The `streambuf' class
14.8.1: Protected `streambuf' members
14.8.1.1: Protected members for input operations
14.8.1.2: Protected members for output operations
14.8.1.3: Protected members for buffer manipulation
14.8.1.4: Deriving classes from `streambuf'
14.8.2: A streambuf used for input and output
14.8.2.1: Keeping track of the offset
14.8.2.2: The members overflow and underflow
14.8.2.3: Overriding uflow?
14.8.2.4: When are which members called?
14.8.2.5: The member xsgetn
14.8.2.6: The member xsputn
14.8.3: The class `filebuf'
14.9: A polymorphic exception class
14.10: How polymorphism is implemented
14.11: Undefined reference to vtable ...
14.12: Virtual constructors
Chapter 15: Friends
15.1: Friend functions
15.2: Extended friend declarations
Chapter 16: Classes Having Pointers To Members
16.1: Pointers to members: an example
16.2: Defining  pointers to members
16.3: Using pointers to members
16.4: Pointers to static members
16.5: Pointer sizes
Chapter 17: Nested Classes
17.1: Defining nested class members
17.2: Declaring nested classes
17.3: Accessing private members in nested classes
17.4: Nesting enumerations
17.4.1: Empty enumerations
17.5: Revisiting virtual constructors
Chapter 18: The Standard Template Library
18.1: Predefined function objects
18.1.1: Arithmetic function objects
18.1.2: Relational function objects
18.1.3: Logical function objects
18.1.4: The `std::not_fn' negator
18.2: Iterators
18.2.1: std::distance and std::size
18.2.2: Insert iterators
18.2.3: Iterators for `istream' objects
18.2.3.1: Iterators for `istreambuf' objects
18.2.4: Iterators for `ostream' objects
18.2.4.1: Iterators for `ostreambuf' objects
18.2.5: Moving elements to another container
18.3: The class 'unique_ptr'
18.3.1: Defining `unique_ptr' objects
18.3.2: Creating a plain `unique_ptr'
18.3.3: Moving  another `unique_ptr'
18.3.4: Pointing to a newly allocated object
18.3.5: Operators and members
18.3.6: Using `unique_ptr' objects for arrays
18.4: The class `shared_ptr'
18.4.1: Defining `shared_ptr' objects
18.4.2: Creating a plain `shared_ptr'
18.4.3: Pointing to a newly allocated object
18.4.4: Operators and members
18.4.5: Casting shared pointers
18.4.6: Using `shared_ptr' objects for arrays
18.5: Smart `smart pointer' construction: `make_shared' and `make_unique'
18.6: Classes having pointer data members
18.7: Comparison classes
18.7.1: The class `weak_equality'
18.7.2: The class `strong_equality'
18.7.3: The class `partial_ordering'
18.7.4: The class `weak_ordering'
18.7.5: The class `strong_ordering'
18.8: Regular Expressions
18.8.1: The regular expression mini language
18.8.1.1: Character classes
18.8.2: Defining regular expressions: std::regex
18.8.3: Retrieving matches: std::match_results
18.8.4: Regular expression matching functions
18.8.4.1: The std::regex_constants::match_flag_type flags
18.8.4.2: Matching full texts: std::regex_match
18.8.4.3: Partially matching text: std::regex_search
18.8.4.4: The member std::match_results::format
18.8.4.5: Modifying target strings: std::regex_replace
18.9: Randomization and Statistical Distributions
18.9.1: Random Number Generators
18.9.2: Statistical distributions
18.9.2.1: Bernoulli distribution
18.9.2.2: Binomial distribution
18.9.2.3: Cauchy distribution
18.9.2.4: Chi-squared distribution
18.9.2.5: Extreme value distribution
18.9.2.6: Exponential distribution
18.9.2.7: Fisher F distribution
18.9.2.8: Gamma distribution
18.9.2.9: Geometric distribution
18.9.2.10: Log-normal distribution
18.9.2.11: Normal distribution
18.9.2.12: Negative binomial distribution
18.9.2.13: Poisson distribution
18.9.2.14: Student t distribution
18.9.2.15: Uniform int distribution
18.9.2.16: Uniform real distribution
18.9.2.17: Weibull distribution
18.10: tie
18.11: Optional return values
Chapter 19: The STL Generic Algorithms
19.1: The Generic Algorithms
19.1.1: Execution policies
19.1.2: accumulate
19.1.3: adjacent_difference
19.1.4: adjacent_find
19.1.5: all_of / any_of / none_of
19.1.6: begin / end
19.1.7: binary_search
19.1.8: copy / copy_if
19.1.9: copy_backward
19.1.10: count / count_if
19.1.11: equal
19.1.12: equal_range
19.1.13: exchange
19.1.14: fill / fill_n
19.1.15: find / find_if / find_if_not
19.1.16: find_end
19.1.17: find_first_of
19.1.18: for_each
19.1.19: generate / generate_n
19.1.20: includes
19.1.21: inner_product
19.1.22: inplace_merge
19.1.23: iota
19.1.24: is_partitioned
19.1.25: is_permutation
19.1.26: is_sorted
19.1.27: is_sorted_until
19.1.28: iter_swap
19.1.29: lexicographical_compare
19.1.30: lower_bound
19.1.31: max / min
19.1.32: max_element / min_element / minmax_element
19.1.33: merge
19.1.34: minmax
19.1.35: mismatch
19.1.36: move / move_backward
19.1.37: next_permutation / prev_permutation
19.1.38: nth_element
19.1.39: partial_sort / partial_sort_copy
19.1.40: partial_sum
19.1.41: partition / partition_point / stable_partition
19.1.42: partition_copy
19.1.43: reduce
19.1.44: remove / remove_if / remove_copy / remove_copy_if
19.1.45: replace / replace_if / replace_copy / replace_copy_if
19.1.46: reverse / reverse_copy
19.1.47: rotate / rotate_copy
19.1.48: sample
19.1.49: search / search_n
19.1.50: set_difference
19.1.51: set_intersection
19.1.52: set_symmetric_difference
19.1.53: set_union
19.1.54: sort / stable_sort
19.1.55: swap / swap_ranges
19.1.56: transform
19.1.57: transform_reduce
19.1.58: handling uninitialized memory
19.1.59: unique
19.1.60: unique_copy
19.1.61: upper_bound
19.1.62: Heap algorithms
19.1.62.1: The `make_heap' function
19.1.62.2: The `pop_heap' function
19.1.62.3: The `push_heap' function
19.1.62.4: The `sort_heap' function
19.1.62.5: An example using the heap functions
Chapter 20: Multi Threading
20.1: Multi Threading
20.1.1: The namespace std::this_thread
20.1.2: The class std::thread
20.1.2.1: Static data and threads: thread_local
20.1.2.2: Exceptions and join()
20.1.3: The class std::jthread
20.1.3.1: std::stop_callback
20.2: Synchronization (mutexes)
20.2.1: Initialization in multi-threaded programs
20.2.2: Shared mutexes
20.3: Locks and lock handling
20.3.1: Name-independent declarations
20.3.2: Deadlocks
20.3.3: Shared locks
20.3.4: Scoped locks
20.4: Event handling (condition variables)
20.4.1: The class std::condition_variable
20.4.2: The class std::condition_variable_any
20.4.3: An example using condition variables
20.5: Atomic actions: mutexes not required
20.6: An example: threaded quicksort
20.7: Shared States
20.8: Asynchronous return objects: std::future
20.8.1: The std::future_error exception and the std::future_errc enum
20.9: Shared asynchronous return objects: std::shared_future
20.10: Starting a new thread: std::async
20.11: Preparing a task for execution: std::packaged_task
20.12: The class `std::promise'
20.13: An example: multi-threaded compilations
20.14: Transactional Memory
20.15: Synchronizing output to streams
20.15.1: The `std::syncbuf' streambuf
20.15.2: Multi-threaded compilations using `osyncstream'
Chapter 21: Function and Variable Templates
21.1: Defining function templates
21.1.1: Considerations regarding template parameters
21.1.2: Auto and decltype
21.1.2.1: declval
21.1.3: Late-specified return type
21.2: Passing arguments by reference (reference wrappers)
21.3: Using local and unnamed types as template arguments
21.4: Template parameter deduction
21.4.1: Lvalue transformations
21.4.2: Qualification transformations
21.4.3: Transformation to a base class
21.4.4: The template parameter deduction algorithm
21.4.5: Template type contractions
21.5: Declaring function templates
21.5.1: Instantiation declarations
21.6: Instantiating function templates
21.6.1: Instantiations: no `code bloat'
21.7: Using explicit template types
21.8: Overloading function templates
21.8.1: An example using overloaded function templates
21.8.2: Ambiguities when overloading function templates
21.8.3: Declaring overloaded function templates
21.9: Specializing templates for deviating types
21.9.1: Avoiding too many specializations
21.9.2: Declaring specializations
21.9.3: Complications when using the insertion operator
21.10: Static assertions
21.11: Numeric limits
21.12: Polymorphous wrappers for function objects
21.13: Compiling template definitions and instantiations
21.14: The function selection mechanism
21.14.1: Determining the template type parameters
21.15: SFINAE: Substitution Failure Is Not An Error
21.16: Conditional function definitions using `if constexpr'
21.17: Summary of the template declaration syntax
21.18: Variables as templates (template variables)
Chapter 22: Class Templates
22.0.1: Template Argument Deduction
22.0.1.1: Simple Definitions
22.0.1.2: Explicit Conversions
22.1: Defining class templates
22.1.1: Constructing the circular queue: CirQue
22.1.2: Non-type parameters
22.1.3: Member templates
22.1.4: CirQue's constructors and member functions
22.1.5: Using CirQue objects
22.1.6: Default class template parameters
22.1.7: Declaring class templates
22.1.8: Preventing template instantiations
22.1.9: Template parameter initialization
22.1.10: Generic lambda expressions
22.2: Static data members
22.2.1: Extended use of the keyword `typename'
22.3: Specializing class templates for deviating types
22.3.1: Example of a class specialization
22.4: Partial specializations
22.4.1: Intermezzo: some simple matrix algebraic concepts
22.4.2: The Matrix class template
22.4.3: The MatrixRow partial specialization
22.4.4: The MatrixColumn partial specialization
22.4.5: The 1x1 matrix: avoid ambiguity
22.5: Variadic templates
22.5.1: Defining and using variadic templates
22.5.2: Perfect forwarding
22.5.3: The unpack operator
22.5.4: Non-type variadic templates
22.5.5: Folding expressions
22.6: Tuples
22.6.1: Tuples and structured bindings
22.7: Computing the return type of function objects
22.8: Instantiating class templates
22.9: Processing class templates and instantiations
22.10: Declaring friends
22.10.1: Non-templates used as friends in templates
22.10.2: Templates instantiated for specific types as friends
22.10.2.1: Free operators as friends of nested classes
22.10.3: Unbound templates as friends
22.10.4: Extended friend declarations
22.11: Class template derivation
22.11.1: Deriving ordinary classes from class templates
22.11.2: Deriving class templates from class templates
22.11.3: Deriving class templates from ordinary classes
22.12: Static Polymorphism
22.12.1: An example of static polymorphism
22.12.2: Converting dynamic polymorphic classes to static             polymorphic classes
22.12.3: Using static polymorphism to avoid reimplementations
22.13: Class templates and nesting
22.14: Constructing iterators
22.14.1: Implementing a `RandomAccessIterator'
22.14.2: Implementing a `reverse_iterator'
Chapter 23: Advanced Template Use
23.1: Subtleties
23.1.1: Type resolution for base class members
23.1.2: ::template, .template and ->template
23.2: Template Meta Programming
23.2.1: Values according to templates
23.2.1.1: Converting integral types to types
23.2.2: Selecting alternatives using templates
23.2.2.1: Defining overloading members
23.2.2.2: Class structure as a function of template parameters
23.2.2.3: An illustrative example
23.2.3: Templates: Iterations by Recursion
23.3: User-defined literals
23.4: Template template parameters
23.4.1: Policy classes - I
23.4.2: Policy classes - II: template template parameters
23.4.2.1: The destructor of Policy classes
23.4.3: Structure by Policy
23.5: Alias Templates
23.6: Trait classes
23.6.1: Distinguishing class from non-class types
23.6.2: Available type traits
23.7: Defining `ErrorCodeEnum' and 'ErrorConditionEnum'                           enumerations
23.7.1: Deriving classes from std::error_category
23.8: Using `noexcept' when offering the `strong guarantee'
23.9: More conversions to class types
23.9.1: Types to types
23.9.2: An empty type
23.9.3: Type convertibility
23.9.3.1: Determining inheritance
23.10: Template TypeList processing
23.10.1: The length of a TypeList
23.10.2: Searching a TypeList
23.10.3: Selecting from a TypeList
23.10.4: Prefixing/Appending to a TypeList
23.10.5: Erasing from a TypeList
23.10.5.1: Erasing the first occurrence
23.10.5.2: Erasing a type by its index
23.10.5.3: Erasing all occurrences of a type
23.10.5.4: Erasing duplicates
23.11: Using a TypeList
23.11.1: The Wrap and Multi class templates
23.11.2: The MultiBase class template
23.11.3: Support templates
23.11.4: Using Multi
23.12: Expression Templates
23.12.1: Designing an Expression Template
23.12.2: Implementing an Expression Template
23.12.3: The BasicType trait class and ordering classes
23.13: Concepts
23.13.1: Defining concepts
23.13.2: Requirements
23.13.2.1: Simple requirements
23.13.2.2: Type requirements
23.13.2.3: Compound requirements
23.13.2.4: Nested requirements
23.13.3: Predefined concepts
23.13.3.1: Concepts specifying one template type parameter
23.13.3.2: Concepts specifying two template type parameters
23.13.3.3: Concepts specifying multiple template type parameters
23.13.4: Applying concepts to template parameter packs
23.13.5: Applying concepts to free functions
23.13.6: Implementing constrained class members
23.13.7: Constrained partial specializations
23.13.7.1: Function- and class template declarations
23.13.7.2: Bound free-operators
Chapter 24: Coroutines
24.1: Defining a coroutine
24.1.1: The coroutine's State class (promise_type)
24.1.1.1: What if `suspend_never' is used?
24.1.2: Simplifying the state class
24.2: Embedding coroutines in classes
24.2.1: The `Reader' coroutine handler
24.2.2: The `Writer' coroutine handler
24.3: `Awaitables', `Awaiters' and `co_await'
24.4: The class `Awaiter'
24.5: Accessing State from inside coroutines
24.6: Finite State Automatons via coroutines
24.6.1: The `Start' handler class
24.6.2: Completing the Finite State Automaton
24.7: Recursive coroutines
24.7.1: Recursively calling recursiveCoro
24.7.2: Beyond a single recursive call
24.8: Coroutine iterators
24.9: Visiting directories using coroutines
24.9.1: The `Dir' class showing directory entries
24.9.2: Visiting directories using coroutines
24.9.3: Functions vs. coroutines
Chapter 25: Modules
25.1: Syntax
25.2: Modules
25.3: The gcm.cache directory
25.3.1: Module-compiled system header files
25.3.2: Module-compiled headers of locally developed libraries
25.3.2.1: Using a relative location
25.3.2.2: Using an absolute location
25.3.3: Local header files
25.4: Namespaces
25.5: Templates
25.5.1: Class templates
25.6: Partitions
25.6.1: The Math:Utility partition
25.6.2: The Math:Add partition
25.6.3: The Math module's members
25.6.4: Using the Math module
25.6.5: Partitioned class templates
25.7: Module mapping
25.7.1: Modifying interface units
25.7.2: External modules
25.8: Design
25.8.1: 'header' and 'frame' files
Chapter 26: Concrete Examples
26.1: Using file descriptors with `streambuf' classes
26.1.1: Classes for output operations
26.1.2: Classes for input operations
26.1.2.1: Using a one-character buffer
26.1.2.2: Using an n-character buffer
26.1.2.3: Seeking positions in `streambuf' objects
26.1.2.4: Multiple `unget' calls in `streambuf' objects
26.1.3: Fixed-sized field extraction from istream objects
26.1.3.1: Member functions and example
26.2: The `fork' system call
26.2.1: A basic Fork class
26.2.2: Parents and Children
26.2.3: Redirection revisited
26.2.4: The `Daemon' program
26.2.5: The class `Pipe'
26.2.6: The class `ParentSlurp'
26.2.7: Communicating with multiple children
26.2.7.1: The class `Selector': interface
26.2.7.2: The class `Selector': implementation
26.2.7.3: The class `Monitor': interface
26.2.7.4: The class `Monitor': s_handler
26.2.7.5: The class `Monitor': the member `run'
26.2.7.6: The class `Monitor': example
26.2.7.7: The class `Child'
26.3: Adding binary operators to classes
26.3.1: Merely using operators
26.3.1.1: To namespace or not to namespace?
26.3.2: The CRTP and defining operator function templates
26.3.3: Insertion and extraction
26.4: Distinguishing lvalues from rvalues with operator[]()
26.5: Implementing a `reverse_iterator'
26.6: Using `bisonc++' and `flexc++'
26.6.1: Using `flexc++' to create a scanner
26.6.1.1: The derived class `Scanner'
26.6.1.2: The lexical scanner specification file
26.6.1.3: Implementing `Scanner'
26.6.1.4: Using a `Scanner' object
26.6.1.5: Building the program
26.6.2: Using `bisonc++' and `flexc++'
26.6.2.1: The `bisonc++' specification file
26.6.2.2: The `flexc++' specification file
26.6.2.3: Building the program







Chapter 1: Overview Of The Chapters

The chapters of the C++ Annotations cover the following topics:
    
    o Chapter 1: This overview of the chapters.
    o Chapter 2: A general introduction to C++.
    o Chapter 3: A first impression: differences between C
        and C++.
    o Chapter 4: Name Spaces: how to avoid name collisions.
    o Chapter 5: The `string' data type.
    o Chapter 6: The C++ I/O library.
    o Chapter 7: The `class' concept: structs having functions. The
        `object' concept: variables of a class.
    o Chapter 8: Static data and functions: members of a class
        not bound to objects.
    o Chapter 9: Allocation and returning unused memory: new,
        delete, and the function  set_new_handler().
    o Chapter 10: Exceptions: handle errors where appropriate, rather
        than where they occur.
    o Chapter 11: Give your own meaning to operators.
    o Chapter 12: Abstract Containers to put stuff into.
    o Chapter 13: Building classes upon classes: setting up class
        hierarchies.
    o Chapter 14: Changing the behavior of member functions
        accessed through base class pointers.
    o Chapter 15: Gaining access to private parts: friend functions and
        classes.
    o Chapter 16: Classes having pointers to members: pointing to
        locations inside objects.
    o Chapter 17: Constructing classes and enums within classes.
    o Chapter 18: The Standard Template Library.
    o Chapter 19: The STL generic algorithms.
    o Chapter 20: Multi Threading.
    o Chapter 21: Function templates: using molds for type independent
        functions.
    o Chapter 22: Class templates: using molds for type independent
        classes.
    o Chapter 23: Advanced Template Use: programming the compiler.
    o Chapter 24: Coroutines.
    o Chapter 26: Several examples of programs written in C++.



Chapter 2: Introduction

This document offers an introduction to the C++ programming language. It
is a guide for C/C++ programming courses, yearly presented by Frank at the
University of Groningen. This document is not a complete C/C++ handbook,
as much of the C-background of C++ is not covered. Other sources
should be referred to for that (e.g., the

 on-line book suggested to
me by George Danchev (danchev at spnet dot net)).

The reader should be forewarned that extensive knowledge of the C
programming language is actually assumed. The C++ Annotations continue where
topics of the C programming language end, such as pointers, basic flow
control and the construction of functions.

Some elements of the language, like specific lexical tokens (like
digraphs (e.g., <: for [, and >: for ])) are not covered
by the C++ Annotations, as these tokens occur extremely seldom in C++ source
code. In addition, trigraphs (using ??< for {, and ??> for
}) have been removed from C++.

The working draft of the C++ standard is freely available, and can be
cloned from the git-repository at https://gitlab.com/cplusplus/draft.git

The version number of the C++ Annotations (currently 13.04.01) is updated when
the content of the document change. The first number is the major number, and
is probably not going to change for some time: it indicates a major
rewriting. The middle number is increased when new information is added to the
document. The last number only indicates small changes; it is increased when,
e.g., series of typos are corrected.

This document is published by the Center of Information Technology,
University of Groningen,
the Netherlands under the 
 GNU General Public License.

The C++ Annotations were typeset using the 
    
    yodl
formatting system.


    
        All correspondence concerning suggestions, additions, improvements or
        changes to this document should be directed to the author:
    


        
Frank B. Brokken 

        University of Groningen,
The Netherlands 

        (email: f.b.brokken@rug.nl)
        

   

In this chapter an overview of C++'s defining features is presented. A few
extensions to C are reviewed and the concepts of object based and object
oriented programming (OOP) are briefly introduced.


2.1: What's new in the C++ Annotations

    This section is modified when the first or second part of the version number
changes (and occasionally also for the third field of the version number). At
a major version upgrade the entries of the previous major version are kept,
and entries referring to older releases are removed.
    
    o  Version 13.04.00: added new sections: the std::quoted manipulator
        (6.3.2.3), and Intermezzo: hashing and unordered containers
        (12.3.12). The unordered_set (12.3.13) is now covered before
        covering the unordered_map (12.3.14).

o  Version 13.03.00: 13.03.00 updated description of lambda expression
        syntax; new section 22.1.9: Template parameter initialization.

o  Version 13.02.00's chapter Modules was rewritten, and adds
        a section to chapter POLYMORPHISM about deriving a
        std::streambuf class that can be used by std::iostream objects
        (cf. section 14.8.2).

o  Version 13.01.00 reorganized section 4.3 (note: changing
        __file_clock to file_clock); removed section 12.3 (allocators,
        removed in the C++20 standard); included missing examples; and
        repaired typos.

o  Version 13.0.0 adds a new chapter (25) about C++
        Modules, covers the _ name-independent declaration,
        upgraded the C++ standard to C++26, clarified and repaired
        typos in several sections.

o  Version 12.5.0 adds a subsection about constructors of polymorphic
        classes (cf. section 14.1.1) and adds a section about reading
        and writing devices using the std::iostream class (replaced by
        section 14.8.2 in version 13.02.00).
        

o  Version 12.4.0 adds a section about the std::byte type
        (cf. chapter 3), offers an alternative
        implementation (cf. section 11.4), and covers the definition of
        multi-argument index operators (chapter 11). The
        description of how to implement class constructors that may throw
        exceptions was updated (cf. section 10.12).

o  Version 12.3.0 updates and reorganizes the coverage of the generic
        algorithms, fixes many typos and unclarities in the Annotations' text,
        removed superfluous sections since C++20, and adds an overview of
        facilities to handle objects constructed in raw memory.

o  Version 12.2.0 takes into account that std::iterator is
        deprecated. Section 22.14 was rewritten; section
        11.1 was updated (operator[] const should return
        Type const & instead of Type values); added section
        5.3 covering std::string_view; added section
        20.15 covering synchronization of output to streams in
        multi-threaded programs; added sections 22.10.2.1 and
        23.13.7.2 about bound-friends; 'typedef' definitions were
        replaced by 'using' declarations.

o  Version 12.1.0 adds a description of the __file_clock::to_sys
        static member,
         repaired the descriptions
        of pop*() members of various abstract containers, and reorganized
        the description of the facilities of the filesystem::path class.

o  Version 12.0.0 adds a new chapter about coroutines (chapter
        24) and a new section (20.1.3) to chapter
        20.


2.2: C++'s history

    The first implementation of C++ was developed in the 1980s at
the AT&T Bell Labs, where the Unix operating system was created.

C++ was originally a `pre-compiler', similar to the preprocessor of C,
converting special constructions in its source code to plain C. Back then
this code was compiled by a standard C compiler. The `pre-code', which was
read by the C++ pre-compiler, was usually located in a file with the
extension .cc, .C or .cpp. This file would then be converted to a
C source file with the extension .c, which was thereupon compiled and
linked.

The nomenclature of C++ source files remains: the extensions .cc and
.cpp are still used. However, the preliminary work of a C++
pre-compiler is nowadays usually performed during the actual compilation
process. Often compilers determine the language used in a source file from its
extension. This holds true for Borland's and Microsoft's C++ compilers,
which assume a C++ source for an extension .cpp. The GNU
compiler g++, which is available on many Unix platforms, assumes for
C++ the extension .cc.

The fact that C++ used to be compiled into C code is also visible
from the fact that C++ is a superset of C: C++ offers the full
C grammar and supports all C-library functions, and adds to this
features of its own. This makes the transition from C to
C++ quite easy. Programmers familiar with C may start
`programming in C++' by using source files having extensions .cc or
.cpp instead of .c, and may then comfortably slip into all the
possibilities offered by C++. No abrupt change of habits is required.


2.2.1: History of the C++ Annotations

        The original version of the C++ Annotations was written by Frank Brokken
and Karel Kubat in Dutch using LaTeX. After some time, Karel rewrote the
text and converted the guide to a more suitable format and (of course) to
English in September 1994.

The first version of the guide appeared on the net in October 1994. By then it
was converted to SGML.

Gradually new chapters were added, and the content was modified and further
improved (thanks to countless readers who sent us their comments).

In major version four Frank added new chapters and converted the document from
SGML to yodl.
    
    The C++ Annotations are freely distributable. Be sure to read
    the legal notes.

Reading the annotations beyond this point implies that you are aware of
    these notes and that you agree with them.

If you like this document, tell your friends about it. Even better, let us
know by sending email to Frank.


2.2.2: Compiling a C program using a C++ compiler

        Prospective C++ programmers should realize that C++ is not a perfect
superset of C. There are some differences you might encounter when you
simply rename a file to a file having the extension .cc and run it through
a C++ compiler:
    
    o  In C, sizeof('c') equals sizeof(int), 'c' being
any ASCII character.  The underlying philosophy is probably that chars,
when passed as arguments to functions, are passed as integers
anyway. Furthermore, the C compiler handles a character constant like
'c' as an integer constant. Hence, in C, the function calls
            putchar(10);

and
            putchar('\n');

are synonymous.

By contrast, in C++, sizeof('c') is always 1 (but see also section
3.4.2). An int is still an int, though. As we shall see later
(section 2.5.4), the two function calls
            somefunc(10);

and
            somefunc('\n');

may be handled by different functions: C++ distinguishes functions not
only by their names, but also by their argument types, which are different in
these two calls. The former using an int argument, the latter a char.
    o  C++ requires very strict prototyping of external
functions. E.g., in C a prototype like
            void func();

means that a function func() exists, returning no value. The
declaration doesn't specify which arguments (if any) are accepted by the
function.

However, in C++ the above declaration means that the function func()
does not accept any arguments at all. Any arguments passed to it
result in a compile-time error.

Note that the keyword extern is not required when declaring functions. A
function definition becomes a function declaration simply by replacing a
function's body by a semicolon. The keyword extern is required,
though, when declaring variables.


2.2.3: Compiling a C++ program

        To compile a C++ program, a C++ compiler is required. Considering
the free nature of this document, it won't come as a surprise that a
 free compiler is suggested here. The
 Free Software Foundation (FSF) provides at 
 http://www.gnu.org a free C++
compiler which is, among other places, also part of the Debian
 
 (http://www.debian.org) distribution of Linux
 ( http://www.linux.org).

Always use the latest C++ standard supported by your compiler. When the
latest standard isn't used by default, but is already partially implemented it
can usually be selected by specifying the appropriate flag. E.g., to use the
C++26 standard specify the flag --std=c++26. 

Note: in the C++ Annotations it is assumed that the lastest available standard
is specified using the --std flag, even if no --std flag has been
specified.


2.2.3.1: C++ under MS-Windows

            For MS-Windows Cygwin 
(http://cygwin.com) or MinGW 
(http://mingw-w64.org/doku.php) provide the foundation
for installing the Windows port of the GNU g++ compiler
(see also https://docs.microsoft.com/en-us/windows/wsl/about).

The GNU g++ compiler's official home page is 
     http://gcc.gnu.org, also containing
information about how to install the compiler in an MS-Windows system.


2.2.3.2: Compiling a C++ source text

            Generally the following command can be used to compile a C++ source file
`source.cc':
                g++ source.cc

This produces a binary program (a.out or a.exe). If the default
name is inappropriate, the name of the executable can be specified using the
-o flag (here producing the program source):
                g++ -o source source.cc

If a mere compilation is required, the compiled module can be produced using
the -c flag:
                g++ -c source.cc

This generates the file source.o, which can later on be linked to
other modules. 

C++ programs quickly become too complex to maintain `by hand'. With all
serious programming projects program maintenance tools are used. Usually the
standard make program is used to maintain C++ programs, but good
alternatives exist, like  the 
        icmake
    or  ccbuild program
maintenance utilities.

It is strongly advised to start using maintenance utilities early in the study
of C++.


2.3: C++: advantages and claims

    Often it is said that programming in C++ leads to `better' programs. Some
of the claimed advantages of C++ are:
    
    o  New programs would be developed in less time because old code can be
reused.
    o  Creating and using new data types would be easier than in C.
    o  The memory management under C++ would be easier and more
transparent.
    o  Programs would be less bug-prone, as C++ uses a stricter syntax
and type checking.
    o  `Data hiding', the usage of data by one program part
while other program parts cannot access the data, would be easier to implement
with C++.

Which of these allegations are true? Originally, our impression was that
the C++ language was somewhat overrated; the same holding true for the
entire object-oriented programming (OOP) approach. The enthusiasm for the
C++ language resembles the once uttered allegations about
Artificial-Intelligence (AI) languages like Lisp and Prolog: these
languages were supposed to solve the most difficult AI-problems `almost
without effort'. New languages are often oversold: in the end, each problem
can be coded in any programming language (say BASIC or
    assembly language).  The advantages and disadvantages of a given
programming language aren't in `what you can do with them', but rather in
`which tools the language offers to implement an efficient and understandable
solution to a programming problem'. Often these tools take the form of
syntactic restrictions, enforcing or promoting certain constructions or
simply suggesting intentions by applying or `embracing' such syntactic
forms. Rather than a long list of plain assembly instructions we now use flow
control statements, functions, objects or even (with C++) so-called
templates to structure and organize code and to express
oneself `eloquently' in the language of one's choice.

Concerning the above allegations of C++, we support the following, however.
    
    o  The development of new programs while existing code is reused can
also be implemented in C by, e.g., using function libraries.  Functions
can be collected in a library and need not be re-invented with each new
program. C++, however, offers specific syntax possibilities for code
reuse, apart from function libraries (see chapters 13 and
21).
    o  Creating and using new data types is certainly possible in C;
e.g., by using structs, typedefs etc.. From these types other types
can be derived, thus leading to structs containing structs and so
on. In C++ these facilities are augmented by defining data types which are
completely `self supporting', taking care of, e.g., their memory management
automatically (without having to resort to an independently operating memory
management system as used in, e.g., Java).
    o  In C++ memory management can in principle be either as easy or as
difficult as it is in C.  Especially when dedicated C functions such as
xmalloc and xrealloc are used (allocating the memory or aborting
the program when the memory pool is exhausted). However, with functions like
malloc it is easy to err. Frequently errors in C programs can be
traced back to  miscalculations when using malloc. Instead, C++
offers facilities to allocate memory in a somewhat safer way, using its
operator new.
    o  Concerning `bug proneness' we can say that C++ indeed uses
stricter type checking than C. However, most modern C compilers
implement `warning levels'; it is then the programmer's choice to disregard or
get rid of the warnings. In C++ many of such warnings become fatal errors
(the compilation stops).
    o  As far as `data hiding' is concerned, C does offer some tools.
E.g., where possible, local or static variables can be used and special
data types such as structs can be manipulated by dedicated functions.
Using such techniques, data hiding can be implemented even in C; though it
must be admitted that C++ offers special syntactic constructions, making
it far easier to implement `data hiding' (and more in general:
`encapsulation') in C++ than in C.

C++ in particular (and OOP in general) is of course not the
solution to all programming problems. However, the language does offer
various new and elegant facilities which are worth investigating. At the
downside, the level of grammatical complexity of C++ has increased
significantly as compared to C. This may be considered a serious drawback
of the language. Although we got used to this increased level of complexity
over time, the transition was neither fast nor painless.

With the C++ Annotations we hope to help the reader when transiting from C
to C++ by focusing on the additions of C++ as compared to C and by
leaving out plain C. It is our hope that you like this document and
may benefit from it.

Enjoy and good luck on your journey into C++!


2.4: What is Object-Oriented Programming?

    Object-oriented (and object-based) programming propagates a slightly different
approach to programming problems than the strategy usually used in C
programs. In C programming problems are usually solved using a
`procedural approach': a problem is decomposed into subproblems and this
process is repeated until the subtasks can be coded. Thus a conglomerate of
functions is created, communicating through arguments and variables, global or
local (or static).

In contrast (or maybe better: in addition) to this, an object-based approach
identifies the keywords used in a problem statement. These keywords are
then depicted in a diagram where arrows are drawn between those keywords to
depict an internal hierarchy. The keywords become the objects in the
implementation and the hierarchy defines the relationship between these
objects. The term object is used here to describe a limited, well-defined
structure, containing all information about an entity: data types and
functions to manipulate the data. As an example of an
  object oriented approach, an illustration follows:
    
    The employees and owner of a car dealer and auto garage company are paid
    as follows. First, mechanics who work in the garage are paid a certain sum
    each month. Second, the owner of the company receives a fixed amount each
    month. Third, there are car salesmen who work in the showroom and receive
    their salary each month plus a bonus per sold car. Finally, the company
    employs second-hand car purchasers who travel around; these employees
    receive their monthly salary, a bonus per bought car, and a restitution of
    their travel expenses.

When representing the above salary administration, the keywords could be
mechanics, owner, salesmen and purchasers. The properties of such units are: a
monthly salary, sometimes a bonus per purchase or sale, and sometimes
restitution of travel expenses. When analyzing the problem in this manner we
arrive at the following representation:
    
    o  The owner and the mechanics can be represented by identical types,
receiving a given salary per month. The relevant information for such a type
would be the monthly amount. In addition this object could contain data as the
name, address and social security number.
    o  Car salesmen who work in the showroom can be represented as the same
type as above but with some extra functionality: the number of
transactions (sales) and the bonus per transaction.

In the hierarchy of objects we would define the dependency between the
first two objects by letting the car salesmen be `derived' from
the owner and mechanics.
    o  Finally, there are the second-hand car purchasers. These share the
functionality of the salesmen except for travel expenses. The additional
functionality would therefore consist of the expenses made and this type would
be derived from the salesmen.

The hierarchy of the identified objects are further illustrated in
Figure 1.
    ------------------------------------------------------------------Insert Figure 1(Hierarchy of objects in the salary administration.)about here (file: intro/objects)------------------------------------------------------------------

The overall process in the definition of a hierarchy such as the above starts
with the description of the most simple type. Traditionally (and still
encountered with some popular object oriented languages) more complex types
are thereupon derived from the basic type, with each derived type adding some
new functionality. From these derived types, more complex types can again be
derived ad infinitum, until a representation of the entire problem can be
made. 

Over the years this approach has become less popular in C++ as it
typically results in very tight coupling among those types, which in
turns reduces rather than enhances the understanding, maintainability and
testability of complex programs. The term coupling refers to the degree of
independence between software components: tight coupling means a strong
dependency, which is frowned upon in C++. In C++ object oriented
programs more and more favor small, easy to understand hierarchies, limited
coupling and a developmental process where design
patterns (cf. Gamma et al.  (1995)) play a central role.

In C++  classes are frequently used to define the
characteristics of objects. Classes contain the necessary functionality to
do useful things. Classes generally do not offer all their functionality (and
typically none of their data) to objects of other classes. As we will see,
classes tend to hide their properties in such a way that they are not
directly modifiable by the outside world. Instead, dedicated functions are
used to reach or modify the properties of objects. Thus class-type objects are
able to uphold their own integrity. The core concept here is encapsulation
of which data hiding is just an example. These concepts are further
elaborated in chapter 7.


2.5: Differences between C and C++

    In this section some examples of C++ code are shown. Some differences
between C and C++ are highlighted.


2.5.1: The function `main'

        In C++ there are only two variants of the function main:
int main() and int main(int argc, char **argv). 

Notes:
    
    o  The return type of main is int, and not void;
    o  The function main cannot be overloaded (for other than the
abovementioned signatures);
    o  It is not required to use an explicit return statement at the
end of  main. If omitted main returns 0;
    o  The value of argv[argc] equals 0;
    o  The `third char **envp parameter' is not defined by the C++
standard and should be avoided.  Instead, the global variable 
extern char **environ should be declared providing access to the program's
environment variables. Its final element has the value 0;
    o  A C++ program ends normally when the main function
returns. Using a function try block (cf. section 10.11) for main
is also considered a normal end of a C++ program. When a C++ ends
normally, destructors (cf. section 9.2) of globally defined
objects are activated. A function like exit(3) does not normally end
a C++ program and using such functions is therefore deprecated.


2.5.2: End-of-line comment

        According to the ANSI/ISO definition, `end of line comment' is
implemented in the syntax of C++. This comment starts with // and ends
at the end-of-line marker. The standard C comment, delimited by /* and
*/ can still be used in C++:
            int main()
    {
        // this is end-of-line comment
        // one comment per line

        /*
            this is standard-C comment, covering
            multiple lines
        */
    }

Despite the example, it is advised not to use C type comment
inside the body of C++ functions. Sometimes existing code must temporarily
be suppressed, e.g., for testing purposes. In those cases it's very practical
to be able to use standard C comment. If such suppressed code itself
contains such comment, it would result in nested comment-lines, resulting in
compiler errors. Therefore, the rule of thumb is not to use C type
comment inside the body of C++ functions (alternatively, #if 0 until
#endif pair of preprocessor directives could of course also be used).


2.5.3: Strict type checking

        C++ uses very strict type checking. A prototype must be known for each
function before it is called, and the call must match the prototype.
The program
            int main()
    {
        printf("Hello World\n");
    }

often compiles under C, albeit with a warning that printf()
is an unknown function. But C++ compilers (should) fail to produce code in
such cases. The error is of course caused by the missing 
 #include <stdio.h> (which in C++ is more commonly included as
  #include <cstdio> directive).

And while we're at it: as we've seen in C++ main always uses
the int return value. Although it is possible to define int
main() without explicitly defining a return statement, within main it is
not possible to use a return statement without an explicit
int-expression.  For example:
            int main()
    {
        return;     // won't compile: expects int expression, e.g.
                    // return 1;
    }

Implicit conversions from void * to non-void pointers are not
allowed. E.g., the following isn't accepted in C++:
            void *none()
    {
        return 0;
    }
    
    int main()
    {
        int *empty = none();
    }


2.5.4: Function Overloading

        In C++ it is possible to define
    functions having identical names but performing different actions. The
functions must differ in their  parameter lists (and/or in
their  const attribute). An example is given
below:
            #include <stdio.h>

    void show(int val)
    {
        printf("Integer: %d\n", val);
    }

    void show(double val)
    {
        printf("Double: %lf\n", val);
    }

    void show(char const *val)
    {
        printf("String: %s\n", val);
    }

    int main()
    {
        show(12);
        show(3.1415);
        show("Hello World!\n");
    }

    In the above program three functions show are defined, only differing
in their parameter lists, expecting an int, double and char *,
respectively. The functions have identical names. Functions having identical
names but different parameter lists are called overloaded. The act of
defining such functions is called `function overloading'.

The C++ compiler implements function overloading in a rather simple
way. Although the functions share their names (in this example show), the
compiler (and hence the linker) use quite different names. The conversion of a
name in the source file to an internally used name is called
 `name mangling'. E.g., the C++ compiler might convert the prototype
void show (int) to the internal name VshowI, while an
analogous function having a char * argument might be called
VshowCP. The actual names that are used internally depend on the compiler
and are not relevant for the programmer, except where these names show up in
e.g., a listing of the content of a library.

Some additional remarks with respect to  function overloading:
    
    o  Do not use function overloading for functions doing conceptually
different tasks.  In the example above, the functions show are still
somewhat related (they print information to the screen).

However, it is also quite possible to define two functions lookup,
one of which would find a name in a list while the other would determine the
video mode. In this case the behavior of those two functions have nothing in
common. It would therefore be more practical to use names which suggest their
actions; say, findname and videoMode.
    o  C++ does not allow identically named functions to differ only in
their return values, as it is always the programmer's choice to either use or
ignore a function's  return value. E.g., the fragment
    printf("Hello World!\n");

provides no information about the return value of the function
printf. Two functions printf which only differ in their
return types would therefore not be distinguishable to the compiler.
    o  In chapter 7 the notion of const member functions is
introduced (cf. section 7.7). Here it is merely mentioned
that classes normally have so-called member functions associated with them
(see, e.g., chapter 5 for an informal introduction to the
concept). Apart from
    
    overloading member functions using different parameter lists, it
is then also possible to
    
    overload member functions by their const attributes. In those cases,
classes may have pairs of identically named member functions, having identical
parameter lists. Then, these functions are overloaded by their const
attribute. In such cases only one of these functions must have the const
attribute.


2.5.5: Default function arguments

        In C++ it is possible to provide `default arguments' when defining a
function. These arguments are supplied by the compiler when they are not
specified by the programmer. For example:
            #include <stdio.h>

    void showstring(char *str = "Hello World!\n");

    int main()
    {
        showstring("Here's an explicit argument.\n");

        showstring();           // in fact this says:
                                // showstring("Hello World!\n");
    }

The possibility to omit arguments in situations where default arguments
are defined is just a nice touch: it is the compiler who supplies the lacking
argument unless it is explicitly specified at the call. The code of the
program will neither be shorter nor more efficient when default arguments are
used.

Functions may be defined with more than one default argument:
            void two_ints(int a = 1, int b = 4);

    int main()
    {
        two_ints();            // arguments:  1, 4
        two_ints(20);          // arguments: 20, 4
        two_ints(20, 5);       // arguments: 20, 5
    }

When the function two_ints is called, the compiler supplies one or two
arguments whenever necessary. A statement like two_ints(,6) is, however,
not allowed: when arguments are omitted they must be on the right-hand side.

Default arguments must be known at compile-time since at that moment
arguments are supplied to functions. Therefore, the default arguments must be
mentioned at the function's declaration, rather than at its
implementation:
            // sample header file
    void two_ints(int a = 1, int b = 4);

    // code of function in, say, two.cc
    void two_ints(int a, int b)
    {
        ...
    }

It is an error to supply default arguments in both function definitions and
function declarations. When applicable default arguments should be provided in
function declarations: when the function is used by other sources the compiler
commonly reads the header file rather than the function definition
itself. Consequently the compiler has no way to determine the values of
default arguments if they are provided in the function definition.


2.5.6: NULL-pointers vs. 0-pointers and nullptr

        In C++ all zero values are coded as 0. In C NULL is often used
in the context of pointers. This difference is purely stylistic, though one
that is widely adopted. In C++ NULL should be avoided (as it is a
macro, and macros can --and therefore should-- easily be avoided in
C++, see also section 8.1.4). Instead 0 can almost always be
used.

Almost always, but not always. As C++ allows function overloading
(cf. section 2.5.4) the programmer might be confronted with
an unexpected function selection in the situation shown in section
2.5.4:
            #include <stdio.h>

    void show(int val)
    {
        printf("Integer: %d\n", val);
    }

    void show(double val)
    {
        printf("Double: %lf\n", val);
    }

    void show(char const *val)
    {
        printf("String: %s\n", val);
    }

    int main()
    {
        show(12);
        show(3.1415);
        show("Hello World!\n");
    }


In this situation a programmer intending to call show(char const *) might
call show(0). But this doesn't work, as 0 is interpreted as int and so
show(int) is called. But calling show(NULL) doesn't work either, as
C++ usually defines NULL as 0, rather than ((void *)0). So,
show(int) is called once again. To solve these kinds of problems the new
C++ standard introduces the keyword nullptr representing the 0
pointer. In the current example the programmer should call show(nullptr)
to avoid the selection of the wrong function. The nullptr value can also
be used to initialize pointer variables. E.g.,
            int *ip = nullptr;      // OK
    int value = nullptr;    // error: value is no pointer


2.5.7: The `void' parameter list

        In C, a function prototype with an empty parameter list, such as
            void func();

means that the argument list of the declared function is not prototyped: for
functions using this prototype the compiler does not warn against calling
func with any set of arguments. In C the keyword void is used when
it is the explicit intent to declare a function with no arguments at all, as
in:
            void func(void);

As C++ enforces strict type checking, in C++ an empty parameter
list indicates the total absence of parameters. The keyword void is
thus omitted.


2.5.8: The `#define __cplusplus'

        Each C++ compiler which conforms to the ANSI/ISO standard defines the
symbol __cplusplus: it is as if each source file were prefixed with the
preprocessor directive #define __cplusplus.

We shall see examples of the usage of this symbol in the following sections.


2.5.9: Using standard C functions

        Normal C functions, e.g., which are compiled and collected in a run-time
library, can also be used in C++ programs. Such functions, however, must be
declared as C functions.

As an example, the following code fragment declares a function xmalloc
as a C function:
            extern "C" void *xmalloc(int size);

This declaration is analogous to a declaration in C, except that the
prototype is prefixed with extern "C".

A slightly different way to declare C functions is the following:
            extern "C"
    {
        // C-declarations go in here
    }

It is also possible to place preprocessor directives at the location of
the declarations. E.g., a C header file myheader.h which declares
C functions can be included in a C++ source file as follows:
            extern "C"
    {
        #include <myheader.h>
    }

Although these two approaches may be used, they are actually seldom
encountered in C++ sources. A more frequently used method to declare
external C functions is encountered in the next section.


2.5.10: Header files for both C and C++

        The combination of the predefined symbol __cplusplus and the
possibility to define extern "C" functions offers the ability to
create header files for both C and C++. Such a header file might,
e.g., declare a group of functions which are to be used in both C and
C++ programs.

The setup of such a header file is as follows:
            #ifdef __cplusplus
    extern "C"
    {
    #endif

        /* declaration of C-data and functions are inserted here. E.g., */
        void *xmalloc(int size);

    #ifdef __cplusplus
    }
    #endif

Using this setup, a normal C header file is enclosed by extern "C"
{ which occurs near the top of the file and by }, which occurs near
the bottom of the file. The #ifdef directives test for the type of the
compilation: C or C++. The `standard' C header files, such as
stdio.h, are built in this manner and are therefore usable for both C
and C++.

In addition C++ headers should support include guards.
In C++ it is usually undesirable to include the same header file twice in
the same source file. Such multiple inclusions can easily be avoided by
including an #ifndef directive in the header file.  For example:
            #ifndef MYHEADER_H_
    #define MYHEADER_H_
        // declarations of the header file is inserted here,
        // using #ifdef __cplusplus etc. directives
    #endif

When this file is initially scanned by the preprocessor, the symbol
MYHEADER_H_ is not yet defined. The #ifndef condition succeeds and all
declarations are scanned. In addition, the symbol MYHEADER_H_ is defined.

When this file is scanned next while compiling the same source file,
the symbol MYHEADER_H_ has been defined and consequently all information
between the #ifndef and #endif directives is skipped by the compiler.

In this context the symbol name MYHEADER_H_ serves only for
recognition purposes. E.g., the name of the header file can be used for this
purpose, in capitals, with an underscore character instead of a dot.

Apart from all this, the custom has evolved to give C header files the
extension .h, and to give C++ header files no extension. For
example, the standard iostreams cin, cout and cerr are available
after including the header file iostream, rather
than iostream.h. In the Annotations this convention is used
with the standard C++ header files, but not necessarily everywhere else.

There is more to be said about header files. Section 7.11 provides
an in-depth discussion of the preferred organization of C++ header files.
In addition, starting with the C++26 standard modules are available
resulting in a somewhat more efficient way of handling declarations than
offered by the traditional header files. 

Currently, the C++ Annotations very briefly covers modules (cf. section
25).


2.5.11: Defining local variables

        Although already available in the C programming language,
    local variables should only be defined once they're needed.  Although
doing so requires a little getting used to, eventually it tends to produce
more readable, maintainable and often more efficient code than defining
variables at the beginning of compound statements. We suggest to apply the
following rules  of thumb when defining local variables:
    
    o  Local variables should be created at `intuitively right' places, such
as in the example below. This does not only entail the for-statement, but
also all situations where a variable is only needed, say, half-way through the
function.
    o  More in general, variables should be defined in such a way that their
scope is as limited and localized as possible.  When avoidable local
variables are not defined at the beginning of functions but rather where
they're first used.
    o  It is considered good practice to 
    avoid global variables. It is fairly easy to lose track of which
global variable is used for what purpose. In C++ global variables are
seldom required, and by localizing variables the risk of using the same
variable for multiple purposes (thereby invalidating the separate purposes of
the variable), can easily be avoided.

If considered appropriate, nested blocks can be used to localize
auxiliary variables. However, situations exist where local variables are
considered appropriate inside nested statements. The just mentioned for
statement is of course a case in point, but local variables can also be
defined within the condition clauses of if-else statements, within
selection clauses of switch statements and condition clauses of while
statements. Variables thus defined are available to the full
statement, including its nested statements. For example, consider the
following switch statement:
            #include <stdio.h>

    int main()
    {
        switch (int c = getchar())
        {
            case 'a':
            case 'e':
            case 'i':
            case 'o':
            case 'u':
                printf("Saw vowel %c\n", c);
            break;

            case EOF:
                printf("Saw EOF\n");
            break;

            case '0' ... '9':
                printf("Saw number character %c\n", c);
            break;

            default:
                printf("Saw other character, hex value 0x%2x\n", c);
        }
    }

    Note the location of the definition of the character `c': it is
defined in the expression part of the switch statement. This implies
that `c' is available only to the switch statement itself,
including its nested (sub)statements, but not outside the scope of the
switch.

The same approach can be used with if and while statements: a
variable that is defined in the condition clause of an if and while
statement is available in their nested statements. There are some caveats,
though:
    
    o  The variable that is defined in the condition clause must be a
variable which is initialized to a numeric or logical value;
    o  The variable definition cannot be nested (e.g., using parentheses)
within a more complex expression.

The former point of attention should come as no big surprise: in order to
be able to evaluate the logical condition of an if or while statement,
the value of the variable must be interpretable as either zero (false) or
non-zero (true). Usually this is no problem, but in C++ objects (like
objects of the type std::string (cf. chapter 5)) are often
returned by functions. Such objects may or may not be interpretable as numeric
values. If not (as is the case with std::string objects), then such
variables can not be defined at the condition or expression clauses of
condition- or repetition statements. The following example will therefore
not compile:
            if (std::string myString = getString())     // assume getString returns
    {                                           // a std::string value
        // process myString
    }

The above example requires additional clarification. Often a variable can
profitably be given local scope, but an extra check is required immediately
following its initialization. The initialization and the test cannot
both be combined in one expression. Instead two nested statements are
required. Consequently, the following example won't compile either:
            if ((int c = getchar()) && strchr("aeiou", c))
        printf("Saw a vowel\n");

If such a situation occurs, either use two nested if statements, or
localize the definition of int c using a nested compound statement:
            if (int c = getchar())             // nested if-statements
        if (strchr("aeiou", c))
            printf("Saw a vowel\n");

    {                                  // nested compound statement
        int c = getchar();
        if (c && strchr("aeiou", c))
           printf("Saw a vowel\n");
    }

The C++26 standard introduced name-independent declarations. They're
covered in section 20.3.1.


2.5.12: The keyword `typedef'

        The keyword typedef is still used in C++, but is not required
anymore when defining  union, struct or enum definitions.
This is illustrated in the following example:
            struct SomeStruct
    {
        int     a;
        double  d;
        char    string[80];
    };

When a struct, union or other compound type is defined, the tag of
this type can be used as type name (this is SomeStruct in the above
example):
            SomeStruct what;

    what.d = 3.1415;


2.5.13: Functions as part of a struct

        In C++ we may define functions as members of structs. Here we
encounter the first concrete example of an object: as previously described
(see section 2.4), an object is a structure containing data while
specialized functions exist to manipulate those data.

A definition of a struct Point is provided by the code fragment below.
In this structure, two int data fields and one function draw are
declared.
            struct Point            // definition of a screen-dot
    {
        int x;              // coordinates
        int y;              // x/y
        void draw();        // drawing function
    };

A similar structure could be part of a painting program and could, e.g.,
represent a pixel. With respect to this struct it should be noted that:
    
    o  The function draw mentioned in the struct definition is a
mere declaration. The actual code of the function defining the actions
performed by the function is found elsewhere (the concept of functions inside
structs is further discussed in section 3.2).
    o  The size of the struct Point is equal to the size of its
two ints.  A function declared inside the structure does not affect its
size. The compiler implements this behavior by allowing the function
draw to be available only in the context of a Point.

The Point structure could be used as follows:
            Point a;                // two points on
    Point b;                // the screen

    a.x = 0;                // define first dot
    a.y = 10;               // and draw it
    a.draw();

    b = a;                  // copy a to b
    b.y = 20;               // redefine y-coord
    b.draw();               // and draw it

As shown in the above example a function that is part of the structure may
be selected using the dot (.) (the arrow (->) operator is used when
pointers to objects are available). This is therefore identical to the way
data fields of structures are selected.

The idea behind this syntactic construction is that several types may
contain functions having identical names. E.g., a structure representing a
circle might contain three int values: two values for the coordinates of
the center of the circle and one value for the radius. Analogously to the
Point structure, a Circle may now have a function draw to draw the
circle.


2.5.14: Evaluation order of operands

            Traditionally, the evaluation order of expressions of operands of binary
operators is, except for the boolean operators and and or, not
defined. C++ changed this for postfix expressions, assignment expressions
(including compound assignments), and shift operators:
    
    o  Expressions using postfix operators (like index operators and member
        selectors) are evaluated from left to right (do not confuse this with
        postfix increment or decrement operators, which cannot be concatenated
        (e.g., variable++++ does not compile)).
    o  Assignment expressions are evaluated from right to left;
    o  Operands of shift operators are evaluated from left to right.

In the following examples first is evaluated before second, before
third, before fourth, whether they are single variables, parenthesized
expressions, or function calls:
            first.second
    fourth += third = second += first
    first << second << third << fourth
    first >> second >> third >> fourth

In addition, when overloading an operator, the function implementing the
overloaded operator is evaluated like the built-in operator it overloads, and
not in the way function calls are generally ordered.



Chapter 3: A First Impression Of C++

In this chapter C++ is further explored. The possibility to
declare functions in structs is illustrated in various examples; the
concept of a class is introduced; casting is covered in detail; many new
types are introduced and several important notational extensions to C are
discussed.


3.1: Notable differences with C

Before we continue with the `real' object-approach to programming, we first
introduce some notable differences with the C programming language: not
mere differences between C and C++, but important syntactic constructs
and keywords not found or differently used in C.


3.1.1: Using the keyword `const'

    Even though the keyword const is part of the C grammar, its use is
more important and much more common and strictly used in C++ than it is in
C.

The const keyword is a modifier stating that the value of a variable
or of an argument may not be modified. In the following example the intent is
to  change the value of a variable ival, which fails:
            int main()
    {
        int const ival = 3;     // a constant int
                                // initialized to 3

        ival = 4;               // assignment produces
                                // an error message
    }

This example shows how ival may be initialized to a given value in its
definition; attempts to change the value later (in an assignment) are not
permitted.

Variables that are declared const can, in contrast to C, be used to
specify the size of an array, as in the following example:
            int const size = 20;
    char buf[size];             // 20 chars big

Another use of the keyword const is seen in the declaration of
pointers, e.g., in pointer-arguments. In the declaration
            char const *buf;

buf is a pointer variable pointing to chars. Whatever is pointed
to by buf may not be changed through buf: the chars are declared
as const. The pointer buf itself however may be changed. A statement
like *buf = 'a'; is therefore not allowed, while ++buf is.

In the declaration
            char *const buf;

buf itself is a const pointer which may not be changed. Whatever
chars are pointed to by buf may be changed at will.

Finally, the declaration
            char const *const buf;

is also possible; here, neither the pointer nor what it points to may be
changed.

The rule of thumb for the placement of the keyword const is the
following: what's written to the left of const may not be changed.

Although simple, this rule of thumb is, unfortunately, not often used. For
example,
    
    Bjarne Stroustrup states (in
    
    https://www.stroustrup.com/bs_faq2.html#constplacement):
    
    Should I put "const" before or after the type?

I put it before, but that's a matter of taste. "const T" and "T const"
were always (both) allowed and equivalent. For example:
            const int a = 1;        // OK
    int const b = 2;        // also OK

My guess is that using the first version will confuse fewer programmers
(``is more idiomatic'').

But we've already seen an example where applying this simple `before'
placement rule for the keyword const produces unexpected (i.e., unwanted)
results as we will shortly see (below). Furthermore, the `idiomatic'
before-placement also conflicts with the notion of const functions, which
we will encounter in section 7.7. With const functions the
keyword const is also placed behind rather than before the name of the
function.

The definition or declaration (either or not containing const) should
always be read from the variable or function identifier back to the type
identifier:
        
    ``Buf is a const pointer to const characters''

This rule of thumb is especially useful in cases where confusion may
occur.  In examples of C++ code published in other places one often
encounters the reverse: const preceding what should not be
altered. That this may result in sloppy code is indicated by our second
example above:
            char const *buf;

What must remain constant here? According to the sloppy interpretation,
the pointer cannot be altered (as const precedes the pointer). In fact,
the char values are the constant entities here, as becomes clear when we try
to compile the following program:
            int main()
    {
        char const *buf = "hello";

        ++buf;                  // accepted by the compiler
        *buf = 'u';             // rejected by the compiler
    }

Compilation fails on the statement *buf = 'u'; and not on the
statement ++buf.


    
    Marshall Cline's
    C++ FAQ gives the
same rule (paragraph 18.5) , in a similar context:
    
    [18.5] What's the difference between "const Fred* p", "Fred* const p" and
"const Fred* const p"?

You have to read pointer declarations right-to-left.

Marshall Cline's advice can be improved, though. Here's a recipe that will
effortlessly dissect even the most complex declaration:
    
    1.  start reading at the variable's name
    2.  read as far as possible until you reach the end of the declaration or
an (as yet unmatched) closing parenthesis.
    3.  return to the point where you started reading, and read backwards
until you reach the beginning of the declaration or a matching opening
parenthesis.
    4.  If you reached an opening parenthesis, continue at step 2 beyond the
parenthesis where you previously stopped.
    
    Let's apply this recipe to the following (by itself irrelevant) complex
declaration. Little arrows indicate how far we should read at each step and
the direction of the arrow indicates the reading direction:
                char const *(* const (*(*ip)())[])[]

                             ip          Start at the variable's name:
                                             'ip' is

                             ip)         Hitting a closing paren: revert
                             -->

                           (*ip)         Find the matching open paren:
                           <-                'a pointer to'

                           (*ip)())      The next unmatched closing par:
                                -->          'a function (not expecting
                                              arguments)'

                         (*(*ip)())      Find the matching open paren:
                         <-                  'returning a pointer to'

                         (*(*ip)())[])   The next closing par:
                                   -->       'an array of'

                (* const (*(*ip)())[])   Find the matching open paren:
                <--------                    'const pointers to'

                (* const (*(*ip)())[])[] Read until the end:
                                      ->     'an array of'

    char const *(* const (*(*ip)())[])[] Read backwards what's left:
    <-----------                             'pointers to const chars'

Collecting all the parts, we get for char const *(* const
(*(*ip)())[])[]: ip is a pointer to a function (not expecting arguments),
returning a pointer to an array of const pointers to an array of pointers to
const chars. This is what ip represents; the recipe can be used to parse
any declaration you ever encounter.


3.1.2: Namespaces

    C++ introduces the notion of a namespace: all symbols are defined
in a larger context, called a namespace. Namespaces are used to avoid
    name conflicts that could arise when a programmer would like to define
a function like sin operating on degrees, but does not want to lose
the capability of using the standard sin function, operating on
radians.

Namespaces are covered extensively in chapter 4. For now it
should be noted that most compilers require the explicit declaration of a
standard namespace: std. So, unless otherwise indicated, it is
stressed that all examples in the Annotations now implicitly use the
    
                using namespace std;

declaration. So, if you actually intend to compile examples given in
the C++ Annotations, make sure that the sources start with the above using
declaration.


3.1.3: The scope resolution operator ::

    C++ introduces several new operators, among which the
scope resolution operator (::). This operator can be
used in situations where a global variable exists having the same name as a
local variable:
            #include <stdio.h>

    double counter = 50;                // global variable

    int main()
    {
        for (int counter = 1;           // this refers to the
             counter != 10;             // local variable
             ++counter)
        {
            printf("%d\n",
                    ::counter           // global variable
                    /                   // divided by
                    counter);           // local variable
        }
    }

In the above program the scope operator is used to address a global
variable instead of the local variable having the same name. In C++ the
scope operator is used extensively, but it is seldom used to reach a global
variable shadowed by an identically named local variable. Its main purpose
is encountered in chapter 7.


3.1.4: `cout', `cin', and `cerr'

    Analogous to C, C++ defines standard input- and output streams
which are available when a program is executed. The streams are:
    
    o  cout, analogous to stdout,
    o  cin, analogous to stdin,
    o  cerr, analogous to stderr.

Syntactically these streams are not used as functions: instead, data are
written to streams or read from them using the operators <<, called
the insertion operator and >>, called the
extraction operator.  This is illustrated in the next example:
            #include <iostream>

    using namespace std;

    int main()
    {
        int     ival;
        char    sval[30];

        cout << "Enter a number:\n";
        cin >> ival;
        cout << "And now a string:\n";
        cin >> sval;

        cout << "The number is: " << ival << "\n"
                "And the string is: " << sval << '\n';
    }

This program reads a number and a string from the cin stream (usually
the keyboard) and prints these data to cout. With respect to streams,
please note:
    
    o  The standard streams are declared in the header file
        iostream. In the examples in the
C++ Annotations this header file is often not mentioned explicitly. Nonetheless,
it must be included (either directly or indirectly) when these streams are
used. Comparable to the use of the using namespace std; clause, the reader
is expected to #include <iostream> with all the examples in which the
standard streams are used.
    o  The streams cout, cin and cerr are variables of so-called
class-types. Such variables are commonly called objects. Classes
are discussed in detail in chapter 7 and are used extensively in
C++.
    o  The stream cin extracts data from a stream and copies the
extracted information to variables (e.g., ival in the above example) using
the extraction operator (two consecutive > characters: >>). Later in
the Annotations we will describe how operators in C++ can perform quite
different actions than what they are defined to do by the language, as is the
case here. Function overloading has already been mentioned.  In C++
operators can also have multiple definitions, which is called operator
overloading.
    o  The operators which manipulate cin, cout and cerr (i.e.,
>> and <<) also manipulate variables of different types. In the
above example cout << ival results in the printing of an integer
value, whereas cout << "Enter a number" results in the printing
of a string. The actions of the operators therefore depend on the types of
supplied variables.
    o  The extraction operator (>>) performs a so called
        type safe assignment to a variable by `extracting' its value from
a text stream. Normally, the extraction operator skips all
        whitespace  characters preceding
the values to be extracted.
    o  Special symbolic constants are used for special
situations. Normally a line is terminated by inserting "\n" or
'\n'. But when inserting the endl symbol the line is terminated
followed by the flushing of the stream's internal buffer. Thus, endl can
usually be avoided in favor of '\n' resulting in somewhat more efficient
code.

The stream objects cin, cout and cerr are not part of the
C++ grammar proper. The streams are part of the definitions in the header
file iostream. This is comparable to functions like printf that are
not part of the C grammar, but were originally written by people who
considered such functions important and collected them in a run-time library.

A program may still use the old-style functions like printf and scanf
rather than the new-style streams.  The two styles can even be mixed. But
streams offer several clear advantages and in many C++ programs have
completely replaced the old-style C functions. Some advantages of using
streams are:
    
    o  Using insertion and extraction operators is
type-safe.  The format strings which are used with printf and
scanf can define wrong format specifiers for their arguments, for which
the compiler sometimes can't warn. In contrast, argument checking with
cin, cout and cerr is performed by the compiler. Consequently it
isn't possible to err by providing an int argument in places where,
according to the format string, a string argument should appear. With streams
there are no format strings.
    o  The functions printf and scanf (and other functions using
format strings) in fact implement a mini-language which is interpreted at
run-time. In contrast, with streams the C++ compiler knows exactly which
in- or output action to perform given the arguments used. No mini-language
here.
    o  In addition the possibilities of the insertion and extraction
operators may be extended allowing objects of classes that didn't exist
when the streams were originally designed to be inserted into or extracted
from streams. Mini languages as used with printf cannot be extended.
    o  The usage of the left-shift and right-shift operators in the context
of the streams illustrates yet another capability of C++: operator
overloading allowing us to redefine the actions an operator performs in
certain contexts. Coming from C operator overloading requires some getting
used to, but after a short little while these overloaded operators feel rather
comfortable.
    o  Streams are independent of the media they operate upon. This (at this
point somewhat abstract) notion means that the same code can be used without
any modification at all to interface your code to any kind of
device. The code using streams can be used when the device is a file on disk;
an Internet connection; a digital camera; a DVD device; a satellite link; and
much more: you name it. Streams allow your code to be decoupled (independent)
of the devices your code is supposed to operate on, which eases maintenance
and allows reuse of the same code in new situations.

The iostream library has a lot more to offer than just cin, cout
and cerr. In chapter 6 iostreams are covered in
greater detail. Even though printf and friends can still be used in
C++ programs, streams have practically replaced the old-style C
I/O functions like printf. If you think you still need to use
printf and related functions, think again: in that case you've probably
not yet completely grasped the possibilities of stream objects.


3.2: Functions as part of structs

Earlier we noted that functions can be part of structs (see
section 2.5.13).  Such functions are called
    member functions.
This section briefly discusses how to define such functions.

The code fragment below shows a struct having data fields for a person's
name and address. A function print is included in the
struct's definition:
            struct Person
    {
        char name[80];
        char address[80];

        void print();
    };

When defining the member function print the structure's name
(Person) and the scope resolution operator (::) are used:
            void Person::print()
    {
        cout << "Name:      " << name << "\n"
                "Address:   " << address << '\n';
    }

The implementation of Person::print shows how the fields of the
struct can be accessed without using the structure's type name. Here the
function Person::print prints a variable name. Since Person::print
is itself a part of struct person, the variable name implicitly
refers to the same type.

This struct Person could be used as follows:
            Person person;

    strcpy(person.name, "Karel");
    strcpy(person.address, "Marskramerstraat 33");
    person.print();

The advantage of member functions is that the called function
automatically accesses the data fields of the structure for which it was
invoked. In the statement person.print() the object person is the
`substrate': the variables name and address that are used in the code
of print refer to the data stored in the person object.


3.2.1: Data hiding: public, private and class

    As mentioned before (see section 2.3), C++ contains
specialized syntactic possibilities to implement data hiding. Data hiding
is the capability of sections of a program to hide its data from other
sections. This results in very clean data definitions. It also allows these
sections to enforce the integrity of their data.

C++ has three keywords that are related to data hiding: private,
protected and public. These keywords can be used in the definition of
structs. The keyword public allows all subsequent fields of a
structure to be accessed by all code; the keyword private only allows code
that is part of the struct itself to access subsequent fields. The keyword
protected is discussed in chapter 13, and is somewhat
outside of the scope of the current discussion.

In a struct all fields are public, unless explicitly stated otherwise.
Using this knowledge we can expand the struct Person:
            struct Person
    {
        private:
            char d_name[80];
            char d_address[80];
        public:
            void setName(char const *n);
            void setAddress(char const *a);
            void print();
            char const *name();
            char const *address();
    };

As the data fields d_name and d_address are in a private
section they are only accessible to the member functions which are defined in
the struct: these are the functions setName, setAddress etc.. As
an illustration consider the following code:
            Person fbb;

    fbb.setName("Frank");         // OK, setName is public
    strcpy(fbb.d_name, "Knarf");  // error, x.d_name is private

Data integrity is implemented as follows: the actual data of a struct
Person are mentioned in the structure definition. The data are accessed by
the outside world using special functions that are also part of the
definition. These member functions control all traffic between the data fields
and other parts of the program and are therefore also called `interface'
functions.  The thus implemented data hiding is illustrated in
Figure 2.
        ------------------------------------------------------------------Insert Figure 2(Private data and public interface functions of the class Person.)about here (file: first/datahiding)------------------------------------------------------------------
    The members setName and setAddress are declared with char const
* parameters. This indicates that the functions will not alter the strings
which are supplied as their arguments.  Analogously, the members name
and address return char const *s: the compiler prevents callers of
those members from modifying  the information made accessible through the
return values of those members.

Two examples of member functions of the struct Person are shown
below:
            void Person::setName(char const *n)
    {
        strncpy(d_name, n, 79);
        d_name[79] = 0;
    }

    char const *Person::name()
    {
        return d_name;
    }

The power of member functions and of the concept of data hiding results
from the abilities of member functions to perform special tasks, e.g.,
checking the validity of the data. In the above example setName copies
only up to 79 characters from its argument to the data member name,
thereby avoiding a buffer overflow.

Another illustration of the concept of data hiding is the following. As an
alternative to member functions that keep their data in memory a library could
be developed featuring member functions storing data on file. To convert a
program storing Person structures in memory to one that stores the
data on disk no special modifications are required. After recompilation
and linking the program to a new library it is converted from storage
in memory to storage on disk. This example illustrates a broader concept than
data hiding; it illustrates encapsulation. Data hiding is a kind of
encapsulation. Encapsulation in general results in reduced coupling of
different sections of a program. This in turn greatly enhances reusability and
maintainability of the resulting software. By having the structure encapsulate
the actual storage medium the program using the structure becomes independent
of the actual storage medium that is used.

Though data hiding can be implemented using structs, more often (almost
always) classes are used instead. A class is a kind of struct, except that
a class uses private access by default, whereas structs use public access by
default. The definition of a class Person is therefore identical to
the one shown above, except that the keyword class has
replaced struct while the initial private: clause can be omitted.  Our
typographic suggestion for class names (and other type names defined by the
programmer) is to start with a capital character to be followed by the
remainder of the type name using lower case letters (e.g., Person).


3.2.2: Structs in C vs. structs in C++

    In this section we'll discuss an important difference between C and
C++ structs and (member) functions. In C it is common to define
several functions to process a struct, which then require a pointer to the
struct as one of their arguments. An imaginary C header file showing
this concept is:
            /* definition of a struct PERSON    This is C   */
    typedef struct
    {
        char name[80];
        char address[80];
    } PERSON;

    /* some functions to manipulate PERSON structs */

    /* initialize fields with a name and address    */
    void initialize(PERSON *p, char const *nm,
                       char const *adr);

    /* print information    */
    void print(PERSON const *p);

    /* etc..    */

In C++, the declarations of the involved functions are put inside
the definition of the struct or class. The argument denoting
which struct is involved is no longer needed.
            class Person
    {
        char d_name[80];
        char d_address[80];

        public:
            void initialize(char const *nm, char const *adr);
            void print();
            // etc..
    };

In C++ the struct parameter is not used. A C function call
such as:
            PERSON x;

    initialize(&x, "some name", "some address");

becomes in C++:
            Person x;

    x.initialize("some name", "some address");


3.3: Several additions to C's grammar



3.3.1: References

    In addition to the common ways to define variables (plain variables or
pointers) C++ introduces references defining synonyms
for variables. A reference to a variable is like an alias; the variable
and the reference can both be used in statements involving the variable:
            int int_value;
    int &ref = int_value;

In the above example a variable int_value is defined. Subsequently a
reference ref is defined, which (due to its initialization) refers to the
same memory location as int_value. In the definition of ref, the
reference operator  & indicates that ref is not
itself an int but a reference to one. The two statements
            ++int_value;
    ++ref;

have the same effect: they increment int_value's value.  Whether that
location is called int_value or ref does not matter.

References serve an important function in C++ as a means to pass
modifiable arguments to functions. E.g., in standard C, a function that
increases the value of its argument by five and returning nothing needs a
pointer parameter:
            void increase(int *valp)    // expects a pointer
    {                           // to an int
        *valp += 5;
    }

    int main()
    {
        int x;

        increase(&x);           // pass x's address
    }

This construction can also be used in C++ but the same effect
is also achieved using a reference:
            void increase(int &valr)    // expects a reference
    {                           // to an int
        valr += 5;
    }

    int main()
    {
        int x;

        increase(x);            // passed as reference
    }

It is arguable whether code such as the above should be preferred over
C's method, though. The statement increase (x) suggests that not
x itself but a copy is passed. Yet the value of x changes because
of the way increase() is defined. However, references can also be used to
pass objects that are only inspected (without the need for a copy or a const
*) or to pass objects whose modification is an accepted side-effect of their
use. In those cases using references are strongly preferred over existing
alternatives like copy by value or passing pointers.

Behind the scenes references are implemented using pointers. So, as far as
the compiler is concerned references in C++ are just const pointers.  With
references, however, the programmer does not need to know or to bother about
levels of indirection. An important distinction between plain pointers and
references is of course that with references no indirection takes place. For
example:
            extern int *ip;
    extern int &ir;

    ip = 0;     // reassigns ip, now a 0-pointer
    ir = 0;     // ir unchanged, the int variable it refers to
                // is now 0.

In order to prevent confusion, we suggest to adhere to the following:
    
    o  In those situations where a function does not alter its
parameters of a built-in or pointer type, value parameters can be used:
        void some_func(int val)
{
    cout << val << '\n';
}

int main()
{
    int x;

    some_func(x);       // a copy is passed
}

o  When a function explicitly must change the values of its arguments, a
pointer parameter is preferred. These pointer parameters should preferably be
the function's initial parameters. This is called return by argument.
        void by_pointer(int *valp)
{
    *valp += 5;
}

o  When a function doesn't change the value of its class- or struct-type
arguments, or if the modification of the argument is a trivial side-effect
(e.g., the argument is a stream) references can be used. Const-references
should be used if the function does not modify the argument:
        void by_reference(string const &str)
{
    cout << str;    // no modification of str
}

int main ()
{
    int x = 7;
    by_pointer(&x);         // a pointer is passed
                            // x might be changed
    string str("hello");
    by_reference(str);      // str is not altered
}

References play an important role in cases where the argument is not
changed by the function but where it is undesirable to copy the argument to
initialize the parameter. Such a situation occurs when a large object is
passed as argument, or is returned by the function.  In these cases the
copying operation tends to become a significant factor, as the entire
object must be copied. In these cases references are preferred.

If the argument isn't modified by the function, or if the caller shouldn't
modify the returned information, the const keyword should be
used. Consider the following example:
        struct Person                   // some large structure
{
    char    name[80];
    char    address[90];
    double  salary;
};

Person person[50];          // database of persons

                            // printperson expects a
                            // reference to a structure
                            // but won't change it
void printperson (Person const &subject)
{
    cout << "Name: " << subject.name << '\n' <<
            "Address: " << subject.address << '\n';

}
                            // get a person by index value
Person const &personIdx(int index)
{
    return person[index];   // a reference is returned,
}                           // not a copy of person[index]

int main()
{
    Person boss;

    printperson(boss);      // no pointer is passed,
                            // so `boss' won't be
                            // altered by the function
    printperson(personIdx(5));
                            // references, not copies
                            // are passed here
}

o  Furthermore, note that there is yet another reason for using
references when passing objects as function arguments. When passing a
reference to an object, the activation of a so called copy constructor
is avoided. Copy constructors are covered in chapter 9.

References could result in extremely `ugly' code. A function may
return a reference to a variable, as in the following example:
            int &func()
    {
        static int value;
        return value;
    }

This allows the use of the following constructions:
            func() = 20;
    func() += func();

It is probably superfluous to note that such constructions should normally
not be used. Nonetheless, there are situations where it is useful to return a
reference.  We have actually already seen an example of this phenomenon in our
previous discussion of streams. In a statement like cout <<
"Hello" << '\n'; the insertion operator returns a reference to
cout. So, in this statement first the "Hello" is inserted into
cout, producing a reference to cout. Through this reference the
'\n' is then inserted in the cout object, again producing a reference
to cout, which is then ignored.

Several differences between pointers and references are pointed out in the
next list below:
    
    o  A reference cannot exist by itself, i.e., without something to
refer to. A declaration of a reference like
        
int &ref;

    is not allowed; what would ref refer to?
    o  References can be declared as external.  These references were
initialized elsewhere.
    o  References may exist as parameters of functions: they are initialized
when the function is called.
    o  References may be used in the return types of functions. In those
cases the function determines what the return value refers to.
    o  References may be used as data members of classes. We return
to this usage later.
    o  Pointers are variables by themselves. They point at
something concrete or just ``at nothing''.
    o  References are aliases for other variables and cannot be re-aliased
to another variable. Once a reference is defined, it refers to its particular
variable.
    o  Pointers (except for const pointers) can be reassigned to point to
different variables.
    o  When an address-of operator  & is used with a
reference, the expression yields the address of the variable to which the
reference applies. In contrast, ordinary pointers are variables themselves, so
the address of a pointer variable has nothing to do with the address of the
variable pointed to.


3.3.2: Rvalue References

    In C++, temporary (rvalue) values are indistinguishable from const &
types. C++ introduces a new reference type called an
    rvalue reference, which is defined as typename &&.

The name rvalue reference is derived from assignment statements, where the
variable to the left of the assignment operator is called an lvalue and
the expression to the right of the assignment operator is called an
rvalue. Rvalues are often temporary, anonymous values, like values
returned by functions.

In this parlance the C++ reference should be considered an
 lvalue reference (using the notation typename &). They can be
contrasted to rvalue references (using the notation typename &&).

The key to understanding rvalue references is the concept of an
 anonymous variable. An anonymous variable has no name and this is the
distinguishing feature for the compiler to associate it automatically with an
rvalue reference if it has a choice. Before introducing some interesting
constructions let's first have a look at some standard situations where
lvalue references are used. The following function returns a temporary
(anonymous) value:
            int intVal()
    {
        return 5;
    }

Although intVal's return value  can be assigned to an int
variable it requires copying, which might become prohibitive when
a function does not return an int but instead some large object. A
reference or pointer cannot be used either to collect the anonymous
return value as the return value won't survive beyond that. So the following
is illegal (as noted by the compiler):
            int &ir = intVal();         // fails: refers to a temporary
    int const &ic = intVal();   // OK: immutable temporary
    int *ip = &intVal();        // fails: no lvalue available

Apparently it is not possible to modify the temporary returned by
intVal. But now consider these functions:
            void receive(int &value)            // note: lvalue reference
    {
        cout << "int value parameter\n";
    }
    void receive(int &&value)           // note: rvalue reference
    {
        cout << "int R-value parameter\n";
    }

and let's call this function from main:
            int main()
    {
        receive(18);
        int value = 5;
        receive(value);
        receive(intVal());
    }

This program produces the following output:
            int R-value parameter
    int value parameter
    int R-value parameter

The program's output shows the compiler selecting receive(int &&value)
in all cases where it receives an anonymous int as its argument. Note that
this includes receive(18): a value 18 has no name and thus receive(int
&&value) is called. Internally, it actually uses a temporary variable to
store the 18, as is shown by the following example which modifies receive:
            void receive(int &&value)
    {
        ++value;
        cout << "int R-value parameter, now: " << value << '\n';
            // displays 19 and 6, respectively.
    }

Contrasting receive(int &value) with receive(int &&value) has
nothing to do with int &value not being a const reference. If
receive(int const &value) is used the same results are obtained. Bottom
line: the compiler selects the overloaded function using the rvalue reference
if the function is passed an anonymous value.

The compiler runs into problems if void receive(int &value) is
replaced by void receive(int value), though. When confronted with the
choice between a value parameter and a reference parameter (either lvalue or
rvalue) it cannot make a decision and reports an ambiguity. In practical
contexts this is not a problem. Rvalue references were added to the language in
order to be able to distinguish the two forms of references: named values
(for which lvalue references are used) and anonymous values (for which
rvalue references are used).

It is this distinction that allows the implementation of
 move semantics and perfect forwarding. At this point the concept of
move semantics cannot yet fully be discussed (but see section 9.7
for a more thorough discussion) but it is very well possible to illustrate
the underlying ideas.

Consider the situation where a function returns a struct Data containing a
pointer to a dynamically allocated NTBS. We agree that Data objects
are only used after initialization, for which two init functions
are available. As an aside: when Data objects are no longer required the
memory pointed at by text must again be returned to the operating
system; assume that that task is properly performed.
            struct Data
    {
        char *text;

        void init(char const *txt);     // initialize text from txt

        void init(Data const &other)
        {
            text = strdup(other.text);
        }
    };

There's also this interesting function:
            Data dataFactory(char const *text);

Its implementation is irrelevant, but it returns a (temporary) Data
object initialized with text. Such temporary objects cease to exist once
the statement in which they are created end.

Now we'll use Data:
            int main()
    {
        Data d1;
        d1.init(dataFactory("object"));
    }

Here the init function duplicates the NTBS stored in the temporary
object. Immediately thereafter the temporary object ceases to exist. If you
think about it, then you realize that that's a bit over the top:
    
    o  the dataFactory function uses init to initialize the text
        variable of its temporary Data object. For that it uses
        strdup; 
    o  the d1.init function then also uses strdup to initialize
        d1.text; 
    o  the statement ends, and the temporary object ceases to exist.

That's two strdup calls, but the temporary Data object thereafter
is never used again.

To handle cases like these rvalue reference were introduced. We add
the following function to the struct Data:
            void init(Data &&tmp)
    {
        text = tmp.text;      // (1)
        tmp.text = 0;         // (2)
    }

Now, when the compiler translates d1.init(dataFactory("object")) it
notices that dataFactory returns a (temporary) object, and because of that
it uses the init(Data &&tmp) function. As we know that the tmp object
ceases to exist after executing the statement in which it is used, the d1
object (at (1)) grabs the temporary object's text value, and then (at
(2)) assigns 0 to other.text so that the temporary object's free(text)
action does no harm.

Thus, struct Data suddenly has become move-aware and implements
move semantics, removing the (extra copy) drawback of the previous
approach, and instead of making an extra copy of the temporary object's NTBS
the pointer value is simply transferred to its new owner.


3.3.3: Lvalues, rvalues and more

    Although this section contains forward references to chapters 5,
7, and 16, its topic best fits the current
chapter. This section can be skipped without loss of continuity, and you might
consider returning to it once you're familiar with the content of these
future chapters.

Historically, the C programming language distinguished between lvalues
and rvalues. The terminology was based on assignment expressions, where
the expression to the left of the assignment operator receives a value (e.g.,
it referred to a location in memory where a value could be written into, like
a variable), while the expression to the right of the assignment operator only
had to represent a value (it could be a temporary variable, a constant value
or the value stored in a variable):
            lvalue = rvalue;

C++ adds to this basic distinction several new ways of referring to
expressions: 
    
    o lvalue: an lvalue in C++ has the same meaning as in
        C. It refers to a location where a value can be stored, like a
        variable, a reference to a variable, or a dereferenced pointer.
    o xvalue: an xvalue indicates an expiring value. An expiring
        value refers to an object (cf. chapter 7) just before
        its lifetime ends. Such objects normally have to make sure that
        resources they own (like dynamically allocated memory) also cease to
        exist, but such resources may, just before the object's lifetime ends,
        be moved to another location, thus preventing their destruction.
    o glvalue: a glvalue is a generalized lvalue. A generalized
        lvalue refers to anything that may receive a value. It is either an
        lvalue or an xvalue.
    o prvalue: a prvalue is a pure rvalue: a literal value (like
        1.2e3) or an immutable object (e.g., the value returned from a
        function returning a constant std::string (cf. chapter
        5)).

An expression's value is an xvalue if it is:
    
    o  the value returned by a function returning an rvalue reference to an
            object;
    o  an object that is cast to an rvalue reference;
    o  an expression accessing a non-static class data member whose object
            is 
        
        o  an xvalue, or
        o  a .* (pointer-to-member) expression (cf. chapter
            16) in which the left-hand side operand is an
            xvalue and the right-hand side operand is a pointer to a data
            member.

The effect of this rule is that named rvalue references are treated as
            lvalues and anonymous rvalue references to objects are treated as
            xvalues.

        Rvalue references to functions are treated as lvalues whether
            anonymous or not.

Here is a small example. Consider this simple struct:
            struct Demo 
    {
        int d_value;
    };

In addition we have these function declarations and definitions:
        Demo &&operator+(Demo const &lhs, Demo const &rhs);
    Demo &&factory();

    Demo demo;
    Demo &&rref = static_cast<Demo &&>(demo);

Expressions like
            factory();
    factory().d_value;
    static_cast<Demo &&>(demo);
    demo + demo

are xvalues. However, the expression
            rref;

is an lvalue.

In many situations it's not particularly important to know what kind of lvalue
or what kind of rvalue is actually used. In the C++ Annotations the term
lhs (left hand side) is frequently used to indicate an operand that's
written to the left of a binary operator, while the term rhs 
    (right hand side) is frequently used to indicate an operand that's
written to the right of a binary operator. Lhs and rhs operands could actually
be gvalues (e.g., when representing ordinary variables), but they could also
be prvalues (e.g., numeric values added together using the addition
operator). Whether or not lhs and rhs operands are gvalues or lvalues can
always be determined from the context in which they are used.


3.3.4: Strongly typed enumerations

    Enumeration values in C++ are in fact int values, thereby bypassing
type safety. E.g., values of different enumeration types may  be
compared for (in)equality, albeit through a (static) type cast.

Another problem with the current enum type is that their values are not
restricted to the enum type name itself, but to the scope where the
enumeration is defined. As a consequence, two enumerations having the same
scope cannot have identical names.

Such problems are solved by defining
    enum classes. An enum class can be defined as in the following
example:
            enum class SafeEnum
    {
        NOT_OK,     // 0, by implication
        OK          = 10,
        MAYBE_OK    // 11, by implication
    };

Enum classes use int values by default, but the used value type can
easily be changed using the : type notation, as in:
            enum class CharEnum: unsigned char
    {
        NOT_OK,
        OK
    };

To use a value defined in an enum class its enumeration name must be
provided as well. E.g., OK is not defined, CharEnum::OK is.

Using the data type specification (noting that it defaults to int) it
is possible to use enum class forward declarations.
        
    E.g.,
            enum Enum1;                 // Illegal: no size available
    enum Enum2: unsigned int;   // Legal: explicitly declared type

    enum class Enum3;           // Legal: default int type is used
    enum class Enum4: char;     // Legal: explicitly declared type

A sequence of symbols of a strongly typed enumeration can also be
indicated in a switch using the ellipsis syntax, as shown in the next
example:
            SafeEnum enumValue();

    switch (enumValue())
    {
        case SafeEnum::NOT_OK ... SafeEnum::OK:
            cout << "Status is known\n";
        break;

        default: 
            cout << "Status unknown\n";
        break;
    }


3.3.5: Initializer lists

    The C language defines the initializer list as a list of values enclosed
by curly braces, possibly themselves containing initializer lists. In C
these initializer lists are commonly used to initialize arrays and structs.

C++ extends this concept by introducing the type
    initializer_list<Type> where Type is replaced by the type name of
the values used in the initializer list. Initializer lists in C++ are,
like their counterparts in C, recursive, so they can also be used with
multi-dimensional arrays, structs and classes.

Before using the initializer_list the <initializer_list> header file
must be included.

Like in C, initializer lists consist of a list of values surrounded by
curly braces. But unlike C, functions can define initializer list
parameters. E.g.,
            void values(std::initializer_list<int> iniValues)
    {
    }

A function like values could be called as follows:
            values({2, 3, 5, 7, 11, 13});

The initializer list appears as an argument which is a  list of values
surrounded by curly braces. Due to the recursive nature of initializer lists a
two-dimensional series of values can also be passes, as shown in the next
example:
            void values2(std::initializer_list<std::initializer_list<int>> iniValues)
    {}

    values2({{1, 2}, {2, 3}, {3, 5}, {4, 7}, {5, 11}, {6, 13}});

Initializer lists are constant expressions and cannot be
modified. However, their size and values may be retrieved using their
size, begin, and end members as follows:

    void values(initializer_list<int> iniValues)
    {
        cout << "Initializer list having " << iniValues.size() << "values\n";
        for
        (
            initializer_list<int>::const_iterator begin = iniValues.begin();
                begin != iniValues.end();
                    ++begin
        )
            cout << "Value: " << *begin << '\n';
    }

Initializer lists can also be used to initialize objects of classes
(cf. section 7.5, which also summarizes the facilities of
initializer lists).

Implicit conversions, also called narrowing conversions 
     are not allowed when specifying values of
initializer lists. Narrowing conversions are encountered when values are used
of a type whose range is larger than the type specified when defining the
initializer list. For example
    
    o  specifying float or double values to define initializer
        lists of int values;
    o  specifying integral values exceeding the range of float to
        define initializer lists of float values;
    o  specifying values of integral types of a wider range than the
        integral type that is specified for the initializer list, except if
        the specified values lie within the range of the initializer list's
        integral type

Some examples:
        initializer_list<int> ii{ 1.2 };        // 1.2 isn't an int value
    initializer_list<unsigned> iu{ ~0ULL }; // unsigned long long doesn't fit
    


3.3.5.1: Designated initialization

        C++, like C, also supports designated initialization. However, as
C++ requires that destruction of data members occurs in the opposite order
as their construction it is required that, when using
designated initialization,  members are initialized in the order in which they
are declared in their class or struct. E.g.,
        struct Data
    {
        int d_first;
        double d_second;
        std::string d_third;
    };

    Data data{ .d_first = 1, .d_third = "hello" };

In this example, d_first and d_third are explicitly initialized,
while d_second is implicitly initialized to its default value (so: 0.0).

In C++ it is not allowed to reorder the initialization of members in a
desginated initialization list. So, Data data{ .d_third = "hello", .d_first
= 1 } is an error, but Data data{ .d_third = "hello" } is OK, as there is
no ordering conflict in the latter example (this also initializes d_first
and d_second to 0).

Likewise, a union 
     can be initialized using designated
initialization, as illustrated by the next example:
            union Data
    {
        int d_first;
        double d_second;
        std::string *d_third;
    };
                        // initialize the union's d_third field:
    Data data{ .d_third = new string{ "hello" } };


3.3.6: Initializers for bit-fields

    Bit-fields  are used to specify series of bits in an integral
value type. For example, in networking software processing IP4 packets, the
first uint32_t value of IP4 packets contain:
    
    o  the version (4 bits);
    o  the header length (4 bits);
    o  the type of service (8 bits);
    o  the total length (16 bits)

Rather than using complex bit and bit-shift operations, these fields inside
integral values can be specified using bit-fields. E.g.,
            struct FirstIP4word
    {
        uint32_t version:   4;
        uint32_t header:    4;
        uint32_t tos:       8;
        uint32_t length:   16;
    };

To total size of a FirstIP4word object is 32 bits, or four bytes. To
show the version of a FirstIP4word first object, simply do:
            cout << first.version << '\n';

and to set its header length to 10 simply do 
            first.header = 10;

Bit fields are already available in C. The C++26 standard allows them
to be initialized by default by using initialization expressions in their
definitions. E.g.,
            struct FirstIP4word
    {
        uint32_t version:   4 = 1;  // version now 1, by default
        uint32_t header:    4 = 10; // TCP header length now 10, by default
        uint32_t tos:       8;
        uint32_t length:   16;
    };

The initialization expressions are evaluated when the object using the
bit-fields is defined. Also, when a variable is used to initialize a bit-field
the variable must at least have been declared when the struct containing
bit-fields is defined. E.g.,
            extern int value;

    struct FirstIP4word
    {
        ...
        uint32_t length:   16 = value;  // OK: value has been declared
    };


3.3.7: Type inference using `auto'

    The keyword auto can be used to simplify type definitions of variables and
return types of functions if  the compiler is able to determine the proper
types of such variables or functions.

Using auto as a storage class specifier is no longer supported by C++:
a variable definition like auto int var results in a compilation error.

The keyword auto is used in situations where it is very hard to determine
the variable's type. These situations are encountered, e.g., in the context of
templates (cf. chapters 18 until 23). It is also used
in situations where a known type is a very long one but also automatically
available to the compiler. In such cases the programmer uses auto to avoid
having to type long type definitions.

At this point in the Annotations only simple examples can be given.  Refer to
section 21.1.2 for additional information about auto (and the
related decltype function).

When defining and initializing a variable int variable = 5 the type of the
initializing expression is well known: it's an int, and unless the
programmer's intentions are different this could be used to define
variable's type (a somewhat contrived example as in this case it
reduces rather than improves the clarity of the code):
            auto variable = 5;

However, it is attractive to use auto.  In chapter 5 the
iterator concept is introduced (see also chapters 12 and
18). Iterators  frequently have long type definitions, like
            std::vector<std::string>::const_reverse_iterator

Functions may return objects having such types. Since the compiler knows
about these types we may exploit this knowledge by using auto. Assume that
a function begin() is declared like this:
            std::vector<std::string>::const_reverse_iterator begin();

Rather than writing a long variable definition (at // 1, below) a much
shorter definition (at // 2) can be used:
            std::vector<std::string>::const_reverse_iterator iter = begin();    // 1
    auto iter = begin();                                                // 2

It's also easy to define and initialize additional variables of such
types. When initializing such variables iter can be used to initialize
those variables, and auto can be used, so the compiler deduces their
types:
            auto start = iter;

When defining variables using auto the variable's type is deduced from
the variable's initializing expression. Plain types and pointer types are used
as-is, but when the initializing expression is a reference type, then the
reference's basic type (without the reference, omitting const or
volatile specifications) is used.

If a reference type is required then auto & or
auto && can be used. Likewise, const and/or pointer specifications can
be used in combination with the auto keyword itself. Here are some
examples:
            int value;
    auto another = value;   // 'int another' is defined

    string const &text();
    auto str = text();      // text's plain type is string, so 
                            // string str, NOT string const str
                            // is defined
    str += "...";           // so, this is OK

    int *ip = &value;
    auto ip2 = ip;          // int *ip2 is defined.

    int *const &ptr = ip;
    auto ip3 = ptr;         // int *ip3 is defined, omitting const &
    auto const &ip4 = ptr;  // int *const &ip4 is defined.

In the next to last auto specification, the tokens (reading right to
left) from the reference to the basic type are omitted: here const & was
appended to ptr's basic type (int *). Hence, int *ip2 is defined.

In the last auto specification auto also produces int *, but
in the type definition const & is added to the type produced by auto,
so int *const &ip4 is defined.

The auto keyword can also be used to postpone the definition of a
function's return type. The declaration of a function intArrPtr returning
a pointer to arrays of 10 ints looks like this:
            int (*intArrPtr())[10];

Such a declaration is fairly complex. E.g., among other complexities it
requires `protection of the pointer' using parentheses
in combination with the function's parameter list. In situations like these
the specification of the return type can be postponed using the auto
return type, followed by the specification of the function's return type after
any other specification the function might receive (e.g., as a const member
(cf. section 7.7) or following its noexcept specification
(cf. section 23.8)).

Using auto to declare the above function, the declaration becomes:
            auto intArrPtr() -> int (*)[10];

A return type specification using auto is called a
        late-specified return type.

Since the C++14 standard late return type specifications are no longer
required for functions returning auto. Such functions can now simply be
declared like this:
            auto autoReturnFunction();

In this case some restrictions apply, both to the function definitions and
the function declarations:
    
    o  If multiple return statements are used in function definitions they
all must return values of identical types;
    o  Functions merely returning auto cannot be used before the
compiler has seen their definitions. So they cannot be used after mere
declarations;
    o  When functions returning auto are implemented as recursive
function then at least one return statement must have been seen before the 
recursive call. E.g.,
            auto fibonacci(size_t n)
    {
        if (n <= 1)
            return n;
        return fibonacci(n - 1) + fibonacci(n - 2);
    }


3.3.7.1: Structured binding declarations

        Usually functions return single-valued results: doubles, ints, strings,
etc. When functions need to return multiple values a return by argument
construction is often used, where addresses of variables that live outside of
the called function are passed to functions, allowing the functions to assign
new values to those variables. 

When multiple values should be returned from a function a struct can
be used, but pairs (cf. section 12.2) or tuples (cf. section
22.6) can also be used. Here's a simple example, where a function
fun returns a struct having two data fields:
            struct Return
    {
        int first;
        double second;
    };
    
    Return fun()
    {
        return Return{ 1, 12.5 };
    }

(Briefly forward referencing to sections 12.2 and 22.6: the
struct definition can completely  be omitted if fun returns a
pair or tuple. In those cases the following code remains valid.)

A function calling fun traditionally defines a variable
of the same type as fun's return type, and then uses that variable's
fields to access first and second. If you don't like the typing,
auto can also be used:
            int main()
    {
        auto r1 = fun();
        cout << r1.first;
    }

Instead of referring to the elements of the returned struct, pair or
tuple structured binding declarations can also be used. Here, auto
is followed by a (square brackets surrounded) comma-separated list of
variables, where each variable is defined, and receives the value of the
corresponding field or element of the called function's return value. So, the
above main function can also be written like this:
            int main()
    {
        auto [one, two] = fun();
        cout << one;                // one and two: now defined
    }

Merely specifying  auto results in fun's return value being
copied, and the structured bindings variables will refer to the copied value. 
But structured binding declarations can also be used in combination with
(lvalue/rvalue) return values. The following ensures that rone and 
rtwo refer to the elements of fun's anonymous return value:
            int main()
    {
        auto &&[rone, rtwo] = fun();
    }

If the called function returns a value that survives the function call
itself, then structured binding declarations can use lvalue
references. E.g.,
            Return &fun2()
    {
        static Return ret{ 4, 5 };
        return ret;
    }

    int main()
    {
        auto &[lone, ltwo] = fun2();    // OK: referring to ret's fields
    }

To use structured binding declarations it is not required to use function
calls. The object providing the data can also anonymously be defined:
            int main()
    {
        auto const &[lone, ltwo] = Return{ 4, 5 }; 
        // or:
        auto &&[lone, ltwo] = Return{ 4, 5 }; 
    }

The object doesn't even have to make its data members publicly
available. In section TUPLES using structured bindings not necessarily
referring to data members is covered.

Another application is found in situations where nested statements of
for or selection statements benefit from using locally defined variables
of various types. Such variables can easily be defined using structured
binding declarations that are initialized from anonymous structs, pairs or
tuples. Here is an example illustrating this:
            // define a struct:
    struct Three
    {
        size_t year;
        double firstAmount;
        double interest;
    };
    // define an array of Three objects, and process each in turn:
    Three array[10];
    fill(array);            // not implemented here

    for (auto &[year, amount, interest]: array)
        cout << "Year " << year << ": amount = " << amount << '\n';

When using structured bindings the structured binding declaration must
specify all elements that are available. So if a struct has four data members
the structured binding declaration must define four elements. To avoid
warnings of unused variables at lease one of the variables of the structured
binding declaration must be used.


3.3.8: Defining types and `using' declarations

    In C++ typedef is commonly used to define shorthand notations for
complex types. Assume we want to define a shorthand for `a pointer to a
function expecting a double and an int, and returning an unsigned long long
int'. Such a function could be:
            unsigned long long int compute(double, int);

A pointer to such a function has the following form:
            unsigned long long int (*pf)(double, int);

If this kind of pointer is frequently used, consider defining it using
typedef: simply put typedef in front of it and the pointer's name is
turned into the name of a type. It could be capitalized to let it stand out
more clearly as the name of a type:
            typedef unsigned long long int (*PF)(double, int);

After having defined this type, it can be used to declare or define such
pointers:
            PF pf = compute;        // initialize the pointer to a function like
                            // 'compute'
    void fun(PF pf);        // fun expects a pointer to a function like
                            // 'compute'

However, including the pointer in the typedef might not be a very good
idea, as it masks the fact that pf is a pointer. After all, PF pf
looks more like `int x' than `int *x'. To document that pf is
in fact a pointer, slightly change the typedef:
            typedef unsigned long long int FUN(double, int);

    FUN *pf = compute;      // now pf clearly is a pointer.

The scope of typedefs is restricted to compilation units. Therefore,
typedefs are usually embedded in header files which are then included by
multiple source files in which the typedefs should be used.

In addition to typedef C++ offers the using keyword to
associate a type and an identifier. In practice typedef and using can
be used interchangeably. The using keyword arguably result in more
readable type definitions. Consider the following three (equivalent)
definitions:
    
    o  The traditional, C style definition of a type, embedding the type
        name in the definition (turning a variable name into a type name):
    typedef unsigned long long int FUN(double, int);

o  Apply using to improve the visibility (for humans) of the type
            name, by moving the type  name to the front of the definition:
        using FUN = unsigned long long int (double, int);

o  An alternative construction, using a late-specified return type
            (cf. section 3.3.7):
        using FUN = auto (double, int) -> unsigned long long int;


3.3.9: Range-based for-loops

    The C++ for-statement is identical to C's for-statement:
            for (init; cond; inc)
        statement

Often the initialization, condition, and increment parts are fairly
obvious, as in situations where all elements of an array or vector must be
processed. Many languages offer the foreach statement for that and C++
offers the std::for_each generic algorithm (cf. section 19.1.18).

In addition to the traditional syntax C++ adds new syntax for the
for-statement: the
    range-based for-loop. This new syntax can be used to process all
element of a range in turn. Three types of ranges are distinguished:
        
        o  Plain arrays (e.g., int array[10]);
        o  Initializer lists;
        o  Standard containers (or comparable) (cf. chapter
            12);
        o  Any other type offering begin() and end() functions
            returning so-called iterators (cf. section 18.2).

The following additional for-statement syntax is available:
            // assume int array[30]
    for (auto &element: array)
        statement

The part to the left of the colon is called the
    for range declaration. The declared variable (element) is a
formal name; use any identifier you like. The variable is only available
within the nested statement, and it refers to (or is a copy of) each of the
elements of the range, from the first element up to the last.

There's no formal requirement to use auto, but using auto is extremely
useful in many situations. Not only in situations where the range refers to
elements of some complex type, but also in situations where you know what you
can do with the elements in the range, but don't care about their exact type
names. In the above example int could also have been used.

The reference symbol (&) is important in the following cases:
    
    o  if you want to modify the elements in the nested statements
    o  if the elements themselves are structs (or classes,
        cf. chapter 7)

When the reference symbol is omitted the variable will be a copy of each
of the subsequent elements of the range. Fine, probably, if you merely need to
look at the variables when they are of primitive types, but needlessly
inefficient if you have an array of BigStruct elements:
            struct BigStruct
    {
        double array[100];
        int    last;
    };

Inefficient, because you don't need to make copies of the array's
elements. Instead, use references to elements:
            BigStruct data[100];    // assume properly initialized elsewhere

    int countUsed()
    {
        int sum = 0;
                            // const &: the elements aren't modified
        for (auto const &element: data)
            sum += element.last;
        return sum;
    }

Range-based for-loops can also benefit from structured bindings. If
struct Element holds a int key and a double value, and all the
values of positive keys should be added then the following code snippet
accomplishes that:
            Element elems[100];     // somehow initialized
    double sum = 0;
    for (auto const &[key, value]: elems)
    {
        if (key > 0)
            sum += value;
    }

The C++26 standard also supports an optional initialization section (like the
ones already available for if and switch statements) for range-based
for-loops. Assume the elements of an array must be inserted into cout, but
before each element we want to display the element's index.  The index
variable is not used outside the for-statement, and the extension offered
in the C++26 standard allows us to localize the index variable. Here is an
example:
                                    // localize idx: only visible in the for-stmnt
    for (size_t idx = 0; auto const &element: data)
        cout << idx++ << ": " << element << '\n';


3.3.10: Raw String Literals

    Standard series of ASCII characters (a.k.a. C strings) are delimited by
double quotes, supporting
  escape sequences like \n, \\ and \", and ending
in 0-bytes. Such series of ASCII-characters are commonly known as
null-terminated byte strings (singular: NTBS, plural: NTBSs).
C's NTBS is the foundation upon which an enormous amount of code has
been built

In some cases it is attractive to be able to avoid having to use escape
sequences (e.g., in the context of XML). C++ allows this using 
     raw string literals.

Raw string literals start with an R, followed by a double quote,
optionally followed by a label (which is an arbitrary sequence of non-blank
characters, followed by (). The raw string ends at the
closing parenthesis ), followed by the label (if specified when
starting the raw string literal), which is in turn followed by a double
quote. Here are some examples:
            R"(A Raw \ "String")"
    R"delimiter(Another \ Raw "(String))delimiter"

In the first case, everything between "( and )" is
part of the string. Escape sequences aren't supported so the text \ "
within the first raw string literal defines three characters: a backslash, a
blank character and a double quote. The second example shows a raw string
defined between the markers "delimiter( and
)delimiter". 

Raw string literals come in very handy when long, complex ascii-character
sequences (e.g., usage-info or long html-sequences) are used. In the end they
are just that: long NTBSs. Those long raw string literals should be separated
from the code that uses them, thus maintaining the readability of the using
code. 

As an illustration: the bisonc++ parser generator supports an option
--prompt. When specified, the code generated by bisonc++ inserts
prompting code when debugging is requested. Directly inserting the raw string
literal into the function processing the prompting code results in code that
is very hard to read:
            void prompt(ostream &out)
    {
        if (d_genDebug)
            out << (d_options.prompt() ? R"(
            if (d_debug__)
            {
                s_out__ << "\n================\n"
                           "? " << dflush__;
                std::string s;
                getline(std::cin, s);
            }
    )"              : R"(
            if (d_debug__)
                s_out__ << '\n';
    )"
                    ) << '\n';        
    }

Readability is greatly enhanced by defining the raw string literals as named
NTBSs, defined in the source file's anonymous namespace (cf. chapter
4):
            namespace {
    
    char const noPrompt[] = 
    R"(
            if (d_debug__)
                s_out__ << '\n';
    )";
    
    char const doPrompt[] = 
    R"(
            if (d_debug__)
            {
                s_out__ << "\n================\n"
                           "? " << dflush__;
                std::string s;
                getline(std::cin, s);
            }
    )";
    
    } // anonymous namespace
    
    void prompt(ostream &out)
    {
        if (d_genDebug)
            out << (d_options.prompt() ? doPrompt : noPrompt) << '\n';        
    }


3.3.11: Binary constants

    In addition to hexadecimal  integral constants
(starting with 0x), octal integral  constants (starting
with 0), and decimal integral  constants (starting
with one of the digits 1..9), binary integral 
constants can be defined using the prefixes 0b or
0B. E.g., to represent the (decimal) value 5 the notation 0b101 can
also be used. 

The binary constants come in handy in the context of, e.g., bit-flags, as
it immediately shows which bit-fields are set, while other notations are less
informative.


3.3.12: Selection statements with initializers

    The standard for repetition statements start with an optional
initialization clause. The initialization clause allows us to localize
variables to the scope of the for statements.  Initialization clauses can also
be used in selection statements.

Consider the situation where an action should be performed if the next line
read from the standard input stream equals go!. Traditionally, when used
inside a function, intending to localize the string to contain the
content of the next line as much as possible, constructions like the
following had to be used:
            void function()
    {
        // ... any set of statements
        {
            string line;    // localize line
            if (getline(cin, line))
                action();
        }
        // ... any set of statements
    }

Since init ; clauses can also be used for selection statements (if
and switch statements) (note that with selection statements the semicolon
is part of the initialization clause, which is different from the optional
init (no semicolon) clause in for statements), we can 
rephrase the above example as follows:
            void function()
    {
        // ... any set of statements
        if (string line; getline(cin, line))
            action();
        // ... any set of statements
    }

Note that a variable may still also be defined in the actual condition
clauses.  This is true for both the extended if and switch
statement. However, before using the condition clauses an initialization
clause may be used to define additional variables (plural, as it may contain a
comma-separated list of variables, similar to the syntax that's available for
for-statements).


3.3.13: Attributes

    Attributes  are compiler directives that are inserted into source
files to inform the compiler of some peculiarity of the code (variable or
function) that follows the specified attribute. Attributes are used to inform
the compiler about situations that are intentional, and thus prevent the
compiler from issuing warnings.

The following attributes are recognized:


    o [[carries_dependency]]:
This attribute is currently not yet covered by the C++ Annotations. At
this point in the C++ Annotations it can safely be ignored.

o [[deprecated]]:
This attribute (and its alternative form
[[deprecated("reason")]]) is available since the C++14 standard.  It
indicates that the use of the name or entity declared with this attribute is
allowed, but discouraged for some reason. This attribute can be used for
classes, typedef-names, variables, non-static data members, functions,
enumerations, and template specializations. An existing non-deprecated entity
may be redeclared deprecated, but once an entity has been declared deprecated
it cannot be redeclared as `undeprecated'. When encountering the
[[deprecated]] attribute the compiler generates a warning, e.g.,
        demo.cc:12:24: warning: 'void deprecatedFunction()' is deprecated 
                    [-Wdeprecated-declarations] deprecatedFunction();

    demo.cc:5:21: note: declared here
                     [[deprecated]] void deprecatedFunction()

When using the alternative form (e.g., 
        [[deprecated("do not use")]] void fun()) the compiler generates a
warning showing the text between the double quotes, e.g.,
        demo.cc:12:24: warning: 'void deprecatedFunction()' is deprecated: 
        do not use [-Wdeprecated-declarations] 
     deprecatedFunction();

    demo.cc:5:38: note: declared here
         [[deprecated("do not use")]] void deprecatedFunction()

o [[fallthrough]]


When statements nested under case entries in switch statements
continue into subsequent case or default entries the compiler issues a
`falling through' warning. If falling through is intentional the attribute
[[fallthrough]], which then must be followed by a semicolon, should be
used. Here is an annotated example:
        void function(int selector)
{
    switch (selector)
    {
        case 1:
        case 2:             // no falling through, but merged entry points
            cout << "cases 1 and 2\n";
        [[fallthrough]];    // no warning: intentionally falling through

        case 3:
            cout << "case 3\n";

        case 4:             // a warning is issued: falling through not
                            // announced.
            cout << "case 4\n";
        [[fallthrough]];    // error: there's nothing beyond 
    }
}

o [[maybe_unused]]


This attribute can be applied to a class, typedef-name, variable,
parameter, non-static data member, a function, an enumeration or an
enumerator. When it is applied to an entity no warning is generated when the
entity is not used. Example:
        void fun([[maybe_unused]] size_t argument)
{
    // argument isn't used, but no warning 
    // telling you so is issued
}

o [[nodiscard]]


The attribute [[nodiscard]] may be specified when declaring a
function, class or enumeration. If a function is declared [[nodiscard]] or
if a function returns an entity previously declared using [[nodiscard]]
then the return value of such a function may only be ignored when explicitly
cast to void. Otherwise, when the return value is not used a warning is
issued. Example:
        int [[nodiscard]] importantInt();
struct [[nodiscard]] ImportantStruct { ... };

ImportantStruct factory();
    
int main()
{
    importantInt();         // warning issued
    factory();              // warning issued
}

o [[noreturn]]:
[[noreturn]] indicates that the function does not
return. [[noreturn]]'s behavior is undefined if the function declared with
this attribute actually returns. The following standard functions have this
attribute: std::_Exit, std::abort, std::exit, std::quick_exit,
std::unexpected, std::terminate, std::rethrow_exception,
std::throw_with_nested, std::nested_exception::rethrow_nested, Here is an
example of a function declaration and definition using the [[noreturn]]
attribute:
            [[noreturn]] void doesntReturn();

    [[noreturn]] void doesntReturn()
    {
        exit(0);
    }


3.3.14: Three-way comparison (<=>)

    The C++26 standard added the three-way comparison operator <=>, also
known as the spaceship operator, to C++. In C++ operators can be
defined for class-types, among which equality and comparison operators (the
familiar set of ==, !=, <, <=, > and >= operators).  To provide
classes with all comparison operators merely the equality and the spaceship
operator need to be defined.

Its priority  is less than the priorities of the bit-shift
operators << and >> and larger than the priorities of the ordering
operators <, <=, >, and >=.

Section 11.7.2 covers the construction of the three-way comparison
operator.


3.4: New language-defined data types

In C the following built-in data types are available: void, char,
short, int, long, float and double. C++ extends these built-in types
with several additional built-in types: the types bool, wchar_t,
long long and long double (Cf. ANSI/ISO draft (1995),
par. 27.6.2.4.1 for examples of these very long types).  The type
 long long is merely a double-long long datatype. The type
 long double is merely a double-long double datatype. These built-in
types as well as pointer variables are called
 primitive types in the C++ Annotations.

There is a subtle issue to be aware of when converting applications developed
for 32-bit architectures to 64-bit architectures. When converting 32-bit
programs to 64-bit programs, only long types and pointer types change in
size from 32 bits to 64 bits; integers of type int remain at their size of
32 bits. This may cause data truncation when assigning pointer or long
types to int types. Also, problems with sign extension can occur when
assigning expressions using types shorter than the size of an int to an
unsigned long or to a pointer.

Except for these built-in types the class-type string is available
for handling character strings. The datatypes bool, and wchar_t are
covered in the following sections, the datatype string is covered in
chapter 5. Note that recent versions of C may also have adopted
some of these newer data types (notably bool and wchar_t).
Traditionally, however, C doesn't support them, hence they are mentioned
here.

Now that these new types are introduced, let's refresh your memory about
letters that can be used in literal constants of various
  types. They are:
    
    o  b or B: in addition to its use as a hexadecimal
value, it can also be used to define a binary constant. E.g., 0b101
equals the decimal value 5. The 0b prefix can be used to specify binary
constants starting with the C++14 standard.
    o  E or e:  the
exponentiation character in floating point literal values. For example:
1.23E+3. Here, E should be pronounced (and interpreted) as: times 10
to the power. Therefore, 1.23E+3 represents the value 1230.
    o  F  can be used as postfix to a
non-integral numeric constant to indicate a value of type float, rather
than double, which is the default. For example: 12.F (the dot
transforms 12 into a floating point value); 1.23E+3F (see the previous
example. 1.23E+3 is a double value, whereas 1.23E+3F is a
float value).
    o  L  can be used as prefix to
indicate a character string whose elements are wchar_t-type
characters. For example: L"hello world".
    o  L  can be used as postfix to an
integral value to indicate a value of type long, rather than int,
which is the default. Note that there is no letter indicating a short
type. For that a static_cast<short>() must be used.
    o  p, to specify the power  in
hexadecimal floating point numbers.  E.g. 0x10p4. The exponent itself is
read as a decimal constant and can therefore not start with 0x. The exponent
part is interpreted as a power of 2. So 0x10p2 is (decimal) equal to 64:
16 * 2^2.
    o  U  can be used as postfix to an
integral value to indicate an unsigned  value, rather than an int.
It may also be combined with the postfix L to produce an unsigned long
int value.

And, of course: the x and a until f characters can be used to
specify hexadecimal constants (optionally using capital letters).


3.4.1: The data type `bool'

        The type bool represents boolean (logical) values, for which the (now
reserved) constants true and false may be used. Except for these
reserved values, integral values may also be assigned to variables of type
bool, which are then implicitly converted to true and false
according to the following conversion rules (assume intValue is an
int-variable, and boolValue is a bool-variable):
                // from int to bool:
    boolValue = intValue ? true : false;

        // from bool to int:
    intValue = boolValue ? 1 : 0;

Furthermore, when bool values are inserted into streams then true
is represented by 1, and false is represented by 0. Consider the
following example:
            cout << "A true value: "  << true << "\n"
            "A false value: " << false << '\n';

The bool data type is found in other programming languages as
well. Pascal has its type Boolean; Java has a boolean
type. Different from these languages, C++'s type bool acts like a kind
of int type. It is primarily a documentation-improving type, having just
two values true and false. Actually, these values can be interpreted
as enum values for 1 and 0. Doing so would ignore the philosophy
behind the bool data type, but nevertheless: assigning true to an
int variable neither produces warnings nor errors.

Using the bool-type is usually clearer than using
int. Consider the following prototypes:
            bool exists(char const *fileName);  // (1)
    int  exists(char const *fileName);  // (2)

With the first prototype, readers expect the function to
return true if the given filename is the name of an existing
file. However, with the second prototype some ambiguity arises: intuitively
the return value 1 is appealing, as it allows constructions like
            if (exists("myfile"))
        cout << "myfile exists";

On the other hand, many system functions (like access, stat, and
many other) return 0 to indicate a successful operation, reserving other
values to indicate various types of errors.

As a rule of thumb I suggest the following: if a function should inform
its caller about the success or failure of its task, let the function return a
bool value. If the function should return success or various types of
errors, let the function return enum values, documenting the situation by
its various symbolic constants. Only when the function returns a conceptually
meaningful integral value (like the sum of two int values), let the
function return an int value.


3.4.2: The data type `wchar_t'

    The wchar_t type is an extension of the char built-in type, to accommodate
wide character values (but see also the next section).  The g++
compiler reports sizeof(wchar_t) as 4, which easily accommodates all 65,536
different Unicode character values.

Note that Java's char data type is somewhat comparable to C++'s
wchar_t type. Java's char type is 2 bytes wide, though. On the
other hand, Java's byte data type is comparable to C++'s char
type: one byte. Confusing?


3.4.3: Unicode encoding

    In C++ string literals can be defined as NTBSs.  Prepending an NTBS
by L (e.g., L"hello") defines a wchar_t string literal.

C++ also supports 8, 16 and 32 bit Unicode encoded
strings. Furthermore, two new data types are introduced: char16_t and
char32_t storing, respectively, a UTF-16 and a UTF-32 unicode
value.

A char type value fits in a utf_8 unicode value. For character sets
exceeding 256 different values wider types (like char16_t or char32_t)
should be used.

String literals for the various types of unicode encodings (and associated
variables) can be defined as follows:
            char     utf_8[] = u8"This is UTF-8 encoded.";
    char16_t utf16[] = u"This is UTF-16 encoded.";
    char32_t utf32[] = U"This is UTF-32 encoded.";

Alternatively, unicode constants may be defined using the \u escape
sequence, followed by a hexadecimal value. Depending on the type of the
unicode variable (or constant) a UTF-8, UTF-16 or UTF-32 value is
used. E.g.,
            char     utf_8[] = u8"\u2018";
    char16_t utf16[] = u"\u2018";
    char32_t utf32[] = U"\u2018";

Unicode strings can be delimited by double quotes but raw string literals
can also be used.


3.4.4: The data type `long long int'

    C++ also supports the type long long int. On 32 bit systems it has at
least 64 usable bits.


3.4.5: The data type `size_t'

    The size_t type is not really a built-in primitive data type, but a data
type that is promoted by POSIX as a typename to be used for non-negative
integral values answering questions like `how much' and `how many', in which
case it should be used instead of unsigned int. It is not a specific
C++ type, but also available in, e.g., C. Usually it is defined
implicitly when a (any) system header file is included. The header file
`officially' defining size_t in the context of C++ is cstddef.

Using size_t has the advantage of being a conceptual type, rather than
a standard type that is then modified by a modifier. Thus, it improves
the self-documenting value of source code.

Several suffixes can be used to expicitly specify the intended representation
of integral constants, like 42UL defining 42 as an unsigned long
int. Likewise, suffixes uz or zu can be used to specify that an
integral constant is represented as a size_t, as in: cout << 42uz.

Sometimes functions explictly require unsigned int to be used. E.g., on
amd-architectures the X-windows function XQueryPointer explicitly
requires a pointer to an unsigned int variable as one of its arguments. In
such situations a pointer to a size_t variable can't be used, but the
address of an unsigned int must be provided. Such situations are
exceptional, though.

Other useful bit-represented types also exists. E.g., uint32_t is
guaranteed to hold 32-bits unsigned values. Analogously, int32_t holds
32-bits signed values. Corresponding types exist for 8, 16 and 64 bits
values. These types are defined in the header file cstdint and can be very
useful when you need to specify or use integral value types of fixed sizes.


3.4.6: The data type `std::byte'

     Quite often 8-bit variables are required, usually to access memory
locations. Traditionally the char type has been
used for that, but char is a signed type and when inserting a char
variable into a stream the character's representation instead of its value is
used. Maybe more important is the inherent confusion when using char type
variables when only using its (unsigned) value: a char documents to the
reader that text is used instead of mere 8-bit values, as used by the smallest
addressable memory locations.

Different from the char type the std::byte type intends to merely
represent an 8-bit value. In order to use std::byte the <cstddef>
header file must be included.

The byte is defined as a strongly typed enum, simply embedding an
unsigned char:
        enum class byte: unsigned char
    {};
    As a byte is an enum without predefined enum values plain assignments
can only be used between byte values. Byte variables can be
initialized using curly braces around an existing byte or around fixed
values of at most 8 bits (see #1 in the following example). If the specified
value doesn't fit in 8 bits (#2) or if the specified value is neither a
byte nor an unsigned char type variable (#3) the compiler reports an
error.

Assignments or assignment-like initializations using rvalues which are
bytes initialized using parentheses with values not fitting in 8 bits are
accepted (#4, #5). In these cases, the specified values are truncated to their
lowest 8 bits. Here are the illustrations:
            byte value{ 0x23 };     // #1 (see the text above)
    //  byte error{ 0x123 };    // #2
    
        char ch = 0xfb;
    //  byte error{ ch };       // #3
    
        byte b1 = byte( ch );   // #4
        value = byte( 0x123 );  // #5

The byte type supports all bit-wise operations, but the right-hand operand
of the bit-wise operator must also be a byte. E.g.,
           value &= byte(0xf0);
 Byte type values can also be ordered and compared for (in)equality.

Unfortunately, no other operations are supported. E.g., bytes cannot be
added and cannot be inserted into or extracted from streams, which somehow
renders the std::byte less useful than ordinary types (like unsigned
int, uint16_t). When needed such operations can be supported using casts
(covered in section 3.5), but it's considered good practice to avoid
casts whenever possible. However, C++ allows us to define a byte-type that
does behave like an ordinary numeric type, including and extracting its
values into and from streams. In section 11.4 such a type is developed.


3.4.7: Digit separators

    To improve the readability of large numbers 
    digit separators for integer and floating point
literals can be used. The digit separator is a single quote which may be
inserted between digits of such literals to enhance human
readability. Multiple digit separators may be used, but only one separator can
be inserted between successive digits. E.g.,
            1'000'000
    3.141'592'653'589'793'238'5
    ''123       // won't compile
    1''23       // won't compile either


3.5: A new syntax for casts

Traditionally, C offers the following cast syntax:
                    (typename)expression

here typename is the name of a valid type, and expression is an
expression.

C style casts are now deprecated. C++ programs should merely use
the new style C++ casts as they offer the compiler facilities to verify
the sensibility of the cast. Facilities which are not offered by the classic
C-style cast.

A cast should not be confused with the often used
    constructor notation:
            typename(expression)

the constructor notation is not a cast, but a request to the compiler to
construct an (anonymous) variable of type typename from expression.

If casts are really necessary one of several new-style casts should be
used. These new-style casts are introduced in the upcoming sections.


3.5.1: The `static_cast'-operator

    The static_cast<type>(expression) is used to convert
`conceptually comparable or related types' to each other. Here as well as in
other C++ style casts type is the type to which the type of
expression should be cast.

Here are some examples of situations where the static_cast can (or should)
be used:
    
    o  When converting an int to a double. 


This happens, for example when the quotient of two int values must be
computed without losing the fraction part of the division. The sqrt
function called in the following fragment returns 2:
    int x = 19;
int y = 4;
sqrt(x / y);

whereas it returns 2.179 when a static_cast is used, as in:
    sqrt(static_cast<double>(x) / y);

The important point to notice here is that a static_cast is allowed to
change the representation of its expression into the representation that's
used by the destination type.

Also note that the division is put outside of the cast expression. If the
division is performed within the cast's expression (as in
static_cast<double>(x / y)) an integer division has already been
performed before the cast has had a chance to convert the type of an
operand to double.

o  When converting enum values to int values (in any
direction). 


Here the two types use identical representations, but different
semantics. Assigning an ordinary enum value to an int doesn't require
a cast, but when the enum is a strongly typed enum a cast is
required. Conversely, a static_cast is required when assigning an int
value to a variable of some enum type. Here is an example:
    enum class Enum
{
    VALUE
};

cout << static_cast<int>(Enum::VALUE);    // show the numeric value

o  When converting related pointers to each other.


The static_cast is used in the context of class inheritance
(cf. chapter 13) to convert a pointer to a so-called `derived
class' to a pointer to its `base class'. It cannot be used for casting
unrelated types to each other (e.g., a static_cast cannot be used to
cast a pointer to a short to a pointer to an int).

A void * is a generic pointer. It is frequently used by
functions in the C library (e.g., memcpy(3)). Since it is the generic
pointer it is related to any other pointer, and a static_cast should be
used to convert a void * to an intended destination pointer. This is a
somewhat awkward left-over from C, which should probably only be used in
that context. Here is an example:

The qsort function from the C library expects a pointer to a
(comparison) function having two void const * parameters. In fact, these
parameters point to data elements of the array to be sorted, and so the
comparison function must cast the void const * parameters to pointers to
the elements of the array to be sorted. So, if the array is an int array[]
and the compare function's parameters are void const *p1 and void const
*p2 then the compare function obtains the address of the int pointed
to by p1 by using:
    static_cast<int const *>(p1);

o  When undoing or introducing the signed-modifier of an int-typed
variable (remember that a static_cast is allowed to change the
expression's representation!). 


Here is an example: the C function tolower requires an int
representing the value of an unsigned char. But char by default is a
signed type. To call tolower using an available char ch we should use:
                tolower(static_cast<unsigned char>(ch))


3.5.2: The `const_cast'-operator

    The const keyword has been given a special place in casting. Normally
anything const is const for a good reason. Nonetheless situations
may be encountered where the const can be ignored. For these special
situations the const_cast should be used. Its syntax is:
                             const_cast<type>(expression)

A const_cast<type>(expression) expression is used to undo the
const attribute of a (pointer) type.

The need for a const_cast may occur in combination with functions from
the standard C library which traditionally weren't always as const-aware
as they should. A function strfun(char *s) might be available, performing
some operation on its char *s parameter without actually modifying the
characters pointed to by s. Passing char const hello[] = "hello"; to
strfun produces the warning
           passing `const char *' as argument 1 of `fun(char *)' discards const

A const_cast is the appropriate way to prevent the warning:
                         strfun(const_cast<char *>(hello));


3.5.3: The `reinterpret_cast'-operator

    The third new-style cast is used to change the interpretation of
information: the reinterpret_cast. It is somewhat reminiscent of the
static_cast, but reinterpret_cast should only be used when it is
known that the information as defined in fact is or can be interpreted as
something completely different. Its syntax is:
                reinterpret_cast<pointer type>(pointer expression)

Think of the reinterpret_cast as a cast offering a poor-man's union:
the same memory location may be interpreted in completely different ways.

The reinterpret_cast is used, for example, in combination with the
write function that is available for streams. In C++ streams are
the preferred interface to, e.g., disk-files. The standard streams like
std::cin and std::cout also are stream objects.

Streams intended for writing (`output streams' like cout) offer write
members having the prototype
                write(char const *buffer, int length)

To write the value stored within a double variable to a stream in its
un-interpreted binary form the stream's write member is used. However, as
a double * and a char * point to variables using different and
unrelated representations, a static_cast cannot be used. In this case a
reinterpret_cast is required. To write the raw bytes of a variable
double value to cout we use:
            cout.write(reinterpret_cast<char const *>(&value), sizeof(double));

All casts are potentially dangerous, but the reinterpret_cast is the
most dangerous of them all. Effectively we tell the compiler: back off, we
know what we're doing, so stop fuzzing. All bets are off, and we'd better
do know what we're doing in situations like these. As a case in point
consider the following code:
            int value = 0x12345678;     // assume a 32-bits int

    cout << "Value's first byte has value: " << hex <<
            static_cast<int>(
                *reinterpret_cast<unsigned char *>(&value)
                            );

The above code produces different results on little and big endian
computers. Little endian computers show the value 78, big endian
computers the value 12. Also note that the different representations used by
little and big endian computers renders the previous example
(cout.write(...)) non-portable over computers of different architectures.

As a rule of thumb: if circumstances arise in which casts have to be
used, clearly document the reasons for their use in your code, making double
sure that the cast does not eventually cause a program to misbehave. Also:
avoid reinterpret_casts unless you have to use them.


3.5.4: The `dynamic_cast'-operator

    Finally there is a new style cast that is used in combination with
polymorphism (see chapter 14). Its syntax is:
                            dynamic_cast<type>(expression)

Different from the static_cast, whose actions are completely determined
compile-time, the dynamic_cast's actions are determined run-time to
convert a pointer to an object of some class (e.g., Base) to a pointer to
an object of another class (e.g., Derived) which is found further down its
so-called class hierarchy (this is also called downcasting).

At this point in the Annotations a dynamic_cast cannot yet be
discussed extensively, but we return to this topic in section
14.6.1.


3.5.5: Casting 'shared_ptr' objects

    This section can safely be skipped without loss of continuity.

In the context of the class shared_ptr, which is covered in section
18.4, several more new-style casts are available.  Actual coverage of
these specialized casts is postponed until section 18.4.5.

These specialized casts are:
    
    o static_pointer_cast, returning a shared_ptr to the base-class
section of a derived class object;
    o const_pointer_cast, returning a shared_ptr to a non-const object
from a shared_ptr to a constant object;
    o dynamic_pointer_cast, returning a shared_ptr to a derived class
object from a shared_ptr to a base class object.


3.6: Keywords and reserved names in C++

C++'s  keywords are a superset of C's  keywords. Here is a
list of all keywords of the language:
    
        alignas         char16_t   double       long      reinterpret_cast true     
alignof         char32_t   dynamic_cast module    requires         try      
and             class      else         mutable   return           typedef  
and_eq          co_await   enum         namespace short            typeid   
asm             co_return  explicit     new       signed           typename 
atomic_cancel   co_yield   export       noexcept  sizeof           union    
atomic_commit   compl      extern       not       static           unsigned 
atomic_noexcept concept    false        not_eq    static_assert    using    
auto            const      float        nullptr   static_cast      virtual  
bitand          const_cast for          operator  struct           void     
bitor           constexpr  friend       or        switch           volatile 
bool            continue   goto         or_eq     synchronized     wchar_t  
break           decltype   if           private   template         while    
case            default    import       protected this             xor      
catch           delete     inline       public    thread_local     xor_eq   
char            do         int          register  throw

Notes:
    

    o  Since the C++17 standard the keyword register is no longer
        used, but it remains a reserved identifier. In other words, definitions
        like     register int index;

result in compilation errors. Also, register is no longer
        considered a storage class specifier (storage class specifiers are
        extern, thread_local, mutable and static).
    o  the operator keywords: 

    and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor and
xor_eq are symbolic alternatives for, respectively, &&, &=, &, |, ~, !,
!=, ||, |=, ^ and ^=.
    o  C++ also recognizes the special identifiers final, override,
transaction_safe, and transaction_safe_override. These identifiers are
special in the sense that they acquire special meanings when declaring classes
or polymorphic functions. Section 14.4 provides further details.

Keywords can only be used for their intended purpose and cannot be used as
names for other entities (e.g., variables, functions, class-names, etc.). In
addition to keywords identifiers starting with an underscore and living in
the global namespace (i.e., not using any explicit namespace or using the
mere :: namespace specification) or living in the std namespace are
 reserved identifiers in the sense that their use is a prerogative of the
implementor.



Chapter 4: Namespaces


4.1: Namespaces

Imagine a math teacher who wants to develop an interactive math program. For
this program functions like cos, sin, tan etc. are to be used
accepting arguments in degrees rather than arguments in
radians. Unfortunately, the function name cos is already in use, and that
function accepts radians as its arguments, rather than degrees.

Problems like these are usually solved by defining another name, e.g., the
function name cosDegrees is defined. C++ offers an alternative
solution through namespaces. Namespaces can be considered as
areas or regions in the code in which identifiers may be defined. Identifiers
defined in a namespace normally won't conflict with names already defined
elsewhere (i.e., outside of their namespaces). So, a function cos
(expecting angles in degrees) could be defined in a namespace
Degrees. When calling cos from within Degrees you would call the
cos function expecting degrees, rather than the standard cos function
expecting radians.


4.1.1: Defining namespaces

    Namespaces are defined according to the following syntax:
            namespace identifier
    {
        // declared or defined entities
        // (declarative region)
    }

The identifier used when defining a namespace is a standard C++
identifier.

Within the declarative region, introduced in the above code example,
functions, variables, structs, classes and even (nested) namespaces can be
defined or declared. Namespaces cannot be defined within a function
body. However, it is possible to define a namespace using multiple
namespace declarations. Namespaces are `open' meaning that
a namespace CppAnnotations could be defined in a file file1.cc and
also in a file file2.cc. Entities defined in the CppAnnotations
namespace of files file1.cc and file2.cc are then united in one
CppAnnotations namespace region. For example:
            // in file1.cc
    namespace CppAnnotations
    {
        double cos(double argInDegrees)
        {
            ...
        }
    }

    // in file2.cc
    namespace CppAnnotations
    {
        double sin(double argInDegrees)
        {
            ...
        }
    }

Both sin and cos are now defined in the same
CppAnnotations namespace.

Namespace entities can be defined outside of their namespaces. This
topic is discussed in section 4.1.4.1.


4.1.1.1: Declaring entities in namespaces

            Instead of defining entities in a namespace, entities may also be
declared in a namespace. This allows us to put all the
     declarations in a header file that can
thereupon be included in sources using the entities defined in the
namespace. Such a header file could contain, e.g.,
            namespace CppAnnotations
    {
        double cos(double degrees);
        double sin(double degrees);
    }


4.1.1.2: A closed namespace

        Namespaces can be defined without a name. Such an 
anonymous namespace restricts the visibility of the defined entities to the
source file defining the anonymous namespace.

Entities defined in the anonymous namespace are comparable to C's
static functions and variables. In C++ the static keyword can
still be used, but its preferred use is in class definitions (see
chapter 7). In situations where in C static variables or
functions would have been used the  anonymous namespace should be used in
C++.

The anonymous namespace is a  closed namespace: it is not
possible to add entities to the same anonymous namespace using different
source files.


4.1.2: Referring to entities

    Given a namespace and its entities, the scope resolution operator can be
used to refer to its entities. For example, the function cos()
defined in the CppAnnotations namespace may be used as follows:
            // assume CppAnnotations namespace is declared in the
    // following header file:
    #include <cppannotations>

    int main()
    {
        cout << "The cosine of 60 degrees is: " <<
                CppAnnotations::cos(60) << '\n';
    }

This is a rather cumbersome way to refer to the cos() function in the
CppAnnotations namespace, especially so if the function is frequently
used. In cases like these an abbreviated form can be
used after specifying a using declaration. Following
            using CppAnnotations::cos;  // note: no function prototype,
                                // just the name of the entity
                                // is required.

calling cos results in a call of the cos function defined in the
CppAnnotations namespace. This implies that the standard cos
function, accepting radians, is not automatically called anymore. To call that
latter cos function the plain
scope resolution operator should be used:
            int main()
    {
        using CppAnnotations::cos;
        ...
        cout << cos(60)         // calls CppAnnotations::cos()
            << ::cos(1.5)       // call the standard cos() function
            << '\n';
    }

A using declaration can have restricted scope. It can be used inside a
block. The using declaration prevents the definition of entities having
the same name as the one used in the using declaration. It is not possible
to specify a using declaration for a variable value in some namespace,
and to define (or declare) an identically named object in a block also
containing a using declaration. Example:
            int main()
    {
        using CppAnnotations::value;
        ...
        cout << value << '\n';  // uses CppAnnotations::value
        int value;              // error: value already declared.
    }


4.1.2.1: The `using' directive

        A generalized alternative to the using declaration is the
using directive:
            using namespace CppAnnotations;

Following this directive, all entities defined in the
CppAnnotations namespace are used as if they were declared by using
declarations.

While the using directive is a quick way to
  import all the names of a namespace (assuming
the namespace has previously been declared or defined), it is at the same time
a somewhat dirty way to do so, as it is less clear what entity is actually
used in a particular block of code.

If, e.g., cos is defined in the CppAnnotations namespace,
CppAnnotations::cos is going to be used when cos is called. However,
if cos is not defined in the CppAnnotations namespace, the
standard cos function will be used. The using directive does not
document as clearly as the using declaration what entity will actually be
used. Therefore use caution when applying the using directive.

Namespace declarations are context sensitive: when a using namespace
declaration is specified inside a compound statement then the declaration is
valid until the compound statement's closing curly brace has been
encountered. In the next example a string first is defined without
explicit specifying std::string, but once the compound statement has ended
the scope of the using namespace std declaration has also ended, and so
std:: is required once again when defining second:
            #include <string>
    int main()
    {
        {
            using namespace std;
            string first;
        }
        std::string second;
    }

A using namespace directive cannot be used within the
declaration block of a class- or enumeration-type. E.g., the following example
won't compile:
            struct Namespace
    {
        using namespace std;      // won't compile
    };


4.1.2.2: `Koenig lookup'

        If Koenig lookup were called the `Koenig principle', it could have been
the title of a new Ludlum novel. However, it is not. Instead it refers to
a C++ technicality.

`Koenig lookup' refers to the fact that if a function is called without
specifying its namespace, then the namespaces of its argument types are used
to determine the function's namespace. If the namespace in which the argument
types are defined contains such a function, then that function is used. This
procedure is called the `Koenig lookup'.

As an illustration consider the next example. The function
FBB::fun(FBB::Value v) is defined in the FBB namespace. It
can be called without explicitly mentioning its namespace:
            #include <iostream>

    namespace FBB
    {
        enum Value        // defines FBB::Value
        {
            FIRST
        };

        void fun(Value x)
        {
            std::cout << "fun called for " << x << '\n';
        }
    }

    int main()
    {
        fun(FBB::FIRST);    // Koenig lookup: no namespace
                            // for fun() specified
    }
    /*
        generated output:
    fun called for 0
    */

    The compiler is rather smart when handling namespaces. If Value in the
namespace FBB would have been defined as using Value = int then
FBB::Value would be recognized as int, thus causing the Koenig lookup
to fail.

As another example, consider the next program. Here two namespaces are
involved, each defining their own fun function. There is no
ambiguity, since the argument defines the namespace and FBB::fun is
called:
            #include <iostream>

    namespace FBB
    {
        enum Value        // defines FBB::Value
        {
            FIRST
        };

        void fun(Value x)
        {
            std::cout << "FBB::fun() called for " << x << '\n';
        }
    }

    namespace ES
    {
        void fun(FBB::Value x)
        {
            std::cout << "ES::fun() called for " << x << '\n';
        }
    }

    int main()
    {
        fun(FBB::FIRST);    // No ambiguity: argument determines
                            // the namespace
    }
    /*
        generated output:
    FBB::fun() called for 0
    */


Here is an example in which there is an ambiguity: fun has two
arguments, one from each namespace. The ambiguity must be resolved by the
programmer:
            #include <iostream>

    namespace ES
    {
        enum Value        // defines ES::Value
        {
            FIRST
        };
    }

    namespace FBB
    {
        enum Value        // defines FBB::Value
        {
            FIRST
        };

        void fun(Value x, ES::Value y)
        {
            std::cout << "FBB::fun() called\n";
        }
    }

    namespace ES
    {
        void fun(FBB::Value x, Value y)
        {
            std::cout << "ES::fun() called\n";
        }
    }

    int main()
    {
        //  fun(FBB::FIRST, ES::FIRST); ambiguity: resolved by
        //                              explicitly mentioning
        //                              the namespace
        ES::fun(FBB::FIRST, ES::FIRST);
    }
    /*
        generated output:
    ES::fun() called
    */


An interesting subtlety with namespaces is that definitions in one
namespace may break the code defined in another namespace. It shows that
namespaces may affect each other and that namespaces may backfire if we're not
aware of their peculiarities. Consider the following example:
            namespace FBB
    {
        struct Value
        {};

        void fun(int x);
        void gun(Value x);
    }

    namespace ES
    {
        void fun(int x)
        {
            fun(x);
        }
        void gun(FBB::Value x)
        {
            gun(x);
        }
    }

    Whatever happens, the programmer'd better not use any of the functions
defined in the ES namespace, since that would result in infinite
recursion. However, that's not the point. The point is that the programmer
won't even be given the opportunity to call ES::fun since the compilation
fails.

Compilation fails for gun but not for fun. But why is that so? Why
is ES::fun flawlessly compiling while ES::gun isn't? In ES::fun
fun(x) is called. As x's type is not defined in a namespace the Koenig
lookup does not apply and fun calls itself with infinite recursion.

With ES::gun the argument is defined in the FBB
namespace. Consequently, the FBB::gun function is a possible candidate to
be called. But ES::gun itself also is possible as ES::gun's prototype
perfectly matches the call gun(x).

Now consider the situation where FBB::gun has not yet been
declared. Then there is of course no ambiguity. The programmer responsible for
the ES namespace is resting happily. Some time after that the programmer
who's maintaining the FBB namespace decides it may be nice to add a
function gun(Value x) to the FBB namespace. Now suddenly the code in
the namespace ES breaks because of an addition in a completely other
namespace (FBB). Namespaces clearly are not completely independent of each
other and we should be aware of subtleties like the above. Later in the
C++ Annotations (chapter 11) we'll return to this issue.

Koenig lookup is only used in the context of namespaces. If a function
is defined outside of a namespace, defining a parameter of a type that's
defined inside a namespace, and that namespace also defines a function with an
identical signature, then the compiler reports an ambiguity when that function
is called. Here is an example, assuming the abovementioned namespace FBB
is also available:
            void gun(FBB::Value x);

    int main(int argc, char **argv)
    {
        gun(FBB::Value{});          // ambiguity: FBB::gun and ::gun can both
                                    // be called.
    }


4.1.3: The standard namespace

    The std namespace is reserved by C++. The standard defines many
entities that are part of the runtime available software (e.g., cout, cin,
cerr); the templates defined in the Standard Template Library (cf.
chapter 18); and the Generic Algorithms (cf. chapter 19)
are defined in the std namespace.

Regarding the discussion in the previous section, using
declarations may be used when referring to entities in the std namespace.
For example, to use the std::cout
stream, the code may declare this object as follows:
            #include <iostream>
    using std::cout;

Often, however, the identifiers defined in the std namespace can all
be accepted without much thought. Because of that, one frequently encounters a
using directive, allowing the programmer to omit a namespace prefix when
referring to any of the entities defined in the namespace specified with the
using directive. Instead of specifying  using declarations the
following using directive is frequently encountered:
construction like
            #include <iostream>
    using namespace std;

Should a using directive, rather than using declarations be used?
As a rule of thumb one might decide to stick to using declarations, up
to the point where the list becomes impractically long, at which point a
using directive could be considered.

Two  restrictions apply to using directives and
declarations:
    
    o  Programmers should not declare or define anything inside the
namespace std. This is not compiler enforced but is imposed upon user
code by the standard;
    o  Using declarations and directives should not be imposed upon
code written by third parties. In practice this means that using
directives and declarations should be banned from header files and should only
be used in source files (cf. section 7.11.1).


4.1.4: Nesting namespaces and namespace aliasing

    Namespaces can be nested. Here is an example:
            namespace CppAnnotations
    {
        int value;
        namespace Virtual
        {
            void *pointer;
        }
    }

The variable value is defined in the CppAnnotations
namespace. Within the CppAnnotations namespace another namespace
(Virtual) is nested. Within that latter namespace the variable
pointer is defined. To refer to these
variable the following options are available:
    
    o  The fully qualified names can be used. A
        fully qualified name of an entity is a list of all the namespaces that
        are encountered until reaching the definition of the entity. The
        namespaces and entity are glued together by the scope resolution
        operator:
   int main()
{
    CppAnnotations::value = 0;
    CppAnnotations::Virtual::pointer = 0;
}

o  A using namespace CppAnnotations directive can be provided. Now
        value can be used without any prefix, but pointer must be used
        with the Virtual:: prefix:
   using namespace CppAnnotations;

int main()
{
    value = 0;
    Virtual::pointer = 0;
}

o  A using namespace directive for the full namespace chain can be
        used. Now value needs its CppAnnotations prefix again, but
        pointer doesn't require a prefix anymore:
   using namespace CppAnnotations::Virtual;

int main()
{
    CppAnnotations::value = 0;
    pointer = 0;
}

o  When using two separate using namespace directives none of the
        namespace prefixes are required anymore:
   using namespace CppAnnotations;
using namespace Virtual;

int main()
{
    value = 0;
    pointer = 0;
}

o  The same can be accomplished (i.e., no namespace prefixes) for
        specific variables by providing specific using declarations:
   using CppAnnotations::value;
using CppAnnotations::Virtual::pointer;

int main()
{
    value = 0;
    pointer = 0;
}

o  A combination of using namespace directives and using
        declarations can also be used. E.g., a using namespace directive
        can be used for the CppAnnotations::Virtual namespace, and a
        using declaration can be used for the CppAnnotations::value
        variable:
   using namespace CppAnnotations::Virtual;
using CppAnnotations::value;

int main()
{
    value = 0;
    pointer = 0;
}

Following a using namespace directive all entities of that namespace
can be used without any further prefix. If a single using namespace
directive is used to refer to a nested namespace, then all entities of that
nested namespace can be used without any further prefix. However, the entities
defined in the more shallow namespace(s) still need the shallow namespace's
name(s). Only after providing specific using namespace directives or
using declarations namespace qualifications can be omitted.

When fully qualified names are preferred but a long name like
            CppAnnotations::Virtual::pointer

is considered too long, a namespace alias may be used:
            namespace CV = CppAnnotations::Virtual;

This defines CV as an alias for the full name. The
variable pointer may now be accessed using:
            CV::pointer = 0;

A namespace alias can also be used in a using namespace directive or
using declaration:
            namespace CV = CppAnnotations::Virtual;
    using namespace CV;

Nested namespace definitions

Starting with the C++17 standard, when nesting namespaces a nested
namespace can directly be referred to using scope resolution operators. E.g.,
            namespace Outer::Middle::Inner
    { 
        // entities defined/declared here are defined/declared in the Inner
        // namespace, which is defined in the Middle namespace, which is
        // defined in the Outer namespace
    }


4.1.4.1: Defining entities outside of their namespaces

        It is not strictly necessary to define members of namespaces inside a
namespace region. But before an entity is defined outside of a namespace
it must have been declared inside its namespace.

To define an entity outside of its namespace its name must be fully
qualified by prefixing the member by its namespaces. The definition may be
provided at the global level or at intermediate levels in the case of nested
namespaces. This allows us to define an entity belonging to namespace A::B
within the region of namespace A.

Assume the type int INT8[8] is defined in the CppAnnotations::Virtual
namespace. Furthermore assume that it is our intent to define a function
squares, inside the namespace 

 CppAnnotations::Virtual returning a
pointer to CppAnnotations::Virtual::INT8.

Having defined the prerequisites within the CppAnnotations::Virtual
namespace, our function could be defined as follows (cf. chapter 9
for coverage of the memory allocation operator new[]):
            namespace CppAnnotations
    {
        namespace Virtual
        {
            void *pointer;

            using INT8 = int[8];

            INT8 *squares()
            {
                INT8 *ip = new INT8[1];

                for (size_t idx = 0; idx != sizeof(INT8) / sizeof(int); ++idx)
                    (*ip)[idx] = (idx + 1) * (idx + 1);

                return ip;
            }
        }
    }

The function squares defines an array of one INT8 vector, and
returns its address after initializing the vector by the squares of the first
eight natural numbers.

Now the function squares can be defined outside of the
CppAnnotations::Virtual namespace:
            namespace CppAnnotations
    {
        namespace Virtual
        {
            void *pointer;

            using INT8 = int[8];

            INT8 *squares();
        }
    }

    CppAnnotations::Virtual::INT8 *CppAnnotations::Virtual::squares()
    {
        INT8 *ip = new INT8[1];

        for (size_t idx = 0; idx != sizeof(INT8) / sizeof(int); ++idx)
            (*ip)[idx] = (idx + 1) * (idx + 1);

        return ip;
    }

In the above code fragment note the following:
    
    o squares is declared inside of the CppAnnotations::Virtual
namespace.
    o  The definition outside of the namespace region requires us to use
the fully qualified name of the function and of its return type.
    o  Inside the body of the function squares we are within the
CppAnnotations::Virtual namespace, so inside the function fully
qualified names (e.g., for INT8) are not required any more.

Finally, note that the function could also have been defined in the
CppAnnotations region. In that case the Virtual namespace would have
been required when defining squares() and when specifying its return type,
while the internals of the function would remain the same:
            namespace CppAnnotations
    {
        namespace Virtual
        {
            void *pointer;

            using INT8 = int[8];

            INT8 *squares();
        }

        Virtual::INT8 *Virtual::squares()
        {
            INT8 *ip = new INT8[1];

            for (size_t idx = 0; idx != sizeof(INT8) / sizeof(int); ++idx)
                (*ip)[idx] = (idx + 1) * (idx + 1);

            return ip;
        }
    }


4.2: The std::chrono namespace (handling time)

The C programming language offers tools like sleep(3) and
select(2) to suspend program execution for a certain amount of time. And
of course the family of time(3) functions for setting and displaying time

Sleep and select can be used for waiting, but as they were designed in
an era when multi threading was unavailable, their usefulness is
limited when used in multi threaded programs. Multi threading has become part
of C++ (covered in detail in chapter 20), and additional
time-related functions are available in the std::filesystem namespace,
covered below in this chapter. 

In multi threaded programs threads are frequently suspended, albeit usually
for a very short time. E.g., when a thread wants to access a variable, but the
variable is currently being updated by another thread, then the former thread
should wait until the latter thread has completed the update. Updating a
variable usually doesn't take much time, but if it takes an unexpectedly long
time, then the former thread may want to be informed about that, so it can do
something else while the latter thread is busy updating the
variable. Interactions between threads like these cannot be realized with
functions like sleep and select.

The std::chrono namespace bridges the gap between the
traditionally available time-related functions and the time-related
requirements of multi-threading and of the std::filesystem name
space. All but the specific std::filesystem related time functionality is
available after including the <chrono> header file. After including the 
<filesystem> header file the facilities of the std::filesystem are
available. 

Time can be measured in various resolutions: in Olympic Games time differences
of hundreds of seconds may make the distinction between gold and silver
medals, but when planning a vacation we might talk about it months before we
go on vacation. Time resolutions are specified by objects of the class
std::ratio, which (apart from including the <chrono> header file) is
also available after including the <ratio> header file.

Different events usually last for different amounts of time (given a specific
time resolution). Amounts of time are specified by objects of the class
std::chrono::duration.

Events can also be characterized by their points in time: midnight, January 1,
1970 GMT is a point in time, as is 19:00, December 5, 2010. Points in time are
specified by objects of the class std::chrono::time_point. 

It's not just that resolutions, durations of events, and points in time of
events may differ, but the devices (clocks) we use for specifying time also
differ. In the past hour glasses were used (and sometimes they're still
used when boiling eggs), but on the other hand we may use atomic clocks when
measurements should be very precise. Four different types of clocks are
available. The commonly used clock is std::chrono::system_clock, but in
the context of the file system there's also a std::chrono::file_clock.

In the upcoming sections the details of the std::chrono namespace are
covered. First we look at characteristics of time resolutions. How to
handle amounts of time given their resolutions is covered next. The next
section describes facilities for defining and handling time-points. The
relationships between these types and the various clock-types are covered
thereafter. 

In this chapter the specification std::chrono:: is often omitted (in
practice using namespace std followed by using namespace chrono is
commonly used; [std::]chrono:: specifications are occasionally used to
avoid ambiguities). Also, every now and then you'll encounter forward
references to later chapters, like the reference to the chapter about
multi-threading. These are hard to avoid, but studying those chapters
at this point can safely be postponed without loss of continuity.


4.2.1: Time resolutions: std::ratio

    Time resolutions (or units of time) are essential components of time
specifications. Time resolutions are defined by objects of the class
    std::ratio. 

Before the class ratio can be used, the <ratio> header file must be
included. But the frquently included <chrono> header file already includes
the ratio header file.

The class ratio has two template arguments. These are positive
integral numbers surrounded by pointed brackets defining, respectively, the
numerator and denominator of a fraction (by default the denominator equals
1). Examples:
            ratio<1>        - representing one; 
    ratio<60>       - representing 60
    ratio<1, 1000>  - representing 1/1000.

The class ratio defines two directly accessible static data
members: num represents its numerator, den
its denominator. A ratio definition by itself simply defines a certain
amount. E.g., when executing the following program
            #include <ratio>
    #include <iostream>
    using namespace std;

    int main()
    {
        cout << ratio<5, 1000>::num << ',' << ratio<5, 1000>::den << '\n';
    }

1,200 is displayed, as that's the `amount' represented by
ratio<5, 1000>: ratio simplifies its fraction whenever possible.

Several predefined ratio types exist. They are, like
ratio itself, defined in the standard namespace and can be used instead of
the more cumbersome ratio<x> or ratio<x, y> specifications:
    


    

    

     yocto    10^-24    
                      zepto    10^-21      2 


    

    

     atto    10^-18    
         femto   10^-15    
         pico    10^-12 


     nano    10^-9    
         micro   10^-6    
         milli   10^-3 


     centi   10^-2    
         deci    10^-1 

    

    

     deca    10^1    
         hecto   10^2    
         kilo    10^3 


     mega    10^6    
         giga    10^9    
         tera    10^12 


     peta    10^15    
         exa     10^18 

    

    

     zetta     10^21    
         yotta     10^24 

    

    


 (note: the definitions of the types yocto, zepto, zetta and yotta
use integral constants exceeding 64 bits. Although these constants are defined
in C++, they are not available on 64 bit or smaller architectures.)

Time related ratios can very well be interpreted as fractions or multiple of
seconds, with ratio<1, 1> representing a resolution of one second.

Here is an example showing how these abbreviations can be used:
                cout << milli::num << ',' << milli::den << '\n' <<
                kilo::num << ',' << kilo::den << '\n';


4.2.2: Amounts of time: std::chrono::duration

    Amounts of time are specified by objects of the class
    std::chrono::duration.

Before using the class duration the <chrono> header file must be
included.

The class duration has two template arguments. A numeric type (int64_t
is normally used) defining the type holding the duration's amount of time, and
a time-resolution ratio (called its resolution).

Often the following predefined duration types are used:
    

    
        ------------------------------------------------------------
        predefined:         duration type    
        ------------------------------------------------------------
        nanoseconds         duration<int64_t, nano>    
        microseconds         duration<int64_t, micro>    
        milliseconds         duration<int64_t, milli>    
        seconds         duration<int64_t>    
        minutes         duration<int64_t, ratio<60>>    
        hours         duration<int64_t, ratio<3600>>    
        ------------------------------------------------------------




E.g., to define a duration of 30 minutes use minutes halfHour{ 30 }.

The suffixes h, min, s, ms, us, ns are available for integral values,
creating the corresponding duration times. E.g., minutes min =
1h stores 60 minutes in min.

Sub-types:


    o rep: the duration's numeric type (commonly int64_t);
    o period: the ratio type (like kilo). E.g.,
        minutes::period::num equals 60.

Constructors:


    o duration():

        the default constructor initializes the object to 0 units;
    o duration(Type const &value):

        the object is initialized to value time units (Type refers to
        the duration's numeric type. E.g., when defining 
        minutes halfHour{ 30 } the argument 30 is stored inside its
        int64_t data member). 

Copy- and move-constructors (cf. chapter 9) are available.

Operators:

Duration types support assignment, negation (e.g., halfHour =
-halfHour), addition and subtraction of duration values, and also
multiplication, division and modulo computations by numeric factors. Compound
assignment operators are also available. E.g.,
     minutes time = 2 * halfHour;    // time: 60 minutes
    time += minutes{ 30 };          // time: 90 minutes 

Members:


    o Type count() const returns the value stored inside the duration
        object. E.g., minutes{ 30 }.count() returns 30.  The following
        members are static members;

o duration::zero() returns an (immutable) duration object whose
        count member returns 0. E.g., seconds::zero().count() returns
        0s;

o duration::min() returns an (immutable) duration object whose
        count member returns the lowest value of its rep type (i.e.,
        std::numeric_limits<duration::rep>::min() (cf. section
        21.11));

o duration::max()  returns an (immutable) duration object whose
        count member returns the maximum value of its rep type.

Conversions:

The precision of left-hand side arguments of assignment operators must be
greater than or equal to the precision of the right-hand side arguments
(the left-hand argument may not lose precision). When using binary
arithmetic operators the resulting duration type has a precision equal to the
finer of the two precisions. E.g.,
     cout << (1min + 1h).count() << '\n';  // shows: 61
    hours hr{ 1 };
    halfHour += hr;                 // OK
    // hr += halfHours;             // won't compile 

Durations can be converted by the 
std::chrono::duration_cast<destination>(source) function template, where
destination is the destination's duration type and source is an
available destination object, but duration_cast truncates the
destination value when destination's precision is less than source's
precision. E.g., duration_cast<minutes>(seconds{ 90 }) returns 
minutes{ 1 }.

When the left-hand side argument's precision is less than the right-hand
side's precision some decision must be made about how to handle the loss of
precision. The following function template can be used to convert a duration
to a less precise type, returning a double value, which can be, e.g.,
truncated or rounded.
     template <typename To, typename From>
    double durationCast(From from)
    {
        return static_cast<double>(from.count()) * 
                            To::period::den * From::period::num /
                            (To::period::num * From::period::den);
    } 

returning a double value (1.5 when called as
durationCast<minutes>(seconds{ 90 })), leaving the decision how to use the
returned double value to the caller.


4.2.3: Clocks measuring time

    Clocks are used to measure time. C++ has several predefined clock
types, defined in the std::chrono namespace.

Before using the chrono clocks the <chrono> header file must be
included.

`Clock' clock-types are used to define points in time and define the
following sub-types:
    
    o  Clock::duration: the clock's time granularity type (by default
        using nanoseconds. E.g., system_clock::duration oneDay{ 24h };

o  Clock::period: the clock's std::ratio type. E.g., 
        system_clock::period::den;

o  Clock::rep: the type storing amounts of time. By default using
        int64_t). E.g., system_clock::rep amount;

o  Clock::time_point: the type storing time points (covered in the
        next section). By default using time_point<system_clock,
        nanoseconds> E.g., system_clock::time_point start.

All clocks have a member now returning the clock type's
time_point corresponding to the current time (relative to the clock's
epoch). It is a static member which can be used like this:
    system_clock::time_point tp = system_clock::now().

Four clock types are defined in the chrono namespace:
    
        o system_clock is the `wall clock', using the system's real time
            clock;
        o steady_clock is a clock whose time increases in parallel with the
            increase of real time;
        o high_resolution_clock is the computer's fastest clock (i.e., the
            clock having the shortest timer-tick interval). In practice this is
            the same clock as system_clock.
        o file_clock: used for time specifications of file-system
            elements. File_clock time specifications use a different
            starting (epoch) time than the other clocks, but it's easy to
            convert file_clock times to/from other clock types.

In addition to the member now the classes system_clock and
high_resolution_clock (referred to as Clock below) provide two static
members:
    
    o std::time_t Clock::to_time_t(Clock::time_point const &tp)
 
       a std::time_t value (the same type as returned by C's
        time(2) function) converting a time_point value to a
        time_t value;

o Clock::time_point Clock::from_time_t(std::time_tvseconds)
 
       the opposite conversion: returning a time_point corresponding to a
        time_t value.

For example:
            system_clock::time_point now = system_clock::now();
    time_t value = system_clock::to_time_t(now);
    system_clock::time_point now2 = system_clock::from_time_t(value);

    // now2 != now since time_t uses seconds, not nanoseconds    


4.2.4: Points in time: std::chrono::time_point

    Points in time are specified by objects of the class
     std::chrono::time_point.

Before using the class time_point the <chrono> header file must be
included.

Like duration the class time_point requires two template arguments: A
clock type and a duration type. The standard clock types define their own
time_point types using nanoseconds as their duration type 
(time_point by default uses nanoseconds).  The
following time point type definitions are therefore identical:
            time_point<standard_clock, nanoseconds>
    time_point<standard_clock>
    standard_clock::time_point 

Constructors:
    
    o time_point():

       the default constructor is initialized to the beginning of the clock's
        epoch. For system_clock it is January, 1, 1970, 00:00h, but
        note that file_clock::time_point has a different starting point
        (see the example at the end of this section);

Copy- and move-constructors as well as copy- and move-assignment operators
        (cf. chapter 9) are available

Operators:
    
    o time_point &operator+=(duration const &amount)
       adds the amount of time represented by amount to the current
        time_point object;

o time_point &operator-=(duration const &amount) subtracts 
       the amount of time represented by amount from the
        current time_point object;

o Type operator<<(std::ostream &, time_point const &) inserts a textual
        representation of the time_point's UTC time into the ostream.

The compound operators are also available as binary arithmetic operators using
a time_point const & and a duration const & operand (in any
order). Example:
           system_clock::now() + seconds{ 5 };

Members:
    
    o duration time_since_epoch() const:

       duration is the duration type used by the time point object for
        which this member is called. To convert the returned duration to a
        size_t value use time_since_epoch().count();

o time_point min() const:
       a static member returning the time point's duration::min
        value. Example:
           cout << 
        time_point<system_clock>::min().time_since_epoch().count() << '\n';
        // shows -9223372036854775808

o time_point max() const: 
       a static member returning the time point's duration::max
        value.

Conversions:

All predefined clocks use nanoseconds for their duration types. Converting to
different duration types is covered in the previous section
(4.2.2). Alternatively, the members to_time_t of the
system_clock and the high_resolution_clock can be used to convert
their time_point values to time_t. Such time_t values are commonly
used to convert time to text, e.g., using the manipulator put_time
(cf. section 6.3.2).  The put_time manipulator must be provided
with the address of a std::tm object, which can be obtained from a
std::time_t value. See figure 3 for a visual representation of
the involved elements.
    ------------------------------------------------------------------Insert Figure 3(Time according to C++)about here (file: threading/time)------------------------------------------------------------------

Use the following blueprint to insert a system_clock::time point
value into a std::ostream (cf. section 6.4.4):
        int main()
    {
        auto now = system_clock::now();         // get a time_point
        cout << now << '\n';                    // now's UTC time
                                                // convert to a time_t
        time_t timeT = system_clock::to_time_t(now);
        tm *local = localtime(&timeT);          // get a tm *
        cout << put_time(local, "%c") << '\n';  // show the time
    }

To convert a time_point to another std::chrono clock's time_point
the template
    
 DestClock::time_point std::chrono::clock_cast<DestClock>(TimePoint tp)
 can be used. The argument tp is an available time_point, and
DestClock specifies the clock type of the returned time_point. E.g.,
the following cout statement displays four identical times:
     auto fNow = file_clock::now();
    auto sNow = file_clock::to_sys(fnow);

    cout << fNow << '\n' <<
        sNow << '\n' <<
        clock_cast<system_clock>(fNow) << '\n' <<
        clock_cast<file_clock>(sNow) << '\n'; 

So, where is the starting point of the file_clock clock? 

 Since time_point's default constructor is initialized to its clock's
starting point it can be obtained this way:
     cout << file_clock::time_point{}<< '\n';
    // shows:  2174-01-01 00:00:00.000000000 


4.3: The std::filesystem namespace

Computers commonly store information that must survive reboots in their file
systems. Traditionally, to manipulate the file system the C programming
language offers functions performing the required system calls.  Such
functions (like rename(2), truncate(2), opendir(2), and
realpath(3)) are of course also available in C++, but their signatures
and way of use are often less attractive as they usually expect char const
* parameters and may use static buffers or memory allocation based on
malloc(3) and free(3).

Since 2003 the Boost library
    offers wrappers around these functions,
offering interfaces to those system calls that are more C++-like.

Currently C++ directly supports these functions in the
std::filesystem namespace. These facilities can be used
after including the <filesystem> header file.

The filesystem namespace is extensive: it contains more than 10 different
classes, and more than 30 free functions. To refer to the identifiers defined
in the std::filesystem namespace their fully qualified names (e.g.,
std::filesystem::path) can be used. Alternatively, after specifying
`using namespace std::filesystem;' the identifiers can be used without
further qualifications. Namespace specifications like `namespace fs =
std::filesystem;' are also encountered, allowing specifications like
fs::path.

Functions in the filesystem namespace may fail. When functions cannot
perform their assigned tasks they may throw exceptions (cf. chapter
10) or they may assign values to error_code objects that
are passed as arguments to those functions (see section 4.3.1 below).


4.3.1: The class 'error_code'

    Objects of the class std::error_code (note: not
std::filesystem::error_code!) encapsulate error values, and
associated error categories (cf. section 10.9; error_code
can be used after including the <system_error> header, but it is also
available after including the <filesystem> header file). Traditionally
error values are available as values assigned to the global  int
errno variable. By convention, when errno's value equals zero there's no
error. This convention was adopted by error_code.

Error codes can be defined for many conceptually different situations. Those
situations are characterized by their own error categories. 

Error categories are used to associate error_code objects with the errors
that are defined by those categories. Default available error categories may
use values like EADDRINUSE (or the equivalent enum class errc value
address_in_use) but new types of error categories, tailored to other
contexts, can also be defined. The enum class errc is defined in the
<bits/error_constants.h> header file. Error categories and how to define
them is covered near the end of the C++ Annotations (section 23.7.1).

Constructors:
    
    o error_code() noexcept:
the object is initialized with error value 0 and the
        system_category error category. Value 0 is not considered an
        error;

o error_code(int ec, error_category const &cat) noexcept:
the object is initialized from  error value ec
        (e.g., errno, set by a failing function), and a const reference to
         the applicable error category (provided by, e.g.,
          std::system_category() or
          std::generic_category()). Here is an example
         defining an error_code object:
                error_code ec{ 5, system_category() };   

o error_code(ErrorCodeEnum value) noexcept:
this is a member template (cf. section 22.1.3), using template
        header template <class ErrorCodeEnum>. It initializes the object
        with the return value of make_error_code(value) (see below). In
        section 23.7 defining ErrorCodeEnums is
        covered.  Note: ErrorCodeEnum as such does not exist. It
        is a mere placeholder for existing ErrorCodeEnum enumerations; 

Copy- and move-constructors are available.

Operators:
    
    o  Standard assignment operators and an assignment operator
        accepting an ErrorCodeEnum are available;

o operator std::errc() const noexcept:
returns the object's errc error-code value. For g++ these are
        defined in the enum class errc in the file
        /usr/include/x86_64-linux-gnu/c++/14/bits/error_constants.h;

o explicit operator bool() const noexcept:
returns true if the object's error value is unequal 0 (i.e., it
        represents and error).

Members:
    
    o void assign(int val, error_category const &cat):
assigns new values to the object's error value and category. E.g,
        ec.assign(0, generic_category());

o error_category const &category() const noexcept:
returns a reference to the object's error category;

o void clear() noexcept:
sets the error_code's value to 0 and its error category to
        system_category;

o error_condition default_error_condition() const noexcept:
returns the current category's default error condition initialized
        with the current object's error value and error category (see section
        10.9.2 for details about the class error_condition);

o string message() const:
the message that is associated with the current object's error value
        is returned (equivalent to category().message(ec.value()));

o int value() const noexcept:
returns the object's error value.

Free functions:
    
    o  Two error_code objects can be compared for (in) equality and can
        be ordered (using operator<). 

Ordering error_codes associated with different error categories
        has no meaning. But when the error categories are identical then they
        are compared by their error code values (cf. this SG14 discussion
        summary);

o error_code make_error_code(errc value) noexcept:
returns an error_code object initialized with
        static_cast<int>(value) and generic_category(). This function
        converts an enum class errc value to an error_code. 

Other error related enums may also be defined with which tailored
        make_error_code functions can be associated (cf. section
        23.7;)

o std::ostream &operator<<(std::ostream & os, error_code const &ec):
executes
    return os << ec.category().name() << ':' << ec.value();

Many filesystem functions define an optional last error_code &ec
parameter. Those functions have noexcept specifications. If those
functions cannot complete their tasks, then ec is set to the appropriate
error code, calling ec.clear() if no error was encountered. If no ec
argument is provided then those functions throw a filesystem_error
exception if they cannot complete their tasks.


4.3.2: Types and permissions of file system elements: 
            file_status

    File system entries (represented by path objects, cf. section 4.3.3
below), have attributes: permissions (e.g., the owner may modify an entry),
and types (like files, directories, and soft-links).  Those attributes are
defined in the class std::filesystem::file_status.

Types:


Defined in the (std::filesystem) enum class file_type.



        ------------------------------------------------------------
         2 

        Type    Meaning    
        ------------------------------------------------------------
        not_found (= -1)    the file system entry was not found

                (not considered an error)    

            none    the file status has not yet been evaluated

                 or an error occurred when evaluating the status    

            regular    a regular file    

            directory    a directory    

            symlink    a symbolic link    

            block    a block device    

            character    a character device    

            fifo     a named pipe    

            socket    a socket file    

            unknown    an unknown file type    

            ------------------------------------------------------------




Permissions:


Defined in the (std::filesystem) enum class perms. 

The enumeration's symbols were defined to make their meanings more descriptive
than the constants defined in the <sys/stat.h> header file, but their
values are identical. The enum class perms supports all bitwise operators.



        ------------------------------------------------------------
         4 

         Symbol    Value  sys/stat.h Meaning    
        ------------------------------------------------------------
        none    0000       ---    No permission bits were set    

            owner_read    0400    S_IRUSR    File owner has read permission    

            owner_write    0200    S_IWUSR    File owner has write permission    

            owner_exec    0100    S_IXUSR    File owner has execute/search

            permissions    

            owner_all    0700    S_IRWXU    File owner has read, write, and

             execute/search permissions    

            group_read    0040    S_IRGRP    The file's group has read permission    

            group_write    0020    S_IWGRP    The file's group has write permission    

            group_exec    0010    S_IXGRP    The file's group has execute/search

            permissions    

            group_all    0070    S_IRWXG    The file's group has read, write, and

            execute/search permissions    

            others_read    0004    S_IROTH    Other users have read permission    

            others_write    0002    S_IWOTH    Other users have write permission    

            others_exec    0001    S_IXOTH    Other users have execute/search

             permissions    

            others_all    0007    S_IRWXO    Other users have read, write, and 

            execute/search permissions    

            all    0777       ---    All users have read, write, and

            execute/search permissions    

            set_uid    04000    S_ISUID    Set user ID to file owner user ID on

            execution    

            set_gid    02000    S_ISGID    Set group ID to file's user group ID on

             execution    

            sticky_bit    01000    S_ISVTX    POSIX XSI specifies that when set on a

            directory only file owners may delete

            files even if the directory is writeable

             by others (used, e.g., with /tmp)    

            mask    07777       ---    All valid permission bits.    

            ------------------------------------------------------------




Perm_options:


The (std::file_system) enum class perm_options specifies the way file
system entries are modified



        ------------------------------------------------------------
         2 

        Value    Meaning    
        ------------------------------------------------------------
        replace    current permisions are replaced new permissions    

            remove    the specified permissions are removed from the

                   file system entry's current permissions    

            nofollow    when the current file system entry refers

                to a symbolic link the permissions of the

                symbolic link itself are updated    

            ------------------------------------------------------------




Constructors:


    o explicit file_status(file_type type = file_type::none,
                         perms permissions = perms::unknown)

Copy and move constructors and assignment operators are avaialble;

Members:


    o perms permissions() const 
        returns the permissions of the file system entry represented by the
        file_status object.

        Refer to section 4.3.3.2 for the function permissions
        changing the permissions of a file system entry; 

o file_type type() const
       returns the type  of the file system entry to which the 
        file_status object refers;

o void type(file_type type)
       changes the type of the file system entry to which the file_tatus
        object refers.


4.3.2.1: The status of file system entries

        The std::path free functions status and symlink_status
(cf. section 4.3.3.2) return the file_status of file system entries.

The following functions provide specific information about the status of
file system entries:


    o bool status_known(file_status const &status)
       returns true if status refers to a determined status
        (status itself may indicate that the entity referred to by
        status does not exist). One way of receiving false is by
        passing it a default status object: status_known(file_status{});

o bool is_WHATEVER(file_status status), 
       replacing WHATEVER by the requested file type, returns true if
        status refers to an entry of the specified type.

These following is_WHATEVER functions are available:
    

        ------------------------------------------------------------
         2 

        Function    Meaning    
        ------------------------------------------------------------
        is_block_file    'entry' is a block device    

            is_character_file    'entry' is a character device    

            is_directory    'entry' is a directory    

            is_empty    'entry' is an empty file or directory    

            is_fifo    'entry' is a named pipe    

            is_other    'entry' is not a directory,

                          regular file or symlink    

            is_regular_file    'entry' is a regular file    

            is_socket    'entry' is a named socket    

            is_symlink    'entry' is a symbolic link    

            ------------------------------------------------------------




Here is a small program showing how file statuses can be obtained and
displayed (cf. sections 4.3.3 and (for the map) section 12.3.7):
        namespace
    {
        std::unordered_map<file_type, char const *> statusMap =
        {
            { file_type::not_found, "an unknown file" },
            { file_type::none,      "not yet or erroneously evaluated "
                                                                "file type" },
            { file_type::regular,   "a regular file" },
            { file_type::directory, "a directory" },
            { file_type::symlink,   "a symbolic link" },
            { file_type::block,     "a block device" },
            { file_type::character, "a character device" },
            { file_type::fifo,      "a named pipe" },
            { file_type::socket,    "a socket file" },
            { file_type::unknown,   "an unknown file type" }
        };
    }
    
    int main()
    {
        cout << oct;
    
        string line;
        while (true)
        {
            cout << "enter the name of a file system entry: ";
            if (not getline(cin, line) or line.empty())
                break;
    
            path entry{ line };
    
            error_code ec;
            file_status stat = status(entry, ec);
    
            if (not status_known(stat))
            {
                cout << "status of " << entry << " is unknown. "
                                                "Ec = " << ec << '\n';
                continue;
            }
    
            cout << "status of " << entry << ": type = " <<
                     statusMap[stat.type()] <<
                     ", permissions: " <<
                        static_cast<size_t>(stat.permissions()) << '\n';
        }
    }


4.3.3: Names of file system entries: path

    Objects of the class filesysten::path commonly contain names of
file system entries. 

Constructors:


    o path(): the default constructor is initialized with an empty
        path; 
    o path(InputIter begin, InputIter end):

       the characters from begin to end define the path's name.

Copy- and move constructors are available, also using argument types
std::string and char const *.

    A path object doesn't have to refer to an existing file system
entry.

    Arguments of path constructors may contain (all optional):
    
    o  a root-name, e.g., a disk-name (like E:) or device indicator
        (like //nfs);
    o  a root-directory, if it is the first character after the (optional)
        root-name;
    o  filename characters (not containing directory separators). In
        addition the `single dot filename' (.) indicates the current
        directory and the `double dot filename' (..) indicates the current
        directory's parent directory;
    o  directory separators (by default the forward slash). Multiple
        consecutive separators are automatically merged into one separator.

The constructors also define a last format ftmp = auto_format
parameter, for which in practice almost never an argument has to be provided
(for its details see cppreference.)


4.3.3.1: Accessors, modifiers and operators

            Operators:
    
    o path &operator/=(Type const &arg):

       the arguments that can be passed to constructors can also be passed
        to this member. The arg argument is separated from the path's
        current content by a directory separator (unless the path is initially
        empty as in cout << path{}.append("entry")). See also the members
        append and concat, below. The free operator / accepts two
        path (promotable) arguments, returning a path containing both
        paths separated by a directory separator (e.g., lhs / rhs returns
        the path object lhs/rhs);

o path &operator+=(Type const &arg):

       similar to /=, but no directory separator is used when
        adding arg to the current path;

o  comparison operators: path objects can be compared using the
        (operators implied by the) == and <=> operators. Path objects
        are compared by lexicographical comparing their ascii-character
        content;

o ostream &operator<<(ostream &out, path const &path) (stream
        insertion) inserts path's content, surrounded by double quotes,
        into out;

o istream &operator>>(istream &in, path &path) extracts
        path's content from in. The extracted path name may optionally
        be surrounded by double quotes. When inserting a previously extracted
        path object only one set of surrounding quotes are shown.

Accessors: 

Accessors return specific path components. If a path doesn't contain
    the requested component then an empty path is returned.
    
    o char const *c_str(): the path's content is returned as an NTBS; 

o path extension() returns the dot-extension of the path's last
        component (including the dot);

o path filename() returns the last path-content of the current path
        object. See also the stem() accessor, below;

o bool is_absolute(): returns true if the path object contains
        an absolute path specification;

o bool is_relative(): returns true if the path object contains
        a relative path specification;

o path parent_path() returns the current path-content from which the
        last element has been removed. Note that if the path object
        contains a filename's path (like "/usr/bin/zip") then
        parent_path removes /zip and returns /usr/bin, so not
        zip's parent directory, but its actual directory;

o path relative_path(): returns the path's content beyond
        the path's root-directory component of the path object. E.g., if
        the path ulb{ "/usr/local/bin" } is defined then
        ulb.relative_path() returns a path containing "usr/local/bin";

o path root_directory(): returns the root-directory component of the
        path object;

o path root_name(): returns the root-name's component of the path
        object;

o path root_path(): returns the root-path component of the path
        object;

o path stem() returns the last path-content of the current path
        object from which the dot-extension hash been removed;

o string(): returns the path's content as a std::string.

       Similar accessors are available for the following string-types:
        wstring, u8string, u16string, , u32string, generic_string,
        generic_wstring, generic_u8string, generic_u16string, and
        generic_u32string;

Except for the family of string() and the is_... accessors, there
    are also bool has_... members returning true if the path
    contains the specified component (e.g., has_extension returns true
    if the path contains an extension).

Member functions:
    
    o path &append(Type const &arg) acts like the /= operator;

o path::iterator begin() returns an iterator containing the first path
        component; Dereferencing a path::iterator returns a path
        object.

       When available root names and root directories are returned as initial
        components. When incrementing path::iterators the individual
        directories and finally filename components are returned. The
        directory separators themselves are not returned when dereferencing
        subsequent path::iterators;

o path::const_iterator begin() const returns a iterator to the
        immutable first path component; Dereferencing a
        path::const_iterator returns a path const object;

o void clear(): the path's content is erased;

o int compare(Type const &other):

       returns the result of lexicographically comparing the current path's
        content with other. Other can be a path, a string-type or
        an NTBS;

o path &concat(Type const &arg) acts like the += operator;

o path::iterator end() returns an iterator beyond the last path
        component;

o path::const_iterator end() const returns an iterator beyond the
        immutable last path component 

o path &remove_filename():

       removes the last component of the stored path not ending in /. If
        the path doesn't contain a slash then the path object's
        cleared;

o path &replace_extension(path const &replacement = path{} ):

       replaces the extension of the last component of the stored path
        (including the extension's dot) with replacement. The extension is
        removed if replacement is empty. If the path calling
        replace_extension has no extension then replacement is added.
        The replacement may optionally start with a dot. The path object's
        extension receives only one dot;

o path &replace_filename(path const &replacement):

       replaces the last component of the stored path with replacement,
        which itself may contain multiple path elements.  If only a
        root-directory is stored, then it is replaced by replacement. The
        member's behavior is undefined if the current path object is empty;


4.3.3.2: Free functions

        Copy_options:


Defined in the (std::filesystem) enum class copy_options.

The enumeration supports bitwise operators.
The copy_options enum values are used to fine-tune the behavior of
filesystem functions copying file system elements.



        ------------------------------------------------------------
         4 

        Symbol     Value        Meaning    
        ------------------------------------------------------------
        none     0           default: copy like cp    



            overwrite_existing     2           replace the destination file    

            update_existing     4           
            replace the destination file only if it is 

            older than the file being copied    



            recursive     8           
        recursively copy subdirectories and their

        content    

    
            copy_symlinks     16           copy symlinks as symlinks    

            skip_symlinks     32           ignore symlinks    



            directories_only     64           
        copy the directory structure, but do not copy 

        any non-directory files    

            create_symlinks     128           
        create symlinks instead of copying files.
 
        The source path must be an absolute path unless 

        the destination path is in the current directory    

            create_hard_links     256           
        instead of copying files create hard links 

        resolving to the same files as the original    

            ------------------------------------------------------------




Functions:


    o path absolute(path const &src,  [, error_code &ec]):
returns src as an absolute path (i.e., starting at the
        filesystem's root (and maybe disk) name). It can be called as, e.g.,
        absolute("tmp/filename"), returning the (absolute) current working
        directory to which absolute's argument is appended as a final
        element, separated by a directory separator.  Relative path indicators
        (like ../ and ./) are kept;  

o path canonical(path const &src [, error_code &ec]):
returns src's canonical path. Src must exist. Example:
            canonical("/usr/local/bin/../../share/man");
     // returns path{ "/usr/share/man" } 

o void copy(path const &src, path const &dest [, copy_options
        opts [, error_code &ec]]):
src must exist. By default copies src to dest if the
            cp program would also succeed. Use opts to fine-tune
            copy's behavior.

       If src is a directory, and dest does not exist, dest is
        created. Directories are recursively copied if copy options
        recursive or none were specified;

o bool copy_file(path const &src, path const &dest [,
        copy_options opts [, error_code &ec]]):
src must exist. Copies src to dest if the cp program
        would also succeed. Symbolic links are followed. The value true is
        returned if copying succeeded;

o void copy_symlink(path const &src, path const &dest [,
        error_code &ec]):
creates the symlink dest as a copy of the symlink src;

o bool create_directories(path const &dest [,
        error_code &ec]):
creates each component of dest, unless already existing. E.g.,
        using argument "a/b/c" and "a" doesn't yet exist then
        "a/b/c" are all created. The value true is returned if
        dest was actually created. If false is returned ec
        contains an error-code, which is zero (ec.value() == 0) if
        dest already existed. See also create_directory below;

o bool create_directory(path const &dest [, path
        const &existing] [, error_code &ec]):
dest's parent directory must exist. This function creates
        directory dest if it does not yet exist. Nested subdirectories are
        not created by create_directory: using argument "a/b/c" and
        neither "a" nor "a/b" exist then this function fails. The
        value true is returned if dest was actually created. If
        false is returned ec contains an error-code, which is zero
        (ec.value() == 0) if dest already existed. If existing is
        specified, then dest receives the same attributes as
        existing;

o void create_directory_symlink(path const
        &dir, path const &link [, error_code &ec]):
like create_symlink (see below), but is used to create a
        symbolic link to a directory;

o void create_hard_link(path const &dest, path const
        &link [, error_code &ec]):
creates a hard link from link to dest. Dest must exist;

o void create_symlink(path const &dest, path const
        &link [, error_code &ec]):
creates a symbolic (soft) link from link to dest; dest
        does not have to exist;

o path current_path([error_code &ec]), void
        current_path(path const &toPath [, error_code &ec]):

       the former function returns the current working directory (cwd),
        the latter changes the cwd to toPath. The returned path's last
        character is not a slash, unless called from the root-directory;

o bool equivalent(path const &path1, path const &path2 [,
        error_code &ec]):
true is returned if path1 and path2 refer to the same file
        or directory, and have identical statuses. Both paths must exist;

o bool exists(path const &dest [, error_code &ec]),
        exists(file_status status):

       true is returned if dest exists (actually: if
        status(dest[, ec]) (see below) returns true). Note: when
        iterating over directories, the iterator usually provides the entries'
        statuses. In those cases calling exists(iterator->status()) is
        more efficient than calling exists(*iterator). When dest is
        the path to a symbolic reference then exists returns whether the
        link's destination exists or not (see also the functions status
        and symlink_status in section 4.3.4);

o std::unintmax_t file_size(path const &dest [, error_code
        &ec]):
returns the size in bytes of a regular file (or symlink destination);

o std::uintmax_t hard_link_count(path const &dest [,
        error_code &ec]):
returns the number of hard links associated with dest;

o file_clock::time_point
        last_write_time(path const &dest [, error_code &ec]),
       void last_write_time(path const &dest,
            file_clock::time_point newTime [, error_code &ec]):

       the former function returns dest's last modification time;
        the latter function changes dest's last modification time to
        newTime.
       last_write_time's return type is defined through a using alias
        for chrono::time_point (cf. section 4.2.4). The returned
        time_point is guaranteed to cover all file time values that may be
        encountered in the current file system. The function
        file_clock::to_sys (see below) can be used to convert
        file_clock time points to system_clock time_points;

o void permissions(Path const &entry, 
                perms newPerms [, perm_options opts = perm_options::replace ] 
                        [, error_code &ec]):
Modifies the permissions of entry. Each of the two last arguments
        is optional. E.g., when specifying an error_code as third argument
        then opts is implicitly specified as replace;

o path read_symlink(path const &src [, error_code &ec]):
src must refer to a symbolic link or an error is generated. The
        link's target is returned;

o bool remove(path const &dest [, error_code &ec]),
         std::uintmax_t remove_all(path const &dest [,
        error_code &ec]):

       remove removes the file, symlink, or empty directory
        dest, returning true if dest could be removed;
        remove_all removes dest if it's a file (or symlink); and
        recursively removes directory dest, returning the number of
        removed entries;

o void rename(path const &src, path const &dest [, error_code
        &ec]):
renames src to dest, as if using the standard mv(1)
        command (if dest exists it is overwritten);

o void resize_file(path const &src, std::uintmax_t size [,
        error_code &ec]):
src's size is changed to size as if using the standard
        truncate(1) command;

o space_info space(path const &src [, error_code &ec]):
returns information about the file system in which src is
        located;

o file_status status(path const &entry [, error_code &ec]):
returns entry's file_status. If entry is the name of a symbolic
        link then the status of the link's destination is returned;

o file_status symlink_status(path const &entry [,
        error_code &ec]):
returns entry's own file_status;

o path system_complete(path const &src[, error_code&
        ec]):
returns the absolute path matching src, using current_path as
        its base;

o path temp_directory_path([error_code& ec]):
 returns the path to a directory that can be used for temporary
        files. The directory is not created, but its name is commonly
        available from the environment variables TMPDIR, TMP, TEMP, or
        TEMPDIR. Otherwise, /tmp is returned.

o system_clock::time_point
        file_clock::to_sys(file_clock::time_point timePoint):
here is how
        the time returned by last_write_time can be represented using the
        system_clock's epoch:
            int main()
{
    time_t seconds = system_clock::to_time_t(
                    file_clock::to_sys(last_write_time("lastwritetime.cc"))
                );

    cout << "lastwritetime.cc's last (UTC) write time: " <<
             put_time(gmtime(&seconds), "%c") << '\n';
}


4.3.4: Handling directories: directory_entry

    The file system is a recursive data structure. Its top-level entry is a
directory (the root directory) containing plain directory entries (files,
(soft) links, named sockets, etc.) and possibly also (sub)directory
entries referring to nested directories which in turn may contiain plain-
and (sub)directory entries.

In the std::filesystem namespace the elements of directories are objects
of the class directory_entry, containing names and statuses of the entries
of that directory. 

Constructors:

The class directory_entry supports all standard constructors (and
assignment operators) and also a constructor expecting a path:
            directory_entry(path const &entry);

Objects of the class directory_entry can be constructed by name, and do
not have to refer to existing entries in the file system. 

Operators:
    
    o ostream &operator<<(ostream &, directory_entry const &) inserts the
        object's path into the ostream;

o  the comparsion operators (==, <=>) compare their std::path
        data members;

o path const &path() const, 
        operator path const &() const
       returns the current object's path name.

Members:


    o void assign(path const &dest):
the current path is replaced by dest (identical to
        directory_entry's assignment operator);

o void replace_filename(path const &dest):
the last element of the current object's path is replaced by
        dest. If that element is empty (e.g., when the object's path ends
        in a directory separator) then dest is appended to the current
        object's path;

o filesystem::file_status status([error_code &ec]):
returns type and attributes of the directory entry referred to by the
        current object. If the current object refers to a symlink then the
        status of the entry the symlink refers to is returned. To obtain the
        status of the entry itself, even if it's a symlink use
        symlink_status (see also section 4.3.2 and 4.3.2.1
        below).


4.3.4.1: Visiting directory entries: (recursive_)directory_iterator

        The filesystem namespace has two classes simplifying directory
processing: objects of the class directory_iterator are (input) iterators
iterating over the entries of directories; and objects of the class
recursive_directory_iterator are (input) iterators recursively visiting
all entries of directories.location 

enum class directory_options

The enum class directory_options defines values that
are used to fine-tune the behavior of recursive_directory_iterator
objects, supporting bitwise operators (the values of its symbols are shown
between parentheses):
    
    o none (0): directory symlinks are skipped, denied permission to enter
        a sub-directory generates an error;
    o follow_directory_symlink (1): symlinks to sub-directories are
        followed;
    o skip_permission_denied (2): directories that cannot be entered are
        silently skipped. 

Constructors:


    o (recursive_)directory_iterator():
the end-iterator of the (recursive_)directory_iterator's
        iterator-range;

o (recursive_)directory_iterator(path const &from [, error_code &ec]):
constructs a (recursive-)directory_iterator using from as the
        starting directory of the iterator. All members of standard input
        iterators (cf. section 18.2) are supported;

o recursive_directory_iterator(path const &from, directory_options opts
            [, error_code &ec]):
constructs a (recursive-)directory_iterator fine-tuning its
        behavior to the options specified by opts.

Copy, and move constructors are available.

These iterators point to directory_entry objects referring to
    entries in the computer's file system. E.g.,
     cout << *directory_iterator{ "/home" } << '\n'; 
                // shows the first entry under /home 

(Recursive_)directory_iterators can be used with range-based for-loops
and in standard for and while statements.

Additional members of recursive_directory_iterators


    o int depth() const:
returns the current iteration depth. The depth of the initial
        directory, specified at construction-time, equals 0;

o void disable_recursion_pending():
when called before incrementing the iterator the next directory entry
        is not recursively visited. Once the iterator is incremented recursion
        is again allowed. If a recursion should end at a specific depth then
        once depth() returns that specific depth this member must be
        called each time before the iterator's increment operator is called;

o recursive_directory_iterator &increment(error_code &ec):
acts identically to the iterator's increment operator. However, at
        errors operator++ throws a filesystem_error exception, while
        increment assigns the error to ec;

o directory_options options() const:
returns the option(s) specified at construction-time;

o void pop():
ends processing the current directory, and continues at the next
        entry in the current directory's parent;

o bool recursion_pending() const:
true is returned if recursive processing of sub-directories of the
        currently processed directory is allowed. If so, and the 
        the iterator points at a sub-directory, then processing
        continues in that sub-directory at the iterator's next increment;

Here's a small program displaying all elements and
all immediate sub-directories of a directory:
            int main()
    {
        recursive_directory_iterator base{ "/var/log",
                                      directory_options::skip_permission_denied };
    
        for (auto entry = base, endIt = end(base); entry != endIt; ++entry)
        {
            cout << entry.depth() << ": " << *entry << '\n';
            if (entry.depth() == 1)
                entry.disable_recursion_pending();
        }
    }

The above program handles entries as they come. Other strategies must be
implemented `by hand'. E.g., a breadth-first strategy first visits all the
non-directory entries and then visits the sub-directories, as illustrated in
the next example by processingthe directories stored in level in turn
(initially it merely contains the starting directory). 

`Processing a directory' means that its non-directory entries are directly
processed storing the names of sub-directories in next. Once
all entries at level have been processed the names of the next level
sub-directories are available (in next). By assigning next to
level all directories at the next level are processed. When reaching the
most deeply nested sub-directories its next is empty and the while
statement ends:
        void breadth(path const &dir)           // starting dir.
    {
        vector<path> level{ dir };          // currently processed level
    
        while (not level.empty())           // process all its dirs.
        {
            vector<path> next;              // dirs of the next level
    
            for (auto const &dir: level)    // visit all dirs at this level
            {
                cout << "At " << dir << '\n';
                                            // at each dir: visit all entries
                for (auto const &entry: directory_iterator{ dir })
                {
                    if (entry.is_directory())   // store all dirs at the current
                        next.push_back(entry);  //  level
                    else                        // or process its non-dir entry
                        cout << "   entry: " << entry << '\n';
                }
            }
    
            level = next;                   // continue at the next level,
        }                                   // which eventually won't exist
    }


4.3.5: Information about the space of file systems: space_info

    Every existing path lives in a file system, Sizes of file systems
typically are quite large, but there is a limit to their sizes.

The size of file systems, the number of bytes that is currently being used
and the remaining number of bytes is made available by the function 
     space(path const &entry [,
error_code &ec]), returning the information about the file system containing
entry in a POD struct space_info.

If the error_code argument is provided then it is cleared if no error
occurs, and set to the operating system's error code if an error has
occurred. If an error occurs and the error_code argument was not provided
then a filesystem_error exception is thrown, receiving path as its
first argument and the operating system's error code as its error_code
argument.  

The returned space_info has three fields:
            uintmax_t capacity;     // total size in bytes
    uintmax_t free;         // number of free bytes on the file system
    uintmax_t available;    // free bytes for a non-privileged process

If a field cannot be determined it is set to -1 (i.e., the max. value of
the type uintmax_t). 

The function can be used this way:
            int main()
    {
        path tmp{ "/tmp" };
    
        auto pod = space(tmp);
    
        cout << "The filesystem containing /tmp has a capacity of " <<
                                                pod.capacity << " bytes,\n"
            "i.e., " << pod.capacity / (1024 * 1024) << " MB.\n"
            "# free bytes: " << pod.free << "\n"
            "# available:  " << pod.available << "\n"
            "free + available:  " << pod.free + pod.available << '\n';
    }


4.3.6: File system exceptions: filesystem_error

    The std::filesystem namespace offers its own exception type
filesystem_error (see also chapter 10). Its constructor has
the following signature (the bracketed parameters are optional):
            filesystem_error(string const &what, 
                    [path const &path1, [path const &path2,]] 
                    error_code ec);

As filesystem facilities are closely related to standard system
functions, errc error code enumeration values can be used to obtain
error_codes to pass to filesystem_error, as illustrated by the
following program:
        int main()
    try
    {
        try
        {
            throw filesystem_error{ "exception encountered", "p1", "p2",
                                        make_error_code(errc::address_in_use) };
        }
        catch (filesystem_error const &fse)
        {
            cerr << "what:  " << fse.what() << "\n"
                    "path1: " << fse.path1() << "\n"
                    "path2: " << fse.path2() << "\n"
                    "code:  " << fse.code() << '\n';
    
            throw;
        }
    }
    catch (exception const &ec)
    {
        cerr << "\n"
                "plain exception's what: " << ec.what() << "\n\n";
    }



Chapter 5: The `string' Data Type

C++ offers many solutions for common problems. Most of these facilities
are part of the Standard Template Library or they are implemented as
generic algorithms (see chapter 19).

Among the facilities C++ programmers have developed over and over again
are those manipulating chunks of text, commonly called strings. The C
programming language offers rudimentary string support.

To process text C++ offers a std::string type. In C++
the traditional C library functions manipulating NTB strings are
deprecated in favor of using string objects. Many problems in C
programs are caused by buffer overruns, boundary errors and allocation
problems that can be traced back to improperly using these traditional C
string library functions. Many of these problems can be prevented using
C++ string objects.

Actually, string objects are class type variables, and in that sense
they are comparable to stream objects like cin and cout. In this
section the use of string type objects is covered. The focus is on their
definition and their use. When using string objects the
    member function syntax is commonly used:
            stringVariable.operation(argumentList)

For example, if string1 and string2 are variables of type
std::string, then
            string1.compare(string2)

can be used to compare both strings.

In addition to the common member functions the string class also offers a
wide variety of operators, like the assignment (=) and the comparison
operator (==). Operators often result in code that is easy to understand
and their use is generally preferred over the use of member functions offering
comparable functionality. E.g., rather than writing
            if (string1.compare(string2) == 0)

the following is generally preferred:
            if (string1 == string2)

To define and use string-type objects, sources must include the header
file <string>. To merely  declare the string type
the header iosfwd can be included.

In addition to std::string, the header file string defines the
following string types:
    
    o std::wstring, a string type consisting of wchar_t
        characters;
    o std::u16string, a string type consisting of char16_t
        characters; 
    o std::u32string, a string type consisting of char32_t
        characters. 


5.1: Operations on strings

Some of the operations that can be performed on strings return indices within
the strings. Whenever such an operation fails to find an appropriate index,
the value string::npos is returned. This value is a symbolic
value of type string::size_type, which is (for all practical
purposes) an (unsigned) int.

All string member functions accepting string objects as arguments also
accept NTBS arguments. The same usually holds true for operators accepting
string objects.

Some string-members use iterators. Iterators are formally introduced
in section 18.2. Member functions using iterators are listed in
the next section (5.2), but the iterator concept itself
is not further covered by this chapter.

Strings support a large variety of members and operators. A short overview
listing their capabilities is provided in this section, with subsequent
sections offering a detailed discussion. The bottom line: C++ strings are
extremely versatile and there is hardly a reason for falling back on the C
library to process text. C++ strings handle all the required memory
management and thus memory related problems, which are the #1 source of
problems in C programs, can be prevented when C++ strings are
used. Strings do come at a price, though. The class's extensive capabilities
have also turned it into a beast. It's hard to learn and master all its
features and in the end you'll find that not all that you expected is actually
there. For example, std::string doesn't offer case-insensitive
comparisons. But in the end it isn't even as simple as that. It is there,
but it is somewhat hidden and at this point in the C++ Annotations it's too
early to study into that hidden corner yet. Instead, realize that C's
standard library does offer useful functions that can be used as long as
we're aware of their limitations and are able to avoid their traps. So for
now, to perform a traditional case-insensitive comparison of the content
of two std::string objects str1 and str2 the following will do:
    
            strcasecmp(str1.c_str(), str2.c_str());

Strings support the following functionality:
    
    o initialization:
when string objects are defined they are always properly
initialized. In other words, they are always in a valid state. Strings may
be initialized empty or already existing text can be used to initialize
strings.
    o assignment:
strings may be given new values. New values may be assigned using
member functions (like assign) but a plain assignment operator (i.e.,
=)may also be used. Furthermore, assignment to a character buffer is
also supported.
    o conversions:
the partial or complete content of string objects may be interpreted
as C strings but the string's content may also be processed as a series
of raw binary bytes, not necessarily terminating in a 0-valued
character. Furthermore, in many situations plain characters and C strings
may be used where std::strings are accepted as well.
    o breakdown:
the individual characters stored in a string can be accessed
using the familiar index operator ([]) allowing us to either access or
modify information in the middle of a string.
    o comparisons:
strings may be compared to other strings (NTBSs) using the
familiar logical comparison operators ==, !=, <, <=, > and >=. There
are also member functions available offering a more fine-grained comparison.
    o modification:
the content of strings may be modified in many ways. Operators are
available to add information to string objects, to insert information in
the middle of string objects, or to replace or erase (parts of) a
string's content.
    o swapping:
the string's swapping capability allows us in principle to exchange
the content of two string objects without a byte-by-byte copying
operation of the string's content.
    o searching:
the locations of characters, sets of characters, or series of
characters may be searched for from any position within the string object
and either searching in a forward or backward direction.
    o housekeeping:
several housekeeping facilities are offered: the string's length,
or its  empty-state may be interrogated. But string objects may also be
resized.
    o stream I/O:
strings may be extracted from or inserted into streams. In addition
to plain string extraction a line of a text file may be read without running
the risk of a buffer overrun. Since extraction and insertion operations are
stream based the I/O facilities are device independent.


5.2: A std::string reference

In this section the string members and string-related operations are
referenced. The subsections cover, respectively the string's initializers,
iterators, operators, and member functions. The following terminology is used
throughout this section:
    
    o  object is always a string-object;
    o  argument is a string const & or a char const * unless
indicated otherwise.  The content of an argument never is modified by the
operation processing the argument;
    o  opos refers to an offset into an object string;
    o  apos refers to an offset into an argument;
    o  on represents a number of characters in an object (starting
at opos);
    o  an represents a number of characters in an argument (starting
at apos).

Both opos and apos must refer to existing offsets, or an exception
(cf. chapter 10) is generated. In contrast, an and on may
exceed the number of available characters, in which case only the available
characters are considered.

Many members declare default values for on, an and apos. Some members
declare default values for opos. Default offset values are 0, the default
values of on and an is string::npos, which can be interpreted as
`the required number of characters to reach the end of the string'.

With members starting their operations at the end of the string object's
content proceeding backwards, the default value of opos is the index of
the object's last character, with on by default equal to opos + 1,
representing the length of the substring ending at opos.

In the overview of member functions presented below it may be assumed that all
these parameters accept default values unless indicated otherwise.  Of course,
the default argument values cannot be used if a function requires additional
arguments beyond the ones otherwise accepting default values.

Some members have overloaded versions expecting an initial argument of type
char const *. But even if that is not the case the first argument can
always be of type char const * where a parameter of std::string is
defined.

Several member functions accept iterators. Section 18.2 covers
the technical aspects of iterators, but these may be ignored at this point
without loss of continuity.  Like apos and opos, iterators must refer
to existing positions and/or to an existing range of characters within the
string object's content.

All string-member functions computing indices return the
predefined constant string::npos on failure.

The s literal suffix to indicate that a std::string constant is
intended when a string literal (like "hello world") is used. It can be
used after declaring using namespace std or, more specific, after
declaring 
        
    using namespace std::literals::string_literals. 

When string literals are used when explicitly defining or using
std::string objects the s-suffix is hardly ever required, but it may
come in handy when using the auto keyword. E.g., auto str = "hello
world"s defines std::string str, whereas it would have been a char
const * if the literal suffix had been omitted.


5.2.1: Initializers

    After defining string objects they are guaranteed to be in a valid
state. At definition time  string objects may be initialized in one of the
following ways:
    The following string constructors  are
available:
    
    o string object:
initializes object to an empty string.  When defining a string
        this way no argument list may be specified;
    o string object(string::size_type count, char ch):
initializes object with count characters ch. Caveat:
        to initialize a string object using this constructor do not use the
        curly braces variant, but use the constructor as shown, to avoid
        selecting the initializer-list constructor (see below);
    o string object(string const &argument):
initializes object with argument;
    o string object(std::string const &argument, string::size_type
            apos, string::size_type an):
initializes object with argument's content starting at index
        position apos, using at most an of argument's characters;
    o string object(InputIterator begin, InputIterator end):
initializes object with the characters in the range of characters
        defined by the two InputIterators.
    o string object(std::initializer_list<char> chars):
initializes object with the characters specified in the 
        initializer list. The string may also directly be initialized, using
        the curly braced initialization. Here is an example showing both
        forms: 
       string str1({'h', 'e', 'l', 'l', 'o'});
string str2{ 'h', 'e', 'l', 'l', 'o' };


5.2.2: Iterators

            See section 18.2 for details about iterators. As a quick
introduction to iterators: an iterator acts like a pointer, and pointers can
often be used in situations where iterators are requested. Iterators usually
come in pairs, defining a range of entities. The begin-iterator points to the
first entity, the end-iterator points just beyond the last entity of the
range. Their difference is equal to the number of entities in the
iterator-range.

Iterators play an important role in the context of generic algorithms
(cf. chapter 19). The class std::string defines the following
iterator types:
    
    o string::iterator and string::const_iterator:
        these iterators are forward iterators. The
const_iterator is returned by string const objects, the plain
iterator is returned by non-const string objects. Characters referred to
by iterators may be modified;
    o string::reverse_iterator and string::const_reverse_iterator:
        these iterators are also forward iterators but when
incrementing the iterator the previous character in the string object
is reached. Other than that they are comparable to, respectively,
string::iterator and string::const_iterator.


5.2.3: Operators

    String objects may be manipulated by member functions but also by
operators. Using operators often results in more natural-looking code. In
cases where operators are available having equivalent functionality as member
function the operator is practically always preferred.

The following operators are available for string objects (in the examples
`object' and `argument' refer to existing std::string objects).
    
    o  plain assignment:
    a character, C or C++ string may be assigned to a string
object. The assignment operator returns its left-hand side operand. Example:
    object =  argument;
object = "C string";
object = 'x';
object = 120;       // same as object = 'x'

o  addition:
    the arithmetic additive assignment operator and the addition
operator add text to a string object. The compound assignment operator
returns its left-hand side operand, the addition operator returns its result
in a temporary string object. When using the addition operator either the
left-hand side operand or the right-hand side operand must be a
std::string object. The other operand may be a char, a C string or a
C++ string. Example:
    object += argument;
object += "hello";
object += 'x';      // integral expressions are OK

argument + otherArgument;   // two std::string objects
argument + "hello";         // using + at least one
"hello" + argument;         // std::string is required
argument + 'a';             // integral expressions are OK
'a' + argument;

o  index operator:
    The index operator may be used to retrieve object's
individual characters, or to assign new values to individual characters of a
non-const string object. There is no range-checking (use the at() member
function for that). This operator returns a char & or char const &.
Example:
    object[3] = argument[5];

o  logical operators:
    the logical comparison operators may be applied to two string
objects or to a string object and a C string to compare their
content. These operators return a bool value.
The ==, !=, >, >=, <, and <= operators are available. The ordering
operators perform a lexicographical comparison of their content
using the ASCII character collating sequence. Example:
    object == object;           // true
object != (object + 'x');   // true
object <= (object + 'x');   // true

o  stream related operators:
    the insertion-operator (cf. section 3.1.4) may be used to
insert a string object into an ostream, the extraction-operator may be
used to extract a string object from an istream. The extraction operator
by default first ignores all whitespace characters and then extracts all
consecutively non-blank characters from an istream. Instead of a string a
character array may be extracted as well, but the advantage of using a string
object should be clear: the destination string object is automatically resized
to the required number of characters. Example:
    cin >> object;
cout << object;


5.2.4: Member functions

    The std::string class offers many member function as well as additional
non-member functions that should be considered part of the string class.
All these functions are listed below in alphabetic order.

The symbolic value string::npos is defined by the string class. It
represents `index-not-found' when returned by member functions returning
string offset positions. Example: when calling `object.find('x')' (see
below) on a string object not containing the character 'x', npos
is returned, as the requested position does not exist.

The final 0-byte used in C strings to indicate the end of an NTBS is
not considered part of a C++ string, and so the member function will
return npos, rather than length() when looking for 0 in a string
object containing the characters of a C string.

Here are the standard functions that operate on objects of the class
string. When a parameter of size_t is mentioned it may be interpreted as a
parameter of type string::size_type, but without defining a default
argument value. The type size_type should be read as
string::size_type. With size_type the default argument values
mentioned in section 5.2 apply. All quoted functions are
member functions of the class std::string, except where indicated
otherwise.
    
    o char &at(size_t opos):
a reference to the character at the indicated position is
        returned. When called with string const objects a char const &
        is returned. The member function performs range-checking, raising an
        exception (that by default aborts the program) if an invalid index is
        passed.
    o string &append(InputIterator begin, InputIterator end):
the characters in the range defined by begin and end are
        appended to the current string object.
    o string &append(string const &argument, size_type apos,
        size_type an):
argument (or a substring) is appended to the current string
        object.
    o string &append(char const *argument, size_type an):
the first an characters of argument are appended to the
        string object.
    o string &append(size_type n, char ch):
n characters ch are appended to the current string object.
    o string &assign(string const &argument, size_type
        apos, size_type an):
argument (or a substring) is assigned to the string object.
        If argument is of type char const * and one additional
        argument is provided the second argument is interpreted as a value
        initializing an, using 0 to initialize apos.
    o string &assign(size_type n, char ch):
n characters ch are assigned to the current string object.
    o char &back():
returns a reference to the last char stored inside the string
        object. The result is undefined for empty strings.
    o string::iterator begin():
an iterator referring to the first character of the current string
        object is returned. With const string objects a const_iterator
        is returned.
    o size_type capacity() const:
the number of characters that can currently be stored in the string
        object without needing to resize it is returned.
    o string::const_iterator cbegin():
a const_iterator referring to the first character of the current
        string object is returned.
    o string::const_iterator cend():
a const_iterator referring to the end of the current
        string object is returned.
    o void clear():
the string's content is erased.
    o int compare(string const &argument) const:
the text stored in the current string object and the text stored in
        argument is compared using a lexicographical comparison using the
        ASCII character collating sequence. zero is returned if the two
        strings have identical content, a negative value is returned if the
        text in the current object should be ordered before the text in
        argument; a positive value is returned if the text in the current
        object should be ordered beyond the text in argument.
    o int compare(size_t opos, size_t on, string const &argument) const:
a substring of the text stored in the current string object is
        compared to the text stored in argument. At most on characters
        starting at offset opos are compared to the text in argument.
    o int compare(size_t opos, size_t on, string const &argument,
            size_type apos, size_type an):
a substring of the text stored in the current string object is
        compared to a substring of the text stored in argument. At most
        on characters of the current string object, starting at offset
        opos, are compared to at most an characters of argument,
        starting at offset apos. In this case argument must be a
        string object.
    o int compare(size_t opos, size_t on, char const *argument, size_t an):
a substring of the text stored in the current string object is
        compared to a substring of the text stored in argument. At most
        on characters of the current string object starting at offset
        opos are compared to at most an characters of
        argument. Argument must have at least an characters. The
        characters may have arbitrary values: 0-valued characters have no
        special meanings.
    o bool contains(argument) const:
returns true if the object contains argument's characters as a
        substring. The argument can be a string, a string_view (see
        section 5.3), a char or an NTBS.
    o size_t copy(char *argument, size_t on, size_type opos) const:
the content of the current string object are (partially) copied into
        argument. The actual number of characters copied is returned.  The
        second argument, specifying the number of characters to copy, from the
        current string object is required. No 0-valued character is appended
        to the copied string but can be appended to the copied text using an
        idiom like the following: 
           argument[object.copy(argument, string::npos)] = 0;
       Of course, the programmer should make sure that argument's size is
        large enough to accommodate the additional 0-byte.
    o string::const_reverse_iterator crbegin():
a const_reverse_iterator referring to the last character of the
        current string object is returned.
    o string::const_reverse_iterator crend():
a const_reverse_iterator referring to the begin of the current
        string object is returned.
    o char const *c_str() const:
the content of the current string object as an NTBS.
    o char const *data() const:
the raw content of the current string object are returned. Since this
        member does not return an NTBS (as
        c_str does), it can be used to retrieve any kind of information
        stored inside the current string object including, e.g., series of
        0-bytes:
           string s(2, 0);
    cout << static_cast<int>(s.data()[1]) << '\n';

o bool empty() const:
true is returned if the current string object contains no data.
    o string::iterator end():
an iterator referring to the position just beyond the last character
        of the current string object is returned. With const string
        objects a const_iterator is returned.
    o bool ends_with(argument) const:
returns true if the object's characters end with argument. The
        argument can be a string, a string_view, a char or an
        NTBS.
    o string &erase(size_type opos, size_type on):
a (sub)string of the information stored in the current string object
        is erased.
    o string::iterator erase(string::iterator begin, string::iterator end):
the parameter end is optional. If omitted the value returned by
        the current object's end member is used. The characters defined by
        the begin and end iterators are erased. The iterator begin
        is returned, which is then referring to the position immediately
        following the last erased character.
    o size_t find(string const &argument, size_type opos) const:
the first index in the current string object is returned where
        argument is found.
    o size_t find(char const *argument, size_type opos, size_type an) const:
the first index in the current string object is returned where
        argument is found. When all three arguments are specified the
        first argument must be a char const *.
    o size_t find(char ch, size_type opos) const:
the first index in the current string object is returned where ch
        is found.
    o size_t find_first_of(string const &argument,
                                               size_type opos) const:
the first index in the current string object is returned whose
        character matches any character from argument.
    o size_type find_first_of(char const *argument, size_type opos,
        size_type an) const:
the first index in the current string object is returned whose
        character matches any character from argument. If opos is
        provided it refers to the first index in the current string object
        where the search for argument should start. If omitted, the string
        object is completely scanned.  If an is provided it indicates the
        number of characters of the char const * argument that should be
        used in the search. It defines a substring starting at the beginning
        of argument. If omitted, all of argument's characters are
        used.
    o size_type find_first_of(char ch, size_type opos):
the first index in the current string object is returned whose
        character is equal to ch.
    o size_t find_first_not_of(string const &argument,
                                               size_type opos) const:
the first index in the current string object is returned whose
        character does not match any character from argument.
    o size_type find_first_not_of(char const *argument, size_type opos,
        size_type an) const:
the first index in the current string object is returned whose
        character does not match any character from argument. The opos
        and an parameters are handled as with find_first_of
    o size_t find_first_not_of(char ch, size_type opos) const:
the first index in the current string object is returned whose
        character is unequal to ch.
    o size_t find_last_of(string const &argument,
                                               size_type opos) const:
the last index in the current string object is returned whose
        character matches any character from argument.
    o size_type find_last_of(char const *argument, size_type opos,
            size_type an) const:
the last index in the current string object is returned whose
        character matches any character from argument. If opos is
        provided it refers to the last index in the current string object
        where the search for argument should start (searching backward
        towards the beginning of the current object). If omitted, the string
        object is scanned completely.  If an is provided it indicates the
        number of characters of the char const * argument that should be
        used in the search. It defines a substring starting at the beginning
        of argument. If omitted, all of argument's characters are
        used.
    o size_type find_last_of(char ch, size_type opos):
the last index in the current string object is returned whose
        character is equal to  ch.
    o size_t find_last_not_of(string const &argument,
                                                size_type opos) const:
the last index in the current string object is returned whose
        character does not match any character from argument.
    o size_type find_last_not_of(char const *argument, size_type opos,
        size_type an) const:
the last index in the current string object is returned whose
        character does not match any character from argument. The opos
        and an parameters are handled as with find_last_of.
    o size_t find_last_not_of(char ch, size_type opos) const:
the last index in the current string object is returned whose
        character is unequal to ch.
    o char &front():
returns a reference to the first char stored inside the string
        object. The result is undefined for empty strings.
    o allocator_type get_allocator():
returns the allocator of the class std::string
    o istream &std::getline(istream &istr, string &object,
            char delimiter = '\n'):
Note: this is not a member function of the class string.

       A line of text is read from istr. All characters until
        delimiter (or the end of the stream, whichever comes first) are
        read from istr and are stored in object. If the delimiter is
        encountered it is removed from the stream, but is not stored in
        object.

       If the delimiter is not found, istr.eof returns true (see
        section 6.3.1). Since streams may be interpreted as bool
        values (cf. section 6.3.1) a commonly encountered idiom to
        read all lines from a stream successively into a string object
        line looks like this:
           while (getline(istr, line))
        process(line);
       The content of the last line, whether or not it was terminated
        by a delimiter, is eventually also assigned to object.
    o string &insert(size_t opos, string const &argument,
        size_type apos, size_type an):
a (sub)string of argument is inserted into the current string
        object at the current string object's index position
        opos. Arguments for apos and an must either both be
        provided or they must both be omitted.
    o string &insert(size_t opos, char const *argument,
                size_type an):
argument (of type char const *) is inserted at index opos
        into the current string object.
    o string &insert(size_t opos, size_t count, char ch):
Count characters ch are inserted at index opos into the
        current string object.
    o string::iterator insert(string::iterator begin, char ch):
the character ch is inserted at the current object's position
        referred to by begin. Begin is returned.
    o string::iterator insert(string::iterator begin, size_t count,
        char ch):
Count characters ch are inserted at the current object's
        position referred to by begin. Begin is returned.
    o string::iterator insert(string::iterator begin, InputIterator
        abegin, InputIterator aend):
the characters in the range defined by the InputIterators abegin
        and aend are inserted at the current object's position referred to
        by begin. Begin is returned.
    o size_t length() const:
the number of characters stored in the current string object is
        returned.
    o size_t max_size() const:
the maximum number of characters that can be stored in the current
        string object is returned.
    o void pop_back():
The string's last character is removed from the string object.
    o void push_back(char ch):
The character ch is appended to the string object.
    o string::reverse_iterator rbegin():
a reverse iterator referring to the last character of the current
        string object is returned. With const string objects a
        const_reverse_iterator is returned.
    o string::reverse_iterator rend():
a reverse iterator referring to the position just before the first
        character of the current string object is returned. With const
        string objects a const_reverse_iterator is returned.
    o string &replace(size_t opos, size_t on, string const
        &argument, size_type apos, size_type an):
a (sub)string of characters in object are replaced by the (subset
        of) characters of argument.  If on is specified as 0
        argument is inserted into object at offset opos.
    o string &replace(size_t opos, size_t on, char const *argument,
        size_type an):
a series of characters in object are replaced by the first an
        characters of char const * argument.
    o string &replace(size_t opos, size_t on, size_type count, char ch):
on characters of the current string object, starting at index
        position opos, are replaced by count characters ch.
    o string &replace(string::iterator begin, string::iterator end,
        string const &argument):
the series of characters in the current string object defined by the
        iterators begin and end are replaced by argument. If
        argument is a char const *, an additional argument an may
        be used, specifying the number of characters of argument that are
        used in the replacement.
    o string &replace(string::iterator begin, string::iterator end,
        size_type count, char ch):
the series of characters in the current string object defined by the
        iterators begin and end are replaced by count characters
        having values ch.
    o string &replace(string::iterator begin, string::iterator end,
        InputIterator abegin, InputIterator aend):
the series of characters in the current string object defined by the
        iterators begin and end are replaced by the characters in the
        range defined by the InputIterators abegin and aend.
    o void reserve(size_t request):
the current string object's capacity is changed to at least
        request. After calling this member, capacity's return value
        will be at least request. A request for a smaller size than the
        value returned by capacity is ignored. A
        std::length_error exception is thrown if
        request exceeds the value returned by max_size
        (std::length_error is defined in the stdexcept
        header). Calling reserve() has the effect of redefining a string's
        capacity: when enlarging the capacity extra memory is allocated, but
        not immediately available to the program. This is illustrated by the
        exception thrown by the string's at() member when trying to access
        an element exceeding the string's size but not the string's
        capacity.
    o void resize(size_t size, char ch = 0):
the current string object is resized to
        size characters. If the string object is resized to a size larger
        than its current size the additional characters will be initialized to
        ch. If it is reduced in size the characters having the highest
        indices are chopped off.
    o size_t rfind(string const &argument, size_type opos) const:
the last index in the current string object where argument is
        found is returned. Searching proceeds from the current object's offset
        opos back to its beginning.
    o size_t rfind(char const *argument, size_type opos, size_type an) const:
the last index in the current string object where argument is
        found is returned.  Searching proceeds from the current object's
        offset opos back to its beginning.  The parameter an specifies
        the length of the substring of argument to look for, starting at
        argument's beginning.
    o size_t rfind(char ch, size_type opos)const:
the last index in the current string object where ch is found
        is returned.  Searching proceeds from the current object's offset
        opos back to its beginning.
    o void shrink_to_fit():
optionally reduces the amount of memory allocated by a vector to its
        current size. The implementor is free to ignore or otherwise optimize
        this request. In order to guarantee a `shrink to fit' operation the
           string{ stringObject }.swap(stringObject)
       idiom can be used.
    o size_t size() const:
the number of characters stored in the current string object is
        returned. This member is a synonym of length().
    o bool starts_with(argument) const:
returns true if the object's character range starts with
        argument. The argument can be a string, a string_view, a
        char or an NTBS.
    o string substr(size_type opos, size_type on) const:
a substring of the current string object of at most on characters
        starting at index opos is returned.
    o void swap(string &argument):
the content of the current string object are swapped with the
        content of argument. For this member argument must be a
        string object and cannot be a char const *.


5.2.5: Conversion functions

    Several string conversion functions are available operating on or producing
std::string objects. These functions are listed below in alphabetic
order. They are not member functions, but class-less (free) functions declared
in the std namespace. The <string> header file must be included
before they can be used.


    o float stof(std::string const &str, size_t *pos = 0):
Initial whitespace characters in str are ignored. Then the
        following sequences of characters are converted to a float value,
        which is returned:
       
        o  A decimal floating point constant:
            
           o  An optional + or - character
           o  A series of decimal digits, possibly containing one decimal
                point character
           o  An optional e or E character, followed by an optional - or +
            character, followed by a series  of decimal digits

o  A hexadecimal floating point constant:
           
           o  An optional + or - character
            o  0x or 0X
            o  A series of hexadecimal digits, possibly containing one
                decimal point character
           o  An optional p or P character, followed by an optional - or +
            character, followed by a series  of decimal digits

o  An infinity expression:
           
           o  An optional + or - character
            o  The words inf or infinity (case insensitive words)

o  A `not a number' expression:
           
           o  An optional + or - character
            o  The words nan or nan(alphanumeric character sequence)
                (nan is a case insensitive word), resulting in a NaN
                floating point value

If pos != 0 the index of the first character in str which was
        not converted is returned in *pos.  A std::invalid_argument
        exception is thrown if the characters in str could not be
        converted to a float, a std::out_of_range exception is thrown
        if the converted value would have exceeded the range of float
        values.
    o double stod(std::string const &str, size_t *pos = 0):
A conversion as described with stof is performed, but now to a
        value of type double.
    o double stold(std::string const &str, size_t *pos = 0):
A conversion as described with stof is performed, but now to a
        value of type long double.
    o int stoi(std::string const &str, size_t *pos = 0,
                                                    int base = 10):
Initial whitespace characters in str are ignored. Then all
        characters representing numeric constants of the number system whose
        base is specified are converted to an int value, which is
        returned. An optional + or - character may prefix the numeric
        characters. Values starting with 0 are automatically interpreted as
        octal values, values starting with 0x or 0X as hexadecimal
        characters. The value base must be between 2 and 36. If pos !=
        0 the index of the first character in str which was not converted
        is returned in *pos. A std::invalid_argument exception is
        thrown if the characters in str could not be converted to an
        int, a std::out_of_range exception is thrown if the converted
        value would have exceeded the range of int values.

Here is an example of its use:
        int value = stoi(" -123"s);  // assigns value -123
value = stoi(" 123"s, 0, 5); // assigns value 38

o long stol(std::string const &str, size_t *pos = 0, 
                                                    int base = 10):
A conversion as described with stoi is performed, but now to a
        value of type long.
    o long long stoll(std::string const &str, size_t *pos = 0, 
                                                    int base = 10):
A conversion as described with stoi is performed, but now to a
        value of type long long.
    o unsigned long stoul(std::string const &str, size_t *pos = 0, 
                                                    int base = 10):
A conversion as described with stoi is performed, but now to a
        value of type unsigned long.
    o unsigned long long stoull(std::string const &str, 
                                            size_t *pos = 0, int base = 10):
A conversion as described with stoul is performed, but now to a
        value of type unsigned long long.
    o std::string to_string(Type value):
Type can be of the types int, long, long long, unsigned, unsigned
        long, unsigned long long, float, double, or long double. The
        value of the argument is converted to a textual representation, which
        is returned as a std::string value.

o std::wstring to_wstring(Type value):
The conversion as described at to_string is performed, returning
        a std::wstring.


5.3: std::string_view

In addition to the class std::string the class std::string_view can be
used as a wrapper-class of char arrays.  The class string_view can
be considered a light-weight string class.  Before using
std::string_view objects the <string_view> header file must have been
included.

In addition to the standard constructors (default, copy, move) it offers the
following constructors:
    
    o constexpr string_view(char const *src, size_t nChars), constructs a
string_view object from the first nChars characters of src. The
characters in the range [src, src + nChars) may be 0-valued
characters;

o constexpr string_view(char const *src), constructs a string_view
object from the NTBS starting at src. The argument passed to this
constructor may not be a null pointer;

o constexpr string_view(Iterator begin, Iterator end), constructs a
string_view object from the characters in the iterator-range
[begin, end).

A string_view object does not contain its own copy of the initialized
data. Instead, it refers to the characters that were used when it was
initially constructed. E.g., the following program produces unpredictable
output, but when the hello array is defined as a static array it shows
hello: 
        #include <string_view>
    #include <iostream>
    using namespace std;
    
    string_view fun()
    {
        char hello[] = "hello";
        return { hello };
    }
    
    int main()
    {
        string_view obj = fun();
        cout << obj << '\n';
    }

The std::string_view class provides the same members as std::string,
except for members extending the string_view's characters (neither
appending nor inserting characters is possible). However, string_view
objects can modify their characters (using the index operator or at
member).

The string_view class also offers some extra members:
    
    o remove_prefix(size_t step):
moves the begin of the object's character range forward by step
        positions.
    o remove_suffix(size_t step):
moves the end of the object's character range backward by step
        positions.
    o constexpr string_view operator""sv(char const *str,
        size_t len):
returns a string_view object containing len characters of
        str.

Like std::string the std::string_view class provides hashing
facilities, so string_view objects can be used as keys in, e.g., map
containers (cf. chapter 12).



Chapter 6: The IO-stream Library

Extending the standard stream (FILE) approach, well known from the
C programming language, C++ offers an input/output (I/O)
library  based on class concepts.

All C++ I/O facilities are defined in the namespace std. The std::
prefix is omitted below, except for situations where this would result in
ambiguities.

Earlier (in chapter 3) we've seen several examples of the
use of the C++ I/O library, in particular showing  insertion
operator (<<) and the extraction operator (>>).
In this chapter we'll cover I/O in more detail.

The discussion of input and output facilities provided by the C++
programming language heavily uses the class concept and the notion of
member functions. Although class construction has not yet been covered (for
that see chapter 7) and although inheritance is not covered
formally before chapter 13, it is quite possible to discuss I/O
facilities long before the technical background of class construction has been
covered.

Most C++ I/O classes have names starting with basic_ (like
basic_ios). However, these basic_ names are not regularly found in
C++ programs, as most classes are also defined through using
declarations  like:
                using ios = basic_ios<char>;

Since C++ supports various kinds of character types (e.g., char,
wchar_t), I/O facilities were developed using the template mechanism
allowing for easy conversions to character types other than the traditional
char type. As elaborated in chapter 21, this also allows the
construction of generic software, that could thereupon be used for any
particular type representing characters. So, analogously to the above
using declaration there exists a
                using wios = basic_ios<wchar_t>;

This way, wios can be used for the wchar_t type. Because of the
existence of these type definitions, the basic_ prefix was omitted from
the C++ Annotations without loss of continuity. The C++ Annotations primarily
focus on the standard 8-bits char type.

Iostream objects cannot be declared  using standard
    forward declarations, like:
            class std::ostream;     // now erroneous

Instead, to declare iostream classes the <iosfwd> header file
should be included:
            #include <iosfwd>       // correct way to declare iostream classes

Using C++ I/O offers the additional advantage of
 type safety. Objects (or plain values) are inserted into
streams. Compare this to the situation commonly encountered in C where the
fprintf function is used to indicate by a format string what kind of
value to expect where. Compared to this latter situation C++'s
iostream approach immediately uses the objects where their values should
appear, as in
            cout << "There were " << nMaidens << " virgins present\n";

The compiler notices the type of the nMaidens variable, inserting
its proper value at the appropriate place in the sentence inserted into
the cout iostream.

Compare this to the situation encountered in C. Although C compilers
are getting smarter and smarter, and although a well-designed
C compiler may warn you for a mismatch between a format specifier and the
type of a variable encountered in the corresponding position of the argument
list of a printf statement, it can't do much more than warn you.
The type safety seen in C++ prevents you from making type
mismatches, as there are no types to match.

Apart from this, iostreams offer more or less the same set of
possibilities as the standard FILE-based I/O used in C: files can be
opened, closed, positioned, read, written, etc.. In C++ the basic FILE
structure, as used in C, is still available. But C++ adds to this I/O
based on classes, resulting in type safety, extensibility, and a clean design.

The ANSI/ISO standard specifies architecture independent I/O. Not all of
the standard's specifications are covered in this chapter, as they often rely
on inheritance and polymorphism, which topics are formally covered by chapters
13 and 14. Some examples are
offered in chapter 26, and in this chapter references to
specific sections in other chapters are given where appropriate.
        ------------------------------------------------------------------Insert Figure 4(Central I/O Classes)about here (file: iostreams/ioclasses)------------------------------------------------------------------
This chapter is organized as follows (see also Figure 4):
    
        o  The class ios_base is the foundation upon which the
iostream I/O library was built. It defines the core of all I/O operations and
offers, among other things, facilities for inspecting the
 state of I/O streams and facilities for output formatting.

o  The class ios is directly derived from
ios_base. Every class of the I/O library doing input or output is itself
derived from this ios class, and therefore inherits its (and, by
implication: ios_base's) capabilities. The reader is urged to keep this in
mind while reading this chapter. The concept of inheritance is not discussed
here, but rather in chapter 13.

    The class ios is important because it implements communication with a
buffer which is used by streams. This buffer is a streambuf object
which is responsible for the actual I/O to/from the actually used device,
which might be a file, a keyboard, a screen, an Internet connection, etc.,
etc.. Consequently iostream objects do not perform I/O operations
themselves, but leave these operations to the (stream)buffer objects with
which they are associated.

o  Next, basic C++ output facilities are discussed. The basic
class used for output operations is ostream, defining the
 insertion operator as well as other facilities writing information to
streams. Apart from inserting information into files it is possible to insert
information into memory buffers, for which the ostringstream class is
available. Formatting output is to a great extent possible using the
facilities defined in the ios class, but it is also possible to
insert formatting commands directly into streams using
    manipulators. This aspect of C++ output is
discussed as well.

o  Basic C++ input facilities are implemented by the istream
class. This class defines the extraction operator and related input
facilities. Comparably to inserting information into memory buffers (using
ostringstream) a class istringstream is available to extract
information from memory buffers.

o  The class iostream combines the facilities offered by
istream and ostream. Thus, iostream objects can be used to read
and write from the same object.

o  The class fstream is a frequently encountered example of an
iostream class: fstream objects are used to read and write from the
same file, which is often used in programs processing data bases. In this
chapter topics like reading and writing from the same stream and
    mixing C and C++ I/O using filebuf objects are also
covered. Other I/O related topics are covered elsewhere in the C++ Annotations
(cf. section 20.15 and chapter 26).

Stream objects have a limited but important role: they are the interface
between, on the one hand, the objects to be input or output and, on the other
hand, the streambuf, which is responsible for the actual input and output
to the device accessed by a streambuf object.

This approach allows us to construct a new kind of streambuf for a new
kind of device, and use that streambuf in combination with the `good old'
istream- and ostream-class facilities. It is important to understand
the distinction between the formatting roles of iostream objects and the
buffering interface to an external device as implemented in a streambuf
object. Interfacing to new devices (like sockets or
 file descriptors) requires the construction of a new
kind of streambuf, rather than a new kind of istream or ostream
object. A wrapper class may be constructed around the istream or
ostream classes, though, to ease the access to a special device. This is
how the stringstream classes were constructed.


6.1: Special header files

    Several iostream related header files are available. Depending on
the situation at hand, the following header files should be used:
    
    o iosfwd: sources should include this header file if only a declaration
of the stream classes is required. For example, if a function defines a
reference parameter to an ostream then the compiler does not need to know
exactly what an ostream is. When declaring such a function
the ostream class merely needs to be be declared. One cannot use
        class std::ostream; // erroneous declaration

void someFunction(std::ostream &str);

but, instead, one should use:
        #include <iosfwd>   // correctly declares class ostream

void someFunction(std::ostream &str);

o <ios>: sources should include this header file when using
types and facilites (like ios::off_type, see below) defined in the
ios class.
    o <streambuf>: sources should include this header file when using
streambuf or filebuf classes. See sections 14.8 and
14.8.3.
    o <istream>: sources should include this preprocessor directive when
using the class istream or when using classes that do both input and
output.  See section 6.5.1.
    o <ostream>: sources should include this header file when using the class
ostream class or when using classes that do both input and output.  See
section 6.4.1.
    o <iostream>: sources should include this header file when using the
global stream objects (like cin and cout) and also when defining a
stream capable of reading and writing from the same device (see also 
section 14.8.2).
    o <fstream>: sources should include this header file when using the file
stream classes.  See sections 6.4.2, 6.5.2, and 6.6.3.
    o <sstream>: sources should include this header file when using the
string stream classes. See sections 6.4.3 and 6.5.3.
    o <iomanip>: sources should include this header file when using
parameterized manipulators. See section 6.3.2.


6.2: The foundation: the class `ios_base'

The class std::ios_base forms the foundation of all I/O
operations, and defines, among other things, facilities for inspecting the
    state of I/O streams and most output formatting facilities. Every
stream class of the I/O library is, through the class ios, derived
from this class, and inherits its capabilities.  As ios_base is the
foundation on which all C++ I/O was built, we introduce it here as the
first class of the C++ I/O library.

Note that, as in C, I/O in C++ is not part of the language
(although it is part of the ANSI/ISO standard on C++).  Although it
is technically possible to ignore all predefined I/O facilities, nobody does
so, and the I/O library therefore represents a de facto I/O standard for
C++. Also note that, as mentioned before, the iostream classes themselves
are not responsible for the eventual I/O, but delegate this to an auxiliary
class: the class streambuf or its derivatives.

It is neither possible nor required to construct an ios_base object
directly. Its construction is always a side-effect of constructing an object
further down the class hierarchy, like std::ios. Ios is the next
class down the iostream hierarchy (see Figure 4). Since all
stream classes in turn inherit from ios, and thus also from ios_base,
the distinction between ios_base and ios is in practice not
important. Therefore, facilities actually provided by ios_base will be
discussed as facilities provided by ios. The reader who is interested in
the true class in which a particular facility is defined should consult the
relevant header files (e.g., ios_base.h and basic_ios.h).


6.3: Interfacing `streambuf' objects: the class `ios'

The std::ios class is derived directly from ios_base, and it
defines de facto the foundation for all stream classes of the C++ I/O
library.

Although it is possible to construct an ios object
directly, this is seldom done. The purpose of the class ios is to
provide the facilities of the class basic_ios, and to add several new
facilites, all related to the streambuf object which is managed
by objects of the class ios.

All other stream classes are either directly or indirectly derived from
ios. This implies, as explained in chapter 13, that all
facilities of the classes ios and ios_base are also available
to other stream classes. Before discussing these additional stream classes,
the features offered by the class ios (and by implication: by
ios_base) are now introduced.

In some cases it may be required to include ios explicitly. An example
is the situations where the formatting flags themselves (cf. section
6.3.2.2) are referred to in source code.

The class ios offers several member functions, most of which are related
to formatting. Other frequently used member functions are:
    
    o std::streambuf *ios::rdbuf():
A pointer to the streambuf object forming the interface between
the ios object and the device with which the ios object communicates
is returned. See sections 14.8 and 26.1.2 for more information
about the class streambuf.
    o std::streambuf *ios::rdbuf(std::streambuf *new):
        The current ios object is associated with another
streambuf object. A pointer to the ios object's original streambuf
object is returned. The object to which this pointer points is not destroyed
when the stream object goes out of scope, but is owned by the caller of
rdbuf.
    o std::ostream *ios::tie():
A pointer to the ostream object that is currently tied to the
ios object is returned (see the next member). The return value 0
indicates that currently no ostream object is tied to the ios
object. See section 6.5.5 for details.
    o std::ostream *ios::tie(std::ostream *outs):
        The ostream object is tied to current ios object. This means
that the ostream object is flushed every time before an input or
output action is performed by the current ios object. A pointer to the
ios object's original ostream object is returned. To break the tie,
pass the argument 0. See section 6.5.5 for an example.


6.3.1: Condition states

    Operations on streams may fail for various reasons.  Whenever an operation
fails, further operations on the stream are suspended. It is possible to
inspect, set and possibly clear the
 condition state of streams, allowing a program to repair the problem
rather than having to abort. The members that are available for interrogating
or manipulating the stream's state are described in the current section.

Conditions are represented by the following condition flags:
    
        o ios::badbit:
if this flag has been raised an illegal operation has been
requested at the level of the streambuf object to which the stream
interfaces. See the member functions below for some examples.
        o ios::eofbit:
if this flag has been raised, the ios object has sensed
end of file.
        o ios::failbit:
if this flag has been raised, an operation performed by the
stream object has failed (like an attempt to extract an int when no
numeric characters are available on input). In this case the stream itself
could not perform the operation that was requested of it.
        o ios::goodbit:
this flag is raised when  none of the other three condition
flags were raised.

Several condition member functions are available to manipulate or
determine the states of ios objects. Originally they returned int
values, but their current return type is bool:
    
    o bool ios::bad():
            the value true is returned when the stream's
badbit has been set and false otherwise.  If true is
returned it indicates that an illegal operation has been requested at the
level of the streambuf object to which the stream interfaces. What does
this mean? It indicates that the streambuf itself is behaving
unexpectedly. Consider the following example:
        std::ostream error(0);

Here an ostream object is constructed without providing it with a
working streambuf object. Since this `streambuf' will never operate
properly, its badbit flag is raised from the very beginning:
error.bad() returns true.

o bool ios::eof():
            the value true is returned when end of file
(EOF) has been sensed (i.e., the eofbit flag has been set) and
false otherwise. Assume we're reading lines line-by-line from cin, but
the last line is not terminated by a final \n character. In that case
std::getline attempting to read the \n delimiter hits
end-of-file first. This raises the eofbit flag and cin.eof() returns
true. For example, assume std::string str and main executing the
statements:
        getline(cin, str);
cout << cin.eof();

Then
        echo "hello world" | program

prints the value 0 (no EOF sensed). But after
        echo -n "hello world" | program

the value 1 (EOF sensed) is printed.
    o bool ios::fail():
            the value true is returned when bad returns true
or when the failbit flag was set. The value false is returned
otherwise.  In the above example, cin.fail() returns false,
whether we terminate the final line with a delimiter or not (as we've read
a line). However, executing another getline results in
raising the failbit flag, causing cin::fail() to return
true. In general: fail returns true if the requested stream
operation failed. A simple example showing this consists of an attempt to
extract an int when the input stream contains the text hello
world. The value not fail() is returned by the bool
interpretation of a stream object (see below).
    o bool ios::good():
            the value of the
goodbit flag is returned. It equals true when none of the other
condition flags (badbit, eofbit, failbit) was raised.
    Consider the following little program:
        #include <iostream>
#include <string>

using namespace std;

void state()
{
    cout << "\n"
            "Bad: " << cin.bad() << " "
            "Fail: " << cin.fail() << " "
            "Eof: " << cin.eof() << " "
            "Good: " << cin.good() << '\n';
}

int main()
{
    string line;
    int x;

    cin >> x;
    state();

    cin.clear();
    getline(cin, line);
    state();

    getline(cin, line);
    state();
}

When this program processes a file having two lines, containing,
respectively, hello and world, while the second
line is not terminated by a \n character the following is shown:
            Bad: 0 Fail: 1 Eof: 0 Good: 0

    Bad: 0 Fail: 0 Eof: 0 Good: 1

    Bad: 0 Fail: 0 Eof: 1 Good: 0

Thus, extracting x fails (good returning false). Then,
the error state is cleared, and the first line is successfully read
(good returning true). Finally the second line is read
(incompletely): good returning false, and eof returning true.

o  
         
        Interpreting streams as bool values:
            streams may be used in expressions expecting logical
values. Some examples are:
        if (cin)                // cin itself interpreted as bool
if (cin >> x)           // cin interpreted as bool after an extraction
if (getline(cin, str))  // getline returning cin

When interpreting a stream as a logical value, it is actually `not
fail()' that is interpreted. The above examples may therefore be rewritten
as:
        if (not cin.fail())
if (not (cin >> x).fail())
if (not getline(cin, str).fail())

The former incantation, however, is used almost exclusively.

The following members are available to manage error states:
    
        o void ios::clear():
    When an error condition has occurred, and the condition can be
repaired, then clear can be used to clear the error state of the file. An
overloaded version exists accepting state flags, that are set after first
clearing the current set of flags: clear(int state). Its return
type is void
        o ios::iostate ios::rdstate():
        
    The current set of flags that are set for an ios object are returned
(as an int). To test for a particular flag, use the bitwise and
operator:
        if (!(iosObject.rdstate() & ios::failbit))
{
    // last operation didn't fail
}

Note that this test cannot be performed for the goodbit flag as its
value equals zero. To test for `good' use a construction
like:
        if (iosObject.rdstate() == ios::goodbit)
{
    // state is `good'
}

o void ios::setstate(ios::iostate state):
        A stream may be assigned a certain set of states using
setstate. Its return type is void. E.g.,
        cin.setstate(ios::failbit);     // set state to `fail'

To set multiple flags in one setstate() call use the bitor
operator:
        cin.setstate(ios::failbit | ios::eofbit)

The member clear is a shortcut to clear all error
flags.  Of course, clearing the flags doesn't automatically mean the
error condition has been cleared too. The strategy should be:
        
            o  An error condition is detected,
            o  The error is repaired
            o  The member clear is called.

C++ supports an exception mechanism to handle exceptional
situations. According to the ANSI/ISO standard, exceptions can be used
with stream objects. Exceptions are covered in chapter 10. Using
exceptions with stream objects is covered in section 10.7.


6.3.2: Formatting output and input

    The way information is written to streams (or, occasionally, read from
streams) is controlled by formatting flags.

Formatting is used when it is necessary to, e.g., set the width of an output
field or input buffer and to determine the form (e.g., the radix) in
which values are displayed. Most formatting features belong to the realm of
the ios class. Formatting is controlled by flags, defined by the ios
class.  These flags may be manipulated in two ways: using specialized
member functions or using manipulators, which are directly inserted into
or extracted from streams. There is no special reason for using either
method; usually both methods are possible. In the following overview
the various member functions are first introduced. Following this the flags
and manipulators themselves are covered. Examples are provided showing how
the flags can be manipulated and what their effects are.

Many manipulators are parameterless and are available once a stream header
file (e.g., iostream) has been included. Some manipulators require
arguments. To use those latter manipulators the header file iomanip must
be included.


6.3.2.1: Format modifying member functions

        Several member functions are available manipulating the I/O formatting
flags. Instead of using the members listed below manipulators are often
available that may directly be inserted into or extracted from streams. The
available members are listed in alphabetical order, but the most important
ones in practice are setf, unsetf and
width.


    o ios &ios::copyfmt(ios &obj):
all format flags of obj are copied to the current ios
object. The current ios object is returned.
    o ios::fill() const:
the current padding character is returned. By default, this is the
blank space.
    o ios::fill(char padding):
        the padding character is redefined, the padding character that
was used before the redefinition is returned. Instead of using this member
function the setfill manipulator may be inserted directly into an
ostream. Example:
        cout.fill('0');         // use '0' as padding char
cout << setfill('+');   // use '+' as padding char

o ios::fmtflags ios::flags() const:
the current set of flags controlling the format state of the stream
for which the member function is called is returned. To inspect whether a
particular flag was set, use the bit_and operator. Example:
            if (cout.flags() & ios::hex)
    cout << "Integral values are printed as hex numbers\n"

o ios::fmtflags ios::flags(ios::fmtflags flagset):
       the previous set of flags are returned and the new set of
flags are defined by flagset. Multiple flags are specified using the
bitor operator. Example:
                // change the representation to hexadecimal
cout.flags(ios::hex | cout.flags() & ~ios::dec);

o int ios::precision() const:
the number of significant digits used when outputting floating point
values is returned (default: 6).
    o int ios::precision(int signif):
        the number of significant digits to use when outputting real
values is set to signif. The previously used number of significant digits
is returned. If the number of required digits exceeds signif then the
number is displayed in `scientific' notation (cf. section
6.3.2.2). Manipulator: setprecision. Example:
            cout.precision(3);          // 3 digits precision
cout << setprecision(3);    // same, using the manipulator

cout << 1.23 << " " << 12.3 << " " << 123.12 << " " << 1234.3 << '\n';
// displays: 1.23 12.3 123 1.23e+03

o  ios::fmtflags ios::setf(ios::fmtflags
flags): 
        sets one or more formatting flags (use the bitor
operator to combine multiple flags). Already set flags are not affected. The
previous set of flags is returned. Instead of using this member function
the manipulator setiosflags may be used. Examples are provided in the next
section (6.3.2.2).
    o ios::fmtflags ios::setf(ios::fmtflags flags, ios::fmtflags mask):
            clears all flags mentioned in mask and sets the flags
specified in flags. The previous set of flags is returned. Some
examples are (but see the next section (6.3.2.2) for a more thorough
discussion):
                    // left-adjust information in wide fields:
cout.setf(ios::left, ios::adjustfield);

        // display integral values as hexadecimal numbers:
cout.setf(ios::hex, ios::basefield);

        // display floating point values in scientific notation:
cout.setf(ios::scientific, ios::floatfield);

o  ios::fmtflags ios::unsetf(fmtflags flags):
        the specified formatting flags are cleared (leaving the
remaining flags unaltered) and returns the previous set of flags.  A
request to unset an active default flag (e.g., cout.unsetf(ios::dec))
is ignored. Instead of this member function the manipulator resetiosflags
may also be used. Example:
        cout << 12.24;              // displays  12.24
cout.setf(ios::fixed);
cout << 12.24;              // displays  12.240000
cout.unsetf(ios::fixed);    // undo a previous ios::fixed setting.
cout << 12.24;              // displays  12.24
cout << resetiosflags(ios::fixed);  // using manipulator rather
                                    // than unsetf

o 
        int ios::width() const:
        the currently active output field width to use on the next
insertion is returned. The default value is 0, meaning `as many characters as
needed to write the value'.
    o int ios::width(int nchars):
        the field width of the next insertion operation is set to
nchars, returning the previously used field width. This setting is not
persistent. It is reset to 0 after every insertion operation. Manipulator:
std::setw(int). Example:
        cout.width(5);
cout << 12;                     // using 5 chars field width
cout << setw(12) << "hello";    // using 12 chars field width


6.3.2.2: Formatting flags

        Most formatting flags are related to outputting information. Information
can be written to output streams in basically two ways: using
 binary output information is written directly to an output stream,
without converting it first to some human-readable format and using
 formatted output by which values stored in the computer's memory are
converted to human-readable text first. Formatting flags are used to define
the way this conversion takes place. In this section all formatting flags are
covered. Formatting flags may be (un)set using member functions, but often
manipulators having the same effect may also be used. For each of the flags it
is shown how they can be controlled by a member function or -if available- a
manipulator.

To display information in wide fields:


    o  ios::internal:
        to add fill characters (blanks by default) between the minus
sign of negative numbers and the value itself. Other values and data types are
right-adjusted. Manipulator: std::internal. Example:
        cout.setf(ios::internal, ios::adjustfield);
cout << internal;       // same, using the manipulator

cout << '\'' << setw(5) << -5 << "'\n";  // displays '-   5'

o ios::left:
to left-adjust values in fields that are wider than needed to display
the values.  Manipulator: std::left. Example:
        cout.setf(ios::left, ios::adjustfield);
cout << left;           // same, using the manipulator

cout << '\'' << setw(5) << "hi" << "'\n";  // displays 'hi   '

o ios::right:
to right-adjust values in fields that are wider than needed to
display the values.  Manipulator: std::right. This is the default.
Example:
        cout.setf(ios::right, ios::adjustfield);
cout << right;          // same, using the manipulator

cout << '\'' << setw(5) << "hi" << "'\n";  // displays '   hi'

Using various number representations:


    o ios::dec:
to display integral values as decimal numbers. Manipulator:
std::dec. This is the default. Example:
        cout.setf(ios::dec, ios::basefield);
cout << dec;            // same, using the manipulator
cout << 0x10;           // displays 16

o ios::hex:
to display integral values as hexadecimal numbers. Manipulator:
std::hex. Example:
        cout.setf(ios::hex, ios::basefield);
cout << hex;            // same, using the manipulator
cout << 16;             // displays 10

o ios::oct:
to display integral values as octal numbers. Manipulator:
std::oct. Example:
        cout.setf(ios::oct, ios::basefield);
cout << oct;            // same, using the manipulator
cout << 16;             // displays 20

o std::setbase(int radix):
This is a manipulator that can be used to change the number
representation to decimal, hexadecimal or octal. Example:
        cout << setbase(8);     // octal numbers, use 10 for
                        // decimal, 16 for hexadecimal
cout << 16;             // displays 20

Fine-tuning displaying values:


    o ios::boolalpha:
logical values may be displayed as text using the text `true' for
the true logical value, and `false' for the false logical value
using boolalpha.  By default this flag is not set. Complementary flag:
ios::noboolalpha. Manipulators: std::boolalpha and
std::noboolalpha.  Example:
        cout.setf(ios::boolalpha);
cout << boolalpha;      // same, using the manipulator
cout << (1 == 1);       // displays true

o ios::showbase:
to display the numeric base of integral values. With hexadecimal
values the 0x prefix is used, with octal values the prefix 0. For the
(default) decimal value no particular prefix is used. Complementary flag:
ios::noshowbase. Manipulators: std::showbase and
std::noshowbase. Example:
        cout.setf(ios::showbase);
cout << showbase;       // same, using the manipulator
cout << hex << 16;      // displays 0x10

o ios::showpos:
to display the + sign with positive decimal (only)
values. Complementary flag:
ios::noshowpos.  Manipulators:
std::showpos and std::noshowpos. Example:
        cout.setf(ios::showpos);
cout << showpos;            // same, using the manipulator
cout << 16;                 // displays +16
cout.unsetf(ios::showpos);  // Undo showpos
cout << 16;                 // displays 16

o ios::uppercase:
to display letters in hexadecimal values using capital
letters. Complementary flag: ios::nouppercase.  Manipulators:
std::uppercase and std::nouppercase.  By default lower
case letters are used. Example:
        cout.setf(ios::uppercase);
cout << uppercase;            // same, using the manipulator
cout << hex << showbase <<
        3735928559;           // displays 0XDEADBEEF

Displaying floating point numbers


    o 
        ios::fixed:
        to display real values using a fixed decimal point (e.g., 12.25
rather than 1.225e+01), the fixed formatting flag is used. It can be used
to set a fixed number of digits behind the decimal point. Manipulator:
fixed. Example:
            cout.setf(ios::fixed, ios::floatfield);
cout.precision(3);          // 3 digits behind the .

    // Alternatively:
cout << setiosflags(ios::fixed) << setprecision(3);

cout << 3.0 << " " << 3.01 << " " << 3.001 << '\n';
     << 3.0004 << " " << 3.0005 << " " << 3.0006 << '\n'
    // Results in:
    // 3.000 3.010 3.001
    // 3.000 3.001 3.001

The example shows that 3.0005 is rounded away from zero, becoming
3.001 (likewise -3.0005 becomes -3.001). First setting precision and then
fixed has the same effect.

o ios::scientific:
to display real values in scientific notation (e.g., 1.24e+03).
Manipulator: std::scientific.  Example:
        cout.setf(ios::scientific, ios::floatfield);
cout << scientific;         // same, using the manipulator
cout << 12.25;              // displays 1.22500e+01

o ios::showpoint:
to display a trailing decimal point and trailing decimal zeros
when real numbers are displayed. Complementary flag:
ios::noshowpoint. Manipulators: std::showpoint,
 std::noshowpoint. Example:
        cout << fixed << setprecision(3);   // 3 digits behind .

cout.setf(ios::showpoint);      // set the flag
cout << showpoint;              // same, using the manipulator

cout << 16.0 << ", " << 16.1 << ", " << 16;
// displays: 16.000, 16.100, 16

Note that the final 16 is an integral rather than a floating point number,
so it has no decimal point. So showpoint has no effect.
    If ios::showpoint is not active trailing zeros are discarded. If
the fraction is zero the decimal point is discarded as well. Example:
        cout.unsetf(ios::fixed, ios::showpoint);    // unset the flags

cout << 16.0 << ", " << 16.1;
// displays: 16, 16.1

Handling whitespace and flushing streams


    o std::endl:
manipulator inserting a newline character and flushing the
stream. Often flushing the stream is not required and doing so would
needlessly slow down I/O processing. Consequently, using endl should be
avoided (in favor of inserting '\n') unless flushing the stream is
explicitly intended. Note that streams are automatically flushed when the
program terminates or when a stream is `tied' to another stream (cf. tie
in section 6.3). Example:
        cout << "hello" << endl;    // prefer: << '\n';

o std::ends:
manipulator inserting a 0-byte into a stream. It is usually used in
combination with memory-streams (cf. section 6.4.3).

o std::flush:
a stream may be flushed using this member. Often flushing the stream
is not required and doing so would needlessly slow down I/O
processing. Consequently, using flush should be avoided unless it is
explicitly required to do so. Note that streams are automatically flushed when
the program terminates or when a stream is `tied' to another stream
(cf. tie in section 6.3). Example:
        cout << "hello" << flush;    // avoid if possible.

o ios::skipws:
leading whitespace characters (blanks, tabs, newlines, etc.) are
skipped when a value is extracted from a stream. This is the default.  If the
flag is not set, leading whitespace characters are not skipped.
Manipulator: std::skipws. Example:
        cin.setf(ios::skipws);  // to unset, use
                        //  cin.unsetf(ios::skipws)

cin >> skipws;          // same, using the manipulator
int value;
cin >> value;           // skips initial blanks

o ios::unitbuf:
the stream for which this flag is set flushes its buffer after
every output operation Often flushing a stream is not required and doing so
would needlessly slow down I/O processing. Consequently, setting unitbuf
should be avoided unless flushing the stream is explicitly intended. Note that
streams are automatically flushed when the program terminates or when a stream
is `tied' to another stream (cf. tie in section 6.3).  Complementary
flag: ios::nounitbuf. Manipulators: std::unitbuf,
std::nounitbuf. Example:
        cout.setf(ios::unitbuf);
cout << unitbuf;            // same, using the manipulator

cout.write("xyz", 3);       // flush follows write.

o std::ws:
manipulator removing all whitespace characters (blanks, tabs,
newlines, etc.) at the current file position. White space characters are
removed if present even if the flag ios::noskipws has been set.  Example
(assume the input contains 4 blank characters followed by the character
X):
        cin >> ws;      // skip whitespace
cin.get();      // returns 'X'


6.3.2.3: The std::quoted manipulator

        The std::quoted manipulator can be used to extract or insert
quoted strings from, c.q. into, streams. The manipulator can, e.g., be used to
read the text in quoted strings from a comma-separated data file or to
surround text by quotes when writing such files.

The quoted manipulator defines three parameters, the second and third
parameters have default values.

When inserting a quoted manipulator into an std::ostream its first
parameter is a char const *, a std::string const &, or a
std::string_view object. It inserts its first argument, surrounded by
quotes, into the std::ostream.

When extracting a quoted manipulator from an std::istream its first
parameter is an std::string &. It extracts a quoted string from the
std::istream and stores the extracted text without the surrounding quotes
into its first argument.

The second and third arguments specify, respectively, the delimiter that is
used surrounding the quoted string (by default a double quote (")), and
an escape character that can be prefixed to characters inside the quoted
string: the escape character will be ignored and the next character will be
used as-is.

The generic form of the quoted manipulator is:
    quoted(source-or-destination, char delim = '"', char escape = '\\')
Within the quoted text blanks are not skipped.

Example, executing a.out < input:
        int main()
    {
        cout << quoted("hello \"world\"") << '\n';
    
        string str = "hello 'world'";
        cout << quoted(str, '\'', '%') << '\n';
    
        cin >> quoted(str);         // 1
        cin.ignore(100, '\n');
        cout << "1: " << str << '\n';
    
        cin >> quoted(str, '\'');   // 2
        cin.ignore(100, '\n');
        cout << "2: " << str << '\n';
    }
    
    // input
    //    "hello \"world\""
    //    'hello \'world\''
    
    // output:
    // "hello \"world\""
    // 'hello %'world%''
    // 1: hello "world"
    // 2: hello 'world'



6.4: Output

In C++ output is primarily based on the std::ostream
class. The ostream class defines the basic operators and members inserting
information into streams: the insertion operator (<<), and special
members like write writing unformatted information to streams.

The class ostream acts as base class for several other classes, all
offering the functionality of the ostream class, but adding their own
specialties. In the upcoming sections the following classes are discussed:
    
    o  The class ostream, offering the basic output facilities;
    o  The class ofstream, allowing us to write files
(comparable to C's  fopen(filename, "w"));
    o  The class ostringstream, allowing us to write information to
memory (comparable to C's sprintf function).


6.4.1: Basic output: the class `ostream'

    The class ostream defines basic output facilities. The cout, clog
and cerr objects are all ostream objects. All facilities related to
output as defined by the ios class are also available in the ostream
class.

We may define ostream objects using the following
 ostream constructor:
    
    o std::ostream object(std::streambuf *sb):
        this constructor creates an ostream object which is a
wrapper around an existing std::streambuf object.  It isn't possible to
define a plain ostream object (e.g., using std::ostream out;) that
can thereupon be used for insertions. When cout or its friends are used,
we are actually using a predefined ostream object that has already been
defined for us and interfaces to the standard output stream using a
(also predefined) streambuf object handling the actual interfacing.

It is, however, possible to define an ostream object passing it
  a 0-pointer. Such an object cannot be
used for insertions (i.e., it raises its ios::bad flag when something
is inserted into it), but it may be given a streambuf later. Thus it may
be preliminary constructed, suspending its use until an appropriate
streambuf becomes available (see also section 14.8).

To define the ostream class in C++ sources, the
    <ostream> header file must be included. To use the predefined
ostream objects (std::cerr, std::cout etc.) the <iostream> header
file must be included.


6.4.1.1: Writing to `ostream' objects

        The class ostream supports both formatted and binary output.

The insertion operator (<<) is used to insert values in
a type safe way into ostream objects.  This is called
    formatted output, as binary values which are stored in the computer's
memory are converted to human-readable ASCII characters according to
certain formatting rules.

The insertion operator points to the ostream object to
receive the information.  The normal associativity of <<
remains unaltered, so when a statement like
            cout << "hello " << "world";

is encountered, the leftmost two operands are evaluated first (cout
<< "hello "), and an ostream & object, which is actually the
same cout object, is returned. Now, the statement is reduced to
            cout << "world";

and the second string is inserted into cout.

The << operator has a lot of (overloaded) variants, so many types of
variables can be inserted into ostream objects. There is an overloaded
<<-operator expecting an int, a double, a pointer, etc. etc..
Each operator returns the ostream object into which the information so far
has been inserted, and can thus immediately be followed by the next insertion.

In addition to insertion operators ostreams also support
std::print. It is called as
      std::ostream& os, std::format_string<Args...> fmt, Args&&... args
    where ... indicates that any number of arguments can be specified.
However, since a print-like functionality is hardly ever required in
C++ programs it's not covered in detail in the C++ Annotations. Refer to
cppreference.com for details about std::print.

When  binary files must be written, normally no
text-formatting is used or required: an int value should be written as a
series of raw bytes, not as a series of ASCII numeric characters 0 to
9. The following member functions of ostream objects may be used to
write `binary files':
    
    o ostream& put(char c):
to write a single character to the output stream. Since a character
is a byte, this member function could also be used for writing a single
character to a text-file.
     o ostream& write(char const *buffer, int length):
to write at most length bytes, stored in the char const
*buffer to the ostream object. Bytes are written as they are stored
in the buffer, no formatting is done whatsoever. Note that the first argument
is a char const *: a type cast is required to write any other
type. For example, to write an int as an unformatted series of
byte-values use:
        int x;
out.write(reinterpret_cast<char const *>(&x), sizeof(int));

The bytes written by the above write call are written to the
ostream in an order depending on the endian-ness of the
underlying hardware. Big-endian computers write the most significant byte(s)
of multi-byte values first, little-endian computers first write the least
significant byte(s).


6.4.1.2: `ostream' positioning

        Although not every ostream object supports repositioning, they usually
do. This means that it is possible to rewrite a section of the stream which
was written earlier. Repositioning is frequently used in database applications
where it must be possible to access the information in the database at random.

The current position can be obtained and modified using the following members:
    
    o ios::pos_type tellp():
the current (absolute) position in the file where the next
write-operation to the stream will take place is returned.
    o ostream &seekp(ios::off_type step, ios::seekdir org):
modifies a stream's actual position. The function expects an
off_type step representing the number of bytes the current stream
position is moved with respect to org. The step value
may be negative, zero or positive.

The origin of the step, org is a value in the
ios::seekdir enumeration. Its values are:
        
        o ios::beg:
the stepsize is computed relative to the beginning of the
stream. This value is used by default.

o ios::cur:
the stepsize is computed relative to the current position of the
stream (as returned by tellp).

o ios::end:
the stepsize is interpreted relative to the current end position
of the stream.

It is OK to
 
    seek or write beyond the last file position.  Writing bytes to a location
beyond EOF will pad the intermediate bytes with 0-valued bytes:
null-bytes.
    Seeking before ios::beg raises the ios::fail flag.

Different from seekg (cf. section 6.5.1.2) seekp does not
clear the stream's ios::eofbit. To reset an ostream's state to `good',
its clear member should be called.


6.4.1.3: `ostream' flushing

        Unless the ios::unitbuf flag has been set, information written to an
ostream object is not immediately written to the physical stream. Rather,
an internal buffer is filled during the write-operations, and when full it
is flushed.

The stream's internal buffer can be flushed under program control:
    
    o ostream& flush():
any buffered information stored internally by the ostream object
is flushed to the device to which the ostream object interfaces. A stream
is flushed automatically when:
        
        o  the object ceases to exist;
        o  the endl or flush manipulators (see section
            6.3.2.2) are inserted into an ostream object;
        o  a stream supporting the close-operation is explicitly closed
(e.g., a std::ofstream object, cf. section 6.4.2).


6.4.2: Output to files: the class `ofstream'

    The std::ofstream class is derived from the ostream class:
it has the same capabilities as the ostream class, but can be used to
access files or create files for writing.

In order to use the ofstream class  in C++ sources, the
    <fstream> header file must be included. Including fstream does not
automatically make available the standard streams cin, cout and
cerr. Include iostream to declare these standard streams.

The following  constructors are available for
ofstream objects:
    
    o ofstream object:
        this is the basic constructor. It defines an ofstream object
which may be associated with an actual file later, using its open() member
(see below).

o ofstream object(char const *name, ios::openmode mode = ios::out):
        this constructor defines an ofstream object and associates
it immediately with the file named name using output mode
mode. Section 6.4.2.1 provides an overview of available output
modes.  Example:
        ofstream out("/tmp/scratch");

It is not possible to open an ofstream using a
 file descriptor. The reason for this is (apparently) that file
descriptors are not universally available over different operating systems.
Fortunately, file descriptors can be used (indirectly) with a
std::streambuf object (and in some implementations: with a
std::filebuf object, which is also a streambuf). Streambuf objects
are discussed in section 14.8, filebuf objects are discussed in
section 14.8.3.

Instead of directly associating an ofstream object with a file, the
object can be constructed first, and opened later.
    
        o void open(char const *name,
                            ios::openmode mode = ios::out):
associates an ofstream object with an actual file. If the
ios::fail flag was set before calling open and opening succeeds the
flag is cleared. Opening an already open stream fails. To reassociate a stream
with another file it must first be closed:
        ofstream out("/tmp/out");
out << "hello\n";
out.close();        // flushes and closes out
out.open("/tmp/out2");
out << "world\n";

o void close():
closes the ofstream object. The function sets the ios::fail
flag of the closed object. Closing the file flushes any buffered information
to the associated file. A file is automatically closed when the associated
ofstream object ceases to exist.

o bool is_open() const:
assume a stream was properly constructed, but it has not yet been
attached to a file. E.g., the statement ofstream ostr was executed. When
we now check its status through good(), a non-zero (i.e., OK) value is
returned.  The `good' status here indicates that the stream object has been
constructed properly. It doesn't mean the file is also open. To test whether a
stream is actually open, is_open should be called. If it returns true,
the stream is open. Example:
            #include <fstream>
    #include <iostream>

    using namespace std;

    int main()
    {
        ofstream of;

        cout << "of's open state: " << boolalpha << of.is_open() << '\n';

        of.open("/dev/null");       // on Unix systems

        cout << "of's open state: " << of.is_open() << '\n';
    }
    /*
        Generated output:
    of's open state: false
    of's open state: true
    */



6.4.2.1: Modes for opening stream objects

        The following file modes or file flags are available when constructing
or opening ofstream (or istream, see section 6.5.2)
objects. The values are of type ios::openmode. Flags may be
combined using the bitor operator.
        
        o ios::app:
reposition the stream to its end before every output command (see
also ios::ate below). The
file is created if it doesn't yet exist. When opening a stream in this mode
any existing content of the file is kept.
        o ios::ate:
start initially at the end of the file.  Note that any existing
content is only kept if some other flag tells the object to do so. For
example ofstream out("gone", ios::ate) rewrites the file gone,
because the implied ios::out causes the rewriting. If rewriting of an
existing file should be prevented, the ios::in
        
    mode should be specified too. However, when ios::in is specified the
file must already exist. The ate mode only initially positions the file at
the end of file position. After that information may be written in the middle
of the file using seekp. When the app mode is used information is
only written at end of file (effectively ignoring seekp operations).
        o ios::binary:
open a file in binary mode (used on systems
distinguishing text- and binary files, like MS-Windows).

o ios::in:
open the file for reading. The file must exist.
        o ios::out:
open the file for writing. Create it if it doesn't yet exist. If
it exists, the file is rewritten.
        o ios::trunc:
start initially with an empty file.  Any existing content of the
file is lost.

The following combinations of file flags have special meanings:
        in | out:           The stream may be read and written. However, the
                    file must exist.
in | out | trunc:   The stream may be read and written. It is
                    (re)created empty first.

An interesting subtlety is that the open members of the ifstream,
ofstream and fstream classes have a second parameter of type
ios::openmode. In contrast to this, the bitor operator returns an
int when applied to two enum-values. The question why the bitor
operator may nevertheless be used here is answered in a later chapter
(cf. section 11.13).


6.4.3: Output to memory: the class `ostringstream'

    To  write information to memory using stream
facilities, std::ostringstream objects should be used.
As the class ostringstream is derived from the class ostream all
ostream's facilities are available to ostringstream objects as
well. To use and define ostringstream objects the header file
<sstream> must be included. In addition the class ostringstream offers
the following constructors and members:
    
        o ostringstream ostr(string const &init,
                ios::openmode mode = ios::out):
when specifying openmode as ios::ate, the
ostringstream object is initialized by the string init and remaining
insertions are appended to the content of the ostringstream object.

o ostringstream ostr(ios::openmode mode = ios::out):

            this constructor can also be used as default
constructor. Alternatively it allows, e.g., forced additions at the end of the
information stored in the object so far (using ios::app). Example:
        std::ostringstream out;
        o std::string str() const:
a copy of the string that is stored inside the ostringstream
            object is returned.
        o void str(std::string const &str):

            the current object is reinitialized with new initial content.

The following example illustrates the use of the ostringstream class:
several values are inserted into the object. Then, the text contained by the
ostringstream object is stored in a std::string, whose length and
content are thereupon printed.  Such ostringstream objects are most often
used for doing `type to string' conversions,
like converting int values to text. Formatting flags can be used with
ostringstreams as well, as they are part of the ostream class.

Here is an example showing an ostringstream object being used:
        #include <iostream>
    #include <sstream>

    using namespace std;

    int main()
    {
        ostringstream ostr("hello ", ios::ate);

        cout << ostr.str() << '\n';

        ostr.setf(ios::showbase);
        ostr.setf(ios::hex, ios::basefield);
        ostr << 12345;

        cout << ostr.str() << '\n';

        ostr << " -- ";
        ostr.unsetf(ios::hex);
        ostr << 12;

        cout << ostr.str() << '\n';

        ostr.str("new text");
        cout << ostr.str() << '\n';

        ostr.seekp(4, ios::beg);
        ostr << "world";
        cout << ostr.str() << '\n';
    }
    /*
        Output from this program:
    hello
    hello 0x3039
    hello 0x3039 -- 12
    new text
    new world
    */



6.4.4: The `put_time' manipulator

    The manipulator  std::put_time(std::tm const *specs, char const
*fmt) can be used to insert time specifications into std::ostream
objects. Before using put_time the <iomanip> header must be included.

Time specifications are provided in std::tm objects, and the way the time
should be displayed is defined by the format string fmt.

Starting with a chrono::time_point the following steps must be performed
to insert the time point's time into a std::ostream:
    
    o  Obtain a time_point (e.g.: system_clock{}.now());

o  Pass the time point to the clock's to_time_t function, saving the
returned time_t value:
        time_t secs = system_clock::to_time_t( system_clock{}.now() );

o  Pass sec's address to either std::localtime or
std::gmtime. These functions return std::tm structs
containing the required time components expressed in, respectively, the
computer's local time or GMT;

o  Pass the return value of either localtime or gmtime together
with a format string (e.g., "%c") to put_time, inserting it into an
std::ostream:
                        // displays, e.g., Mon Nov  4 21:34:59 2019
time_t secs = system_clock::to_time_t( system_clock{}.now() );
std::cout << std::put_time(std::localtime(&secs), "%c") << '\n';

A simple function returning put_time's return value and expecting a
time_point and format string can be defined which handles the above two
statements. E.g., (omitting the std:: and std::chrono:: specifications
for brevity):
            auto localTime(time_point<system_clock> const &tp, char const *fmt)
    {
        time_t secs = system_clock::to_time_t( tp );
        return put_time(localtime(&secs), fmt);
    }
                // used as:
    cout << localTime(system_clock{}.now(), "%c") << '\n';

Many more format specifiers are recognized by put_time.  Specifiers start
with %. To display a percent character as part of the format string write
it twice: %%. In addition to the standard escape sequences, %n can be
used instead of \n, and %t can be used instead of \t.



        ------------------------------------------------------------
         5 


             Specifier    Meaning   std::tm field(s)    

            ------------------------------------------------------------
        %Y         year as a 4 digit decimal number         tm_year    

            %EY         year in an alternative representation         tm_year    

            %y         last 2 digits of year as a decimal number
                (range [00,99])         tm_year    

            %Oy         last 2 digits of year using an alternative numeric
                system         tm_year    

            %Ey         year as offset from locale's alternative calendar
                period %EC (locale-dependent)         tm_year    

            %C         first 2 digits of year as a decimal number
                (range [00,99])         tm_year    

            %EC         name of the base year (period) in the locale's
                alternative representation         tm_year    

            %G         ISO 8601 week-based year, i.e. the year that contains
                the specified week         tm_year,
 tm_wday,
 tm_yday    

            %g         last 2 digits of ISO 8601 week-based year
                (range [00,99])         tm_year,
 tm_wday,
 tm_yday    

            ------------------------------------------------------------






        ------------------------------------------------------------
         5 


             Specifier    Meaning   std::tm field(s)    

            ------------------------------------------------------------
        %b         abbreviated month name, e.g. Oct         tm_mon    
        %m         month as a decimal number (range [01,12])         tm_mon    
        %Om         month using an alternative numeric system         tm_mon    
        ------------------------------------------------------------






        ------------------------------------------------------------
         5 


             Specifier    Meaning   std::tm field(s)    

            ------------------------------------------------------------
        %U         week of the year as a decimal number (Sunday is the first day
            of the week) (range [00,53])         tm_year,
tm_wday,
tm_yday    

            %OU         week of the year, as by %U, using an alternative numeric
            system         tm_year,
 tm_wday,
 tm_yday    

            %W         week of the year as a decimal number (Monday is the first day
            of the week) (range [00,53])         tm_year,
 tm_wday,
 tm_yday    

            %OW         week of the year, as by %W, using an alternative numeric
            system         tm_year,
 tm_wday,
 tm_yday    

            %V         ISO 8601 week of the year (range [01,53])         tm_year,
 tm_wday,
 tm_yday    

            %OV         week of the year, as by %V, using an alternative numeric
            system         tm_year,
 tm_wday,
 tm_yday    

            ------------------------------------------------------------






        ------------------------------------------------------------
         5 


             Specifier    Meaning   std::tm field(s)    

            ------------------------------------------------------------
        %j         day of the year as a decimal number (range
                [001,366])         tm_yday    

            %d         day of the month as a decimal number (range [01,31])         tm_mday    

            %Od         zero-based day of the month using an alternative numeric
                system         tm_mday    

            %e         day of the month as a decimal number (range [1,31])         tm_mday    

            %Oe         one-based day of the month using an alternative numeric
            system         tm_mday    

            ------------------------------------------------------------






        ------------------------------------------------------------
         5 


             Specifier    Meaning   std::tm field(s)    

            ------------------------------------------------------------
        %a         abbreviated weekday name, e.g. Fri         tm_wday    

            %A         full weekday name, e.g. Friday         tm_wday    

            %w         weekday as a decimal number, where Sunday is 0 (range
                [0-6])         tm_wday    

            %Ow         weekday, where Sunday is 0, using an alternative numeric
                system         tm_wday    

            %u         weekday as a decimal number, where Monday is 1 (ISO 8601
                format) (range [1-7])         tm_wday    

            %Ou         weekday, where Monday is 1, using an alternative numeric
                system         tm_wday    

            ------------------------------------------------------------






        ------------------------------------------------------------
         5 

         Specifier    Meaning   std::tm field(s)    
        ------------------------------------------------------------
        %H         hour as a decimal number, 24 hour clock (range
                [00-23])         tm_hour    
        %OH         hour from 24-hour clock using an alternative numeric
                system         tm_hour    
        %I         hour as a decimal number, 12 hour clock (range
                [01,12])         tm_hour    
        %OI         hour from 12-hour clock using the alternative numeric
                system         tm_hour    
        %M         minute as a decimal number (range [00,59])         tm_min    
        %OM         minute using an alternative numeric system         tm_min    
        %S         second as a decimal number (range [00,60])         tm_sec    
        %OS         second using an alternative numeric system         tm_sec    
        ------------------------------------------------------------






        ------------------------------------------------------------
         5 

         Specifier    Meaning   std::tm field(s)    
        ------------------------------------------------------------
        %c         standard date and time string, e.g. Sun Oct 17 04:41:13
                2010         all    
        %Ec         alternative date and time string         all    
        %x         localized date representation         all    
        %Ex         alternative date representation         all    
        %X         localized time representation         all    
        %EX         alternative time representation         all    
        %D         equivalent to "%m/%d/%y"         tm_mon,
 tm_mday,

                tm_year    
        %F         equivalent to "%Y-%m-%d" (the ISO 8601 date
                format)         tm_mon,
 tm_mday,
 tm_year    
        %r         localized 12-hour clock time         tm_hour,
 tm_min,

                tm_sec    
        %R         equivalent to "%H:%M"         tm_hour,
 tm_min    

            %T         equivalent to "%H:%M:%S" (the ISO 8601 time
                format)         tm_hour,
 tm_min,
 tm_sec    
        %p         localized a.m. or p.m.         tm_hour    
        %z         offset from UTC in the ISO 8601 format (e.g. -0430;
 no
                characters if time zone information is not
                available)         tm_isdst    
        %Z         time zone name or abbreviation
(no characters if time zone
                information is not available)         tm_isdst    
        ------------------------------------------------------------





6.5: Input

In C++ input is primarily based on the std::istream
class. The istream class defines the basic operators and members
extracting information from streams: the extraction operator (>>),
and special members like istream::read reading unformatted
information from streams.

The class istream acts as base class for several other classes, all
offering the functionality of the istream class, but adding their own
specialties. In the upcoming sections the following classes are discussed:
    
    o  The class istream, offering the basic facilities for doing input;
    o  The class ifstream, allowing us to read files
(comparable to C's  fopen(filename, "r"));
    o  The class istringstream, allowing us to extract information from
memory rather than from file (comparable to C's sscanf function).


6.5.1: Basic input: the class `istream'

    The class istream defines basic input facilities. The cin object, is
an istream object.  All facilities related to input as defined by the
ios class are also available in the istream class.

We may define istream objects using the following
 istream constructor:
    
        o istream object(streambuf *sb):
            this constructor can be used to construct a wrapper around
an existing std::streambuf object. Similarly to ostream objects,
istream objects may be defined by passing it initially
  a 0-pointer. See section 6.4.1 for
a discussion, see also section 14.8, and see chapter 26
for examples.

To define the istream class in C++ sources, the
    <istream> header file must be included. To use the predefined
istream object cin, the <iostream> header file must be included.


6.5.1.1: Reading from `istream' objects

        The class istream supports both formatted and unformatted (binary)
     input.  The extraction operator (operator>>) is
used to extract values in a type safe way from istream objects.  This
is called formatted input, whereby human-readable ASCII characters are
converted, according to certain formatting rules, to binary values.

The extraction operator points to the objects or variables to receive new
values.  The normal associativity of >> remains unaltered, so when a
statement like
            cin >> x >> y;

is encountered, the leftmost two operands are evaluated first (cin
>> x), and an istream & object, which is actually the same
cin object, is returned. Now, the statement is reduced to
            cin >> y

and the y variable is extracted from cin.

The >> operator has many (overloaded) variants and thus many types of
variables can be extracted from istream objects. There is an overloaded
>> available for the extraction of an int, of a double, of a
string, of an array of characters, possibly to the location pointed at by a
pointer, etc., etc.. String or character array extraction
     
    by default first skips all whitespace characters, and then extracts
all consecutive non-whitespace characters. Once an extraction operator has
been processed the istream object from which the information was extracted
is returned and it can immediately be used for additional istream
operations that appear in the same expression.

Streams do not support facilities for formatted input as offered by
C's scanf and vscanf functions. Although it is not difficult to
add such facilities to the world of streams, scanf-like functionality is
in practice never needed in C++ programs. Furthermore, as it is
potentially type-unsafe, it is better to avoid using C-type formatted
input.

When  binary files must be read, the information should
normally not be formatted: an int value should be read as a series of
unaltered bytes, not as a series of ASCII numeric characters 0 to 9. The
following member functions for reading information from istream objects
are available:
    
        o int gcount() const:
the number of characters read from the input stream
by the last unformatted input operation is returned.
        o int get():
the next available single character is returned
as an unsigned char value using an int return type.
EOF is returned if no more character are available.
        o istream &get(char &ch):
the next single character read from the input stream is
stored in ch. The member function returns the stream itself which may be
inspected to determine whether a character was obtained or not.
        o istream &get(char *buffer, int len, char delim = '\n'):
At most len - 1 characters are read from the input
stream into the array starting at buffer, which should be at least len
bytes long. Reading also stops when the delimiter delim is
encountered. However, the delimiter itself is not removed from the input
stream.

Having stored the characters into buffer, a 0-valued character is
written beyond the last character stored into the buffer.  The functions
eof and fail (see section 6.3.1) return 0 (false) if the
delimiter was encountered before reading len - 1 characters or if the
delimiter was not encountered after reading len - 1 characters.  It is OK
to specifiy a 0-valued character delimiter: this way NTBSs may be read from a
(binary) file.

o istream &getline(char *buffer, int len, char delim = '\n'):
this member function operates analogously to the get member
function, but getline removes delim from the stream if it is actually
encountered. The delimiter itself, if encountered, is not stored in the
buffer. If delim was not found (before reading len - 1
characters) the fail member function, and possibly also eof returns
true. Realize that the std::string class also offers a function
std::getline which is generally preferred over this getline member
function that is described here (see section 5.2.4).

o istream &ignore():
one character is skipped from the input stream.
        o istream &ignore(int n):
n characters are skipped from the input stream.
        o istream &ignore(int n, int delim):
at most n characters are skipped but skipping characters
stops after having removed delim from the input stream.
        o int peek():
this function returns the next available input character,
but does not actually remove the character from the input stream. EOF
is returned if no more characters are available.
        o istream &putback(char ch):
The character ch is `pushed back' into the input stream, to
be read again as the next available character. EOF is returned if this
is not allowed. Normally, it is OK to put back one character. Example:
            string value;
cin >> value;
cin.putback('X');
                // displays: X
cout << static_cast<char>(cin.get());

o istream &read(char *buffer, int len):
At most len bytes are read from the input stream into the
buffer. If EOF is encountered first, fewer bytes are read, with the
member function eof returning true. This function is commonly used
when reading binary files. Section 6.5.2 contains an example in
which this member function is used. The member function gcount() may be
used to determine the number of characters that were retrieved by read.

o istream &readsome(char *buffer, int len):
at most len bytes are read from the input stream into the
buffer. All available characters are read into the buffer, but if EOF
is encountered, fewer bytes are read, without setting the ios::eofbit
or ios::failbit.

o istream &unget():
the last character that was read from the stream is put back.


6.5.1.2: `istream' positioning

        Although not every istream object supports repositioning, some do. This
means that it is possible to read the same section of a stream
repeatedly. Repositioning is frequently used in database applications
where it must be possible to access the information in the database randomly.

The current position can be obtained and modified using the following members:
    
    o ios::pos_type tellg():
the stream's current (absolute) position where the
stream's next read-operation will take place is returned.
    o istream &seekg(ios::off_type step, ios::seekdir org):
modifies a stream's actual position. The function expects an
off_type step representing the number of bytes the current stream
position is moved with respect to org. The step value may be negative,
zero or positive.

The origin of the step, org is a value in the
ios::seekdir enumeration. Its values are:
        
        o ios::beg:
the stepsize is computed relative to the beginning of the
stream. This value is used by default.

o ios::cur:
the stepsize is computed relative to the current position of the
stream (as returned by tellp).

o ios::end:
the stepsize is interpreted relative to the current end position
of the stream.

It is OK to  seek beyond the last file
position.  Seeking before ios::beg raises the ios::failbit
flag.

Calling seekg clears the istream's ios::failbit, but not its
ios::badbit or ios::badbit. To ensure that the stream's state is reset
to `good' its member clear should be called.

To illustrate: in the following example cin's ios::eofbit is
set. Following seekg that bit is cleared, but its ios::goodbit stil
isn't set. Since its goodbit isn't set, extraction fails following
seekg:
            int main()                  // in 'src.cc'
    {
        cin.setstate(ios::eofbit | ios::failbit);
        cerr << cin.good() << ' ' << cin.eof() << '\n';
    
        cin.seekg(0);
        cerr << cin.good() << ' ' << cin.eof() << '\n' <<
                (cin.get() == EOF ? "failed" : "OK") << '\n';
    
        cin.clear();
        cerr << cin.good() << ' ' << cin.eof() << '\n' <<
                (cin.get() == EOF ? "failed" : "OK") << '\n';
    }
    outputs when called as 'a.out < src.cc':
        0 1
        0 0
        failed
        1 0
        OK


6.5.2: Input from files: the class `ifstream'

    The std::ifstream class is derived from the istream class:
it has the same capabilities as the istream class, but can be used to
access files  for reading.

In order to use the ifstream class  in C++ sources, the
    <fstream> header file must be included. Including fstream does not
automatically make available the standard streams cin, cout and
cerr. Include iostream to declare these standard streams.

The following  constructors are available for
ifstream objects:
    
    o ifstream object:
        this is the basic constructor. It defines an ifstream object
which may be associated with an actual file later, using its open() member
(see below).

o ifstream object(char const *name, ios::openmode mode = ios::in):
        this constructor can be used to define an ifstream object
and associate it immediately with the file named name using input mode
mode. Section 6.4.2.1 provides an overview of available input
modes.  Example:
        ifstream in("/tmp/input");

Instead of directly associating an ifstream object with a file, the
object can be constructed first, and opened later.
    
        o void open(char const *name,
                            ios::openmode mode = ios::in):
associates an ifstream object with an actual file. If the
ios::fail flag was set before calling open and opening succeeds the
flag is cleared. Opening an already open stream fails. To reassociate a stream
with another file it must first be closed:
        ifstream in("/tmp/in");
in >> variable;
in.close();        // closes in
in.open("/tmp/in2");
in >> anotherVariable;

o void close():
closes the ifstream object. The function sets the ios::fail
flag of the closed object. Closing the file flushes any buffered information
to the associated file. A file is automatically closed when the associated
ifstream object ceases to exist.

o bool is_open() const:
assume a stream was properly constructed, but it has not yet been
attached to a file. E.g., the statement ifstream ostr was executed. When
we now check its status through good(), a non-zero (i.e., OK) value is
returned.  The `good' status here indicates that the stream object has been
constructed properly. It doesn't mean the file is also open. To test whether a
stream is actually open, is_open should be called. If it returns true,
the stream is open.  Also see the example in section 6.4.2. The
following example  illustrates reading from a binary file (see also section
6.5.1.1):
            #include <fstream>
    using namespace std;

    int main(int argc, char **argv)
    {
        ifstream in(argv[1]);
        double   value;

        // reads double in raw, binary form from file.
        in.read(reinterpret_cast<char *>(&value), sizeof(double));
    }



6.5.3: Input from memory: the class `istringstream'

    To  read information from memory using
stream facilities, std::istringstream objects should
be used.  As the class istringstream is derived from the class istream
all istream's facilities are available to istringstream objects as
well. To use and define istringstream objects the header file
<sstream> must be included. In addition the class istringstream offers
the following constructors and members:
    
        o istringstream istr(string const &init,
                ios::openmode mode = ios::in):
the object is initialized with init's content
        o istringstream istr(ios::openmode mode = ios::in):
this constructor is usually used as the default
            constructor. Example:
        std::istringstream in;

o void str(std::string const &str):
the current object is reinitialized with new initial content.

The following example illustrates the use of the istringstream class:
several values are extracted from the object. Such istringstream objects
are most often used for doing `string to type'
conversions, like converting text to int values (cf. C's atoi
function). Formatting flags can be used with istringstreams as well, as
they are part of the istream class.  In the example note especially the
use of the member seekg:
        #include <iostream>
    #include <sstream>
    using namespace std;

    int main()
    {
        istringstream istr("123 345");  // store some text.
        int x;

        istr.seekg(2);              // skip "12"
        istr >> x;                  // extract int
        cout << x << '\n';          // write it out
        istr.seekg(0);              // retry from the beginning
        istr >> x;                  // extract int
        cout << x << '\n';          // write it out
        istr.str("666");            // store another text
        istr >> x;                  // extract it
        cout << x << '\n';          // write it out
    }
    /*
        output of this program:
    3
    123
    666
    */



6.5.4: Copying streams

    Usually, files are copied  either by reading a source file
character by character or line by line. The basic mold to process streams
  is as follows:
        
            o  Continuous loop:
            
                1.  read from the stream
                2.  if reading did not succeed (i.e., fail returns
                        true), break from the loop
                3.  process the information that was read
            

Note that reading must precede testing, as
it is only possible to know after actually attempting to read from a file
whether the reading succeeded or not. Of course, variations are possible:
getline(istream &, string &) (see section 6.5.1.1) returns an
istream &, so here reading and testing may be contracted using one
expression. Nevertheless, the above mold represents the general case. So,
the following program may be used to copy cin to cout:
        #include <iostream>
using namespace::std;

int main()
{
    while (true)
    {
        char c;

        cin.get(c);
        if (cin.fail())
            break;
        cout << c;
    }
}


Contraction is possible here by combining get with the
if-statement, resulting in:
            if (!cin.get(c))
        break;

Even so, this would still follow the basic rule:
 `read first, test later'.

Simply copying a file isn't required very often. More often a situation is
encountered where a file is processed up to a certain point, followed by plain
copying the file's remaining information. The next program illustrates
this. Using ignore to skip the first line (for the sake of the example it
is assumed that the first line is at most 80 characters long), the second
statement uses yet another overloaded version of the <<-operator, in
which a streambuf pointer  is inserted into
a stream. As the member rdbuf returns a stream's streambuf *, we have
a simple means of inserting a stream's content into an ostream:
            #include <iostream>
    using namespace std;

    int main()
    {
        cin.ignore(80, '\n');   // skip the first line and...
        cout << cin.rdbuf();    // copy the rest through the streambuf *
    }

    This way of copying streams only assumes the existence of a streambuf
object. Consequently it can be used with all specializations of the
streambuf class.


6.5.5: Coupling streams

    Ostream objects  can be coupled to ios objects
using the tie member function. Tying results in flushing the ostream's
buffer whenever an input or output operation is performed on the ios
object to which the ostream object is tied.  By default cout is tied
to cin (using cin.tie(cout)).  This tie means that whenever an
operation on cin is requested, cout is flushed first. To break the
tie, ios::tie(0) can be called. In the example: cin.tie(0).

Another useful coupling of streams is shown by the tie between cerr and
cout. Because of the tie standard output and error messages written to the
screen are shown in sync with the time at which they were generated:
        #include <iostream>
    using namespace std;

    int main()
    {
        cerr.tie(0);        // untie
        cout << "first (buffered) line to cout ";
        cerr << "first (unbuffered) line to cerr\n";
        cout << "\n";

        cerr.tie(&cout);    // tie cout to cerr
        cout << "second (buffered) line to cout ";
        cerr << "second (unbuffered) line to cerr\n";
        cout << "\n";
    }
    /*
        Generated output:

        first (unbuffered) line to cerr
        first (buffered) line to cout
        second (buffered) line to cout second (unbuffered) line to cerr
    */


An alternative way to couple streams is to make streams use a common
streambuf object. This can be implemented using the 
ios::rdbuf(streambuf *) member function. This way two streams can use,
e.g. their own formatting, one stream can be used for input, the other for
output, and redirection using the stream library rather than operating
system calls can be implemented. See the next sections for examples.


6.6: Advanced topics



6.6.1: Moving streams

    Stream classes (e.g.,, all stream classes covered in this chapter)
are movable and can be swapped. This implies that factory functions can be
designed for stream classes. Here is an example:
            ofstream out(string const &name)
    {
        ofstream ret(name);             // construct ofstream
        return ret;                     // return value optimization, but
    }                                   // OK as moving is supported
    
    int main()
    {
        ofstream mine(out("out"));      // return value optimizations, but
                                        // OK as moving is supported

        ofstream base("base");
        ofstream other;

        base.swap(other);               // swapping streams is OK

        other = std::move(base);        // moving streams is OK

        // other = base;                // this would fail: copy assignment
                                        // is not available for streams
    }


6.6.2: Redirecting streams

    Using ios::rdbuf streams can be forced to share their
streambuf objects. Thus information written to one
stream is actually written to another stream; a phenomenon normally
called redirection. Redirection is commonly implemented at the
operating system level, and sometimes that is still necessary (see
section 26.2.3).

A common situation where redirection is useful is when error messages should
be written to file rather than to the standard error stream, usually indicated
by its file descriptor number 2. In the Unix operating system using the
bash shell, this can be realized as follows:
            program 2>/tmp/error.log

Following this command any error messages written by program are
written to /tmp/error.log, instead of appearing on the screen.

Here is an example showing how this can be implemented using streambuf
objects. Assume program expects an argument defining the name
of the file to write the error messages to. It could be called as follows:
            program /tmp/error.log

The program looks like this, an explanation is provided below the
program's source text:
            #include <iostream>
    #include <fstream>
    using namespace std;

    int main(int argc, char **argv)
    {
        ofstream errlog;                                // 1
        streambuf *cerr_buffer = 0;                     // 2

        if (argc == 2)
        {
            errlog.open(argv[1]);                       // 3
            cerr_buffer = cerr.rdbuf(errlog.rdbuf());   // 4
        }
        else
        {
            cerr << "Missing log filename\n";
            return 1;
        }

        cerr << "Several messages to stderr, msg 1\n";
        cerr << "Several messages to stderr, msg 2\n";

        cout << "Now inspect the contents of " <<
                argv[1] << "... [Enter] ";
        cin.get();                                      // 5

        cerr << "Several messages to stderr, msg 3\n";

        cerr.rdbuf(cerr_buffer);                        // 6
        cerr << "Done\n";                               // 7
    }
    /*
        Generated output on file argv[1]

        at cin.get():

    Several messages to stderr, msg 1
    Several messages to stderr, msg 2

        at the end of the program:

    Several messages to stderr, msg 1
    Several messages to stderr, msg 2
    Several messages to stderr, msg 3
    */

    
    o  At lines 1-2 local variables are defined: errlog is the
ofstream to write the error messages to, and cerr_buffer is a pointer
to a streambuf, to point to the original cerr buffer.
    o  At line 3  the alternate error stream is opened.
    o  At line 4  redirection takes place: cerr now writes to
the streambuf defined by errlog. It is important that
the original buffer used by cerr is saved, as explained below.
    o  At line 5  we pause. At this point, two lines were written to
the alternate error file. We get a chance to take a look at its content:
there were indeed two lines written to the file.
    o  At line 6  the redirection is terminated. This is very important, as
the errlog object is destroyed at the end of main. If cerr's
buffer would not have been restored, then at that point
cerr would refer to a non-existing streambuf object, which might
produce unexpected results. It is the responsibility of the programmer to
make sure that an original streambuf is saved before redirection, and is
restored when the redirection ends.
    o  Finally, at line 7, Done is again written to the screen, as
the redirection has been terminated.


6.6.3: Reading AND Writing streams

    Streams can be read and written 
     using std::iostream
objects. Commonly encountered are std::fstream objects and
sometimes std::stringstream objects. Other types of
readable and writable streams can be defined, by deriving such streams from
the std::iostream class (cf. chapter 14, in particular its
section 14.8.2). 

In this section we concentrate on the std::fstream class.  As with
ifstream and ofstream objects, the fstream constructor expects the
name of the file to be opened:
                fstream inout("iofile", ios::in | ios::out);

Note the use of the constants ios::in and ios::out,
indicating that the file must be opened for both reading and writing. Multiple
mode indicators may be used, concatenated by the bitor operator.
Alternatively, instead of ios::out, ios::app could have been
used and mere writing would become appending (at the end of the file).

Reading and writing to the same stream is always a bit awkward: what to do
when the stream may not yet exist, but if it already exists it should not
be rewritten? To realize this the following approach can be used:
        #include <fstream>
    #include <iostream>
    #include <string>

    using namespace std;

    int main()
    {
        fstream rw("fname", ios::out | ios::in);

        if (!rw)            // file didn't exist yet
        {
            rw.clear();     // try again, creating it using ios::trunc
            rw.open("fname", ios::out | ios::trunc | ios::in);
        }

        if (!rw)            // can't even create it: bail out
        {
            cerr << "Opening `fname' failed miserably" << '\n';
            return 1;
        }

        cerr << "We're at: " << rw.tellp() << '\n';

                            // write something
        rw << "Hello world" << '\n';

        rw.seekg(0);        // go back and read what's written

        string s;
        getline(rw, s);

        cout << "Read: " << s << '\n';
    }

    Under this approach if the construction fails fname didn't yet
exist. But then, after clearing the failure flag, open is used also
specifying the ios::trunc flag: this creates an empty file, but
because of ios::in the file is also readable.  In addition
ios::ate could be specified, ensuring that the initial read/write
action would by default occur at EOF.

Under   DOS-like operating systems that use the
multiple character sequence \r\n to separate lines in text files the
flag ios::binary is required to process binary files ensuring
that \r\n combinations are processed as two characters. In general,
ios::binary should be specified when binary (non-text) files are to be
processed. By default files are opened as text files. Unix operating
systems do not distinguish text files from binary files.

With fstream (in general: iostream) objects, combinations of file
flags are used to make sure that a stream is or is not (re)created empty when
opened. See section 6.4.2.1 for details.

Once a file has been opened in read and write mode, the << operator
can be used to insert information into the file, while the >> operator
may be used to extract information from the file. These operations may be
performed in any order, but a seekg or seekp operation is required
when switching between insertions and extractions. The seek operation is used
to activate the stream's data used for reading or those used for writing (and
vice versa). The istream and ostream parts of fstream objects
share the stream's data buffer and by performing the seek operation the stream
either activates its istream or its ostream part. If the seek is
omitted, reading after writing and writing after reading simply fails. The
example shows a whitespace-delimited word being read from a file, writing
another string to the file, just beyond the point where the just read word
terminated. Finally yet another string is read which is found just beyond the
location where the just written strings ended:
            fstream f("filename", ios::in | ios::out);
    string  str;

    f >> str;       // read the first word

                    // write a well-known text
    f.seekp(0, ios::cur);
    f << "hello world";

    f.seekg(0, ios::cur);
    f >> str;       // and read again

Since a seek or clear operation is required when alternating
between read and write (extraction and insertion) operations on the same file
it is not possible to execute a series of << and >> operations in
one expression statement.

Of course, random insertions and extractions are hardly ever used. Generally,
insertions and extractions occur at well-known locations in a file.  In those
cases, the position where insertions or extractions are required can be
controlled and monitored by the seekg, seekp, tellg and tellp
members (see sections 6.4.1.2 and 6.5.1.2).

Error conditions (see section 6.3.1) occurring due to, e.g., reading
beyond end of file, reaching end of file, or positioning before begin of file,
can be cleared by the clear member function.  Following clear
processing may continue. E.g.,
            fstream f("filename", ios::in | ios::out);
    string  str;

    f.seekg(-10);   // this fails, but...
    f.clear();      // processing f continues

    f >> str;       // read the first word

A situation where files are both read and written is seen in
    database applications, using files consisting of records having fixed
sizes, and where locations and sizes of pieces of information are known. For
example, the following program adds text lines to a (possibly existing)
file. It can also be used to retrieve a particular line, given its
order-number in the file. A binary file index allows for the quick
retrieval of the location of lines.
            #include <iostream>
    #include <fstream>
    #include <string>
    #include <climits>
    using namespace std;

    void err(char const *msg)
    {
        cout << msg << '\n';
    }

    void err(char const *msg, long value)
    {
        cout << msg << value << '\n';
    }

    void read(fstream &index, fstream &strings)
    {
        int idx;

        if (!(cin >> idx))                          // read index
        {
            cin.clear();                            // allow reading again
            cin.ignore(INT_MAX, '\n');              // skip the line
            return err("line number expected");
        }

        index.seekg(idx * sizeof(long));            // go to index-offset

        long offset;

        if
        (
            !index.read                             // read the line-offset
            (
                reinterpret_cast<char *>(&offset),
                sizeof(long)
            )
        )
            return err("no offset for line", idx);

        if (!strings.seekg(offset))                 // go to the line's offset
            return err("can't get string offset ", offset);

        string line;

        if (!getline(strings, line))                // read the line
            return err("no line at ", offset);

        cout << "Got line: " << line << '\n';       // show the line
    }

    void write(fstream &index, fstream &strings)
    {
        string line;

        if (!getline(cin, line))                  // read the line
            return err("line missing");

        strings.seekp(0, ios::end);               // to strings
        index.seekp(0, ios::end);                 // to index

        long offset = strings.tellp();

        if
        (
            !index.write                          // write the offset to index
            (
                reinterpret_cast<char *>(&offset),
                sizeof(long)
            )
        )
            return err("Writing failed to index: ", offset);

        if (!(strings << line << '\n'))           // write the line itself
            return err("Writing to `strings' failed");
                                                  // confirm writing the line
        cout << "Write at offset " << offset << " line: " << line << '\n';
    }

    int main()
    {
        fstream index("index", ios::trunc | ios::in | ios::out);
        fstream strings("strings", ios::trunc | ios::in | ios::out);

        cout << "enter `r <number>' to read line <number> or "
                                    "w <line>' to write a line\n"
                "or enter `q' to quit.\n";

        while (true)
        {
            cout << "r <nr>, w <line>, q ? ";       // show prompt

            index.clear();
            strings.clear();

            string cmd;
            cin >> cmd;                             // read cmd

            if (cmd == "q")                         // process the cmd.
                return 0;

            if (cmd == "r")
                read(index, strings);
            else if (cmd == "w")
                write(index, strings);
            else if (cin.eof())
            {
                cout << "\n"
                        "Unexpected end-of-file\n";
                return 1;
            }
            else
                cout << "Unknown command: " << cmd << '\n';
        }
    }

    Another example showing reading and writing of files is provided by
the next program. It also illustrates the processing of NTBSs:
            #include <iostream>
    #include <fstream>
    using namespace std;

    int main()
    {                                       // r/w the file
        fstream f("hello", ios::in | ios::out | ios::trunc);

        f.write("hello", 6);                // write 2 NTB strings
        f.write("hello", 6);

        f.seekg(0, ios::beg);               // reset to begin of file

        char buffer[100];                   // or: char *buffer = new char[100]
        char c;
                                            // read the first `hello'
        cout << f.get(buffer, sizeof(buffer), 0).tellg() << '\n';
        f >> c;                             // read the NTB delim

                                            // and read the second `hello'
        cout << f.get(buffer + 6, sizeof(buffer) - 6, 0).tellg() << '\n';

        buffer[5] = ' ';                    // change asciiz to ' '
        cout << buffer << '\n';             // show 2 times `hello'
    }
    /*
        Generated output:
    5
    11
    hello hello
    */


A completely different way to read and write streams may be implemented
using streambuf members. All considerations mentioned so far remain valid
(e.g., before a read operation following a write operation seekg must be
used). When  streambuf objects are used, either an
istream is associated with the streambuf object of another ostream
object, or an ostream object is associated with the
streambuf object of another istream object. Here is the previous
program again, now using
     associated streams:
            #include <iostream>
    #include <fstream>
    #include <string>
    using namespace std;

    void err(char const *msg);      // see earlier example
    void err(char const *msg, long value);

    void read(istream &index, istream &strings)
    {
        index.clear();
        strings.clear();

        // insert the body of the read() function of the earlier example
    }


    void write(ostream &index, ostream &strings)
    {
        index.clear();
        strings.clear();

        // insert the body of the write() function of the earlier example
    }

    int main()
    {
        ifstream index_in("index", ios::trunc | ios::in | ios::out);
        ifstream strings_in("strings", ios::trunc | ios::in | ios::out);
        ostream  index_out(index_in.rdbuf());
        ostream  strings_out(strings_in.rdbuf());

        cout << "enter `r <number>' to read line <number> or "
                                    "w <line>' to write a line\n"
                "or enter `q' to quit.\n";

        while (true)
        {
            cout << "r <nr>, w <line>, q ? ";       // show prompt

            string cmd;

            cin >> cmd;                             // read cmd

            if (cmd == "q")                         // process the cmd.
                return 0;

            if (cmd == "r")
                read(index_in, strings_in);
            else if (cmd == "w")
                write(index_out, strings_out);
            else
                cout << "Unknown command: " << cmd << '\n';
        }
    }

    In this example
    
    o  the streams associated with the streambuf objects of existing
streams are not ifstream or ofstream objects but basic istream and
ostream objects.
    o  The streambuf object is not defined by an ifstream or
ofstream object. Instead it is defined outside of the streams, using
a filebuf (cf. section 14.8.3) and constructions like:
        filebuf fb("index", ios::in | ios::out | ios::trunc);
istream index_in(&fb);
ostream index_out(&fb);

o  An ifstream object can be constructed using stream modes normally
used with ofstream objects. Conversely, an ofstream objects can be
constructed using stream modes normally used with ifstream objects.
    o  If istream and ostreams share a streambuf, then their
read and write pointers (should) point to the shared buffer: they are tightly
coupled. However, instead of using two objects, defining/using a iostream
object receiving the streambuf automatically ensures that positioning for
reading and writing is synchronized (see also section 14.8.2).
    o  The advantage of using an external (separate) streambuf used by
an iostream object over using a predefined fstream object is that the
stream doesn't have to be a named file (see also section 14.8).

Like fstream objects string-stream objects can also be used for reading
and writing. After including the <sstream> header file a
std::stringstream can be defined which supports both
reading and writing. After inserting information into a stringstream
object seekg(0) can be called to read its info from the beginning of its
content. When a stringstream must repeatedly be used for reading and
writing call its clear and str members before starting a new writing
cycle. Alternatively, a stringstream str can be reinitialized using str
= stringstream{}. Here is an example:
            #include <iostream>
    #include <sstream>
    using namespace std;

    int main(int argc, char **argv)
    {
        stringstream io;
        for (size_t redo = 0; redo != 2; ++redo)
        {
            io.clear();                 // clears the not-good flags
            io.str("");
            io << argv[0] << '\n';

            io.seekg(0);
            string line;
            while (getline(io, line))   // results in io.eof()
                cout << line << '\n';
        }
    }




Chapter 7: Classes

The C programming language offers two methods for structuring data of
different types. The C struct holds data members of various types, and
the C union also defines data members of various types. However, a
union's data members all occupy the same location in memory and the programmer
may decide on which one to use.

In this chapter classes are introduced. A class is a kind of struct,
but its content is by default inaccessible to the outside world, whereas the
content of a C++ struct is by default accessible to the outside
world. In C++ structs find little use: they are mainly used to
aggregate data within the context of classes or to define elaborate return
values. Often a C++ struct merely contains plain old data (POD,
cf. section 9.10). In C++ the class is the main data structuring
device, by default enforcing two core concepts of current-day software
engineering: data hiding and encapsulation (cf. sections 3.2.1
and 7.1.1).

The union is another data structuring device the language offers. The
traditional C union is still available, but C++ also offers
unrestricted unions. Unrestricted unions are unions whose data fields may
be of class types. The C++ Annotations covers these unrestricted unions in
section 9.9, after having introduced several other new concepts of
C++,

C++ extends the C struct and union concepts by allowing the
definition of member
        
    functions (introduced in this chapter) within these data types.  Member
functions are functions that can only be used with objects of these data types
or within the scope of these data types. Some of these member functions are
special in that they are always, usually automatically, called when an object
starts its life (the so-called constructor) or ends its life (the
so-called destructor). These and other types of member functions, as well
as the design and construction of, and philosophy behind, classes are
introduced in this chapter.

We step-by-step construct a class Person, which could be used in a
database application to store a person's name, address and phone number.

 Let's start by creating a class Person right away. From the
onset, it is important to make the distinction between the class
interface and its implementation. A class may loosely be defined as
`a set of data and all the functions operating on those data'. This definition
is later refined but for now it is sufficient to get us started.

A class interface is a definition, defining the organization of objects of
that class. Normally a definition results in memory reservation. E.g., when
defining int variable the compiler ensures that some memory is reserved in
the final program storing variable's values.  Although it is a definition
no memory is set aside by the compiler once it has processed the class
definition. But a class definition follows the one definition rule: in
C++ entities may be defined only once. As a class definition does not
imply that memory is being reserved the term class interface is preferred
instead.

Class interfaces are normally contained in a class header file, e.g.,
person.h. We'll start our class Person interface here (cf section
7.7 for an explanation of the const keywords behind some
of the class's member functions):
            #include <string>

    class Person
    {
        std::string d_name;         // name of person
        std::string d_address;      // address field
        std::string d_phone;        // telephone number
        size_t      d_mass;         // the mass in kg.

        public:                     // member functions
            void setName(std::string const &name);
            void setAddress(std::string const &address);
            void setPhone(std::string const &phone);
            void setMass(size_t mass);

            std::string const &name()    const;
            std::string const &address() const;
            std::string const &phone()   const;
            size_t mass()                const;
    };

 The member functions that are declared in the interface must still be
implemented. The implementation of these members is properly called their
definition.

In addition to member functions classes also commonly define the data
that are manipulated by those member functions. These data are called the
data members. In Person they are d_name, d_address,
d_phone and d_mass. Data members should be given private 
    access rights. Since the class uses private access rights by default
they are usually simply listed at the top of the class interface.

All communication between the outer world and the class data is routed through
the class's member functions. Data members may receive new values (e.g., using
setName) or they may be retrieved for inspection (e.g., using
name). Functions merely returning values stored inside the object, not
allowing the caller to modify these internally stored values, are called
 accessors.

Syntactically there is only a marginal difference between a class and a
struct. Classes by default define private members, structs define
public members. Conceptually, though, there are differences.  In C++
structs are used in the way they are used in C: to aggregate data, which
are all freely accessible. Classes, on the other hand, hide their data from
access by the outside world (which is aptly called data hiding)
and offer member functions to define the communication between the outer world
and the class's data members.

Following Lakos (Lakos, J., 2001)
     Large-Scale C++ Software Design (Addison-Wesley) I
suggest the following setup of class interfaces:
    
    o  All data members have private access rights, and are
placed at the top of the interface.
    o  All data members start with d_, followed by a name suggesting
their meaning (in chapter 8 we'll also
encounter data members starting with s_).
    o  Non-private data members do exist, but one should be hesitant to
define non-private access rights for data members (see also chapter
13).
    o  Two broad categories of member functions are
    manipulators and accessors. Manipulators allow
the users of objects to modify the internal data of the objects. By
convention, manipulators start with set. E.g., setName.
    o  With accessors, a get-prefix is still frequently encountered,
e.g., getName. However, following the conventions promoted by Qt (see
https://doc.qt.io, e.g., its classes QByteArray and
QString), using the get-prefix is deprecated. So, rather than defining
a member getAddress, it should simply be named address.
    o  Normally (exceptions exist) the public member functions of a class
are listed first, immediately following the class's data members. They are the
important elements of the interface as they define the features the class is
offering to its users. It's a matter of convention to list them high up in the
interface.  The keyword private is needed beyond the public members to
switch back from public members to private access rights which
nicely separates the members that may be used `by the general public' from the
class's own support members.

Style conventions usually take a long time to develop. There is nothing
obligatory about them, though. I suggest that readers who have compelling
reasons not to follow the above style conventions use their own. All
others are strongly advised to adopt the above style conventions.

Finally, referring back to section 3.1.2 that
            using namespace std;

must be used in most (if not all) examples of source code. As
explained in sections 7.11 and 7.11.1 the using
directive should follow the preprocessor directive(s) including the header
files, using a setup like the following:
            #include <iostream>
    #include "person.h"

    using namespace std;

    int main()
    {
        ...
    }


7.1: The constructor

C++ classes usually contain two special categories of member functions
which are essential to the proper working of classes. These categories are the
constructors and the destructor. The destructor's primary task is to
return memory allocated by an object to the common pool when an object goes
`out of scope'.  Allocation of memory is discussed in chapter 9, and
an in-depth coverage of destructors is therefore postponed until we reach that
chapter.  In the current chapter the emphasis is on the class's internal
organization and on its constructors.

Constructors are recognized by their names which are equal to their class
names.  Constructors do not specify return values, not even void.  E.g.,
the class Person may define a constructor Person::Person(). The
C++ run-time system ensures that the constructor of a class is called when
a variable of the class is defined. It is possible to define a class lacking
any constructor. In that case the compiler defines a
  default constructor that is called when an object of
that class is defined. What actually happens in that case depends on the data
members that are defined by that class (cf. section 7.3.1).

Objects may be defined locally or globally. However, in C++ most objects
are defined locally. Globally defined objects are hardly ever required and are
somewhat deprecated.

When a function defines a local object, that object's constructor is called
every time the function is called. The object's constructor is activated at
the point where the object is defined (a subtlety is that an object may be
defined implicitly as, e.g., a temporary variable in an expression).

When an object is defined as a static object
  it is constructed when
the program starts. In this case its constructor is called even
before the function main starts. Example:
        #include <iostream>
    using namespace std;

    class Demo
    {
        public:
            Demo();
    };

    Demo::Demo()
    {
        cout << "Demo constructor called\n";
    }

    Demo d;

    int main()
    {}

    /*
        Generated output:
    Demo constructor called
    */

 The program contains one global object of the class Demo with main
having an empty body. Nonetheless, the program produces some output generated
by the constructor of the globally defined Demo object.

Constructors have a very important and well-defined role. They must ensure
that all the class's data members have sensible or at least well-defined
values once the object has been constructed. We'll get back to this important
task shortly. The default constructor has no
argument. It is defined by the compiler unless another constructor is defined
and unless its definition is suppressed (cf. section 7.6). If a
default constructor is required in addition to another constructor then the
default constructor must explicitly be defined as well. C++ provides
special syntax to realize that without much effort, which is also covered by
section 7.6.


7.1.1: A first application

    Our example class Person has three string data members and a size_t
d_mass data member. Access to these data members is controlled by
 interface functions.

Whenever an object is defined the class's constructor(s) ensure that its data
members are given `sensible' values. Thus, objects never suffer from
uninitialized values. Data members may be given new values, but that should
never be directly allowed. It is a core principle (called
 data hiding) of good class design that its data members are private. The
modification of data members is therefore fully controlled by member functions
and thus, indirectly, by the class-designer. The class encapsulates all
actions performed on its data members and due to this
 encapsulation the class object may assume the `responsibility' for its
own data-integrity. Here is a minimal definition of Person's manipulating
members:
        #include "person.h"                 // given earlier
    using namespace std;

    void Person::setName(string const &name)
    {
        d_name = name;
    }
    void Person::setAddress(string const &address)
    {
        d_address = address;
    }
    void Person::setPhone(string const &phone)
    {
        d_phone = phone;
    }
    void Person::setMass(size_t mass)
    {
        d_mass = mass;
    }

 It's a minimal definition in that no checks are performed. But it should be
clear that checks are easy to implement. E.g., to ensure that a phone number
only contains digits one could define:
            void Person::setPhone(string const &phone)
    {
        if (phone.empty())
            d_phone = " - not available -";
        else if (phone.find_first_not_of("0123456789") == string::npos)
            d_phone = phone;
        else
            cout << "A phone number may only contain digits\n";
    }

Note the double negation in this implementation. Double negations are very
hard to read, and an encapsulating member bool hasOnly handles the test,
and improves setPhone's readability:
            bool Person::hasOnly(char const *characters, string const &object)
    {
                        // object only contains 'characters'
        return object.find_first_not_of(characters) == string::npos;
    }

and setPhone becomes:
            void Person::setPhone(string const &phone)
    {
        if (phone.empty())
            d_phone = " - not available -";
        else if (hasOnly("0123456789", phone))
            d_phone = phone;
        else
            cout << "A phone number may only contain digits\n";
    }

Since hasOnly is an encapsulated member function we can ensure that
it's only used with non-empty string objects, so hasOnly itself doesn't
have to check for that.

Access to the data members is controlled by accessor
members. Accessors ensure that data members cannot suffer from uncontrolled
modifications. Since accessors conceptually do not modify the object's data
(but only retrieve the data) these member functions are given the predicate
const. They are called const member functions,
which, as they are guaranteed not to modify their object's data, are available
to both modifiable and constant objects (cf. section 7.7).

To prevent backdoors we must also make sure that the data member is
not modifiable through an accessor's return value. For values of built-in
primitive types that's easy, as they are usually returned by value, which are
copies of the values found in variables.  But since objects may be fairly
large making copies is usually prevented by returning objects by reference. A
backdoor is created by returning a data member by reference, as in the
following example, showing the allowed abuse below the function definition:
            string &Person::name() const
    {
        return d_name;
    }

    Person somebody;
    somebody.setName("Nemo");

    somebody.name() = "Eve";    // Oops, backdoor changing the name

To prevent the backdoor objects are returned as const references from
accessors. Here are the implementations of Person's accessors:
        #include "person.h"                 // given earlier
    using namespace std;

    string const &Person::name() const
    {
        return d_name;
    }
    string const &Person::address() const
    {
       return d_address;
    }
    string const &Person::phone() const
    {
       return d_phone;
    }
    size_t Person::mass() const
    {
       return d_mass;
    }


The Person class interface remains the starting point for the class
design: its member functions define what can be asked of a Person
object. In the end the implementation of its members merely is a technicality
allowing Person objects to do their jobs.

The next example shows how the class Person may be used. An object is
initialized and passed to a function printperson(), printing the person's
data. Note the reference operator in the parameter list of the function
printperson.  Only a reference to an existing Person object is passed
to the function, rather than a complete object.  The fact that
printperson does not modify its argument is evident from the fact that
the parameter is declared const.
        #include <iostream>
    #include "person.h"                 // given earlier
    using namespace std;

    void printperson(Person const &p)
    {
        cout << "Name    : " << p.name()     << "\n"
                "Address : " << p.address()  << "\n"
                "Phone   : " << p.phone()    << "\n"
                "Mass  : " << p.mass()   << '\n';
    }

    int main()
    {
        Person p;

        p.setName("Linus Torvalds");
        p.setAddress("E-mail: Torvalds@cs.helsinki.fi");
        p.setPhone("");
        p.setMass(75);           // kg.

        printperson(p);
    }
/*
    Produced output:

Name    : Linus Torvalds
Address : E-mail: Torvalds@cs.helsinki.fi
Phone   :  - not available -
Mass  : 75

*/



7.1.2: Constructors: with and  without arguments

    The class Person's constructor so far has not received any
parameters. C++ allows constructors to be defined with or without
parameter lists. The arguments are supplied when an object is defined.

For the class Person a constructor expecting three strings and a
size_t might be useful. Representing, respectively, the person's name,
address, phone number and mass. This constructor can be implemented like this
(but see also section 7.3.1):
            Person::Person(string const &name, string const &address,
                   string const &phone, size_t mass)
    {
        d_name = name;
        d_address = address;
        setPhone(phone);
        d_mass = mass;
    }

It must of course also be declared in the class interface:
            class Person
    {
        // data members (not altered)

        public:
            Person(std::string const &name, std::string const &address,
                   std::string const &phone, size_t mass);

            // rest of the class interface (not altered)
    };

Now that this constructor has been declared, the default constructor must
explicitly be declared as well if we still want to be able to construct a
plain Person object without any specific initial values for its data
members.  The class Person would thus support two constructors, and the
part declaring the constructors now becomes:
            class Person
    {
        // data members
        public:
            Person();
            Person(std::string const &name, std::string const &address,
                   std::string const &phone, size_t mass);

            // additional members
    };

In this case, the default constructor doesn't have to do very much, as it
doesn't have to initialize the string data members of the Person
object. As these data members are objects themselves, they are initialized to
empty strings by their own default constructor.  However, there is also a
size_t data member. That member is a variable of a built-in type and such
variables do not have constructors and so are not initialized automatically.
Therefore, unless the value of the d_mass data member is explicitly
initialized its value is:
    
    o  a random value for local Person objects;
    o  0 for global and static Person objects.

The 0-value might not be too bad, but normally we don't want a random
value for our data members. So, even the default constructor has a job to do:
initializing the data members which are not initialized to sensible values
automatically. Its implementation can be:
            Person::Person()
    {
        d_mass = 0;
    }

Using constructors with and without arguments is illustrated next. The
object karel is initialized by the constructor defining a non-empty
parameter list while the default constructor is used for the anon
object. When constructing objects using constructors requiring arguments you
are advised to surround the arguments by curly braces. Parentheses can often
also be used, and sometimes even have to be used (cf. section
12.3.2), but mindlessly using parentheses instead of curly braces may
easily result in unexpected problems (cf. section 7.2). Hence the
advice to prefer curly braces rather than parentheses. Here's the
example showing two constructor-calls:
            int main()
    {
        Person karel{ "Karel", "Rietveldlaan 37", "542 6044", 70 };
        Person anon;
    }

The two Person objects are defined when main starts as they are
local objects, living only for as long as main is active.

If Person objects must be definable using other arguments,
corresponding constructors must be added to Person's interface. Apart from
overloading class constructors it is also possible to provide constructors
with default argument values. These default arguments must be specified with
the constructor declarations in the class interface, like so:
            class Person
    {
        public:
            Person(std::string const &name,
                   std::string const &address = "--unknown--",
                   std::string const &phone   = "--unknown--",
                   size_t mass = 0);

    };

Often, constructors use highly similar implementions. This results from
the fact that the constructor's parameters are often defined for convenience:
a constructor not requiring a phone number but requiring a mass cannot
be defined using default arguments, since phone is not the constructor's
last parameter. Consequently a special constructor is required not having
phone in its parameter list. However, this doesn't necessarily mean that
constructors must duplicate their code, as constructors may call each other
(called constructor delegation). Constructor delegation is illustrated in
section 7.4.1 below.


7.1.2.1: The order of construction

        The possibility to pass arguments to constructors allows us to monitor the
construction order of objects during program execution.  This is illustrated
by the next program using a class Test. The program defines a global
Test object and two local Test objects. The order of construction is
as expected: first global, then main's first local object, then func's
local object, and then, finally, main's second local object:
        #include <iostream>
    #include <string>
    using namespace std;

    class Test
    {
        public:
            Test(string const &name);   // constructor with an argument
    };

    Test::Test(string const &name)
    {
        cout << "Test object " << name << " created" << '\n';
    }

    Test globaltest("global");

    void func()
    {
        Test functest("func");
    }

    int main()
    {
        Test first{ "main first" };
        func();
        Test second{ "main second" };
    }
/*
    Generated output:
Test object global created
Test object main first created
Test object func created
Test object main second created
*/



7.2: Ambiguity resolution

Calling constructors using parentheses may result in unwelcome
surprises. Assume the following class interface is available:
            class Data
    {
        public:
            Data();
            Data(int one);
            Data(int one, int two);

            void display();
    };

The intention is to define two objects of the class Data, using, respectively,
the first and second constructors, while using parentheses in the object
definitions. Your code looks like this (and, ignoring the 'not used' warnings,
compiles correctly):
            #include "data.h"
    int main()
    {
        Data d1();
        Data d2(argc);
    }

Let's get rid of the 'not used' warnings by adding two statements to main:
                    d1.display();
        d2.display();

But, surprise, the compiler complains about the first of these two:

error: request for member 'display' in 'd1', which is of non-class type
'Data()'

What's going on here? First of all, notice the data type the compiler refers
to: Data(), rather than Data. What are those () doing there?

Before answering that question, let's broaden our story somewhat. We know that
somewhere in a library a factory function dataFactory exists. A
factory function creates and returns an object of a certain type. This
dataFactory function returns a Data object, constructed using
Data's default constructor. Hence, dataFactory needs no arguments. We
want to use dataFactory in our program, but must declare the function. So
we add the declaration to main, as that's the only location where
dataFactory will be used. It's a function, not requiring arguments,
returning a Data object:
                    Data dataFactory();

This, however, looks remarkably similar to our d1 object definition:
                    Data d1();

Here we found the solution of our problem: Data d1() apparently is not
the definition of a d1 object, but the declaration of a function,
returning a Data object. So, what's happening here and how should we
define a Data object using Data's default constructor?

First: what's happening here is that the compiler, when confronted with
Data d1(), actually had a choice. It could either define a Data
object, or declare a function. It declares a function. 

Here we're encountering an ambiguity in C++'s grammar which is solved,
according to the language's standard, by always letting a declaration prevail
over a definition. We'll encounter more situations where this ambiguity occurs
later on in this section.

Second: there are several ways we can solve this ambiguity in the way we
want it to be solved. To define an object using its default constructor:
    
    o  merely mention it (like int x): Data d1;
    o  use the curly brace initialization: Data d1{};
    o  use the assignment operator and an anonymous default constructed
        Data object: Data d1 = Data{}, or possibly Data d1 =
        Data(). 


7.2.1: Types `Data' vs. `Data()'

    
    Data() in the last example of the previous section defines a default
constructed anonymous Data object. This takes us back to the compiler
error. According to the compiler, our original d1 apparently was not of
type Data, but of type Data(). So what's that?

Let's first have a look at our second constructor. It expects an
int. We would like to define another Data object, using the second
constructor and want to pass the default int value to the constructor,
using int(). We know this defines a default int value, as cout <<
int() << '\n' nicely displays 0, and int x = int() also initialized x to 
0. So we try `Data di(int())' in main.

Not good: again the compiler complains when we try to use
di. After `di.display()' the compiler tells us:
    error: request for member 'display' in 'di', which is of
non-class type 'Data(int (*)())'

Oops, again not as expected.... Didn't we pass 0? Why the sudden pointer?
Well, we're once again encountering the compiler's same `use a declaration
when possible' strategy. The notation Type() not only represents the
default value of type Type, but it's also a shorthand notation for an
anonymous pointer to a function, not expecting arguments, and returning a
Type value, which you can verify by defining `int (*ip)() = nullptr',
and passing ip as argument to di: di(ip) compiles fine.

So why doesn't the error occur when inserting int() or assigning int()
to int x? In these latter cases nothing is declared. Rather, `cout'
and `int x =' require expressions resulting in values, which is provided
by int()'s `natural' interpretation. But with `Data di(int())' the
compiler again has a choice, and (by design) it chooses a declaration because
the declaration takes priority. Now int()'s interpretation as an anonymous
pointer is available and therefore used.

Likewise, if int x has been defined, `Data b1(int(x))' declares b1
as a function, expecting an int (as int(x) represents a type), while
`Data b2((int)x)' defines b2 as a Data object, using the
constructor expecting a single int value.

Again, to use default entities, values or objects, prefer {} over ():
Data di{ int{} } defines di of type Data, calling the Data(int
x) constructor and uses int's default value 0.

But what about the compiler's original complaint? The compiler told us that
our original d1 was not of type Data, but of type Data(). But
what's that? The parentheses in the error message are important: it indicates
a function: d1, because of the subsequent parentheses, declares a
function, not expecting arguments, and returning a Data object. Shorthand:
Data(), more verbosely: Data (*)(). As a final illustration: the
following program writes hello world twice, and Data d1() in main
simply is an explicit local declaration of the globally defined function
d1, further illustrated by the assignment of d1 to the explicitly
declared pointer to a function pf, returning a Data object:
        #include <iostream>
    #include "data.h"
    
    Data::Data()
    {}

    Data d1()
    {
        std::cout << "hello world\n";
        return Data{};
    }
    
    int main()
    {
        Data d1();
    
        d1();
        Data (*pf)() = d1;
    
        pf();   // or (same): (*pf)()
    }
    Also, as d1() returns a Data object, a statement like
d1().display() correctly compiles.


7.2.2: Superfluous parentheses

    Let's play some more. At some point in our program we defined int b. Then,
in a compound statement we need to construct an anonymous Data object,
initialized by b, and then we're supposed to display b itself:
    
                int b = 18;
    {
        Data(b);
        cout << b;
    }

About that cout statement the compiler tells us (I modified the
error message to reveal its meaning):

error: cannot bind `std::ostream & << Data const &'

Here we didn't insert int b but Data b. Had we omitted the compound
statement, the compiler would have complained about a doubly defined b
entity, as Data(b) simply means Data b, a Data object constructed
by default. Here, the parentheses around b are superfluous and may be
omitted by the compiler when parsing a definition or declaration.

Of course, the question now becomes how a temporary object Data,
initialized with int b can be defined. Remember that the compiler may
remove superfluous parentheses. So, what we need to do is to pass an int
to the anonymous Data object, without using the int's name.
    
    o  We can use a cast: Data(static_cast<int>(b));
    o  We can use a curly brace initialization: Data{ b }.

Values and types make big differences. Consider the following definitions:
            Data (*d4)(int);    // 1
    Data (*d5)(3);      // 2

Definition 1 should cause no problems: it's a pointer to a function,
expecting an int, returning a Data object. Hence, d4 is a pointer
variable.  

Definition 2 is slightly more complex. Yes, it's a pointer. But it has
nothing to do with a function. So what's that argument list containing 3 doing
there? Well, it's not an argument list. It's an initialization that looks like
an argument list. Remember: variables can be initialized using assignment
statements, using parentheses, or using curly braes. In general, they're
interchangeable. So instead of `(3)' we could have written `= 3' or
`{3}'. Let's pick the first alternative, resulting in:
            Data (*d5) = 3;

Now we get to `play compiler' again. Removing some more superfluous
parentheses we get:
            Data *d5 = 3;

It's a pointer to a Data object, initialized to 3. This is
syntactically correct, but semantically incorrect: at address 3
there's no Data object. If we had initially written
             Data (*d5)(&d1);      // 2

the fun resulting from contrasting int and 3 would most likely
have been masked.


7.2.3: Existing types

    Once a type name has been defined it prevails over identifiers representing
variables if the compiler is given a choice. This, too, may result in
interesting constructions.

Assume a function process expecting an int exists in a library. We
want to use this function to process some int data values. So in main
process is declared and called:
            int process(int Data);
    process(argc);

No problems here. But unfortunately we once decided to `beautify' our
code, by throwing in some superfluous parentheses, like so:
            int process(int (Data));
    process(argc);

we're in trouble. Now the compiler generates an error, caused by the rule to
let declarations prevail over definitions. Data now becomes the name of
the class Data, and analogous to int (x) the parameter int (Data)
is parsed as int (*)(Data): a pointer to a function, expecting a Data
object, returning an int.

Here is another example. When, instead of declaring 
            int process(int Data[10]);

we declare, e.g., to emphasize the fact that an array is passed to
process:
            int process(int (Data[10]));

the process function does not expect a pointer to int values, but
a pointer to a function expecting a pointer to Data elements, returning an
int. The size 10 is considered just a number, and is not taken literally,
as it's part of a function parameter specification.

To summarize the findings in the `Ambiguity Resolution' section:
    
    o  The compiler will try to remove superfluous parentheses;
    o  But if the parenthesized construction represents a type, it will try
        to use the type;
    o  More in general: when possible the compiler will interpret a
        syntactic construction as a declaration, rather than as a definition
        (of an object or variable).
    o  Most problems that result from the compiler interpreting
        constructions as declarations are caused by us using parentheses. As a
        rule of thumb: use curly braces, rather than parentheses when
        constructing objects (or values).


7.3: Objects inside objects: composition

In the class Person objects are used as data members. This construction
technique is called composition.

Composition is neither extraordinary nor C++ specific: in C
a struct or union field is commonly used in other compound types. In
C++ it requires some special thought as their initialization sometimes is
subject to restrictions, as discussed in the next few sections.


7.3.1: Composition and (const) objects: (const) member initializers

    Unless specified otherwise object data members of classes are initialized by
their default constructors. Using the default constructor might not always be
the optimal way to intialize an object and it might not even be possible: a
class might simply not define a default constructor.

Earlier we've encountered the following constructor of the Person:
            Person::Person(string const &name, string const &address,
                   string const &phone, size_t mass)
    {
        d_name = name;
        d_address = address;
        d_phone = phone;
        d_mass = mass;
    }

Think briefly about what is going on in this constructor. In the
constructor's body we encounter assignments to string objects. Since
assignments are used in the constructor's body their left-hand side objects
must exist. But when objects are coming into existence constructors must
have been called. The initialization of those objects is thereupon immediately
undone by the body of Person's constructor. That is
not only inefficient but sometimes downright impossible. Assume that the class
interface mentions a string const data member: a data member whose value
is not supposed to change at all (like a birthday, which usually  doesn't
change very much and is therefore a good candidate for a string const data
member). Constructing a birthday object and providing it with an initial value
is OK, but changing the initial value isn't.

The body of a constructor allows assignments to data members. The
initialization of data members happens before that. C++ defines the
 member initializer syntax allowing us to specify the way data members
are initialized at construction time. Member initializers are specified as a
list of constructor specifications between  a colon following a constructor's
parameter list and the opening curly brace of a constructor's body, as
follows:
            Person::Person(string const &name, string const &address,
                   string const &phone, size_t mass)
    :
        d_name(name),
        d_address(address),
        d_phone(phone),
        d_mass(mass)
    {}

In this example the member initialization used parentheses surrounding the
intialization expression. Instead of
parentheses curly braces may also be used. E.g., d_name could also be
initialized this way:
                d_name{ name },

Member initialization always occurs when objects are composed in
classes: if no constructors are mentioned in the member initializer list
the default constructors of the objects are called. Note that this only holds
true for objects. Data members of primitive data types are not
initialized automatically.

Member initialization can, however, also be used for primitive data members,
like int and double. The above example shows the initialization of the
data member d_mass from the parameter mass. When member
initializers are used the data member could even have the same name as the
constructor's parameter (although this is deprecated) as there is no ambiguity
and the first (left) identifier used in a member initializer is always a data
member that is initialized whereas the identifier between parentheses is
interpreted as the parameter.

The order in which class type data members are initialized is defined by
the order in which those members are defined in the composing class
interface. If the order of the initialization in the constructor differs from
the order in the class interface, the compiler complains, and reorders the
initialization so as to match the order of the class interface.

Member initializers should be used as often as possible. As shown it may be
required to use them (e.g., to initialize const data members, or to initialize
objects of classes lacking default constructors) but not using member
initializers also results in inefficient code as the default constructor of a
data member is always automatically called unless an explicit member
initializer is specified. Reassignment in the constructor's body following
default construction is then clearly inefficient. Of course, sometimes it is
fine to use the default constructor, but in those cases the explicit member
initializer can be omitted.

As a rule of thumb: if a value is assigned to a data member in the
constructor's body then try to avoid that assignment in favor of using a
member initializer.


7.3.2: Composition and reference objects: reference member initializers

    Apart from using member initializers to initialize composed objects (be they
const objects or not), there is another situation where member
initializers must be used. Consider the following situation.

A program uses an object of the class Configfile, defined in main
to access the information in a configuration file. The configuration file
contains parameters of the program which may be set by changing the values in
the configuration file, rather than by supplying command line arguments.

Assume another object used in main is an object of the class Process,
doing `all the work'. What possibilities do we have to tell the object of the
class Process that an object of the class Configfile exists?
    
    o  The objects could have been declared as global objects. This
is a possibility, but not a very good one, since all the advantages of
local objects are lost.
    o  The Configfile object may be passed to the Process object at
construction time. Bluntly passing an object (i.e., by value) might not
be a very good idea, since the object must be copied into the Configfile
parameter, and then a data member of the Process class can be used to make
the Configfile object accessible throughout the Process class. This
might involve yet another object-copying task, as in the following situation:
    Process::Process(Configfile conf)   // a copy from the caller
{
    d_conf = conf;                  // copying to d_conf member
}

o  The copy-instructions can be avoided if pointers to
the Configfile objects are used, as in:
    Process::Process(Configfile *conf)  // pointer to external object
{
    d_conf = conf;                  // d_conf is a Configfile *
}

This construction as such is OK, but forces us to use the `->' field
selector operator, rather than the `.' operator, which is (disputably)
awkward. Conceptually one tends to think of the Configfile object as an
object, and not as a pointer to an object. In C this would probably have
been the preferred method, but in C++ we can do better.
    o  Rather than using value or pointer parameters, the Configfile
parameter could be defined as a reference parameter of Process's
constructor. Next, use a Config reference data member in the
class Process.

But a reference variable cannot be initialized using an assignment, and so
the following is incorrect:
            Process::Process(Configfile &conf)
    {
        d_conf = conf;        // wrong: no assignment
    }

The statement d_conf = conf fails, because it is not an
initialization, but an assignment of one Configfile object (i.e.,
conf), to another (d_conf). An assignment to a reference variable is
actually an assignment to the variable the reference variable refers to. But
which variable does d_conf refer to? To no variable at all, since we
haven't initialized d_conf.  After all, the whole purpose of the statement
d_conf = conf was to initialize d_conf....

How to initialize d_conf? We once again use the member initializer
syntax. Here is the correct way to  initialize d_conf:
            Process::Process(Configfile &conf)
    :
        d_conf(conf)      // initializing reference member
    {}

The above syntax must be used in all cases where reference data members
are used. E.g., if d_ir would have been an int reference data member,
a construction like
            Process::Process(int &ir)
    :
        d_ir(ir)
    {}

would have been required.


7.4: Data member initializers

    Non-static data members of classes are usually initialized by the class's
constructors. Frequently (but not always) the same initializations are used by
different constructors, resulting in multiple points where the initializations
are performed, which in turn complicates class maintenance.

Consider a class defining several data members: a pointer to data, a data
member storing the number of data elements the pointer points at, a data
member storing the sequence number of the object. The class also
offer a basic set of constructors, as shown in the following class interface:
            class Container
    {
        Data *d_data;
        size_t d_size;
        size_t d_nr;

        static size_t s_nObjects;

        public:
            Container();
            Container(Container const &other);
            Container(Data *data, size_t size);
            Container(Container &&tmp);
    };

The initial values of the data members are easy to describe, but somewhat
hard to implement. Consider the initial situation and assume the default
constructor is used: all data members should be set to 0, except for d_nr
which must be given the value ++s_nObjects. Since these are
non-default actions, we can't declare the default constructor using =
default, but we must provide an actual implementation:
                Container()
    :
        d_data(0),
        d_size(0),
        d_nr(++s_nObjects)
    {}

In fact, all constructors require us to state the
d_nr(++s_nObjects) initialization. So if d_data's type would have been
a (move aware) class type, we would still have to provide implementations for
all of the above constructors.

C++, however, also supports 
        data member initializers,
simplifying the initialization of non-static data members. Data member
initializers allow us to assign initial values to data members. The compiler
must be able to compute these initial values from initialization expressions,
but the initial values do not have to be constant expressions. So
++s_nObjects can be an initial value.

Using data member initializers for the class Container we get:
            class Container
    {
        Data *d_data = 0;
        size_t d_size = 0;
        size_t d_nr = ++s_nObjects;

        static size_t s_nObjects;

        public:
            Container() = default;
            Container(Container const &other);
            Container(Data *data, size_t size);
            Container(Container &&tmp);
    };

Note that the data member initializations are recognized by the compiler,
and are applied to its implementation of the default constructor. In fact, all
constructors will apply the data member initializations, unless explicitly
initialized otherwise. E.g., the move-constructor may now be implemented like
this:
            Container(Container &&tmp)
    :
        d_data(tmp.d_data),
        d_size(tmp.d_size)
    {
        tmp.d_data = 0;
    }

Although d_nr's intialization is left out of the implementation it
is initialized due to the data member initialization provided in the
class's interface.

An aggregate is an array or a class (usually a struct with no
user-defined constructors, no private or protected non-static data members,
no base classes (cf. chapter 13), and no virtual functions
(cf. chapter 14)). E.g.,
            struct POD      // defining aggregate POD
    {
        int first = 5; 
        double second = 1.28; 
        std::string hello{ "hello" };
    };

To initialize such aggregates braced initializer lists can be used. In
fact, their use is preferred over using the older form (using parentheses), as
using braces avoids confusion with function declarations. E.g.,
            POD pod{ 4, 13.5, "hi there" };

When using  braced-initializer lists not all data members need to be
initialized. Specification may stop at any data member, in which case the
default (or explicitly defined initialization values) of the remaining data
members are used. E.g.,
            POD pod{ 4 };   // uses second: 1.28, hello: "hello"


7.4.1: Delegating constructors

    Often constructors are specializations of each other, allowing objects to be
constructed specifying only subsets of arguments for all of its data members,
using default argument values for the remaining data members.

Before the C++11 standard common practice was to define a member like init
performing all initializations common to constructors. Such an init
function, however, cannot be used to initialize const or reference data
members, nor can it be used to perform so-called base class
initializations (cf. chapter 13).

Here is an example where such an init function might have been used. A
class Stat is designed as a wrapper class around C's stat(2)
function. The class might define three constructors: one expecting no
arguments and initializing all data members to appropriate values; a second
one doing the same, but it calls stat for the filename provided to the
constructor; and a third one expecting a filename and a search path for the
provided file name. Instead of repeating the initialization code in each
constructor, the common code can be factorized into a member init which
is called by the constructors.

C++ offers an alternative by allowing constructors to call each
other. This is called
    
    delegating constructors
        which is illustrated by the next example:
            class Stat
    {
        public:
            Stat()
            :
                Stat("", "")        // no filename/searchpath
            {}
            Stat(std::string const &fileName)
            :
                Stat(fileName, "")  // only a filename
            {}
            Stat(std::string const &fileName, std::string const &searchPath)
            :
                d_filename(fileName),
                d_searchPath(searchPath)
            {
                // remaining actions to be performed by the constructor
            }
    };

C++ allows static const integral data members to be initialized within
the  class interfaces (cf. chapter
8). The C++11 standard adds to this the facility to define
default initializations for plain data members in class interfaces (these data
members may or may not be const or of integral types, but (of course) they
cannot be reference data members).

These default initializations may be overruled by constructors. E.g., if
the class Stat uses a data member bool d_hasPath which is false by
default but the third constructor (see above) should initialize it to true
then the following approach is possible:
            class Stat
    {
        bool d_hasPath = false;

        public:
            Stat(std::string const &fileName, std::string const &searchPath)
            :
                d_hasPath(true)     // overrule the interface-specified
            {}                      // value
    };

Here d_hasPath receives its value only once: it's always initialized
to false except when the shown constructor is used in which case it is
initialized to true.


7.5: Uniform initialization

When defining variables and objects they may immediately be given
initial values. Class type objects are always initialized by one of their
available constructors. C already supports the array and struct
 initializer list consisting of a list of constant
expressions surrounded by a pair of curly braces.

C++ supports a comparable initialization, called 
    uniform initialization. It uses the following syntax:
            Type object{ value list };

When defining objects using a list of objects each individual object
may use its own uniform initialization.

The advantage of uniform initialization over using constructors is that using
constructor arguments sometimes results in an ambiguity as constructing an
object may sometimes be confused with using the object's overloaded function
call operator (cf. section 11.11). As initializer lists can only be used
with plain old data (POD) types (cf. section 9.10) and with classes
that are `initializer list aware' (like std::vector) the ambiguity does
not arise when initializer lists are used.

Uniform initialization can be used to initialize an object or variable, but
also to initialize data members in a constructor or implicitly in the return
statement of functions. Examples (in-class implementations for brevity):
            class Person
    {
        // data members
        public:
            Person(std::string const &name, size_t mass)
            :
                d_name {name},
                d_mass {mass}
            {}

            Person copy() const
            {
                return {d_name, d_mass};
            }
    };

Object definitions may be encountered in unexpected places, easily resulting in
(human) confusion. Consider a function `func' and a very simple
class Fun (struct is used, as data hiding is not an issue here):
            void func();

    struct Fun
    {
        Fun(void (*f)())
        {
            std::cout << "Constructor\n";
        };

        void process()
        {
            std::cout << "process\n";
        }
    };

Assume that in main a Fun object is defined as follows: 
            Fun fun(func);

Running this program displays Constructor, confirming that the object
fun is constructed. Next we change this line of code, calling process
from an anonymous Fun object:
            Fun(func).process();

As expected, Constructor appears, followed by the text process.

What about merely defining an anonymous Fun object? We do:
            Fun(func);

Now we're in for a surprise. The compiler complains that Fun's default
constructor is missing. Why's that? Insert a blank immediately after
Fun and you get Fun (func). Parentheses around an identifier are OK,
and are removed while the parenthesized expression is parsed. In
this case: (func) equals func, and so we have Fun func: the
definition of a Fun func object, using Fun's default constructor (which
isn't declared in Fun's class interface).

So why does Fun(func).process() compile? In this case we have a member
selector operator, whose left-hand operand must be an class-type object. The
object must exist, and Fun(func) represents that object. It's not the name
of an existing object, but a constructor expecting a function (like func)
as its argument. So the compiler creates an anonymous Fun object,
which receives func as its argument.

Clearly, with Fun(func) parentheses cannot be used to create an anonymous
Fun object. However, a uniform initialization can be used. To define
the anonymous Fun object we use this syntax:
            Fun{ func };

(which can also be used to immediately call one of its members. E.g.,
Fun{ func }.process()).

Although the uniform intialization syntax is slightly different from the
syntax  of an initializer list (the latter using the assignment operator) the
compiler nevertheless uses the initializer list if a constructor
supporting an initializer list is available. As an example consider:
            class Vector
    {
        public:
            Vector(size_t size);
            Vector(std::initializer_list<int> const &values);
    };

    Vector vi = {4};

When defining vi the constructor expecting the initializer list is
called rather than the constructor expecting a size_t argument. If the
latter constructor is required the standard constructor
syntax must be used. I.e., Vector vi(4).

Initializer lists are themselves objects that may be constructed using
another initializer list. However, values stored in an initializer list are
immutable. Once the initializer list has been defined their values remain
as-is.

Initializer lists support a basic set of member functions and constructors:
    
    o initializer_list<Type> object:
defines object as an empty initializer list
    o initializer_list<Type> object { list of Type values }:
defines object as an initializer list containing Type values
    o initializer_list<Type> object(other):
initializes object using the values stored in other
    o size_t size() const:
returns the number of elements in the initializer list
    o Type const *begin() const:
returns a pointer to the first element of the initializer list
    o Type const *end() const:
returns a pointer just beyond the location of the last element of the
initializer list


7.6: Defaulted and deleted class members

In everyday class design two situations are frequently encountered:
    
        o  A class offering constructors explicitly has to define a default
constructor;
        o  A class (e.g., a class implementing a stream)
cannot initialize objects by copying the values from an existing object of
that class (called
 copy construction) and cannot assign objects to each other.

Once a class defines at least one constructor its default constructor is
not automatically defined by the compiler. C++ relaxes that
restriction somewhat by offering the `= default'
syntax. A class specifying `= default' with its default constructor
declaration indicates that the trivial default constructor should be
provided by the compiler. A trivial default constructor performs the following
actions:
    
    o  Its data members of built-in or primitive types are not initialized;
    o  Its composed (class type) data members are initialized by their
default constructors.
    o  If the class is derived from a base class (cf. chapter
13) the base class is initialized by its default constructor.

Trivial implementations can also be provided for the copy constructor,
the overloaded assignment operator, and the destructor. Those members
are introduced in chapter 9.

Conversely, situations exist where some (otherwise automatically provided)
members should not be made available. This is realized by specifying
 `= delete'. Using = default and = delete is illustrated
by the following example. The default constructor receives its trivial
implementation, copy-construction is prevented:
            class Strings
    {
        public:
            Strings() = default;
            Strings(std::string const *sp, size_t size);

            Strings(Strings const &other) = delete;
    };


7.7: Const member functions and const objects

The keyword const is often used behind the parameter list of member
functions. This keyword indicates that a member function does not alter the
data members of its object. Such member functions are called
    const member functions. In the class Person, we see that the
accessor functions were declared const:
            class Person
    {
        public:
            std::string const &name()    const;
            std::string const &address() const;
            std::string const &phone()   const;
            size_t mass()              const;
    };

The rule of thumb given in section 3.1.1 applies here too:
whichever appears to the left of the keyword const, is not
altered. With member functions this should be interpreted as `doesn't alter
its own data'.

When implementing a const member function the const attribute must be
repeated:
            string const &Person::name() const
    {
        return d_name;
    }

The compiler prevents the data members of a class from being modified
by one of its const member functions. Therefore a statement like
            d_name[0] = toupper(static_cast<unsigned char>(d_name[0]));

results in a compiler error when added to the above function's definition.

Const member functions are used to prevent inadvertent data
modification. Except for constructors and the destructor (cf. chapter
9) only const member functions can be used with (plain, references
or pointers to) const objects.

Const objects are frequently encountered as const & parameters of
functions. Inside such functions only the object's const members may be
used. Here is an example:
            void displayMass(ostream &out, Person const &person)
    {
        out << person.name() << " weighs " << person.mass() << " kg.\n";
    }

Since person is defined as a Person const & the function
displayMass cannot call, e.g.,

    person.setMass(75).

The const member function attribute can be used to
  overload member functions. When functions are
overloaded by their const attribute the compiler uses the member
function matching most closely the const-qualification of the object:
    
    o  When the object is a const object, only const member
functions can be used.
    o  When the object is not a const object, non-const member
functions are used, unless only a const member function is
available. In that case, the const member function is used.

The next example illustrates how (non) const member functions are
selected:
            #include <iostream>
    using namespace std;

    class Members
    {
        public:
            Members();
            void member();
            void member() const;
    };

    Members::Members()
    {}
    void Members::member()
    {
        cout << "non const member\n";
    }
    void Members::member() const
    {
        cout << "const member\n";
    }

    int main()
    {
        Members const constObject;
        Members       nonConstObject;

        constObject.member();
        nonConstObject.member();
    }
    /*
            Generated output:

        const member
        non const member
    */

    As a general principle of design: member functions should always be given
the const attribute, unless they actually modify the object's data.


7.7.1: Anonymous objects

    Sometimes objects are used because they offer a certain functionality. The
objects only exist because of their functionality, and nothing in the objects
themselves is ever changed. The following class Print offers a facility to
print a string, using a configurable prefix and suffix. A partial class
interface could be:
            class Print
    {
        public:
            Print(ostream &out);
            void print(std::string const &prefix, std::string const &text,
                     std::string const &suffix) const;
    };

An interface like this would allow us to do things like:
            Print print{ cout };
    for (int idx = 0; idx != argc; ++idx)
        print.print("arg: ", argv[idx], "\n");

This works fine, but it could greatly be improved if we could pass
print's invariant arguments to Print's constructor.  This would
simplify print's prototype (only one argument would need to be passed
rather than three) and we could wrap the above code in a function
expecting a Print object:
            void allArgs(Print const &print, int argc, char **argv)
    {
        for (int idx = 0; idx != argc; ++idx)
            print.print(argv[idx]);
    }

The above is a fairly generic piece of code, at least it is with respect
to Print. Since prefix and suffix don't change they can be passed
to the constructor which could be given  the prototype:
            Print(ostream &out, string const &prefix = "", string const &suffix = "");

Now allArgs may be used as follows:
            Print p1{ cout, "arg: ", "\n" };    // prints to cout
    Print p2{ cerr, "err: --", "--\n" };// prints to cerr

    allArgs(p1, argc, argv);            // prints to cout
    allArgs(p2, argc, argv);            // prints to cerr

But now we note that p1 and p2 are only used inside the
allArgs function. Furthermore, as we can see from print's prototype,
print doesn't modify the internal data of the Print object it is
using.

In such situations it is actually not necessary to define objects before
they are used. Instead anonymous objects may be
used. Anonymous objects can be used:
    
    o  to initialize a function parameter which is a const reference to
an object;
    o  if the object is only used inside the function call.

When passing anonymous objects as arguments of const & parameters of
functions they are considered constant as they merely exist for passing the
information of (class type) objects to those functions. This way, they cannot
be modified, nor may their non-const member functions be used.  Of course, a
const_cast could be used to cast away the const reference's constness, but
that's considered bad practice on behalf of the function receiving the
anonymous objects. Also, any modification to the anonymous object is lost once
the function returns as the anonymous object ceases to exist after calling the
function. These anonymous objects used to initialize const references should
not be confused with passing anonymous objects to parameters defined as rvalue
references (section 3.3.2) which have a completely different purpose in
life. Rvalue references primarily exist to be `swallowed' by functions
receiving them. Thus, the information made available by rvalue references
outlives the rvalue reference objects which are also anonymous.

Anonymous objects are defined when a constructor is used without providing
a name for the constructed object. Here is the corresponding example:
            allArgs(Print{ cout, "arg: ", "\n" }, argc, argv);    // prints to cout
    allArgs(Print{ cerr, "err: --", "--\n" }, argc, argv);// prints to cerr

In this situation the Print objects are constructed and immediately
passed as first arguments to the allArgs functions, where they are
accessible as the function's print parameter. While the allArgs
function is executing they can be used, but once the function has completed,
the anonymous Print objects are no longer accessible.


7.7.1.1: Subtleties with anonymous objects

        Anonymous objects can be used to initialize function parameters
that are const references to objects. These objects are created just
before such a function is called, and are destroyed once the function has
terminated. C++'s grammar allows us to use anonymous objects in other
situations as well. Consider the following snippet of code:
            int main()
    {
        // initial statements
        Print{ "hello", "world" };      // assume a matching constructor
                                        // is available
        // later statements
    }

In this example an anonymous Print object is constructed, and it is
immediately destroyed thereafter. So, following the `initial
statements' our Print object is constructed. Then it is destroyed again
followed by the execution of the `later statements'.

The example illustrates that the standard lifetime rules do not apply to
anonymous objects. 
  Their lifetimes are limited to the
statements, rather than to the end of the block in which they are
defined.

Plain anonymous object are at least useful in one situation. Assume we
want to put markers in our code producing some output when the program's
execution reaches a certain point. An object's constructor could be
implemented so as to provide that marker-functionality allowing us to put
markers in our code by defining anonymous, rather than named objects.

C++'s grammar contains another remarkable characteristic illustrated
by the next example:
            int main(int argc, char **argv)
    {
              // assume a matching constructor is available:
        Print p{ cout, "", "" };            // 1
        allArgs(Print{ p }, argc, argv);    // 2
    }

In this example a non-anonymous object p is constructed in statement
1, which is then used in statement 2 to initialize an anonymous
object. The anonymous object, in turn, is then used to initialize
allArgs's const reference parameter. This use of an existing object
to initialize another object is common practice, and is based on the existence
of a so-called
    copy constructor. A copy constructor creates an object (as it is a
constructor) using an existing object's characteristics to initialize the data
of the object that's created. Copy constructors are discussed in depth in
chapter 9, but presently only the concept of a copy constructor is
used.

In the above example a copy constructor is used to initialize an anonymous
object. The anonymous object was then used to initialize a parameter of a
function. However, when we try to apply the same trick (i.e., using an
existing object to initialize an anonymous object) to a plain statement, the
compiler generates an error: the object p can't be redefined (in statement
3, below):
            int main(int argc, char *argv[])
    {
        Print p{ "", "" };                  // 1
        allArgs(Print(p), argc, argv);      // 2
        Print(p);                           // 3 error!
    }

Does this mean that using an existing object to initialize an anonymous
object that is used as function argument is OK, while an existing object can't
be used to initialize an anonymous object in a plain statement?

The compiler actually provides us with the answer to this apparent
contradiction. About statement 3 the compiler reports something like:
            error: redeclaration of 'Print p'

which solves the problem when realizing that within a compound statement
objects and variables may be defined. Inside a compound statement, a
type name followed by a variable name is the grammatical form of a
variable definition. Parentheses can be used to break priorities, but if
there are no priorities to break, they have no effect, and are simply ignored
by the compiler. In statement 3 the parentheses allowed us to get rid of the
blank that's required between a type name and the variable name, but to the
compiler we wrote
                Print (p);

which is, since the parentheses are superfluous, equal to
                Print p;

thus producing p's redeclaration.

As a further example: when we define a variable using a built-in type (e.g.,
double) using superfluous parentheses the compiler quietly removes
these parentheses for us:
            double ((((a))));       // weird, but OK.

To summarize our findings about anonymous variables:
    
    o  Anonymous objects are great for initializing const reference
parameters.
    o  The same syntax, however, can also be used in stand-alone
statements, in which they are interpreted as variable definitions if our
intention actually was to initialize an anonymous object using an existing
object.
    o  Since this may cause confusion, it's probably best to restrict the
use of anonymous objects to the first (and main) form: initializing function
parameters.


7.8: The keyword `inline'

Let us take another look at the implementation of the function
Person::name():
            std::string const &Person::name() const
    {
        return d_name;
    }

This function is used to retrieve the name field of an object of the class
Person. Example:
            void showName(Person const &person)
    {
        cout << person.name();
    }

To insert person's name the following actions are performed:
    
    o  The function Person::name() is called.
    o  This function returns person's d_name as a reference.
    o  The referenced name is inserted into cout.

Especially the first part of these actions causes some time loss, since an
extra function call is necessary to retrieve the value of the name field.
Sometimes a faster procedure immediately making the d_name data member
available is preferred without ever actually calling a function
name. This can be realized using inline functions. An inline
function is a request to the compiler to insert the function's code at the
location of the function's call. This may speed up execution by avoiding a
function call, which typically comes with some (stack handling and parameter
passing) overhead. Note that inline is a request to the compiler: the
compiler may decide to ignore it, and will probably ignore it when the
function's body contains much code. Good programming discipline suggests to be
aware of this, and to avoid inline unless the function's body is 
small (e.g., at most one statement and it's highly unlikely that that
statement will ever change). More on this in section 7.8.2.


7.8.1: Defining members inline

    Inline functions may be implemented in the class interface itself. For the
class Person this results in the following implementation of name:
            class Person
    {
        public:
            std::string const &name() const
            {
                return d_name;
            }
    };

Note that the inline code of the function name now literally
occurs inline in the interface of the class Person. The keyword const
is again added to the function's header.

Although members can be defined  in-class (i.e.,
inside the class interface itself), it is considered bad practice
for the following reasons:
    
    o  Defining members inside the interface contaminates the interface with
implementations. The interface's purpose is to document what functionality the
class offers. Mixing member declarations and implementation details
complicates understanding the interface. Readers need to skip implementation
details which takes time and makes it hard to grab the `broad picture', and
thus to understand at a glance what functionality the class's objects are
offering.
    o  In-class implementations of private member functions may usually be
avoided altogether (as they are private members). They should be moved to the
internal header file (unless inline public members use such inline private
members).
    o  Although members that are eligible for inline-coding should remain
inline, situations do exist where such inline members migrate from an inline
to a non-inline definition. In-class inline definitions still need editing
(sometimes considerable editing) before they can be compiled. This additional
editing is undesirable.

Because of the above considerations inline members should not be defined
in-class. Rather, they should be defined following the class interface. The
Person::name member is therefore preferably defined as follows:
            class Person
    {
        public:
            std::string const &name() const;
    };

    inline std::string const &Person::name() const
    {
        return d_name;
    }

If it is ever necessary to cancel Person::name's inline
implementation, then this becomes its non-inline implementation:
            #include "person.ih"

    std::string const &Person::name() const
    {
        return d_name;
    }

Only the inline keyword needs to be removed to obtain the correct
non-inline implementation.

Defining members inline has the following effect: whenever an inline-defined
function is called, the compiler may insert the function's body at the
location of the function call. It may be that the function itself is never
actually called.

This construction, where the function code itself is inserted rather than a
call to the function, is called an inline function.  Note that using inline
functions may result in multiple occurrences of the code of those functions in
a program: one copy for each invocation of the inline function. This is
probably OK if the function is a small one, and needs to be executed
fast. It's not so desirable if the code of the function is extensive. The
compiler knows this too, and handles the use of inline functions as a
request rather than a command. If the compiler considers the function
too long, it will not grant the request. Instead it will treat the function as
a normal function.

Note that constexpr functions (cf. section 8.1.4) are implicitly
defined as inline functions.


7.8.2: When to use inline functions

    When should inline functions be used, and when not? There are some
     rules of thumb which may be followed:
    
    o  In general inline functions should not be used.
Voila; that's simple, isn't it?
    o  Consider defining a function inline once a fully developed and
tested program runs too slowly and shows `bottlenecks' in certain functions,
and the bottleneck is removed by defining inline members. A profiler, which
runs a program and determines where most of the time is spent, is necessary to
perform such optimizations.
    o  Defining inline functions may be considered when they consist of
one very simple statement (such as the return statement in the function
Person::name).
    o  When a function is defined inline, its implementation is inserted
in the code wherever the function is used. As a consequence, when the
implementation of the inline function changes, all sources using the
inline function must be recompiled. In practice that means that all functions
must be recompiled that include (either directly or indirectly) the header
file of the class in which the inline function is defined. Not a very
attractive prospect, which we call the inline-inconsistency.
    o  It is only useful to implement an inline function when the time spent
during a function call is long compared to the time spent by the function's
body. An example of an inline function which hardly affects the program's
speed is:
    inline void Person::printname() const
{
    cout << d_name << '\n';
}

This function contains only one statement.  However, the statement takes a
relatively long time to execute. In general, functions which perform input and
output take lots of time. The effect of the conversion of this function
printname() to inline would therefore lead to an insignificant gain in
execution time.

All inline functions  have one disadvantage: the
actual code is inserted by the compiler and must therefore be known at
compile-time. Therefore, as mentioned earlier, an inline function can never be
located in a run-time library. Practically this means that an inline function
is found near the interface of a class, usually in the same header file. The
result is a header file which not only shows the declaration of a class,
but also part of its implementation, thus always blurring the distinction
between interface and implementation.


7.8.2.1: A prelude: when NOT to use inline functions

           
As a prelude to chapter 14 (Polymorphism), there is one
situation in which inline functions should definitely be avoided. At this
point in the C++ Annotations it's a bit too early to expose the full details,
but since the keyword inline is the topic of this section this is
considered the appropriate location for the advice.

There are situations where the compiler is confronted with so-called
        vague linkage
    
(cf.
http://gcc.gnu.org/onlinedocs/gcc-4.6.0/gcc/Vague-Linkage.html). These
situations occur when the compiler does not have a clear indication in what
object file to put its compiled code. This happens, e.g., with inline
functions, which are usually encountered in multiple source files. Since the
compiler may insert the code of ordinary inline functions in places where
these functions are called, vague linking is usually no problem with these
ordinary functions.

However, as explained in chapter 14, when using polymorphism
the compiler must ignore the inline keyword and define so-called
virtual members as true (out-of-line)
        
    functions. In this situation the vague linkage may cause problems, as the
compiler must decide in what object files(s) to put their code. Usually that's
not a big problem as long as the function is at least called once. But virtual
functions are special in the sense that they may very well never be explicitly
called. On some architectures (e.g., armel) the compiler may fail to compile
such inline virtual functions. This may result in missing symbols in programs
using them. To make matters slightly more complex: the problem may emerge when
shared libraries are used, but not when static libraries are used.

To avoid all of these problems virtual functions should never be defined
inline, but they should always be defined out-of-line. I.e., they should
be defined in source files.


7.8.3: Inline variables

        In addition to inline functions,  
inline variables can be defined (and identically initialized) in multiple
translation units. E.g., a header file could contain
            inline int value = 15;                      // OK

    class Demo
    {
        // static int s_value = 15;             // ERROR
        static constexpr int s_value = 15;      // OK

        static int s_inline;                    // OK: see below: the inline 
                                                //   definition follows the 
                                                //   class declaration
    };
    inline int Demo::s_inline = 20;             // OK


7.9: Local classes: classes inside functions

Classes are usually defined at the global or namespace level. However, it is
entirely possible to define  a local class, i.e., inside a
function. Such classes are called local classes.

Local classes can be very useful in advanced applications involving
inheritance or templates (cf. section 13.8). At this point in the
C++ Annotations they have limited use, although their main features can be
described. At the end of this section an example is provided.


    o  Local classes may use almost all characteristics of normal classes.
They may have constructors, destructors, data members, and member functions;
    o  Local classes cannot define static data members. Static member
functions, however, can be defined.
    o  Since a local class may define static member functions, it is
possible to define nested functions in C++ somewhat comparable to the
way programming languages like Pascal allow nested functions to be
defined.
    o  If a local class needs access to a constant integral value, a
local enum can be used. The enum may be anonymous, exposing only the
enum values.
    o  Local classes cannot directly access the non-static variables of their
surrounding context. For example, in the example shown below the class
Local cannot directly access main's argc parameter.
    o  Local classes may directly access global data and static variables
defined by their surrounding function. This includes variables defined in the
anonymous namespace of the source file containing the local class.
    o  Local class objects can be defined inside the function body, but they
cannot leave the function as objects of their own type. I.e., a local class
name cannot be used for either the return type or for the parameter types of
its surrounding function.
    o  As a prelude to inheritance (chapter 13):
a local class may be derived from an existing class allowing the surrounding
function to return a dynamically allocated locally constructed  class object,
pointer or reference via a base class pointer or reference.

#include <iostream>
#include <string>

using namespace std;

int main(int argc, char **argv)
{
    static size_t staticValue = 0;

    class Local
    {
        int d_argc;             // non-static data members OK

        public:
            enum                // enums OK
            {
                VALUE = 5
            };
            Local(int argc)     // constructors and member functions OK
            :                   // in-class implementation required
                d_argc(argc)
            {
                                // global data: accessible
                cout << "Local constructor\n";
                                // static function variables: accessible
                staticValue += 5;
            }
            static void hello() // static member functions: OK
            {
                cout << "hello world\n";
            }
    };
    Local::hello();             // call Local static member
    Local loc{ argc };          // define object of a local class.
}



7.10: The keyword `mutable'

Earlier, in section 7.7, the concepts of const member
functions and const objects were introduced.

C++ also allows the declaration of data members which may be modified,
even by const member function. Declarations of such data members start with
the keyword mutable.

Mutable should be used for those data members that may be modified without
logically changing the object, which might therefore still be considered a
constant object.

An example of a situation where mutable is appropriately used is found in
the implementation of a string class. Consider the std::string's c_str
and data members. The actual data returned by the two members are
identical, but c_str must ensure that the returned string is terminated by
an 0-byte. As a string object has both a length and a capacity an easy
way to implement c_str is to ensure that the string's capacity exceeds its
length by at least one character. This invariant allows c_str to be
implemented as follows:
            char const *string::c_str() const
    {
        d_data[d_length] = 0;
        return d_data;
    }

This implementation logically does not modify the object's data as the
bytes beyond the object's initial (length) characters have undefined
values. But in order to use this implementation d_data must be declared
mutable:
            mutable char *d_data;

The keyword mutable is also useful in classes implementing, e.g.,
reference counting. Consider a class implementing reference counting for
strings. The object doing the reference counting might be a const object,
but the class may define a copy constructor. Since const objects can't be
modified, how would the copy constructor be able to increment the reference
count? Here the mutable keyword may profitably be used, as it can be
incremented and decremented, even though its object is a const object.

The keyword mutable should sparingly be used. Data modified by const
member functions should never logically modify the object, and it should be
easy to demonstrate this.  As a rule of thumb: do not use mutable
unless there is a very clear reason (the object is logically not altered) for
violating this rule.


7.11: Header file organization

In section 2.5.10 the requirements for header files when a C++
program also uses C functions were discussed.
  Header files containing class interfaces have additional
requirements.

First, source files. With the exception of the occasional classless
function, source files contain the code of member functions of classes.
Basically,  there are two approaches:
    
    o  All required header files for a member function are included in each
individual source file.
    o  All required header files (for all member functions of a class)
are included in a header file that is included by each of the source
files defining class members.

The first alternative has the advantage of economy for the compiler: it
only needs to read the header files that are necessary for a particular source
file. It has the disadvantage that the program developer must include multiple
header files again and again in source files: it both takes time to type the
include-directives and to think about the header files which are needed in
a particular source file.

The second alternative has the advantage of economy for the program developer:
the header file of the class accumulates header files, so it tends to become
more and more generally useful. It has the disadvantage that the compiler
frequently has to process many header files which aren't actually used by the
function to compile.

With computers running faster and faster (and compilers getting smarter and
smarter) I think the second alternative is to be preferred over the first
alternative. So, as a starting point source files of a particular class
MyClass could be organized according to the following example:
            #include <myclass.h>

    int MyClass::aMemberFunction()
    {}

There is only one include-directive. Note that the directive refers to
a header file in a directory mentioned in the INCLUDE-file environment
variable. Local header files (using #include "myclass.h") could be used
too, but that tends to complicate the organization of the class header file
itself somewhat.

The organization of the header file itself requires some attention. Consider
the following example, in which two classes File and String are
used.

Assume the File class has a member gets(String &destination), while
the class String has a member function getLine(File &file). The
(partial) header file for the class String is then:
            #ifndef STRING_H_
    #define STRING_H_

    #include <project/file.h>   // to know about a File

    class String
    {
        public:
            void getLine(File &file);
    };
    #endif

Unfortunately a similar setup is required for the class File:
            #ifndef FILE_H_
    #define FILE_H_

    #include <project/string.h>   // to know about a String

    class File
    {
        public:
            void gets(String &string);
    };
    #endif

Now we have created a problem. The compiler, trying to compile the source
file of the function File::gets proceeds as follows:
    
    o  The header file project/file.h is opened to be read;
    o  FILE_H_ is defined
    o  The header file project/string.h is opened to be read
    o  STRING_H_ is defined
    o  The header file project/file.h is (again) opened to be read
    o  Apparently, FILE_H_ is already defined, so the remainder of
            project/file.h is skipped.
    o  The interface of the class String is now parsed.
    o  In the class interface a reference to a File object is
            encountered.
    o  As the class File hasn't been parsed yet, a File is still
        an undefined type, and the compiler quits with an error.

The solution to this problem is to use a
 forward class reference before the class interface, and to include
the corresponding class header file beyond the class interface. So we get:
            #ifndef STRING_H_
    #define STRING_H_

    class File;                 // forward reference

    class String
    {
        public:
            void getLine(File &file);
    };

    #include <project/file.h>   // to know about a File

    #endif

A similar setup is required for the class File:
            #ifndef FILE_H_
    #define FILE_H_

    class String;               // forward reference

    class File
    {
        public:
            void gets(String &string);
    };

    #include <project/string.h>   // to know about a String

    #endif

This works well in all situations where either references or pointers to
other classes are involved and with (non-inline) member functions having
    class-type return values or  parameters.

This setup doesn't work with composition, nor with in-class inline
member functions. Assume the class File has a composed data member of
the class String. In that case, the class interface of the class File
must include the header file of the class String before the class
interface itself, because otherwise the compiler can't tell how big a File
object is. A File object contains a String member, but the compiler
can't determine the size of that String data member and thus, by
implication, it can't determine the size of a File object.

In cases where classes contain composed objects (or are derived from other
classes, see chapter 13) the header files of the classes of the
composed objects must have been read before the class interface itself.
In such a case the class File might be defined as follows:
            #ifndef FILE_H_
    #define FILE_H_

    #include <project/string.h>     // to know about a String

    class File
    {
        String d_line;              // composition !

        public:
            void gets(String &string);
    };
    #endif

The class String can't declare a File object as a composed member:
such a situation would again result in an undefined class while compiling the
sources of these classes.

All remaining header files (appearing below the class interface itself)
are required only because they are used by the class's source files.

This approach allows us to introduce yet another refinement:
    
    o  Header files defining a class interface should declare what can
be declared before defining the class interface itself. So, classes that are
mentioned in a class interface  should be specified using
  forward declarations unless
        
        o  They are a base class of the current class (see chapter
13);
        o  They are the class types of composed data members;
        o  They are used in inline member functions.

In particular: additional actual header files are not required for:
        
            o  class-type return values of functions;
            o  class-type value parameters of functions.

Class header files  of objects that are either composed or inherited
or that are used in inline functions, must be known to the compiler before
the interface of the current class starts.  The information in the header
file itself is protected by the #ifndef ... #endif construction introduced
in section 2.5.10.
    o  Program sources in which the class is used only need to include this
header file.  Lakos, (2001) refines this process even
further. See his book Large-Scale C++ Software Design for further
details. This header file should be made available in a well-known location,
such as a directory or subdirectory of the standard INCLUDE path.
    o  To implement member functions the class's header file is required
and usually additional header files (like the string header file) as
well. The class header file itself as well as these additional header files
should be included in a separate internal header file (for which the extension
.ih  (`internal header') is suggested).

The .ih file should be defined in the same directory as the source
files of the class. It has the following characteristics:
        
        o  There is no need for a protective #ifndef .. #endif
shield, as the header file is never included by other header files.
        o  The standard .h header file defining the class interface
is included.
        o  The header files of all classes used  as forward references in
the standard .h header file are included.
        o  Finally, all other header files that are required in the source
files of the class are included.

An example of such a header file organization is:
        
        o  First part, e.g., /usr/local/include/myheaders/file.h:
    #ifndef FILE_H_
#define FILE_H_

#include <fstream>      // for composed 'ifstream'

class Buffer;           // forward reference

class File              // class interface
{
    std::ifstream d_instream;

    public:
        void gets(Buffer &buffer);
};
#endif

o  Second part, e.g., ~/myproject/file/file.ih, where all
sources of the class File are stored:
    #include <myheaders/file.h> // make the class File known

#include <buffer.h>         // make Buffer known to File
#include <string>           // used by members of the class
#include <sys/stat.h>       // File.


7.11.1: Using namespaces in header files

    When entities from  namespaces are used in  header
files, no using directive should be specified in those header
files if they are to be used as general header files declaring classes or
other entities from a library. When the using directive is used in a
header file then users of such a header file are forced to accept and use the
declarations in all code that includes the particular header file.

For example, if in a namespace special an object Inserter cout is
declared, then special::cout is of course a different object than
std::cout. Now, if a class Flaw is constructed, in which the
constructor expects a reference to a special::Inserter, then the class
should be constructed as follows:
            class special::Inserter;

    class Flaw
    {
        public:
            Flaw(special::Inserter &ins);
    };

Now the person designing the class Flaw may be in a lazy mood, and
might get bored by continuously having to prefix special:: before every
entity from that namespace. So, the following construction is used:
            using namespace special;

    class Inserter;
    class Flaw
    {
        public:
            Flaw(Inserter &ins);
    };

This works fine, up to the point where somebody wants to include
flaw.h in other source files: because of the using directive, this
latter person is now by implication also using namespace special, which
could produce unwanted or unexpected effects:
            #include <flaw.h>
    #include <iostream>

    using std::cout;

    int main()
    {
        cout << "starting\n";       // won't compile
    }

The compiler is confronted with two interpretations for cout: first,
because of the using directive in the flaw.h header file, it considers
cout a special::Inserter, then, because of the using directive in
the user program, it considers cout a std::ostream. Consequently,
the compiler reports an error.

As a rule of thumb, header files intended for general use
should not contain using declarations. This rule does not hold true for
header files which are only included by the sources of a class: here the
programmer is free to apply as many using declarations as desired, as
these directives never reach other sources.


7.12: Sizeof applied to class data members

In C++ the well-known sizeof operator can be applied to data
members of classes without the need to specify an object as well. Consider:
            class Data
    {
        std::string d_name;
        ...
    };

To obtain the size of Data's d_name member the following
expression can be used:
            sizeof(Data::d_name);

Note, however, that the compiler observes data protection here as well. It's
only possible to use sizeof(Data::d_name) when d_name is visible,
i.e., it can be used by Data's member functions and friends.



Chapter 8: Static Data And Functions

In the previous chapters we provided examples of classes where each object
had its own set of data members data. Each of the class's
member functions could access any member of any object of its class.

In some situations it may be desirable to define
    common data fields, that may be accessed by all objects of the
class. For example, the name of the startup directory, used by a program that
recursively scans the directory tree of a disk. A second example is a variable
that indicates whether some specific initialization has occurred. In that case
the object that was constructed first would perform the initialization and
would set the flag to `done'.

Such situations are also encountered in C, where several functions need to
access the same variable. A common solution in C is to define all these
functions in one source file and to define the variable static: the
variable name is invisible outside the scope of the source file. This approach
is quite valid, but violates our philosophy of using only one function per
source file. Another C-solution is to give the variable in question an
unusual name, e.g., _6uldv8, hoping that other program parts won't use
this name by accident. Neither the first, nor the second legacy C solution
is elegant.

C++ solves the problem by defining static members: data and functions,
common to all objects of a class and (when defined in the private section)
inaccessible outside of the class.  These static members are this chapter's
topic.

Static members cannot be defined as  virtual
functions. A virtual member function is an ordinary member in that it has a
this pointer. As static member functions have no this pointer, they
cannot be declared virtual.


8.1: Static data

    Any data member of a class can be declared static; be it in the public
or private section of the class interface. Such a data member is created
and initialized only once, in contrast to non-static data members which are
created again and again for each object of the class.

 Static data members are created as soon as the
program starts. Even though they're created at the very beginning of a
program's execution cycle they are nevertheless true members of their classes.

It is suggested to prefix the names of static member with s_ so they may
easily be distinguished (in class member functions) from the class's data
members (which should preferably start with d_).

Public static data members are global variables. They may
be accessed by all of the program's code, simply by using their class
names, the scope resolution operator and their member names. Example:
            class Test
    {
        static int s_private_int;

        public:
            static int s_public_int;
    };

    int main()
    {
        Test::s_public_int = 145;   // OK
        Test::s_private_int = 12;   // wrong, don't touch
                                    // the private parts
    }

The example does not present an executable program.  It merely illustrates
the interface, and not the implementation of static data members,
which is discussed next.


8.1.1: Private static data

        To illustrate the use of a static data member which is a private
variable in a class, consider the following:
            class Directory
    {
        static char s_path[];

        public:
            // constructors, destructors, etc.
    };

The data member s_path[] is a private static data member. During
the program's execution only one Directory::s_path[] exists,
even though multiple objects of the class Directory may exist. This
data member could be inspected or altered by the constructor, destructor or by
any other member function of the class Directory.

Since constructors are called for each new object of a class, static
data members 
     are not initialized by
constructors. At most they are modified. The reason for this is that
static data members exist before any constructor of the class has been
called. Static data members are initialized when they are defined, outside of
any member function, exactly like the initialization of ordinary (non-class)
global variables.

The definition and initialization of a static data member usually occurs
in one of the source files of the class functions, preferably in a source file
dedicated to the definition of static data members, called data.cc.

The data member s_path[], used above, could thus be
defined and initialized as follows in a file data.cc:
            #include "directory.ih"

    char Directory::s_path[200] = "/usr/local";

In the class interface the static member is actually only declared. In
its implementation (definition) its type and class name are explicitly
mentioned. Note also that the size specification can be left out of the
interface, as shown above. However, its size is (either explicitly or
implicitly) required when it is defined.

Note that any source file could contain the definition of the static
data members of a class. A separate data.cc source file is advised, but
the source file containing, e.g., main() could be used as well. Of course,
any source file defining static data of a class must also include the header
file of that class, in order for the static data member to be known to the
compiler.

A second example of a useful private static data member is given below. Assume
that a class Graphics defines the communication of a program with a
graphics-capable device (e.g., a VGA screen). The initialization of the
device, which in this case would be to switch from text mode to graphics mode,
is an action of the constructor and depends on a static flag variable
s_nobjects. The variable s_nobjects simply counts the number of
Graphics objects which are present at one time. Similarly, the destructor
of the class may switch back from graphics mode to text mode when the last
Graphics object ceases to exist.  The class interface for this
Graphics class might be:
            class Graphics
    {
        static int s_nobjects;              // counts # of objects

        public:
            Graphics();
            ~Graphics();                    // other members not shown.
        private:
            void setgraphicsmode();         // switch to graphics mode
            void settextmode();             // switch to text-mode
    };

The purpose of the variable s_nobjects is to count the number of
objects existing at a particular moment in time. When the first object is
created, the graphics device is initialized. At the destruction of the last
Graphics object, the switch from graphics mode to text mode is made:
            int Graphics::s_nobjects = 0;           // the static data member

    Graphics::Graphics()
    {
        if (!s_nobjects++)
            setgraphicsmode();
    }

    Graphics::~Graphics()
    {
        if (!--s_nobjects)
            settextmode();
    }

Obviously, when the class Graphics would define more than one
constructor, each constructor would need to increase the variable
s_nobjects and would possibly have to initialize the graphics mode.


8.1.2: Public static data

        Data members could also be declared in the public section of a class. This,
however, is deprecated (as it violates the principle of data hiding).  The
static data member s_path[] (cf.  section 8.1) could be
declared in the public section of the class definition. This would allow all
the program's code to access this variable directly:
            int main()
    {
        getcwd(Directory::s_path, 199);
    }

A declaration is not a definition. Consequently the variable s_path
still has to be defined.  This implies that some source file still needs to
contain s_path[] array's definition.


8.1.3: Initializing static const data

        Static const  data members should be initialized
like other static data members: in source files defining these data
members.

Usually, if these data members are of integral or built-in primitive data
types the compiler accepts in-class initializations of such data
members. However, there is no formal rule requiring the compiler to do
so. Compilations may or may not succeed depending on the optimizations used by
the compiler (e.g., using -O2 may result in a successful compilation, but
        -O0
    (no-optimizations) may fail to compile, but then maybe only when shared
libraries are used...).

In-class initializations of integer constant values (e.g., of types char,
int, long, etc, maybe unsigned) is nevertheless possible using (e.g.,
anonymous) enums.  The following example illustrates how this can be done:
            class X
    {
        public:
            enum         { s_x = 34 };
            enum: size_t { s_maxWidth = 100 };
    };

To avoid confusion caused by different compiler options static data members
should always explicitly be defined and initialized in a (single) source file,
whether or not they are const data. Additionally, by defining them in a
source file you avoid the inline-inconsistency.


8.1.4: Generalized constant expressions (constexpr)

        In C macros are often used to let the preprocessor perform simple
calculations. These  macro functions may have arguments,
as illustrated in the next example:
            #define xabs(x) ((x) < 0 ? -(x) : (x))

The disadvantages of macros are well known. The main reason for avoiding
macros is that they are not parsed by the compiler, but are processed by the
preprocessor resulting in mere text replacements and thus avoid type-safety
or syntactic checks of the macro definition by itself. Furthermore, since
macros are processed by the preprocessor their use is unconditional, without
acknowledging the context in which they are applied. NULL is an infamous
example. Ever tried to define an enum symbol NULL? or EOF? Chances
are that, if you did, the compiler threw strange error messages at you.

Generalized 
            
    const expressions can be used as an alternative.

Generalized const expressions are recognized by the modifier constexpr (a
keyword), that is applied to the expression's type.

There is a small syntactic difference between the use of the const
modifier and the use of the constexpr modifier.  While the const
modifier can be applied to definitions and declarations alike, the
constexpr modifier can only be applied to definitions:
            extern int const externInt;     // OK: declaration of const int
    extern constexpr int error;     // ERROR: not a definition

Variables defined with the constexpr modifier have constant (immutable)
values. But generalized const expressions are not just used to define constant
variables; they have other applications as well. The constexpr keyword is
usually applied to functions, turning the function into a
    
    constant-expression function.

A constant-expression function should not be confused with a function
returning a const value (although a constant-expression function does
return a (const) value).  A constant expression function has the
following characteristics:
    
    o  it returns a value;
    o  its return type is given the constexpr modifier;

Such functions are also called
        
        named constant expressions with parameters.

These constant expression functions may or may not be called with arguments
that have been evaluated at compile-time (not just `const arguments', as a
const parameter value is not evaluated at compile-time). If they are
called with compile-time evaluated arguments then the returned value is
considered a const value as well.

This allows us to encapsulate expressions that can be evaluated at compile-time
in functions, and it allows us to use these functions in situations where
previously the expressions themselves had to be used. The encapsulation
reduces the number of occurrences of the expressions to one, simplifying
maintenance and reduces the probability of errors.

If arguments that could not be compile-time evaluated are passed to
constant-expression functions, then these functions act like any other
function, in that their return values are no longer considered constant
expressions.

Assume some two-dimensional arrays must be converted to one-dimensional
arrays. The one-dimensional array must have nrows * ncols + nrows +
ncols + 1 elements, to store row, column, and total marginals, as well as the
elements of the source array itself. Furthermore assume that nrows and
ncols have been defined as globally available size_t const values
(they could be a class's static data). The one-dimensional arrays are data
members of a class or struct, or they are also defined as global arrays.

Now that constant-expression functions are available the expression returning
the number of the required elements can be encapsulated in such a function:
            size_t const nRows = 45;
    size_t const nCols = 10;

    constexpr size_t nElements(size_t rows, size_t cols)
    {
        return rows * cols + rows + cols + 1;
    }

        ....

    int intLinear[ nElements(nRows, nCols) ];

    struct Linear
    {
        double d_linear[ nElements(nRows, nCols) ];
    };

If another part of the program needs to use a linear array for an array of
different sizes then the constant-expression function can also be used. E.g.,
            string stringLinear[ nElements(10, 4) ];

Constant-expression functions can be used in other constant expression
functions as well. The following constant-expression function returns half the
value, rounded upwards, that is returned by nElements:
            constexpr size_t halfNElements(size_t rows, size_t cols)
    {
        return (nElements(rows, cols) + 1) >> 1;
    }

Classes should not expose their data members to external software, so as
to reduce coupling between classes and external software. But if a class
defines a static const size_t data member then that member's value could
very well be used to define entities living outside of the class's scope, like
the number of elements of an array or to define the value of some enum. In
situations like these constant-expression functions are the perfect tool to
maintain proper data hiding:
            class Data
    {
        static size_t const s_size = 7;

        public:
            static constexpr size_t size();
            constexpr size_t mSize();
    };

    constexpr size_t Data::size()
    {
        return s_size;
    }

    constexpr size_t Data::mSize()
    {
        return size();
    }

    double data[ Data::size() ];        // OK: 7 elements
    short data2[ Data().mSize() ];      // also OK: see below

Please note the following:
    
    o  Constant-expression functions are implicitly declared inline. As an
        illustration: the file sum.h declares a constexpr
        function: constexpr int sum(int x, int y); It's used by  main:
            #include <iostream>
    #include "sum.h"
    
    int main()
    {
        std::cout << "sum: " << sum(21, 21) << '\n';
    }
        When this file (main.cc) is compiled the compiler reports:
       sum.h:1:15: warning: inline function 
            `constexpr int sum(int, int)' used but never defined.
       Here the compiler explicitly states that sum is an inline
        function. Moreover, even if sum.cc defines sum and
        main.o and sum.o are linked the linker reports:
       undefined reference to `sum(int, int)'. 
       Consequently, the definitions of constexpr functions must
        compile-time be available which in practice means that they're
        defined in (class) header files;
    o  Non-static constant-expression member functions are implicitly
        const, and a const member modifier for them is optional;
    o  Constant values (e.g., static constant data members) used by
        constant-expression functions must be known by the time the compiler
        encounters the functions' definitions. That's why s_size was
        initialized in Data's class interface.

Some final notes: constexpr functions may
    
    o  define any kind of variable except for 
        static or thread_local variables;
    o  define variables  without initializers;
    o  use  conditional statements (if and switch);
    o  use repetition statements, including the range-based for
        statement;
    o  use expressions changing the values of objects that are local to
        the constexpr function;
    o  constexpr member functions can be non-const.  But 
        non-const constexpr member functions can only modify data members
        of objects that were defined local to the constexpr function
        calling the non-const constexpr member function.


8.1.4.1: Constant expression data

            As we've seen, (member) functions and variables of primitive data types can be
defined using the constexpr modifier.  What about class-type objects?

Objects of classes are values of class type, and like values of primitive
types they can be defined with the constexpr specifier. Constant
expression class-type objects must be initialized with constant expression
arguments; the constructor that is actually used must itself have been
declared with the constexpr modifier. Note again that the constexpr
constructor's definition must have been seen by the compiler before the
constexpr object can be constructed:
            class ConstExpr
    {
        public:
            constexpr ConstExpr(int x);
    };

    ConstExpr ok{ 7 };              // OK: not declared as constexpr

    constexpr ConstExpr err{ 7 };   // ERROR: constructor's definition
                                    //        not yet seen

    constexpr ConstExpr::ConstExpr(int x)
    {}

    constexpr ConstExpr ok{ 7 };                // OK: definition seen
    constexpr ConstExpr okToo = ConstExpr{ 7 }; // also OK

A constant-expression constructor has the following characteristics:
    
    o  it is declared with the constexpr modifier;
    o  its member initializers only use constant expressions;
    o  its body is empty.

An object constructed with a constant-expression constructor is called a
        user-defined literal.
    Destructors and copy constructors of user-defined literals must be
trivial.

The constexpr characteristic of user-defined literals may or may not
be maintained by its class's members. If a member is not declared with a
constexpr return value, then using that member does not result in a
constant-expression. If a member does declare a constexpr return value
then that member's return value is considered a constexpr if it is by
itself a constant expression function. To maintain its constexpr
characteristics it can refer to its classes' data members only if its
object has been defined with the constexpr modifier, as illustrated by the
example:
            class Data
    {
        int d_x;

        public:
            constexpr Data(int x)
            :
                d_x(x)
            {}

            int constexpr cMember()
            {
                return d_x;
            }

            int member() const
            {
                return d_x;
            }
    };

    Data d1{ 0 };           // OK, but not a constant expression

    enum e1 {
        ERR = d1.cMember()  // ERROR: cMember(): no constant
    };                      //        expression anymore

    constexpr Data d2{ 0 }; // OK, constant expression

    enum e2 {
        OK = d2.cMember(),  // OK: cMember(): now a constant
                            //                expression
        ERR = d2.member(),  // ERR: member(): not a constant
    };                      //                expression


8.2: Static member functions

    In addition to static data members, C++ allows us to define
    static member functions. Similar to static data that are shared by
all objects of the class, static member functions also exist without any
associated object of their class.

Static member functions  can access all static
members of their class, but also the members (private or public)
of objects of their class if they are informed about the existence of
these objects (as in the upcoming example). As static member functions are not
associated with any object of their class they do not have a
this pointer. In fact, a static member function is completely comparable
to a global function, not associated with any class (i.e., in practice they
are. See the next section (8.2.1) for a subtle note). Since
static member functions do not require an associated object, static member
functions declared in the public section of a class interface may be called
without specifying an object of its class.  The following example illustrates
this characteristic of static member functions:
            class Directory
    {
        string d_currentPath;
        static char s_path[];

        public:
            static void setpath(char const *newpath);
            static void preset(Directory &dir, char const *newpath);
    };
    inline void Directory::preset(Directory &dir, char const *newpath)
    {
                                                    // see the text below
        dir.d_currentPath = newpath;                // 1
    }

    char Directory::s_path[200] = "/usr/local";     // 2

    void Directory::setpath(char const *newpath)
    {
        if (strlen(newpath) >= 200)
            throw "newpath too long";

        strcpy(s_path, newpath);                    // 3
    }

    int main()
    {
        Directory dir;

        Directory::setpath("/etc");                 // 4
        dir.setpath("/etc");                        // 5

        Directory::preset(dir, "/usr/local/bin");   // 6
        dir.preset(dir, "/usr/local/bin");          // 7
    }


    o  at 1 a static member function modifies a private data member
of an object. However, the object whose member must be modified is given to
the member function as a reference parameter.

Note that 
    static member functions can be defined as inline
  functions.
    o  at 2 a relatively long array is defined to be able to accommodate
long paths. Alternatively, a string or a pointer to dynamic memory could
be used.
    o  at 3 a (possibly longer, but not too long) new pathname is stored in
the static data member s_path[]. Note that only static members are used.
    o  at 4, setpath() is called. It is a static member, so no
object is required. But the compiler must know to which class the function
belongs, so the class is mentioned using the scope resolution operator.
    o  at 5, the same is implemented as in 4. Here dir is used to tell
the compiler that we're talking about a function in the Directory
class. Static member functions can be called as normal member
functions, but this does not imply that the static member function receives
the object's address as a this pointer. Here the member-call syntax is
used as an alternative for the classname plus scope resolution operator
syntax.
    o  at 6, currentPath is altered. As in 4, the class and the scope
resolution operator are used.
    o  at 7, the same is implemented as in 6. But here dir is used to
tell the compiler that we're talking about a function in the Directory
class. Here in particular note that this is not using preset() as an
ordinary member function of dir: the function still has no
this-pointer, so dir must be passed as argument to inform the static
member function preset about the object whose currentPath member it
should modify.

In the example only public static member functions were used.  C++
also allows the definition of private static member functions. Such functions
can only be called by member functions of their class.


8.2.1: Calling conventions

            As noted in the previous section, static (public) member functions are
comparable to classless functions. However, formally this statement is not
true, as the C++ standard does not prescribe the same calling conventions
for static member functions as for classless global functions.

In practice the calling conventions are identical, implying that the
address of a static member function could be used as an argument of functions
having parameters that are
     pointers to (global) functions.

If unpleasant surprises must be avoided at all cost, it is suggested to
create global classless wrapper functions around static member functions
that must be used as call back functions for other functions.

Recognizing that the traditional situations in which call back functions
are used in C are tackled in C++ using template algorithms
(cf. chapter 19), let's assume that we have a class Person
having data members representing the person's name, address, phone and
mass. Furthermore, assume we want to sort an array of pointers to Person
objects, by comparing the Person objects these pointers point to. Keeping
things simple, we assume that the following public static member exists:
            int Person::compare(Person const *const *p1, Person const *const *p2);

A useful characteristic of this member is that it may directly inspect the
required data members of the two Person objects passed to the member
function using pointers to pointers (double pointers).

Most compilers allow us to pass this function's address as the address of
the comparison function for the standard C qsort() function. E.g.,
            qsort
    (
        personArray, nPersons, sizeof(Person *),
        reinterpret_cast<int(*)(void const *, void const *)>(Person::compare)
    );

However, if the compiler uses different calling conventions for static
members and for classless functions, this might not work. In such a case, a
classless wrapper function like the following may be used profitably:
            int compareWrapper(void const *p1, void const *p2)
    {
        return
            Person::compare
            (
                static_cast<Person const *const *>(p1),
                static_cast<Person const *const *>(p2)
            );
    }

resulting in the following call of the qsort() function:
            qsort(personArray, nPersons, sizeof(Person *), compareWrapper);

Note:
    
    o  The wrapper function takes care of any mismatch in the calling
conventions of static member functions and classless functions;
    o  The wrapper function handles the required type casts;
    o  The wrapper function might perform small additional services (like
dereferencing pointers if the static member function expects references to
Person objects rather than double pointers);
    o  As an aside: in C++ programs functions like qsort(),
requiring the specification of call back functions are seldom used. Instead
using existing generic template algorithms is preferred (cf. chapter
19).



Chapter 9: Classes And Memory Allocation

In contrast to the set of functions that handle memory allocation in C
(i.e., malloc etc.), memory allocation in C++ is handled by
the operators new and delete.
Important differences between malloc and new are:
    
    o  The function malloc doesn't `know' what the allocated memory
will be used for. E.g., when memory for ints is allocated, the programmer
must supply the correct expression using a multiplication by
sizeof(int). In contrast, new requires a type to be specified; the
sizeof expression is implicitly handled by the compiler. Using new is
therefore type safe.
    o  Memory allocated by malloc is initialized by calloc,
initializing the allocated characters to a configurable initial value.  This
is not very useful when objects are available. As operator new knows about
the type of the allocated entity it may (and will) call the constructor of an
allocated class type object. This constructor may be also supplied with
arguments.
    o  All C-allocation functions must be inspected for
NULL-returns. This is not required anymore when new is used. In fact,
new's behavior when confronted with failing memory allocation is
configurable through the use of a new_handler (cf. section
9.2.2).

A comparable relationship exists between free and delete:
delete makes sure that when an object is deallocated, its
destructor is automatically called.

The automatic calling of constructors and destructors when objects are created
and destroyed has consequences which we shall discuss in this chapter. Many
problems encountered during C program development are caused by incorrect
memory allocation or memory leaks: memory is not allocated, not
freed, not initialized, boundaries are overwritten, etc..  C++ does not
`magically' solve these problems, but it does provide us with tools to
prevent these kinds of problems.

As a consequence of malloc and friends becoming deprecated
the very frequently used str... functions, like
strdup, that are all malloc based, should be avoided in
C++ programs. Instead, the facilities of the string class and
operators new and delete should be used.

Memory allocation procedures influence the way classes dynamically allocating
their own memory should be designed. Therefore, in this chapter these topics
are discussed in addition to discussions about operators new and
delete. We'll first cover the peculiarities of operators new and
delete, followed by a discussion about:
    
    o  the destructor: the member function that's called when an object
ceases to exist;
    o  the assignment operator, allowing us to assign an object to another
object of its own class;
    o  the this pointer, allowing explicit references to the object for
which a member function was called;
    o  the copy constructor: the constructor creating a copy of an object;
    o  the move constructor: a constructor creating an object from
an anonymous temporary object.


9.1: Operators `new' and `delete'

C++ defines two operators to allocate memory and to return it to the
`common pool'. These operators are, respectively, new and delete.

Here is a simple example illustrating their use. An int pointer variable
points to memory allocated by operator new. This memory is later released
by operator delete.
            int *ip = new int;
    delete ip;

Here are some characteristics of operators new and delete:
    
    o  new and delete are operators and therefore do not
require parentheses, as required for functions like malloc and
free;
    o  new returns a pointer to the kind of memory that's asked for by
its operand (e.g., it returns a pointer to an int);
    o  new uses a type as its operand, which has the important
benefit that the correct amount of memory, given the type of the object to be
allocated, is made available;
    o  as a consequence, new is a type safe operator as it always
returns a pointer to the type that was mentioned as its operand. In addition,
the type of the receiving pointer
must match the type specified with operator new;
    o  new may fail, but this is normally of no concern to the
programmer. In particular, the program does not have to test the success
of the memory allocation, as is required for malloc and
friends. Section 9.2.2 delves into this aspect of new;
    o  delete returns void;
    o  for each call to new a matching delete should eventually be
executed, lest a memory leak occurs;
    o  delete can safely operate on a 0-pointer (doing nothing);
    o  otherwise delete must only be used to return memory allocated
by new. It should not be used to return memory allocated by
malloc and friends.
    o  in C++ malloc and friends are deprecated and should be
avoided.

Operator new can be used to allocate primitive types but also to
 allocate objects. When a primitive type
or a struct type without a constructor is allocated the allocated
memory is not guaranteed to be initialized to 0, but an
initialization expression may be provided:
            int *v1 = new int;          // not guaranteed to be initialized to 0
    int *v1 = new int();        // initialized to 0
    int *v2 = new int(3);       // initialized to 3
    int *v3 = new int(3 * *v2); // initialized to 9

When a class-type object is allocated, the arguments of its constructor
(if any) are specified immediately following the type specification in the
new expression and the object is initialized by to the thus specified
constructor. For example, to allocate string objects the following
statements could be used:
            string *s1 = new string;            // uses the default constructor
    string *s2 = new string{};          // same
    string *s3 = new string(4, ' ');    // initializes to 4 blanks.

In addition to using new to allocate memory for a single entity or an
array of entities (see the next section) there also exists a variant
allocating raw memory:
 operator new(sizeInBytes). Raw memory is returned as a void *. Here
new allocates a block of memory for unspecified purpose. Although raw
memory may consist of multiple characters it should not be interpreted as an
array of characters. Since raw memory returned by new is returned as a
void * its return value can be assigned to a void * variable. More
often it is assigned to a char * variable, using a cast. Here is an
example:
            char *chPtr = static_cast<char *>(operator new(numberOfBytes));

The use of raw memory is frequently encountered in combination with the
placement new operator, discussed in section 9.1.5.


9.1.1: Allocating arrays

    Operator new[] is used to allocate arrays. The generic notation
new[] is used in the C++ Annotations. Actually, the number of elements to be
allocated must be specified between the square brackets and it must, in turn,
be prefixed by the type of the entities that must be allocated. Example:
            int *intarr = new int[20];          // allocates 20 ints
    string *stringarr = new string[10]; // allocates 10 strings.

Operator new is a different operator than operator new[]. A
consequence of this difference is discussed in the next section
(9.1.2).

Arrays allocated by operator new[] are called
 dynamic arrays.  They are constructed during the
execution of a program, and their lifetime may exceed the lifetime of the
function in which they were created. Dynamically allocated arrays may last for
as long as the program runs.

When new[] is used to allocate an array of primitive values or an
array of objects, new[] must be specified with a type and an (unsigned)
expression between its square brackets. The type and expression together are
used by the compiler to determine the required size of the block of memory to
make available. When new[] is used the array's elements are stored
consecutively in memory. An array index expression may thereafter be used to
access the array's individual elements: intarr[0] represents the first
int value, immediately followed by intarr[1], and so on until the last
element (intarr[19]).

With non-class types (primitive types, POD
types without constructors) the block of memory returned by operator new[]
is not guaranteed to be initialized to 0. Alternatively, adding () to
the new expression will initialize the block of memory to
zeroes. E.g.,
            struct POD
    {
        int iVal;
        double dVal;
    };
    new POD[5]();       // returns a pointer to 5 0-initialized PODs
    new double[9]();    // returns a pointer to 9 0-initialized doubles

In addition to using new Type[size](), the array's size can also be
inferred by the compiler by specifying comma-separated initialization values
between the parentheses, like new int[](1, 2, 3). This allocates an array
of three ints which are initialized to, respectively, the values 1, 2 and
3. Specifying a larger value between the square brackets is also possible. In
that case the first values are initialized as specified, the remaining values
are initialized to zeroes (or by the objects' default constructors). Instead
of using parentheses curly braces can also be used.

If there are members of the struct POD that are explicitly initialized
in the struct's interface (e.g., int iVal = 12), or if the struct uses
composition, and the composed data member's type defines a default
constructor, then initializations in the struct's interface and
initializations performed by the composed data member's constructor takes
precedence over the 0-initialization. Here is an example:
            struct Data
    {
        int value = 100;
    };
    struct POD
    {
        int iVal = 12;
        double dVal;
        Data data;
    };

    POD *pp = new POD[5]();

Here, pp points to five POD objects, each having their iVal
data members initialized to 12, their dVal data members initialized to 0,
and their data.value members initialized to 100.

When operator new[] is used to allocate arrays of objects of class
types defining default constructors these constructors are automatically
used. Consequently new string[20] results in a block of 20 initialized
string objects. A non-default constructor cannot be called, but often it
is possible to work around that (as discussed in section 13.8).

The expression between brackets of operator new[] represents the
number of elements of the array to allocate. The C++ standard allows
allocation of  0-sized arrays. The statement
 new int[0] is correct C++. However, it is also
pointless and confusing and should be avoided. It is pointless as it doesn't
refer to any element at all, it is confusing as the returned pointer has a
useless non-0 value. A pointer intending to point to an array of values should
be initialized (like any pointer that isn't yet pointing to memory) to 0,
allowing for expressions like if (ptr) ...

Without using operator new[], arrays of variable sizes can also be
constructed as
 local arrays. Such arrays are not dynamic arrays and
their lifetimes are restricted to the lifetime of the block in which they were
defined.

Once allocated, all arrays  have fixed sizes. There is no
simple way to enlarge or shrink arrays. C++ has no operator
`renew'. Section 9.1.3 illustrates how to 
enlarge arrays.


9.1.2: Deleting arrays

    Dynamically allocated arrays are deleted using operator
 delete[]. It expects a pointer to a block of memory, previously allocated
by operator new[].

When operator delete[]'s operand is a pointer to an array of objects
two actions are performed:
    
    o  First, the class's destructor is called for each of the objects in
the array. The destructor, as explained later in this
chapter, performs all kinds of cleanup operations that are
required by the time the object ceases to exist.
    o  Second, the memory pointed at by the pointer is returned to the
common pool.

Here is an example showing how to allocate and delete an array of 10
string objects:
            std::string *sp = new std::string[10];
    delete[] sp;

No special action is performed if a dynamically allocated array of
primitive typed values is deleted. Following int *it = new int[10] the
statement delete[] it simply returns the memory pointed at by it.
Realize that, as a pointer is a primitive type, deleting a dynamically
allocated array of pointers to objects does not result in the proper
destruction of the objects the array's elements point at. So, the following
example results in a memory leak:
            string **sp = new string *[5];
    for (size_t idx = 0; idx != 5; ++idx)
        sp[idx] = new string;
    delete[] sp;            // MEMORY LEAK !

In this example the only action performed by delete[] is to return an
area the size of five pointers to strings to the common pool.

Here's how the destruction in such cases should be performed:
    
    o  Call delete for each of the array's elements;
    o  Delete the array itself

Example:
            for (size_t idx = 0; idx != 5; ++idx)
        delete sp[idx];
    delete[] sp;

One of the consequences is of course that by the time the memory is going
to be returned not only the pointer must be available but also the number of
elements it contains. This can easily be accomplished by storing pointer and
number of elements in a simple class and then using an object of that class.

Operator delete[] is a different operator than operator
delete. The rule of thumb is: if new[] was used, also use
delete[].


9.1.3: Enlarging arrays

    Once allocated, all arrays have fixed  sizes.
There is no simple way to enlarge or shrink arrays. C++ has no
 renew operator. The basic steps to take when enlarging an array are the
following:
    
    o  Allocate a new block of memory of larger size;
    o  Copy the old array content to the new array;
    o  Delete the old array;
    o  Let the pointer to the array point to the newly allocated array.

Static and local arrays cannot be resized. Resizing is only possible for
dynamically allocated arrays. Example:
        #include <string>
    using namespace std;

    string *enlarge(string *old, size_t oldsize, size_t newsize)
    {
        string *tmp = new string[newsize];  // allocate larger array

        for (size_t idx = 0; idx != oldsize; ++idx)
            tmp[idx] = old[idx];            // copy old to tmp

        delete[] old;                       // delete the old array
        return tmp;                         // return new array
    }

    int main()
    {
        string *arr = new string[4];        // initially: array of 4 strings
        arr = enlarge(arr, 4, 6);           // enlarge arr to 6 elements.
    }


The procedure to enlarge shown in the example also has several drawbacks.
    
    o  The new array requires newsize constructors to be called;
    o  Having initialized the strings in the new array, oldsize of them
are immediately reassigned to the corresponding values in the original array;
    o  All the objects in the old arrays are destroyed.

Depending on the context various solutions exist to improve the efficiency
of this rather inefficient procedure. An array of pointers could be used
(requiring only the pointers to be copied, no destruction, no superfluous
initialization) or raw memory in combination with the
 placement new operator could be used (an array of objects
remains available, no destruction, no superfluous construction).


9.1.4: Managing `raw' memory

    As we've seen operator new allocates the memory for an object and
subsequently initializes that object by calling one of its
constructors. Likewise, operator delete calls an object's destructor and
subsequently returns the memory allocated by operator new to the common
pool.

In the next section we'll encounter another use of new, allowing us to
initialize objects in so-called raw memory: memory merely consisting of
bytes that have been made available by either static or dynamic allocation.

Raw memory is made available by
  operator new(sizeInBytes) and also by operator
new[](sizeInBytes). The returned memory should not be interpreted as an array
of any kind but just a series of memory locations that were dynamically made
available. No initialization whatsoever is performed by these variants of
new.

Both variants return void *s so (static) casts are required to use the
return values as memory of some type.

Here are two examples:
                                        // room for 5 ints:
    int *ip = static_cast<int *>(operator new(5 * sizeof(int)));
                                // same as the previous example:
    int *ip2 = static_cast<int *>(operator new[](5 * sizeof(int)));
                                // room for 5 strings:
    string *sp = static_cast<string *>(operator new(5 * sizeof(string)));

As operator new has no concept of data types the size of the intended
data type must be specified when allocating raw memory for a certain number of
objects of an intended type. The use of operator new therefore somewhat
resembles the use of malloc.

The counterpart of operator new is operator delete. Operator
delete (or, equivalently, operator delete[]), expects a void * (so a
pointer to any type can be passed to it). The pointer is interpreted as a
pointer to raw memory which is returned to the common pool without any further
action. In particular, no destructors are called by operator delete. The
use of operator delete therefore resembles the use of free. To return
the memory pointed at by the abovementioned variables ip and sp
operator delete should be used:
                            // delete raw memory allocated by operator new
    operator delete(ip);
    operator delete[](ip2);
    operator delete(sp);


9.1.5: The `placement new' operator

    A remarkable form of operator new is called the placement new
  operator. Before using placement new the <memory>
header file must be included.

Placement new is passed an existing block of memory into which new
initializes an object or value.  The block of memory should be large enough to
contain the object, but apart from that there are no further requirements. It
is easy to determine how much memory is used by an entity (object or variable)
of type Type: the
 sizeof operator returns the number of bytes used by an Type entity.

Entities may of course dynamically allocate memory for their own use.
Dynamically allocated memory, however, is not part of the entity's memory
`footprint' but it is always made available externally to the entity
itself. This is why sizeof returns the same value when applied to
different string objects that return different length and capacity values.

The placement new operator uses the following syntax (using Type to
indicate the used data type):
            Type *new(void *memory) Type{ arguments };

Here, memory is a block of memory of at least sizeof(Type) bytes
and Type(arguments) is any constructor of the class Type.

The placement new operator is useful in situations where classes set
aside memory to be used later. This is used, e.g., by std::string to
change its capacity. Calling string::reserve may enlarge that capacity
without making memory beyond the string's length immediately available to the
string object's users. But the object itself may use its additional
memory. E.g, when information is added to a string object it can draw
memory from its capacity rather than performing a reallocation for each single
character that is added to its content.

Let's apply that philosophy to a class Strings storing std::string
objects. The class defines a string *d_memory accessing the memory holding
its d_size string objects as well as d_capacity - d_size reserved
memory. Assuming that a default constructor initializes d_capacity to 1,
doubling d_capacity whenever an additional string must be stored, the
class must support the following essential operations:
    
    o  doubling its capacity when all its spare memory (e.g., made available
by reserve) has been consumed;
    o  adding another string object
    o  properly deleting the installed strings and memory when a
Strings object ceases to exist.

The private member void Strings::reserve is called when the current
capacity must be enlarged to d_capacity. It operates as follows:
    First new, raw, memory is allocated (line 1). This memory is in no way
initialized with strings. Then the available strings in the old memory are
copied into the newly allocated raw memory using placement new (line 2). Next,
the old memory is deleted (line 3).
    void Strings::reserve()
{
    using std::string;

    string *newMemory = static_cast<string *>(                  // 1
                            operator new(d_capacity * sizeof(string)));
    for (size_t idx = 0; idx != d_size; ++idx)                  // 2
        new (newMemory + idx) string{ d_memory[idx] };
    destroy();                                                  // 3
    d_memory = newMemory;
}


The member append adds another string object to a Strings
object. A (public) member reserve(request) (enlarging d_capacity if
necessary and if enlarged calling reserve()) ensures that the String
object's capacity is sufficient. Then placement new is used to install the
latest string into the raw memory's appropriate location:
    void Strings::append(std::string const &next)
{
    reserve(d_size + 1);
    new (d_memory + d_size) std::string{ next };
    ++d_size;
}


At the end of the String object's lifetime, and during enlarging
operations all currently used dynamically allocated memory must be
returned. This is made the responsibility of the member destroy, which is
called by the class's destructor and by reserve(). More about the
destructor itself in the next section, but the
implementation of the support member destroy is discussed below.

With placement new an interesting situation is encountered. Objects,
possibly themselves allocating memory, are installed in memory that may or may
not have been allocated dynamically, but that is usually not completely filled
with such objects. So a simple delete[] can't be used.  On the other hand,
a delete for each of the objects that are available can't be used
either, since those delete operations would also try to delete the memory
of the objects themselves, which wasn't dynamically allocated.

This peculiar situation is solved in a peculiar way, only encountered in
cases where placement new is used: memory allocated by objects initialized
using placement new is returned by
        
    explicitly calling the object's destructor.  The destructor is
declared as a member having as its name the class name preceded by a tilde,
not using any arguments. So, std::string's destructor is named
~string. An object's destructor only returns memory allocated by the
object itself and, despite of its name, does not destroy its object.  Any
memory allocated by the strings stored in our class Strings is
therefore properly destroyed by explicitly calling their
destructors. Following this d_memory is back to its initial status: it
again points to raw memory. This raw memory is then returned to the common
pool by operator delete:
    void Strings::destroy()
{
    for (std::string *sp = d_memory + d_size; sp-- != d_memory; )
        sp->~string();

    operator delete(d_memory);
}


So far, so good. All is well as long as we're using only one object. What
about allocating an array of objects? Initialization is performed as usual.
But as with delete, delete[] cannot be called when the buffer was
allocated statically. Instead, when multiple objects were initialized using
placement new in combination with a statically allocated
buffer all the objects' destructors must be called explicitly, as in the
following example:
            using std::string;

    char buffer[3 * sizeof(string)];
    string *sp = new(buffer) string [3];

    for (size_t idx = 0; idx != 3; ++idx)
        sp[idx].~string();


Several standard template library functions are available for handling
unitialized (raw) memory. See section 19.1.58 for a description.


9.2: The destructor

Comparable to the constructor, classes may define a destructor.  This
function is the constructor's counterpart in the sense that it is invoked when
an object ceases to exist. A destructor is usually called automatically, but
that's not always true. The destructors of dynamically allocated objects are
not automatically activated, but in addition to that: when a program is
interrupted by an exit call, only the destructors of
already initialized global objects are called.  In that situation destructors
of objects defined locally by functions are also not called. This is
one (good) reason for avoiding exit in C++ programs.

Destructors obey the following syntactical requirements:
    
    o  a destructor's name is equal to its class name prefixed by a tilde;
    o  a destructor has no arguments;
    o  a destructor has no return value.

Destructors are declared in their class interfaces. Example:
            class Strings
    {
        public:
            Strings();
            ~Strings();     // the destructor
    };

By convention the constructors are declared first. The destructor is
declared next, to be followed by other member functions.

A destructor's  main task is to ensure that
memory allocated by an object is properly returned when the object ceases to
exist. Consider the following interface of the class Strings:
            class Strings
    {
        std::string *d_string;
        size_t d_size;

        public:
            Strings();
            Strings(char const *const *cStrings, size_t n);
            ~Strings();

            std::string const &at(size_t idx) const;
            size_t size() const;
    };


The constructor's task is to initialize the data fields of the object. E.g,
its constructors are defined as follows:
            Strings::Strings()
    :
        d_string(0),
        d_size(0)
    {}

    Strings::Strings(char const *const *cStrings, size_t size)
    :
        d_string(new string[size]),
        d_size(size)
    {
        for (size_t idx = 0; idx != size; ++idx)
            d_string[idx] = cStrings[idx];
    }

    As objects of the class Strings allocate memory a destructor is
clearly required. Destructors may or may not be called automatically, but note
that destructors are only called (or, in the case of dynamically allocated
objects: should only be called) for fully constructed objects.

C++ considers objects `fully constructed' once at least one of its
constructors could normally complete. It used to be the constructor, but
as C++ supports constructor delegation, multiple constructors can be
activated for a single object; hence `at least one constructor'.  The
remaining rules apply to fully constructed objects;
    
    o  Destructors of local non-static objects are called automatically when
the execution flow leaves the block in which they were defined; the
destructors of objects defined somewhere in the outer block of a function are
called just before the function terminates.
    o  Destructors of static or global objects are called when the program
itself terminates.
    o  The destructor of a dynamically allocated object is called by
delete using the object's address as its operand;
    o  The destructors of a dynamically allocated array of objects are
called by delete[] using the address of the array's first element as its
operand;
    o  The destructor of an object initialized by placement new is
activated  by explicitly calling the object's destructor.

The destructor's task is to ensure that all memory that is
dynamically allocated and controlled only by the object
itself is returned. The task of the Strings's
destructor would therefore be to delete the memory to which d_string
points. Its implementation is:
            Strings::~Strings()
    {
        delete[] d_string;
    }


The next example shows Strings at work. In process
a Strings store is created, and its data are displayed. It returns a
dynamically allocated Strings object to main. A
Strings * receives the address of the allocated object and deletes the
object again. Another Strings object is then created in a block of
memory made available locally in main, and an
    explicit call to ~Strings is required
to return the memory allocated by that object. In the example only once a
Strings object is automatically destroyed: the local Strings
object defined by process. The other two Strings objects require
explicit actions to prevent memory leaks.
            #include "strings.h"
    #include <iostream>

    using namespace std;;

    void display(Strings const &store)
    {
        for (size_t idx = 0; idx != store.size(); ++idx)
            cout << store.at(idx) << '\n';
    }

    Strings *process(char *argv[], size_t argc)
    {
        Strings store{ argv, argc };
        display(store);
        return new Strings{ argv, argc };
    }

    int main(int argc, char *argv[])
    {
        Strings *sp = process(argv, argc);
        delete sp;

        char buffer[sizeof(Strings)];
        sp = new (buffer) Strings{ argv, static_cast<size_t>(argc) };
        sp->~Strings();
    }



9.2.1: Object pointers revisited

        Operators new and delete are used when an object or variable is
allocated. One of the advantages of the operators new and
delete over functions like malloc and free is that new and
delete call the corresponding object constructors and destructors.

The allocation of an object by operator new is a two-step
process. First the memory for the object itself is allocated. Then its
constructor is called, initializing the object. Analogously to the
construction of an object, the destruction is also a two-step process: first,
the destructor of the class is called deleting the memory controlled by the
object. Then the memory used by the object itself is freed.

Dynamically allocated arrays of objects can also be handled by new and
delete. When allocating an array of objects using operator new the
default constructor is called for each object in the array. In cases like this
operator delete[] must be used to ensure that the destructor is called for
each of the objects in array.

However, the addresses returned by new Type and new Type[size]
are of identical types, in both cases a Type *. Consequently it cannot be
determined by the type of the pointer whether a pointer to dynamically
allocated memory points to a single entity or to an array of entities.

What happens if delete rather than delete[] is used? Consider the
following situation, in which the destructor ~Strings is modified so that
it tells us that it is called.  In a main function an array of two
Strings objects is allocated using new, to be deleted by delete[].
 Next, the same actions are repeated, albeit that the delete operator is
called without []:
            #include <iostream>
    #include "strings.h"
    using namespace std;

    Strings::~Strings()
    {
        cout << "Strings destructor called" << '\n';
    }

    int main()
    {
        Strings *a  = new Strings[2];

        cout << "Destruction with []'s" << '\n';
        delete[] a;

        a = new Strings[2];

        cout << "Destruction without []'s" << '\n';
        delete a;
    }
/*
    Generated output:
Destruction with []'s
Strings destructor called
Strings destructor called
Destruction without []'s
Strings destructor called
*/

    From the generated output, we see that the destructors of the individual
Strings objects are called when delete[] is used, while only the
first object's destructor is called if the [] is omitted.

Conversely, if delete[] is called in a situation where delete
should have been called the results are unpredictable, and the program will
most likely crash. This problematic behavior is caused by the way the run-time
system stores information about the size of the allocated array (usually right
before the array's first element). If a single object is allocated the
array-specific information is not available, but it is nevertheless assumed
present by delete[]. Thus this latter operator encounters bogus values in
the memory locations just before the array's first element. It then dutifully
interprets the value it encounters there as size information, usually causing
the program to fail.

If no destructor is defined, a trivial destructor is defined by the
compiler. The trivial destructor ensures that the destructors of composed
objects (as well as the destructors of base classes if a class is a
derived class, cf. chapter 13) are called. This has serious
implications: objects allocating memory create memory leaks 
unless precautionary measures are taken (by defining an appropriate
destructor). Consider the following program:
            #include <iostream>
    #include "strings.h"
    using namespace std;

    Strings::~Strings()
    {
        cout << "Strings destructor called" << '\n';
    }

    int main()
    {
        Strings **ptr = new Strings* [2];

        ptr[0] = new Strings[2];
        ptr[1] = new Strings[2];

        delete[] ptr;
    }

    This program produces no output at all. Why is this? The variable ptr
is defined as a pointer to a pointer. The dynamically allocated array
therefore consists of pointer variables and pointers are of a primitive type.
No destructors exist for  primitive typed variables. Consequently only the
array itself is returned, and no Strings destructor is called.

Of course, we don't want this, but require the Strings objects
pointed to by the elements of ptr to be deleted too. In this case we have
two options:
    
    o  In a for-statement visit all the elements of the ptr array,
calling delete for each of the array's elements. This procedure was
demonstrated in the previous section.
    o  A wrapper class is designed around a pointer (to, e.g., an object
of some class, like Strings).  Rather than using a pointer to a
pointer to Strings objects a pointer to an array of wrapper-class
objects is used. As a result delete[] ptr calls the destructor of each of
the wrapper class objects, in turn calling the Strings destructor for
their d_strings members. Example:
            #include <iostream>
    using namespace std;

    class Strings   // partially implemented
    {
        public:
            ~Strings();
    };

    inline Strings::~Strings()
    {
        cout << "destructor called\n";
    }

    class Wrapper
    {
        Strings *d_strings;

        public:
            Wrapper();
            ~Wrapper();
    };

    inline Wrapper::Wrapper()
    :
        d_strings(new Strings{})
    {}
    inline Wrapper::~Wrapper()
    {
        delete d_strings;
    }

    int main()
    {
        auto ptr = new Strings *[4];
        // ... code assigning `new Strings' to ptr's elements
        delete[] ptr;               // memory leak: ~Strings() not called

        cout << "===========\n";
        delete[] new Wrapper[4];    // OK: 4 x destructor called
    }
    /*
        Generated output:
    ===========
    destructor called
    destructor called
    destructor called
    destructor called
    */



9.2.2: The function set_new_handler()

    The C++ run-time system ensures that when memory allocation fails an
error function is activated. By default this function throws a
  bad_alloc exception (see section
10.8), terminating the program. Therefore it is not necessary to check
the return value of operator new. Operator new's default behavior may
be modified in various ways. One way to modify its behavior is to redefine the
function that's called when memory allocation fails. Such a function
must comply with the following requirements:
    
    o  it has no parameters;
    o  its return type is void.

A redefined error function might, e.g., print a message and terminate
the program. The user-written error function becomes part of the allocation
system through the function set_new_handler.

Such an error function is illustrated below\ ( This implementation
applies to the GNU C/C++ requirements. Actually using the program given
in the next example is not advised, as it probably enormously slows down your
computer due to the resulting use of the operating system's swap area.):
            #include <iostream>
    #include <string>

    using namespace std;

    void outOfMemory()
    {
        cout << "Memory exhausted. Program terminates." << '\n';
        exit(1);
    }

    int main()
    {
        long allocated = 0;

        set_new_handler(outOfMemory);       // install error function

        while (true)                        // eat up all memory
        {
            new int [100000]();
            allocated += 100000 * sizeof(int);
            cout << "Allocated " << allocated << " bytes\n";
        }
    }

    Once the new error function has been installed it is automatically invoked
when memory allocation fails, and the program is terminated.  Memory
allocation may fail in indirectly called code as well, e.g., when constructing
or using streams or when strings are duplicated by low-level functions.

So far for the theory. On some systems the `out of memory' condition
may actually never be reached, as the operating system may interfere before
the run-time support system gets a chance to stop the program.

The traditional memory allocation functions (like strdup, malloc,
realloc etc.) do not trigger the new handler when memory allocation
fails and should be avoided in C++ programs.


9.3: The assignment operator

In C++ struct and class type objects  can be directly assigned new values
in the same way as this is possible in C.  The default
action of such an assignment for non-class type data members is a straight
byte-by-byte copy from one data member to another.  For now we'll use the
following simple class Person: 
            class Person
    {
        char *d_name;
        char *d_address;
        char *d_phone;

        public:
            Person();
            Person(char const *name, char const *addr, char const *phone);
            ~Person();
        private:
            char *strdupnew(char const *src);   // returns a copy of src.
    };

    // strdupnew is easily implemented, here is its inline implementation:
    inline char *Person::strdupnew(char const *src)
    {
        return strcpy(new char [strlen(src) + 1], src);
    }

Person's data members are initialized to zeroes or to copies of the
NTBSs passed to Person's constructor, using some variant of
strdup. The allocated memory is eventually returned by Person's
destructor.

Now consider the consequences of using Person objects in the following
example:
            void tmpPerson(Person const &person)
    {
        Person tmp;
        tmp = person;
    }

Here's what happens when tmpPerson is called:
    
    o  it expects a reference to a Person as its parameter person.
    o  it defines a local object tmp, whose data members are initialized
to zeroes.
    o  the object referenced by person is copied to tmp:
sizeof(Person) number of bytes are copied from person to tmp.

Now a potentially dangerous situation has been created.  The actual values
in person are pointers, pointing to allocated memory.  After the
assignment this memory is addressed by two objects: person and
tmp.
    
    o  The potentially dangerous situation develops into an acutely
dangerous situation once the function tmpPerson terminates: tmp is
destroyed. The destructor of the class Person releases the memory pointed
to by the fields d_name, d_address and d_phone: unfortunately,
this memory is also pointed at by person....

This problematic assignment is illustrated in Figure 5.

------------------------------------------------------------------Insert Figure 5(Private data and public interface functions of the class Person,
        using byte-by-byte assignment)about here (file: memory/badassign)------------------------------------------------------------------


    Having executed tmpPerson, the object referenced by person now
contains pointers to deleted memory.

This is undoubtedly not a desired effect of using a function like
tmpPerson.  The deleted memory is likely to be reused by subsequent
allocations. The pointer members of person have effectively become
 wild pointers, as they don't point to allocated
memory anymore. In general it can be concluded that
    
every class containing pointer data members is a potential
        candidate for trouble.

    Fortunately, it is possible to prevent these troubles, as discussed next.


9.3.1: Overloading the assignment operator

    Obviously, the right way to assign one Person object to another, is
not to copy the content of the object bytewise. A better way is to
make an equivalent object. One having its own allocated  memory containing
copies of the original strings.

The way to  assign a Person object to another is
illustrated in Figure 6.
    ------------------------------------------------------------------Insert Figure 6(Private data and public interface functions of the class Person,
     using the `correct' assignment.)about here (file: memory/rightass)------------------------------------------------------------------
    There are several ways to assign a Person object to another.  One way
would be to define a special member function to handle the assignment. The
purpose of this member function would be to create a copy of an object having
its own name, address and phone strings.  Such a member function
could be:
            void Person::assign(Person const &other)
    {
            // delete our own previously used memory
        delete[] d_name;
        delete[] d_address;
        delete[] d_phone;

            // copy the other Person's data
        d_name    = strdupnew(other.d_name);
        d_address = strdupnew(other.d_address);
        d_phone   = strdupnew(other.d_phone);
    }

Using assign we could rewrite the offending function tmpPerson:
            void tmpPerson(Person const &person)
    {
        Person tmp;

            // tmp (having its own memory) holds a copy of person
        tmp.assign(person);

            // now it doesn't matter that tmp is destroyed..
    }

This solution is valid, although it only tackles a symptom.  It
requires the programmer to use a specific member function instead of the
assignment operator. The original problem (assignment produces wild pointers)
is still not solved. Since it is hard to `strictly adhere to a rule' a way to
solve the original problem is of course preferred.

Fortunately a solution exists using operator overloading: the
possibility C++ offers to redefine the actions of an operator in a given
context. Operator overloading was briefly mentioned earlier, when the
operators << and >> were redefined to be used with streams (like
cin, cout and cerr), see section 3.1.4.

Overloading the assignment operator is probably the most common form of
operator overloading in C++. A word of warning is appropriate, though.
The fact that C++ allows operator overloading does not mean that this
feature should indiscriminately be used. Here's what you should keep in mind:
    
    o  operator overloading should be used in situations where an operator
has a defined action, but this default action has undesired side effects in a
given context. A clear example is the above assignment operator in the
context of the class Person.
    o  operator overloading can be used in situations where the operator is
commonly applied and no surprise is introduced when it's redefined. An example
where operator overloading is appropriately used is found in the class
std::string: assigning one string object to another provides the
destination string with a copy of the content of the source string. No
surprises here.
    o  in all other cases a member function should be defined instead of
redefining an operator.

An operator should simply do what it is designed to do. The phrase that's
often encountered in the context of operator overloading is do as the
ints do. The way operators behave when applied to ints is what is
expected, all other implementations probably cause surprises and confusion.
Therefore, overloading the insertion (<<) and extraction (>>)
operators in the context of streams is probably ill-chosen: the stream
operations have nothing in common with bitwise shift operations.


9.3.1.1: The member 'operator=()'

    To add operator overloading to  a class, the class interface is simply
provided  with a (usually public) member function naming the particular
operator. That member function is thereupon implemented.

To overload the assignment operator =, a member operator=(Class const
&rhs) is added to the class interface.  Note that the function name consists
of two parts: the keyword operator, followed by the operator itself.  When
we augment a class interface with a member function operator=, then that
operator is redefined for the class, which prevents the default operator
from being used. In the previous section the function
assign was provided to solve the problems resulting from using the
default assignment operator. Rather than using an ordinary member
function C++ commonly uses a dedicated operator generalizing the
operator's default behavior to the class in which it is defined.

The assign member mentioned before may be redefined as follows (the member
operator= presented below is a first, rather unsophisticated, version of
the overloaded assignment operator.  It will shortly be improved):
            class Person
    {
        public:                             // extension of the class Person
                                            // earlier members are assumed.
            void operator=(Person const &other);
    };

Its implementation could be
            void Person::operator=(Person const &other)
    {
        delete[] d_name;                      // delete old data
        delete[] d_address;
        delete[] d_phone;

        d_name = strdupnew(other.d_name);   // duplicate other's data
        d_address = strdupnew(other.d_address);
        d_phone = strdupnew(other.d_phone);
    }

This member's actions are similar to those of the previously mentioned
member assign, but this member is automatically called when the assignment
operator = is used. Actually there are two ways to
call overloaded operators as shown in the next example:
            void tmpPerson(Person const &person)
    {
        Person tmp;

        tmp = person;
        tmp.operator=(person);  // the same thing
    }

Overloaded operators are seldom called explicitly, but explicit calls must
be used (rather than using the plain operator syntax) when you explicitly
want to call the overloaded operator from a pointer to an object (it is
also possible to dereference the pointer first and then use the plain operator
syntax, see the next example):
            void tmpPerson(Person const &person)
    {
        Person *tmp = new Person;

        tmp->operator=(person);
        *tmp = person;          // yes, also possible...

        delete tmp;
    }


9.4: The `this' pointer

A member function of a given class is always called in combination with an
object of its class. There is always an implicit `substrate' for the function
to act on. C++ defines a keyword, this, to reach this substrate.

The this keyword is a pointer variable that always contains the
address
   of the object for which the member
function was called. The this pointer is implicitly declared by each
member function (whether public, protected, or private). The this
pointer is a constant pointer to an object of the member function's
class. For example, the members of the class Person implicitly declare:
            extern Person *const this;

A member function like Person::name could be implemented in two ways:
with or without using the this pointer:
            char const *Person::name() const    // implicitly using `this'
    {
        return d_name;
    }

    char const *Person::name() const    // explicitly using `this'
    {
        return this->d_name;
    }

The this pointer is seldom explicitly used, but situations do exist
where the this pointer is actually required (cf. chapter
16).


9.4.1: Sequential assignments and this

        C++'s syntax allows for  sequential
assignments, with the assignment operator associating from right to left. In
statements like:
            a = b = c;

the expression b = c is evaluated first, and its result in turn is
assigned to a.

The implementation of the overloaded assignment operator we've encountered
thus far does not permit such constructions, as it returns void.

This imperfection can easily be remedied using the this pointer. The
overloaded assignment operator expects a reference to an object of its
class. It can also return a reference to an object of its class. This
reference can then be used as an argument in sequential assignments.

The overloaded assignment operator commonly returns a reference to the
current object (i.e., *this).  The next version of the overloaded
assignment operator for the class Person thus becomes:
            Person &Person::operator=(Person const &other)
    {
        delete[] d_address;
        delete[] d_name;
        delete[] d_phone;

        d_address = strdupnew(other.d_address);
        d_name = strdupnew(other.d_name);
        d_phone = strdupnew(other.d_phone);

        // return current object as a reference
        return *this;
    }

Overloaded operators may themselves be overloaded. Consider the string
class, having overloaded assignment operators operator=(std::string const
&rhs), operator=(char const *rhs), and several more overloaded
versions. These additional overloaded versions are there to handle different
situations which are, as usual, recognized by their argument types. These
overloaded versions all follow the same mold: when necessary dynamically
allocated memory controlled by the object is deleted; new values are assigned
using the overloaded operator's parameter values and *this is returned.


9.5: The copy constructor: initialization vs. assignment

Consider the class Strings, introduced in section 9.2,
once again. As it contains several primitive type data members as well as a
pointer to dynamically allocated memory it needs a constructor, a destructor,
and an overloaded assignment operator. In fact the class offers two
constructors: in addition to the default constructor it offers a constructor
expecting a char const *const * and a size_t.

Now consider the following code fragment. The statement references are
discussed following the example:
            int main(int argc, char **argv)
    {
        Strings s1(argv, argc);     // (1)
        Strings s2;                 // (2)
        Strings s3(s1);             // (3)

        s2 = s1;                        // (4)
    }


    o  At 1 we see  an initialization.  The object
s1 is initialized using main's parameters: Strings's second
constructor is used.
    o  At 2 Strings's default constructor is used, initializing
an empty Strings object.
    o  At 3 yet another Strings object is created, using a
constructor accepting an existing Strings object. This form of
initializations has not yet been discussed. It is called a
  copy construction and the constructor performing the
initialization is called the copy constructor. Copy constructions are also
encountered in the following form:
            Strings s3 = s1;

This is a construction and therefore an initialization. It is not
an assignment as an assignment needs a left-hand operand that has already
been defined. C++ allows the assignment syntax to be used for constructors
having only one parameter. It is somewhat deprecated, though.
    o  At 4 we see a plain assignment.

In the above example three objects were defined, each using a different
constructor. The actually used constructor was deduced from the constructor's
argument list.

The copy constructor encountered here is new. It does not result in a
compilation error even though it hasn't been declared in the class
interface. This takes us to the following rule:
    
    A copy constructor is (almost) always available, even if it isn't declared
    in the class's interface.

The reason for the `(almost)' is given in section 9.7.1.

The copy constructor made available by the compiler is also called the
trivial copy constructor. Its use can easily be suppressed (using the =
delete idiom). The trivial copy constructor performs a byte-wise copy
operation of the existing object's primitive data to the newly created object,
calls copy constructors to intialize the object's class data members from
their counterparts in the existing object and, when inheritance is used, calls
the copy constructors of the base class(es) to initialize the new object's
base classes.

Consequently, in the above example the trivial copy constructor is
used. As it performs a byte-by-byte copy operation of the object's
primitive type data members that is exactly what happens at statement 3.
By the time s3 ceases to exist its destructor deletes its array of
strings. Unfortunately d_string is of a primitive data type and so it also
deletes s1's data. Once again we encounter wild pointers as a result of an
object going out of scope.

The remedy is easy: instead of using the trivial copy constructor a copy
constructor must explicitly be added to the class's interface and its
definition must prevent the wild pointers, comparably to the way this was
realized in the overloaded assignment operator. An object's dynamically
allocated memory is duplicated, so that it contains its own allocated
data. But note that if a class also reserves extra (raw) memory, i.e., if it
supports extra memory capacity, then that unused extra capacity is not made
available in the copy-constructed object. 

 Copy construction can be used to shed excess
capacity. Copy constructors do not have to shed excess capacity. E.g.,
when copy-constructing std::string objects the destination object defines
the same capacity as the source object. The copy constructor is simpler than
the overloaded assignment operator in that it doesn't have to delete
previously allocated memory. Since the object is going to be created no memory
has already been allocated.

Strings's copy constructor can be implemented as follows:
            Strings::Strings(Strings const &other)
    :
        d_string(new string[other.d_size]),
        d_size(other.d_size)
    {
        for (size_t idx = 0; idx != d_size; ++idx)
            d_string[idx] = other.d_string[idx];
    }

The copy constructor is always called when an object is initialized using
another object of its class. Apart from the plain copy construction that we
encountered thus far, here are other situations where the copy constructor is
used:
    
    o  it is used when a function defines a  class type
value parameter rather than a pointer or a reference. The function's argument
initializes the function's parameter using the copy constructor. Example:
    void process(Strings store) // no pointer, no reference
{
    store.at(3) = "modified";   // doesn't modify `outer'
}

int main(int argc, char **argv)
{
    Strings outer(argv, argc);
    process(outer);
}

o  it is used when a function defines a class type value return type.
Example:
    Strings copy(Strings const &store)
{
    return store;
}

Here store is used to initialize copy's return value. The returned
Strings object is a temporary, anonymous object that may be
immediately used by code calling copy but no assumptions can be made about
its lifetime thereafter.


9.6: Revising the assignment operator

 The overloaded assignment operator has characteristics also encountered with
the copy constructor and the destructor:
    
    o  The copying of (private) data occurs (1) in the copy constructor
and (2) in the overloaded assignment function.
    o  Allocated memory is deleted (1) in the overloaded assignment function
and (2) in the destructor.

The copy constructor and the destructor clearly are required. If the
overloaded assignment operator also needs to return allocated memory and to
assign new values to its data members couldn't the destructor and copy
constructor be used for that?

As we've seen in our discussion of the destructor (section
9.2) the destructor can explicitly be called, but that doesn't
hold true for the (copy) constructor. But let's briefly summarize what an
overloaded assignment operator is supposed to do:
    
    o  It should delete the dynamically allocated memory controlled by the
current object;
    o  It should reassign the current object's data members using a provided
existing object of its class.

The second part surely looks a lot like copy construction. Copy
construction becomes even more attractive after realizing that the copy
constructor also initializes any reference data members the class might
have. Realizing the copy construction part is easy: just define a local object
and initialize it using the assignment operator's const reference parameter,
like this:
            Strings &operator=(Strings const &other)
    {
        Strings tmp(other);
        // more to follow
        return *this;
    }

You may think the optimization operator=(Strings tmp) is attractive,
but let's postpone that for a little while (at least until section 9.7).

Now that we've done the copying part, what about the deleting part? And
isn't there another slight problem as well? After all we copied all right, but
not into our intended (current, *this) object.

At this point it's time to introduce swapping. Swapping two variables
means that the two variables exchange their values. We'll discuss swapping in
detail in the next section, but let's for now assume that we've added a member
swap(Strings &other) to our class Strings. This allows us to
complete String's operator= implementation:
            Strings &operator=(Strings const &other)
    {
        Strings tmp(other);
        swap(tmp);
        return *this;
    }

This implementation of operator= is generic: it can be applied to
every class whose objects are swappable. How does it work?
    
    o  The information in the other object is used to initialize a
local tmp object. This takes care of the copying part of the assignment
operator;
    o  Calling swap ensures that the current object receives its new
values (with tmp receiving the current object's original values);
    o  When operator= terminates its local tmp object ceases to
exist and its destructor is called. As it by now contains the data previously
owned by the current object, the current object's original data are now
destroyed, effectively completing the destruction part of the assignment
operation.

Nice?


9.6.1: Swapping

    Many classes (e.g., std::string) offer swap members allowing us to
swap two of their objects. The Standard Template Library (STL, cf. chapter
18) offers various functions related to swapping. There is even a
swap generic algorithm (cf. section 19.1.55), which is commonly
implemented using the assignment operator. When implementing a swap member
for our class Strings it could be used, provided that all of String's
data members can be swapped. As this is true (why this is true is
discussed shortly) we can augment class Strings with a swap member:
            void Strings::swap(Strings &other)
    {
        std::swap(d_string, other.d_string);
        std::swap(d_size, other.d_size);
    }

Having added this member to Strings the copy-and-swap implementation
of String::operator= can now be used.

When two variables (e.g., double one and double two) are swapped,
each one holds the other one's value after the swap. So, if one == 12.50
and two == -3.14 then after swap(one, two) one == -3.14 and two ==
12.50.

Variables of primitive data types (pointers and the built-in types) can be
swapped, class-type objects can be swapped if their classes offer a swap
member.

So should we provide our classes with a swap member, and if so, how should
it be implemented?

The above example (Strings::swap) shows the standard way to implement
a swap member: each of its data members are swapped in turn. But there are
situations where a class cannot implement a swap member this way, even if the
class only defines data members of primitive data types. Consider the
situation depicted in figure 7.

------------------------------------------------------------------Insert Figure 7(Swapping a linked list)about here (file: memory/fswap)------------------------------------------------------------------

In this figure there are four objects, each object has a pointer pointing to
the next object. The basic organization of such a class looks like this:
            class List
    {
        List *d_next;
        ...
    };

Initially four objects have their d_next pointer set to the next
object: 1 to 2, 2 to 3, 3 to 4. This is shown in the upper half of the
figure. At the bottom half it is shown what happens if objects 2 and 3 are
swapped: 3's d_next point is now at object 2, which still points to 4; 2's
d_next pointer points to 3's address, but 2's d_next is now at object
3, which is therefore pointing to itself. Bad news!

Another situation where swapping of objects goes wrong happens with classes
having data members pointing or referring to data members of the same
object. Such a situation is shown in figure 8.

------------------------------------------------------------------Insert Figure 8(Swapping objects with self-referential data)about here (file: memory/fswap2)------------------------------------------------------------------

Here, objects have two data members, as in the following class setup:
            class SelfRef
    {
        size_t *d_ownPtr;       // initialized to &d_data
        size_t d_data;
    };

The top-half of figure 8 shows two objects; their upper data
members pointing to their lower data members. But if these objects are swapped
then the situation shown in the figure's bottom half is encountered. Here the
values at addresses a and c are swapped, and so, rather than pointing to their
bottom data members they suddenly point to other object's data members. Again:
bad news.

The common cause of these failing swapping operations is easily recognized:
simple swapping operations must be avoided when data members point or refer to
data that is involved in the swapping. If, in figure 8 the a and c
data members would point to information outside of the two objects (e.g., if
they would point to dynamically allocated memory) then the simple swapping
would succeed.

However, the difficulty encountered with swapping SelfRef objects does not
imply that two SelfRef objects cannot be swapped; it only means that we
must be careful when designing swap members. Here is an implementation of
SelfRef::swap:
            void SelfRef::swap(SelfRef &other)
    {
        swap(d_data, other.d_data);
    }

In this implementation swapping leaves the self-referential data member
as-is, and merely swaps the remaining data. A similar swap member could be
designed for the linked list shown in figure 7.


9.6.1.1: Fast swapping

            As we've seen with placement new objects can be constructed in blocks
of memory of sizeof(Class) bytes large. And so, two objects of the same
class each occupy sizeof(Class) bytes.

If objects of our class can be swapped, and if our class's data members do
not refer to data actually involved in the swapping operation then a very fast
swapping method that is based on the fact that we know how large our objects
are  can be implemented.

In this fast-swap method we merely swap the content of the sizeof(Class)
bytes. This procedure may be applied to classes whose objects may be swapped
using a member-by-member swapping operation and can (in practice, although
this probably overstretches the allowed operations as described by the
C++ ANSI/ISO standard) also be used in classes having reference data
members. It simply defines a buffer of sizeof(Class) bytes and performs a
circular memcpy operation. Here is its implementation for a hypothetical
class Class. It results in very fast swapping:
            #include <cstring>

    void Class::swap(Class &other)
    {
        char buffer[sizeof(Class)];
        memcpy(buffer, &other, sizeof(Class));
        memcpy(static_cast<void *>(&other), this,   sizeof(Class));
        memcpy(static_cast<void *>(this), buffer, sizeof(Class));
    }

The static_cast for memcpy's destination address is used to prevent a
compiler complaint: since Class is a class-type, the compiler (rightly)
warns against bluntly copying bytes. But using memcpy is fine if you're
Class's developer and know what you're doing.

Here is a simple example of a class defining a reference data member and
offering a swap member implemented like the one above. The reference data
members are initialized to external streams. After running the program one
contains two hello to 1 lines, two contains two hello to 2 lines
(for brevity all members of Reference are defined inline):

    #include <fstream>
    #include <cstring>

    class Reference
    {
        std::ostream &d_out;

        public:
            Reference(std::ostream &out)
            :
                d_out(out)
            {}
            void swap(Reference &other)
            {
                char buffer[sizeof(Reference)];
                memcpy(buffer, this, sizeof(Reference));
                memcpy(static_cast<void *>(this), &other, sizeof(Reference));
                memcpy(static_cast<void *>(&other), buffer,
                                                    sizeof(Reference));
            }
            std::ostream &out()
            {
                return d_out;
            }
    };

    int main()
    {
        std::ofstream one{ "one" };
        std::ofstream two{ "two" };

        Reference ref1{ one };          // ref1/ref2 hold references to
        Reference ref2{ two };          // the streams

        ref1.out() << "hello to 1\n";   // generate some output
        ref2.out() << "hello to 2\n";

        ref1.swap(ref2);

        ref2.out() << "hello to 1\n";   // more output
        ref1.out() << "hello to 2\n";
    }


Fast swapping should only be used for self-defined classes for which it can be
proven that fast-swapping does not corrupt its objects, when swapped.


9.7: Moving data

Traditionally, C++ offered two ways to assign the information pointed to
by a data member of a temporary object to an lvalue object. Either a copy
constructor or reference counting had to be used. In addition to these two
methods C++ now also supports move semantics, allowing transfer
of the data pointed to by a temporary object to its destination.

Moving information is based on the concept of anonymous (temporary)
data. Temporary values are returned by functions like operator-() and
operator+(Type const &lhs, Type const &rhs), and in general by functions
returning their results `by value' instead of returning references or
pointers.

Anonymous values are always short-lived. When the returned values are
primitive types (int, double, etc.) nothing special happens, but if a
class-type object is returned by value then its destructor can be called
immediately following the function call that produced the value. In any case,
the value itself becomes inaccessible immediately after the call. Of course, a
temporary return value may be bound to a reference (lvalue or rvalue), but as
far as the compiler is concerned the value now has a name, which by itself
ends its status as a temporary value.

In this section we concentrate on anonymous temporary values and show how they
can be used to improve the efficiency of object construction and assignment.
These special construction and assignment methods are known as move
construction and move assignment. Classes supporting move operations are
called move-aware.

Classes allocating their own memory usually benefit from becoming
move-aware. But a class does not have to use dynamic memory allocation before
it can benefit from move operations. Most classes using composition (or
inheritance where the base class uses composition) can benefit from move
operations as well.

Movable parameters for class Class take the form Class &&tmp. The
parameter is an rvalue reference, and a rvalue reference only binds to an
anonymous temporary value. The compiler is required to call functions offering
movable parameters whenever possible. This happens when the class defines
functions supporting Class && parameters and an anonymous temporary value
is passed to such functions. Once a temporary value has a name (which already
happens inside functions defining Class const & or Class &&tmp
parameters as within such functions the names of these parameters are
available) it is no longer an anonymous temporary value, and within such
functions the compiler no longer calls functions expecting anonymous
temporary values when the parameters are used as arguments.

The next example (using inline member implementations for brevity) illustrates
what happens if a non-const object, a temporary object and a const object are
passed to functions fun for which these kinds of parameters were
defined. Each of these functions call a function gun for which these kinds
of parameters were also defined. The first time fun is called it (as
expected) calls gun(Class &). Then fun(Class &&) is called as its
argument is an anonymous (temporary) object. However, inside fun the
anonymous value has received a name, and so it isn't anonymous
anymore. Consequently, gun(Class &) is called once again. Finally
fun(Class const &) is called, and (as expected) gun(Class const &) is
now called.
    #include <iostream>

using namespace std;

class Class
{
    public:
    Class()
    {};
    void fun(Class const &other)
    {
        cout << "fun: Class const &\n";
        gun(other);
    }
    void fun(Class &other)
    {
        cout << "fun: Class &\n";
        gun(other);
    }
    void fun(Class &&tmp)
    {
        cout << "fun: Class &&\n";
        gun(tmp);
    }
    void gun(Class const &other)
    {
        cout << "gun: Class const &\n";
    }
    void gun(Class &other)
    {
        cout << "gun: Class &\n";
    }
    void gun(Class &&tmp)
    {
        cout << "gun: Class &&\n";
    }
};

int main()
{
    Class c1;

    c1.fun(c1);
    c1.fun(Class());

    Class const c0;
    c1.fun(c0);
}


Generally it is pointless to define a
  function having an 
 rvalue reference return type. The compiler decides whether or not to use
an overloaded member expecting an rvalue reference on the basis of the
provided argument. If it is an anonymous temporary it calls the function
defining the rvalue reference parameter, if such a function is available. An
rvalue reference return type is used, e.g., with the std::move call, to
keep the rvalue reference nature of its argument, which is known to be a
temporary anonymous object. Such a situation can be exploited also in a
situation where a temporary object is passed to (and returned from) a function
which must be able to modify the temporary object. The alternative, passing a
const &, is less attractive as it requires a const_cast before the
object can be modified. Here is an example:
            std::string &&doubleString(std::string &&tmp)
    {
        tmp += tmp;
        return std::move(tmp);
    }

This allows us to do something like
            std::cout << doubleString("hello "s);

to insert hello hello  into cout.

The compiler, when selecting a function to call applies a fairly simple
algorithm, and also considers copy elision. This is covered shortly (section
9.8).


9.7.1: The move constructor (dynamic data)

    Our class Strings has, among other members, a data member string
*d_string. Clearly, Strings should define a copy constructor, a
destructor and an overloaded assignment operator.

Now consider the following function loadStrings(std::istream &in)
extracting the strings for a Strings object from in. Next, the
Strings object filled by loadStrings is returned by value. The
function loadStrings returns a temporary object, which can then used to
initialize an external Strings object:
            Strings loadStrings(std::istream &in)
    {
        Strings ret;
        // load the strings into 'ret'
        return ret;
    }
    // usage:
    Strings store(loadStrings(cin));

In this example two full copies of a Strings object are required:
    
    o  initializing loadString's value return type from its local
Strings ret object;
    o  initializing store from loadString's return value

We can improve the above procedure by defining a
 move constructor.  Here is the declaration of the Strings class move
constructor:
            Strings(Strings &&tmp);

Move constructors of classes using dynamic memory allocation are allowed
to assign the values of pointer data members to their own pointer data members
without requiring them to make copies of the source's data. Next, the
temporary's pointer value is set to zero to prevent its destructor from
destroying data now owned by the just constructed object. The move constructor
has grabbed or
  stolen the data from the temporary object. This is
OK as the temporary object cannot be referred to again (as it is anonymous, it
cannot be accessed by other code) and we may assume that the temporary objects
cease to exist shortly after the move-constructor's call. Here is an
implementation of Strings move constructor:
            Strings::Strings(Strings &&tmp)
    :
        d_string(tmp.d_string),
        d_size(tmp.d_size),
        d_capacity(tmp.d_capacity)
    {
        tmp.d_string = 0;
        tmp.d_capacity = 0;
        tmp.d_size = 0;
    }

    Move construction (in general: moving) must leave the object from which
information was moved in a valid state. It is not specified in what way that
valid state must be realized, but a good rule of thumb is to return the
object to its default constructed state. In the above illustration of a move
constructor tmp.d_size, tmp.d_capacity and tmp.d_string are all set to
0. It's up to the author of the Strings class to decide whether or not
the String's members can all be set to 0. If there's a default constructor
doing exactly that then assigning zeroes is fine. If d_capacity is doubled
once d_size == d_capacity then setting d_capactiy to 1 and letting
d_string point to a newly allocated block of raw memory the size of a
string might be attractive. The source object's capacity might even
remain as-is. E.g., when moving std::string objects the source object's
capacity isn't altered.

In section 9.5 it was stated that the copy constructor is almost
    
always available. Almost always as the declaration of a move constructor
suppresses the default availability of the copy constructor. The default copy
constructor is also suppressed if a move assignment operator is declared
(cf. section 9.7.3).

The following example shows a simple class Class, declaring a move
constructor. In the main function following the class interface a
Class object is defined which is then passed to the constructor of a
second Class object. Compilation fails with the compiler reporting:
            error: cannot bind 'Class' lvalue to 'Class&&'
    error:   initializing argument 1 of 'Class::Class(Class&&)'

class Class
{
    public:
        Class() = default;
        Class(Class &&tmp)
        {}
};

int main()
{
    Class one;
    Class two{ one };
}


The cure is easy: after declaring a (possibly default) copy
constructor the error disappears:
        class Class
{
    public:
        Class() = default;
        Class(Class const &other) = default;
        Class(Class &&tmp)
        {}
};

int main()
{
    Class one;
    Class two{ one };
}



9.7.2: The move constructor (composition)

        Classes not using pointer members pointing to memory controlled by its
objects (and not having base classes doing so, see chapter 13)
may also  benefit from overloaded members expecting rvalue references. The
class benefits from move operations when one or more of the composed data
members themselves support move operations.

Move operations cannot be implemented if the class type of a composed data
member does not support moving or copying. Currently, stream classes fall
into this category.

An example of a move-aware class is the class std:string. A class
Person could use composition by defining std::string d_name and
std::string d_address. Its move constructor would then have the following
prototype:
            Person(Person &&tmp);

However, the following implementation of this move constructor is
incorrect:
            Person::Person(Person &&tmp)
    :
        d_name(tmp.d_name),
        d_address(tmp.d_address)
    {}

It is incorrect as string's copy constructors rather than
string's move constructors are called. If you're wondering why this
happens then remember that move operations are only performed for
anonymous objects. To the compiler anything having a name isn't anonymous. And
so, by implication, having available a rvalue reference does not mean that
we're referring to an anonymous object. But we know that the move
constructor is only called for anonymous arguments. To use the corresponding
string move operations we have to inform the compiler that we're talking
about anonymous data members as well. For this a cast could be used (e.g.,
static_cast<Person &&>(tmp)), but the C++-0x standard provides the
function
 std::move to anonymize a named object. The correct
implementation of Person's move construction is, therefore:
            Person::Person(Person &&tmp)
    :
        d_name( std::move(tmp.d_name) ),
        d_address( std::move(tmp.d_address) )
    {}

The function std::move is (indirectly) declared by many header
files.  If no header is already declaring std::move then include
utility.

When a class using composition not only contains class type data members
but also other types of data (pointers, references, primitive data types),
then these other data types can be initialized as usual. Primitive data type
members can simply be copied; references can be initialized as usual and
pointers may use move operations as discussed in the previous section.

The compiler never calls move operations for variables having names.  Let's
consider the implications of this by looking at the next example, assuming
the class Class offers a move constructor and a copy constructor:
            Class factory();

    void fun(Class const &other);   // a
    void fun(Class &&tmp);          // b

    void callee(Class &&tmp)
    {
        fun(tmp);                   // 1
    }

    int main()
    {
        callee(factory());
    }


    o  At 1 function a is called. At first sight this might be surprising,
but fun's argument is not an anonymous temporary object but a
named temporary object.

Realizing that fun(tmp) might be called twice the compiler's choice is
understandable. If tmp's data would have been grabbed at the first call,
the second call would receive tmp without any data. But at the last call
we might know that  tmp is never used again and so we might like to ensure
that fun(Class &&) is called. For this, once again, std::move is used:
            fun(std::move(tmp));            // last call!


9.7.3: Move-assignment

    In addition to the overloaded assignment operator a move assignment
operator may be implemented for classes supporting move operations. In this
case, if the class supports swapping the implementation is surprisingly
simple. No copy construction is required and the move assignment operator can
simply be implemented like this:
            Class &operator=(Class &&tmp)
    {
        swap(tmp);
        return *this;
    }

If swapping is not supported then the assignment can be performed for each
of the data members in turn, using std::move as shown in the previous
section with a class Person. Here is an example showing how to do this
with that class  Person:
            Person &operator=(Person &&tmp)
    {
        d_name = std::move(tmp.d_name);
        d_address = std::move(tmp.d_address);
        return *this;
    }

As noted previously (section 9.7.1) declaring a move assignment
operator suppresses the default availability of the copy constructor. It is
made available again by declaring the copy constructor in the class's
interface (and of course by providing an explicit implementation or by using
the = default default implementation).


9.7.4: Revising the assignment operator (part II)

    Now that we've familiarized ourselves with the overloaded assignment operator
and the move-assignment, let's once again have a look at their
implementations for a class Class, supporting swapping through its
swap member. Here is the generic implementation of the overloaded
assignment operator:
            Class &operator=(Class const &other)
    {
        Class tmp{ other };
        swap(tmp);
        return *this;
    }

and this is the move-assignment operator:
            Class &operator=(Class &&tmp)
    {
        swap(tmp);
        return *this;
    }

They look remarkably similar in the sense that the overloaded assignment
operator's code is identical to the move-assignment operator's code once a
copy of the other object is available. Since the overloaded assignment
operator's tmp object really is nothing but a temporary Class object
we can use this fact by implementing the overloaded assignment operator in
terms of the move-assignment. Here is a second revision of the overloaded
assignment operator:
            Class &operator=(Class const &other)
    {
        Class tmp{ other };
        return *this = std::move(tmp);
    }


9.7.5: Moving and the destructor

        Once a class becomes a move-aware class one should realize that its
destructor still performs its job as implemented. Consequently, when moving
pointer values from a temporary source to a destination the move constructor
commonly ensures that the temporary object's pointer value is set to zero, to
prevent doubly freeing memory.

If a class defines pointers to pointer data members there usually is not
only a pointer that is moved, but also a size_t defining the number of
elements in the array of pointers.

Once again, consider the class Strings. Its destructor is implemented
like this:
            Strings::~Strings()
    {
        for (string **end = d_string + d_size; end-- != d_string; )
            delete *end;
        delete[] d_string;
    }

The move constructor (and other move operations!) must realize that the
destructor not only deletes d_string, but also considers d_size. When
d_size and d_string are set to 0, the destructor (correctly) won't
delete anything. In addition, when the class uses 
capacity-doubling once d_size ==
d_capacity then the move constructor can still reset the source's
(d_capacity) to 0, since it's known that the tmp object ceases to
exist following the move-assignment:
            Strings::Strings(Strings &&tmp)
    :
        d_string(tmp.d_string),
        d_size(tmp.d_size),
        d_capacity(tmp.d_capacity)
    {
        tmp.d_string = 0;
        tmp.d_capacity = 0;
        tmp.d_size = 0;
    }

Other variations are possible as well. The bottom line: the move construcor
must ensure that after the destination object has grabbed the source object's
data the source object remains in a valid state. That's easily accomplished by
assigning the same values to its data members as set by the default
constructor.


9.7.6: Move-only classes

    Classes may very well allow move semantics without offering copy
semantics. Most stream classes belong to this category. Extending their
definition with move semantics greatly enhances their usability. Once move
semantics becomes available for such classes, so called
 factory functions 
    (functions returning an object constructed by the function) can easily be
implemented. E.g.,
            // assume char *filename
    ifstream inStream(openIstream(filename));

For this example to work an ifstream constructor must offer a move
constructor. This ensures that only one object refers to the open istream.

Once classes offer move semantics their objects can also safely be stored
in standard containers (cf. chapter 12). When such containers
perform reallocations (e.g., when their sizes are enlarged) they use the
object's move constructors rather than their copy constructors.  As move-only
classes suppress copy semantics containers storing objects of move-only
classes implement the correct behavior in that it is impossible to assign such
containers to each other.


9.7.7: Default move constructors and assignment operators

    As we've seen, classes by default offer a copy constructor and assignment
operator. These class members are implemented so as to provide basic support:
data members of primitive data types are copied byte-by-byte, but for class
type data members their corresponding copy constructors c.q. assignment
operators are called.  The compiler also attempts to provide default
implementations for move constructors and move assignment operators. However,
the default constructors and assignment operators cannot always be provided.

These are the rules the compiler applies when deciding what to provide or not
to provide:
    
    o  If the copy constructor, the copy assignment operator,
or a destructor is declared (even if it's declared using = default),
then the default move constructor and move assignment operator are suppressed;
their use is replaced by the corresponding copy operation (constructor or
assignment operator);

o  If the move constructor or the move assignment operator is declared
then the copy constructor and the copy assignment operator are implicitly
declared as deleted, and can therefore not be used anymore;

o  If either the move constructor or the move assignment
operator is declared, then (in addition to suppressing the copy operations)
the default implementation of the other move-member is also suppressed;

o  In all other cases the default copy and move constructors and the
default copy and assignment operators are provided.

If default implementations of copy or move constructors or assignment
operators are suppressed, but they should be available, then it's easy to
provide the default implementations by specifying the required signatures, to
which the specification `= default' is added.

Here is an example of a class offering all defaults: constructor, copy
constructor, move constructor, copy assignment operator and move assignment
operator:
            class Defaults
    {
        int d_x;
        Mov d_mov;
    };

Assuming that Mov is a class offering move operations in addition
to the standard copy operations, then the following actions are performed
on the destination's d_mov and d_x:
            Defaults factory();

    int main()
    {                              Mov operation:    d_x:
                                   ---------------------------
      Defaults one;                Mov(),            undefined
      Defaults two(one);           Mov(Mov const &), one.d_x
      Defaults three(factory());   Mov(Mov &&tmp),   tmp.d_x

      one = two;                   Mov::operator=(   two.d_x
                                        Mov const &),

      one = factory();             Mov::operator=(   tmp.d_x
                                        Mov &&tmp)
    }

If, Defaults declares at least one constructor (not being the copy- or
move constructor) as well as the copy assignment operators then only the
default copy- and declared assignment operator are available. E.g.:

    class Defaults
    {
        int d_x;
        Mov d_mov;

        public:
            Defaults(int x);
            Defaults &operator=(Default const &rhs);
    };

    Defaults factory();

    int main()
    {                              Mov operation:    resulting d_x:
                                   --------------------------------
      Defaults one;                ERROR: not available
      Defaults two(one);           Mov(Mov const &),        one.d_x
      Defaults three(factory());   Mov(Mov const &),        one.d_x

      one = two;                   Mov::operatpr=(          two.d_x
                                        Mov const &)
      one = factory();             Mov::operator=(          tmp.d_x
                                        Mov const &)
    }

To reestablish the defaults, append = default to the appropriate
declarations:
            class Defaults
    {
        int d_x;
        Mov d_mov;

        public:
            Defaults()               = default;
            Defaults(int x);
            // Defaults(Default const &) remains available (by default)

            Defaults(Defaults &&tmp) = default;

            Defaults &operator=(Defaults const &rhs);
            Defaults &operator=(Defaults &&tmp) = default;
    };

Be cautious declaring defaults, as default implementations copy data members
of primitive types byte-by-byte from the source object to the destination
object. This is likely to cause problems with pointer type data members.

The = default suffix can only be used when declaring constructors or
assignment operators in the class's public section.


9.7.8: Moving: implications for class design

    Here are some general rules to apply when designing classes offering value
semantics (i.e., classes whose objects can be used to initialize other
objects of their class and that can be asssigned to other objects of their
class):
    
    o  Classes using pointers to dynamically allocated memory, owned by the
class's objects must be provided with a copy constructor, an overloaded copy
assignment operator and a destructor;
    o  Classes using pointers to dynamically allocated memory, owned by the
class's objects, should be provided with a move constructor and a move
assignment operator;
    o  Classes using composition may benefit from move constructors and
move assignment operators as well. Some classes support neither move nor copy
construction and assignment (for example: stream classes don't). If your
class contains data members of such class types then defining move operations
is pointless.

In the previous sections we've also encountered an important  design
        
    principle that can be applied to move-aware classes:
    
    Whenever a member of a class receives a const & to an object of its
own class and creates a copy of that object to perform its actual actions on,
then that function's implementation can be implemented by an overloaded
function expecting an rvalue reference.

The former function can now call the latter by passing std::move(tmp)
to it. The advantages of this design principle should be clear: there is only
one implementation of the actual actions, and the class automatically becomes
move-aware with respect to the involved function.

We've seen an initial example of the use of this principle in section
9.7.4. Of course, the principle cannot be applied to the copy
constructor itself, as you need a copy constructor to make a copy. The copy-
and move constructors must always be implemented independently from each
other.


9.8: Copy Elision and Return Value Optimization

When the compiler selects a member function (or constructor) it applies a
simple set of rules, matching arguments with parameter types.

Below two tables are shown. The first table should be used in cases where a
function argument has a name, the second table should be used in cases where
the argument is anonymous. In each table select the const or non-const column
and then use the topmost overloaded function that is available having the
specified parameter type.

The tables do not handle functions defining value parameters.  If a function
has overloads expecting, respectively, a value parameter and some form of
reference parameter the compiler reports an ambiguity when such a function is
called. In the following selection procedure we may assume, without loss of
generality, that this ambiguity does not occur and that all parameter types
are reference parameters.

Parameter types matching a function's argument of type T if the argument
is:
    
    o  a named  argument (an lvalue or a named rvalue)
        

    

        

             2 


        
                 non-const    const 

        

        

        
             Use the topmost   (T &) 
        

        
             available function   (T const &)    (T const &) 

        

        

    

        

        Example: for an int x argument a function fun(int &) is
selected rather than a function fun(int const &). If no fun(int &) is
available the fun(int const &) function is used. If neither is available
(and fun(int) hasn't been defined instead) the compiler reports an error.

o  an anonymous argument (an anonymous temporary or a literal value)
        

    

        

             2 


        
                non-const    const 

        

        

        
             Use the topmost   (T &&) 
        

        
             available function   (T const &)    (T const &) 

        

        

        


        Example: when the return value of an int arg() function is passed
to a function fun for which various overloaded versions are available
fun(int &&) is selected. If this function is unavailable but fun(int
const &) is, then the latter function is used.  If none of these two
functions is available the compiler reports an error.

The tables show that eventually all arguments can be used with a
function specifying a T const & parameter. For anonymous arguments a
similar catch all is available having a higher priority: T const &&
matches all anonymous arguments. Functions having this signature are normally
not defined as their implementations are (should be) identical to the
implementations of the functions expecting a T const & parameter. Since
the temporary can apparently not be modified a function defining a T const
&& parameter has no alternative but to copy the temporary's resources. As
this task is already performed by functions expecting a T const &, there
is no need for implementing functions expecting T const && parameters, and
it's considered bad style if you do.

As we've seen the move constructor grabs the information from a temporary
for its own use. That is OK as the temporary is going to be destroyed after
that anyway. It also means that the temporary's data members are
modified.

Having defined appropriate copy and/or move constructors it may be
somewhat surprising to learn that the compiler may decide to stay clear of a
copy or move operation. After all making no copy and not moving is
more efficient than copying or moving.

The option the compiler has to avoid making copies (or perform move
operations) is called copy elision or return value optimization. In
all situations where copy or move constructions are appropriate the compiler
may apply copy elision. Here are the rules. In sequence the compiler considers
the following options, stopping once an option can be selected:
    
    o  if a copy or move constructor exists, try copy elision
    o  if a move constructor exists, move.
    o  if a copy constructor exists, copy.
    o  report an error

All modern compilers apply copy elision. Here are some examples where it may
be encountered:
            class Elide;

    Elide fun()         // 1
    {
        Elide ret;
        return ret;
    }

    void gun(Elide par);

    Elide elide(fun()); // 2

    gun(fun());         // 3


    o  At 1 ret may never exist. Instead of using ret and copying
ret eventually to fun's return value it may directly use the area used
to contain fun's return value.
    o  At 2 fun's return value may never exist. Instead of defining an
area containing fun's return value and copying that return value to
elide the compiler may decide to use elide to create fun's return
value in.
    o  At 3 the compiler may decide to do the same for gun's par
parameter: fun's return value is directly created in par's area, thus
eliding the copy operation from fun's return value to par.


9.9: Unrestricted Unions

Standard (C-type) unions can only have fields of basic types, like int,
double and pointers. C++ extends the C-type union concept by offering 
    unrestricted unions.

Unrestricted unions also allow data fields of types for which non-trivial copy
constructors were defined. Such data fields commonly are of class-types. Here
is an example of such an unrestricted union:
            union Union
    {
        int u_int;
        std::string u_string;
    };

One of its fields is defined as a std::string (having a constructor),
turning this union into an unrestricted union.  As an unrestricted union
defines at least one field of a type having a constructor the question becomes
how these unions can be constructed and destroyed.

The destructor of a union consisting of, e.g. a std::string and an
int should of course not call the string's destructor if the
union's last (or only) use referred to its int field. Likewise, when
the std::string field is used, and processing switches from the
std::string to the int field, std::string's destructor
should be called before any assignment to the int field takes place.

The compiler does not solve the issue for us, and in fact does not at all
implement default constructors or destructors for unrestricted unions. If we
try to define an unrestricted union like the one shown above, an error message
is issued. E.g.,
            error: use of deleted function 'Union::Union()'
    error: 'Union::Union()' is implicitly deleted because the default
            definition would be ill-formed:
    error: union member 'Union::u_string' with non-trivial
            'std::basic_string<...>::basic_string() ...'


9.9.1: Implementing the destructor

    Although the compiler won't provide (default) implementations for constructors
and destructors of unrestricted unions, we can (and must). The task
isn't difficult, but there are some caveats.

Consider our unrestricted union's destructor. It clearly should destroy
u_string's data if that is its currently active field; but it should do
nothing if u_int is its currently active field. But how does the
destructor know what field to destroy? It doesn't, as the unrestricted union
contains no information about what field is currently active.

This problem is tackled by embedding the unrestricted union in a larger
aggregate (like a class or a struct) where it becomes a regular data
member. We still consider the unrestricted union a data type by itself, but
its use requires caution. The surrounding class is provided with a d_field
data member keeping track of the currently active union-field. The d_field
value is an enumeration value which is defined by the union. The actual use of
the unrestricted union is completely controlled by the aggregate, freeing the
aggregate's users from any administration related to the unrestricted union.

Using this design we start out with an explicit and empty implementation of
the destructor, as there's no way to tell the destructor itself what field to
destroy:
            Data::Union::~Union()
    {};

Nevertheless, unrestricted unions must properly destroy their class-type
fields. Since an unrestricted union itself doesn't know what its active field
is, it must be informed about that by its surrounding class. To simplify the
generalization to other types a static array of pointers to functions
destroying the current field's value is used. This array is defined in the
union's private section as
            static void (Union::*s_destroy[])();

and it is initialized as:
            void (Union::*Union::s_destroy[])() = 
    {
        &Union::destroyText,
        &Union::destroyValue
    };

Primitive data types normally don't need any special attention when they
go out of scope, so destroyValue can be defined as an empty function:
            void Union::destroyValue()
    {}

On the other hand, the member destroyText must explicitly call
u_text's destructor:
            void Union::destroyText()
    {
        u_text.std::string::~string();
    }

Proper destruction can now be realized by a single function void
destroy(Field field) which simply calls the appropriate function:
            void Union::destroy(Field type)
    {
        (this->*s_destroy[type])();
    }

Since the unrestricted union is defined as a data member of a surrounding
class, the surrounding class's destructor is responsible for the proper
destruction of its unrestricted union. As the surrounding class keeps track of
the currently active unrestricted union's field its implementation is easy:
            Data::~Data()
    {
        d_union.destroy(d_field);
    }


9.9.2: Embedding an unrestricted union in a surrounding class

    The unrestricted union becomes a data member of the surrounding aggregate
(e.g., class Data). The class Data is provided with a data member
Union::Field d_field and Data's users might query the currently active
field from, e.g., an accessor field:

    class Data
    {
        Union::Field d_field;
        Union d_union;
    
        public:
            Data(int value = 0);
            Data(Data const &other);
            Data(Data &&tmp);
            Data(std::string const &text);

            ~Data();                // empty body

            Union::Field field() const;
            ...
    };

Data's constructors receive int or string values. To pass these
values to d_union, we need Union constructors for the various union
fields.

The unrestricted union itself starts out like this:
            union Union
    {
        enum Field
        {
            TEXT,
            VALUE
        };
    
        private:
            std::string u_text;
            int u_value;
    
        public:
            Union(Union const &other) = delete;
            ~Union();               // empty
    
            Union(int value);
            Union(std::string const &text);

            Union(Union const &other, Field type);
            Union(Union &&tmp, Field type);
            ...
    };

The last two Union constructors are comparable to the standard copy-
and move constructors. With unrestricted unions, however, the existing union's
actual type needs to be specified so that the correct field is initialized.
To simplify the generalization to other types we apply a procedure that is
comparable to the procedure we followed for destroying an understricted union:
we define a static array of pointers to copy-functions. This array is declared
in the union's private section as
            static void (Union::*s_copy[])(Union const &other);

and it is defined as:
            void (Union::*Union::s_copy[])(Union const &other) = 
    {
        &Union::copyText,
        &Union::copyValue
    };

The copyText and copyValue private members are responsible for
copying other's data fields. However, there is a little snag. Although
basic types can directly be assigned, class-type fields cannot.  Destination
fields cannot be initialized using member initializers as the field to
initialize depends on the Field type that's passed to the
constructor. Because of that the initialization must be performed inside the
constructors' bodies. At that point the data fields are
merely a series of uninitialized bytes, and so placement new is used to
copy-construct class-type fields. Here are the implementations of the copy
functions:
            void Union::copyValue(Union const &other)
    {
        u_value = other.u_value;
    }

    void Union::copyText(Union const &other)
    {
        new(&u_text) string{ other.u_text };
    }

When implementing the union's move constructor other considerations must be
taken into account. Since we're free to do whatever we want with the move
constructor's Union &&tmp object, we can simply grab its current field,
and store a VALUE type of value into tmp. For that we use the
Union's swap facility, the current object's field, another Union
object, and the other Union's field type (swapping is discussed in the
next section). Of course, if there isn't any primitive typed field this
doesn't work. In that case field-specific move functions must be used,
comparable to the ones used when copy-constructing a Union object.

Now we're ready for the constructors' implementations:
            Union::Union(std::string const &text)
    :
        u_text(text)
    {}
    
    Union::Union(int value)
    :
        u_value(value)
    {}

    Union::Union(Union &&tmp, Field type)
    {
        swap(VALUE, tmp, type);
    }

    Union::Union(Union const &other, Field type)
    {
        (this->*s_copy[type])(other);
    }

    Data::Data(Data const &other)
    :
        d_field(other.d_field),
        d_union(other.d_union, d_field)
    {}
    
    Data::Data(Data &&tmp)
    :
        d_field(Union::VALUE),
        d_union(0)
    {
        swap(tmp);      // see the next section
    }

    Data::Data(int value)
    :
        d_field(Union::VALUE),
        d_union(value)
    {}


9.9.3: Swapping unrestricted unions

    Unrestricted unions should define a non-throwing swap member. It needs
three arguments: the current object's field, another union object, and that
union's field. The prototype of our unrestricted union's swap member,
therefore, is:
            void swap(Field current, Union &other, Field next);

To implement it similar considerations as encountered with the copy
constructor apply. An unrestricted union having k fields must support k
* k different swap situations. Representing these in a k * k matrix we
note that the diagonal elements refer to swapping identical elements for which
no special considerations apply (assuming swapping of identical data types is
supported). The lower-triangle elements are identical to their transposed
upper-triangle elements, and so they can use those elements after reverting
the current and other union objects. All field-specific swap functions can be
organized in a k x k static matrix of pointers to swapping members. For
Union the declaration of that matrix is
            static void (Union::*s_swap[][2])(Union &other);

and its definition is
            void (Union::*Union::s_swap[][2])(Union &other) = 
    {
      {  &Union::swap2Text,     &Union::swapTextValue},
      {  &Union::swapValueText, &Union::swap2Value},
    };

The diagonal and lower-triangle elements are
straightforwardly implemented. E.g.,
            void Union::swap2Text(Union &other)
    {
        u_text.swap(other.u_text);
    }

    void Union::swapValueText(Union &other)
    {
        other.swapTextValue(*this);
    }

but implementing the upper-triangle elements requires some thought. To
install a class-type field placement new must again be used. But this time
we're not copying but moving, as the current object is going to lose its
content. Like swapping, moving should always succeed. Following the move
construction the other object has received the current object's data. As the
current object keeps its valid state after the move, it must also explicitly
be destroyed to properly end its lifetime. Here is the implementation of
swapTextValue:
            void Union::swapTextValue(Union &other)
    {
        int value = other.u_value;  // save the int value
                                    // install string at other
        new(&other.u_text) string{ std::move(u_text) }; 
        u_text.~string();           // remove the old field's union
    
        u_value = value;            // store current's new value
    }

When an unrestricted union has multiple class-type fields then when
swapping move construction must be applied to both unrestricted unions. This
requires a temporary. Assume an unrestricted union supports fields of classes
This and That then to swap unrestricted unions using, respectively the
This and That fields we do as follows:
            void ThisThat::swapThisThat(ThisThat &other)
    {
        This tmp{ std::move(u_this) };  // save the current object
        u_this.~This();                 // properly destroy it

                                        // install the other object at
                                        // this object
        new(&u_that) That{ std::move(other.u_that) };
        other.u_that.~That();           // properly destroy the other 
                                        // object

                                        // install this object's original 
                                        // value at the other object
        new(&other.u_this) This{ std::move(tmp) };

    }                                   // tmp is automatically destroyed

Now that unrestricted unions can be swapped, their swap member can be
used by swap members of surrounding classes. E.g.,
            void Data::swap(Data &other)
    {
        d_union.swap(d_field, other.d_union, other.d_field);
    
        Union::Field field = d_field;   // swap the fields
        d_field = other.d_field;
        other.d_field = field;
    }


9.9.4: Assignment

    There are two ways to assign a Data object to another one: copy assignment
and move assignment. Their implementations are standard:
            Data &Data::operator=(Data const &other)    // copy-assignment
    {
        Data tmp{ other };
        swap(tmp);
        return *this;
    }

    Data &Data::operator=(Data &&tmp)           // move-assignment
    {
        swap(tmp);
        return *this;
    }

Since swap has already been defined the assignment operators need no
further attention: they are implemented using their standard implementations.

When unrestricted unions are used outside of surrounding classes a
situation may arise where two unrestricted unions are directly assigned to
each other. In that case the unions' active fields must somehow be
available. Since operator= can only be defined having one parameter,
simply passing an unrestricted union as its rvalue would lack information
about the lvalue's and rvalue's active fields. Instead two members are
suggested: copy, doing copy assignment and move, doing move
assignment. Their implementations closely resemble those of the standard
assignment operators:
            void Union::copy(Field type, Union const &other, Field next)
    {
        Union tmp{ other, next };   // create a copy
        swap(type, tmp, next);      // swap *this and tmp
        tmp.destroy(type);          // destroy tmp
    }
        
    void Union::move(Field type, Union &&tmp, Field next)
    {
        swap(type, tmp, next);
    }

In the source distribution you'll find a directory
yo/memory/examples/unions. It contains a small demo-program in which
Union and Data are used.


9.10: Aggregate Data Types

C++ inherited the struct concept from C and extended it with the class
concept. Structs are still used in C++, mainly to store and pass around
aggregates of different data types. A commonly used term for these structs is
aggregate (in some languages known as plain old data
(pod)). Aggregates are commonly used in C++ programs to merely combine
data in dedicated (struct) types. E.g., when a function must return a
double, a bool and std::string these three different data types
may be aggregated using a struct that merely exists to pass along
values. Data protection and functionality is hardly ever an issue. For such
cases C and C++ use structs. But as a C++ struct is just a
class with special access rights some members (constructors, destructor,
overloaded assignment operator) may implicitly be defined. Aggregates
capitalize on this concept by requiring that their definitions remain as
simple as possible, showing the following characteristics:
    
    o  there are no user provided constructors or user provided inherited
        constructors  (cf. chapter 13); 
    o  their non-static data members have public access rights;
    o  they have no virtual members;
    o  when using inheritance, the base classes aren't virtual and only
        public inheritance is used.

Aggregates can also be arrays, in which case the array elements are the
aggregate's elements. If an aggregate is a struct its direct base classes
are its elements (if any), followed by the struct's data members, in their
declaration order. Here is an example:
            struct Outer
    {
        struct Inner
        {
            int     d_int;
            double  d_double;
        };

        std::string d_string;
        Inner d_inner;
    };

    Outer out{ "hello", { 1, 12.5} };

Outer out's d_string is initialized with hello", its d_inner
member has two data members: d_int is initialized to 1, d_double to
12.5.

(Designated) initializer lists can also be used (cf. section
3.3.5). Also, often structured binding declarations (cf. section
3.3.7.1) can be used to avoid explicitly defining an aggregate data
type.


9.11: Conclusion

Four important extensions to classes were introduced in this chapter: the
destructor, the copy constructor, the move constructor and
the overloaded assignment operator. In addition the importance of
swapping, especially in combination with the overloaded assignment
operator, was stressed.

Classes having pointer data members, pointing to dynamically allocated memory
controlled by the objects of those classes, are
potential sources of memory leaks. The extensions introduced in this
chapter implement the  standard defense against such memory leaks.

Encapsulation (data hiding) allows us to ensure that the object's data
integrity is maintained. The automatic activation of constructors and
destructors greatly enhance our capabilities to ensure the data integrity of
objects doing dynamic memory allocation.

A simple conclusion is therefore that classes whose objects allocate memory
controlled by themselves must at least implement a
destructor, an overloaded assignment operator and a
 copy constructor. Implementing a move constructor remains
optional, but it allows us to use factory functions with classes not
allowing copy construction and/or assignment.

In the end, assuming the availability of at least a copy or move constructor,
the compiler might avoid them using copy elision. The compiler is free to
use copy elision wherever possible; it is, however, never a requirement. The
compiler may therefore always decide not to use copy elision.  In all
situations where otherwise a copy or move constructor would have been used the
compiler may consider to use copy elision.



Chapter 10: Exceptions

C supports several ways for a program to react to situations breaking
the normal unhampered flow of a program:
    
    o  The function may notice the abnormality and issue a message. This is
probably the least disastrous reaction a program may show.
    o  The function in which the abnormality is observed may decide to stop
its intended task, returning an error code to its caller. This is a great
example of postponing decisions: now the calling function is faced with
a problem. Of course the calling function may act similarly, by passing the
error code up to its caller.
    o  The function may decide that things are going out of hand, and may
call exit to terminate the program completely. A tough way to handle a
problem if only because the destructors of local objects aren't activated.
    o  The function may use a combination of the functions setjmp and
longjmp to enforce non-local exits. This mechanism implements a kind
of goto jump, allowing the program to continue at an outer level,
skipping the intermediate levels which would have to be visited if a series of
returns from nested functions would have been used.

In C++ all these flow-breaking methods are still
available. However, of the mentioned alternatives, setjmp and
longjmp  isn't frequently encountered in C++ (or even in C)
programs, due to the fact that the program flow is completely disrupted.

C++ offers exceptions as the preferred alternative to, e.g.,
setjmp and longjmp. Exceptions allow C++ programs to perform a
controlled non-local return, without the disadvantages of longjmp and
setjmp.

Exceptions are the proper way to bail out of a situation which cannot be
handled easily by a function itself, but which is not disastrous enough for
a program to terminate completely. Also, exceptions provide a flexible layer
of control between the short-range return and the crude exit.

In this chapter exceptions are covered. First an example is given of the
different impact exceptions and the setjmp/longjmp combination have on
programs. This example is followed by a discussion of the formal aspects
of exceptions. In this part the guarantees our software should be able
to offer when confronted with exceptions are presented. Exceptions and their
guarantees have consequences for constructors and destructors. We'll encounter
these consequences at the end of this chapter.


10.1: Exception syntax

Before contrasting the traditional C way of handling non-local gotos with
exceptions let's introduce the  syntactic elements that are involved when
using exceptions.
    
    o  Exceptions are generated by a throw statement. The keyword
throw, followed by an expression of a certain type, throws the expression
value as an exception. In C++ anything having value semantics may be
thrown as an exception: an int, a bool, a string, etc. However,
there also exists a standard exception type (cf. section 10.8) that
may be used as base class (cf. chapter 13) when defining new
exception types.

o  Exceptions are generated within a well-defined local environment,
called a try-block. The run-time support system ensures that all of the
program's code is itself surrounded by a global try block. Thus, every
exception generated by our code will always reach the boundary of at least one
try-block.  A program terminates when an exception reaches
the boundary of the global try block, and when this happens destructors of
local and global objects that were alive at the point where the exception was
generated are not called. This is not a desirable situation and therefore all
exceptions should be generated within a try-block explicitly defined by
the program. Here is an example of a string exception thrown from within a
try-block:
    try
{
    // any code can be defined here
    if (someConditionIsTrue)
        throw "this is the std::string exception"s;
    // any code can be defined here
}

o catch: Immediately following the try-block, one or more
catch-clauses must be defined. A catch-clause consists of a
catch-header defining the type of the exception it can catch followed by a
compound statement defining what to do with the caught exception:
    catch (string const &msg)
{
    // statements in which the caught string object are handled
}

Multiple catch clauses may appear underneath each other, one for each
exception type that has to be caught. In general the catch clauses may
appear in any order, but there are exceptions requiring a specific order. To
avoid confusion it's best to put a catch clause for the most general
exception last. At most one exception clause will be activated. C++
does not support a Java-style finally-clause activated after
completing a catch clause.


10.2: An example using exceptions

In the following examples the same basic program is used. The program uses
two classes, Outer and Inner.

First, an Outer object is defined in main, and its member
Outer::fun is called.  Then, in Outer::fun an Inner object is
defined. Having defined the Inner object, its member Inner::fun is
called.

That's about it. The function Outer::fun terminates calling
inner's destructor. Then the program terminates, activating
outer's destructor. Here is the basic program:
            #include <iostream>
    using namespace std;

    class Inner
    {
        public:
            Inner();
            ~Inner();
            void fun();
    };
    Inner::Inner()
    {
        cout << "Inner constructor\n";
    }
    Inner::~Inner()
    {
        cout << "Inner destructor\n";
    }
    void Inner::fun()
    {
        cout << "Inner fun\n";
    }

    class Outer
    {
        public:
            Outer();
            ~Outer();
            void fun();
    };
    Outer::Outer()
    {
        cout << "Outer constructor\n";
    }
    Outer::~Outer()
    {
        cout << "Outer destructor\n";
    }
    void Outer::fun()
    {
        Inner in;
        cout << "Outer fun\n";
        in.fun();
    }

    int main()
    {
        Outer out;
        out.fun();
    }

    /*
        Generated output:
    Outer constructor
    Inner constructor
    Outer fun
    Inner fun
    Inner destructor
    Outer destructor
    */

    After compiling and running, the program's output is entirely as expected:
the destructors are called in their correct order (reversing the calling
sequence of the constructors).

Now let's focus our attention on two variants in which we simulate a non-fatal
disastrous event in the Inner::fun function. This event must supposedly be
handled near main's end.

We'll consider two variants. In the first variant the event is handled by
setjmp and longjmp; in the second variant the event is handled using
C++'s exception mechanism.


10.2.1: Anachronisms: `setjmp' and `longjmp'

    The basic program from the previous section is slightly modified to contain a
variable
  jmp_buf jmpBuf used by setjmp and longjmp.

The function Inner::fun calls longjmp, simulating a disastrous
event, to be handled near main's end. In main a target location for
the long jump is defined through the function setjmp. Setjmp's zero
return indicates the initialization of the jmp_buf variable, in which case
Outer::fun is called. This situation represents the `normal flow'.

The program's return value is zero only if Outer::fun terminates
normally. The program, however, is designed in such a way that this won't
happen: Inner::fun calls longjmp. As a result the execution flow
returns to the setjmp function. In this case it does not return a zero
return value. Consequently, after calling Inner::fun from Outer::fun
main's if-statement is entered and the program terminates with return
value 1.  Try to follow these steps when studying the following program
source, which is a direct modification of the basic program given in section
10.2:
            #include <iostream>
    #include <setjmp.h>
    #include <cstdlib>

    using namespace std;

    jmp_buf jmpBuf;

    class Inner
    {
        public:
            Inner();
            ~Inner();
            void fun();
    };

    Inner::Inner()
    {
        cout << "Inner constructor\n";
    }
    void Inner::fun()
    {
        cout << "Inner fun\n";
        longjmp(jmpBuf, 0);
    }
    Inner::~Inner()
    {
        cout << "Inner destructor\n";
    }

    class Outer
    {
        public:
            Outer();
            ~Outer();
            void fun();
    };

    Outer::Outer()
    {
        cout << "Outer constructor\n";
    }
    Outer::~Outer()
    {
        cout << "Outer destructor\n";
    }
    void Outer::fun()
    {
        Inner in;
        cout << "Outer fun\n";
        in.fun();
    }

    int main()
    {
        Outer out;

        if (setjmp(jmpBuf) != 0)
            return 1;

        out.fun();
    }
    /*
        Generated output:
    Outer constructor
    Inner constructor
    Outer fun
    Inner fun
    Outer destructor
    */

    This program's output clearly shows that inner's destructor is not
called. This is a direct consequence of the non-local jump performed by
longjmp. Processing proceeds immediately from the longjmp call inside
Inner::fun to setjmp in main. There, its return value is unequal
zero, and the program terminates with return value 1. Because of the non-local
jump Inner::~Inner is never executed: upon return to main's setjmp
the existing stack is simply broken down disregarding any destructors waiting
to be called.

This example illustrates that the destructors of objects can easily be skipped
when longjmp and setjmp are used and C++ programs should therefore
avoid those functions like the plague.


10.2.2: Exceptions: the preferred alternative

    Exceptions are C++'s answer to the problems caused by setjmp and
longjmp. Here is an example using exceptions. The program is once again
derived from the basic program of section 10.2:
            #include <iostream>
    using namespace std;

    class Inner
    {
        public:
            Inner();
            ~Inner();
            void fun();
    };
    Inner::Inner()
    {
        cout << "Inner constructor\n";
    }
    Inner::~Inner()
    {
        cout << "Inner destructor\n";
    }
    void Inner::fun()
    {
        cout << "Inner fun\n";
        throw 1;
        cout << "This statement is not executed\n";
    }

    class Outer
    {
        public:
            Outer();
            ~Outer();
            void fun();
    };

    Outer::Outer()
    {
        cout << "Outer constructor\n";
    }
    Outer::~Outer()
    {
        cout << "Outer destructor\n";
    }
    void Outer::fun()
    {
        Inner in;
        cout << "Outer fun\n";
        in.fun();
    }

    int main()
    {
        Outer out;
        try
        {
            out.fun();
        }
        catch (int x)
        {}
    }
    /*
        Generated output:
    Outer constructor
    Inner constructor
    Outer fun
    Inner fun
    Inner destructor
    Outer destructor
    */

    Inner::fun now throws an int exception where a longjmp was
previously used. Since in.fun is called by out.fun, the exception is
generated within the try block surrounding the out.fun call. As an
int value was thrown this value reappears in the catch clause beyond
the try block.

Now Inner::fun terminates by throwing an exception instead of calling
longjmp. The exception is caught in main, and the program
terminates. Now we see that inner's destructor is properly called.  It is
interesting to note that Inner::fun's execution really terminates at the
throw statement: The cout statement, placed just beyond the throw
statement, isn't executed.

What did this example teach us?
    
    o  Exceptions provide a means to break a function's (and program's)
normal flow without having to use a cascade of return-statements, and
without the need to terminate the program using blunt tools like the function
exit.
    o  Exceptions do not disrupt the proper activation of destructors. Since
setjmp and longjmp do distrupt the proper activation of
destructors their use is strongly deprecated in C++.


10.3: Throwing exceptions

Exceptions are generated by throw statements. The throw keyword is
followed by an expression, defining the thrown exception value.  Example:
            throw "Hello world";        // throws a char *
    throw 18;                   // throws an int
    throw string{ "hello" };    // throws a string

Local objects cease to exist when a function terminates. This is no
different for exceptions.

Objects defined locally in functions are automatically destroyed once
exceptions thrown by these functions leave these functions. This also happens
to objects thrown as exceptions. However, just before leaving the function
context the object is copied and it is this copy that eventually reaches the
appropriate catch clause.

The following examples illustrates this process.
Object::fun defines a local Object toThrow, that is
thrown as an exception. The exception is caught
in main. But by then the object originally thrown doesn't exist anymore,
and main received a copy:
            #include <iostream>
    #include <string>
    using namespace std;

    class Object
    {
        string d_name;

        public:
            Object(string name)
            :
                d_name(name)
            {
                cout << "Constructor of " << d_name << "\n";
            }
            Object(Object const &other)
            :
                d_name(other.d_name + " (copy)")
            {
                cout << "Copy constructor for " << d_name << "\n";
            }
            ~Object()
            {
                cout << "Destructor of " << d_name << "\n";
            }
            void fun()
            {
                Object toThrow("'local object'");
                cout << "Calling fun of " << d_name << "\n";
                throw toThrow;
            }
            void hello()
            {
                cout << "Hello by " << d_name << "\n";
            }
    };

    int main()
    {
        Object out{ "'main object'" };
        try
        {
            out.fun();
        }
        catch (Object o)
        {
            cout << "Caught exception\n";
            o.hello();
        }
    }

    Object's copy constructor is special in that it defines its name as
the other object's name to which the string " (copy)" is appended. This
allow us to monitor the construction and destruction of objects more closely.
Object::fun generates an exception, and throws its locally defined
object. Just before throwing the exception the program has produced the
following output:
            Constructor of 'main object'
    Constructor of 'local object'
    Calling fun of 'main object'

When  the exception is generated the next line of output is produced:
            Copy constructor for 'local object' (copy)

The local object is passed to throw where it is treated  as a value
argument, creating a copy of toThrow. This copy is thrown as the
exception, and the local toThrow object ceases to exist. The thrown
exception is now caught by the catch clause, defining an
Object value parameter. Since this is a value parameter yet another
copy is created. Thus, the program writes the following text:
            Destructor of 'local object'
    Copy constructor for 'local object' (copy) (copy)

The catch block now displays:
            Caught exception

Following this o's  hello member is called, showing us that we
indeed received a copy of the copy of the original toThrow object:
            Hello by 'local object' (copy) (copy)

Then the program terminates and its remaining  objects are now
destroyed, reversing  their order of creation:
            Destructor of 'local object' (copy) (copy)
    Destructor of 'local object' (copy)
    Destructor of 'main object'

The copy created by the catch clause clearly is superfluous. It can be
avoided by defining object reference parameters in catch clauses:
`catch (Object &o)'. The program now produces the following output:
            Constructor of 'main object'
    Constructor of 'local object'
    Calling fun of 'main object'
    Copy constructor for 'local object' (copy)
    Destructor of 'local object'
    Caught exception
    Hello by 'local object' (copy)
    Destructor of 'local object' (copy)
    Destructor of 'main object'

Only a single copy of toThrow was created.

It's a bad idea to  throw a pointer to a locally defined
object. The pointer is thrown, but the object to which the pointer refers
ceases to exist once the exception is thrown. The catcher receives a
wild pointer. Bad news....

Let's summarize the above findings:
    
    o  Local objects are thrown as copied objects;
    o  Don't throw pointers to local objects;
    o  It is possible to throw pointers to dynamically generated
objects. In this case one must take care that the generated object is properly
deleted by the exception handler to prevent a memory leak.

Exceptions are thrown in situations where a function can't complete its
assigned task, but the program is still able to continue. Imagine a program
offering an interactive calculator. The program expects numeric expressions,
which are evaluated.  Expressions may show syntactic errors or it may be
mathematically impossible to evaluate them. Maybe the calculator allows us to
define and use variables and the user might refer to non-existing variables:
plenty of reasons for the expression evaluation to fail, and so many reasons
for exceptions to be thrown. None of those should terminate the
program. Instead, the program's user is informed about the nature of the
problem and is invited to enter another expression. Example:
            if (!parse(expressionBuffer))           // parsing failed
        throw "Syntax error in expression";

    if (!lookup(variableName))              // variable not found
        throw "Variable not defined";

    if (divisionByZero())                   // unable to do division
        throw "Division by zero is not defined";

Where these throw statements are located is irrelevant: they may be
found deeply nested inside the program, or at a more superficial level.
Furthermore, functions may be used to generate the exception to be
thrown. An Exception object might support stream-like insertion operations
allowing us to do, e.g.,
            if (!lookup(variableName))
        throw Exception() << "Undefined variable '" << variableName << "';


10.3.1: The empty `throw' statement

    Sometimes it is required to inspect a thrown exception. An exception catcher
may decide to ignore the exception, to process the exception, to rethrow it
after inspection or to change it into another kind of exception. For example,
in a server-client application the client may submit requests to the server by
entering them into a queue. Normally every request is eventually answered by
the server. The server may reply that the request was successfully processed,
or that some sort of error has occurred. On the other hand, the server may
have died, and the client should be able to discover this calamity, by not
waiting indefinitely for the server to reply.

In this situation an intermediate exception handler is called for. A thrown
exception is first inspected at the middle level. If possible it is processed
there. If it is not possible to process the exception at the middle level, it
is passed on, unaltered, to a more superficial level, where the really tough
exceptions are handled.

By placing an  empty throw statement in the exception
handler's code the received exception is passed on to the next level that
might be able to process that particular type of exception. The rethrown
exception is never handled by one of its neighboring exception handlers; it
is always transferred to an exception handler at a more superficial level.

In our server-client situation a function
            initialExceptionHandler(string &exception)

could be designed to handle the string exception. The received message
is inspected. If it's a simple message it's processed, otherwise the exception
is passed on to an outer level. In initialExceptionHandler's
implementation the empty throw statement is used:
            void initialExceptionHandler(string &exception)
    {
        if (!plainMessage(exception))
            throw;

        handleTheMessage(exception);
    }

Below (section 10.5), the empty throw statement is used
to pass on the exception received by a catch-block. Therefore, a function
like initialExceptionHandler can be used for a variety of thrown
exceptions, as long as their types match initialExceptionHandler's
parameter, which is a string.

The next example jumps slightly ahead, using some of the topics covered in
chapter 14. The example may be skipped, though, without
loss of continuity.

A basic exception handling class can be constructed from which specific
exception types are derived.  Suppose we have a class Exception, having a
member function ExceptionType Exception::severity.  This member function
tells us (little wonder!) the severity of a thrown exception. It might be
Info, Notice, Warning, Error or Fatal.  The information contained in
the exception depends on its severity and is processed by a function
handle. In addition, all exceptions support a member function like
textMsg, returning textual information about the exception in a
string.

By defining a polymorphic function handle it can be made to behave
differently, depending on the nature of a thrown exception, when called
from a basic Exception pointer or reference.

In this case, a program may throw any of these five exception types.  Assuming
that the classes Message and Warning were derived from the class
Exception, then the handle function matching the exception type will
automatically be called by the following exception catcher:
            //
    catch (Exception &ex)
    {
        cout << e.textMsg() << '\n';

        if
        (
            ex.severity() != ExceptionType::Warning
            &&
            ex.severity() != ExceptionType::Message
        )
            throw;              // Pass on other types of Exceptions

        ex.handle();            // Process a message or a warning
    }

Now anywhere in the try block preceding the exception handler
Exception objects or objects of one of its derived classes may be
thrown. All those exceptions will be caught by the above handler. E.g.,
            throw Info{};
    throw Warning{};
    throw Notice{};
    throw Error{};
    throw Fatal{};


10.4: The try block

The try-block surrounds throw statements. Remember that a program is
always surrounded by a global try block, so throw statements may
appear anywhere in your code. More often, though, throw statements are
used in function bodies and such functions may be called from within try
blocks.

A try block is defined by the keyword try followed by a compound
statement. This block, in turn,  must be followed by at least one
catch handler:
            try
    {
                // any statements here
    }
    catch (...)  // at least one catch clause here
    {}

Try-blocks are commonly nested, creating exception levels. For
example, main's code is surrounded by a try-block, forming an outer
level handling exceptions.  Within main's try-block functions are
called which may also contain try-blocks, forming the next exception
level. As we have seen (section 10.3.1), exceptions thrown in
inner level try-blocks may or may not be processed at that level. By
placing an empty throw statement in an exception handler, the
thrown exception is passed on to the next (outer) level.


10.5: Catching exceptions

A catch clause consists of the keyword catch followed by a parameter
list defining one parameter specifying type and (parameter) name of the
exception caught by that particular catch handler. This name may then be
used as a variable in the compound statement following the catch clause.
Example:
            catch (string &message)
    {
        // code to handle the message
    }

Primitive types and objects may be thrown as exceptions. It's a bad idea
to throw a pointer or reference to a local object, but a pointer to a
dynamically allocated object may be thrown if the exception handler
deletes the allocated memory to prevent a memory leak. Nevertheless,
throwing such a pointer is dangerous as the exception handler won't be able to
distinguish dynamically allocated memory from non-dynamically allocated
memory, as illustrated by the next example:
            try
    {
        static int x;
        int *xp = &x;

        if (condition1)
            throw xp;

        xp = new int(0);
        if (condition2)
            throw xp;
    }
    catch (int *ptr)
    {
        // delete ptr or not?
    }

Close attention should be paid to the nature of the parameter of the
exception handler, to make sure that when pointers to dynamically allocated
memory are thrown the memory is returned  once the handler has processed
the pointer. In general pointers should not be thrown as exceptions. If
dynamically allocated memory must be passed to an exception handler then the
pointer should be wrapped in a smart pointer, like unique_ptr or
shared_ptr (cf. sections 18.3 and 18.4).

Multiple catch handlers may follow a try block, each handler
defining its own exception type.  The order 
of the exception handlers is important. When an exception is thrown, the first
exception handler matching the type of the thrown exception is used and
remaining exception handlers are ignored. Eventually at most one exception
handler following a try-block is activated. Normally this is of no
concern as each exception has its own unique type.

Example: if exception handlers are defined for char *s and void *s
then NTBSs are caught by the former handler. Note that a char
* can also be considered a void *, but the exception type matching
procedure is smart enough to use the char * handler with the thrown
NTBS. Handlers should be designed very type specific to catch the
correspondingly typed exception. For example, int-exceptions are not
caught by double-catchers, char-exceptions are not caught by
int-catchers. Here is a little example illustrating that the order of the
catchers is not important for types not having any hierarchal relationship to
each other (i.e., int is not derived from double; string is not
derived from an NTBS):
        #include <iostream>
using namespace std;

int main()
{
    while (true)
    {
        try
        {
            string s;
            cout << "Enter a,c,i,s for ascii-z, char, int, string "
                                                      "exception\n";
            getline(cin, s);
            switch (s[0])
            {
                case 'a':
                    throw "ascii-z";
                case 'c':
                    throw 'c';
                case 'i':
                    throw 12;
                case 's':
                    throw string{};
            }
        }
        catch (string const &)
        {
            cout << "string caught\n";
        }
        catch (char const *)
        {
            cout << "ASCII-Z string caught\n";
        }
        catch (double)
        {
            cout << "isn't caught at all\n";
        }
        catch (int)
        {
            cout << "int caught\n";
        }
        catch (char)
        {
            cout << "char caught\n";
        }
    }
}

    Rather than defining specific exception handlers a specific class can be
designed whose objects contain information about the exception. Such an
approach was mentioned earlier, in section 10.3.1. Using this
approach, there's only one handler required, since we know we don't throw
other types of exceptions:
            try
    {
        // code throws only Exception objects
    }
    catch (Exception &ex)
    {
        ex.handle();
    }

When the code of an exception handler has been processed, execution continues
beyond the last exception handler directly following the matching
try-block (assuming the handler doesn't itself use flow control statements
(like return or throw) to break the default flow of execution). The
following cases can be distinguished:
    
    o  If no exception was thrown within the try-block no exception
handler is activated, and execution continues from the last statement in
the try-block to the first statement beyond the last catch-block.
    o  If an exception was thrown within the try-block but neither
the current level nor another level contains an appropriate exception handler,
the program's default exception handler is called, aborting the program.
    o  If an exception was thrown from the try-block and an appropriate
exception handler is available, then the code of that exception handler is
executed. Following that, the program's execution continues at the first
statement beyond the last catch-block.

All statements in a try block following an executed
throw-statement are ignored. However, objects that were successfully
constructed within the try block before executing the throw statement
are destroyed before any exception handler's code is executed.


10.5.1: The default catcher

    At a certain level of the program only a limited set of handlers may actually
be required. Exceptions whose types belong to that limited set are processed,
all other exceptions are passed on to exception handlers of an outer level
try block.

An intermediate type of exception handling may be implemented using the
default exception handler, which must be (due to the hierarchal nature of
exception catchers, discussed in section 10.5) placed beyond
all other, more specific exception handlers.

This default exception handler cannot determine the actual type of the thrown
exception and cannot determine the exception's value but it may execute some
statements, and thus do some default processing. Moreover, the caught
exception is not lost, and the default exception handler may use the empty
throw statement (see section 10.3.1) to pass the exception on to
an outer level, where it's actually processed.  Here is an example showing
this use of a default exception handler:
        #include <iostream>
    using namespace std;

    int main()
    {
        try
        {
            try
            {
                throw 12.25;    // no specific handler for doubles
            }
            catch (int value)
            {
                cout << "Inner level: caught int\n";
            }
            catch (...)
            {
                cout << "Inner level: generic handling of exceptions\n";
                throw;
            }
        }
        catch (double d)
        {
            cout << "Outer level may use the thrown double: " << d << '\n';
        }
    }
    /*
        Generated output:
    Inner level: generic handling of exceptions
    Outer level may use the thrown double: 12.25
    */

    The program's output illustrates that an empty throw statement in a
default exception handler throws the received exception to the next (outer)
level of exception catchers, keeping type and value of the thrown exception.

Thus, basic or generic exception handling can be accomplished at an inner
level, while specific handling, based on the type of the thrown expression,
can be provided at an outer level. Additionally, particularly in
multi-threaded programs (cf. chapter 20), thrown exceptions can be
transferred between threads after converting std::exception objects to
std::exception_ptr objects. This proceduce can even be used from inside
the default catcher. Refer to section 10.9.4 for further coverage of the
class std::exception_ptr.


10.6: Functions unable to throw exceptions: the `noexcept' keyword

Once a function has been defined it's often called from other functions. If
called functions are not defined in the same source file as calling functions
the called functions must be declared, for which header files are often
used. Those called functions might throw exceptions, which might be
unacceptible to the function calling those other functions. E.g., functions
like swap and destructors may not throw exceptions. 

Functions that may not throw exceptions can be declared and defined by
specifying the noexcept keyword (see section 10.9 for examples
of function declarations specifying noexcept).

When using noexept there's a slight run-time overhead penalty because the
function needs an over-all try-catch block catching any exception that
might be thrown by its (called) code. When an exception is caught (violating
the noexcept specification) then the catch clause calls
std::terminate, ending the program.

In addition to using a plain noexcept, it can also be given an argument
that is evaluated compile-time (e.g., void fun() noexcept(sizeof(int) ==
4)): if the evaluation returns true then the noexcept requirement is
used; if the evaluation returns false, then the noexcept requirement
is ignored. Examples of this advanced use of noexcept are provided in
section 23.8.


10.7: Iostreams and exceptions

The C++ I/O library was used well before exceptions were available in
C++. Hence, normally the classes of the iostream library do not throw
exceptions. However, it is possible to modify that behavior using the
 ios::exceptions member function. This function has two overloaded
versions:
    
    o ios::iostate exceptions():
this member returns the state flags for which the
         stream will throw exceptions;
    o void exceptions(ios::iostate state)

        this member causes the stream to throw an exception
            when state state is observed.

In the I/O library, exceptions are objects of the class
  ios::failure, derived from 
ios::exception. A std::string const &message may be specified when
defining a failure object. Its message may then be  retrieved using its
  virtual char const *what() const member.

Exceptions should be used in exceptional circumstances. Therefore, we
think it is questionable to have stream objects throw exceptions for fairly
normal situations like EOF. Using exceptions to handle input errors
might be defensible (e.g., in situations where input errors should not occur
and imply a corrupted file) but often aborting the program with an appropriate
error message would probably be the more appropriate action. As an example
consider the following interactive program using exceptions to catch incorrect
input:
            #include <iostream>
    #include <climits>
    using namespace::std;

    int main()
    {
        cin.exceptions(ios::failbit);   // throw exception on fail
        while (true)
        {
            try
            {
                cout << "enter a number: ";
                int value;
                cin >> value;
                cout << "you entered " << value << '\n';
            }
            catch (ios::failure const &problem)
            {
                cout << problem.what() << '\n';
                cin.clear();
                cin.ignore(INT_MAX, '\n');  // ignore the faulty line
            }
        }
    }


By default, exceptions raised from within ostream objects are caught by
these objects, which set their ios::badbit as a result. See also the
paragraph on this issue in section 14.8.


10.8: Standard exceptions

All data types may be thrown as exceptions. Several additional exception
classes are now defined by the C++ standard. Before using those additional
exception classes the <stdexcept> header file must be included.

All of these standard exceptions
   are class types by themselves, but also offer
all facilities of the  std::exception class and objects
of the standard exception classes may also be considered objects of the
std::exception class.

The std::exception class offers the member
            char const *what() const;

 describing in a short textual message the nature of the
exception.

C++ defines the following standard exception classes:
    
    o std::bad_alloc (this requires the <new> header file):
        thrown when operator new fails;
    o std::bad_array_new_length (this requires the 
        <new> header file): thrown when an illegal array size is requested
        when using new Type[...]. Illegal sizes are negative values,
        values that exceed an implementation defined maximum, the number of
        initializer clauses exceeds the specified number of array elements
        (e.g., new int[2]{ 1, 2, 3 });
    o std::bad_cast (this requires the <typeinfo> header
        file): thrown in the context of polymorphism
        (see section 14.6.1);
    o std::bad_exception (this requires the 
        <exception> header file): thrown when a function tries to
        generate another type of exception than declared in its
        function throw list;
    o std::bad_typeid (this requires the <typeinfo> header
        file): also thrown in the context of
        polymorphism (see section 14.6.2);

All additional exception classes were derived from std::exception. The
constructors of all these additional classes accept std::string const &
arguments summarizing the reason for the exception (retrieved by the
exception::what member).  The additionally defined exception classes are:
    
    o std::domain_error: a (mathematical) domain error is
detected;
    o std::invalid_argument: the argument of a function
has an invalid value;
    o std::length_error: thrown when an object would have
exceeded its maximum permitted length;
    o std::logic_error: a logic error should be thrown when a
problem is detected in the internal logic of the program. Example: a function
like C's printf is called with more arguments than there are format
specifiers in its format string;
    o std::out_of_range: thrown when an argument exceeds its
permitted range. Example: thrown by at members when their arguments exceed
the range of admissible index values;
    o std::overflow_error: an overflow error should be
thrown when an arithmetic overflow is detected. Example: dividing a
value by a very small value;
    o std::range_error: a range error should be thrown when
an internal computation results in a value exceeding a permissible range;
    o std::runtime_error: a runtime error should be thrown
when a problem is encountered that can only be detected while the program is
being executed.  Example: a non-integral is entered when the program's input
expects an integral value.
    o std::underflow_error: an underflow error should be
thrown when an arithmetic underflow is detected. Example: dividing a very
small value by a very large value.
    o std::tx_exception<Type>: derived from
std::runtime_error. This exception can be thrown from an atomic_cancel
compound statement (cf. section 20.14) to undo statements executed so
far. 


10.8.1: Standard exceptions: to use or not to use?

    Since values of any type may be thrown as exceptions, you may wonder when to
throw values of standard exception types and (if ever) when to throw values of
other types.

Current practice in the C++ community is to throw exceptions only in
exceptional situations. In that respect C++'s philosophy about using
exceptions differs markedly from the way exceptions are used in, e.g., Java,
where exceptions are often encountered in situations C++ doesn't consider
exceptional. Another common practice is to follow a `conceptual' style when
designing software. A nice characteristic of exceptions is that exceptions can
be thrown at a point where your source shows what's happening: throwing an
std::out_of_range exception is nice for the software maintainer, as
the reason for the exception is immediately recognized.

At the catch-clause the semantical context usually isn't very relevant anymore
and by catching a std::exception and showing its what() content the
program's user is informed about what happened.

But throwing values of other types can also be useful. What about a situation
where you want to throw an exception and catch it at some shallow level?  In
between there may be various levels of software provided by external software
libraries over which the software engineer has no control.  At those levels
exceptions (std::exceptions) could be generated too, and those exceptions
might also be caught by the library's code. When throwing a standard exception
type it may be hard to convince yourself that that exception isn't caught by
the externally provided software. Assuming that no catch-alls are used (i.e.,
catch (...))  then throwing an exception from the std::exception
family might not be a very good idea. In such cases throwing a value from a
simple, maybe empty, enum works fine:
            enum HorribleEvent 
    {};      

    ... at some deep level:
        throw HorribleEvent{};

    ... at some shallow level:
    catch (HorribleEvent hs)
    {
        ...
    }

Other examples can easily be found: design a class holding a message and
an error (exit) code: where necessary throw an object of that class, catch it
in the catch clause of main's try block and you can be sure that all objects
defined at intermediate levels are neatly destroyed, and at the end you show
the error message and return the exit code embedded in your non-exception
object.

So, the advice is to use std::exception types when available, and
clearly do the required job. But if an exception is used to simply bail out
of an unpleasant situation, or if there's a chance that externally provided
code might catch std:exceptions then consider throwing objects or values
of other types.


10.9: System error, error_category, and error_condition

The class std::system_error is derived from
std::runtime_error, which in turn is derived from std::exception


    
Before using the class system_error or related classes the
<system_error> header file must be included.

System_error exceptions can be thrown when errors occur having
associated (system) error values. Such errors are typically associated
with low-level (like operating system) functions, but other types of errors
(e.g., bad user input, non-existing requests) can also be handled. 

In addition to error codes (cf. section 4.3.1) and error categories
(covered below) error conditions are distinguished. Error
conditions specify platform independent types of errors like syntax errors or
non-existing requests.

When constructing system_error objects error codes and error categories
may be specified. First we'll look at the classes error_condition and
error_category, then system_error itself is covered in more detail.

Figure 9 illustrates how the various components interact.

------------------------------------------------------------------Insert Figure 9(System_error: associated components)about here (file: exceptions/systemerror)------------------------------------------------------------------

As shown in figure 9 the class error_category uses the class
error_condition and the class error_condition uses the class
error_category. As a consequence of this circular dependency between these
two classes these classes should be approached as one single class:
when covering error_category the class error_condition should be known
and vice versa. This circular dependency among these classes is unfortunate
and an example of bad class design.

As system_error is eventually derived from exception it offers the
standard what member. It also contains an error_code.

In POSIX systems the errno variable is associated with many, often rather
cryptic, symbols. The predefined enum class errc attempts to provide
intuitively more appealing symbols. Since its symbols are defined in a
strongly typed enumeration, they cannot directly be used when defining a
matching error_code. Instead, a make_error_code function converts
    enum class errc values and values of newly defined
error code enumerations (called ErrorCodeEnum below) to error_code
objects.

The enum class errc defined in the std namespace defines symbols whose
values are equal to the traditional error code values used by C but 
describe the errors in a less cryptic way. E.g.,
            enum class errc 
    {
        address_family_not_supported, // EAFNOSUPPORT
        address_in_use,               // EADDRINUSE
        address_not_available,        // EADDRNOTAVAIL
        already_connected,            // EISCONN
        argument_list_too_long,       // E2BIG
        argument_out_of_domain,       // EDOM
        bad_address,                  // EFAULT
        ...
    };

Values of ErrorCodeEnums can be passed to matching make_error_code
functions. Defining your own ErrorCodeEnum enumeration is covered in
section 23.7.

Now that the general outline has been presented, it's time to have a closer
look at the various components shown in figure 9.


10.9.1: The class `std::error_category'

    Objects of the class std::error_category identify sources of sets of error
codes. New error categories for new error code enumerations can also be
defined (cf. section 23.7).

Error categories are designed as singletons: only one object of each class
can exist. Because of this error_categories are equal when the addresses
of error_category objects are equal. Error category objects are returned
by functions (see below) or by static instance() members of error category
classes.

Error category classes define several members. Most are declared virtual
(cf. chapter 14), meaning that those members may be redefined
in error category classes we ourselves design:
    
    o virtual error_condition
        default_error_condition(int ev) const noexcept:
returns an error_condition object (cf. section 10.9.2)
        initialized with error value ev and the current 
        (i.e., *this) error_category;

o virtual bool equivalent(error_code const &code, int condition) const
        noexcept:
returns true if the equivalence between 
        the error condition that is associated with the error_code object
        and the error_condition_enum value that is specified (as an
        int value) as the function's second argument could be
        establisted;

o virtual bool equivalent(int ev, error_condition const &condition)
        const noexcept:
returns true if the equivalence of an error_condition object
        that is constructed from the ErrorConditionEnum value that is
        associated with the ErrorCategoryEnum value that was passed (as
        int) to the function and the error_condition object that was
        passed to the function as its second argument could be established;


    o virtual string message(int ev) const:
This member returns a string describing the error condition denoted by
        ev, which should be a (cast to int) value of the category's
        error condition enumeration;


    o virtual char const *name() const noexcept:
This member returns the name of the error category as NTBS (like
        generic); 

o bool operator<(error_category const &rhs) const noexcept:
returns less<const error_category*>()(this, &rhs).

o  Error category objects can be compared for (in)equality.

The functions returning predefined error categories are:
    
    o error_category const &generic_category() noexcept:
returns a reference to the generic error_category object.
        The returned object's name member returns a pointer to the string
        "generic";

o error_category const &system_category() noexcept:
returns a reference to the operating system error_category
        object: it is used for errors reported by the operating system. The
        object's name member returns a pointer to the string
        "system";

o error_category const &iostream_category() noexcept:
returns a reference to the iostream error_category
        object: it is used for errors reported by stream objects. The
        object's name member returns a pointer to the string
        "iostream";

o error_category const &future_category() noexcept:
returns a reference to the future error_category object: it is
        used for errors reported by `future' objects (cf. section
        20.8). The object's name member returns a pointer to the
        string "future";


10.9.2: The class `std::error_condition'

    Error_condition objects contain information about
`higher level' types of errors. They are supposed to be platform independent
like syntax errors or non-existing requests.

Error condition objects are returned by the member default_error_condition
of the classes error_code and error_category, and they are returned by
the function std::error_condition
make_error_condition(ErrorConditionEnum ec). The type name
ErrorConditionEnum is a formal name for an enum class that enumerates
the `higher level' error types. The error_condition objects returned by
make_error_condition are initialized with ec and the
error_category that uses the ErrorConditionEnum.  Defining your own
ErrorConditionEnum is covered in section 23.7.

Constructors:
    
    o error_condition() noexcept:
the object's value is initialized to `no error' (i.e., 0) and
        a system_category error category;

o  The copy constructor is available;

o error_condition(int ec, error_category const &cat) noexcept:
the object's value is initialized to ec and error category
        cat. It is the responsibility of the caller to ensure that ec
        represents a (cast to int) value of cat's error condition
        enumeration;

o error_condition(ErrorConditionEnum value) noexcept:
this is a member template (cf. section 22.1.3), using template
        header template <class ErrorConditionEnum>. It initializes the
        object with the return value of
        make_error_condition(value);

Members:
    
    o  The copy assignment operator and an assignment operator accepting an
        ErrorConditionEnum are available;

o void assign(int val, error_category const &cat):
assigns new values to the current object's value and
        category data members;

o error_category const &category() const noexcept:
returns a reference to the object's error category (note that this is
        a reference to the category's singleton object);

o void clear() noexcept:
after calling this member value is set to 0 and the object's error
        category set to generic_category;

o string message() const:
returns category().message(value());

o explicit operator bool() const noexcept:
returns true if value() returns a non-zero value (so its
        semantic meaning is `the object represents an error');

o int value() const noexcept:
returns the object's error value.

Two error_condition objects can be compared for (in)equality, and can be
ordered using operator<. Ordering is pointless if the two objects refer to
different error categories. If the categories of two objects are different 
they are considered different.


10.9.3: The class system_error

    System_error objects can be constructed from error_codes or from 
error values (ints) and matching error category objects, optionally
followed by a standard textual description of the nature of the encountered
error. 

Here is the class's public interface:
           class system_error: public runtime_error 
    {
        public:
            system_error(error_code ec);
            system_error(error_code ec, string const &what_arg);
            system_error(error_code ec, char const *what_arg);
    
            system_error(int ev, error_category const &ecat);
            system_error(int ev, error_category const &ecat,
                             char const *what_arg);
            system_error(int ev, error_category const &ecat,
                             string const &what_arg);
    
            error_code const &code() const noexcept;
            char const *what() const noexcept;
    }

The ev values often are the values of the errno variable as set
upon failure by system level functions like chmod(2).

Note that the first three constructors shown in the interface receive an
error_code object as their first arguments. As one of the error_code
constructors also expects an int and and error_category argument,
the second set of three constructors could also be used instead of the first
set of three constructors. E.g.,
            system_error(errno, system_category(), "context of the error");
    // identical to:
    system_error(error_code(errno, system_category()), 
                                            "context of the error");

The second set of three constructors are primarily used when an existing
function already returns an error_code. E.g.,
            system_error(make_error_code(errc::bad_address), 
                                            "context of the error");
    // or maybe:
    system_error(make_error_code(static_cast<errc>(errno)), 
                                            "context of the error");

In addition to the standard what member, the system_error class also
offers a member code returning a const reference to the exception's error
code. 

The NTBS returned by system_error's what member may be formatted by a
system_error object:
            what_arg + ": " + code().message()

Note that, although system_error was derived from runtime_error,
you'll lose the code member when catching a std::exception object. Of
course, downcasting is possible, but that's a stopgap. Therefore, if a
system_error is thrown, a matching catch (system_error const &) clause
must be provided to retrieve the value returned by the code member. This,
and the rather complex organization of the classes that are involved when
using system_error result in a very complex, and hard to generalize
exception handling. In essence, what you obtain at the cost of high
complexity is a facility for categorizing int or enum error
values. Additional coverage of the involved complexities is provided in
chapter 23, in particular section 23.7
(for a flexible alternative, see the class 
     FBB::Exception in the author's 
        Bobcat library).


10.9.4: Exception propagation: std::exception_ptr

    In practice almost everything can be used as an exception. But at the same
time any exception that is thrown can be reached using the function
(std::current_exception), and access to any exception
can be standardized using std::make_exception_ptr.
These functions expect or use objects of the class
std::exception_ptr, and in this section we take a closer
look at tha class.

The class exception_ptr's default constructor initializes it to a
null-pointer. In the following code snippet the variable isNull is set to
true:
            std::exception_ptr obj;
    bool isNull =  obj == nullptr && obj == 0;

The class exception_ptr provides copy and move constructors as well as
copy and move assignment operators.

Two exception_ptr objects can be compared for equality. They are equal
if they refer to the same exception. Move assignment transfers the exception
referred to by the right-hand side operand to the left-hand side operand, and
turns the right-hand side operand into a null pointer.

There is no published method directly retrieving the exception to which an
exception_ptr object refers. However, there are some free functions
constructing or handling exception_ptr objects:
    
    o std::exception_ptr std::current_exception() noexcept:
An exception_ptr object is returned referring to the currently
        handled exception (or a copy of the currently handled exception, or a
        default constructed exception_ptr object if no current exception
        is available). This function can also be called when a default
        exception catcher is used. 


The exception referred to by current_exception does not have to
        be an object of the class std::exception. Any type of object or
        value thrown as an exception is retrieved as an exception_ptr by
        current_exception. The exception referred to by an
        exception_ptr object remains valid for at least as long as there
        exists an exception_ptr object that refers to it. Calling
        current_exception twice in a row then the two returned
        exception_ptr objects may or may not refer to the same exception
        object.

o std::exception_ptr make_exception_ptr(Type
        value) noexcept:
This function template constructs an exception_ptr from a value of
        any type which is passed as its argument. Type does not
        necessarily have to be a std::exception but can be anything that
        can be thrown as an exception: an int, a std::string, a
        std::exception, you name it.


        Here are
        some examples, showing how values of different types can be passed as
        arguments to make_exception_ptr

           auto ptr = make_exception_ptr(exception());
    ptr = make_exception("hello world"s);
    ptr = make_exception(12);


o void std::rethrow_exception(exception_ptr obj):
The exception to which obj refers is thrown. Note: obj
        cannot be a nullptr.


10.10: Exception guarantees

Software should be exception safe: the program should continue to work
according to its specifications in the face of exceptions. It is not always
easy to realize exception safety. In this section some guidelines and
terminology is introduced when discussing exception safety.

Since exceptions may be generated from within all C++ functions,
exceptions may be generated in many situations. Not all of these situations
are immediately and intuitively recognized as situations where exceptions can
be thrown. Consider the following function and ask yourself at which points
exceptions may be thrown:
            void fun()
    {
        X x;
        cout << x;
        X *xp = new X{ x };
        cout << (x + *xp);
        delete xp;
    }

If it can be assumed that cout as used above does not throw an
exception there are at least 13 opportunities for exceptions to be thrown:
    
    o  X x: the default constructor could throw an exception (#1)
    o  cout << x: the overloaded insertion operator could throw an
exception (#2), but its rhs argument might not be an X but, e.g., an
int, and so X::operator int() const could be called which offers yet
another opportunity for an exception (#3).
    o  *xp = new X{ x }: the copy constructor may throw an exception
(#4) and operator new (#5a) too. But did you realize that this latter
exception might not be thrown from ::new, but from, e.g., X's own
overload of operator new? (#5b)
    o  cout << (x + *xp): we might be seduced into thinking that two
X objects are added. But it doesn't have to be that way. A separate class
Y might exist and X may have a conversion operator operator Y() const,
and operator+(Y const &lhs, X const &rhs), operator+(X const &lhs, Y const
&rhs), and operator+(X const &lhs, X const &rhs) might all exist. So, if
the conversion operator exists, then depending on the kind of overload of
operator+ that is defined either the addition's left-hand side operand
(#6), right-hand side operand (#7), or operator+ itself (#8) may throw an
exception. The resulting value may again be of any type and so the overloaded
cout << return-type-of-operator+ operator may throw an exception
(#9). Since operator+ returns a temporary object it is destroyed shortly
after its use. X's destructor could throw an exception (#10).
    o  delete xp: whenever operator new is overloaded operator
delete should be overloaded as well and may throw an exception (#11). And of
course, X's destructor might again throw an exception (#12).
    o  }: when the function terminates the local x object is
destroyed: again an exception could be thrown (#13).

It is stressed here (and further discussed in section 10.12)
that although it is possible for exceptions to leave
destructors this would violate the C++ standard and so it must be
prevented in well-behaving C++ programs.

How can we expect to create working programs when exceptions might be thrown
in so many situations?

Exceptions may be generated in a great many situations, but serious
problems are prevented when we're able to provide at least one of the
following exception guarantees:
    
    o  The basic guarantee: no resources are leaked. In practice this
means: all allocated memory is properly returned when exceptions are thrown.
    o  The strong guarantee: the program's state remains unaltered when
an exception is thrown (as an example: the canonical form of the overloaded
assignment operator provides this guarantee)
    o  The nothrow guarantee: this applies to code for which it can be
proven that no exception can be thrown from it.


10.10.1: The basic guarantee

    The basic guarantee dictates that functions that fail to complete their
assigned tasks must return all allocated resources, usually memory, before
terminating. Since practically all functions and operators may throw
exceptions and since a function may repeatedly allocate resources the
blueprint of a function allocating resources shown below defines a try block
to catch all exceptions that might be thrown. The catch handler's task is to
return all allocated resources and then rethrow the exception.
            void allocator(X **xDest, Y **yDest)
    {
        X *xp = 0;              // non-throwing preamble
        Y *yp = 0;

        try                     // this part might throw
        {
            xp = new X[nX];     // alternatively: allocate one object
            yp = new Y[nY];
        }
        catch (...)
        {
            delete xp;
            throw;
        }

        delete[] *xDest;        // non-throwing postamble
        *xDest = xp;
        delete[] *yDest;
        *yDest = yp;
    }

In the pre-try code the pointers to receive the addresses returned by the
operator new calls are initialized to 0. Since the catch handler must be
able to return allocated memory they must be available outside of the try
block. If the allocation succeeds the memory pointed to by the destination
pointers is returned and then the pointers are given new values.

Allocation and or initialization might fail. If allocation fails new
throws a std::bad_alloc exception and the catch handler
simply deletes 0-pointers which is OK.

If allocation succeeds but the construction of (some) of the objects fails
by throwing an exception then the following is
     guaranteed to happen:
    
    o  The destructors of all successfully allocated objects are called;
    o  The dynamically allocated memory to contain the objects is returned

Consequently, there is no memory leak when new fails. Inside the above
try block new X may fail: this does not affect the 0-pointers
and so the catch handler merely deletes 0 pointers. When new Y fails
xp points to allocated memory and so it must be returned. This happens
inside the catch handler. The final pointer (here: yp) will only be
unequal zero when new Y properly completes, so there's no need for the
catch handler to return the memory pointed at by yp.


10.10.2: The strong guarantee

    The strong guarantee dictates that an object's state should not change in
the face of exceptions. This is realized by performing all operations that
might throw on a separate copy of the data. If all this succeeds then the
current object and its (now successfully modified) copy are swapped. An
example of this approach can be observed in the canonical overloaded
assignment operator:
            Class &operator=(Class const &other)
    {
        Class tmp(other);
        swap(tmp);
        return *this;
    }

The copy construction might throw an exception, but this keeps the current
object's state intact. If the copy construction succeeds swap swaps the
current object's content with tmp's content and returns a reference to
the current object. For this to succeed it must be guaranteed that swap
won't throw an exception. Returning a reference (or a value of a primitive
data type) is also guaranteed not to throw exceptions. The canonical form of
the overloaded assignment operator therefore meets the requirements of the
strong guarantee.

Some  rules of thumb were formulated that relate to the
strong guarantee
 (cf. Sutter, H., Exceptional C++, Addison-Wesley, 2000). E.g.,
    
    o  All the code that might throw an exception affecting the current
state of an object should perform its tasks separately from the data
controlled by the object. Once this code has performed its tasks without
throwing an exception replace the object's data by the new data.
    o  Member functions modifying their object's data should not return
original (contained) objects by value.

The canonical assignment operator is a good example of the first rule of
thumb. Another example is found in classes storing objects. Consider a class
PersonDb storing multiple Person objects. Such a class might offer a
member void add(Person const &next). A plain implementation of this
function (merely intended to show the application of the first rule of thumb,
but otherwise completely disregarding efficiency considerations) might be:
            Person *PersonDb::newAppend(Person const &next)
    {
        Person *tmp = 0;
        try
        {
            tmp = new Person[d_size + 1];
            for (size_t idx = 0; idx < d_size; ++idx)
                tmp[idx] = d_data[idx];
            tmp[d_size] = next;
            return tmp;
        }
        catch (...)
        {
            delete[] tmp;
            throw;
        }
    }

    void PersonDb::add(Person const &next)
    {
        Person *tmp = newAppend(next);
        delete[] d_data;
        d_data = tmp;
        ++d_size;
    }

The (private) newAppend member's task is to create a copy of the
currently allocated Person objects, including the data of the next
Person object. Its catch handler catches any exception that might be
thrown during the allocation or copy process and  returns all memory
allocated so far, rethrowing the exception at the end. The function is
 exception neutral as it propagates all its exceptions to its caller. The
function also doesn't modify the PersonDb object's data, so it meets the
strong exception guarantee. Returning from newAppend the member add
may now modify its data. Its existing data are returned and its d_data
pointer is made to point to the newly created array of Person
objects. Finally its d_size is incremented. As these three steps don't
throw exceptions add too meets the strong guarantee.

The second rule of thumb (member functions modifying their object's data
should not return original (contained) objects by value) may be illustrated
using a member PersonDb::erase(size_t idx). Here is an implementation
attempting to return the original d_data[idx] object:
            Person PersonData::erase(size_t idx)
    {
        if (idx >= d_size)
            throw "Array bounds exceeded"s;
        Person ret(d_data[idx]);
        Person *tmp = copyAllBut(idx);
        delete[] d_data;
        d_data = tmp;
        --d_size;
        return ret;
    }

Although copy elision usually prevents the use of the copy constructor
when returning ret, this is not guaranteed to happen. Furthermore, a copy
constructor may throw an exception. If that happens the function has
irrevocably mutated the PersonDb's data, thus losing the strong guarantee.

Rather than returning d_data[idx] by value it might be assigned to an
external Person object before mutating PersonDb's data:
            void PersonData::erase(Person *dest, size_t idx)
    {
        if (idx >= d_size)
            throw "Array bounds exceeded"s;
        *dest = d_data[idx];
        Person *tmp = copyAllBut(idx);
        delete[] d_data;
        d_data = tmp;
        --d_size;
    }

This modification works, but changes the original assignment of creating a
member returning the original object. However, both functions suffer from a
task overload as they modify PersonDb's data and also return an original
object. In situations like these the one-function-one-responsibility
rule of thumb should be kept in mind: a function should have a single, well
defined responsibility.

The preferred approach is to retrieve PersonDb's objects using a member
like Person const &at(size_t idx) const and to erase an object using a
member like void PersonData::erase(size_t idx).


10.10.3: The nothrow guarantee

    Exception safety can only be realized if some functions and operations are
guaranteed not to throw exceptions. This is called the
 nothrow guarantee. An example of a function that must offer the nothrow
guarantee is the swap function. Consider once again the canonical
overloaded assignment operator:
            Class &operator=(Class const &other)
    {
        Class tmp(other);
        swap(tmp);
        return *this;
    }

If swap were allowed to throw exceptions then it would most likely
leave the current object in a partially swapped state. As a result the current
object's state would most likely have been changed. As tmp has been
destroyed by the time a catch handler receives the thrown exception it becomes
very difficult (as in: impossible) to retrieve the object's original
state. Losing the strong guarantee as a consequence.

The swap function must therefore offer the nothrow guarantee. It must
have been designed as if using the following prototype (see also section
23.8): 
            void Class::swap(Class &other) noexcept;

Likewise, operator delete and operator delete[] offer the nothrow
guarantee, and according to the C++ standard destructors may themselves
not throw exceptions (if they do their behavior is formally undefined, see
also section 10.12 below).

Since the C programming language does not define the exception concept
functions from the standard C library offer the nothrow guarantee
by implication. This allowed us to define the generic swap function in
section 9.6 using memcpy.

Operations on primitive types offer the nothrow guarantee. Pointers may be
reassigned, references may be returned etc. etc. without having to worry about
exceptions that might be thrown.


10.11: Function try blocks

    Exceptions may be generated from inside constructors. How can exceptions
generated in such situations be caught by the constructor itself, rather than
outside the constructor? The intuitive solution, nesting the object
construction in a try block does not solve the problem. The exception by
then has left the constructor and the object we intended to construct isn't
visible anymore.

Using a nested try block is illustrated in the next example, where
main defines an object of class PersonDb. Assuming that
PersonDb's constructor throws an exception, there is no way we can access
the resources that might have been allocated by PersonDb's constructor
from the catch handler as the pdb object is out of scope:
            int main(int argc, char **argv)
    {
        try
        {
            PersonDb pdb{ argc, argv }; // may throw exceptions
            ...                         // main()'s other code
        }
        catch (...)                      // and/or other handlers
        {
            ...                         // pdb is inaccessible from here
        }
    }

Although all objects and variables defined inside a try block are
inaccessible from its associated catch handlers, object data members were
available before starting the try block and so they may be accessed from a
catch handler. In the following example the catch handler in
PersonDb's constructor is able to access its d_data member:
            PersonDb::PersonDb(int argc, char **argv)
    :
        d_data(0),
        d_size(0)
    {
        try
        {
            initialize(argc, argv);
        }
        catch (...)
        {
            // d_data, d_size: accessible
        }
    }

Unfortunately, this does not help us much. The initialize member is
unable to reassign d_data and d_size if PersonDb const pdb
was defined; the initialize member should at least offer the basic
exception guarantee and return any resources it has acquired before
terminating due to a thrown exception; and although d_data and d_size
offer the nothrow guarantee as they are of primitive data types a class type
data member might throw an exception, possibly resulting in violation of the
basic guarantee.

In the next implementation of PersonDb assume that constructor
receives a pointer to an already allocated block of Person objects. The
PersonDb object takes ownership of the allocated memory and it is
therefore responsible for the allocated memory's eventual destruction.
Moreover, d_data and d_size are also used by a composed object
PersonDbSupport, having a constructor expecting a Person const * and
size_t argument. Our next implementation may then look something like
this:
            PersonDb::PersonDb(Person *pData, size_t size)
    :
        d_data(pData),
        d_size(size),
        d_support(d_data, d_size)
    {
        // no further actions
    }

This setup allows us to define a PersonDb const &pdb. Unfortunately,
PersonDb cannot offer the basic guarantee. If PersonDbSupport's
constructor throws an exception it isn't caught although d_data already
points to allocated memory.

The function try block offers a solution for this problem. A function
try block consists of a try block and its associated handlers. The
function try block starts immediately after the function header, and
its block defines the function body. With constructors base class and data
member initializers may be placed between the try keyword and the opening
curly brace. Here is our final implementation of PersonDb, now offering
the basic guarantee:
            PersonDb::PersonDb(Person *pData, size_t size)
    try
    :
        d_data(pData),
        d_size(size),
        d_support(d_data, d_size)
    {}
    catch (...)
    {
        delete[] d_data;
    }

Let's have a look at a stripped-down example. A constructor defines a
function try block. The exception thrown by the Throw object is initially
caught by the object itself. Then it is rethrown. The surrounding
Composer's constructor also defines a function try block, Throw's
rethrown exception is properly caught by Composer's exception handler,
even though the exception was generated from within its member initializer
list:
            #include <iostream>

    class Throw
    {
        public:
            Throw(int value)
            try
            {
                throw value;
            }
            catch (...)
            {
                std::cout << "Throw's exception handled locally by Throw()\n";
                throw;
            }
    };

    class Composer
    {
        Throw d_t;
        public:
            Composer()
            try             // NOTE: try precedes initializer list
            :
                d_t(5)
            {}
            catch (...)
            {
                std::cout << "Composer() caught exception as well\n";
            }
    };

    int main()
    {
        Composer c;
    }


When running this example, we're in for a nasty surprise: the program runs
and then breaks with an abort exception. Here is the output it produces,
the last two lines being added by the system's final catch-all handler,
catching all remaining uncaught exceptions:
            Throw's exception handled locally by Throw()
    Composer() caught exception as well
    terminate called after throwing an instance of 'int'
    Abort

The reason for this is documented in the C++ standard: at the end of a
catch-handler belonging to a constructor or destructor function try block, the
original exception is automatically rethrown.

The exception is not rethrown if the handler itself throws another exception,
offering the constructor or destructor a way to
  replace a thrown exception by another one. The
exception is only rethrown if it reaches the end of the catch handler of a
constructor or destructor function try block. Exceptions caught by nested
catch handlers are not automatically rethrown.

As only constructors and destructors rethrow exceptions caught by their
function try block catch handlers the run-time error encountered in the above
example may simply be repaired by providing main with its own function try
block:
            int main()
    try
    {
        Composer c;
    }
    catch (...)
    {}

Now the program runs as planned, producing the following output:
            Throw's exception handled locally by Throw()
    Composer() caught exception as well

A final note: if a function defining a function try block also declares an
exception throw list then only the types of rethrown exceptions must match
the types mentioned in the throw list.


10.12: Exceptions in constructors

    Object destructors are only activated for completely constructed objects.
Although this may
sound like a truism, there is a subtlety here. If the construction of an
object fails for some reason, the object's destructor is not called when
the object goes out of scope. This could happen if an exception that is
 
    generated by the constructor is not caught by the constructor. If the
exception is thrown when the object has already allocated some memory, then
that memory is not returned: its destructor isn't called as the object's
construction wasn't successfully completed.

The following example illustrates this situation in its prototypical
form. The constructor of the class Incomplete first displays a message
and then throws an exception. Its destructor also displays a message:
            class Incomplete
    {
        public:
            Incomplete()
            {
                cerr << "Allocated some memory\n";
                throw 0;
            }
            ~Incomplete()
            {
                cerr << "Destroying the allocated memory\n";
            }
    };

    Next, main() creates an Incomplete object inside a try
block. Any exception that may be generated is subsequently caught:
            int main()
    {
        try
        {
            cerr << "Creating `Incomplete' object\n";
            Incomplete{};
            cerr << "Object constructed\n";
        }
        catch (...)
        {
            cerr << "Caught exception\n";
        }
    }

    When this program is run, it produces the following output:
            Creating `Incomplete' object
    Allocated some memory
    Caught exception

Thus, if Incomplete's constructor would actually have allocated some
memory, the program would suffer from a memory leak. To prevent this from
happening, the following counter measures are available:
    
    o  Prevent the exceptions from leaving the constructor.

        If part of the constructor's body may generate exceptions, then this
part may be surrounded by a try block, allowing the exception to be caught
by the constructor itself. This approach is defensible when the constructor
is able to repair the cause of the exception and to complete its construction
as a valid object.
    o  If an exception is generated by a base class constructor or by a
member initializing constructor then a try block within the constructor's
 body won't be able to catch the thrown exception. This always results in
the exception leaving the constructor and the object is not considered to have
been properly constructed. A try block may include the member
initializers, and the try block's compound statement becomes the
constructor's body as in the following example:
    class Incomplete2
{
    Composed d_composed;
    public:
        Incomplete2()
        try
        :
            d_composed(/* arguments */)
        {
            // body
        }
        catch (...)
        {}
};

An exception thrown by either the member initializers or the body
results in the execution never reaching the body's closing curly brace. Instead
the catch clause is reached. Since the constructor's body isn't properly
completed the object is not considered properly constructed and eventually the
object's destructor won't be called.

The catch clause of a constructor's function try block behaves
slightly different than a catch clause of an ordinary function try
block. An exception reaching a constructor's function try block may be
transformed into another exception (which is thrown from the catch clause) but
if no exception is explicitly thrown from the catch clause the exception
originally reaching the catch clause is always rethrown. Consequently, there's
no way to confine an exception thrown from a base class constructor or from a
member initializer to the constructor: such an exception always propagates
to a more shallow block and in that case the object's construction is always
considered incomplete.

Therefore, if incompletely constructed objects throw exceptions then
the constructor remains responsible for preventing memory
(generally: resource) leaks. There are several ways to realize this:
    
    o  If the constructor defines a function try block, then the
constructor's catch clause cannot use any of its data members
anymore. By implication: if the constructor has allocated memory pointed to by
one of its data members, then the catch-clause cannot delete that
memory. Instead, the constructor's body itself must ensure that the memory is
properly deleted;
    o  When multiple inheritance is used: if initial base classes have
properly been constructed and a later base class throws, then the initial base
class objects are automatically destroyed (as they are themselves fully
constructed objects)
    o  When composition is used: already constructed composed objects are
automatically destroyed (as they are fully constructed objects)
    o  Instead of using plain pointers smart pointers (cf. section
18.4) should be used to manage dynamically allocated memory. In this
case, if the constructor throws either before or after the allocation of the
dynamic memory, then allocated memory is properly returned as shared_ptr
objects are, after all, objects.
    o  If plain pointer data members must be used then the constructor's
body should first, in its member initialization section, initialize its plain
pointer data members. Then, in its body it can dynamically allocate memory,
reassigning the plain pointer data members. In these cases the
pointer-handling must be embedded in a try-block, allowing the constructor to
free the allocated memory before a final exception is thrown, possibly 
reaching the catch clause of the constructor's function try block. That final
catch clause can complete whatever actions are required (e.g., write a
log-file entry). Example:
    class Incomplete2
{
    Composed d_composed;
    char *d_cp;         // plain pointers
    int *d_ip;

    public:
        Incomplete2(size_t nChars, size_t nInts)
        try
        :
            d_composed(/* arguments */),    // might throw
            d_cp(0),
            d_ip(0)
        {
            try
            {
                preamble();                     // might throw
                d_cp = new char[nChars];        // might throw
                d_ip = new int[nChars];         // might throw
                postamble();                    // might throw
            }
            catch (...)
            {
                delete[] d_cp;                  // clean up
                delete[] d_ip;
                throw;                          // retrow the exception
            }
        }
        catch (...)
        {
            // maybe write a log-entry, but also throws 
            // the original exception
        }
};

On the other hand, C++ supports constructor delegation, so an object
may have been completely constructed according to the C++ run-time system,
but yet its (delegating) constructor may throw an exception, as illustrated by
the next example:
             1: #include <iostream>
     2: using namespace std;
     3: 
     4: class Delegate
     5: {
     6:     char *d_p1;
     7:     char *d_p2;
     8: 
     9:     public:
    10:         Delegate()      // succeeds -> object constructed
    11:         :
    12:             Delegate(0)
    13:         {
    14:             d_p2 = new char[10];
    15:             cout << "default, throws...\n";
    16:             throw 12;   // but considered constructed
    17:         }
    18:         ~Delegate()
    19:         {
    20:             delete[] d_p1;
    21:             delete[] d_p2;
    22:             cout << "destructor\n";
    23:         }
    24: 
    25:     private:
    26:         Delegate(int x)         // completes OK
    27:         :
    28:             d_p1(0),
    29:             d_p2(0)
    30:         {
    31:             cout << "delegated\n";
    32:         }
    33: };
    34: 
    35: int main()
    36: try
    37: {
    38:     Delegate del;           // throws
    39: 
    40:     cout << "never reached\n";
    41: } // del's destructor is called here
    42: catch (...)
    43: {
    44:     cout << "main's catch clause\n";
    45: }

Here it is the responsibility of Delegate's designer to ensure that
the throwing default constructor does not invalidate the actions performed by
the Delegate(int x) constructor. The latter constructor is called (line
12) by the default constructor, and merely initializes (lines 28, 29) the data
members at lines 6 and 7. Next, the default constructor, after allocating some
memory, throws an exception (line 16). In fact, an exception may be called at
any point, since the destructor (line 18) will be called automatically anyway
(line 41). If multiple exceptions could be thrown then Delegate can define
an enumeration and a data member of that enumeration type, which is set to the
enum value indication the nature of the next exception (if it is thrown), so
the destructor can handle the exception according to its type.


10.13: Exceptions in destructors

    According to the C++ standard exceptions thrown by destructors may
not leave their bodies. Providing a destructor with a function try
block is therefore a violation of the standard: exceptions caught by a
function try block's catch clause have already left the destructor's body.
If --in violation of the standard-- the destructor is provided with a
function try block and an exception is caught by the try block then
that exception is rethrown, similar to what happens in catch clauses of
constructor functions' try blocks.

The consequences of  an exception leaving the destructor's body is not
defined, and may result in unexpected behavior. Consider the following example:

Assume a carpenter builds a cupboard having a single drawer. The cupboard
is finished, and a customer, buying the cupboard, finds that the cupboard can
be used as expected. Satisfied with the cupboard, the customer asks the
carpenter to build another cupboard, this time having two
drawers. When the second cupboard is finished, the customer takes it home and
is utterly amazed when the second cupboard completely collapses immediately
after it is used for the first time.

Weird story? Then consider the following program:
            int main()
    {
        try
        {
            cerr << "Creating Cupboard1\n";
            Cupboard1{};
            cerr << "Beyond Cupboard1 object\n";
        }
        catch (...)
        {
            cerr << "Cupboard1 behaves as expected\n";
        }
        try
        {
            cerr << "Creating Cupboard2\n";
            Cupboard2{};
            cerr << "Beyond Cupboard2 object\n";
        }
        catch (...)
        {
            cerr << "Cupboard2 behaves as expected\n";
        }
    }

    When this program is run it produces the following output:
            Creating Cupboard1
    Drawer 1 used
    Cupboard1 behaves as expected
    Creating Cupboard2
    Drawer 2 used
    Drawer 1 used
    terminate called after throwing an instance of 'int'
    Abort

The final Abort indicates that the program has aborted instead of
displaying a message like Cupboard2 behaves as expected.

Let's have a look at the three classes involved. The class Drawer has no
particular characteristics, except that its destructor throws an exception:
            class Drawer
    {
        size_t d_nr;
        public:
            Drawer(size_t nr)
            :
                d_nr(nr)
            {}
            ~Drawer()
            {
                cerr << "Drawer " << d_nr << " used\n";
                throw 0;
            }
    };

    The class Cupboard1 has no special characteristics at all. It merely
has a single composed Drawer object:
            class Cupboard1
    {
        Drawer left;
        public:
            Cupboard1()
            :
                left(1)
            {}
    };

    The class Cupboard2 is constructed comparably, but it has two
composed Drawer objects:
            class Cupboard2
    {
        Drawer left;
        Drawer right;
        public:
            Cupboard2()
            :
                left(1),
                right(2)
            {}
    };


When Cupboard1's destructor is called Drawer's destructor is
eventually called to destroy its composed object. This destructor throws an
exception, which is caught beyond the program's first try block. This
behavior is completely as expected.

A subtlety here is that Cupboard1's destructor (and hence Drawer's
destructor) is activated immediately subsequent to its construction. Its
destructor is called immediately subsequent to its construction as
Cupboard1() defines an anonymous object. As a result the Beyond
Cupboard1 object text is never inserted into std::cerr.

Because of Drawer's destructor throwing an exception a problem occurs
when Cupboard2's destructor is called. Of its two composed objects, the
second Drawer's destructor is called first.  This destructor throws an
exception, which ought to be caught beyond the program's second try
block. However, although the flow of control by then has left the context of
Cupboard2's destructor, that object hasn't completely been destroyed yet
as the destructor of its other (left) Drawer still has to be called.

Normally that would not be a big problem: once an exception is thrown from
Cupboard2's destructor any remaining actions would simply be ignored,
albeit that (as both drawers are properly constructed objects) left's
destructor would still have to be called.

This happens here too and left's destructor also needs to throw an
exception. But as we've already left the context of the second try block,
the current flow control is now thoroughly mixed up, and the program has no
other option but to abort. It does so by calling terminate(), which in
turn calls abort(). Here we have our collapsing cupboard having two
drawers, even though the cupboard having one drawer behaves perfectly.

The program aborts since there are multiple composed objects whose
destructors throw exceptions leaving the destructors. In this situation one of
the composed objects would throw an exception by the time the program's flow
control has already left its proper context causing the program to abort.

The C++ standard therefore understandably stipulates that exceptions
may never leave destructors. Here is the skeleton of a destructor whose
  code might throw
exceptions. No function try block but all the destructor's actions are
encapsulated in a try block nested under the destructor's body.
            Class::~Class()
    {
        try
        {
            maybe_throw_exceptions();
        }
        catch (...)
        {}
    }



Chapter 11: More Operator Overloading

Having covered the overloaded assignment operator in chapter 9, and
having shown several examples of other overloaded operators as well (i.e., the
insertion and extraction operators in chapters 3 and
6), we now take a look at operator overloading in general.


11.1: Overloading `operator[]()'

As our next example of operator overloading, we introduce a class IntArray
encapsulating an array of ints. Indexing the array elements is possible
using the standard array index operator [], but additionally checks for
 array bounds overflow are performed (note, however, that index checking is
not normally done by index operators. Since it's good practice to avoid
surprises array bound checks should normally not be performed by overloaded
index operators). The
 index operator (operator[]) is interesting because it can be used  in
expressions as both lvalue and as rvalue.

Here is an example illustrating the basic use of the class:
            int main()
    {
        IntArray x{ 20 };               // 20 ints

        for (int idx = 0; idx < 20; ++idx)
            x[idx] = 2 * idx;                   // assign the elements

        for (int idx = 0; idx <= 20; ++idx)     // result: boundary overflow
            cout << "At index " << idx << ": value is " << x[idx] << '\n';
    }
    First, the constructor is used to create an object containing 20
ints. The elements stored in the object can be assigned or retrieved. The
first for-loop assigns values to the elements using the index operator,
the second for-loop retrieves the values but also results in a run-time
error once the non-existing value x[20] is addressed. The IntArray
class interface is:
            #include <cstddef>

    class IntArray
    {
        size_t d_size;
        int     *d_data;

         public:
            IntArray(size_t size = 1);
            IntArray(IntArray const &other);
            ~IntArray();
            IntArray &operator=(IntArray const &other);

                                                // overloaded index operators:
            int &operator[](size_t index);              // first
            int const &operator[](size_t index) const;  // second

            void swap(IntArray &other);         // trivial

        private:
            void boundary(size_t index) const;
            int &operatorIndex(size_t index) const;
    };

    This class has the following characteristics:
    
    o  One of its constructors has a size_t parameter having a
default argument value, specifying the number of int elements in the
object.
    o  The class internally uses a pointer to reach allocated memory.
Hence, the necessary tools are provided: a copy constructor, an overloaded
assignment operator and a destructor.
    o  That there are two overloaded index operators. Why are there
two?

The first overloaded index operator allows us to reach and modify the
elements of non-constant IntArray objects.  This overloaded operator's
prototype is a function returning a reference to an int, allowing
us to use an expression like x[10] as rvalue or lvalue.

With non-const IntArray objects operator[] can therefore be used
to retrieve and to assign values.  Therefore, the return value of the
non-const operator[] member is an int &, to allow modification of the
elements when used as lvalue, whereas the return value of the const
operator[] member is preferably an int const &, rather than a mere
int. In this situation we prefer the use of a const & return value to
allow immediate writing of the return value to, e.g., a binary file, as in:
        void writeValue(Intarray const &iarr, size_t idx)
    {
        cout.write(reinterpret_cast<char const *>(&iarr[idx]));
    }

This whole scheme fails if there's nothing to assign. Consider the
situation where we have an IntArray const stable(5). Such an object is an
immutable const object. The compiler detects this and refuses to compile
this object definition if only the non-const operator[] is
available. Hence the second overloaded index operator is added to the class's
interface. This second form of the overloaded index operator is automatically
used by the compiler with const objects. It is used for value
retrieval instead of value assignment. That, of course, is precisely what
we want when using const objects. In this situation members are overloaded
only by their const attribute. This form of function overloading was
introduced earlier in the C++ Annotations (sections 2.5.4 and
7.7).

o  As there is only one pointer data member, the destruction of the
memory allocated by the object is a simple delete[] data.

Now, the implementation of the members (omitting the trivial
implementation of swap, cf. chapter 9) are:
            #include "intarray.ih"

    IntArray::IntArray(size_t size)
    :
        d_size(size)
    {
        if (d_size < 1)
            throw "IntArray: size of array must be >= 1"s;

        d_data = new int[d_size];
    }

    IntArray::IntArray(IntArray const &other)
    :
        d_size(other.d_size),
        d_data(new int[d_size])
    {
        memcpy(d_data, other.d_data, d_size * sizeof(int));
    }

    IntArray::~IntArray()
    {
        delete[] d_data;
    }

    IntArray &IntArray::operator=(IntArray const &other)
    {
        IntArray tmp(other);
        swap(tmp);
        return *this;
    }

    int &IntArray::operatorIndex(size_t index) const
    {
        boundary(index);
        return d_data[index];
    }

    int &IntArray::operator[](size_t index)
    {
        return operatorIndex(index);
    }

    int const &IntArray::operator[](size_t index) const
    {
        return operatorIndex(index);
    }

    void IntArray::swap(IntArray &other)
    {
        // swaps the d_size and d_data data members
        // of *this and other
    }


    void IntArray::boundary(size_t index) const
    {
        if (index < d_size)
            return;
        ostringstream out;
        out  << "IntArray: boundary overflow, index = " <<
                index << ", should be < " << d_size << '\n';
        throw out.str();
    }

    Note how the operator[] members were implemented: as non-const members
may call const member functions and as the implementation of the const
member function is identical to the non-const member function's implementation
both operator[] members could be defined inline using an auxiliary
function int &operatorIndex(size_t index) const.  A const member
function may return a non-const reference (or pointer) return value, referring
to one of the data members of its object. Of course, this is a potentially
dangerous backdoor that may break data hiding. However, the members in the
public interface prevent this breach and so the two public operator[]
members may themselves safely call the same int &operatorIndex() const
member, that defines a
 private backdoor.


11.1.1: Multi-argument `operator[]()'

    Consider a standard two-dimensional array. It has nRows rows and nCols
columns. Such an array is a generalization of a one-dimensional array: each
row consists of an array of nCols elements. If a type DoubleArray is
available, defined like the IntArray of the previous section, but
containing double values instead of int values, then designing a class
Matrix could start as follows:
        
    class Matrix
    {
        size_t d_nRows;
        size_t d_nCols;
        DoubleArray *d_row;

        public:
            Matrix(size_t nRows, size_t nCols);
        ...
    };
    and its constructor allocates nRows DoubleArrays, each having
nCols columns, with each DoubleArray being initialized to a nCols
elements, initialized to 0 by the DoubleArray's default constructor:
        
    Matrix::Matrix(size_t nRows, size_t nCols)
    :
        d_nRows(nRows),
        d_nCols(nCols),
        d_row(new DoubleArray[nRows])
    {}

Traditionally accessing elements of a Matrix could be realized in three
ways (plus optionally corresponding const variants):
    
    o  A member double &element(size_t row, size_t col), returning the
        value of element [row, col];
    o  A member DoubleArray &row(size_t row), returning row's
        DoubleArray;
    o  A member DoubleArray &operator[](size_t row), also returning
        row's DoubleArray;

The members element and row work fine, but at the disadvantage that
the standard syntax for referring to matrix elements is not used. E.g, to
access element matrix[3, 4] we would write matrix.element(3, 4) or
matrix.row(3)[4], whereas the third element requires us to use two index
operators: matrix[3][4]. Moreover, with the second and third members data
hiding of DoubleArray row is abandoned if the only reason for these
members is to access Matrix elements.

operator[]
The overloaded index operator, however, can also be defined having multiple
arguments, allowing the use of the (standard mathematical) syntax
matrix[row, col] (and in general: the overloaded index operator can also
have more arguments, which might come in useful when defining, e.g., arrays of
arrays). 

Providing Matrix with an index operator accepting two arguments is simple:
just add a member double &operator[](size_t row, size_t col) (and
optionally a comparable const member) to the class's interface. Its
implementation may then directly return the requested array element:
        
    double &Matrix::operator[](size_t row, size_t col)
    {
        return d_row[row][col];
    }

As an aside: note that this implementation does not check whether the provided
indices are valid. Traditionally index operators don't perform such checks,
improving the efficiency of programs when it's known that indices
cannot be invalid. E.g., to initialize all elements of a Matrix with
subsequent integral values the following function could be used:
        
    void init(Matrix &matrix, size_t value)
    {
        for (size_t row = 0; row != matrix.nRows(); ++row)
            for (size_t col = 0; col != matrix.nCols(); ++col)
                matrix[row, col] = value++;
    }


If, on the other hand, checking the validity of the indices is necessary
then a multi-argument at member like the following can be defined:
        
    double &Matrix::at(size_t row, size_t col)
    {
        if (row >= d_nrows or col >= d_nCols)
            throw runtime_error("Matrix::at: invalid indices");
        return d_row[row][col];
    }


11.2: Overloading insertion and extraction operators

    Classes may be adapted in such a way that their objects may be inserted
into and extracted from, respectively, a std::ostream and
std::istream.

The class std::ostream defines insertion operators for primitive
types, such as int, char *, etc.. In this section we learn how to
extend the existing functionality of classes (in particular std::istream
and std::ostream) in such a way that they can be used also in combination
with classes developed much later in history.

In particular we will show how the
 insertion operator can be overloaded allowing the insertion of any type of
object, say Person (see chapter 9), into an ostream. Having
defined such an overloaded operator we're able to use the following code:
            Person kr("Kernighan and Ritchie", "unknown", "unknown");

    cout << "Name, address and phone number of Person kr:\n" << kr << '\n';

The statement cout << kr uses operator<<.  This member
function has two operands: an ostream & and a Person &. The required
action is defined in an overloaded free function operator<< expecting two
arguments:
                                        // declared in `person.h'
    std::ostream &operator<<(std::ostream &out, Person const &person);

                                // defined in some source file
    ostream &operator<<(ostream &out, Person const &person)
    {
        return
            out <<
                "Name:    " << person.name() << ", "
                "Address: " << person.address() << ", "
                "Phone:   " << person.phone();
    }

The free function operator<< has the following noteworthy characteristics:
    
    o  The function returns a reference to an ostream object,
to enable `chaining' of the insertion operator.
    o  The two operands of operator<< are passed to the free function as its
arguments. In the example, the parameter out was initialized by cout,
the parameter person by kr.

In order to overload the extraction operator for, e.g., the Person
class, members are needed modifying the class's private data members. Such
 modifiers are normally offered by the class interface. For
the Person class these members could be the following:
            void setName(char const *name);
    void setAddress(char const *address);
    void setPhone(char const *phone);

These members may easily be implemented: the memory
pointed to by the corresponding data member must be deleted, and the data
member should point to a copy of the text pointed to by the parameter. E.g.,
            void Person::setAddress(char const *address)
    {
        delete[] d_address;
        d_address = strdupnew(address);
    }

A more elaborate function should check the reasonableness of the new
address (address also shouldn't be a 0-pointer). This
however, is not further pursued here. Instead, let's have a look at the final
 operator>>. A simple implementation is:
            istream &operator>>(istream &in, Person &person)
    {
        string name;
        string address;
        string phone;

        if (in >> name >> address >> phone)    // extract three strings
        {
            person.setName(name.c_str());
            person.setAddress(address.c_str());
            person.setPhone(phone.c_str());
        }
        return in;
    }

Note the stepwise approach that is followed here. First, the required
information is extracted using available extraction operators. Then, if that
succeeds, modifiers are used to modify the data members of the object to
be extracted. Finally, the stream object itself is returned as a reference.


11.3: Conversion operators

    A class may be constructed around a built-in type. E.g., a class
String, constructed around the char * type. Such a class may define
all kinds of operations, like assignments. Take a look at the following class
interface, designed after the string class:
            class String
    {
        char *d_string;

        public:
            String();
            String(char const *arg);
            ~String();
            String(String const &other);
            String &operator=(String const &rvalue);
            String &operator=(char const *rvalue);
    };

Objects of this class can be initialized from a char const *, and also
from a String itself. There is an overloaded assignment operator, allowing
the assignment from a String object and from a char const
*\ (Note that the assignment from a char const * also allows the
null-pointer. An assignment like stringObject = 0 is perfectly in order.).

Usually, in classes that are less directly linked to their data than this
String class, there will be an accessor member function,
like a member char const *String::c_str() const. However, the need to use
this latter member doesn't appeal to our intuition when an array of String
objects is defined by, e.g., a class StringArray. If this latter class
provides the operator[] to access individual String members, it would
most likely offer at least the following class interface:
            class StringArray
    {
        String *d_store;
        size_t d_n;

        public:
            StringArray(size_t size);
            StringArray(StringArray const &other);
            StringArray &operator=(StringArray const &rvalue);
            ~StringArray();

            String &operator[](size_t index);
    };

This interface allows us to assign String elements to each other:
            StringArray sa{ 10 };

    sa[4] = sa[3];  // String to String assignment

But it is also possible to assign a char const * to an element of
sa:
            sa[3] = "hello world";

Here, the following steps are taken:
    
    o  First, sa[3] is evaluated. This results in a String reference.
    o  Next, the String class is inspected for an overloaded assignment,
        expecting a char const * to its right-hand side. This operator is
        found, and the string object sa[3] receives its new value.

Now we try to do it the other way around: how to access the
char const * that's stored in sa[3]? The following attempt fails:
            char const *cp = sa[3];

It fails since we would need an overloaded assignment operator for the
'class' char const *. Unfortunately, there isn't such a class, and
therefore we can't build that overloaded assignment operator (see also section
11.15). Furthermore, casting won't work as the
compiler doesn't know how to cast a String to a char const *.  How
to proceed?

One possibility is to define an accessor member function c_str():
            char const *cp = sa[3].c_str()

This compiles fine but looks clumsy.... A far better approach would be to
use a conversion operator.

A conversion operator is a kind of overloaded operator, but this time
the overloading is used to cast the object to another type.
In class interfaces, the general form of a
conversion operator is:
            operator <type>() const;

Conversion operators usually are const member functions: they are
automatically called when their objects are used as rvalues in expressions
having a type lvalue.  Using a conversion operator a String
object may be interpreted as a char const * rvalue, allowing us to perform
the above assignment.

Conversion operators are somewhat dangerous. The conversion is automatically
performed by the compiler and unless its use is perfectly transparent it may
confuse those who read code in which conversion operators are used. E.g.,
novice C++ programmers are frequently confused by statements like `if
(cin) ...'.

As a rule of thumb: classes should define at most one conversion
operator. Multiple conversion operators may be defined but frequently result
in ambiguous code. E.g., if a class defines operator bool() const and
operator int() const then passing an object of this class to a function
expecting a size_t argument results in an ambiguity as an int and a
bool may both be used to initialize a size_t.

In the current example, the class String could define the following
conversion operator for char const *:
            String::operator char const *() const
    {
        return d_string;
    }

Notes:
    
    o  Conversion operators do not define return types. The conversion
operator returns a value of the type specified beyond the operator
keyword.
    o  In certain situations (e.g., when a String argument is passed to
a function specifying an ellipsis parameter) the compiler needs a hand to
disambiguate our intentions. A static_cast solves the problem.
    o  With template functions conversion operators may not work
immediately as expected. For example, when defining a conversion operator
X::operator std::string const() const then cout << X() won't
compile. The reason for this is explained in section 21.9, but a
shortcut allowing the conversion operator to work is to define the following
overloaded operator<< function:
    std::ostream &operator<<(std::ostream &out, std::string const &str)
{
    return out.write(str.data(), str.length());
}

Conversion operators are also used when objects of classes defining
conversion operators are inserted into streams. Realize that the right hand
sides of insertion operators are function parameters that are initialized by
the operator's right hand side arguments. The rules are simple:
    
    o  If a class X defining a conversion operator also defines an
insertion operator accepting an X object the insertion operator is used;
    o  Otherwise, if the type returned by the conversion operator is
insertable then the conversion operator is used;
    o  Otherwise, a compilation error results. Note that this happens if the
type returned by the conversion operator itself defines a conversion operator
to a type that may be inserted into a stream.

In the following example an object of class Insertable is directly
inserted; an object of the class Convertor uses the conversion operator;
an object of the class Error cannot be inserted since it does not define
an insertion operator and the type returned by its conversion operator cannot
be inserted either (Text does define an operator int() const, but
the fact that a Text itself cannot be inserted causes the error):
            #include <iostream>
    #include <string>
    using namespace std;

    struct Insertable
    {
        operator int() const
        {
            cout << "op int()\n";
            return 0;
        }
    };
    ostream &operator<<(ostream &out, Insertable const &ins)
    {
        return out << "insertion operator";
    }
    struct Convertor
    {
        operator Insertable() const
        {
            return Insertable();
        }
    };
    struct Text
    {
        operator int() const
        {
            return 1;
        }
    };
    struct Error
    {
        operator Text() const
        {
            return Text{};
        }
    };

    int main()
    {
        Insertable insertable;
        cout << insertable << '\n';
        Convertor convertor;
        cout << convertor << '\n';
        Error error;
        cout << error << '\n';
    }


Some final remarks regarding conversion operators:
    
    o  A conversion operator should be a `natural extension' of the
facilities of the object. For example, the stream classes define operator
bool(), allowing constructions like if (cin).
    o  A conversion operator usually returns an rvalue. It does so
to enforce data-hiding and because it is the intended use of the conversion
operator. Defining a conversion operator as an lvalue (e.g., defining an
operator int &() conversion operator) opens up a back door, and the
operator can only be used as lvalue when explicitly called (as in:
x.operator int&() = 5). Don't normally use it, although there are
exceptions to this rule (cf. section 11.4).
    o  Conversion operators are normally defined as const member
functions as they normally don't modify their object's data members (but
again: see section 11.4).
    o  Conversion operators returning composed objects should return const
references to these objects whenever possible to avoid calling the composed
object's copy constructor.


11.4: An alternative implementation of the `byte' type

In chapter 3 the std::byte type was introduced. It
offers bit-wise and comparison operators but lacks other arithmetic operators
as well as insertion and extraction operators, which may render it less
useful. Fortunately, using the facilities provided by operator overloading a
more generically useful byte-type can be developed.

In this section we develop a class Byte offering all facilities of numeric
types, as well as insertion and extraction operators, while the size of a
Byte object equals the size of an unsigned char: 1 byte.

A size 1 Byte type is realized by defining a class Byte having a
single uint8_t data member. Insertion and extraction operators are
provided by free functions. The class Byte, therefore, starts as follows:
        #include <iostream>
    #include <cstdint>          // uint8_t
    
    class Byte
    {
        uint8_t d_byte;

All the class's member functions are public: Byte objects can be defined
using the default constructor, the copy constructor, and a constructor
accepting any argument that can be converted to a uint8_t:
                Byte();                     // CC available by default
            Byte(uint8_t byte);

Byte objects can be used as lvalues, so assignment operators must be
provided. The first (default) assignment operator handles assignment of
Byte objects, while the second assignment operator handles assignment of
numeric types which can be converted to uint8_t values:
                Byte &operator=(Byte const &rhs) = default;
            Byte &operator=(uint8_t rhs);

Conversion operators are provided so Byte objects can be used in
situations where uint8_t values are used:
                operator uint8_t &();
            operator uint8_t() const;

Athough conversion operators usually return const references, an excption
is made for Byte because of its semantics: as a Byte is essentially a
wrapper around a uint8_t a non-const conversion operator is provided
allowing the use of arithmetic assignment operators. A statement like byte
+= 13 (having defined Byte byte) is compiled as byte.operator() = 13,
which requires the non-const conversion operator.

Most members require a single statement and can very well be implemented
inline. Here are their implementations:
        inline Byte::Byte()
    :
        d_byte(0)
    {}
    
    inline Byte::Byte(uint8_t byte)
    :
        d_byte(byte)
    {}
    
    Byte &Byte::operator=(uint8_t rhs)
    {
        d_byte = rhs;
        return *this;
    }
    
    inline Byte::operator uint8_t &()
    {
        return d_byte;
    }
    
    inline Byte::operator uint8_t () const // 16_t
    {
        return d_byte;
    }

The insertion and extraction operators act identically to the standard
insertion and extraction operators for uint8_t type values: they insert
and extract the char representation of the Byte's value, because
that's what insertion and extraction operators do: they process text. Here are
their one-line implementations, using the conversion operators to insert or
assign the Byte's d_byte value:
        inline std::ostream &operator<<(std::ostream &out, Byte const &byte)
    {
        return out << byte.operator uint8_t();
    }
    
    inline std::istream &operator>>(std::istream &in, Byte &byte)
    {
        return in >> byte.operator uint8_t &();
    }

To write the d_byte's binary value the stream's write member
should be used in the way it's always used: reinterpret a Byte as a
char const * and write a single byte. Reading a Byte's binary value is
implemented analogously, using the stream's read member.

Finally, here are some examples of how Byte variables can be used in
practice: 
        using namespace std;
    
    int main()
    {
        Byte b1;            // default: d_byte = 0
        Byte b2{ 12 };      // construct from an int
        Byte b3{ b2 };      // copy-construct
    
        b1 = 65;            // direct assignment
        b1 += 20;           // arithmetic assignment
    
        b1 <<= 1;           // shift-assignments
        b1 >>= 1;
    
        b1 |= 1;            // bit-or assignment
    
        uint8_t u8 = b1;    // assign Byte to an uint8_t
    
                            // some stream insertions
        cout << sizeof(Byte) << ',' << (b1 < b2) << ',' <<
                b1 << ',' << b3 << ',' << u8 << '\n';
    
                            // using 'write'
        cout.write(reinterpret_cast<char const *>(&b1), 1) << '\n';
    }
    
    // output:
    //  hex values                                 text ( . : not printable)
    //  31 2C 30 2C 55 2C 0C 2C 55 0A 55 0A        1,0,U,.,U.U.


11.5: The keyword `explicit'

    Conversions are not only performed by conversion operators, but also by
constructors accepting one argument (i.e., constructors having one or multiple
parameters, specifying default argument values for all parameters or for all
but the first parameter).

Assume a data base class DataBase is defined in which Person
objects can be stored. It defines a Person *d_data pointer, and so it
offers a copy constructor and an overloaded assignment operator.

In addition to the copy constructor DataBase offers a default
constructor and several additional constructors:
    
    o DataBase(Person const &): the DataBase initially contains a
single Person object;
    o DataBase(istream &in): the data about multiple persons are read from
in.
    o DataBase(size_t count, istream &in = cin): the data of count
persons are read from in, by default the standard input stream.

The above constructors all are perfectly reasonable. But they also allow
the compiler to compile the following code without producing any warning at
all:
            DataBase db;
    DataBase db2;
    Person person;

    db2 = db;           // 1
    db2 = person;       // 2
    db2 = 10;           // 3
    db2 = cin;          // 4

Statement 1 is perfectly reasonable: db is used to redefine
db2. Statement 2 might be understandable since we designed DataBase to
contain Person objects. Nevertheless, we might question the logic that's
used here as a Person is not some kind of DataBase. The logic becomes
even more opaque when looking at statements 3 and 4. Statement 3 in effect
waits for the data of 10 persons to appear at the standard input
stream. Nothing like that is suggested by db2 = 10.

Implicit promotions are used with statements 2 through 4. Since
constructors accepting, respectively a Person, an istream, and a
size_t and an istream have been defined for DataBase and since the
assignment operator expects a DataBase right-hand side (rhs) argument the
compiler first converts the rhs arguments to anonymous DataBase objects
which are then assigned to db2.

It is good practice to prevent implicit promotions by using the
 explicit modifier when declaring a constructor. Constructors using the
explicit modifier can only be used to construct objects
explicitly. Statements 2-4 would not have compiled if the constructors
expecting one argument would have been declared using explicit. E.g.,
            explicit DataBase(Person const &person);
    explicit DataBase(size_t count, std:istream &in);

Having declared all constructors accepting one argument as explicit
the above assignments would have required the explicit specification of
the appropriate constructors, thus clarifying the programmer's intent:
            DataBase db;
    DataBase db2;
    Person person;

    db2 = db;                   // 1
    db2 = DataBase{ person };   // 2
    db2 = DataBase{ 10 };       // 3
    db2 = DataBase{ cin };      // 4

As a rule of thumb prefix one argument constructors with the
explicit keyword unless implicit promotions are perfectly natural
(string's char const * accepting constructor is a case in point).


11.5.1: Explicit conversion operators

    In addition to explicit constructors, C++ supports
 
explicit conversion operators.

For example, a class might define operator bool() const returning true
if an object of that class is in a usable state and false if not.
Since the type bool is an arithmetic type this could result in unexpected
or unintended behavior. Consider:
            void process(bool value);

    class StreamHandler
    {
        public:
            operator bool() const;      // true: object is fit for use
            ...
    };

    int fun(StreamHandler &sh)
    {
        int sx;

        if (sh)                         // intended use of operator bool()
            ... use sh as usual; also use `sx'

        process(sh);                    // typo: `sx' was intended
    }

In this example process unintentionally receives the value returned by
operator bool using the implicit conversion from bool to int.

When defining explicit conversion operators implicit conversions like the
one shown in the example are prevented. Such conversion operators can only be
used in situations where the converted type is explicitly required (as in the
condition clauses of if or while statements), or is explicitly
requested using a static_cast. To declare an explicit bool conversion
operator in class StreamHandler's interface replace the above declaration
by:
            explicit operator bool() const;

Since the C++14 standard istreams define an 
    explicit operator bool() const. As a consequence:
        while (cin.get(ch)) // compiles OK
        ;

    bool fun1()
    {
        return cin;     // 'bool = istream' won't compile as 
    }                   // istream defines 'explicit operator bool'

    bool fun1()
    {
        return static_cast<bool>(cin); // compiles OK
    }


11.6: Overloading increment and decrement operators

Overloading the 
    increment operator (operator++) and
    decrement operator ( operator--) introduces a
small problem: there are two versions of each operator, as they may be used as
postfix operator (e.g., x++) or as prefix operator (e.g.,
++x).

Used as postfix operator, the value's object is returned as an
rvalue, temporary const object and the post-incremented variable itself
disappears from view. Used as prefix operator, the variable is
incremented, and its value is returned as lvalue and it may be altered
again by modifying the prefix operator's return value. Whereas these
characteristics are not required when the operator is overloaded, it is
strongly advised to implement these characteristics in any overloaded
increment or decrement operator.

Suppose we define a wrapper class around the size_t value
type. Such a class could offer the following (partially shown) interface:
            class Unsigned
    {
        size_t d_value;

        public:
            Unsigned();
            explicit Unsigned(size_t init);

            Unsigned &operator++();
    }

The class's last member declares the prefix overloaded increment
operator. The returned lvalue is Unsigned &. The member is easily
implemented:
            Unsigned &Unsigned::operator++()
    {
        ++d_value;
        return *this;
    }

To define the postfix operator, an overloaded version of the operator
is defined, expecting a (dummy) int argument. This might be considered a
kludge, or an acceptable application of function overloading. Whatever
your opinion in this matter, the following can be concluded:
    
    o  Overloaded increment and decrement operators without parameters
are prefix operators, and should return references to the current
object.
    o  Overloaded increment and decrement operators having an int
parameter are postfix operators, and should return a value which
is a copy of the object at the point where its postfix operator is used.

The postfix increment operator is declared as follows in the class
Unsigned's interface:
            Unsigned operator++(int);

It may be implemented as follows:
            Unsigned Unsigned::operator++(int)
    {
        Unsigned tmp{ *this };
        ++d_value;
        return tmp;
    }

Note that the operator's parameter is not used. It is only part of the
implementation to disambiguate the prefix- and postfix operators in
implementations and declarations.

In the above example the statement incrementing the current object offers
the nothrow guarantee as it only involves an operation on a
primitive type. If the initial copy construction throws then the original
object is not modified, if the return statement throws the object has safely
been modified. But incrementing an object could itself throw exceptions. How
to implement the increment operators in that case? Once again, swap is our
friend. Here are the pre- and postfix operators offering the strong guarantee
when the member increment performing the increment operation may throw:
            Unsigned &Unsigned::operator++()
    {
        Unsigned tmp{ *this };
        tmp.increment();
        swap(tmp);
        return *this;
    }
    Unsigned Unsigned::operator++(int)
    {
        Unsigned tmp{ *this };
        tmp.increment();
        swap(tmp);
        return tmp;
    }

Both operators first create copies of the current objects. These copies
are incremented and then swapped with the current objects. If increment
throws the current objects remain unaltered; the swap operations ensure that
the correct objects are returned (the incremented object for the prefix
operator, the original object for the postfix operator) and that the current
objects become the incremented objects.

When calling the increment or decrement operator using its full member
function name then any int argument passed to the function results in
calling the postfix operator. Omitting the argument results in calling the
prefix operator. Example:
            Unsigned uns{ 13 };

    uns.operator++();     // prefix-incrementing uns
    uns.operator++(0);    // postfix-incrementing uns

Both the prefix and postfix increment and decrement operators are deprecated
when applied to bool type of variables. In situations where a postfix
increment operator could be useful the std::exchange (cf. section
19.1.13) should be used.


11.7: Overloading binary operators

    In various classes overloading binary operators (like operator+) can
be a very natural extension of the class's functionality. For example, the
std::string class has various overloaded operator+ members.

Most binary operators come in two flavors: the plain binary operator (like
the + operator) and the compound binary assignment operator (like
operator+=). Whereas the plain binary operators return values, the
compound binary assignment operators usually return references to the objects
for which the operators were called. For example, with std::string objects
the following code (annotations below the example) may be used:
            std::string s1;
    std::string s2;
    std::string s3;

    s1 = s2 += s3;                  // 1
    (s2 += s3) + " postfix";        // 2
    s1 = "prefix " + s3;            // 3
    "prefix " + s3 + "postfix";     // 4

    
    o  at // 1 the content of s3 is added to s2. Next, s2
is returned, and its new content is assigned to s1. Note that +=
returns s2.
    o  at // 2 the content of s3 is also added to s2, but as
+= returns s2 itself, it's possible to add some more to s2
    o  at // 3 the + operator returns a std::string containing
the concatenation of the text prefix and the content of s3. This
string returned by the + operator is thereupon assigned to s1.
    o  at // 4 the + operator is applied twice. The effect is:
        
        1.  The first + returns a std::string containing
the concatenation of the text prefix and the content of s3.
        2.  The second + operator takes this returned string as its left
hand value, and returns a string containing the concatenated text of its left
and right hand operands.
        3.  The string returned by the second + operator represents the
value of the expression.
        

Now consider the following code, in which a class Binary supports an
overloaded operator+:
            class Binary
    {
        public:
            Binary();
            Binary(int value);
            Binary operator+(Binary const &rhs);
    };

    int main()
    {
        Binary b1;
        Binary b2{ 5 };

        b1 = b2 + 3;            // 1
        b1 = 3 + b2;            // 2
    }

    Compilation of this little program fails for statement // 2, with the
compiler reporting an error like:
            error: no match for 'operator+' in '3 + b2'

Why is statement // 1 compiled correctly whereas statement // 2
won't compile?

In order to understand this remember promotions. As we have seen in
section 11.5, constructors expecting single arguments may implicitly
be activated when an argument of an appropriate type is provided. We've
already encountered this with std::string objects, where NTBSs may be used
to initialize std::string objects.

Analogously, in statement // 1, operator+ is called, using b2
as its left-hand side operand. This operator expects another Binary object
as its right-hand side operand. However, an int is provided. But as a
constructor Binary(int) exists, the int value can be promoted to a
Binary object. Next, this Binary object is passed as argument to the
operator+ member.

Unfortunately, in statement // 2 promotions are not available: here
the + operator is applied to an int-type lvalue. An int is a
primitive type and primitive types have no knowledge of `constructors',
`member functions' or `promotions'.

How, then, are promotions of left-hand operands implemented in statements
like "prefix " + s3? Since promotions can be applied to function
arguments, we must make sure that both operands of binary operators are
arguments. This implies that plain binary operators supporting promotions for
either their left-hand side operand or right-hand side operand should be
declared as
        free operators,
    also called free functions.

Functions like the plain binary operators conceptually belong to the class
for which they implement these operators. Consequently they should be
declared in the class's header file. We cover their implementations
shortly, but here is our first revision of the declaration of the class
Binary, declaring an overloaded + operator as a free function:
            class Binary
    {
        public:
            Binary();
            Binary(int value);
    };

    Binary operator+(Binary const &lhs, Binary const &rhs);


After defining binary operators as free functions, several promotions are
available:
    
    o  If the left-hand operand is of the intended class type, the right
hand argument is promoted whenever possible;
    o  If the right-hand operand is of the intended class type, the left
hand argument is promoted whenever possible;
    o  No promotions occur when neither operand is of the intended
class type;
    o  An ambiguity occurs when promotions to different classes are possible
for the two operands. For example:
            class A;

    class B
    {
        public:
            B(A const &a);
    };

    class A
    {
        public:
            A();
            A(B const &b);
    };

    A operator+(A const &a, B const &b);
    B operator+(B const &b, A const &a);

    int main()
    {
        A a;

        a + a;
    };

    Here, both overloaded + operators are possible candidates when
compiling the statement a + a. The ambiguity must be solved by explicitly
promoting one of the arguments, e.g., a + B{a}, which enables the compiler
to resolve the ambiguity to the first overloaded + operator.

The next step consists of implementing the required overloaded binary
compound assignment operators, having the form @=, where @ represents
a binary operator. As these operators always have left-hand side operands
which are object of their own classes, they are implemented as genuine member
functions. Compound assignment operators usually return references to the
objects for which the binary compound assignment operators were requested, as
these objects might be modified in the same statement. E.g.,
    (s2 += s3) + " postfix". 

Here is our second revision of the class Binary, showing the
declaration of the plain binary operator as well as the corresponding compound
assignment operator:
            class Binary
    {
        public:
            Binary();
            Binary(int value);

            Binary &operator+=(Binary const &rhs);
    };

    Binary operator+(Binary const &lhs, Binary const &rhs);


How should the compound addition assignment operator be implemented?  When
implementing compound binary assignment operators the strong guarantee should
always be kept in mind: if the operation might throw use a temporary object
and swap. Here is our implementation of the compound assignment operator:
            Binary &Binary::operator+=(Binary const &rhs)
    {
        Binary tmp{ *this };
        tmp.add(rhs);           // this might throw
        swap(tmp);
        return *this;
    }

It's easy to implement the free binary operator: the lhs argument is
copied into a Binary tmp to which the rhs operand is added. Then
tmp is returned, using copy elision. The class Binary declares the
free binary operator as a friend (cf. chapter 15), so it can call
Binary's add member:
            class Binary
    {
        friend Binary operator+(Binary const &lhs, Binary const &rhs);

        public:
            Binary();
            Binary(int value);

            Binary &operator+=(Binary const &other);

        private:
            void add(Binary const &other);
    };


The binary operator's implementation becomes:
            Binary operator+(Binary const &lhs, Binary const &rhs)
    {
        Binary tmp{ lhs };
        tmp.add(rhs);
        return tmp;
    }


If the class Binary is move-aware then it's attractive to add move-aware
binary operators. In this case we also need operators whose left-hand side
operands are rvalue references. When a class is move aware various interesting
implementations are suddenly possible, which we encounter below, and in
the next (sub)section. First have a look at the signature of such a binary
operator (which should also be declared as a friend in the class interface):
            Binary operator+(Binary &&lhs, Binary const &rhs);

Since the lhs operand is an rvalue reference, we can modify it ad lib. 
Binary operators are commonly designed as factory functions, returning objects
created by those operators. However, the (modified) object referred to by 
lhs should itself not be returned. As stated in the C++ standard,
     
    A temporary object bound to a reference parameter in a function call
    persists until the completion of the full-expression containing the call.

and furthermore: 
     
    The lifetime of a temporary bound to the returned value in a function
    return statement is not extended; the temporary is destroyed at the end of
    the full-expression in the return statement.

In other words, a temporary object cannot itself be returned as the
function's return value: a Binary && return type should therefore not be
used. Therefore functions implementing binary operators are factory functions
(note, however, that the returned object may be constructed using the class's
move constructor whenever a temporary object has to be returned).

Alternatively, the binary operator can first create an object by move
constructing it from the operator's lhs operand, performing the binary
operation on that object and the operator's rhs operand, and then return the
modified object (allowing the compiler to apply copy elision). It's a matter
of taste which one is preferred.

Here are the two implementations. Because of copy elision the explicitly
defined ret object is created in the location of the return value. Both
implementations, although they appear to be different, show identical run-time
behavior:
                        // first implementation: modify lhs
    Binary operator+(Binary &&lhs, Binary const &rhs)   
    {
        lhs.add(rhs);
        return std::move(lhs);
    }
                // second implementation: move construct ret from lhs
    Binary operator+(Binary &&lhs, Binary const &rhs)   
    {
        Binary ret{ std::move(lhs) };
        ret.add(rhs);
        return ret;
    }

Now, when executing expressions like (all Binary objects) 
b1 + b2 + b3 the following functions are called:
            copy operator+          = b1 + b2 
    Copy constructor        = tmp(b1) 
        adding              = tmp.add(b2)
    copy elision            : tmp is returned from b1 + b2
        
    move operator+          = tmp + b3 
    adding                  = tmp.add(b3)
    Move construction       = tmp2(move(tmp)) is returned

But we're not there yet: in the next section we encounter possibilities
for several more interesting implementations, in the context of compound
assignment operators.


11.7.1: Member function reference bindings (& and &&)

    We've seen that binary operators (like operator+) can be implemented very
efficiently, but require at least move constructors.

An expression like
            Binary{} + varB + varC + varD

therefore returns a move constructed object representing Binary{} +
varB, then another move constructed object receiving the first return value
and varC, and finally yet another move constructed object receiving the
second returned object and varD as its arguments.

Now consider the situation where we have a function defining a Binary &&
parameter, and a second Binary const & parameter. Inside that function
these values need to be added, and their sum is then passed as argument to two
other functions. We could do this:
            void fun1(Binary &&lhs, Binary const &rhs)
    {
        lhs += rhs;
        fun2(lhs);
        fun3(lhs);
    }

But realize that when using operator+= we first construct a copy of
the current object, so a temporary object is available to perform the addition
on, and then swap the temporary object with the current object to commit the
results. But wait! Our lhs operand already is a temporary object. So why
create another?

In this example another temporary object is indeed not required: lhs
remains in existence until fun1 ends. But different from the binary
operators the binary compound assignment operators don't have explicitly
defined left-hand side operands. But we still can inform the compiler that a
particular member (so, not merely compound assignment operators) should
only be used when the objects calling those members is an anonymous temporary
object, or a non-anonymous (modifiable or non-modifiable) object. For this
we use
    reference bindings a.k.a.  
    reference qualifiers. 

Reference bindings consist of a reference token (&), optionally
preceded by const, or an rvalue reference token (&&). Such reference
qualifiers are immediately affixed to the function's head (this applies to the
declaration and the implementation alike). Functions provided with rvalue
reference bindings are selected by the compiler when used by anonymous
temporary objects, whereas functions provided with lvalue reference bindings
are selected by the compiler when used by other types of objects.

Reference qualifiers allow us to fine-tune our implementations of compound
assignment operators like operator+=. If we know that the object calling
the compound assignment operator is itself a temporary, then there's no need
for a separate temporary object. The operator may directly perform its
operation and could then return itself as an rvalue reference. Here is the
implementation of operator+= tailored to being used by temporary objects:
            Binary &&Binary::operator+=(Binary const &rhs) &&
    {
        add(rhs);                   // directly add rhs to *this, 
        return std::move(*this);    // return the temporary object itself
    }

This implementation is about as fast as it gets. But be careful: in the
previous section we learned that a temporary is destroyed at the end of the
full expression of a return stattement. In this case, however, the temporary
already exists, and so (also see the previous section) it should persist until
the expression containing the (operator+=) function call is completed. As
a consequence,
                cout << (Binary{} += existingBinary) << '\n';

is OK, but
                Binary &&rref = (Binary{} += existingBinary);
    cout << rref << '\n';

is not, since rref becomes a dangling reference immediately after its
initialization. 

A full-proof alternative implementation of the rvalue-reference bound
operator+= returns a move-constructed copy:
            Binary Binary::operator+=(Binary const &rhs) &&
    {
        add(rhs);                   // directly add rhs to *this, 
        return std::move(*this);    // return a move constructed copy
    }

The price to pay for this full-proof implementation is an extra move
construction. Now, using the previous example (using rref), operator+=
returns a copy of the Binary{} temporary, which is still a temporary
object which can safely be referred to by rref.

Which implementation to use may be a matter of choice: if users of
Binary know what they're doing then the former implementation can be used,
since these users will never use the above rref initialization. If you're
not so sure about your users, use the latter implementation: formally your
users will do something they shouldn't do, but there's no penalty for that.

For the compound assignment operator called by an lvalue reference (i.e.,
a named object) we use the implementation for operator+= from the previous
section (note the reference qualifier):
            Binary &Binary::operator+=(Binary const &rhs) &
    {
        Binary tmp(*this);
        tmp.add(rhs);           // this might throw
        swap(tmp);
        return *this;
    }

With this implementation adding Binary objects to each other
(e.g., b1 += b2 += b3) boils down to
            operator+=    (&)       = b2 += b3
    Copy constructor        = tmp(b2) 
        adding              = tmp.add(b3)
        swap                = b2 <-> tmp
    return                  = b2

    operator+=    (&)       = b1 += b2
    Copy constructor        = tmp(b1) 
        adding              = tmp.add(b2)
        swap                = b1 <-> tmp
    return                  = b1

When the leftmost object is a temporary then a copy construction and swap
call are replaced by the construction of an anonymous object. E.g.,
with Binary{} += b2 += b3 we observe:
            operator+=    (&)       = b2 += b3
    Copy constructor        = tmp(b2) 
        adding              = tmp.add(b3)
        swap                = b2 <-> tmp
    
    Anonymous object        = Binary{}

    operator+=    (&&)      = Binary{} += b2
        adding              = add(b2)

    return                  = move(Binary{})

For Binary &Binary::operator+=(Binary const &rhs) & an alternative
implementation exists, merely using a single return statement, but in fact
requiring two extra function calls. It's a matter of taste whether you prefer
writing less code or executing fewer function calls:  
            Binary &Binary::operator+=(Binary const &rhs) &
    {
        return *this = Binary{ *this } += rhs;
    }

Notice that the implementations of operator+ and operator+= are
independent of the actual definition of the class Binary. Adding standard
binary operators to a class (i.e., operators operating on arguments of their
own class types) can therefore easily be realized.


11.7.2: The three-way comparison operator `<=>'

    The C++26 standard added the three-way comparison operator <=>,
also known as the spaceship operator, to the language.

This operator is closely related to comparison classes, covered
in section 18.7. At this point we focus on using the 
std::strong_ordering class: the examples of the spaceship operator
presented in this section all return strong_ordering objects. These
objects are 
    
    o strong_ordering::equal if both operands are equal;
    o strong_ordering::less if the left-hand side operand is smaller than
        the right-hand side operand; 
    o strong_ordering::greater if the left-hand side operand is greater than
        the right-hand side operand.

Standard operand conversions are handled by the compiler. Note that
    
    o  if one of the operands is of type bool, then the other operand
        must also be of type bool;
    o  narrowing conversions, except from integral types to floating point
        types, are not allowed;
    o  when the operands are of identical enumeration types their values are
        converted to the underlying numeric integral type, which values are
        then compared.

Other standard conversions, like lvalue transformations and qualification
conversions (cf. section 21.4), are automatically performed.

Now about the spaceship operator itself. Why would you want it? Of course, if
it's defined then you can use it. As it's available for integral numeric types
the following correctly compiles:
            auto isp =    3 <=> 4;
    whereafter isp's value can be compared to available
outcome-values: 
     cout << ( isp == strong_ordering::less ? "less\n" : "not less\n" );

But that by itself doesn't make the spaceship operator all too
interesting. What does make it interesting is that, in combination with
operator==, it handles all comparison operators. So after providing a
class with operator== and operator<=> its objects can be compared for
equality, inequality, and they can be ordered by <, <=, >, and >=. As
an example consider books. To book owners the titles and author names are the
books' important characterstics. To sort them on book shelfs we must use
operator<, to find a particular book we use operator==, to determine
whether two books are different we use operator!= and if you want to order
them in an country where Arabic is the main language you might want to sort
them using operator> considering that the prevalent reading order in those
countries is from right to left. Ignoring constructors, destructors and other
members, then this is the interface of our class Book (note the inclusion
of the <compare> header file, containing the declarations of the
comparison classes):
            #include <string>
    #include <compare>

    class Book
    {
        friend bool operator==(Book const &lhs, Book const &rhs);
        friend std::strong_ordering operator<=>(Book const &lhs, 
                                                Book const &rhs);
        std::string d_author;
        std::string d_title;

        // ...
    };
    Both friend-functions are easy to implement:
            bool operator==(Book const &lhs, Book const &rhs)
    {
        return lhs.d_author == rhs.d_author and lhs.d_title == rhs.d_title;
    }
    
    strong_ordering operator<=>(Book const &lhs, Book const &rhs)
    {
        return lhs.d_author < rhs.d_author  ? strong_ordering::less     :
               lhs.d_author > rhs.d_author  ? strong_ordering::greater  :
               lhs.d_title  < rhs.d_title   ? strong_ordering::less     :
               lhs.d_title  > rhs.d_title   ? strong_ordering::greater  :
                                              strong_ordering::equal;
    }
    And that's it! Now all comparison operators (and of course the spaceship
operator itself) are available. The following now compiles flawlessly:
        
    void books(Book const &b1, Book const &b2)
    {
        cout << (b1 == b2) << (b1 != b2) << (b1 <  b2) << 
                (b1 <= b2) << (b1 >  b2) << (b1 >= b2) << '\n';
    }
        
    calling books for two identical books inserts 100101 into cout.

The spaceship operator is available for integral numeric types and may have
been defined for class types. E.g., it is defined for std::string. It is
not automatically available for floating point types. 


11.8: Overloading `operator new(size_t)'

When operator new is overloaded, it must define a void * return
type, and its first parameter must be of type size_t. The default
operator new defines only one parameter, but overloaded versions may
define multiple parameters. The first one is not explicitly specified but is
deduced from the size of objects of the class for which operator new
is overloaded. In this section overloading operator new is discussed.
Overloading new[] is discussed in section 11.10.

It is possible to define multiple versions of the operator new, as long as
each version defines its own unique set of arguments. When overloaded
operator new members must dynamically allocate memory they can do so using
the global operator new, applying the scope resolution operator
::. In the next example the overloaded operator new of the class
String initializes the substrate of dynamically allocated String
objects to 0-bytes:
            #include <cstring>
    #include <iosfwd>

    class String
    {
        std::string *d_data;

        public:
            void *operator new(size_t size)
            {
                return memset(::operator new(size), 0, size);
            }
            bool empty() const
            {
                return d_data == 0;
            }
    };

The above operator new is used in the following program, illustrating
that even though String's default constructor does nothing the object's
data member d_data is initialized to zero:
            #include "string.h"
    #include <iostream>
    using namespace std;

    int main()
    {
        String *sp = new String;

        cout << boolalpha << sp->empty() << '\n';   // shows: true
    }

At new String the following took place:
    
    o  First, String::operator new was called, allocating and
initializing a block of memory, the size of a String object.
    o  Next, a pointer to this block of memory was passed to the
        (default) String constructor. Since no constructor was defined,
        the constructor itself didn't do anything at all.

As String::operator new initialized the allocated memory to zero bytes
the allocated String object's d_data member had already been
initialized to a 0-pointer by the time it started to exist.

All member functions (including constructors and destructors) we've
encountered so far define a (hidden) pointer to the object on which they
should operate. This hidden pointer becomes the function's this pointer.

In the next example of pseudo C++ code, the pointer is explicitly
shown to illustrate what's happening when operator new is used. In the
first part a String object str is directly defined, in the second
part of the example the (overloaded) operator new is used:
            String::String(String *const this);     // real prototype of the default
                                            // constructor

    String *sp = new String;                // This statement is implemented
                                            // as follows:

    String *sp = static_cast<String *>(            // allocation
                        String::operator new(sizeof(String))
                 );
    String::String{ sp };                          // initialization

In the above fragment the member functions were treated as
 object-less member functions of the class String. Such members are
called static member functions (cf. chapter 8). Actually,
operator new is such a static member function. Since it has no
this pointer it cannot reach data members of the object for which it is
expected to make memory available. It can only allocate and initialize the
allocated memory, but cannot reach the object's data members by name as there
is as yet no data object layout defined.

Following the allocation, the memory is passed (as the this pointer)
to the constructor for further processing.

Operator new can have multiple parameters. The first parameter is
initialized as an implicit argument and is always a size_t
parameter. Additional overloaded operators may define additional
parameters. An interesting additional operator new is the
    placement new operator. With the placement new
operator a block of memory has already been set aside and one of the class's
constructors is used to initialize that memory. Overloading placement new
requires an operator new having two parameters: size_t and char *,
pointing to the memory that was already available. The size_t
parameter is implicitly initialized, but the remaining parameters must
explicitly be initialized using arguments to operator new. Hence we reach
the familiar syntactical form of the placement new operator in use:
            char buffer[sizeof(String)];        // predefined memory
    String *sp = new(buffer) String;    // placement new call

The declaration of the placement new operator in our class String
looks like this:
            void *operator new(size_t size, char *memory);

It could be implemented like this (also initializing the String's
memory to 0-bytes):
            void *String::operator new(size_t size, char *memory)
    {
        return memset(memory, 0, size);
    }

Any other overloaded version of operator new could also be
defined. Here is an example showing the use and definition of an overloaded
operator new storing the object's address immediately in an existing array
of pointers to String objects (assuming the array is large enough):
                // use:
    String *next(String **pointers, size_t *idx)
    {
        return new(pointers, (*idx)++) String;
    }

        // implementation:
    void *String::operator new(size_t size, String **pointers, size_t idx)
    {
        return pointers[idx] = ::operator new(size);
    }


11.9: Overloading `operator delete(void *)'

The delete operator may also be overloaded. In fact it's good practice to
overload operator delete whenever operator new is also overloaded.

Operator delete must define a void * parameter.  A second overloaded
version defining a second parameter of type size_t is related to
overloading operator new[] and is discussed in section
11.10.

Overloaded operator delete members return void.

The `home-made' operator delete is called when deleting a dynamically
allocated object after executing the destructor of the associated
class. So, the statement
            delete ptr;

with ptr being a pointer to an object of the class String for
which the operator delete was overloaded, is a shorthand for the following
statements:
            ptr->~String(); // call the class's destructor

                    // and do things with the memory pointed to by ptr
    String::operator delete(ptr);

The overloaded operator delete may do whatever it wants to do with the
memory pointed to by ptr. It could, e.g., simply delete it. If that would
be the preferred thing to do, then the default
delete operator can be called using the ::
 scope resolution operator. For example:
            void String::operator delete(void *ptr)
    {
        // any operation considered necessary, then, maybe:
        ::delete ptr;
    }

To declare the above overloaded operator delete simply add the
following line to the class's interface:
            void operator delete(void *ptr);

Like operator new operator delete is a static member function
(see also chapter 8).


11.10: Operators `new[]' and `delete[]'

In sections 9.1.1, 9.1.2 and 9.2.1
    operator new[] and operator delete[] were introduced.  Like
operator new and operator delete the
operators new[] and delete[] may be overloaded.

As it is possible to overload new[] and delete[] as well as
operator new and operator delete, one should be careful in selecting
the appropriate set of operators. The following rule of thumb should always
be applied:
    If new is used to allocate memory, delete should be used
        to deallocate memory. If new[] is used to allocate memory,
        delete[] should be used to deallocate memory.

By default these operators act as follows:
    
    o operator new is used to allocate a single object or
        primitive value. With an object, the object's constructor is
called.
    o operator delete is used to return the memory allocated by operator
new. Again, with class-type objects, the class's destructor is
called.
    o operator new[] is used to allocate a series of primitive values or
objects. If a series of objects is allocated, the class's default constructor
is called to initialize each object individually.
    o operator delete[] is used to delete the memory previously allocated
by new[]. If objects were previously allocated, then the destructor
is called for each individual object. Be careful, though, when pointers to
objects were allocated. 
     If pointers to objects were allocated the
destructors of the objects to which the allocated pointers point won't
automatically be called. A pointer is a primitive type and so no further
action is taken when it is returned to the common pool.


11.10.1: Overloading `new[]'

        To overload operator new[] in a class (e.g., in the class
String) add the following line to the class's interface:
            void *operator new[](size_t size);

The member's size parameter is implicitly provided and is initialized
by C++'s run-time system to the amount of memory that must be allocated.
Like the simple one-object operator new it
should return a void *. The number of objects that must be initialized can
easily be computed from size / sizeof(String) (and of course replacing
String by the appropriate class name when overloading operator new[]
for another class). The overloaded new[] member may allocate raw memory
using e.g., the default operator new[] or the default operator new:
            void *operator new[](size_t size)
    {
        return ::operator new[](size);
        // alternatively:
        // return ::operator new(size);
    }

Before returning the allocated memory the overloaded operator new[]
has a chance to do something special. It could, e.g., initialize the memory to
zero-bytes.

Once the overloaded operator new[] has been defined, it is
automatically used in statements like:
            String *op = new String[12];

Like operator new additional overloads of operator new[] may be
defined. One opportunity for an operator new[] overload is overloading
 placement new specifically for arrays of objects. This operator is
available by default but becomes  unavailable once at least one overloaded
operator new[] is defined. Implementing placement new is not
difficult. Here is an example, initializing the available memory to 0-bytes
before returning:
            void *String::operator new[](size_t size, char *memory)
    {
        return memset(memory, 0, size);
    }

To use this overloaded operator, the second parameter must again be
provided, as in:
            char buffer[12 * sizeof(String)];
    String *sp = new(buffer) String[12];


11.10.2: Overloading `delete[]'

    To overload operator delete[] in a class String add the following line
to the class's interface:
            void operator delete[](void *memory);

Its parameter is initialized to the address of a block of memory
previously allocated by String::new[].

There are some subtleties to be aware of when implementing
operator delete[]. Although the addresses returned by new and
new[] point to the allocated object(s), there is an additional size_t
value available immediately before the address returned by new and
new[]. This size_t value is part of the allocated block and contains
the actual size of the block. This of course does not hold true for the
 placement new operator.

When a class defines a destructor the size_t value preceding the
address returned by new[] does not contain the size of the allocated
block, but the number of objects specified when calling
new[]. Normally that is of no interest, but when overloading operator
delete[] it might become a useful piece of information. In those cases
operator delete[] does not receive the address returned by new[]
but rather the address of the initial size_t value. Whether this is at all
useful is not clear. By the time delete[]'s code is executed all objects
have already been destroyed, so operator delete[] is only to determine how
many objects were destroyed but the objects themselves cannot be used anymore.

Here is an example showing this behavior of operator delete[] for a
minimal Demo class:
            struct Demo
    {
        size_t idx;
        Demo()
        {
            cout << "default cons\n";
        }
        ~Demo()
        {
            cout << "destructor\n";
        }
        void *operator new[](size_t size)
        {
            return ::operator new(size);
        }
        void operator delete[](void *vp)
        {
            cout << "delete[] for: " << vp << '\n';
            ::operator delete[](vp);
        }
    };

    int main()
    {
        Demo *xp;
        cout << ((int *)(xp = new Demo[3]))[-1] << '\n';
        cout << xp << '\n';
        cout << "==================\n";
        delete[] xp;
    }
    // This program displays (your 0x?????? addresses might differ, but
    // the difference between the two should be sizeof(size_t)):
    //  default cons
    //  default cons
    //  default cons
    //  3
    //  0x8bdd00c
    //  ==================
    //  destructor
    //  destructor
    //  destructor
    //  delete[] for: 0x8bdd008

Having overloaded operator delete[] for a class String, it will be
used automatically in statements like:
                delete[] new String[5];

Operator delete[] may also be overloaded using an additional
size_t parameter:
            void operator delete[](void *p, size_t size);

Here size is automatically initialized to the size (in bytes) of the
block of memory to which void *p points. If this form is defined, then
void operator[](void *) should not be defined, to avoid ambiguities.
An example of this latter form of operator delete[] is:
            void String::operator delete[](void *ptr, size_t size)
    {
        cout << "deleting " << size << " bytes\n";
        ::operator delete[](ptr);
    }

Additional overloads of operator delete[] may be defined, but to use
them they must explicitly be called as static member functions (cf. chapter
8). Example:
                // declaration:
    void String::operator delete[](void *ptr, ostream &out);
        // usage:
    String *xp = new String[3];
    String::operator delete[](xp, cout);


11.10.3: The `operator delete(void *, size_t)' family

    As we've seen classes may overload their operator delete and operator
delete[] members. 

Since the C++14 standard the global void operator delete(void *, size_t
size) and void operator delete[](void *, size_t size) functions can also
be overloaded.

When a global sized deallocation function is defined, it is automatically
used instead of the default, non-sized deallocation function. The performance
of programs may improve if a sized deallocation function is available (cf. 
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3663.html).


11.10.4: `new[]', `delete[]' and exceptions

    When an exception is thrown while executing a new[] expression, what will
happen? In this section we'll show that new[] is
  exception safe even when
only some of the objects were properly constructed.

To begin, new[] might throw while trying to allocate the required
memory. In this case a bad_alloc is thrown and we don't leak as nothing
was allocated.

Having allocated the required memory the class's default constructor is going
to be used for each of the objects in turn. At some point a constructor might
throw. What happens next is defined by the C++ standard: the destructors
of the already constructed objects are called and the memory allocated for
the objects themselves is returned to the common pool. Assuming that the
failing constructor offers the basic guarantee new[] is therefore
exception safe even if a constructor may throw.

The following example illustrates this behavior. A request to allocate and
initialize five objects is made, but after constructing two objects
construction fails by throwing an exception. The output shows that the
destructors of properly constructed objects are called and that the allocated
substrate memory is properly returned:
        #include <iostream>
    using namespace std;

    static size_t count = 0;

    class X
    {
        int x;

        public:
            X()
            {
                if (count == 2)
                    throw 1;
                cout << "Object " << ++count << '\n';
            }
            ~X()
            {
                cout << "Destroyed " << this << "\n";
            }
            void *operator new[](size_t size)
            {
                cout << "Allocating objects: " << size << " bytes\n";
                return ::operator new(size);
            }
            void operator delete[](void *mem)
            {
                cout << "Deleting memory at " << mem << ", containing: " <<
                    *static_cast<int *>(mem) << "\n";
                ::operator delete(mem);
            }
    };

    int main()
    try
    {
        X *xp = new X[5];
        cout << "Memory at " << xp << '\n';
        delete[] xp;
    }
    catch (...)
    {
        cout << "Caught exception.\n";
    }
    // Output from this program (your 0x??? addresses might differ)
    //  Allocating objects: 24 bytes
    //  Object 1
    //  Object 2
    //  Destroyed 0x8428010
    //  Destroyed 0x842800c
    //  Deleting memory at 0x8428008, containing: 5
    //  Caught exception.



11.11: Function Objects

Function Objects are created by overloading the
 function call operator operator(). By defining the function call
operator an object masquerades as a function, hence the term
         function objects.
    Function objects are also known as
        functors.

Function objects are important when using
    generic algorithms. The use of function objects is
preferred over alternatives like
  pointers to functions. The fact that they are
important in the context of generic algorithms leaves us with a didactic
dilemma. At this point in the C++ Annotations it would have been nice if generic
algorithms would already have been covered, but for the discussion of the
generic algorithms knowledge of function objects is required. This
bootstrapping problem is solved in a well-known way: by ignoring the
dependency for the time being, for now concentrating on the function object
concept.

Function objects are objects for which operator() has been
defined. Function objects are not just used in combination with generic
algorithms, but also as a (preferred) alternative to pointers to
functions.

Function objects are frequently used to implement
 predicate functions.  Predicate functions return boolean values.
Predicate functions and predicate function objects are commonly referred to as
`predicates'. Predicates are frequently used by generic algorithms such as the
count_if generic algorithm, covered in chapter 19,
returning the number of times its function object has returned true. In
the standard template library two kinds of predicates are used: unary
predicates receive one argument, binary predicates receive two arguments.

Assume we have a class Person and an array of Person objects. Further
assume that the array is not sorted. A well-known procedure for finding a
particular Person object in the array is to use the function 
lsearch, which performs a linear search in an array. Example:
            Person &target = targetPerson();    // determine the person to find
    Person *pArray;
    size_t n = fillPerson(&pArray);

    cout << "The target person is";

    if (not lsearch(&target, pArray, &n, sizeof(Person), compareFunction))
        cout << " not";
    cout << " found\n";

The function targetPerson determines the person we're looking for, and
fillPerson is called to fill the array.  Then lsearch is used to
locate the target person.

The comparison function must be available, as its address is one of the
arguments of lsearch. It must be a real function having an address. If it
is defined inline then the compiler has no choice but to ignore that request
as inline functions don't have addresses. CompareFunction could be
implemented like this:
            int compareFunction(void const *p1, void const *p2)
    {
        return *static_cast<Person const *>(p1)     // lsearch wants 0
                !=                                  // for equal objects
                *static_cast<Person const *>(p2);
    }

This, of course, assumes that the operator!= has been overloaded in
the class Person. But overloading operator!= is no big deal, so
let's assume that that operator is actually available.

On average n / 2 times at least the
following actions take place:
    
    1.  The two arguments of the compare function are pushed on the stack;
    2.  The value of the final parameter of lsearch is determined,
producing compareFunction's address;
    3.  The compare function is called;
    4.  Then, inside the compare function the address of the right-hand
argument of the 
        Person::operator!= argument is pushed on the stack;
    5.  Person::operator!= is evaluated;
    6.  The argument of the Person::operator!= function is popped off
the stack;
    7.  The two arguments of the compare function are popped off the stack.
    
    Using function objects results in a different picture. Assume we have
constructed a function PersonSearch, having the following prototype (this,
however, is not the preferred approach. Normally a generic algorithm is
preferred over a home-made function. But for now we focus on PersonSearch
to illustrate the use and implementation of a function object):
            Person const *PersonSearch(Person *base, size_t nmemb,
                               Person const &target);

This function can be used as follows:
            Person &target = targetPerson();
    Person *pArray;
    size_t n = fillPerson(&pArray);

    cout << "The target person is";

    if (!PersonSearch(pArray, n, target))
        cout << " not";

    cout << "found\n";

So far, not much has been changed. We've replaced the call to lsearch
with a call to another function: PersonSearch.  Now look at
PersonSearch itself:
            Person const *PersonSearch(Person *base, size_t nmemb,
                                Person const &target)
    {
        for (int idx = 0; idx < nmemb; ++idx)
            if (target(base[idx]))
                return base + idx;
        return 0;
    }

PersonSearch implements a plain linear search. However, in the
for-loop we see target(base[idx]). Here target is used as a
function object. Its implementation is simple:
            bool Person::operator()(Person const &other) const
    {
        return *this == other;
    }

Note the somewhat peculiar syntax: operator(). The first set of
parentheses define the operator that is overloaded: the function call
operator. The second set of parentheses define the parameters that are
required for this overloaded operator. In the class header file this
overloaded operator is declared as:
            bool operator()(Person const &other) const;

Clearly Person::operator() is a simple function. It contains but one
statement, and we could consider defining it inline. Assuming we do,
then this is what happens when operator() is called:
    
    1.  The address of the right-hand argument of the
        Person::operator== argument is pushed on the stack;
    2.  The operator== function is evaluated (which probably also is a
semantic improvement over calling operator!= when looking for an
object equal to a specified target object);
    3.  The argument of Person::operator== argument is popped off the
        stack.
    
    Due to the fact that operator() is an inline function, it is not
actually called. Instead operator== is called immediately. Moreover, the
required stack operations are fairly modest.

Function objects may truly be defined inline.  Functions that are called
indirectly (i.e., using pointers to functions) can never be defined inline as
their addresses must be known.  Therefore, even if the function object needs
to do very little work it is defined as an ordinary function if it is going to
be called through pointers. The overhead of performing the indirect call may
annihilate the advantage of the flexibility of calling functions
indirectly. In these cases using inline function objects can result in an
increase of a program's efficiency.

An added benefit of function objects is that they may access the private data
of their objects. In a search algorithm where a compare function is used (as
with lsearch) the target and array elements are passed to the compare
function using pointers, involving extra stack handling. Using function
objects, the target person doesn't vary within a single search
task. Therefore, the target person could be passed to the function object's
class constructor. This is in fact what happens in the expression
target(base[idx]) receiving as its only argument the subsequent elements
of the array to search.


11.11.1: Constructing manipulators

    In chapter 6 we saw constructions like cout << hex
<< 13 << to display the value 13 in hexadecimal format. One
may wonder by what magic the hex manipulator accomplishes this. In this
section the construction of manipulators like hex is covered.

Actually the construction of a manipulator is rather simple. To start, a
definition of the manipulator is needed. Let's assume we want to create a
manipulator w10 which sets the field width of the next field to be
written by the ostream object to 10. This manipulator is constructed as a
function. The w10 function needs to know about the ostream object
in which the width must be set. By providing the function with an ostream
& parameter, it obtains this knowledge. Now that the function knows about the
ostream object we're referring to, it can set the width in that object.

Next, it must be possible to use the manipulator in an insertion
sequence. This implies that the return value of the manipulator must be
a reference to an ostream object also.

From the above considerations we're now able to construct our w10
function:
            #include <ostream>
    #include <iomanip>

    std::ostream &w10(std::ostream &str)
    {
        return str << std::setw(10);
    }

    The w10 function can of course be used in a `stand alone' mode, but it
can also be used as a manipulator. E.g.,
            #include <iostream>
        #include <iomanip>

        using namespace std;

        extern ostream &w10(ostream &str);

        int main()
        {
            w10(cout) << 3 << " ships sailed to America\n";
            cout << "And " << w10 << 3 << " more ships sailed too.\n";
        }

        The w10 function can be used as a manipulator because the class
ostream has an overloaded operator<< accepting a 
     pointer to a function expecting an ostream &
and returning an ostream &. Its definition is:
    
            ostream &operator<<(ostream &(*func)(ostream &str))
    {
        return (*func)(*this);
    }
    E.g., this std::ostream &(*func)(std::ostream &str) function is the
signature of the std::endl manipulator.

In addition to the above overloaded operator<< another one is defined
            ios_base &operator<<(ios_base &(*func)(ios_base &base))
    {
        (*func)(*this);
        return *this;
    }

This latter function is used when inserting, e.g., hex or
internal.

The above procedure does not work for manipulators requiring arguments.
It is of course possible to overload operator<< to accept an ostream
reference and the address of a function expecting an ostream & and, e.g.,
an int, but while the address of such a function may be specified with the
<<-operator, the arguments itself cannot be specified. So, one wonders
how the following construction has been implemented:
            cout << setprecision(3)

In this case the manipulator is defined as a macro. Macro's, however,
are the realm of the preprocessor, and may easily suffer from unwelcome
side-effects. In C++ programs they should be avoided whenever
possible. The following section introduces a way to implement manipulators
requiring arguments without resorting to macros, but using
  anonymous objects.


11.11.1.1: Manipulators requiring arguments

        Manipulators taking arguments are implemented as  macros: they are
handled by the preprocessor, and are not available beyond the preprocessing
stage.  

Manipulators, maybe requiring arguments, can also be defined without using
macros. One solution, suitable for modifying globally available objects (like
cin, or cout) is based on using  anonymous
objects:
    
    o  First, a class is defined, e.g. Align, whose constructor
expects the arguments configuring the required manipulation. In our example
representing, respectively, a field width and an alignment type.
    o  The class also supports an overloaded insertion (or extraction)
operator. E.g.,
            ostream &operator<<(ostream &ostr, Align const &align)

o  Next, the (anonymous) object is inserted into a stream. The insertion
operator passes the stream to Align::align, allowing that member to
configure (and return) the provided stream.

Here is an example of a little program using such a home-made
manipulator expecting multiple arguments:
            #include <iostream>
    #include <iomanip>

    class Align
    {
        unsigned d_width;
        std::ios::fmtflags d_alignment;

        public:
            Align(unsigned width, std::ios::fmtflags alignment);
            std::ostream &operator()(std::ostream &ostr) const;
    };

    Align::Align(unsigned width, std::ios::fmtflags alignment)
    :
        d_width(width),
        d_alignment(alignment)
    {}

    std::ostream &Align::operator()(std::ostream &ostr) const
    {
        ostr.setf(d_alignment, std::ios::adjustfield);
        return ostr << std::setw(d_width);
    }

    std::ostream &operator<<(std::ostream &ostr, Align &&align)
    {
        return align(ostr);
    }

    using namespace std;

    int main()
    {
        cout
            << "`" << Align{ 5, ios::left } << "hi" << "'"
            << "`" << Align{ 10, ios::right } << "there" << "'\n";
    }

    /*
        Generated output:

        `hi   '`     there'
    */


When (local) objects must be manipulated, then the class that must provide
manipulators may define function call operators receiving the required
arguments. E.g., consider a class Matrix that should allow its users to
specify the value and line separators when inserting the matrix into an
ostream. 

Two data members (e.g., char const *d_valueSep and char const
*d_lineSep) are defined (and initialized to acceptable values). The
insertion function inserts d_valueSep between values, and d_lineSep at
the end of inserted rows. The member operator()(char const *valueSep, char
const *lineSep) simply assigns values to the corresponding data members.

Given an object Matrix matrix, then at this point matrix(" ", "\n")
can be called. The function call operator should probably not insert the
matrix, as the responsibility of manipulators is to manipulate, not to
insert. So, to insert a matrix a statement like 
            cout << matrix(" ", "\n") << matrix << '\n';

should probably be used. The manipulator (i.e., function call operator)
assigns the proper values to d_valueSep and d_lineSep, which are then
used during the actual insertion.

The return value of the function call operator remains to be specified. The
return value should be insertable, but in fact should not insert anything at
all. An empty NTBS could be returned, but that's a bit kludge-like. Instead
the address of a manipulator function, not performing any action, can be
returned. Here's the implementation of such an empty manipulator:
            // static       (alternatively a free function could be used)
        std::ostream &Matrix::nop(std::ostream &out)
        {
            return out;
        }

Thus, the implementation of the Matrix's manipulator becomes:
            std::ostream &( 
            *Matrix::operator()(char const *valueSep, char const *lineSep) ) 
                                                            (std::ostream &)
        {
            d_valueSep = valueSep;
            d_lineSep = lineSep;
            return nop;
        }

Instead (probably a matter of taste) of returning the address of an empty
function the manipulator could first set the required insertion-specific
values and could then return itself: the Matrix would be inserted
according to the just assigned values to the insertion variables:
            Matrix const &Matrix::operator()
            (char const *valueSep, char const *lineSep)
        {
            d_valueSep = valueSep;
            d_lineSep = lineSep;
            return *this;
        }

In this case the insertion statement is simplified to
            cout << matrix(" ", "\n") << '\n';


11.12: Lambda expressions

C++ supports  lambda expressions. As we'll see in
chapter 19 generic algorithms often accept arguments that can
either be function objects or plain functions. Examples are the sort
(cf. section 19.1.54) and find_if (cf. section 19.1.15) generic
algorithms. As a rule of thumb: when a called function must remember its
state a function object is appropriate, otherwise a plain function can be
used.

Frequently the function or function object is not readily available, and it
must be defined in or near the location where it is used.  This is commonly
realized by defining a class or function in the anonymous namespace (say:
class or function A), passing an A to the code needing A. If that code is
itself a member function of the class B, then A's implementation might benefit
from having access to the members of class B.

This scheme usually results in a significant amount of code (defining the
class), or it results in complex code (to make available software elements
that aren't automatically accessible to A's code). It may also result in code
that is irrelevant at the current level of specification. Nested classes don't
solve these problems either. Moreover, nested classes can't be used in
templates.

lambda expressions solve these problems.  A lambda expression defines an
    anonymous function object which may immediately be passed to functions
expecting function object arguments, as explained in the following sections.

According to the C++ standard, lambda expressions provide a concise way to
create simple function objects. The emphasis here is on simple: a lambda
expression's size should be comparable to the size of inline-functions: just
one or maybe two statements. If you need more code, then encapsulate that code
in a separate function which is then called from inside the lambda
expression's compound statement, or consider designing a separate function
object. 


11.12.1: Lambda expressions: syntax

    A lambda expression defines an
    anonymous function object, also called a
    closure object or simply a closure. 

When a lambda expression is evaluated it results in a temporary function
object (the closure object). This temporary function object is of a unique
anonymous class type, called its closure type.

Lambda expressions are used inside blocks, classes or namespaces (i.e., pretty
much anywhere you like). Their implied closure type is defined in the smallest
enclosing scope (e.g., block, class or namespace) containing the lambda
expression. The closure object's visibility starts at its point of definition
and ends where its closure type ends (their visibility is identical to the
visibility of plain variables).

The closure type defines a  public inline const function call
operator. Here is an example of a lambda expression:
    
    
            []                    // the `lambda-introducer'
    (int lhs, int rhs)          // the `lambda-declarator'
    {                       // the function's body
        return lhs * rhs;
    }

This function (formally: the function call operator of the closure type
created by this lambda expression) expects two int arguments and returns
their product. It is an inline const member of its closure type. The
function's const attribute is removed if the lambda expression specifies
 mutable. E.g.,
            [](int lhs, int rhs) mutable
    ...

When specifying static instead of mutable the lambda-expression is
defined as a static function.

The lambda-declarator may be omitted if no parameters are defined.

    The parameters in a lambda declarator can also be given default arguments.

Lambda declarator specifiers can be specified as constexpr, and also as
mutable or static. When specifying constexpr the lambda-expression
is a constexpr, which may be compile-time evaluated if its arguments
qualify as const-expressions. If a lambda-expression is defined inside a
constexpr function then the lambda-expression itself automatically becomes
a constexpr itself, and the constexpr lambda declarator specifier is
not required. Thus, the following two function definitions are identical:
            constexpr int change10(int value)
    {
        return [value] 
               { 
                   return value > 10 ? value - 10 : value + 10; 
               }();
    }
    
    constexpr int change10(int value)
    {
        return [value] () constexpr 
               { 
                   return value > 10 ? value - 10 : value + 10; 
               }();
    }

The lambda expression defined at the top of this section could for example be
used in combination with the accumulate generic algorithm (cf. section
19.1.2) to compute the product of a series of int values stored in a
vector:
            cout << accumulate(vi.begin(), vi.end(), 1,
                [] (int lhs, int rhs) 
                { 
                    return lhs * rhs; 
                }
            );

This lambda expression implicitly defines its return
        
    type as decltype(lhs * rhs). Implicit return types can be used in these
cases:
    
    o  the lambda expression does not contain a return statement (i.e.,
        it's a void lambda expression);
    o  the lambda expression contains a single return statement; or
    o  the lambda expression contains multiple return statements
        returning values of identical types (e.g., all int values).

If there are multiple return statements returning values of different
types then the lambda expression's return type must explicitly be specified
using a
        late-specified return type,
(cf. section 3.3.7):
            [](bool neg, double rhs) -> int
    {
        return neg ? -rhs : rhs;
    }

Variables which are visible at the location of a
lambda expression may be accessed by the lambda expression's compound
statement. Which variables and how they are accessed depends on specifications
in the lambda-introducer. In the following overview: when it's stated that
`variables can be modified' then that only applies to variables that
themselves allow modifications.

 
    o  Global variables are always accessible, and can be modified if their
        definitions allow modifications;
    o  If the lambda expression is defined inside a function then that
        function's local variables may be accessed by the lambda expression's
        compound statement if their names are listed inside the
        lambda-introducer.

       The specification local is used below as a shorthand for a
        (comma-separated) list of names of local variables of the surrounding
        function that are visible in the lambda expression's compound
        statement;
    o  Local variables specified in the lambda introducer are in fact data
        members of the lambda expression which are initialized by the
        corresponding local variables of the function surrounding the lambda
        expression. This is illustrated by the following example showing
        different addresses for var (however, copying can be prevented as
        covered below):
           #include <iostream>

    int main()
    {
        int var = 12;
        std::cout << &var << '\n';
        [var]               // lambda expr. using 'var'
        {
            std::cout << &var << '\n';
        }();                // directly call the lambda expr.
    }

    o  When the lambda expression is defined inside a class member function
        the lambda-introducer may specify this or *this allowing the
        lambda function to access/use the class's members (not defining copies
        as when accessing local variables). When this is specified the
        class's members can be accessed and the class's (non-const) data
        members can be modified. When *this is specified the lambda
        expression uses a copy of the class's object. 

By default lambda expressions define constant (immutable) function objects,
but by appending mutable to the lambda declarator they are defined as
mutable function objects.  Sometimes using mutable or not is
irrelevant. In those cases `mutable_opt' is specified below. If
mutable_opt is not specified then specifying mutable indicates that
the lambda expression's function isn't a const member function anymore, so
it may modify its (modifiable) data members.

Below specifications (...) refer to the lambda expression's declarator,
and lambda introducers ending in  , ...] indicate that they may
specify , this or , *this.

    There is no required ordering of the this, *this and local
specifications. 


    o [...] (...) mutable_opt:
       only global variables can be accessed;

o [local, ...] (...):
       like the previous specification, with local being defined as the
        lambda expression's data member(s) initialized by copying
        the surrounding function's local variable(s).  When mutable is
        specified the (copied) local variable(s) can be modified;

o [&local, ...] (...):
       like the previous specification, but the local variable(s) specified
        using &-prefixes are available as references to the class's data
        members. They can be modified if those members are non-const;

o [&d_local = std::as_const(local), ...] (...) 
       defines the lambda expression's d_local data member as a reference
        to the (immutable) local variable of the surrounding function. The
        d_local data members remains a reference to an immutable local
        variable whether or not mutable is specified.

       To use std::as_const the <utility> header file must be
        included;

o [&, ...] (...)  mutable_opt:
       the `local reference specifier': local variables are visible as
        modifiable references. If some of those references should not be able
        to modify their local variables then use specifications like
        &d_local = std::as_const(local) for those variables.

o [vars, ...] (...): 
       in addition to the above specifications lambda expressions may define
        their own data members. E.g., to define a lambda expression having its
        own int count and double value data members, also allowing it
        to access the caller's local variables its lambda-introducers can be
        specified as [&, count = int(0), value = double(0)].

       Such local variables are defined by specifying their names, followed by
        their type-specifying initializations;

o [=, ...] (...): 
       if the lambda introducer starts with = then all local variables are
        available as copies of the surrounding function's local variables;

o [=, &local, ...]: 
       like the previous specification, but the local variable(s) prefixed by
        & are available as references. If those local variables are
        non-const they can be modified. If some of those references should not
        be able to modify their local variables then use specifications like
        &d_local = std::as_const(local);

Even when not specified, lambda expressions defined inside class member
functions implicitly capture
     their this
pointers, and class members are always accessed relative to this. 

Although lambda expressions are anonymous function objects, they can be
assigned to variables. Often, the variable is defined using the keyword
auto. E.g.,
            auto sqr = [](int value)
               {
                   return value * value;
               };

The lifetime of such a lambda expression is equal to the lifetime of sqr,
which is the variable receiving the lambda expression as its value.

Defining a lambda expression is different from calling its function
operator. The function S2::f() returns what the lamba expression (1)'s
function call operator returns: its function call operator is called by using
() (at (3)). What it in fact returns is another anonymous function object
(defined at (2)). As that's just a function object, to retrieve its value
it must still be called from f's return value using something like this:
            S2 s2;      // define the object
    s2.f()();   // call the function call operator (see below)

Here, the second set of parentheses activates the returned function
object's function call operator. Had the parentheses been omitted at (3) then
S2::f() would have returned a mere anonymous function object (defined at
(1)), in which case it would require three sets of parentheses to retrieve
ohseven's value: s2.f()()().


11.12.2: Using lambda expressions

        Now that the syntax of lambda expressions has been covered let's see how
they can be used in various situations.

First we consider named lambda expressions. Named lambda expressions nicely
fit in the niche of 
        
    local functions: when a function needs to perform computations which are
at a conceptually lower level than the function's task itself, then it's
attractive to encapsulate these computations in a separate support function
and call the support function where needed.  Although support functions can be
defined in anonymous namespaces, that quickly becomes awkward when the
requiring function is a class member and the support function also must access
the class's members.

In that case a named lambda expression can be used: it can be defined inside
a requiring function, and it may be given full access to the surrounding
class. The name to which the lambda expression is assigned becomes the name of
a function which can be called from the surrounding function. Here is an
example, converting a numeric IP address to a dotted decimal string, which can
also be accessed directly from an Dotted object (all implementations
in-class to conserve space):
            class Dotted
    {
        std::string d_dotted;
        
        public:
            std::string const &dotted() const
            {
                return d_dotted;
            }
            std::string const &dotted(size_t ip)
            {
                auto octet = 
                    [](size_t idx, size_t numeric)
                    {
                        return to_string(numeric >> idx * 8 & 0xff);
                    };

                d_dotted = 
                        octet(3, ip) + '.' + octet(2, ip) + '.' +
                        octet(1, ip) + '.' + octet(0, ip);

                return d_dotted;
            }
    };

Next we consider the use of generic algorithms, like 
the for_each (cf. section 19.1.18):
            void showSum(vector<int> const &vi)
    {
        int total = 0;
        for_each(
            vi.begin(), vi.end(),
            [&](int x)
            {
                total += x;
            }
        );
        std::cout << total << '\n';
    }

Here the variable int total is passed to the lambda expression by
reference and is directly accessed by the function. Its parameter list merely
defines an int x, which is initialized in sequence by each of the values
stored in vi. Once the generic algorithm has completed showSum's
variable total has received a value that is equal to the sum of all the
vector's values. It has outlived the lambda expression and its value is
displayed.

But although generic algorithms are extremely useful, there may not always be
one that fits the task at hand. Furthermore, an algorithm like for_each
looks a bit unwieldy, now that the language offers range-based for-loops. So
let's try this, instead of the above implementation:
            void showSum(vector<int> const &vi)
    {
        int total = 0;
        for (auto el: vi)
            [&](int x)
            {
                total += x;
            };

        std::cout << total << '\n';
    }

But when showSum is now called, its cout statement consistently
reports 0. What's happening here?

When a generic algorithm is given a lambda function, its implementation
instantiates a reference to a function. The referenced function is thereupon
called from within the generic algorithm. But, in the above example the
range-based for-loop's nested statement merely represents the definition
of a lambda function. Nothing is actually called, and hence total remains
equal to 0. 

Thus, to make the above example work we not only must define the
lambda expression, but we must also call the lambda function. We can do
this by  giving the lambda function a name, and then call the
lambda function by its given name:
            void showSum(vector<int> const &vi)
    {
        int total = 0;
        for (auto el: vi)
        {
            auto lambda = [&](int x)
                            {
                                total += x;
                            };

            lambda(el);
        }
        std::cout << total << '\n';
    }

In fact, there is no need to give the lambda function a name: the auto
lambda definition represents the lambda function, which could also
directly be called. The syntax for doing this may look a
bit weird, but there's nothing wrong with it, and it allows us to drop the
compound statement, required in the last example, completely. Here goes:
            void showSum(vector<int> const &vi)
    {
        int total = 0;
        for (auto el: vi)
            [&](int x)
            {
                total += x;
            }(el);          // immediately append the 
                            // argument list to the lambda
                            // function's definition
        std::cout << total << '\n';
    }

Lambda expressions can also be used to prevent spurious returns from
condition_variable's wait calls (cf. section 20.4.3). 

The class condition_variable allows us to do so by offering wait
members expecting a lock and a predicate. The predicate checks the data's
state, and returns true if the data's state allows the data's
processing. Here is an alternative implementation of the down member shown
in section 20.4.3, checking for the data's actual availability:
            void down()
    {
        unique_lock<mutex> lock(sem_mutex);
        condition.wait(lock, 
            [&]()
            {
                return semaphore != 0
            }
        );
        --semaphore;
    }

The lambda expression ensures that wait only returns once
semaphore has been incremented.

Lambda expression are primarily used to obtain functors that are used in a
very localized section of a program. Since they are used inside an existing
function we should realize that once we use lambda functions multiple
aggregation levels are mixed. Normally a function implements a task which can
be described at its own aggregation level using just a few sentences. E.g.,
``the function std::sort sorts a data structure by comparing its elements
in a way that is appropriate to the context where sort is called''. By
using an existing comparison method the aggregation level is kept, and the
statement is clear by itself. E.g.,
            sort(data.begin(), data.end(), greater<DataType>());

If an existing comparison method is not available, a tailor-made function
object must be created. This could be realized using a lambda
expression. E.g.,
            sort(data.begin(), data.end(), 
        [&](DataType const &lhs, DataType const &rhs)
        {
            return lhs.greater(rhs);
        }
    );

Looking at the latter example, we should realize that here two different
aggregation levels are mixed: at the top level the intent is to sort the
elements in data, but at the nested level (inside the lambda expression)
something completely different happens. Inside the lambda expression we define
how a the decision is made about which of the two objects is the greater. Code
exhibiting such mixed aggregation levels is hard to read, and should be
avoided. 

On the other hand: lambda expressions also simplify code because the overhead
of defining tailor-made functors is avoided. The advice, therefore, is to use
lambda expressions sparingly. When they are used make sure that their
sizes remain small. As a rule of thumb: lambda expressions should be
treated like in-line functions, and should merely consist of one, or maybe
occasionally two expressions.

A special group of lambda expressions is known as generic lambda
expressions. As generic lambda expressions are in fact class templates,
their coverage is postponed until chapter 22.


11.13: The case of [io]fstream::open()

Earlier, in section 6.4.2.1, it was noted that the
[io]fstream::open members expect an ios::openmode value as their
final argument. E.g., to open an fstream object for writing you could do
as follows:
            fstream out;
    out.open("/tmp/out", ios::out);

Combinations are also possible. To open an fstream object for
both reading and writing the following stanza is often seen:
            fstream out;
    out.open("/tmp/out", ios::in | ios::out);

When trying to combine enum values using a `home made' enum we may run
into problems. Consider the following:
            enum Permission
    {
        READ =      1 << 0,
        WRITE =     1 << 1,
        EXECUTE =   1 << 2
    };

    void setPermission(Permission permission);

    int main()
    {
        setPermission(READ | WRITE);
    }

When offering this little program to the compiler it replies with an
error message like this:
    
invalid conversion from 'int' to 'Permission'


The question is of course: why is it OK to combine ios::openmode
values passing these combined values to the stream's open member, but
not OK to combine Permission values.


    Combining enum values using arithmetic operators results in int-typed
values. Conceptually this never was our intention. Conceptually it can be
considered correct to combine enum values if the resulting value conceptually
makes sense as a value that is still within the original enumeration
domain. Note that after adding a value READWRITE = READ | WRITE to the
above enum we're still not allowed to specify READ | WRITE as an
argument to setPermission.

To answer the question about combining enumeration values and yet stay
within the enumeration's domain we turn to operator overloading. Up to this
point operator overloading has been applied to class types. Free functions
like operator<< have been overloaded, and those overloads are conceptually
within the domain of their class.

As C++ is a strongly typed language realize that defining an enum is
really something beyond the mere association of int-values with symbolic
names. An enumeration type is really a type of its own, and as with any type
its operators can be overloaded. When writing READ | WRITE the compiler
performs the default conversion from enum values to int values and
applies the operator to ints. It does so when it has no alternative.

But it is also possible to overload the enum type's operators. Thus we may
ensure that we'll remain within the enum's domain even though the resulting
value wasn't defined by the enum. The advantage of type-safety and conceptual
clarity is considered to outweigh the somewhat peculiar introduction of values
hitherto not defined by the enum.

Here is an example of such an overloaded operator:
            Permission operator|(Permission left, Permission right)
    {
        return static_cast<Permission>(static_cast<int>(left) | right);
    }

Other operators can easily and analogously be constructed.

Operators like the above were defined for the  ios::openmode
enumeration type, allowing us to specify ios::in | ios::out as argument to
open while specifying the corresponding parameter as ios::openmode
as well. Clearly, operator overloading can be used in many situations, not
necessarily only involving class-types.


11.14: User-defined literals

In addition to the well-known literals, like numerical constants (with or
without suffixes), character constants and string (textual) literals, C++
also supports
    user-defined literals, also known as extensible literals. 

A user-defined literal is defined by a function (see also section 23.3)
that must be defined at namespace scope. Such a function is called a
        literal operator.
    A literal operator cannot be a class member function.  The names of a
literal operator must start with an
     underscore, and a literal operator is
used (called) by suffixing its name (including the underscore) to the
argument that must be passed to it . Assuming _NM2km (nautical mile to km)
is the name of a literal operator, then it could be called as 100_NM2km,
producing, e.g., the value 185.2.

Using Type to represent the return type of the literal operator
its generic declaration looks like this:
            Type operator "" _identifier(parameter-list);

The blank space trailing the empty string is required. The parameter lists
of literal operators can be:
    
    o unsigned long long int. It is used as, e.g., 123_identifier. The
        argument to this literal operator can be decimal constants,
        binary constants (initial 0b), octal constants (initial 0) and
        hexadecimal constants (initial 0x);
    o long double. It is used as, e.g., 12.25_NM2km;
    o char const *text. The text argument is an NTBS. It is
        used as, e.g., 1234_pental. The argument must not be given
        double quotes, and must represent a numeric constant, as also expected
        by literal operators defining  unsigned long long int
        parameters.
    o char const *text, size_t len. Here, the compiler determines len
        as if it had called strlen(text). It is used as, e.g.,
        "hello"_nVowels;
    o wchar_t const *text, size_t len, same as the previous one, but
        accepting a string of wchar_t characters. It is used as, e.g.,
        L"1234"_charSum;
    o char16_t const *text, size_t len, same as the previous one, but
        accepting a string of char16_t characters. It is used as, e.g.,
        u"utf 16"_uc;
    o char32_t const *text, size_t len, same as the previous one, but
        accepting a string of char32_t characters. It is used as, e.g.,
        U"UTF 32"_lc;

If literal operators are overloaded the compiler will pick the literal
operator requiring the least `effort'. E.g., 120 is processed by a literal
operator defining an unsigned long long int parameter and not by its
overloaded version, defining a char const * parameter. But if overloaded
literal operators exist defining char const * and long double
parameters then the operator defining a char const * parameter is used
when the argument 120 is provided, while the operator defining a long
double parameter is used with the argument 120.3.

A literator operator can define any return type. Here is
an example of a definition of the _NM2km literal operator:
            double operator "" _NM2km(char const *nm)
    {
        return std::stod(nm) * 1.852;
    }

    double value = 120_NM2km;   // example of use

Of course, the argument could also have been a long double
constant. Here's an alternative implementation, explicitly expecting a long
double:

    constexpr double operator "" _NM2km(long double nm)
    {
        return nm * 1.852;
    }

    double value = 450.5_NM2km;   // example of use

A numeric constant can also be processed completely at compile-time.
Section 23.3 provides the details of this type of literal operator.

Arguments to literal operators are themselves always constants. A literal
operator like _NM2km cannot be used to convert, e.g., the value of a
variable. A literal operator, although it is defined as a function, cannot be
called like a function. The following examples therefore
result in compilation errors:
            double speed;

    speed_NM2km;        // no identifier 'speed_NM2km'
    _NM2km(speed);      // no function _NM2km
    _NM2km(120.3);      // no function _NM2km


11.15: Overloadable operators

The following  operators can be overloaded:
            +       -       *       /       %       ^       &       |
    ~       !       ,       =       <=>     <       >       <=
    >=      ++      --      <<      >>      ==      !=      &&
    ||      +=      -=      *=      /=      %=      ^=      &=
    |=      <<=     >>=     []      ()      ->      ->*     new
    new[]   delete  delete[]

Several operators have textual alternatives:



    
        
    

    

     textual alternative                 operator 


    

       and          &&  


       and_eq    &=  


       bitand    &   


       bitor      |   


       compl      ~   


       not          !   


       not_eq    !=  


       or            ||  


       or_eq      |=  


       xor          ^   


       xor_eq    ^=  


    

    

    


`Textual' alternatives of operators are also overloadable (e.g.,
operator and). However, note that textual alternatives are not
additional operators. So, within the same context operator&& and
operator and can not both be overloaded.

Several of these operators may only be overloaded as member functions
     within a class. This
holds true for the '=', the '[]', the '()' and the '->'
operators. Consequently, it isn't possible to redefine, e.g., the assignment
operator globally in such a way that it accepts a char const * as an
lvalue and a String & as an rvalue. Fortunately, that isn't
necessary either, as we have seen in section 11.3.

Finally, the following operators cannot be  overloaded:
            .       .*      ::      ?:      sizeof  typeid



Chapter 12: Abstract Containers

C++ offers several predefined datatypes, all part of the
 Standard Template Library,
which can be used to implement solutions to frequently occurring problems. The
datatypes discussed in this chapter are all
  containers: you can put stuff inside them, and you can
retrieve the stored information from them.

The interesting part is that the kind of data that can be stored inside these
containers has been left unspecified at the time the containers were
constructed. That's why they are spoken of as abstract containers.

Abstract containers rely heavily on templates, covered in chapter
21 and beyond. To use abstract containers, only a minimal grasp of
the template concept is required. In C++ a template is in fact a recipe
for constructing a function or a complete class. The recipe tries to abstract
the functionality of the class or function as much as possible from the data
on which the class or function operates. As the data types on which the
templates operate were not known when the template was implemented, the
datatypes are either inferred from the context in which a function template is
used, or they are mentioned explicitly when a class template is used (the term
that's used here is instantiated). In situations where the types are
explicitly mentioned, the angle bracket notation is used to indicate
which data types are required. For example, below (in section 12.2) we'll
encounter the pair container, which requires the
explicit mentioning of two data types. Here is a pair object
containing both an int and a string:
            pair<int, string> myPair;

The object myPair is defined as an object holding both an int and
a string.

The angle bracket notation is used intensively in the upcoming
discussion of abstract containers. Actually, understanding this part of
templates is the only real requirement for using abstract containers. Now
that we've introduced this notation, we can postpone the more thorough
discussion of templates to chapter 21, and concentrate on
their use in this chapter.

Most abstract containers are sequential containers: they contain
data that can be stored and retrieved in some sequential
way. Examples are the array, implementing a fixed-sized array; a 
vector, implementing an extendable array; the
list, implementing a data structure that allows for the easy insertion or
deletion of  data; the queue, also called a FIFO
    (first in, first out) structure, in which the first element that is
entered is the first element to be retrieved again; and the stack,
which is a
    first in, last out (FILO or LIFO) structure.

In addition to sequential containers several special containers are
available. The pair is a basic container in which a pair of values (of
types that are left open for further specification) can be stored, like two
strings, two ints, a string and a double, etc.. Pairs are often used to return
data elements that naturally come in pairs. For example, the map is an
abstract container storing keys and their associated values. Elements
of these maps are returned as pairs.

A variant of the pair is the complex container, implementing
operations that are defined on complex numbers.

A tuple (cf. section 22.6) generalizes the pair container to a
data structure accommodating any number of different data types.

All abstract containers described in this chapter as well as the string
and stream datatypes (cf. chapters 5 and 6) are part of
the Standard Template Library.

All but the unordered containers support
        
        
        
    the following basic set of operators:
    
    o  The overloaded assignment operator, so we can assign two
containers of the same types to each other. If the container's data type
supports move assignment, then assignment of an anonymous temporary container
to a destination container will use move assignment when assigning new values
to the destination container's element. Overloaded assignment is also
supported by the unordered containers;
    o   Tests for equality: == and !=
The equality operator applied to two containers returns true if the two
containers have the same number of elements, which are pairwise equal
according to the equality operator of the contained data type. The
inequality operator does the opposite;
    o   Ordering operators: <, <=, > and
>=.  The < operator returns true if each element in the
left-hand side container is less than each corresponding element in the
right-hand side container. Additional elements in either the left-hand side
container or the right-hand side container are ignored.
    container left;
container right;

left = {0, 2, 4};
right = {1, 3};             // left < right

right = {1, 3, 6, 1, 2};    // left < right

Note that  before a
user-defined type (usually a class-type) can be stored in a container, the
user-defined type should at least support:
    
    o  A default value (e.g., a default constructor)
    o  The equality operator (==)
    o  The less-than operator (<)

Sequential containers can also be initialized using initializer lists.

Most containers (exceptions are the stack (section
12.3.11), priority_queue (section
12.3.5), and queue (section
12.3.4) containers) support members to determine their maximum sizes
(through their member function max_size).

Virtually all containers support copy construction.  If the container
supports copy construction and the container's data type supports move
construction, then move construction is automatically used for the container's
data elements when a container is initialized with an anonymous temporary
container.

Closely linked to the standard template library are the
    generic algorithms. These algorithms may be used to perform
frequently occurring tasks or more complex tasks than is possible with the
containers themselves, like counting, filling, merging, filtering etc.. An
overview of generic algorithms and their applications is given in chapter
19. Generic algorithms usually rely on the availability of
    iterators, representing begin and
end-points for processing data stored inside containers. The abstract
containers usually support constructors and members expecting iterators, and
they often have members returning iterators (comparable to the
string::begin and string::end members). In this chapter the
iterator concept is not further investigated. Refer to chapter 18 for
this.

Containers often collect data during their lifetimes. When a container
goes out of scope, its destructor tries to destroy its data elements. This
only succeeds if the data elements themselves are stored inside the
container. If the  data elements of containers
are pointers to dynamically allocated memory then the memory pointed to by
these pointers is not destroyed, resulting in a memory leak. A consequence
of this scheme is that the data stored in a container should often be
considered the `property' of the container: the container should be able to
destroy its data elements when the container's destructor is called. So,
normally containers should not contain pointers to data. Also, a container
should not be required 
     to contain const data, as const data
prevent the use of many of the container's members, like the assignment
operator.


12.1: Notations used in this chapter

    In this chapter about containers, the following notational conventions are
used:
    
    o  Containers live in the standard namespace. In code examples this will
be clearly visible, but in the text std:: is usually omitted.
    o  A container without angle brackets represents any container of
that type. Mentally add the required type in
    angle bracket notation. E.g., pair may represent pair<string,
int>.
    o  The notation Type represents the generic type. Type could
be int, string, etc.
    o  Identifiers object and container represent objects of the
container type under discussion.
    o  The identifier value represents a value of the type that is
stored in the container.
    o  Simple, one-letter identifiers, like n represent unsigned
values.
    o  Longer identifiers represent iterators. Examples
are pos, from, beyond

Some containers, e.g., the map container, contain pairs of
values, usually called `keys' and `values'. For such containers the following
notational convention is used in addition:
    
    o  The identifier key indicates a value of the used key-type
    o  The identifier keyvalue indicates a value of the `value_type'
used with the particular container.


12.2: The `pair' container

The  pair container is a rather basic container. It is
used to store two elements, called first and second, and that's about
it. Before using pair containers the header file <utility> must be
included.

The pair's data types are specified when the pair object is
defined (or declared) using the template's angle bracket notation (cf. chapter
21). Examples:
            pair<string, string> piper("PA28", "PH-ANI");
    pair<string, string> cessna("C172", "PH-ANG");

here, the variables piper and cessna are defined as pair
variables containing two strings. Both strings can be retrieved using the
first and second fields of the pair type:
            cout << piper.first << '\n' <<      // shows 'PA28'
            cessna.second << '\n';      // shows 'PH-ANG'

The first and second members can also be used to reassign values:
            cessna.first = "C152";
    cessna.second = "PH-ANW";

If a pair object must be completely reassigned, an anonymous
 pair object can be used as the right-hand operand of
the assignment. An anonymous variable defines a temporary variable (which
receives no name) solely for the purpose of (re)assigning another variable of
the same type. Its  generic form is
            type(initializer list)

Note that when a pair object is used the type specification is not
completed by just mentioning the containername pair. It also requires the
specification of the data types which are stored within the pair. For this the
(template) angle bracket notation is used again. E.g., the reassignment
of the cessna pair variable could have been accomplished as follows:
            cessna = pair<string, string>("C152", "PH-ANW");

In cases like these, the type specification can become quite
elaborate, in which case using declarations can be used to improve
readability. If many pair<type1, type2> clauses are
used in a source, the typing effort may be reduced and readability might be
improved by first defining a name for the clause, and then using the defined
name later. E.g.,
            using pairStrStr = pair<string, string>;

    cessna = pairStrStr("C152", "PH-ANW");

All abstract containers are class templates, and the
types for which class templates are initialized are commonly specified between
pointed brackets following the class template's name. However, the compiler
may be able to 
     deduce the container's types from
the types of arguments that are specified when constructing the
container. E.g., when defining
            pair values{ 1, 1.5 };

the compiler deduces that values.first is an int and
values.second is a double. Sometimes the class template's types cannot
be deduced. In those cases the intended types must explicitly be specified:
            pair<int, double> values;

Although the compiler will deduce types whenever it can, it might not
deduce the types we had in mind. Had we defined
            pair cessna{ "C172", "PH-BVL" };

then the compilation would succeed, but an expression  like 
        cout << cessna.first.length() would not compile, as "C172" is
a NTBS, and hence cessna.first is a char *. In this case simply
appending an s to the NTBSs fixes the problem, but such a simple fix might
not always be available. Section 12.3.2 provides more information about
deducing template parameter types.

Apart from this (and the basic set of operations (assignment and
comparisons)) the pair offers no further functionality. It is, however,
a basic ingredient of the upcoming abstract containers map, multimap and
hash_map.

C++ also offers a generalized pair container: the tuple, covered
in section 22.6.


12.3: Available Containers



12.3.1: The `array' container

    The array class implements a 
     fixed-size array.  Before using the array
container the <array> header file must be included.

To define a std::array both the data type of its elements and its size
must be specified: the data type is given after an opening angle bracket,
immediately following the `array' container name. The array's size is
provided after the data type specification. Finally, a closing angle bracket
completes the array's type. Specifications like this are common practice with
containers.  The combination of array, type and size defines a
type. As a result, array<string, 4> defines another type than
array<string, 5>, and a function explicitly defining an array<Type, N>
parameter will not accept an array<Type, M> argument if N and M
are unequal.

The array's size may may be defined as 0 (although such an array probably has
little use as it cannot store any element). The elements of an array are
stored contiguously. If array<Type, N> arr has been defined, then
&arr[n] + m == &arr[n + m], assuming than 0 <= n < N and 0 <= n + m
< N.

The following constructors, operators, and member functions are available:
    
    o   Constructors:
        
        o  The copy and move constructors are available;
        o  An array may be constructed with a fixed number N of
default elements:
        array<string, N> object;

o  An initial subset of the elements of an array may be 
initialized using a brace delimited initializer list:
    array<double, 4> dArr = {1.2, 2.4};

Here dArr is defined as an array of 4 element, with dArr[0] and
dArr[1] initialized to, respectively 1.2 and 2.4, and dArr[2] and
dArr[3] initialized to 0.  An attractive characteristic of arrays (and
other containers) is that containers initialize 
their data elements to the data type's default value. The data type's
default constructor is used for this initialization. With non-class
data types the value 0 is used.  So, for an array<double, 4> array we know
that all but its explicitly initialized elements are initialized to
zero. 

o  In addition to the standard operators for containers, the array
supports the index operator, which can be used to retrieve or reassign
individual elements of the array. Note that the elements which are indexed
must exist. For example, having defined an empty array a statement like
iarr[0] = 18 produces an error, as the array is empty. Note that 
operator[] does not respect its
 array bounds. If you want run-time array bound checking, use the array's
at member.
    o  The array class offers the following
  member functions:
        
        o Type &at(size_t idx):
returns a reference to the array's element at index position
            idx. If idx exceeds the array's size a
            std::out_of_range exception is thrown.
        o Type &back():
returns a reference to the last element in the array. It is the
            responsibility of the programmer to use the member only if the
            array is not empty.
        o array::iterator begin():
returns an iterator pointing to the first
            element in the array, returning end if the array is empty.
        o array::const_iterator cbegin():
returns a const_iterator pointing to the first element in the
            array, returning cend if the array is empty.
        o array::const_iterator cend():
returns a const_iterator pointing just beyond the array's last
            element.
        o array::const_reverse_iterator crbegin():
returns a const_reverse_iterator pointing to the last element
            in the array, returning crend if the array is empty.
        o array::const_reverse_iterator crend():
returns a const_reverse_iterator pointing just before the
            array's first element.
        o value_type *data():
returns a pointer to the array's first data element. With a const
            array a value_type const * is returned.
        o bool empty():
returns true if the array contains no elements.
        o array::iterator end():
returns an iterator pointing beyond the last element in the
            array.
        o void fill(Type const &item):
fills all the array's elements with a copy of item
        o Type &front():
returns a reference to the first element in the array. It is the
            responsibility of the programmer to use the member only if
            the array is not empty.
        o array::reverse_iterator rbegin():
 this member returns a reverse_iterator
            pointing to the last element in the array.
        o array::reverse_iterator rend():
returns a reverse_iterator pointing before the first element in
            the array.
        o constexpr size_t size():
returns the number of elements the array contains.
        o void swap(array<Type, N> &other):
swaps the content of the current and other array. The array
            other's data type and size must be equal to the data type and size
            of the object calling swap.

Using an array rather than a standard C style array offers several
advantages:
    
    o  All its elements are immediately initialized;
    o  Introspection is possible (e.g., size can be used);
    o  The array container can be used in the context of templates,
        there code is developed that operates on data types that become
        available only after the code itself has been developed;
    o  Since array supports reverse iterators, it can immediately be
        used with generic algorithms performing `reversed' operations (e.g.,
        to perform a descending rather than ascending sort (cf. section
        19.1.54))

In general, when looking for a sequential data structure, the array or
vector (introduced in the next section) should be your `weapon of
choice'. Only if these containers demonstrably do not fit the problem at hand
you should use another type of container.


12.3.2: The `vector' container

    The vector class implements an 
    expandable array.  Before using the vector
container the <vector> header file must be included.

The following constructors, operators, and member functions are available:

o   Constructors:
    
    o  The copy and move constructors are available;

o  A vector may be constructed empty:
    vector<string> object;

o  A vector may be initialized to a certain number of elements:
    vector<string> object(5, "Hello"s); // initialize to 5 Hello's,
vector<string> container(10);       // and to 10 empty strings
vector<string> names = {"george", "frank", "tony", "karel"};

Note the difference between vector<int> first(5) and vector<int>
second{ 5 }. The vector first contains five elements, initialized to 0,
while the vector second contains one element, initialized to 5. Referring
back to section 12.2: with the latter definition the compiler is able to
deduce 
     the vector's template parameter
type (int), so the latter definition could also have been written as
vector second{ 5 }. 

An ambiguity might be observed when looking at 
        vector object{ vector{ 1 } };

Did we define a vector<int> or a vector<vector<int>>? The standard
considers this a vector<int>: it is initialized using the vector's move
constructor from an abstract vector<int>.

o  A vector may be initialized using iterators. To
initialize a vector with elements 5 until 10 (including the last one) of an
existing vector<string> the following construction may be used:
    extern vector<string> container;
vector<string> object(&container[5], &container[11]);

Note here that the last element pointed to by the second iterator
(&container[11]) is not stored in object.  This is a simple
example of the use of iterators, in which the used
    range of values starts at the first value, and includes all elements up
to but not including the element to which the second iterator refers. The
standard notation for this is [begin, end).

o  In addition to the standard operators for containers, the vector
supports the index operator, which can be used to retrieve or reassign
individual elements of the vector. Note that the elements which are indexed
must exist. For example, having defined an empty vector a statement like
ivect[0] = 18 produces an error, as the vector is empty.  So, the vector
is not automatically
  expanded, and operator[] does not respect its
 array bounds. In this case the vector should be resized first, or
ivect.push_back(18) should be used (see below). If you need run-time array
bound checking, use the vector's at member.
    o  The vector class offers the following
  member functions:
        
        o void assign(...):
assigns new content to the vector:
            
            o assign(iterator begin, iterator end) assigns the values at
the iterator range [begin, end) to the vector;
            o assign(size_type n, value_type const &val) assigns n
copies of val to the vector;
            o assign(initializer_list<value_type> values) assigns the
values in the initializer list to the vector.

o Type &at(size_t idx):
returns a reference to the vector's element at
index position idx. If idx exceeds the vector's size a
std::out_of_range exception is thrown.
        o Type &back():
returns a reference to the last element in the
vector. It is the responsibility of the programmer to use the member only
if the vector is not empty.
        o vector::iterator begin():
returns an iterator pointing to the first
element in the vector, returning end if the vector is empty.
        o size_t capacity():
Number of elements for which memory has been
allocated. It returns at least the value returned by size
        o vector::const_iterator cbegin():
returns a const_iterator pointing to the first
element in the vector, returning cend if the vector is empty.
        o vector::const_iterator cend():
returns a const_iterator pointing just beyond the
vector's last element.
        o void clear():
erases all the vector's elements.
        o vector::const_reverse_iterator crbegin():
returns a const_reverse_iterator pointing to the last
element in the vector, returning crend if the vector is empty.
        o vector::const_reverse_iterator crend():
returns a const_reverse_iterator pointing just before the
vector's first element.
        o value_type *data():
returns a pointer to the vector's first data element.
        o iterator emplace(const_iterator position, Args &&...args):
a value_type object is constructed from the arguments
specified after position, and the newly created element is inserted at
position. Different from insert, which expects an existing object of
the container's value type (and inserts 
the provided argument into the container) using copy or move construction or
assignment, emplace uses its arguments to 
construct such an object immediately at the intended location of the
container, without requiring copy or move construction or assignment.
        o constexpr &emplace_back(Args &&...args):
a value_type object is constructed from the member's
arguments, and the newly created element is inserted beyond the vector's last
element, returning a reference to the newly added element.
        o bool empty():
returns true if the vector contains no
elements.
        o vector::iterator end():
returns an iterator pointing beyond the last
element in the vector.
        o vector::iterator erase():
erases a specific range of elements in the vector:
            
            o erase(pos) erases the element pointed to by the iterator
pos. The iterator ++pos is returned.
            o erase(first, beyond) erases elements indicated by the iterator
range [first, beyond), returning beyond.

o Type &front():
returns a reference to the first element in the
vector. It is the responsibility of the programmer to use the member only if
the vector is not empty.
        o allocator_type get_allocator() const:
returns a
copy of the allocator object used by the vector object.
        o ... insert():
elements may be inserted starting at a certain position. The
return value depends on the version of insert() that is called:
            
            o vector::iterator insert(pos) inserts a default value of type
Type at pos, pos is returned.
            o vector::iterator insert(pos, value) inserts value at
pos, pos is returned.
            o void insert(pos, first, beyond) inserts the elements in the
                iterator range [first, beyond).
            o void insert(pos, n, value) inserts n elements having value
value at position pos.

o size_t max_size():
returns the maximum number of
elements this vector may contain.
        o void pop_back():
removes the last element from the vector. Avoid calling this
member on an empty vector: although nothing is returned its internally
maintained count of number of available elements is reduced, causing the
vector's size() member to return the (when cast to int) value -1, and
then -2, etc., etc.
        o void push_back(value):
adds value to the end of the vector.
        o vector::reverse_iterator rbegin():
this member returns a reverse_iterator pointing to the last
element in the vector.
        o vector::reverse_iterator rend():
returns an iterator pointing before the first element in the
vector. 
        o void reserve(size_t request):
if request is less than or equal to capacity, this
call has no effect. Otherwise, it is a request to allocate additional
memory. If the call is successful, then capacity returns a value of at
least request. Otherwise, capacity is unchanged. In either case,
size's return value won't change, until a function like resize is
called, actually changing the number of accessible elements.
        o void resize():
can be used to alter the number of elements that
are currently stored in the vector:
            
            o resize(n, value) may be used to resize the vector to a size
of n. Value is optional. If the vector is expanded and value is
not provided, the additional elements are initialized to the default value
of the used data type, otherwise value is used to initialize extra
elements.

o void shrink_to_fit():
optionally reduces the
amount of memory allocated by a vector to its current size. The
implementor is free to ignore or otherwise optimize this request. In order to
guarantee a `shrink to fit' operation the
    vector<Type>(vectorObject).swap(vectorObject)

idiom can be used.
        o size_t size():
returns the number of elements in the vector.
        o void swap(vector<Type> &other):
swaps two vectors using identical data types. Example:
    #include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> v1(7);
    vector<int> v2(10);

    v1.swap(v2);
    cout << v1.size() << " " << v2.size() << '\n';
}
/*
    Produced output:
10 7
*/


12.3.3: The `list' container

    The list container  implements a list data structure.
Before using a list container the header file <list> must be included.

The organization of a list is shown  in figure 10.
        ------------------------------------------------------------------Insert Figure 10(A list data-structure)about here (file: containers/list)------------------------------------------------------------------
    Figure 10  shows that a list consists of separate
list-elements, connected by pointers. The list can be
     traversed in two directions: starting at Front the
list may be traversed from left to right, until the 0-pointer is reached at
the end of the rightmost list-element. The list can also be traversed from
right to left: starting at Back, the list is traversed from right to left,
until eventually the 0-pointer emanating from the leftmost list-element is
reached.

As a subtlety note that the representation given in figure 10 is not
necessarily used in actual implementations of the list. For example, consider
the following little program:
            int main()
    {
        list<int> l;
        cout << "size: " << l.size() << ", first element: " <<
                l.front() << '\n';
    }

When this program is run it might actually produce the output:
            size: 0, first element: 0

Its front element can even be assigned a value. In this case the
implementor has chosen to provide the list with a hidden element. The list
actually is a circular
  list, where the hidden element serves as terminating
element, replacing the 0-pointers in figure 10. As noted, this is a
subtlety, which doesn't affect the conceptual notion of a list as a data
structure ending in 0-pointers. Note also that it is well known that various
implementations of list-structures are possible (cf.
    Aho, A.V., Hopcroft J.E. and Ullman, J.D., (1983)
    Data Structures and Algorithms (Addison-Wesley)).

Both lists and vectors are often appropriate data structures in situations
where an  unknown number of data elements must be
stored. However, there are some  rules of thumb to follow
when selecting the appropriate  data structure.
    
    o  When most accesses are random, a vector is the preferred data
            structure. Example: in a program counting character frequencies in
            a textfile, a vector<int> frequencies(256) is the
            datastructure of choice, as the values of the received characters
            can be used as indices into the frequencies vector.
    o  The previous example illustrates a second rule of thumb, also
            favoring the vector: if the number of elements is known in
            advance (and does not notably change during the lifetime of the
            program), the vector is also preferred over the list.
    o  In cases where insertions or deletions prevail and the data
            structure is large the list is generally preferred.

At present lists aren't as useful anymore as they used to be (when
computers were much slower and more memory-constrained).  Except maybe for
some rare cases, a vector should be the preferred container; even when
implementing algorithms traditionally using lists.

Other considerations related to the choice between lists and vectors
should also be given some thought. Although it is true that the vector is able
to grow dynamically, the dynamic growth requires data-copying.
Clearly, copying a million large data structures takes a considerable amount
of time, even on fast computers. On the other hand, inserting a large number
of elements in a list doesn't require us to
    copy non-involved data. Inserting a new element in a list merely
requires us to juggle some pointers. In figure 11 this is shown: a
new element is inserted between the second and third element, creating a new
list of four elements.
        ------------------------------------------------------------------Insert Figure 11(Adding a new element to a list)about here (file: containers/insertlist)------------------------------------------------------------------
    Removing an element from a list is also fairly easy. Starting again
from the situation shown in figure 10, figure 12 shows
what happens if element two is removed from our list. Again: only pointers
need to be juggled. In this case it's even simpler than adding an element:
only two pointers need to be rerouted.
        ------------------------------------------------------------------Insert Figure 12(Removing an element from a list)about here (file: containers/dellist)------------------------------------------------------------------
    To summarize the comparison between lists and vectors: it's probably best
to conclude that there is no clear-cut answer to the question what data
structure to prefer. There are rules of thumb, which may be adhered to. But if
worse comes to worst, a profiler may be required to find out what's best.

The list container offers the following constructors, operators, and
member functions:
    
    o   Constructors:
        
        o  The copy and move constructors are available;
        o  A list may be constructed empty:
    list<string> object;

As with the vector, it is an error to refer to an element of an
empty list.
        o  A list may be initialized to a certain number of elements. By
            default, if the initialization value is not explicitly mentioned,
            the default value or default constructor for the actual data type
            is used. For example:
    list<string> object(5, "Hello"s); // initialize to 5 Hello's
list<string> container(10);       // and to 10 empty strings

o  A list may be initialized using a two iterators. To initialize a
            list with elements 5 until 10 (including the last one) of a
            vector<string> the following construction may be used:
    extern vector<string> container;
list<string> object(&container[5], &container[11]);

o  The list does not offer specialized operators, apart from the
            standard operators for containers.
    o  The following member functions are available:
        
        o void assign(...):
            assigns new content to the list:
            
            o assign(iterator begin, iterator end) assigns the values at
                the iterator range [begin, end) to the list;
            o assign(size_type n, value_type const &val) assigns n
                copies of val to the list;

o Type &back():
returns a reference to the last element in
            the list. It is the responsibility of the programmer to use
            this member only if the list is not empty.
        o list::iterator begin():
returns an iterator pointing to
            the first element in the list, returning end if the list is
            empty.
        o void clear():
erases all elements from the list.
        o value_type &emplace_back(Args &&...args):
a value_type object is constructed from the member's
            arguments, and the newly created element is inserted beyond the
            last element of the list, returning a reference to the newly added
            element. 
        o value_type &emplace_front(Args &&...args):
a value_type object is constructed from the member's
            arguments, and the newly created element is inserted before the
            first element of the list, returning a reference to the newly added
            element. 
        o bool empty():
returns true if the list contains no
            elements.
        o list::iterator end():
returns an iterator pointing beyond
            the last element in the list.
        o list::iterator erase():
erases a specific range of
            elements in the list:
            
            o erase(pos) erases the element pointed to by pos. The
                iterator ++pos is returned.
            o erase(first, beyond) erases elements indicated by the iterator
                range [first, beyond). Beyond is returned.

o Type &front():
returns a reference to the first element
            in the list. It is the responsibility of the programmer to use
            this member only if the list is not empty.
        o allocator_type get_allocator() const:
returns a
            copy of the allocator object used by the list object.
        o ... insert():
inserts elements into the list. The return
            value depends on the version of insert that is called:
            
            o list::iterator insert(pos) inserts a default value of type
                Type at pos, pos is returned.
            o list::iterator insert(pos, value) inserts value at
                pos, pos is returned.
            o void insert(pos, first, beyond) inserts the elements in the
                iterator range [first, beyond).
            o void insert(pos, n, value) inserts n elements having value
                value at position pos.

o size_t max_size():
returns the maximum number of
            elements this list may contain.
        o void merge(list<Type> other):
this member function
            assumes that the current and other lists are sorted (see below,
            the member sort). Based on that assumption, it inserts the
            elements of other into the current list in such a way that the
            modified list remains sorted.  If both list are not sorted, the
            resulting list will be ordered `as much as possible', given the
            initial ordering of the elements in the two
            lists. list<Type>::merge uses Type::operator< to sort the
            data in the list, which operator must therefore be available.  The
            next example illustrates the use of the merge member: the list
            `object' is not sorted, so the resulting list is ordered 'as
            much as possible'.  
               #include <iostream>
    #include <string>
    #include <list>
    using namespace std;

    void showlist(list<string> &target)
    {
        for
        (
            list<string>::iterator from = target.begin();
            from != target.end();
            ++from
        )
            cout << *from << " ";

        cout << '\n';
    }

    int main()
    {
        list<string> first;
        list<string> second;

        first.push_back("alpha"s);
        first.push_back("bravo"s);
        first.push_back("golf"s);
        first.push_back("quebec"s);

        second.push_back("oscar"s);
        second.push_back("mike"s);
        second.push_back("november"s);
        second.push_back("zulu"s);

        first.merge(second);
        showlist(first);
    }
    // shows:
    // alpha bravo golf oscar mike november quebec zulu
 
           A subtlety is that merge doesn't alter the list if the list
            itself is used as argument: object.merge(object) won't change
            the list `object'.
        o void pop_back():
removes the last element from the
            list. The program aborts when called on an empty list.
        o void pop_front():
removes the first element from the
            list. The program aborts when called on an empty list.
        o void push_back(value):
adds value to the end of
            the list.
        o void push_front(value):
adds value before the
            first element of the list.
        o list::reverse_iterator rbegin():
 returns a
            reverse_iterator pointing to the last element in the list.
        o void remove(value):
removes all occurrences of value
            from the list. In the following example, the two strings
            `Hello' are removed from the list object:
               #include <iostream>
    #include <string>
    #include <list>
    using namespace std;

    int main()
    {
        list<string> object;

        object.push_back("Hello"s);
        object.push_back("World"s);
        object.push_back("Hello"s);
        object.push_back("World"s);

        object.remove("Hello"s);

        while (object.size())
        {
            cout << object.front() << '\n';
            object.pop_front();
        }
    }
    /*
            Generated output:
        World
        World
    */

        o void remove_if(Predicate pred):
removes all
            occurrences from the list for which the predicate function or
            function object pred returns true. For each of the objects
            stored in the list the predicate is called as pred(*iter),
            where iter represents the iterator used internally by
            remove_if. If a function pred is used, its prototype
            should be bool pred(value_type const &object).
        o list::reverse_iterator rend():
this member returns a
            reverse_iterator pointing before the first element in the list.
        o void resize():
alters the number of elements that are
            currently stored in the list:  o resize(n, value)
            may be used to resize the list to a size of n. Value is
            optional. If the list is expanded and value is not provided,
            the extra elements are initialized to the default value of the
            used data type, otherwise value is used to initialize extra
            elements.
        o void reverse():
reverses the order of the elements in
            the list. The element back becomes front and vice
            versa.
        o size_t size():
returns the number of elements in the
            list.
        o void sort():
sorts the list. An example of its use is
            given at the description of the unique member function
            below. list<Type>::sort uses Type::operator< to sort the
            data in the list, which must therefore be available.
        o void splice(pos, object):
transfers the content of
            object to the current list, starting the insertion at the
            iterator position pos of the object using the splice
            member. Following splice, object is empty. For example:
         #include <iostream>
#include <string>
#include <list>
using namespace std;

int main()
{
    list<string> object;

    object.push_front("Hello"s);
    object.push_back("World"s);

    list<string> argument(object);

    object.splice(++object.begin(), argument);

    cout << "Object contains " << object.size() << " elements, " <<
            "Argument contains " << argument.size() <<
            " elements,\n";

    while (object.size())
    {
        cout << object.front() << '\n';
        object.pop_front();
    }
}

           Alternatively, argument may be followed by an iterator of
            argument, indicating the first element of argument that
            should be spliced, or by two iterators begin and end
            defining the iterator-range [begin, end) on argument
            that should be spliced into object.
        o void swap():
swaps two lists using identical data types.
        o void unique():
operating on a sorted list, this member
            function removes all consecutively identical elements from the
            list. list<Type>::unique uses Type::operator== to identify
            identical data elements, which operator must therefore be
            available.  Here's an example removing all multiply occurring
            words from the list:     #include <iostream>
    #include <string>
    #include <list>
    using namespace std;
                            // see the merge() example
    void showlist(list<string> &target)
    {
        for
        (
            list<string>::iterator from = target.begin();
            from != target.end();
            ++from
        )
            cout << *from << " ";

        cout << '\n';
    }


    int main()
    {
        string
            array[] =
            {
                "charlie",
                "alpha",
                "bravo",
                "alpha"
            };

        list<string>
            target
            (
                array, array + sizeof(array)
                / sizeof(string)
            );

        cout << "Initially we have:\n";
        showlist(target);

        target.sort();
        cout << "After sort() we have:\n";
        showlist(target);

        target.unique();
        cout << "After unique() we have:\n";
        showlist(target);
    }
    /*
        Generated output:

        Initially we have:
        charlie alpha bravo alpha
        After sort() we have:
        alpha alpha bravo charlie
        After unique() we have:
        alpha bravo charlie
    */



12.3.4: The `queue' container

    The queue class implements a queue data structure.  Before using a
queue container the header file <queue> must be included.

A queue is depicted in figure 13.
        ------------------------------------------------------------------Insert Figure 13(A queue data-structure)about here (file: containers/queue)------------------------------------------------------------------
    In figure 13 it is shown that a queue has one point (the
back) where items can be added to the queue, and one point (the front)
where items can be removed (read) from the queue. A queue is therefore
also called a FIFO data structure, for first in, first out. It is
most often used in situations where events should be handled in the same order
as they are generated.

The following constructors, operators, and member functions are available
for the queue container:
    
    o   Constructors:
        
        o  The copy and move constructors are available;
        o  A queue may be constructed empty:
            queue<string> object;

As with the vector, it is an error to refer to an element of an
empty queue.

o  The queue container only supports the basic container operators.
    o  The following member functions are available for
queues:
        
        o Type &back():
returns a
reference to the last element in the queue. It is the
    responsibility of the programmer to use the member only if the queue is
not empty.

o value_type &emplace(Args &&...args):
a value_type object is constructed from the member's
            arguments, and the newly created element is inserted beyond the
            end of the queue, returning a reference to (or copy of) the newly
            added element. 
        o bool empty():
returns
true if the queue contains no elements.
        o Type &front():
returns a
reference to the first element in the queue. It is the responsibility of the
programmer to use the member only if the queue is not empty.
        
        o void pop():
removes the element at the front of
the queue. Note that the element is not returned by this member. 
Avoid calling this member on an empty queue: although nothing is returned its
internally maintained count of number of available elements is reduced,
causing the queue's size() member to return the (when cast to int)
value -1, and then -2, etc., etc.  One might wonder why
pop returns void, instead of a value of type Type
(cf. front). One reason is found in the principles of good software
design: functions should perform one task. Combining the removal and return of
the removed element breaks this principle. Moreover, when this principle is
abandoned pop's implementation is always flawed. Consider the
prototypical implementation of a pop member that is expected to return the
queue's front-value:
            Type queue::pop()
        {
            Type ret{ front() };
            erase_front();
            return ret;
        }
   Since queue has no control over Type's behavior the first statement
(Type ret{ front() }) might throw. Although this still supports the
'commit or roll-back' principle, queue cannot guarantee that Type
offers copy-construction. Hence, pop does not return the queue's
front-value, but simply erases that element: 
            Type queue::pop()
        {
            erase_front();
        }
    Note that push doesn't require copy construction: push can also be
implemented when move-construction is supported. E.g.,
            void queue::push(Type &&tmp)
        {
            d_data.push_back(std::move(tmp));   // uses move construction
        }
    Because of all this, we must first use front and then pop to
obtain and remove the queue's front element.
        o void push(value):
this member
adds value to the back of the queue.
        o size_t size():
returns the
number of elements in the queue.

Note that the queue does not support iterators or an index operator. The
only elements that can be accessed are its front and back element.  A queue
can be emptied by:
    
    o  repeatedly removing its front element;
    o  assigning an empty queue using the same data type to it;
    o  having its destructor called.


12.3.5: The `priority_queue' container

    The priority_queue class implements a priority queue data structure.
Before using a priority_queue container the <queue> header file must
have been included.

A priority queue is identical to a queue, but allows the entry of data
elements according to priority rules. A real-life priority queue is
found, e.g., at airport check-in terminals. At a terminal the passengers
normally stand in line to wait for their turn to check in, but late passengers
are usually allowed to jump the queue: they receive a higher priority than
other passengers.

The priority queue uses operator< of the data type stored in the
priority queue to decide about the priority of the data elements. The
smaller the value, the lower the priority. So, the priority queue
could be used to sort values while they arrive.  A simple example of such
a priority queue application is the following program: it reads words from
cin and writes a sorted list of words to cout:
        #include <iostream>
#include <string>
#include <queue>
using namespace std;

int main()
{
    priority_queue<string> q;
    string word;

    while (cin >> word)
        q.push(word);

    while (q.size())
    {
        cout << q.top() << '\n';
        q.pop();
    }
}


Unfortunately, the words are listed in reversed order: because of the
underlying <-operator the words appearing later in the ASCII-sequence
appear first in the priority queue. A solution to that problem is to define a
wrapper class around the string datatype, reversing string's
operator<. Here is the modified program:
        #include <iostream>
#include <string>
#include <queue>

class Text
{
    std::string d_s;

    public:
        Text(std::string const &str)
        :
            d_s(str)
        {}
        operator std::string const &() const
        {
            return d_s;
        }
        bool operator<(Text const &right) const
        {
            return d_s > right.d_s;
        }
};

using namespace std;

int main()
{
    priority_queue<Text> q;
    string word;

    while (cin >> word)
        q.push(word);

    while (q.size())
    {
        word = q.top();
        cout << word << '\n';
        q.pop();
    }
}


Other possibilities to achieve the same exist. One would be to store the
content of the priority queue in, e.g., a vector, from which the elements can
be read in reversed order.

The following constructors, operators, and member functions are available
for the priority_queue container:
    
    o   Constructors:
        
        o  The copy and move constructors are available;
        o  A priority_queue may be constructed empty:
    priority_queue<string> object;

o  The priority_queue only supports the basic operators of
containers.
    o  The following member functions are available for priority queues:
        
        o bool empty():
returns true if the
priority queue contains no elements.
        o void pop():
removes the element at the top of the priority
queue. Note that the element is not returned by this member. As with the
queue avoid calling this member on an empty priority queue (cf. section
12.3.4.)
        o void push(value):
inserts value at the
appropriate position in the priority queue.
        o size_t size():
returns the number of elements
in the priority queue.
        o Type &top():
returns a reference to the first
element of the priority queue. It is the
    responsibility of the programmer to use the member only if the priority
queue is not empty.

Note that the priority queue does not support iterators or an index
operator. The only element that can be accessed is its top element.  A
priority queue can be emptied by:
    
    o  repeatedly removing its top element;
    o  assigning an empty queue using the same data type to it;
    o  having its destructor called.


12.3.6: The `deque' container

    The deque (pronounce: `deck') class implements a
    doubly ended queue data structure (deque).  Before using a deque
container the header file <deque> must be included.

A deque is comparable to a queue, but it allows for reading and
writing at both ends. Actually, the deque data type supports a lot more
functionality than the queue, as illustrated by the following overview of
available member functions. A deque is a combination of a vector and
two queues, operating at both ends of the vector. In situations where random
insertions and the addition and/or removal of elements at one or both sides of
the vector occurs frequently using a deque should be considered.

The following constructors, operators, and member functions are available for
deques:
    
    o   Constructors:
        
        o  The copy and move constructors are available;
        o  A deque may be constructed empty:
    deque<string> object;

As with the vector, it is an error to refer to an element of an
empty deque.
        o  A deque may be initialized to  a certain number of
elements. By default, if the initialization value is not explicitly mentioned,
the default value or default constructor for the actual data type is used. For
example:
    deque<string> object(5, "Hello"s), // initialize to 5 Hello's
deque<string> container(10);       // and to 10 empty strings

o  A deque may be initialized using two iterators. To
initialize a deque with elements 5 until 10 (including the last one) of a
vector<string> the following construction may be used:
    extern vector<string> container;
deque<string> object(&container[5], &container[11]);

o  In addition to the standard operators for containers, the deque
supports the index operator, which may be used to retrieve or reassign
random elements of the deque. Note that the indexed elements must exist.
    o  The following member functions are available for deques:
        
        o void assign(...):
            assigns new content to the deque:
            
            o assign(iterator begin, iterator end) assigns the values at
the iterator range [begin, end) to the deque;
            o assign(size_type n, value_type const &val) assigns n
copies of val to the deque;

o Type &at(size_t idx):
            returns a reference to the deque's element at index position
idx. If idx exceeds the deque's size a std::out_of_range exception
is thrown.
        o Type &back():
returns a
reference to the last element in the deque. It is the
    responsibility of the programmer to use the member only if the deque is
not empty.
        o deque::iterator begin():
returns an iterator pointing
to the first element in the deque. 
        o deque::const_iterator cbegin():
            returns a const_iterator pointing to the first
element in the deque, returning cend if the deque is empty.
        o deque::const_iterator cend():
            returns a const_iterator pointing just beyond the
deque's last element.
        o void clear():
erases all
elements in the deque.
        o deque::const_reverse_iterator crbegin():
            returns a const_reverse_iterator pointing to the last
element in the deque, returning crend if the deque is empty.
        o deque::const_reverse_iterator crend():
            returns a const_reverse_iterator pointing just before the
deque's first element.
        o iterator emplace(const_iterator position, Args &&...args)
            a value_type object is constructed from the arguments
specified after position, and the newly created element is inserted at
position.
        o void emplace_back(Args &&...args)
            a value_type object is constructed from the member's
arguments, and the newly created element is inserted beyond the deque's last
element.
        o void emplace_front(Args &&...args)
            a value_type object is constructed from the member's
arguments, and the newly created element is inserted before the deque's first
element.
        o bool empty():
returns
true if the deque contains no elements.
        o deque::iterator end():
returns an iterator pointing beyond
the last element in the deque. 
        o deque::iterator erase():
the member can
be used to erase a specific range of elements in the deque:
            
            o erase(pos) erases the element pointed to by pos. The
iterator ++pos is returned.
            o erase(first, beyond) erases elements indicated by the iterator
range [first, beyond). Beyond is returned.

o Type &front():
returns a
reference to the first element in the deque. It is the responsibility of the
programmer to use the member only if the deque is not empty.
        o allocator_type get_allocator() const:
returns a
copy of the allocator object used by the deque object.

o ... insert():
inserts elements starting at a certain
position. The return value depends on the version of insert that is
called:
            
            o deque::iterator insert(pos) inserts a default value of type
Type at pos, pos is returned.
            o deque::iterator insert(pos, value) inserts value at
pos, pos is returned.
            o void insert(pos, first, beyond) inserts the elements in the
                iterator range [first, beyond).
            o void insert(pos, n, value) inserts n elements having value
value starting at iterator position pos.

o size_t max_size():
returns the maximum number of
elements this deque may contain.
        o void pop_back():
removes the last element from the
deque. Avoid calling this member on an empty deque: although nothing is
returned its internally maintained count of number of available elements is
reduced, causing the deque's size() member to return the (when cast to
int) value -1, and then -2, etc., etc.
        o void pop_front():
removes the first element from the
deque. Avoid calling this member on an empty deque: as with pop_back() its
internally maintained count of number of available elements is reduced
        o void push_back(value):
adds value to the end of
the deque.
        o void push_front(value):
adds value before the
first element of the deque.
        o deque::reverse_iterator rbegin():

            returns a reverse_iterator pointing to the last element in the
deque.
        o deque::reverse_iterator rend():
this member returns a
reverse_iterator pointing before the first element in the deque.
        o void resize():
alters the number of elements that are
currently stored in the deque:
            
            o resize(n, value) may be used to resize the deque to a size of
n. Value is optional. If the deque is expanded and value is not
provided, the additional elements are initialized to the default value of the
used data type, otherwise value is used to initialize extra elements.

o void shrink_to_fit():
optionally reduces the
amount of memory allocated by a deque to its current size. The implementor is
free to ignore or otherwise optimize this request. In order to guarantee a
`shrink to fit' operation deque<Type>(dequeObject).swap(dequeObject) idiom
can be used.
        o size_t size():
returns the
number of elements in the deque.
        o void swap(argument):
swaps two deques using identical data
types.


12.3.7: The `map' container

    The map class offers a (sorted) associative array. Before using a
map container the <map> header file must be included.

A map is filled with key, value pairs, which may be of any
container-accepted type. Since types are associated with both the key and the
value, we must specify two types in the angle bracket notation, comparable
to the specification we've seen with the pair container (cf. section
12.2). The first type represents the key's type, the second type
represents the value's type. The key is used to access its associated
information. The map sorts the keys using their operator<, where the
smallest key values appears as first element in the map.  For example, a
map in which the key is a string and the value is a double can be
defined as follows:
            map<string, double> object;
That information is called the value. For example, a phone book uses the
names of people as the key, and uses the telephone number and maybe other
information (e.g., the zip-code, the address, the profession) as value. Since
a map sorts its keys, the key's operator< must be defined, and it
must be sensible to use it. For example, it is generally a bad idea to use
pointers for keys, as sorting pointers is something different than sorting the
values pointed at by those pointers. In addition to the key, value types,
a third type defines the comparison class, used to compare two keys. By
default, the comparison class is std::less<KeyType> (cf. section
18.1.2), using the key type's operator< to compare two key
values. For key type KeyType and value type ValueType the map's type
definition, therefore, looks like this:
            map<KeyType, ValueType, std::less<KeyType>>

The two fundamental operations on maps are the storage of key, value
combinations, and the retrieval of values, given their keys. The index
operator using a key as the index, can be used for both. If the index operator
is used as lvalue, the expression's rvalue is inserted into the
map. If it is used as rvalue, the key's associated value is retrieved.
Each key can be stored only once in a map. If the same key is entered
again, the new value replaces the formerly stored value, which is lost.

A specific key, value combination can implicitly or explicitly be inserted
into a map. If explicit insertion is required, the key, value
combination must be constructed first. For this, every map defines a
value_type which may be used to create values that can be stored in the
map. For example, a value for a map<string, int> can be constructed as
follows:
            map<string, int>::value_type siValue{ "Hello", 1 };

The value_type is associated with the map<string, int>: the
type of the key is string, the type of the value is int. Anonymous
value_type objects are also often used. E.g.,
            map<string, int>::value_type{ "Hello", 1 };

Instead of using the line map<string, int>::value_type(...) over and
over again, a using declaration is frequently used to reduce typing and to
improve readability:
            using StringIntValue = map<string, int>::value_type;

Now values for the map<string, int> may be specified this way:
            StringIntValue{ "Hello", 1 };

Alternatively, pairs may be used to represent key, value
combinations used by maps:
            pair<string, int>{ "Hello", 1 };


12.3.7.1: The `map' constructors

            The following  constructors are available for the
map container:
    
    o  The copy and move constructors are available;
    o  A map may be constructed empty:
    map<string, int> object;

Note that the values stored in maps may be containers themselves. For
example, the following defines a map in which the value is a pair: a
container
   nested under another container:
            map<string, pair<string, string>> object;

Note the use of the two
 
 consecutive closing angle brackets, which does not result in ambiguities
as their syntactical context differs from their use as binary operators in
expressions.
    o  A map may be initialized using two iterators.  The iterators may
either point to value_type values for the map to be constructed, or to
plain pair objects. If pairs are used, their first element represents
the type of the keys, and their second element represents the type of the
values. Example:
    pair<string, int> pa[] =
{
    pair<string,int>("one", 1),
    pair<string,int>("two", 2),
    pair<string,int>("three", 3),
};

map<string, int> object(&pa[0], &pa[3]);

In this example, map<string, int>::value_type could have been written
instead of pair<string, int> as well.

If begin represents the first iterator that is used to construct a map
and if end represents the second iterator, [begin, end) will be
used to initialize the map. Maybe contrary to intuition, the map
constructor only enters new keys.  If the last element of pa would
have been "one", 3, only two elements would have entered the map:
"one", 1 and "two", 2. The value "one", 3 would silently have been
ignored.

The map receives its own copies of the data to which the iterators
point as illustrated by the following example:
            #include <iostream>
    #include <map>
    using namespace std;

    class MyClass
    {
        public:
            MyClass()
            {
                cout << "MyClass constructor\n";
            }
            MyClass(MyClass const &other)
            {
                cout << "MyClass copy constructor\n";
            }
            ~MyClass()
            {
                cout << "MyClass destructor\n";
            }
    };

    int main()
    {
        pair<string, MyClass> pairs[] =
        {
            pair<string, MyClass>{ "one", MyClass{} }
        };
        cout << "pairs constructed\n";

        map<string, MyClass> mapsm{ &pairs[0], &pairs[1] };
        cout << "mapsm constructed\n";
    }
    /*
        Generated output:
    MyClass constructor
    MyClass copy constructor
    MyClass destructor
    pairs constructed
    MyClass copy constructor
    mapsm constructed
    MyClass destructor
    MyClass destructor
    */


When tracing the output of this program, we see that, first, the
constructor of a MyClass object is called to initialize the anonymous
element of the array pairs. This object is then copied into the first
element of the array pairs by the copy constructor. Next, the original
element is not required anymore and is destroyed. At that point the array
pairs has been constructed. Thereupon, the map constructs a temporary
pair object, which is used to construct the map element. Having
constructed the map element, the temporary pair object is
destroyed. Eventually, when the program terminates, the pair element
stored in the map is destroyed too.


12.3.7.2: The `map' operators

            The map supports, in addition to the standard operators for containers,
the index operator.

The index operator may be used to retrieve or reassign individual elements of
the map. The argument of the index operator is called a key.

If the provided key is not available in the map, a new data element is
automatically added to the map using the default value or default
constructor to initialize the value part of the new element. This default
value is returned if the index operator is used as an rvalue.

When initializing a new or reassigning another element of the map, the type of
the right-hand side of the assignment operator must be equal to (or promotable
to) the type of the map's value part. E.g., to add or change the value of
element "two" in a map, the following statement can be used:
            mapsm["two"] = MyClass{};


12.3.7.3: The `map' public members

        The following member functions are available for the map container:
    
        o mapped_type &at(key_type const &key):
returns a reference to the map's mapped_type
associated with key. If the key is not stored in the map an
std::out_of_range exception is thrown.
    o map::iterator begin():
returns an iterator pointing to the
first element of the map. 
        o map::const_iterator cbegin():
returns a const_iterator pointing to the first
element in the map, returning cend if the map is empty.
        o map::const_iterator cend():
returns a const_iterator pointing just beyond the
map's last element.
    o void clear():
erases all elements from
the map.
    o size_t count(key):
returns 1 if
the provided key is available in the map, otherwise 0 is returned.
    o map::reverse_iterator crbegin() const:

    reverse_iterator returns a reverse_iterator pointing to the last
element of the map.
    o map::reverse_iterator crend():
returns an iterator pointing
before the first element of the map.
        o pair<iterator, bool> emplace(Args &&...args):
a value_type object is constructed from emplace's
arguments. If the map already contained an object using the same
key_type value, then a std::pair is returned containing an iterator
pointing to the object using the same key_type value and the value
false. If no such key_type value was found, the newly constructed
object is inserted into the map, and the returned std::pair
contains an iterator pointing to the newly inserted value_type
as well as the value true.
        o iterator emplace_hint(const_iterator position,
                            Args &&...args):
a value_type object is constructed from the member's
arguments, and the newly created element is inserted into the
map, unless the (at args) provided key already exists. The
implementation may or may not use position as a hint to start looking
for an insertion point. The returned iterator points to the value_type
using the provided key. It may refer to an already existing value_type or
to a newly added value_type; an existing value_type is not
replaced. If a new value was added, then the container's size has been
incremented when emplace_hint returns.
    o bool empty():
returns true if
the map contains no elements.
    o map::iterator end():
returns an iterator pointing beyond the
last element of the map.
    o pair<map::iterator, map::iterator> equal_range(key):
this
member returns a pair of iterators, being respectively the return values of
the member functions lower_bound and upper_bound, introduced below.
An example illustrating these member functions is given at the
discussion of the member function upper_bound.
    o ... erase():
erases a specific element or range of elements
from the map:
        
        o bool erase(key) erases the element having the
given key from the map. True is returned if the value was removed,
false if the map did not contain an element using the given key.
        o void erase(pos) erases the element pointed to by the iterator
pos.
        o void erase(first, beyond) erases all elements indicated by
the iterator range [first, beyond).

o map::iterator find(key):
returns
an iterator to the element having the given key. If the element isn't
available, end is returned. The following example illustrates the use of
the find member function:
            #include <iostream>
    #include <map>
    using namespace std;

    int main()
    {
        map<string, int> object;

        object["one"] = 1;

        map<string, int>::iterator it = object.find("one");

        cout << "`one' " <<
                (it == object.end() ? "not " : "") << "found\n";

        it = object.find("three");

        cout << "`three' " <<
                (it == object.end() ? "not " : "") << "found\n";
    }
    /*
        Generated output:
    `one' found
    `three' not found
    */

        o allocator_type get_allocator() const:
returns a
copy of the allocator object used by the map object.
    o ... insert():
inserts elements into the map. Values
associated with already existing keys, however, are not replaced by new
values. Its return value depends on the version of insert that is called:
        
        o pair<map::iterator, bool> insert(keyvalue) inserts
a new value_type into the map. The return value is a
pair<map::iterator, bool>.  If the returned bool field is true,
keyvalue was inserted into the map. The value false indicates that the
key that was specified in keyvalue was already available in the map, and
so keyvalue was not inserted into the map.  In both cases the
map::iterator field points to the data element having the
key that was specified in keyvalue. The use of this variant of
insert is illustrated by the following example:
            #include <iostream>
    #include <string>
    #include <map>
    using namespace std;

    int main()
    {
        pair<string, int> pa[] =
        {
            pair<string,int>("one", 10),
            pair<string,int>("two", 20),
            pair<string,int>("three", 30),
        };
        map<string, int> object(&pa[0], &pa[3]);

                // {four, 40} and `true' is returned
        pair<map<string, int>::iterator, bool>
            ret = object.insert
                    (
                        map<string, int>::value_type
                        ("four", 40)
                    );

        cout << boolalpha;

        cout << ret.first->first << " " <<
            ret.first->second << " " <<
            ret.second << " " << object["four"] << '\n';

                // {four, 40} and `false' is returned
        ret = object.insert
                    (
                        map<string, int>::value_type
                        ("four", 0)
                    );

        cout << ret.first->first << " " <<
            ret.first->second << " " <<
            ret.second << " " << object["four"] << '\n';
    }
    /*
        Generated output:

        four 40 true 40
        four 40 false 40
    */

    Note the somewhat peculiar constructions like
            cout << ret.first->first << " " << ret.first->second << ...

Note that `ret' is equal to the pair returned by the
insert member function. Its `first' field is an iterator into the
map<string, int>, so it can be considered a pointer to a map<string,
int>::value_type. These value types themselves are pairs too, having
`first' and `second' fields. Consequently, `ret.first->first' is
the key of the map value (a string), and `ret.first->second' is
the value (an int).
        o map::iterator insert(pos, keyvalue). This way a
map::value_type may also be inserted into the map. pos is ignored, and
an iterator to the inserted element is returned.
        o void insert(first, beyond) inserts the (map::value_type)
elements pointed to by the iterator range [first, beyond). Values that
were already present are not replaced.

o key_compare key_comp():
returns a copy of the object used by the map to compare keys. The
        type map<KeyType, ValueType>::key_compare is defined by the map
        container and key_compare's parameters have types KeyType const
        &. The comparison function returns true if the first key
        argument should be ordered before the second key argument. To compare
        keys and values, use value_comp, listed below.
    o map::iterator lower_bound(key):
returns an iterator pointing to the first
keyvalue element of which the key is at least equal to the specified
key.  If no such element exists, the function returns end.
    o size_t max_size():
returns the maximum number of
elements this map may contain.
    o map::reverse_iterator rbegin():

    returns a reverse_iterator pointing to the last element of the map.
    o map::reverse_iterator rend():
returns an iterator pointing
before the first element of the map.
    o size_t size():
returns the
number of elements in the map.
    o void swap(argument):
swaps two maps using identical key/value
types.
    o map::iterator upper_bound(key):
returns an iterator pointing to the first
keyvalue element having a key exceeding the specified key.  If no
such element exists, the function returns end.  The following
example illustrates the member functions equal_range, lower_bound
and upper_bound:
            #include <iostream>
    #include <map>
    using namespace std;

    int main()
    {
        pair<string, int> pa[] =
        {
            pair<string,int>("one", 10),
            pair<string,int>("two", 20),
            pair<string,int>("three", 30),
        };
        map<string, int> object(&pa[0], &pa[3]);
        map<string, int>::iterator it;

        if ((it = object.lower_bound("tw")) != object.end())
            cout << "lower-bound `tw' is available, it is: " <<
                    it->first << '\n';

        if (object.lower_bound("twoo") == object.end())
            cout << "lower-bound `twoo' not available" << '\n';

        cout << "lower-bound two: " <<
                object.lower_bound("two")->first <<
                " is available\n";

        if ((it = object.upper_bound("tw")) != object.end())
            cout << "upper-bound `tw' is available, it is: " <<
                    it->first << '\n';

        if (object.upper_bound("twoo") == object.end())
            cout << "upper-bound `twoo' not available" << '\n';

        if (object.upper_bound("two") == object.end())
            cout << "upper-bound `two' not available" << '\n';

        pair
        <
            map<string, int>::iterator,
            map<string, int>::iterator
        >
            p = object.equal_range("two");

        cout << "equal range: `first' points to " <<
                    p.first->first << ", `second' is " <<
            (
                p.second == object.end() ?
                    "not available"
                :
                    p.second->first
            ) <<
            '\n';
    }
    /*
        Generated output:

            lower-bound `tw' is available, it is: two
            lower-bound `twoo' not available
            lower-bound two: two is available
            upper-bound `tw' is available, it is: two
            upper-bound `twoo' not available
            upper-bound `two' not available
            equal range: `first' points to two, `second' is not available
    */


o value_compare value_comp():
returns a copy of the object used by the map to compare keys. The
        type map<KeyType, ValueType>::value_compare is defined by the map
        container and value_compare's parameters have types value_type
        const &. The comparison function returns true if the first
        key argument should be ordered before the second key argument. The
        Value_Type elements of the value_type objects passed to this
        member are not used by the returned function.


12.3.7.4: The `map': a simple example

            As mentioned at the beginning of section 12.3.7, the map represents
a sorted associative array. In a map the keys are sorted. If an
application must visit all elements in a map the begin and end
iterators must be used.

The following example illustrates how to make a simple table listing all keys
and values found in a map:
            #include <iostream>
    #include <iomanip>
    #include <map>

    using namespace std;

    int main()
    {
        pair<string, int>
            pa[] =
            {
                pair<string,int>("one", 10),
                pair<string,int>("two", 20),
                pair<string,int>("three", 30),
            };
        map<string, int>
            object(&pa[0], &pa[3]);

        for
        (
            map<string, int>::iterator it = object.begin();
                it != object.end();
                    ++it
        )
            cout << setw(5) << it->first.c_str() <<
                    setw(5) << it->second << '\n';
    }
    /*
        Generated output:
      one   10
    three   30
      two   20
    */



12.3.8: The `multimap' container

    Like the map, the multimap class implements a (sorted)
    associative array. Before using a multimap container the header
file <map> must be included.

The main difference between the map and the multimap is that the
multimap supports multiple values associated with the same key, whereas the
map contains single-valued keys. Note that the multimap also accepts multiple
identical values associated with identical keys.

The map and the multimap have the same set of constructors and member
functions, with the exception of the index operator which is not supported
  with the multimap. This is understandable: if
multiple entries of the same key are allowed, which of the possible values
should be returned for object[key]?

Refer to section 12.3.7 for an overview of
the multimap member functions. Some member functions, however, deserve
additional attention when used in the context of the multimap
container. These members are discussed below.
    
        o size_t map::count(key):
returns the
number of entries in the multimap associated with the given key.
        o ... erase():
erases elements from the map:
            
            o size_t erase(key) erases all elements having the
given key. The number of erased elements is returned.
            o void erase(pos) erases the single element pointed to by
pos. Other elements possibly having the same keys are not erased.
            o void erase(first, beyond) erases all elements indicated by
the iterator range [first, beyond).

o pair<multimap::iterator, multimap::iterator> equal_range(key):
returns a pair of iterators, being respectively the return values of
lower_bound and upper_bound, introduced below.  The function provides
a simple means to determine all elements in the multimap that have the
same keys. An example illustrating the use of these member functions is
given at the end of this section.
        o multimap::iterator find(key):
this
member returns an iterator pointing to the first value whose key is
key. If the element isn't available, end is returned. The
iterator could be incremented to visit all elements having the same key
until it is either end, or the iterator's first member is
not equal to key anymore.
        o multimap::iterator insert():
this
member function normally succeeds, and so a multimap::iterator is
returned, instead of a pair<multimap::iterator, bool> as returned with the
map container. The returned iterator points to the newly added element.

Although the functions lower_bound and upper_bound act
identically in the map and multimap containers, their operation in a
multimap deserves some additional attention. The next example illustrates
lower_bound, upper_bound and
equal_range applied to a multimap:
            #include <iostream>
    #include <map>
    using namespace std;

    int main()
    {
        pair<string, int> pa[] =
        {
            pair<string,int>("alpha", 1),
            pair<string,int>("bravo", 2),
            pair<string,int>("charlie", 3),
            pair<string,int>("bravo", 6),   // unordered `bravo' values
            pair<string,int>("delta", 5),
            pair<string,int>("bravo", 4),
        };
        multimap<string, int> object(&pa[0], &pa[6]);

        using msiIterator = multimap<string, int>::iterator;

        msiIterator it = object.lower_bound("brava");

        cout << "Lower bound for `brava': " <<
                it->first << ", " << it->second << '\n';

        it = object.upper_bound("bravu");

        cout << "Upper bound for `bravu': " <<
                it->first << ", " << it->second << '\n';

        pair<msiIterator, msiIterator>
            itPair = object.equal_range("bravo");

        cout << "Equal range for `bravo':\n";
        for (it = itPair.first; it != itPair.second; ++it)
            cout << it->first << ", " << it->second << '\n';
        cout << "Upper bound: " << it->first << ", " << it->second << '\n';

        cout << "Equal range for `brav':\n";
        itPair = object.equal_range("brav");
        for (it = itPair.first; it != itPair.second; ++it)
            cout << it->first << ", " << it->second << '\n';
        cout << "Upper bound: " << it->first << ", " << it->second << '\n';
    }
    /*
        Generated output:

        Lower bound for `brava': bravo, 2
        Upper bound for `bravu': charlie, 3
        Equal range for `bravo':
        bravo, 2
        bravo, 6
        bravo, 4
        Upper bound: charlie, 3
        Equal range for `brav':
        Upper bound: bravo, 2
    */

    In particular note the following characteristics:
    
    o lower_bound and upper_bound produce the same result for
non-existing keys: they both return the first element having a key that
exceeds the provided key.
    o  Although the keys are ordered in the multimap, the values for
equal keys are not ordered: they are retrieved in the order in which they were
entered.


12.3.9: The `set' container

    The set class implements a sorted collection of values. Before using
set containers the <set> header file must be included.

A set contains unique values (of a container-acceptable type). Each value
is stored only once, and the set sorts its values using their
operator<, where the smallest values appears as first element in the set.

A specific value can be explicitly created: Every set defines a
value_type which may be used to create values that can be stored in the
set. For example, a value for a set<string> can be constructed as
follows:
            set<string>::value_type setValue{ "Hello" };

Like the std::map container, the std::set also has an additional
parameter declaring the class that is used for comparing values in the set.
For value type ValueType the set's type definition, therefore, looks like
this:
            set<ValueType, std::less<ValueType>>

The value_type is associated with the set<string>. Anonymous
value_type objects are also often used. E.g.,
            set<string>::value_type{ "Hello" };

Instead of using the line set<string>::value_type(...) over
and over again, a using declaration is often used to reduce typing and to
improve readability:
            using StringSetValue = set<string>::value_type;

Now values for the set<string> may be constructed as follows:
            StringSetValue{ "Hello" };

Alternatively, values of the set's type may be used
immediately. In that case the value of type Type is implicitly
converted  to a set<Type>::value_type.

The following constructors, operators, and member functions are available
for the set container:
    
    o  Constructors:
        
        o  The copy and move constructors are available;
        o  A set may be constructed empty:
            set<int> object;

o  A set may be initialized using two iterators. For example:
                int intarr[] = {1, 2, 3, 4, 5};

    set<int> object{ &intarr[0], &intarr[5] };

Note that all values in the set must be different: it is not possible to
store the same value repeatedly when the set is constructed. If the same value
occurs repeatedly, only the first instance of the value is entered into the
set; the remaining values are silently ignored.

Like the map, the set receives its own copy of the data it
contains.
    o  The set container only supports the standard set of operators
that are available for containers.
    o  The set class has the following member functions:
        
        o set::iterator begin():
returns an iterator pointing to
the first element of the set. If the set is empty end is returned.
        o void clear():
erases all elements from the set.
        o size_t count(value):
returns 1 if
the specified value is available in the set, otherwise 0 is returned.
        o pair<iterator, bool> emplace(Args &&...args):
a value_type object is constructed from the member's
            arguments, and the newly created element is inserted into the
            set. The return value's second member is true if the
            element is inserted, and false if the element was already
            available. The pair's first element is an iterator referring
            to the set's eement. 
        o bool empty():
returns true if
the set contains no elements.
        o set::iterator end():
returns an iterator pointing beyond
the last element of the set.
        o pair<set::iterator, set::iterator> equal_range(value):
this
member returns a pair of iterators, being respectively the return values of
the member functions lower_bound and upper_bound, introduced
below.
        o ... erase():
erases a specific element or range of
elements from the set:
            
            o bool erase(value) erases the element having the given
value from the set. True is returned if the value was removed,
false if the set did not contain an element `value'.
            o void erase(pos) erases the element pointed to by the iterator
pos.
            o void erase(first, beyond) erases all elements
indicated by the iterator range [first, beyond).

o set::iterator find(value):
returns
an iterator to the element having the given value. If the element isn't
available, end is returned.
        o allocator_type get_allocator() const:
returns a
copy of the allocator object used by the set object.
        o ... insert():
inserts elements into the set. If the
element already exists, the existing element is left untouched and the element
to be inserted is ignored.  The return value depends on the version of
insert that is called:
            
            o pair<set::iterator, bool> insert(value) inserts
a new set::value_type into the set. The return value is a
pair<set::iterator, bool>.  If the returned bool field is true,
value was inserted into the set. The value false indicates that the
value that was specified was already available in the set, and
so the provided value was not inserted into the set.  In both cases the
set::iterator field points to the data element in the set having the
specified value.
            o set::iterator insert(pos, value). This way a
set::value_type may also be inserted into the set. pos is ignored, and
an iterator to the inserted element is returned.
            o void insert(first, beyond) inserts the (set::value_type)
elements pointed to by the iterator range [first, beyond) into the
set. Values that were already present are not replaced.

o key_compare key_comp():
returns a copy of the object used by the set to compare keys. The
        type 

        set<ValueType>::key_compare is defined by the set container
        and key_compare's parameters have types ValueType const &. The
        comparison function returns true if its first argument should
        be ordered before its second argument.
    o set::iterator lower_bound(value):
returns an iterator pointing to the first value element of
        which the value is at least equal to the specified value.  If no
        such element exists, the function returns end.
    o size_t max_size():
returns the maximum number of
        elements this set may contain.
    o set::reverse_iterator rbegin():

        returns a reverse_iterator pointing to the last element of the set.
    o set::reverse_iterator rend:
returns a reverse_iterator
        pointing before the first element of the set.
    o size_t size():
returns the number of elements in the set.
    o void swap(argument):
swaps two sets (argument being
the second set) that use identical data types.
    o set::iterator upper_bound(value):
returns an iterator pointing to the first
value element having a value exceeding the specified value.  If no
such element exists, the function returns end.
    o value_compare value_comp():
returns a copy of the object used by the set to compare values. The
        type 

       set<ValueType>::value_compare is defined by the set container and
        value_compare's parameters have types ValueType const &. The
        comparison function returns true if its first argument should be
        ordered before its second argument. Its operation is identical to that
        of a key_compare object, returned by key_comp.


12.3.10: The `multiset' container

    Like the set, the multiset class implements a
     sorted collection of values. Before
using multiset containers the header file <set> must be included.

The main difference between the set and the multiset is that the
multiset supports multiple entries of the same value, whereas the set
contains unique values.

The set and the multiset have the same set of constructors and member
functions.  Refer to section 12.3.9 for an overview of the member functions
that can be used with the multiset.  Some member functions, however,
behave slightly different than their counterparts of the set
container. Those members are:
    
        o size_t count(value):
returns the number of
entries in the multiset associated with the given value.
        o ... erase():
erases elements from the set:
            
            o size_t erase(value) erases all elements having the given
value. The number of erased elements is returned.
            o void erase(pos) erases the element pointed to by the iterator
pos. Other elements possibly having the same values are not erased.
            o void erase(first, beyond) erases all elements indicated by
the iterator range [first, beyond).

o pair<multiset::iterator, multiset::iterator> equal_range(value):
returns a pair of iterators, being respectively the return values of
lower_bound and upper_bound, introduced below.  The function provides
a simple means to determine all elements in the multiset that have the
same values.
        o multiset::iterator find(value):
returns an
iterator pointing to the first element having the specified value. If the
element isn't available, end is returned. The iterator could be
incremented to visit all elements having the given value until it is
either end, or the iterator doesn't point to `value' anymore.
        o ... insert():
this member function normally succeeds and
returns a multiset::iterator rather than a pair<multiset::iterator,
bool> as returned with the set container. The returned iterator points to
the newly added element.

Although the functions lower_bound and upper_bound act identically
in the set and multiset containers, their operation in a multiset
deserves some additional attention. With a multiset container
lower_bound and upper_bound produce the same result for non-existing
keys: they both return the first element having a key exceeding the provided
key.

Here is an example showing the use of various member functions of a
multiset:
        #include <iostream>
    #include <set>

    using namespace std;

    int main()
    {
        string
            sa[] =
            {
                "alpha",
                "echo",
                "hotel",
                "mike",
                "romeo"
            };

        multiset<string>
            object(&sa[0], &sa[5]);

        object.insert("echo");
        object.insert("echo");

        multiset<string>::iterator
            it = object.find("echo");

        for (; it != object.end(); ++it)
            cout << *it << " ";
        cout << '\n';

        cout << "Multiset::equal_range(\"ech\")\n";
        pair
        <
            multiset<string>::iterator,
            multiset<string>::iterator
        >
            itpair = object.equal_range("ech");

        if (itpair.first != object.end())
            cout << "lower_bound() points at " << *itpair.first << '\n';
        for (; itpair.first != itpair.second; ++itpair.first)
            cout << *itpair.first << " ";

        cout << '\n' <<
                object.count("ech") << " occurrences of 'ech'" << '\n';

        cout << "Multiset::equal_range(\"echo\")\n";
        itpair = object.equal_range("echo");

        for (; itpair.first != itpair.second; ++itpair.first)
            cout << *itpair.first << " ";

        cout << '\n' <<
                object.count("echo") << " occurrences of 'echo'" << '\n';

        cout << "Multiset::equal_range(\"echoo\")\n";
        itpair = object.equal_range("echoo");

        for (; itpair.first != itpair.second; ++itpair.first)
            cout << *itpair.first << " ";

        cout << '\n' <<
                object.count("echoo") << " occurrences of 'echoo'" << '\n';
    }
    /*
        Generated output:

        echo echo echo hotel mike romeo
        Multiset::equal_range("ech")
        lower_bound() points at echo

        0 occurrences of 'ech'
        Multiset::equal_range("echo")
        echo echo echo
        3 occurrences of 'echo'
        Multiset::equal_range("echoo")

        0 occurrences of 'echoo'
    */



12.3.11: The `stack' container

    The stack class implements a stack data structure.  Before using
stack containers the header file <stack> must be included.

A stack is also called a first in, last out (FILO or LIFO) data
structure as the first item to enter the stack is the last item to leave.  A
stack is an extremely useful data structure in situations where data must
temporarily remain available. For example, programs maintain a stack to store
local variables of functions: the lifetime of these variables is determined by
the time these functions are active, contrary to global (or static local)
variables, which live for as long as the program itself lives. Another example
is found in calculators using the
 Reverse Polish Notation (RPN), in which the operands of operators are
kept  in a stack, whereas operators pop their operands off the stack and
push the results of their work back onto the stack.

As an example of the use of a stack, consider figure 14, in which
the content of the stack is shown while the expression (3 + 4) * 2 is
evaluated. In the RPN this expression becomes 3 4 + 2 *, and figure
14 shows the stack content after each token (i.e., the
operands and the operators) is read from the input. Notice that each operand
is indeed pushed on the stack, while each operator changes the content of the
stack.
    ------------------------------------------------------------------Insert Figure 14(The content of a stack while evaluating 3 4 + 2 *)about here (file: containers/stack/stack)------------------------------------------------------------------
    The expression is evaluated in five steps. The caret between the tokens
in the expressions shown on the first line of figure 14 shows what
token has just been read. The next line shows the actual stack-content, and
the final line shows the steps for referential purposes. Note that at step 2,
two numbers have been pushed on the stack. The first number (3) is now at
the bottom of the stack. Next, in step 3, the + operator is read. The
operator pops two operands (so that the stack is empty at that moment),
calculates their sum, and pushes the resulting value (7) on the
stack. Then, in step 4, the number 2 is read, which is dutifully pushed on
the stack again. Finally, in step 5 the final operator * is read, which
pops the values 2 and 7 from the stack, computes their product, and
pushes the result back on the stack. This result (14) could then be popped
to be displayed on some medium.

From figure 14 we see that a stack has one location (the top)
where items can be pushed onto and popped off the stack. This top element is
the stack's only immediately visible element. It may be accessed and modified
directly.

Bearing this model of the stack in mind, let's see what we formally can do
with the stack container. For the stack, the following constructors,
operators, and member functions are available:
    
    o   Constructors:
        
        o  The copy and move constructors are available;
        o  A stack may be constructed empty:
    stack<string> object;

o  Only the basic set of container operators are supported by the
stack
    o  The following member functions are available for stacks:
        
        o value_type &emplace(Args &&...args):
a value_type object is constructed from the member's
            arguments, and the newly created element is pushed on the stack.
            The return value is a reference to (or copy of) the newly pushed
            value. 
        o bool empty():
this
member returns true if the stack contains no elements.
        o void pop():
removes the element at the top of the
stack. Note that the popped element is not returned by this member.
Refer to section 12.3.4 for a discussion about the reason why pop has
return type void and why pop should not be called on an empty
stack.
        o void push(value):
places
value at the top of the stack, hiding the other elements from view.
        o size_t size():
this
member returns the number of elements in the stack.
        o Type &top():
this member
returns a reference to the stack's top (and only visible) element. It is the
    responsibility of the programmer to use this member only if the stack
is not empty.

The stack does not support iterators or an index
operator. The only elements that can be accessed is its top element.
  To empty a stack:
    
    o  repeatedly remove its front element;
    o  assign an empty stack to it;
    o  have its destructor called (e.g., by ending its lifetime).


12.3.12: Intermezzo: hashing and unordered containers

    Hashing is a very attractive way to find an element in a
container. For example, to find an element in an unsorted std::vector of
N elements takes (on average) N / 2 tests before the element is
found. If the elements in the vector are sorted a binary search algorithm
(cf. section 19.1.7) can be used, requiring about 2 log N tests
before the element is found. But hashing usually requires just one test, and
occasionally one or two additional tests.

Hashing is used by some of the standard containers, like the unordered_set
(section 12.3.13) and the unordered_map (section 12.3.14)
containers. For example, when using the unordered_set container four types
must be specified: a key type, a hash type, a predicate type, and
an allocator type. In practice, the default provided allocator type 
can be used (section 23.4 covers the construction of your
own allocator class). The key is used by the hash type to determine the
location of the key in the container, and the predicate type doesn't have to
be specified if the key type itself supports the equality operator
(i.e., operator==).


 For all built-in types and many standard (std::) types hashing is already
available, e.g., std::bitset, std::chrono, std::error_code,
std::optional, std::string, std::string_view, std::unique_ptr,
std::vector<bool>
 (a complete overview is provided by
cppreference.com).
When using those key types unordered_sets can be constructed by merely
specifying the key type. E.g.,
        std::unordered_set<double>
    std::unordered_set<std::string>
    

When a hash value cannot already be computed for a Key type then a hash
class, and (if not already provided by the key type itself) a predicator type
must be defined.

 
 Consider the situation where an unordered_set for a Key class type
containing two std::string data members must be constructed. To define and
use such an unordered_set the following must be available:
    
    
    o  The Key class. In this case, where the class's data members can
        all be compared for equality, a default operator== can be used. If
        not all its data members can be compared for equality the Key
        class can implement its own operator== member. If, when using an
        unordered container, a special operator== is required, then a
        separate predicate class must be defined which is then specified when
        defining the unordered container.

       Such a predicate class defines a bool operator()(Key const &lhs, Key
        const &rhs), performing the special comparison of two Key
        objects, which apparently could not be defined that way by the Key
        class itself.

       In the example used in this section the Key class's data members
        all can be compared for equality, and thus the compiler can provide a
        default implementation for its operator== member. Here is the
        class's interface, note the = default in the declaration of its
        operator== member:
           class Key
    {
        std::string d_name;
        std::string d_phone;
    
        public:
            Key(std::string const &name, std::string const &phone);
            bool operator==(Key const &other) const = default;
    
            std::string const &name() const;
            std::string const &phone() const;
    };


o  Hash values (also known as bucket numbers) are computed and
        returned either by a KeyHash class defining a function call
        operator (also known as the hash function), or by a specialization
        for the Key type of the std::hash template.

       Collisions  occur when a computed hash value happens
        to be equal to the hash value computed for another key which is
        already available in the container.  Those cases are seldom
        encountered and are solved by the container, hence the on average
        slightly more than one required test to retrieve a key.  A simple
        solution consists of using linear chaining, using linked list to
        store colliding elements.

A KeyHash class only needs a size_t operator()(Key const &key) const
member function (optionally noexcept can be added to its declaration). For
example:
        struct KeyHash
    {
        size_t operator()(Key const &key) const;        // noexcept;
    };
    
    size_t KeyHash::operator()(Key const &key) const    // noexcept
    {
        std::hash<std::string> hashObj;
    
        return hashObj(key.name()) ^ (hashObj(key.phone()) << 1);
    }


Using Key and KeyHash an unordered_set can be defined as, e.g.,
        std::unordered_set<Key, KeyHash> keySet;
    

Alternatively a specialization of the std::hash class template
(cf. chapter 22) can be defined.  Interestingly, when the
std::hash specialization is defined it's OK to define it as std::hash,
so inside the std namespace, which normally is off-limits for user-defined
software. it's easy to define this specialization. Here are its class and
member function:
                template<>
    struct std::hash<Key>
    {
        std::size_t operator()(Key const &key) const;           // noexcept;
    };
    
    size_t std::hash<Key>::operator()(Key const &key) const     // noexcept
    {
        std::hash<std::string> hashObj;
    
        return hashObj(key.name()) ^ (hashObj(key.phone()) << 1);
    }


When this specialization is available the unordered_set can be defined by
merely specifying the Key class:
        std::unordered_set<Key> keySet;
    

The std::hash class is included by quite a few standard header files:
    bitset, chrono, coroutine, filesystem, 
        functional, memory, optional, stacktrace, string,
        string_view, system_error, text_encoding, thread,
        typeindex, utility, variant, vector.

Include the <functional> header file when std::hash is required but
none of the other header files are included.

Key values frequently consists of text. Be careful not to use a plain char
const * key type as two char const * values pointing to equal
C-strings stored at different locations are different char const *
keys, as their pointer values rather than their textual content are compared.


12.3.13: The `unordered_set' container

    The set container, like the map container, orders its elements. If
ordering is not an issue, but fast lookups is, then a hash-based set and/or
multi-set may be used (cf. section 12.3.12). C++ offers such
hash-based sets and multi-sets: the unordered_set and
unordered_multiset.

    Before using a hash-based set container the header file
<unordered_set> must be included.

Elements stored in the unordered_set are immutable, but they can be
inserted and removed from the container. The set merely stores elements, and
the stored element itself acts as its key.


12.3.13.1: The `unordered_set' constructors

        When defining an unordered_set four template arguments must be specified
(often, except for the first template argument default arguments can be used):
    
    
    o  a Key type: the type of the values storing in the set (becoming
        the sub-types unordered_set::key_type and
         unordered_set::value_types);
    o  a Hash type: the type of an object computing a hash value from a key
        value (becoming the sub-type unordered_set::hasher);
    o  a Predicator type: the type of an object that can compare two keys
        for equality (becoming the sub-type unordered_set::key_equal).
    o  an Allocator type: the type of its allocator. This is usually left
        unspecified, using the allocator provided by default by the
        implementor.

The generic definition of an unordered_set container looks like this:
            std::unordered_set <Key type, Hash type, Predicate type, Allocator type>

For all built-in types and many standard (std::) types hashing and the
predicator are already available (cf. section 12.3.12).  In these cases an
unordered_set object can be defined by merely specifying the key
type. E.g.,
            std::unordered_set<std::string> strSet;

When necessary the set's size is automatically enlarged, in which case the
container rehashes all its elements.

The unordered_set supports the following constructors:
    
    o  Copy and move constructors (and corresponding assignment operators)
        are available; 
    o explicit unordered_set(size_type size = implSize,
             hasher const &hf = hasher(),
             key_equal const &eql = key_equal(),
             allocator_type const &alloc = allocator_type()):

        this constructor can also be used as default constructor.
        Here, implSize is the container's default initial size, which is
        specified by the implementor. In practice the default size
        argument provided by the implementor is completely satisfactory;
    o unordered_set(const_iterator begin, const_iterator end,
                    size_type n = implSize, hasher const &hf = hasher()),
            key_equal const &eql = key_equal(),
            allocator_type const &alloc = allocator_type():

        this constructor expects two iterators specifying
        a range of unordered_set::value_type const objects;
    o unordered_set(initializer_list<value_type> initList,
                    size_type n = implSize, hasher const &hf = hasher()),
            key_equal const &eql = key_equal(),
            allocator_type const &alloc = allocator_type():

        a constructor expecting an initializer_list of
            unordered_set::value_type values.


12.3.13.2: The `unordered_set' public members

        The unordered_set provides the following
  member functions:


    o unordered_set::iterator begin():
returns an iterator pointing to the first element in the
        unordered set, returning end if the unordered set is empty;
    o size_t bucket(key_type const &key):
returns the hash value where key is stored. If key wasn't
        stored yet bucket adds value_type(key, Value()) before
        returning its index position;
    o size_t bucket_count():
returns the number of slots used by the container. Each slot may
        contain one (or more, in case of collisions) value_type objects;
    o size_t bucket_size(size_t index):
returns the number of value_type objects stored at bucket position
        index;
    o unordered_set::const_iterator cbegin():
returns a const_iterator pointing to the first element in the
        unordered set, returning cend if the unordered set is empty;
    o unordered_set::const_iterator cend():
returns a const_iterator pointing just beyond the unordered set's
        last element;
    o void clear():
erases all the unordered set's elements;
    o size_t count(key_type const &key):
returns the number of times a value_type object using key_type
        key is stored in the unordered_set (which is either one or
        zero);
    o iterator emplace(Args &&...args):
a value_type object is constructed from emplace's
        arguments. It is added to the set if it is unique, and an iterator to
        the value_type is returned;
    o iterator emplace_hint(const_iterator position,
                        Args &&...args):
a value_type object is constructed from the member's arguments,
        and if the newly created element is unique it is inserted into the
        unordered_set. The implementation may or may not use position
        as a hint to start looking for an insertion point. The returned
        iterator points to the value_type;
    o bool empty():
returns true if the unordered set contains no elements;
    o unordered_set::iterator end():
returns an iterator pointing beyond the last element in the
        unordered set;
    o pair<iterator, iterator> equal_range(key):
this member returns a pair
        of iterators defining the range of elements having a key that is equal
        to key;
    o unordered_set::iterator erase():
erases a specific range of elements in the unordered set:
        
        o erase(key_type const &key) erases key from the set. It
           returns true if the key was removed and false if the
            key wasn't available in the set.
        o erase(pos) erases the element pointed to by the iterator
            pos. The iterator ++pos is returned.
        o erase(first, beyond) erases elements indicated by the
            iterator range [first, beyond), returning beyond.

o iterator find(key):
returns an iterator to the element having the given key. If the
        element isn't available, end is returned;
    o allocator_type get_allocator() const:
returns a copy of the allocator object used by the unordered_set
        object;
    o hasher hash_function() const:
returns a copy of the hash function object used by the
        unordered_set object;
    o ... insert():
       elements may be inserted starting at a certain position. No
        insertion is performed if the provided key is already in use. The
        return value depends on the version of insert() that is
        called. When a pair<iterator, bool> is returned, then the
        pair's first member is an iterator pointing to the element having
        a key that is equal to the key of the provided value_type, the
        pair's second member is true if value was actually
        inserted into the container, and false if not;
       
        o pair<iterator, bool> insert(value_type const &value)
            attempts to insert value.
        o pair<iterator, bool> insert(value_type &&tmp)
           attempts to insert value using value_type's move
            constructor.
        o pair<iterator, bool> insert(const_iterator hint, value_type
            const &value)
           attempts to insert value, possibly using hint as a starting
            point when trying to insert value.
        o pair<iterator, bool> insert(const_iterator hint, value_type &&tmp)
           attempts to insert a value using value_type's move
            constructor, and possibly using hint as a starting point when
            trying to insert value.
        o void insert(first, beyond)
           tries to insert the elements in the iterator range [first,
            beyond).
        o void insert(initializer_list <value_type> iniList)
           attempts to insert the elements in iniList into the container.

o hasher key_eq() const:
returns a copy of the key_equal function object used by the
        unordered_set object;
    o float load_factor() const:
returns the container's current load factor, i.e. size /
        bucket_count;
    o size_t max_bucket_count():
returns the maximum number of buckets this unordered_set may
        contain;
    o float max_load_factor() const:
identical to load_factor;
    o void max_load_factor(float max):
changes the current maximum load factor to max. When a load factor
        of max is reached, the container will enlarge its
        bucket_count, followed by a rehash of its elements. Note that the
        container's default maximum load factor equals 1.0
    o size_t max_size():
returns the maximum number of elements this unordered_set may
        contain;
    o void rehash(size_t size):
if size exceeds the current bucket count, then the bucket count is
        increased to size, followed by a rehash of its elements;
    o void reserve(size_t request):
if request is less than or equal to the current bucket count, this
        call has no effect. Otherwise, the bucket count is increased to a
        value of at least request, followed by a rehash of the container's
        elements;
    o size_t size():
returns the number of elements in the unordered set;
    o void swap(unordered_set &other):
swaps the content of the current and the other unordered_set.  


12.3.13.3: The `unordered_multiset' container

        The unordered_multiset allows multiple objects using the same keys to be
stored in an unordered set. The unordered_multiset container offers the
same set of members and constructors as the unordered_set, but without the
unique-key restriction imposed upon the unordered_set.

Below all members are described whose behavior differs from the behavior of
the corresponding unordered_set members:


        o size_t count(key_type const &key):
returns the number of times a value_type object using
key_type key is stored in the unordered_set. This member is
commonly used to verify whether key is available in the
unordered_multiset;
        o iterator emplace(Args &&...args):
a value_type object is constructed from emplace's
arguments. The returned iterator points to the newly inserted inserted
value_type;
        o iterator emplace_hint(const_iterator position,
                            Args &&...args):
a value_type object is constructed from the member's
arguments, and the newly created element is inserted into the
unordered_multiset. The
implementation may or may not use position as a hint to start looking
for an insertion point. The returned iterator points to the value_type
using the provided key;
    o pair<iterator, iterator> equal_range(key):
this
member returns a pair of iterators  defining the range of elements having a
key that is equal to key;
    o iterator find(key):
returns an iterator to an element having the given key. If no
such element is available, end is returned;
        o ... insert():
            elements may be inserted starting at a certain position. The
return value depends on the version of insert() that is called. When an
iterator is returned, then it points to the element that was inserted:
            
            o iterator insert(value_type const &value)
                inserts value.
            o iterator insert(value_type &&tmp)
                inserts value using value_type's move
                constructor.
            o iterator insert(const_iterator hint, value_type const &value)
                inserts value, possibly using hint as a
starting point when trying to insert value.
            o iterator insert(const_iterator hint, value_type &&tmp)
                inserts value using value_type's move
constructor, and possibly using hint as a starting point when trying to
insert value.
            o void insert(first, beyond) inserts the elements in
the iterator range [first, beyond).
            o void insert(initializer_list <value_type> iniList)
                inserts the elements in iniList into the container.


12.3.14: The `unordered_map' container (`hash table')

    In C++ hash tables are available as objects of the class
std::unordered_map. 

Before using unordered_map or unordered_multimap containers the header
file <unordered_map> must be included.

The class unordered_map implements an associative array in which the
elements are stored according to some hashing scheme.  While the map
container defines a sorted data structure whose keys are sorted using the key
class's operator<, the unordered_map uses hashing to located its
elements (cf. section 12.3.12). Hashing is a by far faster way to store and
retrieve data than using a sorted data structure.

Retrieval of a key/value pair is as simple as computing the key's
 hash value, and locating it in the table at the computed index location:
if the key is present, the key/value combination is retrieved. If the key
isn't available then the key/value pair may automatically be stored in the
table at the computed bucket location, returning the value.

The term unordered_map is used rather than hash to avoid name
collisions with hash tables developed before they were added to the language.

The unordered_map supports the index operator operating identically to the
map's index operator: a (const) reference to the ValueType associated
with the provided KeyType's value is returned. If not yet available, the
key is added to the unordered_map, and a default ValueType value is
returned. In addition, it supports operator==.

The unordered_map also offers the  ValueType &at(KeyType const
&key) member returning a reference to the unordered_map's ValueType
associated with key. If key is not stored in the unordered_map a
std::out_of_range exception is thrown.

Other than those, it offers the same members as the unordered_set
(cf. section 12.3.13), but some of these members show different behavior.
Those members are listed below. For a description of the
remaining members, please refer to section 12.3.13.2.


        o pair<iterator, bool> emplace(Args &&...args):
a value_type object is constructed from emplace's
arguments. If the unordered_map already contained an object using the same
key_type value, then a std::pair is returned containing an iterator
pointing to the object using the same key_type value and the value
false. If no such key_type value was found, the newly constructed
object is inserted into the unordered_map, and the returned std::pair
contains an iterator pointing to the newly inserted inserted value_type
as well as the value true;

o iterator emplace_hint(const_iterator position,
                            Args &&...args):
a value_type object is constructed from the member's
arguments, and the newly created element is inserted into the
unordered_map, unless the (at args) provided key already exists. The
implementation may or may not use position as a hint to start looking
for an insertion point. The returned iterator points to the value_type
using the provided key. It may refer to an already existing value_type or
to a newly added value_type; an existing value_type is not
replaced. If a new value was added, then the container's size has been
incremented when emplace_hint returns;


12.3.14.1: The `unordered_map' constructors

        When defining an unordered_map type five template arguments must be
specified :
    
    
    o  a Key type: the type of the values storing in the set (becoming
        the sub-type unordered_map::key_type);
    o  a Value type: the value associated with a key (becoming the sub-type
        unordered_map::mapped_type);
    o  a Hash type: the type of an object computing a hash value from a key
        value (becoming the sub-type unordered_map::hasher);
    o  a Predicator type: the type of an object that can compare two keys
        for equality (becoming the sub-type unordered_map::key_equal);
    o  an Allocator type: the type of its allocator. This is usually left
        unspecified, using the allocator provided by default by the
        implementor.

The generic definition of an unordered_map container type looks like this:
        std::unordered_map <Key type, Value type, Hash type, 
                        Predicate type, Allocator type>

For all built-in types and many standard (std::) types hashing and the
predicator are already available. In practice the allocator type is not
specified, as the default allocator suffices.  In these cases an
unordered_map object can be defined by merely specifying its key- and
value types, like this:
            std::unordered_map<std::string, ValueType> hash;

The unordered_map supports the following constructors:
    
    o  Copy and move constructors (and corresponding assignment operators)
        are available; 
    o explicit unordered_map(size_type n = implSize,
             hasher const &hf = hasher(),
             key_equal const &eql = key_equal(),
             allocator_type const &alloc = allocator_type()):

       this constructor can also be used as default constructor.  Here,
        implSize is the container's default initial size, which is
        specified by the implementor. In practice the default size
        argument provided by the implementor is completely satisfactory;
    o explicit unordered_map(const_iterator begin, const_iterator
        end,size_type n = implSize,
             hasher const &hf = hasher(),
             key_equal const &eql = key_equal(),
             allocator_type const &alloc = allocator_type()):

            this constructor expects two iterators providing
        a range of unordered_map::value_type const objects;
    o unordered_map(initializer_list<value_type> initList,
            size_type n = implSize,
            hasher const &hf = hasher(),
            key_equal const &eql = key_equal(),
            allocator_type const &alloc = allocator_type()):

        a constructor expecting an initializer_list of
        unordered_map::value_type values.

The following example shows a program using an unordered_map
containing the names of the months of the year and the number of days these
months (usually) have. Then, using the subscript operator the days in several
months are displayed (the predicate used here is the generic algorithm
equal_to<string>, which is provided by the compiler as the default fourth
argument of the unordered_map constructor):
            #include <unordered_map>
    #include <iostream>
    #include <string>
    using namespace std;

    int main()
    {
        unordered_map<string, int> months;

        months["january"] = 31;
        months["february"] = 28;
        months["march"] = 31;
        months["april"] = 30;
        months["may"] = 31;
        months["june"] = 30;
        months["july"] = 31;
        months["august"] = 31;
        months["september"] = 30;
        months["october"] = 31;
        months["november"] = 30;
        months["december"] = 31;

        cout << "september -> " << months["september"] << '\n' <<
                "april     -> " << months["april"] << '\n' <<
                "june      -> " << months["june"] << '\n' <<
                "november  -> " << months["november"] << '\n';
    }
    /*
        Generated output:
    september -> 30
    april     -> 30
    june      -> 30
    november  -> 30
    */



        
12.3.14.2: The `unordered_multimap' container

        The unordered_multimap allows multiple objects using the same keys to be
stored in an unordered map. The unordered_multimap container offers the
same set of members and constructors as the unordered_map, but without the
unique-key restriction imposed upon the unordered_map.

The unordered_multimap does not offer operator[] and does not offer
at members.

Below all members are described whose behavior differs from the behavior of
the corresponding unordered_map members:


        o at(key_type const &key)

            not supported by the unordered_multimap container;
        o size_t count(key_type const &key):
returns the number of times a value_type object using
            key_type key is stored in the unordered_map. This
            member is commonly used to verify whether key is available in
            the unordered_multimap;
        o iterator emplace(Args &&...args):
a value_type object is constructed from emplace's
arguments. The returned iterator points to the newly inserted inserted
value_type;
        o iterator emplace_hint(const_iterator position,
                            Args &&...args):
a value_type object is constructed from the member's
arguments, and the newly created element is inserted into the
unordered_multimap. The
implementation may or may not use position as a hint to start looking
for an insertion point. The returned iterator points to the value_type
using the provided key;
    o pair<iterator, iterator> equal_range(key):
this
member returns a pair of iterators  defining the range of elements having a
key that is equal to key;
    o iterator find(key):
returns an iterator to an element having the given key. If no
such element is available, end is returned;
        o ... insert():
            elements may be inserted starting at a certain position. The
return value depends on the version of insert() that is called. When an
iterator is returned, then it points to the element that was inserted.
            
            o iterator insert(value_type const &value)
                inserts value.
            o iterator insert(value_type &&tmp)
                inserts value using value_type's move
                constructor.
            o iterator insert(const_iterator hint, value_type const &value)
                inserts value, possibly using hint as a
starting point when trying to insert value.
            o iterator insert(const_iterator hint, value_type &&tmp)
                inserts value using value_type's move
constructor, and possibly using hint as a starting point when trying to
insert value.
            o void insert(first, beyond) inserts the elements in
the iterator range [first, beyond).
            o void insert(initializer_list <value_type> iniList)
                inserts the elements in iniList into the container.


12.3.15: Heterogeneous lookup

    The associative containers offered by C++ allow us to find a value (or
values) matching a given key. Traditionally, the type of the key used for the
lookup must match the container's key type. 

Arbitrary lookup key types can be used provided a comparison operator is
available to compare that type with the container's key type.  Thus, a char
const * key (or any other type for which an operator< overload for
std::string is available) can be used to lookup values in a
map<std::string, ValueType>. This is called
    heterogeneous lookup.

Heterogeneous lookup is allowed when the comparator given to the associative
container does allow this. The standard library classes std::less and
std::greater were augmented to allow heterogeneous lookup.


12.4: The `complex' container

The complex container defines the standard operations that can be
performed on complex numbers. Before using complex containers the
header file <complex> must be included.

The complex number's real and imaginary types are specified as the container's
data type. Examples:
            complex<double>
    complex<int>
    complex<float>

Note that the real and imaginary parts of complex numbers have the same
datatypes.

When initializing (or assigning) a complex object, the imaginary part
may be omitted from the initialization or assignment resulting in its value
being 0 (zero). By default, both parts are zero.

Below it is silently assumed that the used complex type is
complex<double>. Given this assumption, complex numbers may be initialized
as follows:
    
    o target:   A default initialization: real and imaginary parts are 0.
    o target(1): The real part is 1, imaginary part is 0
    o target(0, 3.5): The real part is 0, imaginary part is 3.5
    o target(source): target is initialized with the values of
source.

Anonymous complex values may also be used. In the next example two
anonymous complex values are pushed on a stack of complex numbers, to be
popped again thereafter:
            #include <iostream>
    #include <complex>
    #include <stack>

    using namespace std;

    int main()
    {
        stack<complex<double>>
            cstack;

        cstack.push(complex<double>(3.14, 2.71));
        cstack.push(complex<double>(-3.14, -2.71));

        while (cstack.size())
        {
            cout << cstack.top().real() << ", " <<
                    cstack.top().imag() << "i" << '\n';
            cstack.pop();
        }
    }
    /*
        Generated output:
    -3.14, -2.71i
    3.14, 2.71i
    */


The following member functions and operators are defined for complex
numbers (below, value may be either a primitive scalar type or a
complex object):
    
        o  Apart from the standard container operators, the following
operators are supported from the complex container.
        
        o complex operator+(value):
 this
member returns the sum of the current complex container and value.
        o complex operator-(value):
 this
member returns the difference between the current complex container and
value.
        o complex operator*(value):
 this
member returns the product of the current complex container and
value.
        o complex operator/(value):
 this
member returns the quotient of the current complex container and
value.
        o complex operator+=(value):
 this
member adds value to the current complex container, returning the
new value.
        o complex operator-=(value):

this member subtracts value from the current complex container,
returning the new value.
        o complex operator*=(value):
 this
member multiplies the current complex container by value, returning
the new value
        o complex operator/=(value):
 this
member divides the current complex container by value, returning the
new value.

o Type real():
returns the real part of a
complex number.
        o Type imag():
returns the imaginary part
of a complex number.
        o  Several mathematical functions are available for the
complex container, such as abs, arg, conj, cos,
cosh, exp, log, norm, polar, pow,
sin, sinh and sqrt. All these functions are free functions,
not member functions, accepting complex numbers as their arguments. For
example,
    abs(complex<double>(3, -5));
pow(target, complex<int>(2, 3));

o  Complex numbers may be  extracted from istream
objects and inserted  into ostream objects. The insertion
results in an ordered pair (x, y), in which x represents the real
part and y the imaginary part of the complex number. The same form may
also be used when extracting a complex number from an istream
object. However, simpler forms are also allowed. E.g., when extracting
1.2345 the imaginary part is set to 0.



Chapter 13: Inheritance

When programming in C, programming problems are commonly approached using
a top-down structured approach: functions and actions of the program are
defined in terms of sub-functions, which again are defined in
sub-sub-functions, etc..  This yields a hierarchy of code: main at
the top, followed by a level of functions which are called from main,
etc..

In C++ the relationship between code and data is also frequently
defined in terms of dependencies among classes. This looks like
composition (see section 7.3), where objects of a class
contain objects of another class as their data. But the relation described
here is of a different kind: a class can be defined in terms of an older,
pre-existing, class. This produces a new class having all the functionality of
the older class, and additionally defining its own specific functionality.
Instead of composition, where a given class contains another class, we
here refer to derivation, where a given class is or
is-implemented-in-terms-of another class.

Another term for derivation is inheritance: the new class inherits the
functionality of an existing class, while the existing class does not appear
as a data member in the interface of the new class. When discussing
inheritance the existing class is called the base class, while the new
class is called the derived class.

Derivation of classes is often used when the methodology of C++ program
development is fully exploited. In this chapter we first address the
syntactic possibilities offered by C++ for deriving classes. Following
this we address some of the specific possibilities offered by
class derivation (inheritance).

As we have seen in the introductory chapter (see section 2.4), in the
object-oriented approach to problem solving classes are identified during the
problem analysis. Under this approach objects of the defined classes
represent entities that can be observed in the problem at hand. The classes
are placed in a hierarchy, with the top-level class containing limited
functionality.  Each new derivation (and hence descent in the
 class hierarchy) adds new functionality compared to yet existing classes.

In this chapter we shall use a simple vehicle classification system to build a
hierarchy of classes. The first class is Vehicle, which implements as its
functionality the possibility to set or retrieve the mass of a vehicle. The
next level in the object hierarchy are land-, water- and air vehicles.

The initial object hierarchy is illustrated in Figure 15.
    ------------------------------------------------------------------Insert Figure 15(Initial object hierarchy of vehicles.)about here (file: inheritance/hierarchy)------------------------------------------------------------------

This chapter mainly focuses on the technicalities of class derivation. The
distinction between inheritance used to create derived classes whose objects
should be considered objects of the base class and inheritance used to
implement derived classes in-terms-of their base classes is postponed
until the next chapter (14).

Inheritance (and polymorphism, cf. chapter 14) can be used with
classes and structs. It is not defined for unions.


13.1: Related types

    The relationship between the proposed classes representing different kinds of
vehicles is further investigated here. The figure shows the object hierarchy:
a Car is a special case of a Land vehicle, which in turn is a
special case of a Vehicle.

The class Vehicle represents the `greatest common divisor' in the
classification system. Vehicle is given limited functionality: it can
store and retrieve a vehicle's mass:
            class Vehicle
    {
        size_t d_mass;

        public:
            Vehicle();
            Vehicle(size_t mass);

            size_t mass() const;
            void setMass(size_t mass);
    };

Using this class, the vehicle's mass can be defined as soon as the
corresponding object has been created. At a later stage the mass can be
changed or retrieved.

To represent vehicles traveling over land, a new class Land can be
defined offering Vehicle's functionality and adding its own specific
functionality. Assume we are interested in the speed of land vehicles and
in their mass. The relationship between Vehicles and Lands could of
course be represented by composition but that would be awkward: composition
suggests that a Land vehicle is-implemented-in-terms-of, i.e.,
contains, a Vehicle, while the natural relationship clearly is that
the Land vehicle is a kind of Vehicle.

A relationship in terms of composition would also somewhat complicate our
Land class's design.  Consider the following example showing a class
Land using composition (only the setMass functionality is shown):
            class Land
    {
        Vehicle d_v;        // composed Vehicle
        public:
            void setMass(size_t mass);
    };

    void Land::setMass(size_t mass)
    {
        d_v.setMass(mass);
    }

Using composition, the Land::setMass function only passes its
argument on to Vehicle::setMass. Thus, as far as mass handling is
concerned, Land::setMass introduces no extra functionality, just extra
code. Clearly this code duplication is superfluous: a Land object is a
Vehicle; to state that a Land object contains a Vehicle is at
least somewhat peculiar.

The intended relationship is represented better by inheritance.
        A rule of thumb for choosing between inheritance and composition
distinguishes between is-a and has-a relationships. A truck is a
vehicle, so Truck should probably derive from Vehicle.  On the other
hand, a truck has an engine; if you need to model engines in your system,
you should probably express this by composing an Engine class inside the
Truck class.

Following the above rule of thumb, Land is derived from the base class
Vehicle:
            class Land: public Vehicle
    {
        size_t d_speed;
        public:
            Land();
            Land(size_t mass, size_t speed);

            void setSpeed(size_t speed);
            size_t speed() const;
    };

To derive a class (e.g., Land) from another class (e.g., Vehicle)
postfix the class name Land in its interface by : public Vehicle:
            class Land: public Vehicle

The class Land now contains all the functionality of its base class
Vehicle as well as its own features. Here those features are a constructor
expecting two arguments and member functions to access the d_speed data
member. Here is an example showing the possibilities of the derived class
Land:
            Land veh{ 1200, 145 };

    int main()
    {
        cout << "Vehicle weighs " << veh.mass() << ";\n"
                "its speed is " << veh.speed() << '\n';
    }

This example illustrates two features of derivation.
    
    o  First, mass is not mentioned as a member in Land's
interface. Nevertheless it is used in veh.mass.  This member function is
an implicit part of the class, inherited from its `parent' vehicle.
    o  Second, although the derived class Land contains the
functionality of Vehicle, the Vehicle's private members remain
private: they can only be accessed by Vehicle's own member functions. This
means that Land's member functions must use Vehicle's member
functions (like mass and setMass) to address the mass
field. Here there's no difference between the access rights granted to
Land and the access rights granted to other code outside of the class
Vehicle. The class Vehicle  encapsulates the
specific Vehicle characteristics, and data hiding is one way to realize
encapsulation.

Encapsulation is a core principle of good class design. Encapsulation
reduces the dependencies among classes improving the maintainability and
testability of classes and allowing us to modify classes without the need to
modify depending code. By strictly complying with the principle of data hiding
a class's internal data organization may change without requiring depending
code to be changed as well. E.g., a class Lines originally storing
C-strings could at some point have its data organization changed. It could
abandon its char ** storage in favor of a vector<string> based
storage. When Lines uses perfect data hiding depending source code
may use the new Lines class without requiring any modification at all.

As a rule of thumb, derived classes must be fully recompiled (but don't
have to be modified) when the data organization (i.e., the data members)
of their base classes change.  Adding new member functions to the base
class doesn't alter the data organization so no recompilation is needed
when new member functions are added.

There is one subtle exception to this rule of thumb: if a new member function
is added to a base class and that function happens to be declared as the first
virtual member function of the base class (cf. chapter 14
for a discussion of the virtual member function concept) then that
also changes the data organization of the base class.

Now that Land has been derived from Vehicle we're ready for our next
class derivation. We'll define a class Car to represent
automobiles. Agreeing that a Car object is a Land vehicle, and that
a Car has a brand name it's easy to design the class Car:
            class Car: public Land
    {
        std::string d_brandName;

        public:
            Car();
            Car(size_t mass, size_t speed, std::string const &name);

            std::string const &brandName() const;
    };

In the above class definition, Car was derived from Land, which in
turn is derived from Vehicle. This is called nested derivation:
Land is called Car's direct base class, while Vehicle is
called Car's  indirect base class.


13.1.1: Inheritance depth: desirable?

        Now that Car has been derived from Land and Land has been derived
from Vehicle we might easily be seduced into thinking that these class
hierarchies are the way to go when designing classes. But maybe we should
temper our enthusiasm.

Repeatedly deriving classes from classes quickly results in big, complex class
hierarchies that are hard to understand, hard to use and hard to
maintain. Hard to understand and use as users of our derived class now also
have to learn all its (indirect) base class features as well. Hard to maintain
because all those classes are very closely coupled. While it may be true that
when data hiding is meticulously adhered to derived classes do not have to be
modified when their base classes alter their data organization, it also
quickly becomes practically infeasible to change those base classes once more
and more (derived) classes depend on their current organization.

What initially looks like a big gain, inheriting the base class's interface,
thus becomes a liability. The base class's interface is hardly ever completely
required and in the end a class may benefit from explicitly defining its own
member functions rather than obtaining them through inheritance.

Often classes can be defined in-terms-of existing classes: some of their
features are used, but others need to be shielded off. Consider the stack
container: it is commonly implemented in-terms-of a deque, returning
deque::back's value as stack::top's value.

When using inheritance to implement an is-a relationship make sure to get
the `direction of use' right: inheritance aiming at implementing an is-a
relationship should focus on the base class: the base class facilities aren't
there to be used by the derived class, but the derived class facilities should
redefine (reimplement) the base class facilities using polymorphism (which is
the topic of the next chapter), allowing
code to use the derived class facilities polymorphically through the base
class.  We've seen this approach when studying streams: the base class (e.g.,
ostream) is used time and again. The facilities defined by classes derived
from ostream (like ofstream and ostringstream) are then used by
code only relying on the facilities offered by the ostream class, never
using the derived classes directly.

When designing classes always aim at the lowest possible coupling. Big class
hierarchies usually indicate poor understanding of robust class design. When a
class's interface is only partially used and if the derived class is
implemented in terms of another class consider using composition rather than
inheritance and define the appropriate interface members in terms of the
members offered by the composed objects.


13.2: Access rights: public, private, protected

    Early in the C++ Annotations (cf. section 3.2.1) we encountered two
important design principles when developing classes: data hiding and
encapsulation.  Data hiding restricts control over an object's data to the
members of its class, encapsulation is used to restrict access to the
functionality of objects. Both principles are invaluable tools for maintaining
data integrity.

The keyword private starts sections in class interfaces in which members
are declared which can only be accessed by members of the class itself. This
is our main tool for realizing data hiding. According to established good
practices of class design the public sections are populated with member
functions offering a clean interface to the class's functionality. These
members allow users to communicate with objects; leaving it to the objects how
requests sent to objects are handled. In a well-designed class its objects are
in full control of their data.

Inheritance doesn't change these principles, nor does it change the way the
`private' and `protected' keywords operate. A derived class does not
have access to a base class's private section.

Sometimes this is a bit too restrictive. Consider a class implementing a
random number generating streambuf (cf. chapter 6). Such a
streambuf can be used to construct an istream irand, after which
extractions from irand produces series of random numbers, like in the next
example in which 10 random numbers are generated using stream I/O:
                RandBuf buffer;
    istream irand(&buffer);

    for (size_t idx = 0; idx != 10; ++idx)
    {
        size_t next;
        irand >> next;
        cout << "next random number: " << next << '\n';
    }

The question is, how many random numbers should irand be able to
generate? Fortunately, there's no need to answer this question, as
RandBuf can be made responsible for generating the next random
number. RandBuf, therefore, operates as follows:
        
        o  It generates a random number;
        o  It is passed in textual form to its base class streambuf;
        o  The istream object extracts this random number, merely
            using streambuf's interface;

(this process is repeated for subsequent random numbers).

Once RandBuf has stored the text representation of the next
random number in some buffer, it must tell its base class (streambuf)
where to find the random number's characters. For this streambuf offers a
member setg, expecting the location and size of the buffer holding the
random number's characters.

The member setg clearly cannot be declared in streambuf's
private section, as RandBuf must use it to prepare for the
extraction of the next random number. But it should also not be in
streambuf's public section, as that could easily result in unexpected
behavior by irand. Consider the following hypothetical example:
            RandBuf randBuf;
    istream irand(&randBuf);

    char buffer[] = "12";
    randBuf.setg(buffer, ...);  // setg public: buffer now contains 12

    size_t next;
    irand >> next;              // not a *random* value, but 12.

Clearly there is a close connection between streambuf and its derived
class RandBuf. By allowing RandBuf to specify the buffer from
which streambuf reads characters RandBuf remains in control,
denying other parts of the program to break its well-defined behavior.

This close connection between base- and derived-classes is realized by a third
keyword related to the accessibility of class members: protected. Here is
how the member setg could have been be declared in a class streambuf:
            class streambuf
    {
        // private data here (as usual)
        protected:
            void setg(... parameters ...);  // available to derived classes

        public:
            // public members here
    };

Protected members are members that can be accessed by derived classes, but are
not part of a class's public interface.

Avoid the temptation to declare data members in a class's protected
section: it's a sure sign of bad class design as it needlessly results in
tight coupling of base and derived classes. The principle of data hiding
should not be abandoned now that the keyword protected has been
introduced. If a derived class (but not other parts of the software) should
be given access to its base class's data, use member functions:
accessors and modifiers declared in the base class's protected
section. This enforces the intended restricted access without resulting in
tightly coupled classes.


13.2.1: Public, protected and private derivation

        With inheritance public derivation is frequently used. When public derivation
is used the access rights of the base class's interface remains unaltered in
the derived class. But the type of inheritance may also be defined as
private or protected.

Protected derivation is used when the keyword protected is put in front of
the derived class's base class:
            class Derived: protected Base

When protected derivation is used all the base class's public and
protected members become protected members in the derived class. The derived
class may access all the base class's public and protected members. Classes
that are in turn derived from the derived class view the base class's members
as protected. Any other code (outside of the inheritance tree) is unable to
access the base class's members.

Private derivation is used when the keyword private is put in front of the
derived class's base class:
            class Derived: private Base

When private derivation is used all the base class's members turn into
private members in the derived class. The derived class members may access
all base class public and protected members but base class members cannot be
used elsewhere.

Public derivation should be used to define an is-a relationship
between a derived class and a base class: the derived class object is-a
base class object allowing the derived class object to be used polymorphically
as a base class object in code expecting a base class object. Private
inheritance is used in situations where a derived class object is defined
in-terms-of the base class where composition cannot be used. There's little
documented use for protected inheritance, but one could maybe encounter
protected inheritance when defining a base class that is itself a derived
class making its base class members available to classes derived from it.

Combinations of inheritance types do occur. For example, when designing a
stream-class it is usually derived from std::istream or
std::ostream. However, before a stream can be constructed, a
std::streambuf must be available. Taking advantage of the fact that the
inheritance order is defined in the class interface, we use multiple
inheritance (see section 13.6) to derive the class from both
std::streambuf and (then) from std::ostream. To the class's users it
is a std::ostream and not a std::streambuf. So private derivation is
used for the latter, and public derivation for the former class:
            class Derived: private std::streambuf, public std::ostream


13.2.2: Promoting access rights

        When private or protected derivation is used, users of derived class objects
are denied access to the base class members. Private derivation denies access
to all base class members to users of the derived class, protected derivation
does the same, but allows classes that are in turn derived from the derived
class to access the base class's public and protected members.

In some situations this scheme is too
    
    
        restrictive. Consider a class RandStream derived privately from a
class RandBuf which is itself derived from std::streambuf and also
publicly from istream:
            class RandBuf: public std::streambuf
    {
        // implements a buffer for random numbers
    };
    class RandStream: private RandBuf, public std::istream
    {
        // implements a stream to extract random values from
    };

Such a class could be used to extract, e.g., random numbers using the
standard istream interface.

Although the RandStream class is constructed with the
functionality of istream objects in mind, some of the members of the class
std::streambuf may be considered useful by themselves. E.g., the function
streambuf::in_avail returns a lower bound to the number of characters
that can be read immediately. The standard way to make this function available
is to define a shadow member calling the base class's member:
            class RandStream: private RandBuf, public std::istream
    {
        // implements a stream to extract random values from
        public:
            std::streamsize in_avail();
    };
    inline std::streamsize RandStream::in_avail()
    {
        return std::streambuf::in_avail();
    }

This looks like a lot of work for just making available a member from the
protected or private base classes. If the intent is to make available the
     in_avail member
access promotion can be used. Access promotion allows us to specify which
members of private (or protected) base classes become available in the
protected (or public) interface of the derived class. Here is the above
example, now using access promotion:
            class RandStream: private RandBuf, public std::istream
    {
        // implements a stream to extract random values from
        public:
            using std::streambuf::in_avail;
    };

It should be noted that access promotion makes available all overloaded
versions of the declared base class member. So, if streambuf would offer
not only in_avail but also, e.g., in_avail(size_t *) both
members would become part of the public interface.


13.3: The constructor of a derived class

    A derived class inherits functionality from its base class (or base classes,
as C++ supports multiple inheritance, cf. section 13.6). When a
derived class object is constructed it is built on top of its base class
object. As a consequence the base class must have been constructed before the
actual derived class elements can be initialized. This results in some
requirements that must be observed when defining
  derived class constructors.

A constructor exists to initialize the object's data members. A derived class
constructor is also responsible for the proper initialization of its base
class. Looking at the definition of the class Land introduced earlier
(section 13.1), its constructor could simply be defined as
follows:
            Land::Land(size_t mass, size_t speed)
    {
        setMass(mass);
        setSpeed(speed);
    }

However, this implementation has several disadvantages.
    
    o  When constructing a derived class object a base class constructor is
always called before any action is performed on the derived class object
itself. By default the base class's default constructor is going to be
called.
    o  Using the base class constructor only to reassign new values to its
data members in the derived class constructor's body usually is inefficient,
but sometimes sheer impossible as in situations where base class reference or
const data members must be initialized. In those cases a specialized base
class constructor must be used instead of the base class default constructor.

A derived class's base class may be initialized using a dedicated base
class constructor by calling the base class constructor in the derived class
constructor's initializer clause. Calling a base class constructor in a
constructor's initializer clause is called a
 base class initializer. The base class initializer must be called before
initializing any of the derived class's data members and when using the base
class initializer none of the derived class data members may be used. When
constructing a derived class object the base class is constructed first and
only after that construction has successfully completed
the derived class data members are available for initialization. Land's
constructor may therefore be improved:
            Land::Land(size_t mass, size_t speed)
    :
        Vehicle(mass),
        d_speed(speed)
    {}

Derived class constructors always by default call their base class's
default constructor. This is of course not correct for a derived class's
 copy constructor. Assuming that the class Land must be provided with a
copy constructor its Land const &other parameter also represents the other
object's base class:
            Land::Land(Land const &other)   // assume a copy constructor is needed
    :
        Vehicle(other),             // copy-construct the base class part.
        d_speed(other.d_speed)      // copy-construct Land's data members
    {}


13.3.1: Move construction

        As with classes using composition derived classes may benefit from defining a
 move constructor.
A derived class may offer a move constructor for two reasons:
    
    o  it supports move construction for its  data members
    o  its base class is move-aware

The design of move constructors moving data members was covered in section
9.7. A move constructor for a derived class whose base class is
move-aware must anonymize the rvalue reference before passing it to the
base class move constructor. The std::move function should be used when
implementing the move constructor to move the information in base classes or
composed objects to their new destination object.

The first example shows the move constructor for the class Car,
assuming it has a movable char *d_brandName data member and
assuming that Land is a move-aware class. The second example shows the
move constructor for the class Land, assuming that it does not itself have
movable data members, but that its Vehicle base class is move-aware:
            Car::Car(Car &&tmp)
    :
        Land(std::move(tmp)),           // anonymize `tmp'
        d_brandName(tmp.d_brandName)    // move the char *'s value
    {
        tmp.d_brandName = 0;
    }

    Land(Land &&tmp)
    :
        Vehicle(std::move(tmp)),    // move-aware Vehicle
        d_speed(tmp.d_speed)        // plain copying of plain data
    {}


13.3.2: Move assignment

        Derived classes may also benefit from move assignment operations. If the
derived class and its base class support swapping then the implementation is
simple, following the standard shown earlier in section 9.7.3. For the
class Car this could boil down to:
            Car &Car::operator=(Car &&tmp)
    {
        swap(tmp);
        return *this;
    }

If swapping is not supported then std::move can be used to call the
base class's move assignment operator:
            Car &Car::operator=(Car &&tmp)
    {
        static_cast<Land &>(*this) = std::move(tmp);
        // move Car's own data members next
        return *this;
    }


13.3.3: Inheriting constructors

        Derived classes can be constructed without
        
    explicitly defining derived class constructors. In those cases the
available base class constructors are called.

This feature is either used or not. It is not possible to omit some of the
derived class constructors, using the corresponding base class constructors
instead. To use this feature for classes that are derived from multiple base
classes (cf. section 13.6) all the base class constructors must
have different signatures. Considering the complexities that are involved here
it's probably best to avoid using base class constructors for classes using
multiple inheritance.

The construction of derived class objects can be delegated to
  base class constructor(s) using
the following syntax:
            class BaseClass
    {
        public:
            // BaseClass constructor(s)
    };

    class DerivedClass: public BaseClass
    {
        public:
            using BaseClass::BaseClass; // No DerivedClass constructors
                                        // are defined
    };


13.3.4: Aggregate Initializations

            Aggregates (e.g., structs) can be initialized
using the familiar curly brace notation. Curly brace notations can also be
used when initializing base-structs of derived-structs. Each base-level struct
receives its own set of curly braces while initializing the derived-level
struct. Here is an example:
            struct Base
    {
        int value;
    };
    struct Derived: public Base
    {
        string text;
    };

    // Initializiation of a Derived object:
    
    Derived der{{value}, "hello world"};
    //          -------
    //          initialization of Derived's base struct.


13.4: The destructor of a derived class

    Destructors
  of classes are automatically called when an
object is destroyed. This also holds true for objects of classes derived from
other classes. Assume we have the following situation:
            class Base
    {
        public:
            ~Base();
    };

    class Derived: public Base
    {
        public:
            ~Derived();
    };

    int main()
    {
        Derived derived;
    }

At the end of main, the derived object ceases to exists. Hence,
its destructor (~Derived) is called. However, since derived is also a
Base object, the ~Base destructor is called as well.  The base class
destructor is never explicitly called from the derived class destructor.

Constructors
   and destructors
are called in a stack-like fashion: when derived is constructed, the
appropriate base class constructor is called first, then the appropriate
derived class constructor is called. When the object derived is destroyed,
its destructor is called first, automatically followed by the activation of
the Base class destructor. A derived class destructor is always called
before its base class destructor is called.

When the construction of a derived class object did not successfully complete
(i.e., the constructor threw an exception) then its destructor is not
called. However, the destructors of properly constructed base classes will
be called if a derived class constructor throws an exception. This, of course,
is how it should be: a properly constructed object should also be destroyed,
eventually. Example:
            #include <iostream>
    struct Base
    {
        ~Base()
        {
            std::cout << "Base destructor\n";
        }
    };
    struct Derived: public Base
    {
        Derived()
        {
            throw 1;    // at this time Base has been constructed
        }
    };
    int main()
    {
        try
        {
            Derived d;
        }
        catch (...)
        {}
    }
    /*
        This program displays `Base destructor'
    */


13.5: Redefining member functions

    Derived classes may redefine base class members.  Let's assume that a vehicle
classification system must also cover trucks, consisting of two parts: the
front part, the tractor, pulls the rear part, the trailer. Both the
tractor and the trailer have their own mass, and the mass function
should return the combined mass.

The definition of a Truck starts with a class definition. Our initial
Truck class is derived from Car but it is then expanded to hold one
more size_t field representing the additional mass information. Here we
choose to represent the mass of the tractor in the Car class and to store
the mass of a full truck (tractor + trailer) in its own d_mass data
member: 
            class Truck: public Car
    {
        size_t d_mass;

        public:
            Truck();
            Truck(size_t tractor_mass, size_t speed, char const *name,
                  size_t trailer_mass);

            void setMass(size_t tractor_mass, size_t trailer_mass);
            size_t mass() const;
    };

    Truck::Truck(size_t tractor_mass, size_t speed, char const *name,
                 size_t trailer_mass)
    :
        Car(tractor_mass, speed, name),
        d_mass(tractor_mass + trailer_mass)
    {}

Note that the class Truck now contains two functions already
present in the base class Car: setMass and mass.
    
    o  The redefinition of setMass poses no problems: this
function is simply redefined to perform actions which are specific to a
Truck object.
    o  Redefining setMass, however,
      hides
Car::setMass. For a Truck only the setMass function having
two size_t arguments can be used.
    o  The Vehicle's setMass function remains available for a
Truck, but it must now be
  called explicitly, as
Car::setMass is hidden from view.  This latter function is hidden,
even though Car::setMass has only one size_t argument. To implement
Truck::setMass we could write:
     void Truck::setMass(size_t tractor_mass, size_t trailer_mass)
{
    d_mass = tractor_mass + trailer_mass;
    Car::setMass(tractor_mass);     // note: Car:: is required
}

o  Outside of the class Car::setMass is
accessed using the scope resolution operator. So, if a Truck truck needs
to set its Car mass, it must use
    truck.Car::setMass(x);

o  An alternative to using the scope resolution operator is to add a
member having the same function prototype as the base class member to the
derived class's interface. This derived class member could be implemented
inline to call the base class member. E.g., we add the following member to the
class Truck:
    // in the interface:
void setMass(size_t tractor_mass);

// below the interface:
inline void Truck::setMass(size_t tractor_mass)
{
    (d_mass -= Car::mass()) += tractor_mass;
    Car::setMass(tractor_mass);
}

Now the single argument setMass member function can be used by
Truck objects without using the scope resolution operator. As the
function is defined inline, no overhead of an additional function call is
involved.

o   To
prevent hiding the base class members a using declaration may be added to
the derived class interface. The relevant section of Truck's class
interface then becomes:
    class Truck: public Car
{
    public:
        using Car::setMass;
        void setMass(size_t tractor_mass, size_t trailer_mass);
};

A using declaration imports (all overloaded versions of) the mentioned
member function directly into the derived class's interface. If a base class
member has a signature that is identical to a derived class member then
compilation fails (a using Car::mass declaration cannot be added to
Truck's interface). Now code may use truck.setMass(5000) as well as
truck.setMass(5000, 2000).

Using declarations obey access rights. To prevent non-class members from
using setMass(5000) without a scope resolution operator but allowing
derived class members to do so the using Car::setMass declaration
should be put in the class Truck's private section.

o  The function mass is also already defined in Car, as
it was inherited from Vehicle. In this case, the class Truck 
redefines this member function to return the truck's full mass:
     size_t Truck::mass() const
{
    return d_mass;
}

Example:
            int main()
    {
        Land vehicle{ 1200, 145 };
        Truck lorry{ 3000, 120, "Juggernaut", 2500 };
    
        lorry.Vehicle::setMass(4000);
    
        cout << '\n' << "Tractor weighs " <<
                        lorry.Vehicle::mass() << '\n' <<
            "Truck + trailer weighs " << lorry.mass() << '\n' <<
            "Speed is " << lorry.speed() << '\n' <<
            "Name is " << lorry.name() << '\n';
    }

The class Truck was derived from Car. However, one might question
this class design. Since a truck is conceived of as a combination of a
tractor and a trailer it is probably better defined using a mixed design,
using inheritance for the tractor part (inheriting from Car, and
composition for the trailer part).

This redesign changes our point of view from a Truck being a Car
(and some strangely added data members) to a Truck still being an
Car (the tractor) and containing a Vehicle (the trailer).

Truck's interface is now very specific, not requiring users to study
Car's and Vehicle's interfaces and it opens up possibilities for
defining `road trains': tractors towing multiple trailers. Here is an example
of such an alternate class setup:
            class Truck: public Car    // the tractor
    {
        Vehicle d_trailer;      // use vector<Vehicle> for road trains

        public:
            Truck();
            Truck(size_t tractor_mass, size_t speed, char const *name,
                  size_t trailer_mass);

            void setMass(size_t tractor_mass, size_t trailer_mass);
            void setTractorMass(size_t tractor_mass);
            void setTrailerMass(size_t trailer_mass);

            size_t tractorMass() const;
            size_t trailerMass() const;
        // consider:
            Vehicle const &trailer() const;
    };


13.6: Multiple inheritance

    Except for the class Randbuf classes thus far have always been derived
from a single base class. In addition to single inheritance 
     
    C++ also supports multiple inheritance. In multiple inheritance a
class is derived from several base classes and hence inherits functionality
from multiple parent classes at the same time.

When using multiple inheritance it should be defensible to consider the
newly derived class an instantiation of both base classes. Otherwise,
composition is more appropriate. In general, linear derivation (using only
one base class) is used much more frequently than multiple derivation. Good
class design dictates that a class should have a single, well described
responsibility and that principle often conflicts with multiple inheritance
where we can state that objects of class Derived are both Base1
and Base2 objects.

But then, consider the prototype of an object for which
multiple inheritance was used to its extreme: the
    Swiss army knife! This object is a knife, it is a pair of
scissors, it is a can-opener, it is a corkscrew, it is ....

The `Swiss army knife' is an extreme example of multiple inheritance. In
C++ there are various good arguments for using multiple inheritance as
well, without violating the `one class, one responsibility' principle. We
postpone those arguments until the next chapter. The
current section concentrates on the technical details of constructing classes
using multiple inheritance.

How to construct a `Swiss army knife' in C++? First we need (at least)
two base classes. For example, let's assume we are designing a toolkit
allowing us to construct an instrument panel of an aircraft's cockpit. We
design all kinds of instruments, like an artificial horizon and an
altimeter. One of the components that is often seen in aircraft is a
nav-com set: a combination of a navigational beacon receiver (the `nav'
part) and a radio communication unit (the `com'-part). To define the nav-com
set, we start by designing the NavSet class (assume the existence of the
classes Intercom, VHF_Dial and Message):
            class NavSet
    {
        public:
            NavSet(Intercom &intercom, VHF_Dial &dial);

            size_t activeFrequency() const;
            size_t standByFrequency() const;

            void setStandByFrequency(size_t freq);
            size_t toggleActiveStandby();
            void setVolume(size_t level);
            void identEmphasis(bool on_off);
    };

Next we design the class  ComSet:
            class ComSet
    {
        public:
            ComSet(Intercom &intercom);

            size_t frequency() const;
            size_t passiveFrequency() const;

            void setPassiveFrequency(size_t freq);
            size_t toggleFrequencies();

            void setAudioLevel(size_t level);
            void powerOn(bool on_off);
            void testState(bool on_off);
            void transmit(Message &message);
    };

Using objects of this class we can receive messages, transmitted
though the Intercom, but we can also transmit messages using a
Message object that's passed to the ComSet object using its
transmit member function.

Now we're ready to construct our NavCom set:
            class NavComSet: public ComSet, public NavSet
    {
        public:
            NavComSet(Intercom &intercom, VHF_Dial &dial);
    };

Done. Now we have defined a NavComSet which is both a NavSet
and a ComSet: the facilities of both base classes  are now
available in the derived class using multiple inheritance.

Please note the following:
    
    o  The keyword public is present before both base class names
(NavSet and ComSet). By default inheritance uses
 private derivation and the keyword public must be repeated before
each of the base class specifications. Base classes are not required to use
the same derivation type. One base class could have public derivation and
another base class could use private derivation.
    o  The multiply derived class NavComSet introduces no additional
functionality of its own, but merely combines two existing classes into a new
aggregate class. Thus, C++ offers the possibility to simply sweep
multiple simple classes into one more complex class.
    o  Here is the implementation of The NavComSet constructor:
    NavComSet::NavComSet(Intercom &intercom, VHF_Dial &dial)
:
    ComSet(intercom),
    NavSet(intercom, dial)
{}

The constructor requires no extra code: Its purpose is to activate
the constructors of its base classes. The order in which the base class
initializers 
     are called is not
dictated by their calling order in the constructor's code, but by the ordering
of the base classes in the class interface.
    o  The NavComSet class definition requires no additional data
members or member functions: here (and often) the inherited interfaces provide
all the required functionality and data for the multiply derived class to
operate properly.

Of course, while defining the base classes, we made life easy on ourselves
by strictly using different member function names. So, there is a function
setVolume in the NavSet class and a function setAudioLevel in the
ComSet class. A bit cheating, since we could expect that both units in
fact have a composed object Amplifier, handling the volume setting. A
revised class might offer an Amplifier &amplifier() const member function,
and leave it to the application to set up its own interface to the
amplifier. Alternatively, a revised class could define members for setting the
volume of either the NavSet or the ComSet parts.

In situations where two base classes offer identically named members
 
special provisions need to be made to prevent ambiguity:
    
    o  The intended base class can explicitly be specified using the base
class name and scope resolution operator:
    NavComSet navcom(intercom, dial);

navcom.NavSet::setVolume(5);    // sets the NavSet volume level
navcom.ComSet::setVolume(5);    // sets the ComSet volume level

o  The class interface is provided with member functions that can be
called unambiguously. These additional members are usually defined inline:
    class NavComSet: public ComSet, public NavSet
{
    public:
        NavComSet(Intercom &intercom, VHF_Dial &dial);
        void comVolume(size_t volume);
        void navVolume(size_t volume);
};
inline void NavComSet::comVolume(size_t volume)
{
    ComSet::setVolume(volume);
}
inline void NavComSet::navVolume(size_t volume)
{
    NavSet::setVolume(volume);
}

o  If the NavComSet class is obtained from a third party, and cannot
be modified, a disambiguating wrapper class may be used:
    class MyNavComSet: public NavComSet
{
    public:
        MyNavComSet(Intercom &intercom, VHF_Dial &dial);
        void comVolume(size_t volume);
        void navVolume(size_t volume);
};
inline MyNavComSet::MyNavComSet(Intercom &intercom, VHF_Dial &dial)
:
    NavComSet(intercom, dial);
{}
inline void MyNavComSet::comVolume(size_t volume)
{
    ComSet::setVolume(volume);
}
inline void MyNavComSet::navVolume(size_t volume)
{
    NavSet::setVolume(volume);
}


13.7: Conversions between base classes and derived classes

    When public inheritance is used to define classes, an object of a derived
class is at the same time an object of the base class.  This has important
consequences for object assignment and for the situation where pointers or
references to such objects are used. Both situations are now discussed.


13.7.1: Conversions with object assignments

        Continuing our discussion of the NavCom class, introduced in section
13.6, we now define two objects, a base class and a derived class
object:
            ComSet com(intercom);
    NavComSet navcom(intercom2, dial2);

The object navcom is constructed using an Intercom and a
VHF_Dial object. However, a NavComSet is at the same time a
ComSet, allowing the assignment from navcom (a derived class
object) to com (a base class object):
            com = navcom;

The effect of this assignment is that the object com now
communicates with intercom2.  As a ComSet does not have a VHF_Dial,
the navcom's dial is ignored by the assignment. When assigning a
base class object from a derived class object only the base class data members
are assigned, other data members are dropped, a phenomenon called
slicing. In situations like these slicing probably does not have serious
consequences, but when passing derived class objects to functions defining
base class parameters or when returning derived class objects from functions
returning base class objects slicing also occurs and might have unwelcome
side-effects.

The assignment from a base class object to a derived class object is
problematic. In a statement like
            navcom = com;

it isn't clear how to reassign the NavComSet's VHF_Dial data
member as they are missing in the ComSet object com. Such an
assignment  is therefore refused by the
compiler. Although derived class objects are also base class objects, the
reverse does not hold true: a base class object is not also a derived class
object.

The following general rule applies: in assignments in which base class
objects and derived class objects are involved, assignments in which data are
dropped are legal (called slicing). Assignments in which data remain
unspecified are not allowed.  Of course, it is possible to overload an
assignment operator to allow the assignment of a derived class object from a
base class object.  To compile the statement
            navcom = com;

the class NavComSet must have defined an overloaded assignment
operator accepting a ComSet object for its argument. In that case it's up
to the programmer to decide what the assignment operator will do with the
missing data.


13.7.2: Conversions with pointer assignments

            We return to our Vehicle classes, and define the following objects and
pointer variable:
            Land land(1200, 130);
    Car car(500, 75, "Daf");
    Truck truck(2600, 120, "Mercedes", 6000);
    Vehicle *vp;

Now we can assign the  addresses of the three objects of
the derived classes to the Vehicle pointer:
            vp = &land;
    vp = &car;
    vp = &truck;

Each of these assignments is acceptable. However, an
    implicit conversion of the derived class to the base class
Vehicle is used, since vp is defined as a pointer to a
Vehicle. Hence, when using vp only the member functions manipulating
mass can be called as this is the Vehicle's only functionality.
As far as the compiler can tell this is the object vp points to.

The same holds true for  references to
Vehicles. If, e.g., a function is defined having a Vehicle reference
parameter, the function may be passed an object of a class derived from
Vehicle. Inside the function, the specific Vehicle members remain
accessible. This analogy between pointers and references holds true in
general. Remember that a reference is nothing but a pointer in disguise: it
mimics a plain variable, but actually it is a pointer.

This restricted functionality has an important consequence
for the class Truck. Following  vp = &truck, vp points to
a Truck object. So, vp->mass() returns 2600 instead of
8600 (the combined mass of the cabin and of the trailer: 2600 + 6000),
which would have been returned by truck.mass().


    When a function is called using a pointer to an object, then the
    type of the pointer (and not the type of the object) determines
which
      member
functions are available and can be executed.  In other words, C++
implicitly converts the type of an object reached through a pointer to the
pointer's type.

If the actual type of the object pointed to by a pointer is known, an
explicit type cast can be used to access the full set of member functions
that are available for the object: 
            Truck truck;
    Vehicle *vp;

    vp = &truck;        // vp now points to a truck object

    Truck *trp;

    trp = static_cast<Truck *>(vp);
    cout << "Make: " << trp->name() << '\n';

Here, the second to last statement specifically casts a Vehicle *
variable to a Truck *. As usual (when using casts), this code
is not without risk. It only works  if vp really points to a
Truck. Otherwise the program may produce unexpected results.


13.8: Using non-default constructors with new[]

    An often heard complaint is that operator new[] calls the default
constructor
  of a class to initialize the
allocated objects. For example, to allocate an array of 10 strings we can do
            new string[10];

but it is not possible to use another constructor. Assuming that we'd want
to initialize the strings with the text hello world, we can't write
something like:
            new string{ "hello world" }[10];

The initialization of a dynamically allocated object usually consists of a
two-step process: first the array is allocated (implicitly calling the default
constructor); second the array's elements are initialized, as in the following
little example:
            string *sp = new string[10];
    fill(sp, sp + 10, string{ "hello world" });

These approaches all suffer from `double initializations', comparable
to not using member initializers in constructors.

One way to avoid double initialization is to use inheritance.
Inheritance can profitably be used to call non-default constructors in
combination with operator new[]. The approach capitalizes on the
following:
    
    o  A base class pointer may point to a derived class object;
    o  A derived class without (non-static)
            
            
        data members has the same size as its base class.

The above also suggests a possible approach:
    
    o  Derive a simple, member-less class from the class we're interested
in;
    o  Use the appropriate base class initializer in its default
constructor;
    o  Allocate the required number of derived class objects, and assign
new[]'s return expression to a pointer to base class objects.

Here is a simple example, producing 10 lines containing
the text hello world:
        #include <iostream>
#include <string>
#include <algorithm>
#include <iterator>

using namespace std;

struct Xstr: public string
{
    Xstr()
    :
        string("hello world")
    {}
};

int main()
{
    string *sp = new Xstr[10];
    copy(sp, sp + 10, ostream_iterator<string>{ cout, "\n" });
}

    Of course, the above example is fairly unsophisticated, but it's easy to
polish the example: the class Xstr can be defined in
an anonymous namespace, accessible only to a function getString() which
may be given a size_t nObjects parameter, allowing users to specify the
number of hello world-initialized strings they would like to allocate.

Instead of hard-coding the base class arguments it's also possible to use
variables or functions providing the appropriate values for the base class
constructor's arguments. In the next example a
    local class Xstr is defined inside a function
nStrings(size_t nObjects, char const *fname), expecting the number of
string objects to allocate and the name of a file whose subsequent lines
are used to initialize the objects. The local class
is invisible outside of the function nStrings, so no special namespace
safeguards are required.

As discussed in section 7.9, members of local classes cannot access
local variables from their surrounding function. However, they can access
global and static data defined by the surrounding function.

Using a local class neatly allows us to hide the implementation details
within the function nStrings, which simply opens the file, allocates the
objects, and closes the file again. Since the local class is derived from
string, it can use any string constructor for its base class
initializer. In this particular case it doesn't even do that, as copy elision
ensures that Xstr's base class string in fact is the string
returned by nextLine. That latter function's string subsequently
receives the lines of the just opened stream. As nextLine is a static
member function, it's available to Xstr default constructor's member
initializers even though at that time the Xstr object isn't available yet.
        #include <fstream>
#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>

using namespace std;

string *nStrings(size_t size, char const *fname)
{
    static thread_local ifstream in;

    struct Xstr: public string
    {
        Xstr()
        :
            string(nextLine())
        {}
        static string nextLine()
        {
            string line;

            getline(in, line);
            return line;        // copy elision turns this
        }                       // into Xstr's base class string
    };
    in.open(fname);
    string *sp = new Xstr[size];
    in.close();

    return sp;
}

int main()
{
    string *sp = nStrings(10, "nstrings.cc");
    copy(sp, sp + 10, ostream_iterator<string>{ cout, "\n" });
}

    When this program is run, it displays the first 10 lines of the file
nstrings.cc.

Note that the example defines a static thread_local ifstream
object. Thread_local variables are formally introduced in chapter
20. The thread_local specification assures that the function
can safely be used, even in multithreaded programs.

A completely different way to avoid the double initialization (not using
inheritance) is to use placement new (cf. section 9.1.5): simply
allocate the required amount of memory followed by the proper in-place
allocation of the objects, using the appropriate constructors. In the next
example a pair of static construct/destroy members are used to perform the
required initialization. In the example construct expects an istream
that provides the initialization strings for objects of a class String
simply containing a std::string object. Construct first allocates
enough memory for the n String objects plus room for an initial
size_t value. This initial size_t value is then initialized with
n. Next, in a for statement, lines are read from the provided stream
and the lines are passed to the constructors, using placement new
calls. Finally the address of the first String object is returned. Then,
the destruction of the objects is handled by the member destroy. It
retrieves the number of objects to destroy from the size_t it finds just
before the location of the address of the first object to destroy. The objects
are then destroyed by explicitly calling their destructors. Finally the raw
memory, originally allocated by construct is returned.
        #include <fstream>
#include <iostream>
#include <string>
using namespace std;

class String
{
    union Ptrs
    {
        void *vp;
        String *sp;
        size_t *np;
    };

    std::string d_str;

    public:
        String(std::string const &txt)
        :
            d_str(txt)
        {}
        ~String()
        {
            cout << "destructor: " << d_str << '\n';
        }
        static String *construct(istream &in, size_t n)
        {
            Ptrs p = {operator new(n * sizeof(String) + sizeof(size_t))};
            *p.np++ = n;

            string line;
            for (size_t idx = 0; idx != n; ++idx)
            {
                getline(in, line);
                new(p.sp + idx) String{ line };
            }

            return p.sp;
        }
        static void destroy(String *sp)
        {
            Ptrs p = {sp};
            --p.np;
            for (size_t n = *p.np; n--; )
                sp++->~String();

            operator delete(p.vp);
        }
};

int main()
{
    String *sp = String::construct(cin, 5);

    String::destroy(sp);
}

/*
    After providing 5 lines containing, respectively
        alpha, bravo, charley, delta, echo
    the program displays:
                destructor: alpha
                destructor: bravo
                destructor: charley
                destructor: delta
                destructor: echo
*/




Chapter 14: Polymorphism

Using inheritance classes may be derived from other classes, called base
classes. In the previous chapter we saw that base class pointers may be used
to point to derived class objects. We also saw that when a base class pointer
points to an object of a derived class the pointer's type, rather than the
object's type, determines which member functions are visible. So when a
Vehicle *vp, points to a Car object Car's speed or
brandName members can't be used.

In the previous chapter two fundamental ways classes may be related to each
other were discussed: a class may be implemented-in-terms-of another
class and it can be stated that a derived class is-a base class. The
former relationship is usually implemented using composition, the latter
is usually implemented using a special form of inheritance, called
polymorphism, the topic of this chapter.

An is-a relationship between classes allows us to apply the
 Liskov Substitution Principle (LSP) according to which a derived
class object may be passed to and used by code expecting a pointer or
reference to a base class object. In the C++ Annotations so far the LSP has been
applied many times. Every time an ostringstream, ofstream or fstream
was passed to functions expecting an ostream we've been applying this
principle. In this chapter we'll discover how to design our own classes
accordingly.

LSP is implemented using a technique called polymorphism: although a base
class pointer is used it performs actions defined in the (derived) class
of the object it actually points to. So, a Vehicle *vp might behave like
a Car * when pointing to a Car\ (In one of the StarTrek
movies, Capt.  Kirk was in trouble, as usual. He met an extremely beautiful
lady who, however, later on changed into a hideous troll. Kirk was quite
surprised, but the lady told him: ``Didn't you know I am a polymorph?'').

Polymorphism is implemented using a feature called late binding. It's
called that way because the decision which function to call (a base class
function or a function of a derived class) cannot be made at compile-time,
but is postponed until the program is actually executed: only then it is
determined which member function will actually be called.

In C++ late binding is not the default way functions are called. By
default static binding (or early binding) is used. With static
binding the functions that are called are determined by the compiler, merely
using the class types of objects, object pointers or object references.

Late binding is an inherently different (and slightly slower) process as it is
decided at run-time, rather than at compile-time what function is going
to be called. As C++ supports both late- and early-binding C++
programmers are offered an option as to what kind of binding to use.  Choices
can be optimized to the situations at hand. Many other languages offering
object oriented facilities (e.g., Java) only or by default offer late
binding. C++ programmers should be keenly aware of this. Expecting early
binding and getting late binding may easily produce nasty bugs.

Let's look at a simple example to start appreciating the differences between
late and early binding. The example merely illustrates. Explanations of
why things are as shown are shortly provided.

Consider the  following little program:
            #include <iostream>
    using namespace std;

    class Base
    {
        protected:
            void hello()
            {
                cout << "base hello\n";
            }
        public:
            void process()
            {
                hello();
            }
    };
    class Derived: public Base
    {
        protected:
            void hello()
            {
                cout << "derived hello\n";
            }
    };
    int main()
    {
        Derived derived;

        derived.process();
    }

    The important characteristic of the above program is the Base::process
function, calling hello. As process is the only member that is defined
in the public interface it is the only member that can be called by code not
belonging to the two classes. The class Derived, derived from Base
clearly inherits Base's interface and so process is also available in
Derived. So the Derived object in main is able to call
process, but not hello.

So far, so good. Nothing new, all this was covered in the previous
chapter.  One may wonder why Derived was defined at all.  It was
presumably defined to create an implementation of hello that's appropriate
for Derived but differing from Base::hello's
implementation. Derived's author's reasoning was as follows: Base's
implementation of hello is not appropriate; a Derived class object can
remedy that by providing an appropriate implementation. Furthermore our author
reasoned:
    ``since the type of an
object determines the interface that is used, process must call
Derived::hello as hello is called via process from a Derived
class object''.

Unfortunately our author's reasoning is flawed, due to static binding. When
Base::process was compiled static binding caused the compiler to bind
the hello call to Base::hello().

The author intended to create a Derived class that is-a Base
class. That only partially succeeded: Base's interface was inherited, but
after that Derived has relinquished all control over what happens. Once
we're in process we're only able to see Base's member
implementations. Polymorphism offers a way out, allowing us to redefine (in a
derived class) members of a base class allowing these redefined members to be
used from the base class's interface.

This is the essence of LSP: public inheritance should not be used to reuse the
base class members (in derived classes) but to be reused (by the base class,
polymorphically using derived class members reimplementing base class
members).

Take a second to appreciate the implications of the above little program. The
hello and process members aren't too impressive, but the implications
of the example are. The process member could implement directory travel,
hello could define the action to perform when encountering a
file. Base::hello might simply show the name of a file, but
Derived::hello might delete the file; might only list its name if its
younger than a certain age; might list its name if it contains a certain text;
etc., etc.. Up to now Derived would have to implement process's
actions itself; Up to now code expecting a Base class reference or pointer
could only perform Base's actions. Polymorphism allows us to reimplement
members of base classes and to use those reimplemented members in code
expecting base class references or pointers. Using polymorphism existing code
may be reused by derived classes reimplementing the appropriate members of
their base classes. It's about time to uncover how this magic can be realized.

Polymorphism, which is not the default in C++, solves the problem and
allows the author of the classes to reach its goal. For the curious reader:
prefix void hello() in the Base class with the keyword virtual and
recompile. Running the modified program produces the intended and expected
derived hello. Why this happens is explained next.


14.1: Virtual functions

By default the behavior of a member function called via a pointer or reference
is determined by the implementation of that function in the pointer's or
reference's class. E.g., a Vehicle * activates Vehicle's member
functions, even when pointing to an object of a derived class. This is known
as as early or
      static binding: the function to
call is determined at
 compile-time. In C++ late
  or  dynamic binding is realized using
virtual member functions.

A member function becomes a virtual member function when its declaration
starts with the keyword virtual. It is stressed once again that in
C++, different from several other object oriented languages, this is
not the default situation. By default static binding is used.

Once a function is declared virtual in a base class, it remains virtual in
all derived classes. The keyword virtual should not be mentioned for
members in derived classes which are declared virtual in base classes. In
derived classes those members should be provided with the override
indicator, allowing the compiler to verify that you're indeed referring to an
existing virtual member function.

In the vehicle classification system (see section 13.1), let's
concentrate on the members mass and setMass. These members define the
user interface of the class Vehicle. What we would like to accomplish
is that this user interface can be used for Vehicle and for any class
inheriting from Vehicle, since objects of those classes are themselves
also Vehicles. 

If we can define the user interface of our base class (e.g., Vehicle) such
that it remains usable irrespective of the classes we derive from Vehicle
our software achieves an enormous reusability: we design our software around
Vehicle's user interface, and our software will also properly function for
derived classes. Using plain inheritance doesn't accomplish this. If we define 
            std::ostream &operator<<(std::ostream &out, Vehicle const &vehicle)
    {
        return out << "Vehicle's mass is " << vehicle.mass() << " kg.";
    }

and Vehicle's member mass returns 0, but Car's member
mass returns 1000, then twice a mass of 0 is reported when the following
program is executed:
            int main()
    {
        Vehicle vehicle;
        Car vw{ 1000, 160, "Golf" };

        cout << vehicle << '\n' << vw << '\n';
    }

We've defined an overloaded insertion operator, but since it only knows
about Vehicle's user interface, `cout << vw' will use vw's
Vehicle's user interface as well, thus displaying a mass of 0. 

Reusability is enhanced if we add a redefinable interface to the base
class's interface. A redefinable interface allows derived classes to fill in
their own implementation, without affecting the user interface. At the same
time the user interface will behave according to the derived class's wishes,
and not just to the base class's default implementation.

Members of the reusable interface should be declared in the class's
private sections: conceptually they merely belong to their own classes
(cf. section 14.7). In the base class these members should be
declared virtual. These members can be redefined (overridden) by derived
classes, and should there be provided with override indicators.

We keep our user interface (mass), and add the redefinable member 
vmass to Vehicle's interface:
            class Vehicle
    {
        public:
            size_t mass() const;
            size_t si_mass() const;    // see below

        private:
            virtual size_t vmass() const;
    };

Separating the user interface from the redefinable interface is a sensible
thing to do. It allows us to fine-tune the user interface (only one point of
maintenance), while at the same time allowing us to standardize the expected
behavior of the members of the redefinable interface. E.g., in many countries
the International system of units is used, using the kilogram as the unit for
mass. Some countries use other units (like the lbs: 1 kg being
approx. 2.2046 lbs). By separating the user interface from the redefinable
interface we can use one standard for the redefinable interface, and keep the
flexibility of transforming the information ad-lib in the user
interface. 

Just to maintain a clean separation of user- and redefinable interface we
might consider adding another accessor to Vehicle, providing the
si_mass, simply implemented like this:
            size_t Vehicle::si_mass() const
    {
        return vmass();
    }

If Vehicle supports a member d_massFactor then its mass member can
be implemented like this:
            size_t Vehicle::mass()
    {
        return d_massFactor * si_mass();
    }

Vehicle itself could define vmass so that it returns a token
value. E.g.,
            size_t Vehicle::vmass()
    {
        return 0;
    }

Now let's have a look at the class Car. It is derived from
Vehicle, and it inherits Vehicle's user interface. It also has a data
member size_t d_mass, and it implements its own reusable interface:
            class Car: public Vehicle
    {
        ...
        private:
            size_t vmass() override;
    }

If Car constructors require us to specify the car's mass (stored
in d_mass), then Car simply implements its vmass member like
this:
            size_t Car::vmass() const
    {
        return d_mass;
    }

The class Truck, inheriting from Car needs two mass values: the
tractor's mass and the trailer's mass. The tractor's mass is passed to its
Car base class, the trailor's mass is passed to its Vehicle d_trailor
data member. Truck, too, overrides vmass, this time returning the sum
of its tractor and trailor masses:
            size_t Truck::vmass() const
    {
        return Car::si_mass() + d_trailer.si_mass();
    }

Once a class member has been declared virtual it becomes
a virtual member in all derived classes, whether or not these members are
provided with the override indicator. But override should be used,
as it allows to compiler to catch typos when writing down the derived class
interface. 

A member function may be declared virtual anywhere in a
    class hierarchy, but this probably defeats the underlying polymorphic
class design, as the original base class is no longer capable of completely
covering the redefinable interfaces of derived classes. If, e.g, mass is
declared virtual in Car, but not in Vehicle, then the specific
characteristics of virtual member functions would only be available for
Car objects and for objects of classes derived from Car. For a
Vehicle pointer or reference static binding would remain to be used.  

The effect of late binding (polymorphism) is illustrated below:
            void showInfo(Vehicle &vehicle)
    {
        cout << "Info: " << vehicle << '\n';
    }

    int main()
    {
        Car car(1200);            // car with mass 1200
        Truck truck(6000, 115,      // truck with cabin mass 6000, 
              "Scania", 15000);     // speed 115, make Scania, 
                                    // trailer mass 15000

        showInfo(car);             // see (1) below
        showInfo(truck);            // see (2) below

        Vehicle *vp = &truck;
        cout << vp->speed() << '\n';// see (3) below
    }

Now that mass is defined virtual, late binding is used:
    
    o  at (1), Car's mass is displayed;
    o  at (2) Truck's mass is displayed;
    o  at (3) a syntax error is generated. The member
        speed is not a member of Vehicle, and hence not callable via
        a Vehicle*.

The example illustrates that when a pointer to a class is used only the
members of that class can be called.  A member's virtual characteristic
only influences the type of binding (early vs. late), not the set of member
functions that is visible to the pointer.

Through virtual members derived classes may redefine the behavior performed by
functions called from base class members or from pointers or references to
base class objects. This redefinition of base class members by derived classes
is called overriding members.


14.1.1: Constructors of polymorhic classes

    Although constructors of polymorphic classes may (indirectlly) call virtual
members, that's probably not what you want as constructors of polymorphic
classes don't consider that those members may be overridden by derived
classes. As an opening example: if the class Vehicle would
define these members:
        public:
        void Vehicle::prepare()
        {
            vPrepare();
        }
    private:
        virtual void Vehicle::vPrepare()
        {
            cout << "Preparing the Vehicle\n";
        }
    and Car would override vPrepare:
        virtual void Car::vPrepare()
        {
            cout << "Preparing the Car\n";
        }
    then Preparing the Car would be shown by the following code fragment:
    Car car{1200};
    Vehicle &veh = car;
    veh.prepare();

Maybe a preparation is always required. So why not do it in the base
class's constructor? Thus, the Vehicle's constructor could be defined as:
        Vehicle::Vehicle()
    {
        prepare();
    } 
However, the following code fragment shows Preparing the Vehicle,
and not Preparing the Car:
    Car car{1200};
As base classes' constructors do not recognize overridden virtual members
Vehicle's constructor simply calls its own vPrepare member instead of
Vehicle::vPrepare. 

There is clear logic to base class constructors not recognizing overridden
member functions: polymorphism allows us to tailor the base class's interface
to derived classes. Virtual members exist to realize this tailoring process.
But that's completely different from not being able to call derived classes'
members from base classes' constructors: at that point the derived class
objects haven't yet properly been initialized. When derived class objects are
constructed their base class parts are constructed before the derived class
objects themselves are in a valid state. Therefore, if a base class
constructor would be allowed to call an overridden virtual member then that
member would most likely use data of the derived class, which at that point
haven't properly been initialized yet (often resulting in undefined behavior
like segmentation faults).


14.2: Virtual destructors

When an object ceases to exist the object's destructor is called. Now
consider the following code fragment (cf. section 13.1):
            Vehicle *vp = new Land{ 1000, 120 };

    delete vp;          // object destroyed

Here delete is applied to a base class pointer. As the base class
defines the available interface delete vp calls ~Vehicle and ~Land
remains out of sight. Assuming that Land allocates memory a
 memory leak results. Freeing memory is not the only action destructors can
perform. In general they may perform any action that's necessary when an
object ceases to exist. But here none of the actions defined by ~Land are
performed. Bad news....

In C++ this problem is solved by
  virtual destructors.  A
destructor can be declared virtual. When a base class destructor is
declared virtual then the destructor of the actual class pointed to by a base
class pointer bp is going to be called when delete bp is
executed. Thus, late binding is realized for destructors even though the
destructors of derived classes have unique names. Example:
            class Vehicle
    {
        public:
            virtual ~Vehicle();     // all derived class destructors are
                                    // now virtual as well.
    };

By declaring a virtual destructor, the above delete operation
(delete vp) correctly calls Land's destructor, rather than
Vehicle's destructor.

Once a destructor is called it performs as usual, whether or not it
is a virtual destructor. So, ~Land first executes its own statements
and then calls ~Vehicle. Thus, the above delete vp statement
uses late binding to call ~Vehicle and from this point on the object
destruction proceeds as usual.

Destructors should always be defined virtual in classes designed as a
base class from which other classes are going to be derived. Often those
destructors themselves have no tasks to perform. In these cases the virtual
     
    destructor is given an empty body. For example, the definition of
Vehicle::~Vehicle() may be as simple as:
            Vehicle::~Vehicle()
    {}

Resist the temptation to define virtual destructors (even empty
destructors)
  inline as this complicates class
maintenance. Section 14.11 discusses the reason behind this
 rule of thumb.


14.3: Pure virtual functions

The base class Vehicle is provided with its own concrete implementations
of its virtual members (mass and setMass).  However, virtual member
functions do not necessarily have to be implemented in base classes.

When the implementations of virtual members are omitted from base classes the
class imposes requirements upon derived classes. The derived classes are
required to provide the `missing implementations'.

This approach, in some languages (like C#, Delphi
  and Java) known as an interface, defines a
 protocol. Derived classes must obey the protocol by implementing the
as yet not implemented members. If a class contains at least one member whose
implementation is missing no objects of that class can be defined.

Such incompletely defined classes are always base classes. They enforce a
protocol by merely declaring names, return values and arguments of some of
their members. These classes are call 
 abstract classes or abstract base classes.  Derived classes become
non-abstract classes by implementing the as yet not implemented members.

Abstract base classes are the foundation of many design patterns
  (cf. Gamma et al. (1995))
 , allowing the programmer to create highly
 reusable software. Some of these design patterns are covered by the
C++ Annotations (e.g, the Template Method in section 26.2), but for a
thorough discussion of design patterns the reader is referred to Gamma et
al.'s book.

Members that are merely declared in base classes are called
 pure virtual functions. A virtual member becomes a pure virtual member
by postfixing = 0 to its declaration (i.e., by replacing the semicolon
ending its declaration by `= 0;'). Example:
            #include <iosfwd>
    class Base
    {
        public:
            virtual ~Base();
            virtual std::ostream &insertInto(std::ostream &out) const = 0;
    };
    inline std::ostream &operator<<(std::ostream &out, Base const &base)
    {
        return base.insertInto(out);
    }

All classes derived from Base must implement the insertInto
member function, or their objects cannot be constructed. This is neat: all
objects of class types derived from Base can now always be inserted into
ostream objects.

Could the virtual destructor of a base class ever be a pure virtual
function? The answer to this question is no. First of all, there is no need to
enforce the availability of destructors in derived classes as destructors are
provided by default (unless a destructor is declared with the = delete
attribute). Second, if it is a pure virtual member its implementation does not
exist. However, derived class destructors eventually call their base class
destructors. How could they call base class destructors if their
implementations are lacking? More about this in the next section.

Often, but not necessarily, pure virtual member functions are
const  member functions. This allows the
construction of constant derived class objects. In other situations this might
not be necessary (or realistic), and
 non-constant member functions might be required. The general rule for
const member functions also applies to pure virtual functions: if the
member function alters the object's data members, it cannot be a const
member function.

Abstract base classes frequently don't have
  data members.  However, once a base class
declares a pure virtual member it must be declared identically in derived
classes. If the implementation of a pure virtual function in a derived class
alters the derived class object's data, then that function cannot be
declared as a const member. Therefore, the author of an abstract base
class should carefully consider whether a pure virtual member function should
be a const member function or not.


14.3.1: Implementing pure virtual functions

    Pure virtual member functions may be implemented. To implement a pure virtual
    
    
 member function, provide it with its normal = 0; specification, but
implement it as well. Since the = 0; ends in a semicolon, the pure virtual
member is always at most a declaration in its class, but an implementation may
either be provided outside from its interface (maybe using inline).

Pure virtual member functions may be called from derived class objects or
from its class or derived class members by specifying the base class and scope
resolution operator together with the member to be called. Example:

#include <iostream>

class Base
{
    public:
        virtual ~Base();
        virtual void pureimp() = 0;
};
Base::~Base()
{}
void Base::pureimp()
{
    std::cout << "Base::pureimp() called\n";
}
class Derived: public Base
{
    public:
        void pureimp() override;
};
inline void Derived::pureimp()
{
    Base::pureimp();
    std::cout << "Derived::pureimp() called\n";
}
int main()
{
    Derived derived;

    derived.pureimp();
    derived.Base::pureimp();

    Derived *dp = &derived;

    dp->pureimp();
    dp->Base::pureimp();
}
// Output:
//      Base::pureimp() called
//      Derived::pureimp() called
//      Base::pureimp() called
//      Base::pureimp() called
//      Derived::pureimp() called
//      Base::pureimp() called


Implementing a pure virtual member has limited use. One could argue that
the pure virtual member function's implementation may be used to perform tasks
that can already be performed at the base class level. However, there is no
guarantee that the base class virtual member function is actually going to be
called.  Therefore base class specific tasks could as well be offered by a
separate member, without blurring the distinction between a member doing some
work and a pure virtual member enforcing a protocol.


14.4: Explicit virtual overrides

Consider the following situations:
    
    o  A class Value is a value class. It offers a copy constructor,
an overloaded assignment operator, maybe move operations, and a public,
non-virtual constructor. In section 14.7 it is argued that such
classes are not suited as base classes. New classes should not inherit from
Value. How to enforce this?
    o  A polymorphic class Base defines a virtual member
v_process(int32_t). A class derived from Base needs to override this
member, but the author mistakingly defined v_proces(int32_t). How to
prevent such errors, breaking the polymorphic behavior of the derived class?
    o  A class Derived, derived from a polymorphic Base class
overrides the member Base::v_process, but classes that are in turn derived
from Derived should no longer override v_process, but may override
other virtual members like v_call and v_display. How to enforce this
restricted polymorphic character for classes derived from Derived?

Two special identifiers, final and override are used to realize
the above. These identifiers are special in the sense that they only require
their special meanings in specific contexts. Outside of this context they are
just plain identifiers, allowing the programmer to define a variable like
bool final.

The identifier final can be applied to class declarations to indicate
that the class cannot be used as a base class. E.g.:
            class Base1 final               // cannot be a base class
    {};
    class Derived1: public Base1    // ERR: Base1 is final
    {};

    class Base2                     // OK as base class
    {};
    class Derived2 final: public Base2  // OK, but Derived2 can't be
    {};                                 //     used as a base class
    class Derived: public Derived2      // ERR: Derived2 is final
    {};

The identifier final can also be added to virtual member
declarations. This indicates that those virtual members cannot be overridden
by derived classes. The restricted polymorphic character of a class, mentioned
above, can thus be realized as follows:
            class Base
    {
        virtual int v_process();    // define polymorphic behavior
        virtual int v_call();
        virtual int v_display();
    };
    class Derived: public Base      // Derived restricts polymorphism
    {                               // to v_call and v_display
        virtual int v_process() final;
    };
    class Derived2: public Derived
    {
        // int v_process();            No go: Derived:v_process is final
        virtual int v_display();    // OK to override
    };

To allow the compiler to detect typos, differences in parameter types, or
differences in member function modifiers (e.g., const vs. non-const)
the identifier override can (should) be appended to derived class members
overriding base class members. E.g.,
            class Base
    {
        virtual int v_process();
        virtual int v_call() const;
        virtual int v_display(std::ostream &out);
    };
    class Derived: public Base
    {
        virtual int v_proces() override;    // ERR: v_proces != v_process
        virtual int v_call() override;      // ERR: not const
                                            // ERR: parameter types differ
        virtual int v_display(std::istream &out) override;
    };


14.5: Virtual functions and multiple inheritance

    In chapter 6 we encountered the class fstream, one class
offering features of ifstream and ofstream. In chapter
13 we learned that a class may be derived from multiple base
classes. Such a derived class inherits the properties of all its base
classes. Polymorphism can also be used in combination with multiple
inheritance.

Consider what would happen if more than one `path' leads from the derived
class up to its (base) classes. This is illustrated in the next (fictitious)
example where a class Derived is doubly derived from Base:
            class Base
    {
        int d_field;
        public:
            void setfield(int val);
            int field() const;
    };
    inline void Base::setfield(int val)
    {
        d_field = val;
    }
    inline int Base::field() const
    {
        return d_field;
    }
    class Derived: public Base, public Base
    {};

Due to the double derivation, Base's functionality now occurs twice in
Derived. This results in ambiguity: when the function setfield() is
called for a Derived class object, which function will that be as there
are two of them? The scope resolution operator won't come to the rescue and so
the C++ compiler cannot compile the above example and (correctly)
identifies an error.

The above code clearly duplicates its base class in the derivation, which can
of course easily be avoided by not doubly deriving from Base (or by using
composition (!)). But duplication of a base class can also occur through
 nested inheritance, where an object is derived from, e.g., a Car and
from an Air (cf. section 13.1). Such a class would be needed
to represent, e.g., a flying car\ (such as the one in James Bond
vs. the Man with the Golden Gun...). An AirCar would ultimately contain
two Vehicles, and hence two mass fields, two setMass()
functions and two mass() functions. Is this what we want?


14.5.1: Ambiguity in multiple inheritance

    Let's investigate closer why an AirCar introduces ambiguity, when
derived from Car and Air.
    
    o  An AirCar is a Car, hence a Land, and hence a
Vehicle.
    o  However, an AirCar is also an Air, and hence a Vehicle.

The duplication of Vehicle data is further illustrated in
Figure 16.
    ------------------------------------------------------------------Insert Figure 16(Duplication of a base class in multiple derivation.)about here (file: polymorphism/ambiguity)------------------------------------------------------------------
    The internal organization of an AirCar is shown in
Figure 17
    ------------------------------------------------------------------Insert Figure 17(Internal organization of an AirCar object.)about here (file: polymorphism/internal)------------------------------------------------------------------
    The C++ compiler detects the ambiguity in an AirCar object,
and will therefore not  compile  statements like:
            AirCar jBond;
    cout << jBond.mass() << '\n';

Which member function mass to call cannot be determined by the
compiler but the programmer has two possibilities to resolve the ambiguity for
the compiler:
    
    o  First, the function call where the ambiguity originates can be
modified. The ambiguity is resolved using the scope resolution operator:
    // let's hope that the mass is kept in the Car
// part of the object..
cout << jBond.Car::mass() << '\n';

The scope resolution operator and the class name are put right before
the name of the member function.
    o  Second, a dedicated function mass could be created for
the class AirCar:
    int AirCar::mass() const
{
    return Car::mass();
}

The second possibility is preferred as it does not require the compiler to
flag an error; nor does it require the programmer using the class
AirCar to take special precautions.

However, there exists a more elegant solution, discussed in the next
section.


14.5.2: Virtual base classes

        As illustrated in Figure 17, an AirCar represents
two Vehicles. This not only results in an ambiguity about which
function to use to access the mass data, but it also defines two
mass fields in an AirCar. This is slightly redundant, since we can
assume that an AirCar has but one mass.

It is, however, possible to define an AirCar as a class consisting of
but one Vehicle and yet using multiple derivation.  This is realized by
defining the base classes that are multiply mentioned in a derived class's
inheritance tree as a
  virtual base class.

For the class AirCar this implies a small change when deriving an
AirCar from Land and Air classes:
            class Land: virtual public Vehicle
    {
        // etc
    };
    class Car: public Land
    {
        // etc
    };
    class Air: virtual public Vehicle
    {
        // etc
    };
    class AirCar: public Car, public Air
    {
    };

Virtual derivation  ensures that a Vehicle is
only added once to a derived class. This means that the route along which a
Vehicle is added to an AirCar is no longer depending on its direct
base classes; we can only state that an AirCar is a Vehicle.  The
internal organization of an AirCar after virtual derivation is shown in
Figure 18.
    ------------------------------------------------------------------Insert Figure 18(Internal organization of an AirCar object when the base
            classes are virtual.)about here (file: polymorphism/virtbase)------------------------------------------------------------------

When a class Third inherits from a base class Second which in turn
inherits from a base class First then the First class constructor
called by the Second class constructor is also used when this Second
constructor is used when constructing a Third object. Example:
            class First
    {
        public:
            First(int x);
    };
    class Second: public First
    {
        public:
            Second(int x)
            :
                First(x)
            {}
    };
    class Third: public Second
    {
        public:
            Third(int x)
            :
                Second(x)           // calls First(x)
            {}
    };

The above no longer holds true when Second uses virtual derivation.
When Second uses virtual derivation its base class constructor is
ignored when Second's constructor is called from Third. Instead
Second by default calls First's default constructor. This is
illustrated by the next example:
            class First
    {
        public:
            First()
            {
                cout << "First()\n";
            }
            First(int x);
    };
    class Second: public virtual First      // note: virtual
    {
        public:
            Second(int x)
            :
                First(x)
            {}
    };
    class Third: public Second
    {
        public:
            Third(int x)
            :
                Second(x)
            {}
    };
    int main()
    {
        Third third{ 3 };   // displays `First()'
    }

    When constructing Third First's default constructor is used by
default. Third's constructor, however, may overrule this default behavior
by explicitly specifying the constructor to use. Since the First object
must be available before Second can be constructed it must be specified
first. To call First(int) when constructing Third(int) the latter
constructor can be defined as follows:
            class Third: public Second
    {
        public:
            Third(int x)
            :
                First(x),           // now First(int) is called.
                Second(x)
            {}
    };

This behavior may seem puzzling when simple linear inheritance is used but
it makes sense when multiple inheritance is used with base classes using
virtual inheritance. Consider AirCar: when Air and Car both
virtually inherit from Vehicle will Air and Car both initialize
the common Vehicle object? If so, which one is going to be called first?
What if Air and Car use different Vehicle constructors? All these
questions can be avoided by passing the responsibility for the initialization
of a common base class to the class eventually using the common base class
object. In the above example Third. Hence Third is provided an
opportunity to specify the constructor to use when initializing First.

Multiple inheritance may also be used to inherit from classes that do not all
use virtual inheritance. Assume we have two classes, Derived1 and
Derived2, both (possibly virtually) derived from Base.

We now address the question which constructors will be called when calling a
constructor of the class Final: public Derived1, public Derived2.

To distinguish the involved constructors Base1 indicates the Base
class constructor called as base class initializer for Derived1 (and
analogously: Base2 called from Derived2). A plain Base  indicates
Base's  default constructor.

Derived1 and Derived2 indicate the base class initializers used when
constructing a Final object.

Now we're ready to distinguish the various cases when constructing an object
of the class Final: public Derived1, public Derived2:
        
        o  classes:
                Derived1: public Base
Derived2: public Base

This is normal, non virtual multiple derivation. The
                following constructors are called in the order shown:
                Base1,
Derived1,
Base2,
Derived2

o  classes:
                Derived1: public Base
Derived2: virtual public Base

Only Derived2 uses virtual derivation.
               Derived2's base class constructor is ignored.  Instead,
                Base is called and it is called prior to any other
                constructor:
               Base,
Base1,
Derived1,
Derived2

As only one class uses virtual derivation, two Base
                class objects remain available in the eventual Final
                class.

o  classes:
                Derived1: virtual public Base
Derived2: public Base

Only Derived1 uses virtual derivation.
               Derived1's base class constructor is ignored.  Instead,
                Base is called and it is called prior to any other
                constructor. Different from the first (non-virtual) case
                Base is now called, rather than Base1:
               Base,
Derived1,
Base2,
Derived2

o  classes:
                Derived1: virtual public Base
Derived2: virtual public Base

Both base classes use virtual derivation and so
                only one Base class object will be present in the
                Final class object. The following constructors are called
                in the order shown:
                Base,
Derived1,
Derived2

Virtual derivation is, in contrast to virtual functions, a pure
compile-time issue. Virtual inheritance merely defines
how the compiler defines a class's data organization and construction process.


14.5.3: When virtual derivation is not appropriate

    Virtual inheritance can be used to merge multiply occurring base
classes. However, situations may be encountered where multiple occurrences of
base classes is appropriate. Consider the definition of a Truck (cf.
section 13.5):
            class Truck: public Car
    {
        int d_trailer_mass;

        public:
            Truck();
            Truck(int engine_mass, int sp, char const *nm,
                   int trailer_mass);

            void setMass(int engine_mass, int trailer_mass);
            int mass() const;
    };
    Truck::Truck(int engine_mass, int sp, char const *nm,
                  int trailer_mass)
    :
        Car(engine_mass, sp, nm)
    {
        d_trailer_mass = trailer_mass;
    }
    int Truck::mass() const
    {
        return                  // sum of:
            Car::mass() +    //   engine part plus
            trailer_mass;         //   the trailer
    }

This definition shows how a Truck object is constructed to contain two
mass fields: one via its derivation from Car and one via its own int
d_trailer_mass data member. Such a definition is of course valid, but it
could also be rewritten. We could derive a Truck from a Car
and from a Vehicle, thereby explicitly requesting the double presence
of a Vehicle; one for the mass of the engine and cabin, and one for the
mass of the trailer. A slight complication is that a class organization like
            class Truck: public Car, public Vehicle

is not accepted by the C++ compiler. As a Vehicle is already part
of a Car, it is therefore not needed once again. This organization may,
however, be accepted using a small trick. By creating an additional class
inheriting from Vehicle and deriving Truck from that additional class
rather than directly from Vehicle the problem is solved. Simply derive a
class TrailerVeh from Vehicle, and then Truck from Car and
TrailerVeh:
            class TrailerVeh: public Vehicle
    {
        public:
            TrailerVeh(int mass)
            :
                Vehicle(mass)
            {}
    };
    class Truck: public Car, public TrailerVeh
    {
        public:
            Truck();
            Truck(int engine_mass, int sp, char const *nm, int trailer_mass);
            void setMass(int engine_mass, int trailer_mass);
            int mass() const;
    };
    inline Truck::Truck(int engine_mass, int sp, char const *nm,
                        int trailer_mass)
    :
        Car(engine_mass, sp, nm),
        TrailerVeh(trailer_mass)
    {}
    inline int Truck::mass() const
    {
        return                      // sum of:
            Car::mass() +        //   engine part plus
            TrailerVeh::mass();   //   the trailer
    }


14.6: Run-time type identification

C++ offers two ways to  retrieve types of
objects and expressions at run-time. The possibilities of C++'s run-time
type identification are limited compared to languages like
  Java. Usually
    static type checking and static type identification is used in
C++. Static type checking is possibly safer and certainly more efficient
than run-time type identification and should therefore be preferred over
run-time type identification. But situations exist where run-time type
identification is appropriate. C++ offers run-time type identification
through the dynamic cast and typeid operators.
    
    o  A dynamic_cast  is used  to convert a base
class  pointer or reference to a
derived class pointer or reference. This is also known as down-casting.
        o  The typeid operator returns the actual type of an expression.
            

These operators can be used with objects of classes having at least one
virtual member function.


14.6.1: The dynamic_cast operator

        The dynamic_cast<> operator is used to convert a base
class pointer or  reference to,
respectively, a derived class pointer or reference. This is also called
down-casting as direction of the cast is down the inheritance tree.

A dynamic cast's actions are determined run-time; it can only be used if
the base class declares at least one virtual member function. For the dynamic
cast to succeed, the destination class's Vtable must be equal to the
Vtable to which the dynamic cast's argument refers to, lest the cast fails
and returns 0 (if a dynamic cast of a pointer was requested) or throws a
std::bad_cast exception (if a dynamic cast of a reference was requested).

In the following example a pointer to the class Derived is obtained from
the Base class pointer bp:
            class Base
    {
        public:
            virtual ~Base();
    };
    class Derived: public Base
    {
        public:
            char const *toString();
    };
    inline char const *Derived::toString()
    {
        return "Derived object";
    }
    int main()
    {
        Base *bp;
        Derived *dp,
        Derived d;

        bp = &d;

        dp = dynamic_cast<Derived *>(bp);

        if (dp)
            cout << dp->toString() << '\n';
        else
            cout << "dynamic cast conversion failed\n";
    }

In the condition of the above if statement the success of the dynamic
cast is verified. This verification is performed at run-time, as the
actual class of the objects to which the pointer points is only known by then.

If a base class pointer is provided, the dynamic cast operator returns 0 on
failure and a pointer to the requested derived class on success.

Assume a vector<Base *> is used. The pointers of such a vector may
point to objects of various classes, all derived from Base. A dynamic cast
returns a pointer to the specified class if the base class pointer indeed
points to an object of the specified class and returns 0 otherwise.

We could determine the actual class of an object a pointer points to by
performing a series of checks to find the derived class to which a base class
pointer points. Example:
            class Base
    {
        public:
            virtual ~Base();
    };
    class Derived1: public Base;
    class Derived2: public Base;

    int main()
    {
        vector<Base *> vb(initializeBase());

        Base *bp = vb.front();

        if (dynamic_cast<Derived1 *>(bp))
            cout << "bp points to a Derived1 class object\n";
        else if (dynamic_cast<Derived2 *>(bp))
            cout << "bp points to a Derived2 class object\n";
    }

Alternatively, a reference to a base class object may be available. In
this case the dynamic_cast operator throws an exception if the down
casting fails. Example:
            #include <iostream>
    #include <typeinfo>

    class Base
    {
        public:
            virtual ~Base();
            virtual char const *toString();
    };
    inline char const *Base::toString()
    {
        return "Base::toString() called";
    }
    class Derived1: public Base
    {};
    class Derived2: public Base
    {};

    Base::~Base()
    {}
    void process(Base &b)
    {
        try
        {
            std::cout << dynamic_cast<Derived1 &>(b).toString() << '\n';
        }
        catch (std::bad_cast)
        {}
        try
        {
            std::cout << dynamic_cast<Derived2 &>(b).toString() << '\n';
        }
        catch (std::bad_cast)
        {
            std::cout << "Bad cast to Derived2\n";
        }
    }
    int main()
    {
        Derived1 d;
        process(d);
    }
    /*
        Generated output:

        Base::toString() called
        Bad cast to Derived2
    */

    In this example the value std::bad_cast  is used. A
std::bad_cast exception is thrown if the dynamic cast of a reference to a
derived class object fails.

Note the form of the catch clause: bad_cast is the name of a type.
Section 17.4.1 describes how  such a type can be defined.

The dynamic cast operator is a useful tool when an existing base class cannot
or should not be modified (e.g., when the sources are not available), and a
derived class may be modified instead. Code receiving a base class pointer or
reference may then perform a dynamic cast to the derived class to access the
derived class's functionality.

You may wonder in what way the behavior of the dynamic_cast differs from
that of the static_cast.

When the static_cast is used, we tell the compiler that it must convert a
pointer or reference to its expression type to a pointer or reference of its
destination type. This holds true whether the base class declares virtual
members or not. Consequently, all the static_cast's actions can be
determined by the compiler, and the following compiles fine:
            class Base
    {
        // maybe or not virtual members
    };
    class Derived1: public Base
    {};
    class Derived2: public Base
    {};

    int main()
    {
        Derived1 derived1;
        Base *bp = &derived1;

        Derived1 &d1ref = static_cast<Derived1 &>(*bp);
        Derived2 &d2ref = static_cast<Derived2 &>(*bp);
    }

Pay attention to the second static_cast: here the Base class
object is cast to a Derived2 class reference. The compiler has no problems
with this, as Base and Derived2 are related by inheritance.

Semantically, however, it makes no sense as bp in fact points to a
Derived1 class object. This is detected by a dynamic_cast.  A
dynamic_cast, like the static_cast, converts related pointer or
reference types, but the dynamic_cast provides a run-time safeguard. The
dynamic cast fails when the requested type doesn't match the actual type of
the object we're pointing at. In addition, the dynamic_cast's use is much
more restricted than the static_cast's use, as the dynamic_cast can
only be used for downcasting to derived classes having virtual members.

In the end a dynamic cast is a cast, and casts should be avoided whenever
possible. When the need for dynamic casting arises ask yourself whether the
base class has correctly been designed. In situations where code expects a
base class reference or pointer the base class interface should be all that is
required and using a dynamic cast should not be necessary. Maybe the base
class's virtual interface can be modified so as to prevent the use of dynamic
casts. Start frowning when encountering code using dynamic casts. When using
dynamic casts in your own code always properly document why the dynamic cast
was appropriately used and was not avoided.


14.6.2: The `typeid' operator

        As with the dynamic_cast operator, typeid is usually applied to
references to base class objects that refer to derived class
objects. Typeid should only be used with base classes offering virtual
members. 

Before using typeid the <typeinfo> header file must be included.

The typeid operator returns an object of type type_info.
Different compilers may offer different implementations of the class
type_info, but at the very least typeid must offer the following
interface:
            class type_info
    {
        public:
            virtual ~type_info();
            int operator==(type_info const &other) const;
            int operator!=(type_info const &other) const;
            bool before(type_info const &rhs) const;
            char const *name() const;
        private:
            type_info(type_info const &other);
            type_info &operator=(type_info const &other);
    };

Note that this class has a private copy constructor and a private
overloaded assignment operator. This prevents code from  constructing
type_info objects and prevents code from assigning type_info objects
to each other. Instead, type_info objects are
constructed and returned by the typeid operator.

If the typeid operator is passed a base class reference it is able to
return the actual name of the type the reference refers to. Example:
            class Base;
    class Derived: public Base;

    Derived d;
    Base    &br = d;

    cout << typeid(br).name() << '\n';

In this example the typeid operator is given a base class reference.
It prints the text ``Derived'', being the class name of the class
br actually refers to. If Base does not contain virtual functions, the
text ``Base'' is printed.

The typeid operator can be used to determine the name of the actual
type of  expressions, not just of class type
objects. For example:
            cout << typeid(12).name() << '\n';     // prints:  int
    cout << typeid(12.23).name() << '\n';  // prints:  double

Note, however, that the above example is suggestive at most. It may
print int and double, but this is not necessarily the case. If
portability is required, make sure no tests against these static, built-in
text-strings are required. Check out what your compiler produces in case of
doubt.

In situations where the typeid operator is applied to
determine the type of a derived class, a base class reference
        
    should be used as the argument of the typeid operator. Consider
the following example:
            class Base;     // contains at least one virtual function
    class Derived: public Base;

    Base *bp = new Derived;     // base class pointer to derived object

    if (typeid(bp) == typeid(Derived *))    // 1: false
        ...
    if (typeid(bp) == typeid(Base *))       // 2: true
        ...
    if (typeid(bp) == typeid(Derived))      // 3: false
        ...
    if (typeid(bp) == typeid(Base))         // 4: false
        ...
    if (typeid(*bp) == typeid(Derived))     // 5: true
        ...
    if (typeid(*bp) == typeid(Base))        // 6: false
        ...

    Base &br = *bp;

    if (typeid(br) == typeid(Derived))      // 7: true
        ...
    if (typeid(br) == typeid(Base))         // 8: false
        ...

Here, (1) returns false as a Base * is not a Derived
*. (2) returns true, as the two pointer types are the same, (3)
and (4) return false as pointers to objects are not the objects
themselves.

On the other hand, if *bp is used in the above expressions, then
(1) and (2) return false as an object (or reference to an object)
is not a pointer to an object, whereas (5) now returns true: *bp
actually refers to a Derived class object, and typeid(*bp) returns
typeid(Derived).  A similar result is obtained if a base class reference
is used: 7 returning true and 8 returning false.

The type_info::before(type_info const &rhs) member is used to
determine the collating order of classes. This is useful when comparing
two types for equality.  The function returns a nonzero value if *this
precedes rhs in the hierarchy or collating order of the used types. When a
derived class is compared to its base class the comparison returns 0,
otherwise a non-zero value. E.g.:
            cout << typeid(ifstream).before(typeid(istream)) << '\n' << // 0
            typeid(istream).before(typeid(ifstream)) << '\n';   // not 0

With built-in types the implementor may implement that non-0 is returned
when a `wider' type is compared to a `smaller' type and 0 otherwise:
            cout << typeid(double).before(typeid(int)) << '\n' <<   // not 0
            typeid(int).before(typeid(double)) << '\n';     // 0

When two equal types are compared, 0 is returned:
            cout << typeid(ifstream).before(typeid(ifstream)) << '\n';   // 0

When a 0-pointer is passed to the operator typeid a bad_typeid
exception is thrown.


14.7: Inheritance: when to use to achieve what?

Inheritance should not be applied automatically and thoughtlessly. Often
composition can be used instead, improving on a class's design by reducing
coupling. When inheritance is used public inheritance should not
automatically be used but the type of inheritance that is selected should
match the programmer's intent.

We've seen that polymorphic classes on the one hand offer interface members
defining the functionality that can be requested of base classes and on the
other hand offer virtual members that can be overridden. One of the signs of
good class design is that member functions are designed according to the
principle of `one function, one task'. In the current context: a class member
should either be a member of the class's public or protected interface or it
should be available as a virtual member for reimplementation by derived
classes. Often this boils down to virtual members that are defined in the base
class's private section. Those functions shouldn't be called by code using
the base class, but they exist to be overridden by derived classes using
polymorphism to redefine the base class's behavior.

The underlying principle was mentioned before in the introductory paragraph
of this chapter: according to the Liskov Substitution Principle
(LSP) an is-a relationship between classes (indicating that a
derived class object is a base class object) implies that a derived class
object may be used in code expecting a base class object.

In this case inheritance is used not to let the derived class use the
facilities already implemented by the base class but to reuse the base class
polymorphically by reimplementing the base class's virtual members in the
derived class.

In this section we'll discuss the reasons for using inheritance. Why should
inheritance (not) be used? If it is used what do we try to accomplish by it?

Inheritance often competes with composition. Consider the following two
alternative class designs:
            class Derived: public Base
    { ... };

    class Composed
    {
        Base d_base;
        ...
    };

Why and when prefer Derived over Composed and vice versa? What
kind of inheritance should be used when designing the class Derived?
    
    o  Since Composed and Derived are offered as alternatives we are
        looking at the design of a class (Derived or Composed) that
        is-implemented-in-terms-of another class.
    o  Since Composed does itself not make Base's interface
        available, Derived shouldn't do so either. The underlying
        principle is that private inheritance should be used when
        deriving a classs Derived from Base where Derived
        is-implemented-in-terms-of Base.
    o  Should we use inheritance or composition? Here are some arguments:
        
        o  In general terms composition results in looser coupling and
            should therefore be preferred over inheritance.
        o  Composition allows us to define classes having multiple members
            of the same type (think about a class having multiple
            std::string members) which can not be realized using
            inheritance.
        o  Composition allows us to separate the class's interface from its
            implementation. This allows us to modify the class's data
            organization without the need to recompile code using our
            class. This is also known as the bridge design pattern or the
            compiler firewall or pimpl (pointer to the
            implementation) idiom.
        o  If Base offers members in its protected interface that
            must be used when implementing Derived inheritance must also
            be used. Again: since we're implementing-in-terms-of the
            inheritance type should be private.
        o  Protected inheritance may be considered when the derived class
            (D) itself is intended as a base class that should only make
            the members of its own base class (B) available to classes
            that are derived from it (i.e., D).

Private inheritance should also be used when a derived class is-a certain
type of base class, but in order to initialize that base class an object of
another class type must be available. Example: a new istream class-type
(say: a stream IRandStream from which random numbers can be extracted) is
derived from std::istream.  Although an istream can be constructed
empty (receiving its streambuf later using its rdbuf member), it is
clearly preferable to initialize the istream base class right away.

Assuming that a Randbuffer: public std::streambuf has been created for
generating random numbers then IRandStream can be derived from
Randbuffer and std::istream. That way the istream base class can
be initialized using the Randbuffer base class.

As a RandStream is definitely not a Randbuffer public inheritance
is not appropriate. In this case IRandStream
is-implemented-in-terms-of a Randbuffer and so private inheritance
should be used.

IRandStream's class interface should therefore start like this:
            class IRandStream: private Randbuffer, public std::istream
    {
        public:
            IRandStream(int lowest, int highest)    // defines the range
            :
                Randbuffer(lowest, highest),
                std::istream(this)                  // passes &Randbuffer
            {}
        ...
    };

Public inheritance should be reserved for classes for which the LSP holds
true. In those cases the derived classes can always be used instead of the
base class from which they derive by code merely using base class references,
pointers or members (I.e., conceptually the derived class is-a base
class). This most often applies to classes derived from base classes offering
virtual members. To separate the user interface from the redefinable interface
the base class's public interface should not contain virtual members
(except for the virtual destructor) and the virtual members should all be in
the base class's private section. Such virtual members can still be overridden
by derived classes (this should not come as a surprise, considering how
polymorphism is implemented) and this design offers the base class full
control over the context in which the redefined members are used. Often the
public interface merely calls a virtual member, but those members can always
be redefined to perform additional duties.

The prototypical form of a  base class therefore
looks like this:
            class Base
    {
        public:
            virtual ~Base();
            void process();             // calls virtual members (e.g.,
                                        // v_process)
        private:
            virtual void v_process();   // overridden by derived classes
    };

Alternatively a base class may offer a non-virtual destructor, which
should then be protected. It shouldn't be public to prevent deleting objects
through their base class pointers (in which case virtual destructors should be
used). It should be protected to allow derived class destructors to call their
base class destructors. Such base classes should, for the same reasons, have
non-public constructors and overloaded assignment operators.


14.8: The `streambuf' class

The class std::streambuf receives the character sequences
processed by streams and defines the interface between stream objects and
devices (like a file on disk). A streambuf object is usually not directly
constructed, but usually it is used as base class of some derived class
implementing the communication with some concrete device.

The primary reason for existence of the class streambuf is to decouple
the stream classes from the devices they operate upon. The rationale here
is to add an extra layer between the classes allowing us to communicate with
devices and the devices themselves. This implements a chain of command
which is seen regularly in software design.

The chain of command is considered a generic pattern when designing
 reusable software, encountered also in, e.g., the
 TCP/IP stack.

A streambuf provides yet another example of the chain of command
pattern: the program talks to stream objects, which in turn forward
requests to streambuf objects, which in turn communicate with the
devices. Thus, as we will see shortly, we are able to do in user-software what
had to be done via (expensive) system calls before.

The class streambuf has no public constructor, but does make available
several public member functions. In addition to these public member functions,
several member functions are only available to classes derived from
streambuf.  In section 14.8.3 a predefined specialization of the
class streambuf is introduced. All public members of streambuf
discussed here are also available in filebuf.

The next section shows the streambuf members that may be overridden
when deriving classes from streambuf. Chapter 26 offers
concrete examples of classes derived from streambuf.

The class streambuf is used by streams performing input operations and
by streams performing output operations and their member functions can be
ordered likewise. The type std::streamsize used below may,
for all practical purposes, be considered equal to the type size_t.


    
    When inserting information into ostream objects the information is
eventually passed on to the ostream's streambuf. The streambuf may
decide to throw an exception. However, this exception does not leave the
ostream using the streambuf. Rather, the exception is caught by the
ostream, which sets its ios::bad_bit. Exceptions thrown by
manipulators which are inserted into ostream objects are not caught by
ostream objects.

Public members for input operations


    o std::streamsize in_avail():
Returns a lower bound on the number of characters that can currently
        be read from the streambuf (e.g., all characters from the stream's
        current offset position to its EOF position).
    o int sbumpc():
The next available character or EOF is returned. The returned
        character is removed from the streambuf object. If no input is
        available, sbumpc calls the (protected) member uflow (see
        section 14.8.1 below) to make new characters
        available. EOF is returned if no more characters are
        available.
    o int sgetc():
The next available character or EOF is returned. The character
        is not removed from the streambuf object (i.e., the
        streambif's offset position isn't incremented). To remove a
        character from the streambuf object, sbumpc (or sgetn) can
        be used.
    o int sgetn(char *buffer, std::streamsize n):
At most n characters are retrieved from the input buffer, and
        stored in buffer. The actual number of characters read is
        returned. The (protected) member xsgetn (see section
        14.8.1 below) is called to obtain the requested number of
        characters.
    o int snextc():
The current character is obtained from the input buffer and returned as
        the next available character or EOF is returned. The character
        is not removed from the streambuf object.
    o int sputbackc(char c):
Inserts c into the streambuf's buffer to be returned as the
        next character to read from the streambuf object. Caution should
        be exercised when using this function: often there is a maximum of
        just one character that can be put back.
    o int sungetc():
Returns the last character read to the input buffer, to be read again
        at the next input operation.  Caution should be exercised when using
        this function: often there is a maximum of just one character that
        can be put back.

Public members for output operations


    o int pubsync():
Synchronizes (i.e., flushes) the buffer by writing any information
        currently available in the streambuf's buffer to the
        device. Normally only used by classes derived from streambuf.
    o int sputc(char c):
Character c is inserted into the streambuf object.  If, after
        writing the character, the buffer is full, the function calls the
        (protected) member function overflow to flush the buffer to the
        device (see section 14.8.1 below).
    o int sputn(char const *buffer, std::streamsize n):
At most n characters from buffer are inserted into the
        streambuf object. The actual number of characters inserted is
        returned. This member function calls the (protected) member xsputn
        (see section 14.8.1 below) to insert the requested number of
        characters.

Public members for miscellaneous operations

The next three members are normally only used by classes derived from
streambuf.
    
    o ios::pos_type pubseekoff(ios::off_type offset, ios::seekdir way,
            ios::openmode mode = ios::in | ios::out):
Sets the offset of the next character to be read or written to
        offset, relative to the standard  ios::seekdir
        values indicating the direction of the seeking operation.
    o ios::pos_type pubseekpos(ios::pos_type pos,
            ios::openmode mode = ios::in | ios::out):
Sets the absolute position of the next character to be read or
        written to pos.
    o streambuf *pubsetbuf(char* buffer, std::streamsize n):
The streambuf object is going to use buffer, which may contain
        at least n characters.


14.8.1: Protected `streambuf' members

    The protected members of the class streambuf are important for
understanding and using streambuf objects. Although there are both
protected data members and protected member functions
     defined in the class streambuf using the protected
data members is strongly discouraged as using them violates the principle
of data hiding. As streambuf's set of member functions is quite
extensive, it is hardly ever necessary to use its data members directly. The
following subsections do not even list all protected member functions but only
those are covered that are useful for constructing specializations.

Streambuf objects control a buffer, used for input and/or output, for
which begin-, actual- and end-pointers have been defined, as depicted in
figure 19.
    ------------------------------------------------------------------Insert Figure 19(Input- and output buffer pointers of the class `streambuf')about here (file: iostreams/sbbuffers)------------------------------------------------------------------

Streambuf offers two protected constructors:
    
    o streambuf::streambuf():
Default (protected) constructor of the class streambuf.
    o streambuf::streambuf(streambuf const &rhs):
(Protected) copy constructor of the class streambuf. Note that
this copy constructor merely copies the values of the data members of rhs:
after using the copy constructor both streambuf objects refer to the same
data buffer and initially their pointers point at identical positions. Also
note that these are not shared pointers, but only `raw copies'.


14.8.1.1: Protected members for input operations

        Several protected member functions are available for input operations. The
member functions marked virtual may of course be redefined in derived
classes:
    
    o char *eback():
Streambuf maintains three pointers controlling its input buffer:
        eback points to the `end of the putback' area: characters can
        safely be put back up to this position. See also figure
        19. Eback points to the beginning of the input
        buffer.
    o char *egptr():
Egptr points just beyond the last character that can be retrieved
        from the input buffer. See also figure 19. If gptr
        equals egptr the buffer must be refilled which is
        handled by member  underflow, see below.
    o void gbump(int n):
The object's gptr (see below) is advanced over  n positions.
    o char *gptr():
Gptr points to the next character to be
        retrieved from the object's input buffer. See also figure
        19.
    o virtual int pbackfail(int c):
This member function may be overridden by derived classes to do
        something intelligent when putting back character c fails. One
        might consider restoring the old read pointer when input buffer's
        begin has been reached. This member function is called when ungetting
        or putting back a character fails. In particular, it is called when
            
            o gptr() == 0: no buffering used,
            o gptr() == eback(): no more room to push back,
            o *gptr() != c: a different character than the next character
                to be read must be pushed back.

If c == endOfFile() then the input device must be reset by one
        character position. Otherwise c must be prepended to the
        characters to be read. The function should return EOF on
        failure. Otherwise 0 can be returned.
    o void setg(char *beg, char *next, char *beyond):
Initializes an input buffer. beg points to the beginning of the
        input area, next points to the next character to be retrieved, and
        beyond points to the location just beyond the input buffer's last
        character. Often next is at least beg + 1, to allow a put back
        operation. No input buffering is used when this member is called as
        setg(0, 0, 0). See also the member uflow, below.
    o virtual streamsize showmanyc():
(Pronounce: s-how-many-c) This member function may be overridden by
        derived classes. It must return a guaranteed lower bound on the number
        of characters that can be read from the device before uflow or
        underflow returns EOF. By default 0 is returned (meaning
        no or some characters are returned before the latter two functions
        return EOF). When a positive value is returned then the next
        call of u(nder)flow does not return EOF.
    o virtual int uflow():
This member function may be overridden by derived classes to reload an
        input buffer with fresh characters. By default it calls underflow
        (see below) to reload the input buffer. If underflow fails,
        EOF is returned.  Otherwise, the next available character
        (*gptr()) is returned as an unsigned char, and then increments
        gptr. This is different from
        underflow, which merely returns the next available character,
        without incrementing gptr's position.

When the streambuf doesn't use input buffering this function,
        rather than underflow, can be overridden to produce the next
        available character from the device.
    o virtual int underflow():
This member function may be overridden by derived classes to read
        another character from the device. The default implementation is to
        return EOF.

It is called when
            
            o  there is no input buffer (eback() == 0)
            o gptr() >= egptr(): the input buffer is exhausted.

Often, when buffering is used, the complete buffer
        is not refreshed as this would make it impossible to put back
        characters immediately following a reload. Instead, buffers are often
        refreshed in halves. This system is called a split buffer.

Classes derived from streambuf for reading normally at least
        override underflow. The prototypical example of an overridden
        underflow function looks like this:
            int underflow()
{
    if (not refillTheBuffer())  // assume a member d_buffer is available
        return EOF;
                                // reset the input buffer pointers
    setg(d_buffer, d_buffer, d_buffer + d_nCharsRead);

                                // return the next available character
                                // (the cast is used to prevent
                                // misinterpretations of 0xff characters
                                // as EOF)
    return static_cast<unsigned char>(*gptr());
}
       This example can be used by streams reading the information made
        available by devices. Section 14.8.2 covers a more
        complex situation: stream supporting both input and output.
    o virtual streamsize xsgetn(char *buffer, streamsize n):
This member function may be overridden by derived classes to retrieve
        at once n characters from the input device. The default
        implementation is to call sbumpc for every single character
        meaning that by default this member (eventually) calls underflow
        for every single character. The function returns the actual number of
        characters read or EOF. Once EOF is returned the
        streambuf stops reading the device (see also section
        14.8.2.)


14.8.1.2: Protected members for output operations

        The following  protected members are available for output
operations. Again, some members may be overridden by derived classes:
    
    o virtual int overflow(int c):
This member function may be overridden by derived classes to flush the
        characters currently stored in the output buffer to the output device,
        and then to reset the output buffer pointers so as to represent an
        empty buffer. Its parameter c is initialized to the next character
        to be processed. If no output buffering is used overflow is called
        for every single character that is written to the streambuf
        object. No output buffering is accomplished by setting the buffer
        pointers (using, setp, see below) to 0. The
        default implementation returns EOF, indicating that no
        characters can be written to the device.

Classes derived from streambuf for writing normally at least
        override overflow. The prototypical example of an overridden
        overflow function looks like this (see also section
        14.8.2):
 int OFdStreambuf::overflow(int c)
{
    sync();                             // flush the buffer
    if (c != EOF)                       // write a character?
    {
        *pptr() = static_cast<char>(c); // put it into the buffer
        pbump(1);                       // advance the buffer's pointer
    }
    unsigned char ch = c;
    return ch;
}

o char *pbase():
Streambuf maintains three pointers controlling its output buffer:
        pbase points to the beginning of the output buffer area. See also
        figure 19.
    o char *epptr():
Streambuf maintains three pointers controlling its output buffer:
        epptr points just beyond the output buffer's last available
        location. See also figure 19. If pptr (see below)
        equals epptr the buffer must be flushed. This is implemented by
        calling overflow, see before.
    o void pbump(int n):
The location returned by pptr (see below) is advanced by n
        positions. The next character will be written at that location.
    o char *pptr():
Streambuf maintains three pointers controlling its output buffer:
        pptr points to the location in the output buffer where the next
        available character will be written (note that in order to write a
        character pptr() must point to a location in the range pbase()
        to epptr()). See also figure 19.
    o void setp(char *beg, char *beyond):
Streambuf's output buffer is initialized to the locations passed
        to setp. Beg points to the beginning of the output buffer and
        beyond points just beyond the last available location of the
        output buffer. Use setp(0, 0) to indicate that no buffering
        should be used. In that case overflow is called for every single
        character to be written to the device.
    o virtual streamsize xsputn(char const *buffer, streamsize n):
This member function may be overridden by derived classes to write a
        series of at most n characters to the output buffer. The actual
        number of inserted characters is returned. If EOF is returned
        writing to the device stops. The default implementation calls
        sputc for each individual character. Redefine this member if,
        e.g., the streambuf should support the ios::openmode ios::app.
        Assuming the class MyBuf, derived from streambuf, features a
        data member ios::openmode d_mode (representing the requested
        ios::openmode), and a member write(char const *buf, streamsize
        len) (writing len bytes at pptr()), then the following code
        acknowledges the ios::app mode (see also section
        14.8.2):
 std::streamsize MyStreambuf::xsputn(char const *buf, std::streamsize len)
{
    if (d_openMode & ios::app)
        seekoff(0, ios::end);

    return write(buf, len);
}


14.8.1.3: Protected members for buffer manipulation

        Several protected members are related to buffer management and positioning:
    
    o virtual streambuf *setbuf(char *buffer, streamsize n):
This member function may be overridden by derived classes to install a
        buffer. The default implementation performs no actions. It is called
        by pubsetbuf.
    o virtual ios::pos_type seekoff(ios::off_type offset, 
                   ios::seekdir way, ios::openmode mode = ios::in | ios::out):
This member function may be overridden by derived classes to
        reset the next pointer for input or output to a new relative
        position (using
       ios::beg, ios::cur or ios::end). The
        default implementation indicates failure by returning -1. This
        function is called when tellg or tellp are called. When
        derived class supports seeking, then it should also define this
        function to handle repositioning requests. It is called by
        pubseekoff. The new position or (by default) an invalid position
        (i.e., -1) is returned (see also section 14.8.2).
    o virtual ios::pos_type seekpos(ios::pos_type offset,
            ios::openmode mode = ios::in | ios::out):
This member function may be overridden by derived classes to reset the
        next pointer for input or output to a new absolute position (i.e,
        relative to ios::beg). This function is called when seekg or
        seekp are called. The new position or (by default) an invalid
        position (i.e., -1) is returned.
    o virtual int sync():
This member function may be overridden by derived classes to flush the
        output buffer to the output device or to reset the input device just
        beyond the position of the character that was returned last. It
        returns 0 on success, -1 on failure. The default implementation (not
        using a buffer) is to return 0, indicating successful syncing. This
        member is used to ensure that any characters that are still buffered
        are written to the device or to put unconsumed characters back to the
        device when the streambuf object ceases to exist.


14.8.1.4: Deriving classes from `streambuf'

            When classes are derived from streambuf at least underflow should
be overridden by classes intending to read information from devices, and
overflow should be overridden by classes intending to write information to
devices. Several examples of classes derived from streambuf are provided
in chapter 26.

Fstream class type objects use a combined input/output buffer,
resulting from istream and ostream being virtually derived from
ios, which class defines a streambuf. To construct a class
supporting both input and output using separate buffers, the streambuf
itself may define two buffers. When seekoff is called for reading, a
mode parameter can be set to ios::in, otherwise to ios::out. Thus
the derived class knows whether it should access the read buffer or the
write buffer. Of course, underflow and overflow do not have to
inspect the mode flag as they by implication know on which buffer they should
operate.


14.8.2: A streambuf used for input and output

    Several complexities might be encountered when overriding underflow
especially when buffers must repeatedly be refreshed and loaded. They are:
    
    o  How to keep track of the over-all current offset?
    o  The actions performed by underflow and overflow.
    o  Should uflow be overridden?
    o  Which members are called by which stream read/write requests?
    o  Overriding xsgetn.
    o  Overriding xsputn.

------------------------------------------------------------------Insert Figure 20(Multi-buffer I/O handling)about here (file: polymorphism/buffers)------------------------------------------------------------------

Figure 20 shows a situation where multiple buffers are used: the
device's information is made available in a buffer which is processed and
managed by the derived streambuf class. In this figure the following
variables are introduced:
    
    o offset is the device's current position. Its lower limit is 0,
        and for all practical purposes there is no upper limit.
    o maxEnd, however, is the device's current physical maximum offset
        value. Such a physical maximum may not exist, but it does exist if,
        e.g., a physical buffer in memory is used which cannot contain more
        than a fixed number of bytes. In those cases maxEnd is set to that
        maximum value, representing the offset just beyond the highest offset
        where a byte may be written to the device;
    o getEnd is the current maximum number of characters that can be read
        from the device. With a newly defined device its value is 0 (zero),
        but once bytes are written to the device getEnd is updated to the
        position just beyond to the highest ever offset where a byte was
        written. When disk-files are used getEnd would be equal to the
        file's size;
    o  All of the device's information cannot at once be made available in
        the streambuf's buffer. Instead it is split up in blocks of a
        fixed size. The device's offset of the first byte of such a
        block is available in bufBeg, and the offset just beyond the last
        byte of such a block is available in bufEnd; bufEnd never
        exceeds maxEnd.

This section covers how such multi-buffer data can be handled by iostream
objects: streams supporting reading and writing. Such streams offer
seekg and seekp members, but the device's offset position applies to
both seekg and seekp: after either seekg or seekp reading and
writing both start at the position defined by either of these two seek
members. Furthermore, when switching between reading and writing no
seekg/seekp call is required: by default new read or write requests
continue at the device's current offset, set by the last read, write, or seek
request.


14.8.2.1: Keeping track of the offset

        Keeping track of the current offset is not a trivial problem. As long as
neither reading nor writing has been used seekg and seekp simply
compute the requested offset. Seek requests specifying ios::beg change
offset to the requested value. When ios::end is specified the offset
is computed relative to getEnd (since getEnd corresponds to the file
size of a physical disk-file ios::end should use getEnd as the current
end-offset and not maxEnd).

Computing offsets as shifts relative to the current offset is slightly
more complicated. When so far neither reading nor writing has been used
things are easy: the new offset equals the current offset plus the specified
ios::off_type sthift. But once information has just been read or written
things get complicated because offset doesn't correspond anymore to
the actual offset. 

For example, initially, when information is written to the device's offset,
bufBeg and bufEnd are computed so that offset is located inside the
buffer starting at bufBegin and continuing to bufEnd, but thereafter
subsequent write requests are handled by the stream itself, and therefore
offset isn't updated. Instead only pptr()'s location, is updated,
invalidating offset.

Assume buffers are 100 bytes large, and in a concrete situation the buffer
covers the device's offsets 500 to 600 while offset equals 510. Then,
after writing "hello" pptr() is 515, but offset is still at
510. Consequently, in that situation issuing seekp(-5, ios::cur).tellp()
should not return 5 (i.e., offset - 5), but 10: bufBegin + pptr() -
pbase() - 5. A similar situation is encountered when reading: gptr() is
updated by read operations.

This problem is solved by introducing three states: the streambuf object
starts in the SEEK-state: the last-used operation wasn't reading or
writing. Once reading is used the state changes to READ, and to WRITE
once writing is used.

When s seek-request is issued the relative position depends on the current
state: in state SEEK the seek's shift value (as in seekg(shift,
ios::cur))is added to offset, in state READ it's added to bufBegin
+ gptr() - eback(), and in state WRITE it's added to bufBegin + pptr()
- pbase(). Seek-requests also change the state to SEEK, so subsequent
seek requests are computed relative to the last computed offset. Finally,
to ensure that underflow and overflow are called when subsequent read
or write operations are requested seek requests also reset setg and
setp by calling them with 0 (zero) arguments. Here is a skeleton
implementation of seekoff (assuming using namespace std):
    ios::pos_type StreamBuf::seekoff(ios::off_type step,
                                 ios::seekdir way, ios::openmode mode)
{
    off_type offs;

    switch (way)
    {
        default:                    // ios::beg: buffOffset is step
            offs = step;
        break;

        case ios::cur:
            switch (d_last)
            {
                                    // default: case SEEK
                default:            // no read/write used so far
                    offs = offset;
                break;              // add step to bufOffset (below)

                case READ:          // setg was used, set bufOffset to
                                    // the abs offset of gptr()
                    offs = bufbeg + gptr() - eback();
                break;

                case WRITE:         // setp was used, set bufOffset to
                                    // the abs offset of pptr()
                    offs = bufbeg + pptr() - pbase();

                                    // may extend the writing area
                    if (offs > static_cast<off_type>(getend))
                        getend = offs;
                break;
            }

            offs += step;         // add the step
        break;

        case ios::end:
            offs = getend + step; // shift from the last write position
        break;
    }

    if (offs < 0)
        offs = 0;                 // offset always >= 0

    d_last = SEEK;

    setg(0, 0, 0);                  // reset the buffers
    setp(0, 0);

    return offset = offs;      // the updated offset
}



14.8.2.2: The members overflow and underflow

        The member overflow is called then the stream's write buffer is empty or
exhausted. The member underflow is called then the stream's read buffer is
empty or exhausted.

In both cases a new buffer (from bufBeg to bufEnd) is computed
containing offset. But offset depends on the streambuf's state. When
called in the SEEK state offset is up-to-date; when called in the
WRITE state the offset is at the last-used write offset; when called in
the READ state the current offset is at the end of the current read
buffer. 

When called in the SEEK state the read and write buffers are already
empty. When called in the READ state the actual offset is determined and
the read buffer is reset to ensure that underflow is called at the next
read operation.

Likewise, when called in the WRITE state the write buffer is reset to
ensure that overflow is called at the next write request. 

Both underflow and overflow therefore start by determining the current
offset, computing the corresponding buffer boundaries. The member getOffset
is called by both underflow and overflow. Here's its skeleton
implementation: 
    size_t StreamBuf::getOffset()
{
    size_t offs;

    switch (d_last)
    {
        default:        // no buffers so far: use offset
            offs = offset;
        break;

        case READ:      // use the lastused read offset
            offs = bufbeg + (gptr() - eback());
            setg(0, 0, 0);
        break;

        case WRITE:     // use the lastused write offset
            offs = bufbeg + (pptr() - pbase());
            setp(0, 0);
        break;
    }

    bufLimits(offs);  // set the buffer limits

    return offs;
}


The member bufLimits simply ensures that offset is located inside a
buffer: 
    void StreamBuf::bufLimits(size_t offset)
{
    bufbeg = offset / blockSize * blockSize;
    bufend = bufbeg + blockSize;

    if (bufend > maxend)        // never exceed maxend
        bufend = maxend;
}


The member overflow returns EOF or initializes a new read
buffer. Since overflow is guaranteed to be called when writing is
requested from states SEEK and READ it calls getOffset to obtain
the current absolute offset and the corresponding bufBeg en bufEnd
values. Writing can only be used if offset < maxEnd. If so, a new write
buffer is installed whose pptr() points at the offset position in the
physical device. After calling setp overflow must return ch on
success. Here's overflow's skeleton:
    int StreamBuf::overflow(int ch)     // writing
{
    size_t offs = getOffset();

    if (offs >= maxend)           // at maxend: no more space
        return EOF;
                                    // define the writing buffer
    setp(allData + bufbeg, allData + bufend);
    pbump(offs - bufbeg);       // go to the pos. to write the next ch

    *pptr() = ch;                   // write ch to the buffere
    pbump(1);
    ++offset;
                                    // maybe enlarge getend
    getend = max(getend, bufbeg + (pptr() - pbase()));

    d_last = WRITE;                 // change to writing mode
                                    // return the last written char
    return static_cast<unsigned char>(ch);
}


The member underflow returns EOF or initializes a new read
buffer. Since underflow is guaranteed to be called when reading is
requested from states SEEK and WRITE it calls, like overflow,
getOffset to obtain the current absolute offset as well as the
matching bufBeg en bufEnd values. Reading can only be used if
offset < getEnd. If so, a new read buffer is installed whose gptr()
points at the offset position in the physical device.

As with overflow, after calling setg it's essential that the first
available character (i.e., *gptr()) is returned. If not and the buffer
contains just one character then that character might not be processed by
underflow's caller. Here's underflow's skeleton:
    int StreamBuf::underflow()
{
    offset = getOffset();

    if (offset >= getend)    // beyond the reading area
        return EOF;
                                // define the reading buffer
    setg(allData + bufbeg, allData + offset,
         allData + min(getend, bufend));

    d_last = READ;
    return static_cast<unsigned char>(*gptr());
}



14.8.2.3: Overriding uflow?

        Should uflow be overridden?  The function uflow is called when it's
available to return the next character from the device. In practice this is
handled by underflow, so there's probably little need for overriding
uflow. But if uflow is overridden then it must return the next
avaialble character and update gptr() to the next available
character. If the update isn't performed then the returned characters is
received twice by the stream. Here's a skeleton:
    int StreamBuf::uflow()
{
    if (gptr() == egptr() and underflow() == EOF)
        return EOF;

    unsigned char ch = *gptr();
    gbump(1);
    return ch;
}



14.8.2.4: When are which members called?

        The following overview shows which members are called by which stream
read/write/seek requests:
    
    o seek requests seekg, seekp call seekoff. The member
        streambuf::seekpos is maybe also called, but in practice
        seekpos calls seekoff;

o get calls underflow if there's no (or an exhausted) read
        buffer. Otherwise it returns the character at gptr(), incrementing
        its position;

o >>  calls underflow if there's no (or an exhausted) read
        buffer. Otherwise if white-space characters are ignored (which is the
        default) all white-space characters are skipped and the stream reads
        the matching bytes from the read buffer, refreshing the buffer when
        needed;

o read calls xsgetn (which itself calls underflow) if there's
        no or an exhausted read buffer, and tries to read the requested number
        of characters from the device;

o rdbuf() is used to insert the device's content from its current
        offset position. It calls underflow and reads all the buffer's
        characters until underflow returns EOF.

o put calls overflow if there's no (or a  filled up) write
        buffer. Otherwise it returns the character at pptr(), incrementing
        its position;

o << calls overflow if there's no (or a filled up) write
        buffer. Otherwise the argument may be converted to characters (like
        when insterting an int value) and the resulting characters are
        inserted into the device, refreshing the buffer when needed;

o write calls xsputn (which itself calls overflow) if there's
        no or a filled up wrte buffer, and tries to write the requested number
        of characters to the device.


14.8.2.5: The member xsgetn

        A stream's read member calls xsgetn to read nChars characters from
the device. The nChar characters might not be available in the current
buffer. In those cases underflow is called to refresh the
buffer. Initially some bytes may already be available. At each cycle the
number of available characters are copied to the next location of its buf
parameter, calling underflow if there are no available characters
anymore. So xsgetn, while there are (still) characters to be read from the
device, must
    
    o  determine whether the current buffer contains available characters;
    o  if not, it must call underflow;
    o  the number of available characters are copied to the next position of
        the buf parameter;
    o  the number of bytes to read is reduced by the number of available
        bytes; 
    o  the buf pointer and the counters are updated using the
        number of read bytes. 

Here is a skeleton of xsgetn:
    streamsize StreamBuf::xsgetn(char *buf, streamsize nChars)
{
    size_t toRead = nChars;
    size_t nRead = 0;

    while (toRead)
    {
        size_t avail = egptr() - gptr();    // available buffer space

                                            // no or empty memory buffer
                                            // but no more readable chars
        if (avail == 0 and underflow() == EOF)
            return nRead;

        avail = min(getend, bufend) - offset;
        size_t next = min(avail, toRead);   // next #bytes to write

        memcpy(buf, gptr(), next);          // write to the buffer
        gbump(next);                        // update gptr

        buf += next;                        // update the buf location
        toRead -= next;                     // and update the counters
        nRead += next;
        offset += next;
    }

    d_last = READ;                  // now reading: also if underflow
                                    // wasn't called.
    return nRead;
}



14.8.2.6: The member xsputn

        A stream's write member calls xputn to write nChars characters
into the device. As with xsgetn the nChar characters might not be
available in the current buffer. In those cases overflow is called to
refresh the buffer. Room for some bytes may still be available, and at each
cycle the number of available locations are copied from the member's buf
parameter to the streambuf's write buffer, calling overflow if there's
no space available anymore in the current write buffer. So xsputn, while
there are (still) characters to be written to the device, must
    
    o  determine whether the current buffer contains some space;
    o  if not, it must call overflow;
    o  the number of available characters are copied to the device's 
        write-buffer;
    o  the number of bytes to write is reduced by the number of available
        bytes; 
    o  the buf pointer and the counters are updated using the
        number of written bytes. 

Here is a skeleton of xsputn:
    streamsize StreamBuf::xsputn(char const *buf, streamsize nChars)
{
    size_t toWrite = nChars;
    size_t nWritten = 0;

    size_t offs = getOffset();
    size_t avail = epptr() - pptr();        // available buffer space

    while (toWrite)
    {
        if (avail == 0)                     // no space: try to reload
        {                                   // the buffer
            if (overflow(*buf) == EOF)      // storage space exhausted?
                break;                      // yes: done

            ++buf;                          // no: 1 byte was written
            ++nWritten;
            ++offs;

            --toWrite;

            avail = epptr() - pptr();       // remaining buffer space

            if (avail == 0)                 // next cycle if avail == 0
                continue;
        }

        size_t next = min(avail, toWrite);  // next #bytes to write

        memcpy(pptr(), buf, next);          // write to the buffer
        pbump(next);                        // update pptr

        buf += next;                        // update the buf location
        nWritten += next;                   // and update the counters
        toWrite -= next;
        offset = offs += next;
    }

    if (getend < offs + nWritten)           // maybe enlarge the reading
        getend = offs + nWritten;           // area

    d_last = WRITE;                         // WRITE state: now writing,
                                            // maybe overflow wasn't used
    return nWritten;
}



14.8.3: The class `filebuf'

    The class filebuf is a specialization of streambuf used by the
file stream classes. Before using a filebuf the header file
<fstream> must be included.

In addition to the (public) members that are available through the class
streambuf, filebuf offers the following (public) members:
    
    o filebuf():
Filebuf offers a public constructor. It initializes
        a plain filebuf object that is not yet connected to a stream.
    o bool is_open():
True is returned if the filebuf is actually connected to an
        open file, false otherwise. See the open member, below.
    o filebuf *open(char const *name, ios::openmode mode):
Associates the filebuf object with a file
        whose name is provided. The file is opened according to the provided
        openmode.
    o filebuf *close():
Closes the association between the filebuf
        object and its file. The association is automatically closed when the
        filebuf object ceases to exist.


14.9: A polymorphic exception class

Earlier in the C++ Annotations (section 10.3.1) we hinted at the
possibility of designing a class Exception whose process member
would behave differently, depending on the kind of exception that was
thrown. Now that we've introduced polymorphism we can further develop this
example.

It probably does not come as a surprise that our class Exception should be
a polymorphic base class from which special exception handling classes can be
derived. In section 10.3.1 a member severity was used offering
functionality that may be replaced by members of the Exception base class.

The base class Exception may be  designed as follows:
            #ifndef INCLUDED_EXCEPTION_H_
    #define INCLUDED_EXCEPTION_H_
    #include <iostream>
    #include <string>

    class Exception
    {
        std::string d_reason;

        public:
            Exception(std::string const &reason);
            virtual ~Exception();

            std::ostream &insertInto(std::ostream &out) const;
            void handle() const;

        private:
            virtual void action() const;
    };

    inline void Exception::action() const
    {
        throw;
    }
    inline Exception::Exception(std::string const &reason)
    :
        d_reason(reason)
    {}
    inline void Exception::handle() const
    {
        action();
    }
    inline std::ostream &Exception::insertInto(std::ostream &out) const
    {
        return out << d_reason;
    }

    inline std::ostream &operator<<(std::ostream &out, Exception const &e)
    {
        return e.insertInto(out);
    }

    #endif

    Objects of this class may be inserted into ostreams but the core
element of this class is the virtual member function action, by default
rethrowing an exception.

A derived class Warning simply prefixes the thrown warning text by the
text Warning:, but a derived class Fatal overrides
Exception::action by calling std::terminate, forcefully
terminating the program.

Here are the classes Warning and Fatal
            #ifndef WARNINGEXCEPTION_H_
    #define WARNINGEXCEPTION_H_

    #include "exception.h"

    class Warning: public Exception
    {
        public:
            Warning(std::string const &reason)
            :
                Exception("Warning: " + reason)
            {}
    };
    #endif

            #ifndef FATAL_H_
    #define FATAL_H_

    #include "exception.h"

    class Fatal: public Exception
    {
        public:
            Fatal(std::string  const &reason);
        private:
            void action() const override;
    };

    inline Fatal::Fatal(std::string  const &reason)
    :
        Exception(reason)
    {}

    inline void Fatal::action() const
    {
        std::cout << "Fatal::action() terminates" << '\n';
        std::terminate();
    }

    #endif


When the example program is started without arguments it throws a
Fatal exception, otherwise it throws a Warning exception. Of
course, additional exception types could also easily be defined. To make the
example compilable the Exception destructor is defined above main. The
default destructor cannot be used, as it is a virtual destructor. In practice
the destructor should be defined in its own little source file:
            #include "warning.h"
    #include "fatal.h"

    Exception::~Exception()
    {}

    using namespace std;

    int main(int argc, char **argv)
    try
    {
        try
        {
            if (argc == 1)
                throw Fatal("Missing Argument") ;
            else
                throw Warning("the argument is ignored");
        }
        catch (Exception const &e)
        {
            cout << e << '\n';
            e.handle();
        }
    }
    catch (...)
    {
        cout << "caught rethrown exception\n";
    }



14.10: How polymorphism is implemented

This section briefly describes how  polymorphism is
implemented in C++.  It is not necessary to understand how polymorphism is
implemented if you just want to use polymorphism. However, we think it's
nice to know how polymorphism is possible. Also, knowing how polymorphism is
implemented clarifies why there is a (small) penalty to using polymorphism in
terms of memory usage and efficiency.

The fundamental idea behind polymorphism is that the compiler does not know
which function to call at compile-time. The appropriate function is
selected at run-time. That means that the address of the function must be
available somewhere, to be looked up prior to the actual call. This
`somewhere' place must be accessible to the object in question. So when a
Vehicle *vp points to a Truck object, then vp->mass() calls
Truck's member function. The address of this function is obtained through
the actual object to which vp points.

Polymorphism is commonly implemented as follows: an object containing virtual
member functions also contains, usually as its first data member a
 hidden data member, pointing to an array containing the addresses of the
class's virtual member functions. The hidden data member is usually called the
vpointer, the array of virtual member function addresses the vtable.

The class's vtable is shared by all objects of that class.  The overhead of
polymorphism in terms of memory consumption is therefore:
    
    o  one vpointer data member per object pointing to:
    o  one vtable per class.

Consequently, a statement like vp->mass first inspects the hidden
data member of the object pointed to by vp. In the case of the vehicle
classification system, this data member points to a table containing two
addresses: one pointer to the function mass and one pointer to the
function setMass (three pointers if the class also defines (as it
should) a virtual destructor). The actually called function is determined from
this table.

The internal organization of the objects having virtual functions is
illustrated in Figure 21 and Figure 22
(originals provided by Guillaume Caumon).

------------------------------------------------------------------Insert Figure 21(Internal organization objects when virtual functions are defined.)about here (file: polymorphism/implementation)------------------------------------------------------------------

------------------------------------------------------------------Insert Figure 22(Complementary figure, provided by Guillaume Caumon)about here (file: polymorphism/caumon)------------------------------------------------------------------

As shown by Figure 21 and Figure 22,
objects potentially using virtual member functions must have one (hidden) data
member to address a table of function pointers. The objects of the classes
Vehicle and Car both address the same table. The class Truck,
however, overrides mass. Consequently, Truck needs its own vtable.

A small complication arises when a class is derived from multiple base
classes, each defining virtual functions. Consider the following example:
            class Base1
    {
        public:
            virtual ~Base1();
            void fun1();        // calls vOne and vTwo
        private:
            virtual void vOne();
            virtual void vTwo();
    };
    class Base2
    {
        public:
            virtual ~Base2();
            void fun2();        // calls vThree
        private:
            virtual void vThree();
    };
    class Derived: public Base1, public Base2
    {
        public:
            ~Derived() override;
        private:
            void vOne() override;
            void vThree() override;
    };

In the example Derived is multiply derived from Base1 and Base2,
each supporting virtual functions. Because of this, Derived also has
virtual functions, and so Derived has a vtable allowing a base class
pointer or reference to access the proper virtual member.

When Derived::fun1 is called (or a Base1 pointer pointing to a
Derived object calls fun1) then fun1 calls Derived::vOne and
Base1::vTwo. Likewise, when Derived::fun2 is called
Derived::vThree is called.

The complication
  occurs
with Derived's vtable. When fun1 is called its class type determines
the vtable to use and hence which virtual member to call. So when vOne is
called from fun1, it is presumably the second entry in Derived's
vtable, as it must match the second entry in Base1's vtable. However, when
fun2 calls vThree it apparently is also the second entry in
Derived's vtable as it must match the second entry in Base2's vtable.

Of course this cannot be realized by a single vtable. Therefore, when multiple
inheritance is used (each base class defining virtual members) another
approach is followed to determine which virtual function to call. In this
situation (cf. figure Figure 23) the class Derived receives
two vtables, one for each of its base classes and each Derived
class object harbors two hidden vpointers, each one pointing to its
corresponding vtable.

------------------------------------------------------------------Insert Figure 23(Vtables and vpointers with multiple base classes)about here (file: polymorphism/multivtable)------------------------------------------------------------------

Since base class pointers, base class references, or base class interface
members unambiguously refer to one of the base classes the compiler can
determine which vpointer to use.

The following therefore holds true for classes multiply derived from base
classes offering virtual member functions:
    
    o  the derived class defines a vtable for each of its base classes
offering virtual members;
    o  Each derived class object contains as many hidden vpointers as it has
vtables.
    o  Each of a derived class object's vpointers points to a unique vtable
and the vpointer to use is determined by the class type of the base class
pointer, the base class reference, or the base class interface function that
is used.


14.11: Undefined reference to vtable ...

Occasionaly, the linker generates an error like the following:
  
            In function `Derived::Derived()':
        : undefined reference to `vtable for Derived'

This error is generated when a virtual function's implementation is
missing in a derived class, but the function is mentioned in the derived
class's interface.

Such a situation is easily encountered:
    
    o  Construct a (complete) base class defining a virtual member function;
    o  Construct a Derived class mentioning the virtual function in its
        interface;
    o  The Derived class's virtual function is not implemented. Of course,
the compiler doesn't know that the derived class's function is not implemented
and will, when asked, generate code to create a derived class object;
    o  Eventually, the linker is unable to find the derived class's virtual
member function. Therefore, it is unable to construct the derived class's
vtable;
    o  The linker complains with the message:
    undefined reference to `vtable for Derived'

Here is an example producing the error:
            class Base
    {
        virtual void member();
    };
    inline void Base::member()
    {}
    class Derived: public Base
    {
        void member() override;      // only declared
    };
    int main()
    {
        Derived d;  // Will compile, since all members were declared.
                    // Linking will fail, since we don't have the
                    // implementation of Derived::member()
    }

    It's of course easy to correct the error: implement the derived class's
missing virtual member function.

Virtual functions should never be implemented inline. Since the vtable
contains the addresses of the class's virtual functions, these functions must
have addresses and so they must have been compiled as real (out-of-line)
functions. By defining virtual functions inline you run the risk that the
compiler simply overlooks those functions as they may very well never be
explicitly called (but only polymorphically, from a base class pointer or
reference). As a result their addresses may never enter their class's vtables
(and even the vtable itself might remain undefined), causing linkage problems
or resulting in programs showing unexpected behavior. All these kinds of
problems are simply avoided: never define virtual members inline (see also
section 7.8.2.1).


14.12: Virtual constructors

In section 14.2 we learned that C++ supports virtual
destructors. Like many other object oriented languages (e.g., Java),
however, the notion of a virtual constructor is not supported. Not having
virtual constructors becomes a liability when only base class references or
pointers are available, and a copy of a derived class object is
required. Gamma et al. (1995)  discuss the
 Prototype design pattern  to deal with this
situation.

According to the Prototype Design Pattern each derived class is given
the responsibility of implementing a member function returning a pointer to a
copy of the object for which the member is called. The usual name for this
function is clone. Separating the user interface from the reimplementation
interface clone is made part of the interface and newCopy is defined
in the reimplementation interface. A base class supporting `cloning' defines a
virtual destructor, clone, returning newCopy's return value and the
virtual copy constructor, a pure virtual function, having the prototype
virtual Base *newCopy() const = 0. As newCopy is a pure virtual
function all derived classes must now implement their own `virtual
constructor'.

This setup suffices in most situations where we have a pointer or
reference to a base class, but it fails when used with abstract
containers. We can't create a vector<Base>, with Base featuring the
pure virtual copy member in its interface, as Base is called to
initialize new elements of such a vector. This is impossible as newCopy is a
pure virtual function, so a Base object can't be constructed.

The intuitive solution, providing newCopy with a default
implementation, defining it as an ordinary virtual function, fails too as the
container calls Base(Base const &other), which would have to call
newCopy to copy other.  At this point it is unclear what to do with
that copy, as the new Base object already exists, and contains no Base
pointer or reference data member to assign newCopy's return value to.

Alternatively (and preferred) the original Base class (defined as an
abstract base class) is kept as-is and a wrapper class Clonable is used
to manage the Base class pointers returned by newCopy. In chapter
17 ways to merge Base and Clonable into one class are
discussed, but for now we'll define Base and Clonable as separate
classes.

The class Clonable is a very standard class. It contains a pointer member
so it needs a copy constructor, destructor, and overloaded assignment
operator. It's given at least one non-standard member: Base &base() const,
returning a reference to the derived object to which Clonable's Base *
data member refers. It is also provided with an additional constructor to
initialize its Base * data member.

Any non-abstract class derived from Base must implement Base
*newCopy(), returning a pointer to a newly created (allocated) copy of the
object for which newCopy is called.

Once we have defined a derived class (e.g., Derived1), we can put our
Clonable and Base facilities to good use.  In the next example we see
main defining a vector<Clonable>. An anonymous Derived1
object is then inserted into the vector using the following steps:
    
    o  A new anonymous Derived1 object is created;
    o  It initializes a Clonable using Clonable(Base *bp);
    o  The just created Clonable object is inserted into the vector,
using Clonable's move constructor. There are only temporary Derived
and Clonable objects at this point, so no copy construction is required.

In this sequence, only the Clonable object containing the Derived1
* is used. No additional copies need to be made (or destroyed).

Next, the base member is used in combination with typeid to show
the actual type of the Base & object: a Derived1 object.

Main then contains the interesting definition vector<Clonable>
v2(bv). Here a copy of bv is created. This copy construction observes
the actual types of the Base references, making sure that the appropriate
types appear in the vector's copy.

At the end of the program, we have created two Derived1 objects, which
are correctly deleted by the vector's destructors. Here is the full program,
illustrating the `virtual constructor' concept\ (
   Jesse van den Kieboom created an alternative implementation of a class
    Clonable, implemented as a class template. His
    implementation is found in the source archive under
    contrib/classtemplates/.): 
            #include <iostream>
    #include <vector>
    #include <algorithm>
    #include <typeinfo>

// Base and its inline member:
    class Base
    {
        public:
            virtual ~Base();
            Base *clone() const;
        private:
            virtual Base *newCopy() const = 0;
    };
    inline Base *Base::clone() const
    {
        return newCopy();
    }

// Clonable and its inline members:
    class Clonable
    {
        Base *d_bp;

        public:
            Clonable();
            explicit Clonable(Base *base);
            ~Clonable();
            Clonable(Clonable const &other);
            Clonable(Clonable &&tmp);
            Clonable &operator=(Clonable const &other);
            Clonable &operator=(Clonable &&tmp);

            Base &base() const;
    };
    inline Clonable::Clonable()
    :
        d_bp(0)
    {}
    inline Clonable::Clonable(Base *bp)
    :
        d_bp(bp)
    {}
    inline Clonable::Clonable(Clonable const &other)
    :
        d_bp(other.d_bp->clone())
    {}
    inline Clonable::Clonable(Clonable &&tmp)
    :
        d_bp(tmp.d_bp)
    {
        tmp.d_bp = 0;
    }
    inline Clonable::~Clonable()
    {
        delete d_bp;
    }
    inline Base &Clonable::base() const
    {
        return *d_bp;
    }

// Derived and its inline member:
    class Derived1: public Base
    {
        public:
            ~Derived1() override;
        private:
            Base *newCopy() const override;
    };
    inline Base *Derived1::newCopy() const
    {
        return new Derived1(*this);
    }

// Members not implemented inline:
    Base::~Base()
    {}
    Clonable &Clonable::operator=(Clonable const &other)
    {
        Clonable tmp(other);
        std::swap(d_bp, tmp.d_bp);
        return *this;
    }
    Clonable &Clonable::operator=(Clonable &&tmp)
    {
        std::swap(d_bp, tmp.d_bp);
        return *this;
    }
    Derived1::~Derived1()
    {
        std::cout << "~Derived1() called\n";
    }

// The main function:
    using namespace std;

    int main()
    {
        vector<Clonable> bv;

        bv.push_back(Clonable(new Derived1()));
        cout << "bv[0].name: " << typeid(bv[0].base()).name() << '\n';

        vector<Clonable> v2(bv);
        cout << "v2[0].name: " << typeid(v2[0].base()).name() << '\n';
    }
    /*
        Output:
            bv[0].name: 8Derived1
            v2[0].name: 8Derived1
            ~Derived1() called
            ~Derived1() called
    */




Chapter 15: Friends

In all examples discussed up to now, we've seen that private members are
only accessible by the members of their class. This is good, as it
enforces encapsulation and data hiding. By encapsulating functionality
within a class we prevent that a class exposes multiple responsibilities; by
hiding data we promote a class's data integrity and we prevent that other
parts of the software become implementation dependent on the data that
belong to a class.

In this (very) short chapter we introduce the friend keyword and the
principles that underly its use. The bottom line being that by using the
friend keyword functions are granted access to a class's private
members. Even so, this does not imply that the principle of data hiding is
abandoned when the friend keyword is used.

In this chapter the topic of
    friendship among classes is not discussed. Situations in which it is
natural to use friendship among classes are discussed in chapters
17 and 21 and such situations are natural
extensions of the way friendship is handled for functions.

There should be a well-defined conceptual reason for declaring friendship
(i.e., using the friend keyword). The traditionally offered definition of
the  class concept usually looks something like
this:
    A class is a set of data together with the functions that operate
        on that data.

As we've seen in chapter 11 some functions have to be defined
outside of a class interface. They are defined outside of the class interface
to allow promotions for their operands or to extend the facilities of existing
classes not directly under our control. According to the above traditional
definition of the class concept those functions that cannot be defined in the
class interface itself should nevertheless be considered functions belonging
to the class. Stated otherwise: if permitted by the language's syntax they
would certainly have been defined inside the class interface. There are two
ways to implement such functions. One way consists of implementing those
functions using available public member functions. This approach was used,
e.g., in section 11.2. Another approach applies the definition of
the class concept to those functions. By stating that those functions in fact
belong to the class they should be given direct access to the data members of
objects. This is accomplished by the friend keyword.

As a general principle
     we state that all functions
operating on the data of objects of a class that are declared in the same file
as the class interface itself belong to that class and may be granted direct
access to the class's data members.


15.1: Friend functions

        In section 11.2 the insertion operator of the class
Person (cf. section 9.3) was implemented like this:
            ostream &operator<<(ostream &out, Person const &person)
    {
        return
            out <<
                "Name:    " << person.name() <<  ", "
                "Address: " << person.address() <<  ", "
                "Phone:   " << person.phone();
    }

Person objects can now be inserted into streams.

However, this implementation required three member functions to be called,
which may be considered a source of inefficiency. An improvement would be
reached by defining a member Person::insertInto and let operator<<
call that function.  These two functions could be defined as follows:
            std::ostream &operator<<(std::ostream &out, Person const &person)
    {
        return person.insertInto(out);
    }
    std::ostream &Person::insertInto(std::ostream &out)
    {
        return
            out << "Name:    " << d_name << ", "
                   "Address: " << d_address << ", "
                   "Phone:   " << d_phone;
    }

As insertInto is a member function it has direct access to the
object's data members so no additional member functions must be called when
inserting person into out.

The next step consists of realizing that insertInto is only defined
for the benefit of operator<<, and that operator<<, as it is declared
in the header file containing Person's class interface should be
considered a function belonging to the class Person. The member
insertInto can therefore be omitted when operator<< is declared as a
friend.

Friend functions must be declared as friends in the class interface. These
 friend declarations are not member
functions, and so they are independent of the class's private, protected
and public sections. Friend declaration may be placed anywhere in the
class interface. Convention dictates that friend declarations are listed
directly at the top of the class interface. The class Person, using
friend declaration for its extraction and insertion operators starts like
this:
            class Person
    {
        friend std::ostream &operator<<(std::ostream &out, Person &pd);
        friend std::istream &operator>>(std::istream &in, Person &pd);

        // previously shown interface (data and functions)
    };

The insertion operator may now directly access a Person object's data
members:
            std::ostream &operator<<(std::ostream &out, Person const &person)
    {
        return
            cout << "Name:    " << person.d_name << ", "
                    "Address: " << person.d_address << ", "
                    "Phone:   " << person.d_phone;
    }

Friend declarations are true declarations. Once a class contains friend
declarations these friend functions do not have to be declared again below the
class's interface. This also clearly indicates the class designer's intent:
the friend functions are declared by the class, and can thus be considered
functions belonging to the class.


15.2: Extended friend declarations

    C++ has added 
    extended friend declarations
    
        to the language. When a class is declared as a friend, then the
class keyword no longer has to be provided. E.g.,
            class Friend;                   // declare a class
    using FriendClass = Friend;           // and maybe a using declaration

    class Class1
    {
        friend FriendClass;              // Friend: also OK
    };

In the pre-C++11 standards the friend declaration required an explicit
class; e.g., friend class Friend.

The explicit use of class remains required if the compiler hasn't seen
the friend's name yet. E.g.,
            class Class1
    {
        // friend Unseen;           // fails to compile: Unseen unknown.
        friend class Unseen;        // OK
    };

Section 22.10 covers the use of extended friend declarations in
class templates.



Chapter 16: Classes Having Pointers To Members

Classes having pointer data members have been discussed in detail in
chapter 9.  Classes defining pointer data-members deserve some
special attention, as they usually require the definitions of copy
constructors, overloaded assignment operators and destructors


Situations exist where we do not need a pointer to an object but rather a
pointer to members of a class. Pointers to members can profitably be used
to configure the behavior of objects of classes. Depending on which member a
pointer to a member points to objects will show certain behavior.

Although pointers to members have their use, polymorphism can frequently be
used to realize comparable behavior. Consider a class having a member
process performing one of a series of alternate behaviors. Instead of
selecting the behavior of choice at object construction time the class could
use the interface of some (abstract) base class, passing an object of some
derived class to its constructor and could thus configure its behavior. This
allows for easy, extensible and flexible configuration, but access to the
class's data members would be less flexible and would possibly require the use
of `friend' declarations. In such cases pointers to members may actually be
preferred as this allows for (somewhat less flexible) configuration as well as
direct access to a class's data members.

So the choice apparently is between on the one hand ease of configuration and
on the other hand ease of access to a class's data members. In this chapter
we'll concentrate on pointers to members, investigating what these pointers
have to offer.


16.1: Pointers to members: an example

    Knowing how pointers to variables and objects are used does not intuitively
lead to the concept of pointers to members . Even if
the return types and parameter types of member functions are taken into
account, surprises can easily be encountered.  For example, consider the
following class:
            class String
    {
        char const *(*d_sp)() const;

        public:
            char const *get() const;
    };

For this class, it is not possible to let char const *(*d_sp)() const
point to the String::get member function as d_sp cannot be given the
address of the member function get.

One of the reasons why this doesn't work is that the variable d_sp has
global scope (it is a pointer to a function, not a
pointer to a function within String), while the member function get is
defined within the String class, and thus has class scope. The fact
that d_sp is a data member of the class String is irrelevant
here. According to d_sp's definition, it points to a function living
somewhere outside of the class.

Consequently, to define a pointer to a member (either data or function, but
usually a function) of a class, the scope of the pointer must indicate
class scope.  Doing so, a pointer to the member
String::get is defined like this:
            char const *(String::*d_sp)() const;

So, by prefixing the *d_sp pointer data member by String::, it is
defined as a pointer in the context of the class String. According to its
definition it is a pointer to a function in the class String, not
expecting arguments, not modifying its object's data, and returning a pointer
to constant characters.


16.2: Defining  pointers to members

         
    Pointers to members are defined by prefixing the normal pointer notation
with the appropriate class plus scope resolution operator. Therefore, in the
previous section, we used char const * (String::*d_sp)() const to indicate
that d_sp
    
    o  is a pointer (*d_sp);
    o  points to something in the class String (String::*d_sp);
    o  is a pointer to a const function, returning a char const *
            (char const * (String::*d_sp)() const).

The prototype of a matching function is therefore:
            char const *String::somefun() const;

which is any const parameterless function in the class
String, returning a char const *.

When defining pointers to members the standard procedure for constructing
pointers to functions can still be applied:
    
    o  put parentheses around the fully qualified function name (i.e., the
function's header, including the function's  class name):
        char const * ( String::somefun ) () const

o  Put a pointer (a star (*)) character immediately before the
function name itself:
        char const * ( String:: * somefun ) () const

o  Replace the function name with the name of the pointer variable:
        char const * (String::*d_sp)() const

 
    Here is another example, defining a pointer
to a data member. Assume the class String contains a string d_text
member. How to construct a pointer to this member? Again we follow standard
procedure:
    
    o  put parentheses around the fully qualified variable name:
        std::string (String::d_text)

o  Put a pointer (a star (*)) character immediately before the
variable-name itself:
        std::string (String::*d_text)

o  Replace the variable name with the name of the pointer variable:
        std::string (String::*tp)

In this case, the parentheses are superfluous and may be omitted:
        string String::*tp

Alternatively, a very simple rule of thumb is
    
    o  Define a normal (i.e., global) pointer variable,
    o  Prefix the class name to the pointer character, once you point to
something inside a class

For example, the following pointer to a global function
            char const * (*sp)() const;

becomes a pointer to a member function after prefixing the class-scope:
            char const * (String::*sp)() const;

Nothing forces us to define pointers to members in their target
(String) classes. Pointers to members may be defined in their target
classes (so they become data members), or in another class, or as a local
variable or as a global variable. In all these cases the pointer to member
variable can be given the address of the kind of member it points to. The
important part is that a pointer to member can be initialized or assigned
without requiring the existence of an object of the pointer's target class.

Initializing or assigning an address to such a pointer merely indicates to
which member the pointer points. This can be considered some kind of
 relative address; relative to the object for which the function is
called. No object is required when pointers to members are initialized or
assigned. While it is allowed to initialize or assign a pointer to member, it
is (of course) not possible to call those members without specifying an
object of the correct type.


     
In the following example initialization of and assignment to
pointers to members is illustrated (for illustration purposes all members of
the class PointerDemo are defined public). In the example itself the
&-operator is used to determine the addresses of the members. These
operators as well as the class-scopes are required. Even when used inside
member implementations:
            #include <cstddef>

    class PointerDemo
    {
        public:
            size_t d_value;
            size_t get() const;
    };

    inline size_t PointerDemo::get() const
    {
        return d_value;
    }

    int main()
    {                                           // initialization
        size_t (PointerDemo::*getPtr)() const = &PointerDemo::get;
        size_t PointerDemo::*valuePtr         = &PointerDemo::d_value;

        getPtr   = &PointerDemo::get;           // assignment
        valuePtr = &PointerDemo::d_value;
    }

    This involves nothing special. The difference with pointers at
global scope is that we're now restricting ourselves to the scope of the
PointerDemo class. Because of this restriction, all pointer
definitions and all variables whose addresses are used must be given the
PointerDemo class scope.


    Pointers to members can also be used with virtual
    member functions. No special syntax is required when pointing to virtual
members. Pointer construction, initialization and assignment is done
identically to the way it is done with non-virtual members.


16.3: Using pointers to members

    Using pointers to members to call a member function requires the existence of
an object of the class of the members to which the pointer to member refers
to.  With pointers operating at global scope, the dereferencing operator *
is used.  With pointers to objects the field selector operator operating on
pointers (->) or the field selector operating operating on objects (.)
can be used to select appropriate members.

 
To use a pointer to member in combination with an object the
    pointer to member field selector (.*) must be specified.  To use a
pointer to a member via a pointer to an object the `pointer to member field
selector through a pointer to an object' (->*) must be specified. These
two operators combine the notions of a field selection (the . and ->
parts) to reach the appropriate field in an object and of dereferencing: a
dereference operation is used to reach the function or variable the pointer to
member points to.

Using the example from the previous section, let's see how we can use
pointers to member functions and pointers to data members:
            #include <iostream>

    class PointerDemo
    {
        public:
            size_t d_value;
            size_t get() const;
    };

    inline size_t PointerDemo::get() const
    {
        return d_value;
    }

    using namespace std;

    int main()
    {                                           // initialization
        size_t (PointerDemo::*getPtr)() const = &PointerDemo::get;
        size_t PointerDemo::*valuePtr   = &PointerDemo::d_value;

        PointerDemo object;                     // (1) (see text)
        PointerDemo *ptr = &object;

        object.*valuePtr = 12345;               // (2)
        cout << object.*valuePtr << '\n' <<
                object.d_value << '\n';

        ptr->*valuePtr = 54321;                 // (3)
        cout << object.d_value << '\n' <<
                (object.*getPtr)() << '\n' <<   // (4)
                (ptr->*getPtr)() << '\n';
    }

    We note:
    
    o  At (1) a PointerDemo object and (in the next line) a pointer to
such an object is defined.
    o  At (2) we specify an object (and hence the .* operator) to reach
the member valuePtr points to. This member is given a value.
    o  At (3) the same member is assigned another value, but this time using
the pointer to a PointerDemo object. Hence we use the ->* operator.
    o  At (4) the .* and ->* are used once again, this time to call
a function through a pointer to member. As the function argument list
has a higher priority than the pointer to member field selector
operator, the latter must be protected by parentheses.

Pointers to members can be used profitably in situations where a class has
a member that behaves differently depending on a configuration
setting. Consider once again the class Person from section
9.3. Person defines data members holding a person's name,
address and phone number. Assume we want to construct a Person
database of employees.  The employee database can be queried, but depending on
the kind of person querying the database either the name, the name and phone
number or all stored information about the person is made available. This
implies that a member function like address must return something like
`<not available>' in cases where the person querying the database is not
allowed to see the person's address, and the actual address in other cases.

The employee database is opened specifying an argument reflecting the
status of the employee who wants to make some queries. The status could
reflect his or her position in the organization, like BOARD,
SUPERVISOR, SALESPERSON, or CLERK. The first two categories are
allowed to see all information about the employees, a SALESPERSON is
allowed to see the employee's phone numbers, while the CLERK is only
allowed to verify whether a person is actually a member of the organization.

We now construct a member string personInfo(char const *name) in the
database class. A standard implementation of this class could be:
            string PersonData::personInfo(char const *name)
    {
        Person *p = lookup(name);   // see if `name' exists

        if (!p)
            return "not found";

        switch (d_category)
        {
            case BOARD:
            case SUPERVISOR:
                return allInfo(p);
            case SALESPERSON:
                return noPhone(p);
            case CLERK:
                return nameOnly(p);
        }
    }

Although it doesn't take much time, the switch must nonetheless be
evaluated every time personInfo is called. Instead of using a switch, we
could define a member d_infoPtr as a pointer to a member function of the
class PersonData returning a string and expecting a pointer to a
Person as its argument.

Instead of evaluating the switch this pointer can be used to point to
allInfo, noPhone or nameOnly. Furthermore, the member function
the pointer points to will be known by the time the PersonData
object is constructed and so its value needs to be determined only once (at
the PersonData object's construction time).

Having initialized d_infoPtr the personInfo member function is now
implemented simply as:
            string PersonData::personInfo(char const *name)
    {
        Person *p = lookup(name);       // see if `name' exists

        return p ? (this->*d_infoPtr)(p) :  "not found";
    }

The member d_infoPtr is defined as follows (within the class
PersonData, omitting other members):
            class PersonData
    {
        std::string (PersonData::*d_infoPtr)(Person *p);
    };

Finally, the constructor initializes d_infoPtr. This could be realized
using a simple switch:
            PersonData::PersonData(PersonData::EmployeeCategory cat)
    {
        switch (cat)
        {
            case BOARD:
            case SUPERVISOR:
                d_infoPtr = &PersonData::allInfo;
            break;
            case SALESPERSON:
                d_infoPtr = &PersonData::noPhone;
            break;
            case CLERK:
                d_infoPtr = &PersonData::nameOnly;
            break;
        }
    }

Note how addresses of member functions are determined. The class
PersonData scope must be specified, even though we're already inside
a member function of the class PersonData.

Since the EmployeeCategory values are known, the switch in the
above constructor can also easily be avoided by defining a static array of
pointers to functions. The class PersonData defines the static array:
            class PersonData
    {
        std::string (PersonData::*d_infoPtr)(Person *p);
        static std::string (PersonData::*s_infoPtr[])(Person *p);
    };

and s_infoPtr[] can be initialized compile-time:
            string (PersonData::*PersonData::s_infoPtr[])(Person *p) = 
    {
        &PersonData::allInfo,       // BOARD
        &PersonData::allInfo,       // SUPERVISOR
        &PersonData::noPhone,       // SALESPERSON
        &PersonData::nameOnly       // CLERK
    };

The constructor, instead of using a switch, now directly calls the
required member from the appropriate array element:
            PersonData::PersonData(PersonData::EmployeeCategory cat)
    :
        d_infoPtr(s_infoPtr[cat])
    {}

An example using pointers to data members is provided in section
19.1.54, in the context of the stable_sort generic algorithm.


16.4: Pointers to static members

    Static  members of a class can be used without having
available an object of their class. Public static members can be called like
free functions, albeit that their class names must be specified when they are
called.

Assume a class String has a public static member function
count, returning the number of string objects created so
far. Then, without using any String object the function
String::count may be called:
            void fun()
    {
        cout << String::count() << '\n';
    }

Public static members can be called like free functions (but see also
section 8.2.1).  Private static members can only be
called within the context of their class, by their class's member or friend
functions.

Since static members have no associated objects their addresses can be
stored in ordinary function pointer variables, operating at the global
level. Pointers to members cannot be used to store addresses of static
members. Example:
            void fun()
    {
        size_t (*pf)() = String::count;
                // initialize pf with the address of a static member function

        cout << (*pf)() << '\n';
                // displays the value returned by String::count()
    }


16.5: Pointer sizes

         An interesting
characteristic of pointers to members is that their sizes differ from those of
`normal' pointers. Consider the following little program:
            #include <string>
    #include <iostream>

    class X
    {
        public:
            void fun();
            std::string d_str;
    };
    inline void X::fun()
    {
        std::cout << "hello\n";
    }

    using namespace std;
    int main()
    {
        cout <<
           "size of pointer to data-member:     " << sizeof(&X::d_str) << "\n"
           "size of pointer to member function: " << sizeof(&X::fun) << "\n"
           "size of pointer to non-member data: " << sizeof(char *) << "\n"
           "size of pointer to free function:   " << sizeof(&printf) << '\n';
    }

    /*
        generated output (on 32-bit architectures):

        size of pointer to data-member:     4
        size of pointer to member function: 8
        size of pointer to non-member data: 4
        size of pointer to free function:   4
    */

    On a 32-bit architecture a pointer to a member function requires eight
bytes, whereas other kind of pointers require four bytes (Using GNU's g++
compiler).

Pointer sizes are hardly ever explicitly used, but their sizes may cause
confusion in statements like:
            printf("%p", &X::fun);

Of course, printf is likely not the right tool for displaying the
value of these C++ specific pointers. The values of these pointers can be
inserted into streams when a union, reinterpreting the 8-byte pointers as
a series of size_t char values, is used:
            #include <string>
    #include <iostream>
    #include <iomanip>

    class X
    {
        public:
            void fun();
            std::string d_str;
    };
    inline void X::fun()
    {
        std::cout << "hello\n";
    }

    using namespace std;
    int main()
    {
        union
        {
            void (X::*f)();
            unsigned char *cp;
        }
            u = { &X::fun };

        cout.fill('0');
        cout << hex;
        for (unsigned idx = sizeof(void (X::*)()); idx-- > 0; )
            cout << setw(2) << static_cast<unsigned>(u.cp[idx]);
        cout << '\n';
    }


But why are their sizes different from the sizes of ordinary pointers? To
answer this question let's first have a look at the familiar
std::fstream. It is derived from std::ifstream and
std::ofstream. An fstream, therefore, contains both an ifstream and
an ofstream. An fstream will be organized as shown in figure
24. 

------------------------------------------------------------------Insert Figure 24(std::fstream object organization)about here (file: pointermembers/sizes)------------------------------------------------------------------

In fstream (a) the first base class was std::istream, and the second
baseclass was std::ofstream. But it could also very well be the other way
around, as illustrated in fstream (b): first the std::ofstream, then
the std::ifstream. And that's the crux of the biscuit.

If we have an fstream fstr{"myfile"} object and do fstr.seekg(0), then
we call ifstream's seekg function. But if we do fstr.seekp(0), then we
call ofstream's seekp function. These functions have their own addresses,
say &seekg and &seekp. But when we call a member function (like
fstr.seekp(0)) then what we in fact are doing is seekp(&fstr, 0).

But the problem here is that &fstr does not represent the correct object
address: seekp operates on an ofstream, and that object does not start
at &fstr, so (in fstream (a)), at &(fstr + sizeof(ifstream)).

So, the compiler, when calling a member function of a class using inheritance,
must make a correction for the relative location of an object whose members we
are calling.

However, when we're defining something like
            ostream &(fstream::*ptr)(ios::off_type step, ios::seekdir org) = &seekp;

and then do (fstr->*)ptr(0) the compiler doesn't know anymore which
function is actually being called: it merely receives the function's
address. To solve the compiler's problem the shift (for the location of the
ofstream object) is now stored in the member pointer itself. That's one reason
why the extra data field is needed when using function pointers.

Here is a concrete illustration: first we define 2 structs, each having a
member function (all inline, using single line implementations to save some
space):
            struct A
    {
        int a;
    };

    struct B
    {
        int b;
        void bfun() {}
    };

Then we define C, which is derived from both A (first) and B (next)
(comparable to fstream, which embeds ifstream and ofstream):
            struct C: public A, public B
    {};

Next, in main we define objects of two different unions and assign the
address of B::bfun to their ptr fields, but BPTR.ptr looks at it
as a member in the struct B world, while CPTR.ptr looks at it as a
member in the struct C world.

Once the unions' pointer fields have been assigned their value[] arrays are
used to display the content of the ptr fields (see below):
            int main()
    {
        union BPTR
        {
            void (B::*ptr)();
            unsigned long value[2];
        };
        BPTR bp;
        bp.ptr = &B::bfun;
        cout << hex << bp.value[0] << ' ' << bp.value[1] << dec << '\n';

        union CPTR
        {
            void (C::*ptr)();
            unsigned long value[2];
        };
        CPTR cp;
        cp.ptr = &C::bfun;
        cout << hex << cp.value[0] << ' ' << cp.value[1] << dec << '\n';
    }

When this program is run, we see
            400b0c 0
    400b0c 4

(your address values (the first ones on the two lines) may differ). Note
that the functions' addresses are the same, but since in the C world the B
object lives beyond the A object, and the A object is 4 bytes large, we must
add 4 to the value of the `this' pointer when calling the function from a C
object. That's exactly what the shift value in the pointer's second field is
telling the compiler.



Chapter 17: Nested Classes

Classes can be defined inside other classes. Classes defined inside
other classes are called  nested classes. Nested classes
are used in situations where the nested class has a close conceptual
relationship to its surrounding class. For example, with the class string
a type string::iterator is available which provides all characters
that are stored in the string. This string::iterator type could be
defined as an object iterator, defined as nested class in the class
string. 

Since  nested classes are defined inside other classes their members, when
provided with references or pointers to objects of their surrounding classes,
may access all members of those objects, even their private members.

A class can be nested in every part of the surrounding class: in the
public, protected or private section. If a class is nested in the
public section of a class, it is
     visible outside the surrounding class. If
it is nested in the protected section it is visible in subclasses, derived
from the surrounding class, if it is nested in the private section, it is
only visible for the members of the surrounding class.

The surrounding class has no special privileges with respect to the nested
class. For example, consider the following class definition:
            class Surround
    {
        public:
            class FirstWithin
            {
                int d_variable;

                public:
                    FirstWithin();
                    int var() const;
            };
        private:
            class SecondWithin
            {
                int d_variable;

                public:
                    SecondWithin();
                    int var() const;
            };
    };
    inline int Surround::FirstWithin::var() const
    {
        return d_variable;
    }
    inline int Surround::SecondWithin::var() const
    {
        return d_variable;
    }

In the C++ Annotations, in order to save space, nested class interfaces are
usually declared inside their surrounding class, as shown above. Often this
can be avoided, which is desirable as it more clearly separates the outer
class's interface and the nested class's interface. Likewise, in-class member
implementations should be avoided. Here is an illustration of how outer- and
nested class interfaces can be separated:
            class Surround
    {
        class SecondWithin;

        public:
            class FirstWithin;
    };

    class Surround::FirstWithin
    {
        int d_variable;

        public:
            FirstWithin();
            int var() const;
    };

    class Surround::SecondWithin
    {
        int d_variable;

        public:
            SecondWithin();
            int var() const;
    };

For these three classes access to members is defined as follows:
        
        o  The class Surround::FirstWithin is visible outside and inside
Surround. The class Surround::FirstWithin thus has  global visibility.
        o  FirstWithin's constructor and its member function
var are also globally visible.
        o  The data member d_variable is only visible to the members of
the class Surround::FirstWithin. Neither the members of Surround nor
the members of Surround::SecondWithin can directly access
Surround::FirstWithin::d_variable.
        o  The class Surround::SecondWithin is only visible inside
Surround. The public members of the class Surround::SecondWithin can
also be used by the members of the class Surround::FirstWithin, as nested
classes can be considered members of their surrounding class.
        o  Surround::SecondWithin's constructor and its member function
var also can only be reached by the members of Surround (and by the
members of its nested classes).
        o  Surround::SecondWithin::d_variable is only visible to
Surround::SecondWithin's members. Neither the members of Surround nor
the members of Surround::FirstWithin can access d_variable of the
class Surround::SecondWithin directly.
        o  As always, an object of the class type is required before
its members can be called. This also holds true for nested classes.

To grant the surrounding class access rights to the private members of a
nested class the nested class may declare its surrounding class as a
friend. Conversely, as nested classes can be considered members of their
surrounding class their member functions have full access to the outer class
members, if they are provided with an outer class object (see section
17.3).

Although nested classes can be considered members of the surrounding
class, members of nested classes are not members of the surrounding
class: members of the class Surround may not directly call
FirstWithin::var. This is understandable considering that a
Surround object is not also a FirstWithin or SecondWithin
object. In fact, nested classes are just typenames. It is not implied that
objects of such classes automatically exist in the surrounding class. If a
member of the surrounding class should use a (non-static) member of a nested
class then the surrounding class must define a nested class object, which can
thereupon be used by the members of the surrounding class to use members of
the nested class.

For example, in the following class definition there is a surrounding
class Outer and a nested class Inner. The class Outer contains a
member function caller. The member function caller uses the
d_inner object that is composed within Outer to call
Inner::infunction:
            class Outer
    {
        public:
            void caller();

        private:
            class Inner
            {
                public:
                    void infunction();
            };
            Inner d_inner;      // class Inner must be known
    };
    void Outer::caller()
    {
        d_inner.infunction();
    }


17.1: Defining nested class members

Member functions of nested classes may be defined as inline functions.  Inline
  member functions can be defined as if they were
defined outside of the class definition. To define the member function
Outer::caller outside of the class Outer, the function's
  fully qualified name (starting
from the outermost class scope (Outer)) must be provided to the
compiler. Inline and in-class functions can be defined accordingly. They can
be defined and they can use any nested class. Even if the nested class's
definition appears later in the outer class's interface.

When (nested) member functions are defined inline, their definitions should be
put below their class interface. Static nested data members are also usually
defined outside of their classes.
    
        If the class FirstWithin would have had a static size_t
datamember epoch, it could have been initialized as follows:
            size_t Surround::FirstWithin::epoch = 1970;

Furthermore, multiple scope resolution operators are needed to refer to
public static members in code outside of the surrounding class:
            void showEpoch()
    {
        cout << Surround::FirstWithin::epoch;
    }

Within the class Surround only the FirstWithin::
scope must be used; within the class FirstWithin there is
no need to refer explicitly to the scope.

What about the members of the class SecondWithin? The classes
FirstWithin and SecondWithin are both nested within Surround, and
can be considered members of the surrounding class. Since members of a class
may directly refer to each other, members of the class SecondWithin can
refer to (public) members of the class FirstWithin. Consequently, members
of the class SecondWithin could refer to the epoch member of
FirstWithin as FirstWithin::epoch.


17.2: Declaring nested classes

Nested classes  may be declared before they are
actually defined in a surrounding class. Such forward declarations are
required if a class contains multiple nested classes, and the nested classes
contain pointers, references, parameters or return values to objects of the
other nested classes.

For example, the following class Outer contains two nested classes
Inner1 and Inner2. The class Inner1 contains a pointer to
Inner2 objects, and Inner2 contains a pointer to Inner1
objects. Cross references require forward declarations. Forward declarations
must be given an access specification that is identical to the access
specification of their definitions. In the following example the Inner2
forward declaration must be given in a private section, as its definition
is also part of the class Outer's private interface:
            class Outer
    {
        private:
            class Inner2;       // forward declaration

            class Inner1
            {
                Inner2 *pi2;    // points to Inner2 objects
            };
            class Inner2
            {
                Inner1 *pi1;    // points to Inner1 objects
            };
    };


17.3: Accessing private members in nested classes

To grant nested classes access rights to the private members of other nested
classes, or to grant a surrounding class access to the private members of its
nested classes the friend keyword must be used.

No friend declaration is required to grant a nested class access to the
private members of its surrounding class. Static members of the surrounding
class can directly be accessed, other members can be accessed if a surrounding
class object is defined by or passed to members of the nested class. After
all, a nested class is a type defined by its surrounding class and as such
objects of the nested class are members of the outer class and thus can access
all the outer class's members. Here is an example showing this principle. The
example won't compile as members of the class Extern are denied access to
Outer's private members, but Outer::Inner's members can access
Outer's private members:
            class Outer
    {
        int d_value;
        static int s_value;

        public:
            Outer()
            :
                d_value(12)
            {}
            class Inner
            {
                public:
                    Inner()
                    {
                        cout << "Outer's static value: " << s_value << '\n';
                    }
                    Inner(Outer &outer)
                    {
                        cout << "Outer's value: " << outer.d_value << '\n';
                    }
            };
    };
    class Extern            // won't compile!
    {
        public:
            Extern(Outer &outer)
            {
                cout << "Outer's value: " << outer.d_value << '\n';
            }

            Extern()
            {
                cout << "Outer's static value: " << Outer::s_value << '\n';
            }
    };

    int Outer::s_value = 123;
    int main()
    {
        Outer outer;
        Outer::Inner in1;
        Outer::Inner in2{ outer };
    }

Now consider the situation where a class Surround has two nested
classes FirstWithin and SecondWithin. Each of the three classes has a
static data member int s_variable:
            class Surround
    {
        static int s_variable;
        public:
            class FirstWithin
            {
                static int s_variable;
                public:
                    int value();
            };
            int value();
        private:
            class SecondWithin
            {
                static int s_variable;
                public:
                    int value();
            };
    };

If the class Surround should be able to access FirstWithin and
SecondWithin's private members, these latter two classes must declare
Surround to be their friend. The function Surround::value can
thereupon access the private members of its nested classes. For example (note
the friend declarations in the two nested classes):
            class Surround
    {
        static int s_variable;
        public:
            class FirstWithin
            {
                friend class Surround;
                static int s_variable;
                public:
                    int value();
            };
            int value();
        private:
            class SecondWithin
            {
                friend class Surround;
                static int s_variable;
                public:
                    int value();
            };
    };
    inline int Surround::FirstWithin::value()
    {
        FirstWithin::s_variable = SecondWithin::s_variable;
        return (s_variable);
    }

Friend declarations may be provided beyond the definition of the
entity that is to be considered a friend. So a class can be declared a friend
beyond its definition. In that situation in-class code may already use the
fact that it is going to be declared a friend by the upcoming class. As an
example, consider an in-class implementation of the function
Surround::FirstWithin::value. The required friend declaration can also
be inserted after the implementation of the function value:
            class Surround
    {
        public:
            class FirstWithin
            {
                static int s_variable;
                public:
                    int value();
                    {
                        FirstWithin::s_variable = SecondWithin::s_variable;
                        return s_variable;
                    }
                friend class Surround;
            };
        private:
            class SecondWithin
            {
                friend class Surround;
                static int s_variable;
            };
    };

Note that members named identically in outer and inner classes
(e.g., `s_variable')
  
    may be accessed using the proper scope resolution expressions, as
illustrated below:
            class Surround
    {
        static int s_variable;
        public:
            class FirstWithin
            {
                friend class Surround;
                static int s_variable;  // identically named
                public:
                    int value();
            };
            int value();

        private:
            class SecondWithin
            {
                friend class Surround;
                static int s_variable;  // identically named
                public:
                    int value();
            };
            static void classMember();
    };
    inline int Surround::value()
    {                                   // scope resolution expression
        FirstWithin::s_variable = SecondWithin::s_variable;
        return s_variable;
    }
    inline int Surround::FirstWithin::value()
    {
        Surround::s_variable = 4;       // scope resolution expressions
        Surround::classMember();
        return s_variable;
    }
    inline int Surround::SecondWithin::value()
    {
        Surround::s_variable = 40;      // scope resolution expression
        return s_variable;
    }

Nested classes aren't automatically each other's friends. Here friend
declarations must be provided to grant one nested classes access to another
nested class's private members. 

To grant FirstWithin access to SecondWithin's private members,
SecondWithin must contain a friend declaration.

Likewise, the class FirstWithin simply uses friend class
SecondWithin to grant SecondWithin access to FirstWithin's private
members. Even though the compiler hasn't seen SecondWithin yet, a friend
declaration is also considered a forward declaration.

Note that SecondWithin's forward declaration cannot be specified inside
FirstWithin by using `class Surround::SecondWithin;', as this would
generate an error message like:
        `Surround' does not have a nested type named `SecondWithin'

Now assume that in addition to the nested class SecondWithin there also
exists an outer-level class SecondWithin. To declare that class a friend
of FirstWithin's declare friend ::SecondWithin inside class
FirstWithin. In that case, an outer level class declaration of
FirstWithin must be provided before the compiler encounters the friend
::SecondWithin declaration.

Here is an example in which all classes have full access to all private
members of all involved classes: , and a
outer level FirstWithin has also been declared:
            class SecondWithin;

    class Surround
    {
        // class SecondWithin;      not required (but no error either): 
        //                          friend declarations (see below)
        //                          are also forward declarations

        static int s_variable;
        public:
            class FirstWithin
            {
                friend class Surround;
                friend class SecondWithin;
                friend class ::SecondWithin;

                static int s_variable;
                public:
                    int value();
            };
            int value();            // implementation given above
        private:
            class SecondWithin
            {
                friend class Surround;
                friend class FirstWithin;

                static int s_variable;
                public:
                    int value();
            };
    };
    inline int Surround::FirstWithin::value()
    {
        Surround::s_variable = SecondWithin::s_variable;
        return s_variable;
    }
    inline int Surround::SecondWithin::value()
    {
        Surround::s_variable = FirstWithin::s_variable;
        return s_variable;
    }


17.4: Nesting enumerations

Enumerations   may also be
nested in classes. Nesting enumerations is a good way to show the close
connection between the enumeration and its class. Nested enumerations have the
same controlled visibility as other class members. They may be defined in the
private, protected or public sections of classes and are inherited by derived
classes.  In the class ios we've seen values like ios::beg and
ios::cur. In the current GNU C++ implementation these values are
defined as values of the seek_dir enumeration:
            class ios: public _ios_fields
    {
        public:
            enum seek_dir
            {
                beg,
                cur,
                end
            };
    };

As an illustration assume that a class DataStructure represents a data
structure that
may be traversed in a forward or backward direction. Such a class can define
an enumeration Traversal having the values FORWARD and
BACKWARD. Furthermore, a member function setTraversal can be defined
requiring a Traversal type of argument. The class can be defined as
follows:
            class DataStructure
    {
        public:
            enum Traversal
            {
                FORWARD,
                BACKWARD
            };
            setTraversal(Traversal mode);
        private:
            Traversal
                d_mode;
    };

Within the class DataStructure the values of the Traversal
enumeration can be used directly. For example:
            void DataStructure::setTraversal(Traversal mode)
    {
        d_mode = mode;
        switch (d_mode)
        {
            FORWARD:
                // ... do something
            break;

            BACKWARD:
                // ... do something else
            break;
        }
    }

Outside of the class DataStructure the name of the enumeration type is
not used to refer to the values of the enumeration. Here the classname is
sufficient. Only if a variable of the enumeration type is required the name of
the enumeration type is needed, as illustrated by the following piece of code:
            void fun()
    {
        DataStructure::Traversal                // enum typename required
            localMode = DataStructure::FORWARD; // enum typename not required

        DataStructure ds;
                                                // enum typename not required
        ds.setTraversal(DataStructure::BACKWARD);
    }

In the above example the constant DataStructure;:FORWARD was used to
specify a value of an enum defined in the class DataStructure. Instead of
DataStructure::FORWARD the construction ds.FORWARD is also
accepted. In my opinion this syntactic liberty is ugly: FORWARD is a
symbolic value that is defined at the class level; it's not a member of ds,
which is suggested by the use of the member selector operator.

Only if DataStructure defines a nested class Nested, in
turn defining the enumeration Traversal, the two class scopes are
required. In that case the latter example should have been coded as follows:
            void fun()
    {
        DataStructure::Nested::Traversal
            localMode = DataStructure::Nested::FORWARD;

        DataStructure ds;

        ds.setTraversal(DataStructure::Nested::BACKWARD);
    }

Here a construction like DataStructure::Nested::Traversal localMode =
ds.Nested::FORWARD could also have been used, although I personally would
avoid it, as FORWARD is not a member of ds but rather a symbol that is
defined in DataStructure.


17.4.1: Empty enumerations

    Enum types usually define symbolic values. However, this is not
required. In section 14.6.1 the std::bad_cast type was
introduced. A bad_cast is thrown by the dynamic_cast<> operator
when a reference to a base class object cannot be cast to a derived
class reference. The bad_cast could be caught as type, irrespective
of any value it might represent.

Types may be defined  without any associated
values. An empty enum can be defined which is an enum not defining
any values. The empty enum's type name may thereupon be used as a legitimate
type in, e.g. a  catch clause.

The example shows how an empty enum is defined (often, but not necessarily
within a class) and how it may be thrown (and caught) as exceptions:
            #include <iostream>

    enum EmptyEnum
    {};

    int main()
    try
    {
        throw EmptyEnum();
    }
    catch (EmptyEnum)
    {
        std::cout << "Caught empty enum\n";
    }



17.5: Revisiting virtual constructors

In section 14.12 the notion of virtual constructors was introduced. In
that section a class Base was defined as an abstract base class.  A class
Clonable was defined to manage Base class pointers in containers like
vectors.

As the class Base is a minute class, hardly requiring any implementation,
it can very well be defined as a nested class in Clonable. This
emphasizes the close relationship between Clonable and
Base. Nesting Base under Clonable changes
            class Derived: public Base

into:
            class Derived: public Clonable::Base

Apart from defining Base as a nested class and deriving from
Clonable::Base rather than from Base (and providing Base members
with the proper Clonable:: prefix to complete their fully qualified
names), no further modifications are required. Here are the modified parts of
the program shown earlier (cf. section 14.12), now using Base
nested under Clonable:
        // Clonable and nested Base, including their inline members:
    class Clonable
    {
        public:
            class Base;
        private:
            Base *d_bp;
        public:
            class Base
            {
                public:
                    virtual ~Base();
                    Base *clone() const;
                private:
                    virtual Base *newCopy() const = 0;
            };
            Clonable();
            explicit Clonable(Base *base);
            ~Clonable();
            Clonable(Clonable const &other);
            Clonable(Clonable &&tmp);
            Clonable &operator=(Clonable const &other);
            Clonable &operator=(Clonable &&tmp);

            Base &base() const;
    };
    inline Clonable::Base *Clonable::Base::clone() const
    {
        return newCopy();
    }
    inline Clonable::Base &Clonable::base() const
    {
        return *d_bp;
    }

// Derived and its inline member:
    class Derived1: public Clonable::Base
    {
        public:
            ~Derived1();
        private:
            virtual Clonable::Base *newCopy() const;
    };
    inline Clonable::Base *Derived1::newCopy() const
    {
        return new Derived1(*this);
    }

// Members not implemented inline:
    Clonable::Base::~Base()
    {}




Chapter 18: The Standard Template Library

The Standard Template Library (STL) is a general purpose library
consisting of containers, generic algorithms, iterators, function objects,
allocators, adapters and data structures. The data structures used by the
algorithms are abstract in the sense that the algorithms can be used with
(practically) any data type.

The algorithms can process these abstract data types because they are
template based. This chapter does not cover template construction
(see chapter 21 for that). Rather, it focuses on the use
of the algorithms.

Several elements also used by the standard template library have already been
discussed in the C++ Annotations. In chapter 12 abstract containers
were discussed, and in section 11.11 function objects were introduced.
Also, iterators were mentioned at several places in this document.

The main components of the STL are covered in this and the next
chapter. Iterators, adapters, smart pointers, multi threading and other
features of the STL are discussed in coming sections. Generic
algorithms are covered in the next chapter (19).

Allocators take care of the memory allocation within the
STL. The default allocator class suffices for most applications, and is
not further discussed in the C++ Annotations.

All elements of the STL  are defined in the
standard namespace. Therefore, a using namespace std or a comparable
directive is required unless it is preferred to specify the required namespace
explicitly. In header files  the std namespace should explicitly
be used (cf. section 7.11.1).

In this chapter the empty angle bracket notation is frequently used. In
code a typename must be supplied between the angle brackets. E.g., plus<>
is used in the C++ Annotations, but in code plus<string> may be encountered.


18.1: Predefined function objects

Before using the predefined function objects presented in this section the
<functional> header file must be included.

Function objects play important roles in generic
algorithms. For example, there exists a generic algorithm sort
expecting two iterators defining the range of objects that should be sorted,
as well as a function object calling the appropriate comparison operator for
two objects. Let's take a quick look at this situation. Assume strings are
stored in a vector, and we want to sort the vector in descending order. In
that case, sorting the vector stringVec is as simple as:
                    sort(stringVec.begin(), stringVec.end(), greater<string>());

The last argument is recognized as a constructor: it is an
instantiation of the greater<> class template, applied to
strings. This object is called as a function object by the sort
generic algorithm. The generic algorithm calls the function object's
operator() member to compare two string objects. The function object's
operator() will, in turn, call operator> of the string data
type. Eventually, when sort returns, the first element of the vector will
contain the string having the greatest string value of all.

The function object's operator() itself is not visible at this
point. Don't confuse the parentheses in the `greater<string>()' argument
with calling operator(). When operator() is actually used inside
sort, it receives two arguments: two strings to compare for
`greaterness'. Since greater<string>::operator() is defined inline, the
call itself is not actually present in the above sort call.  Instead
sort calls string::operator> through greater<string>::operator().

Now that we know that a constructor is passed as argument to (many) generic
algorithms, we can design our own function objects. Assume we want to sort our
vector case-insensitively. How do we proceed? First we note that the default
string::operator< (for an incremental sort) is not appropriate, as it does
case sensitive comparisons. So, we provide our own CaseInsensitive class,
which compares two strings case insensitively. Using the POSIX function
strcasecmp, the following program performs the trick. It
case-insensitively sorts its command-line arguments in ascending alphabetic
order:
            #include <iostream>
    #include <string>
    #include <cstring>
    #include <algorithm>
    using namespace std;

    class CaseInsensitive
    {
        public:
            bool operator()(string const &left, string const &right) const
            {
                return strcasecmp(left.c_str(), right.c_str()) < 0;
            }
    };
    int main(int argc, char **argv)
    {
        sort(argv, argv + argc, CaseInsensitive{});
        for (int idx = 0; idx < argc; ++idx)
            cout << argv[idx] << " ";
        cout << '\n';
    }

    The default constructor of the class CaseInsensitive is used to
provide sort with its final argument. So the only member function that
must be defined is CaseInsensitive::operator(). Since we know it's called
with string arguments, we define it to expect two string arguments,
which are used when calling strcasecmp. Furthermore, the function call
operator operator() is defined inline, so that it does not produce
overhead when called by the sort function. The sort function calls the
function object with various combinations of strings. If the compiler
grants our inline requests, it will in fact call strcasecmp, skipping two
extra function calls.

The comparison function object is often a predefined function object.
Predefined function object classes are available for many commonly used
operations.  In the following sections the available predefined function
objects are presented, together with some examples showing their use. Near the
end of the section about function objects function adapters are
introduced.

Predefined function objects are used predominantly with generic
algorithms. Predefined function objects exists for arithmetic, relational, and
logical operations.


18.1.1: Arithmetic function objects

    The  arithmetic
function objects support the standard arithmetic operations: addition,
subtraction, multiplication, division, modulo and negation. These function
objects invoke the corresponding operators of the data types for which they
are instantiated. For example, for addition the function object plus<Type>
 is available. If we replace Type by size_t then the addition
operator for size_t values is used, if we replace Type by string,
the addition operator for strings is used. For example:
            #include <iostream>
    #include <string>
    #include <functional>
    using namespace std;

    int main(int argc, char **argv)
    {
        plus<size_t> uAdd;              // function object to add size_ts

        cout << "3 + 5 = " << uAdd(3, 5) << '\n';

        plus<string> sAdd;              // function object to add strings

        cout << "argv[0] + argv[1] = " << sAdd(argv[0], argv[1]) << '\n';
    }
    /*
        Output when called as: a.out going

        3 + 5 = 8
        argv[0] + argv[1] = a.outgoing
    */

    Why is this useful? Note that the function object can be used with all
kinds of data types (not only with the predefined datatypes) supporting the
operator called by the function object.

Suppose we want to perform an operation on a left hand side operand which is
always the same variable and a right hand side argument for which, in turn,
all elements of an array should be used. E.g., we want to compute the sum of
all elements in an array; or we want to concatenate all the strings in a
text-array. In situations like these function objects come in handy.

As stated, function objects are heavily used in the context of the generic
algorithms, so let's take a quick look ahead at yet another one.

The generic algorithm accumulate visits all elements specified by an
iterator-range, and performs a requested binary operation on a common element
and each of the elements in the range, returning the accumulated result after
visiting all elements specified by the iterator range. It's easy to use this
algorithm. The next program accumulates all command line arguments and prints
the final string:
            #include <iostream>
    #include <string>
    #include <functional>
    #include <numeric>
    using namespace std;

    int main(int argc, char **argv)
    {
        string result =
                accumulate(argv, argv + argc, string{}, plus<string>());

        cout << "All concatenated arguments: " << result << '\n';
    }

    The first two arguments define the (iterator) range of elements to visit,
the third argument is string. This anonymous string object provides an
initial value. We could also have used
            "All concatenated arguments: "s

in which case the cout statement could simply have been
cout << result << '\n'.
    The string-addition operation is used, called from plus<string>. The
final concatenated string is returned.

Now we define a class Time, overloading operator+. Again, we can
apply the predefined function object plus, now tailored to our newly
defined datatype, to add times:
            #include <iostream>
    #include <string>
    #include <vector>
    #include <functional>
    #include <numeric>
    using namespace std;

    class Time
    {
        friend ostream &operator<<(ostream &str, Time const &time);
        size_t d_days;
        size_t d_hours;
        size_t d_minutes;
        size_t d_seconds;
        public:
            Time(size_t hours, size_t minutes, size_t seconds);
            Time &operator+=(Time const &rhs);
    };

    Time operator+(Time const &lhs, Time const &rhs)
    {
        Time ret(lhs);
        return ret += rhs;
    }

    Time::Time(size_t hours, size_t minutes, size_t seconds)
    :
        d_days(0),
        d_hours(hours),
        d_minutes(minutes),
        d_seconds(seconds)
    {}
    Time &Time::operator+=(Time const &rhs)
    {
        d_seconds   += rhs.d_seconds;
        d_minutes   += rhs.d_minutes   + d_seconds / 60;
        d_hours     += rhs.d_hours     + d_minutes / 60;
        d_days      += rhs.d_days      + d_hours   / 24;
        d_seconds   %= 60;
        d_minutes   %= 60;
        d_hours     %= 24;
        return *this;
    }
    ostream &operator<<(ostream &str, Time const &time)
    {
        return str << time.d_days << " days, " << time.d_hours <<
                                                    " hours, " <<
                      time.d_minutes << " minutes and " <<
                      time.d_seconds << " seconds.";
    }
    int main(int argc, char **argv)
    {
        vector<Time> tvector;

        tvector.push_back(Time( 1, 10, 20));
        tvector.push_back(Time(10, 30, 40));
        tvector.push_back(Time(20, 50,  0));
        tvector.push_back(Time(30, 20, 30));

        cout <<
            accumulate
            (
                tvector.begin(), tvector.end(), Time(0, 0, 0), plus<Time>()
            ) <<
            '\n';
    }
    //  Displays: 2 days, 14 hours, 51 minutes and 30 seconds.

    The design of the above program is fairly straightforward.  Time
defines a constructor, it defines an insertion operator and it defines its own
operator+, adding two time objects. In main four Time objects are
stored in a vector<Time> object. Then, accumulate is used to compute
the accumulated time. It returns a Time object, which is inserted into
cout.

While this section's first example illustrated using a named function
object, the last two examples illustrate how anonymous objects can be
passed to the (accumulate) function.

The STL supports the following set of arithmetic function objects.  The
function call operator (operator()) of these function objects calls the
matching arithmetic operator for the objects that are passed to the function
call operator, returning that arithmetic operator's return value. The
arithmetic operator that is actually called is mentioned below:
    
    o plus<>: calls the binary operator+;
    o minus<>: calls the binary operator-;
    o multiplies<>: calls the binary operator*;
    o divides<>: calls operator/;
    o modulus<>:  calls operator%;
    o negate<>: calls the unary operator-. This arithmetic
function object is a unary function object as it expects one argument.

In the next example the transform generic algorithm is used to toggle
the signs of all elements of an array. Transform
expects two iterators, defining the range of objects to be transformed; an
iterator defining the begin of the destination range (which may be the same
iterator as the first argument); and a function object defining a unary
operation for the indicated data type.
            #include <iostream>
    #include <string>
    #include <functional>
    #include <algorithm>
    using namespace std;

    int main(int argc, char **argv)
    {
        int iArr[] = { 1, -2, 3, -4, 5, -6 };

        transform(iArr, iArr + 6, iArr, negate<int>());

        for (int idx = 0; idx < 6; ++idx)
            cout << iArr[idx] << ", ";
        cout << '\n';
    }
    // Displays:  -1, 2, -3, 4, -5, 6,



18.1.2: Relational function objects

    The relational operators are called by the
  relational
function objects. All standard relational operators are supported: ==, !=,
>, >=, < and <=.

The STL supports the following set of relational function objects.  The
function call operator (operator()) of these function objects calls the
matching relational operator for the objects that are passed to the function
call operator, returning that relational operator's return value. The
relational operator that is actually called is mentioned below:
    
    o equal_to<>: calls operator==;
    o not_equal_to<>: calls operator!=;
    o greater<>: calls operator>;
    o greater_equal<>: calls operator>=;
    o less<>: this object's member operator() calls
        operator<;
    o less_equal<>: calls operator<=.

An example using the relational function objects in combination with
sort is:
            #include <iostream>
    #include <string>
    #include <functional>
    #include <algorithm>
    using namespace std;

    int main(int argc, char **argv)
    {
        sort(argv, argv + argc, greater_equal<string>());

        for (int idx = 0; idx < argc; ++idx)
            cout << argv[idx] << " ";
        cout << '\n';

        sort(argv, argv + argc, less<string>());

        for (int idx = 0; idx < argc; ++idx)
            cout << argv[idx] << " ";
        cout << '\n';
    }

    The example illustrates how strings may be sorted alphabetically and
reversed alphabetically. By passing greater_equal<string> the strings are
sorted in decreasing order (the first word will be the 'greatest'), by
passing less<string> the strings are sorted in increasing order (the
first word will be the 'smallest').

Note that argv contains char * values, and that the relational
function object expects a string.  The promotion from char const * to
string is silently performed.


18.1.3: Logical function objects

    The logical operators are called by the
  logical function
objects. The standard logical operators are supported: and, or, and
not.

The STL supports the following set of logical function objects. The
function call operator (operator()) of these function objects calls the
matching logical operator for the objects that are passed to the function
call operator, returning that logical operator's return value. The
logical operator that is actually called is mentioned below:
    
    o logical_and<>: calls operator&&;
    o logical_or<>: calls operator||;
    o logical_not<>: calls operator!.

An example using operator! is provided in the following trivial
program, using transform  to transform
the logical values stored in an array:
            #include <iostream>
    #include <string>
    #include <functional>
    #include <algorithm>
    using namespace std;

    int main(int argc, char **argv)
    {
        bool bArr[] = {true, true, true, false, false, false};
        size_t const bArrSize = sizeof(bArr) / sizeof(bool);

        for (size_t idx = 0; idx < bArrSize; ++idx)
            cout << bArr[idx] << " ";
        cout << '\n';

        transform(bArr, bArr + bArrSize, bArr, logical_not<bool>());

        for (size_t idx = 0; idx < bArrSize; ++idx)
            cout << bArr[idx] << " ";
        cout << '\n';
    }
    /*
      Displays:

        1 1 1 0 0 0
        0 0 0 1 1 1
    */



18.1.4: The `std::not_fn' negator

    A negator is a function object toggling the truth value of
a function that's called from the negator: if the function returns true
the negator returns false and vv.

The standard negator is std::not_fn, declared in the
<functional> header file.

The function not_fn expects a (movable) object as its argument, returning
the negated value of the return value of its argument's function call
operator. 

As an example consider a main function defining an array of int
values:
            int main()
    {
        int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    }

To count the number of even values count_if, using a lambda function can
be used:
                cout << 
            count_if(arr, arr + size(arr),
                [&](int value)
                {
                    return (value & 1) == 0;
                }
            ) << '\n';

To count the number of odd values, not_fn can be used in the above code
like so:
                cout << 
            count_if(arr, arr + size(arr),
                not_fn(
                    [&](int value)
                    {
                        return (value & 1) == 0;
                    }
                )
            ) << '\n';

Of course, in this simple example the lambda function could also easily have
been modified. But if instead of a lambda function an existing class
implementing a function object had been used it would have been difficult or
impossible to change the behavior of that class. If the class offers moving
operations then not_fn can be used to negate the values returned by that
class's function call operator.


18.2: Iterators

In addition to the conceptual iterator types presented in this section the STL
defines several adapters  allowing objects to
be passed as iterators. These adapters are presented in the upcoming
sections. Before those adapters can be used the <iterator> header file
must be included. 

The standard iterator  (std::iterator) is
now
deprecated\ (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0174r1.html#2.1),
and the compiler issues a corresponding warning. Consequently,
std::iterator should no longer be used when designing your own iterators
(section 22.14 describes how to design your own).

Iterators are objects acting like pointers. Iterators have the following
general   characteristics:
    
    o  Two iterators may be compared for (in)equality using the == and
!= operators. The ordering operators (e.g., >, <)
can usually not be used.
    o  Given an iterator iter, *iter represents the object the
iterator points to (alternatively, iter-> can be used to reach the members
of the object the iterator points to).
    o  Given an iterator iter, iter.base() returns the
address of *iter. It returns the same type as &*iter. E.g.,
                vector<int> vi{ 1, 2, 3 };
    int *ip = vi.begin().base();
    cout << *ip << '\n';        // shows: 1
    o  ++iter or iter++ advances the iterator to the next
element. The notion of advancing an iterator to the next element is
consequently applied: several containers support reverse_iterator types,
in which the ++iter operation actually reaches a previous element in a
sequence.
    o  Pointer arithmetic may be used with iterators of containers
storing their elements consecutively in memory like vector and
deque. For such containers iter + 2 points to the second element
beyond the one to which iter points. See also section 18.2.1,
covering std::distance.
    o  Merely defining an iterator is comparable to having a
        0-pointer. Example:
        #include <vector>
#include <iostream>
using namespace std;

int main()
{
    vector<int>::iterator vi;

    cout << &*vi;       // prints 0
}

STL containers usually define members offering iterators (i.e., they
define their own type iterator). These members are commonly called
begin and end and (for reversed iterators (type reverse_iterator))
rbegin and rend.


    Whereas reverse iterators can be constructed from ordinary (forward)
iterators using reverse_iterator constructors as in:
        string str;
    auto revit = string::reverse_iterator{ str.begin() };

the opposite is not accomplished that way. To retrieve the forward
iterator corresponding to a reverse iterator, the reverse_iterator.base()
    
member can be used. E.g., to obtain the forward iterator corresponding to
revit use
            auto forward { revit.base() };

Standard practice requires  iterator ranges to be
left inclusive.  The notation [left, right) indicates that left
is an iterator pointing to the first element, while right is an iterator
pointing just beyond the last element. The iterator range is empty
when left == right.

The following example shows how all elements of a vector of strings can be
inserted into cout using its iterator ranges [begin(), end()), and
[rbegin(), rend()). Note that the for-loops for both ranges are
identical. Furthermore it nicely illustrates how the auto keyword can be
used to define the type of the loop control variable instead of using a much
more verbose variable definition like vector<string>::iterator (see also
section 3.3.7):
        #include <iostream>
    #include <vector>
    #include <string>
    using namespace std;

    int main(int argc, char **argv)
    {
        vector<string> args(argv, argv + argc);

        for (auto iter = args.begin(); iter != args.end(); ++iter)
            cout << *iter << " ";
        cout << '\n';

        for (auto iter = args.rbegin(); iter != args.rend(); ++iter)
            cout << *iter << " ";
        cout << '\n';
    }


Furthermore, the STL defines
 const_iterator types that must be used
when visiting a series of elements in a constant container. Whereas the
elements of the vector in the previous example could have been altered, the
elements of the vector in the next example are immutable, and
const_iterators are required:
            #include <iostream>
    #include <vector>
    #include <string>
    using namespace std;

    int main(int argc, char **argv)
    {
        vector<string> const args(argv, argv + argc);

        for
        (
            vector<string>::const_iterator iter = args.begin();
                iter != args.end();
                    ++iter
        )
            cout << *iter << " ";
        cout << '\n';

        for
        (
            vector<string>::const_reverse_iterator iter = args.rbegin();
                iter != args.rend();
                    ++iter
        )
            cout << *iter << " ";
        cout << '\n';
    }

    The examples also illustrate that plain
  pointers can be used as iterators. The
initialization vector<string> args(argv, argv + argc) provides the
args vector with a pair of pointer-based iterators: argv points to the
first element to initialize args with, argv + argc points just beyond
the last element to be used, ++argv reaches the next command line
argument. This is a general pointer characteristic, which is why they too can
be used in situations where iterators are expected.

The STL defines six types  of iterators. These
iterator types are expected by generic algorithms, and in order to create a
particular type of iterator yourself it is important to know their
characteristics. In general, iterators  (see also
section 22.14) must define:
    
    o operator==, testing two iterators for equality,
    o operator!=, testing two iterators for inequality,
    o operator++, incrementing the iterator, as prefix operator,
    o operator*, to access the element the iterator refers to,

The following types of iterators are used when describing generic
algorithms  in chapter 19:
    
    o  InputIterators:
        InputIterators are used to read from a container.  The
dereference operator is guaranteed to work as rvalue in
expressions. Instead of an InputIterator it is also possible to use (see
below) Forward-, Bidirectional- or RandomAccessIterators.  Notations like
InputIterator1 and InputIterator2 may be used as well. In these cases,
numbers are used to indicate which iterators `belong together'. E.g., the
generic algorithm inner_product has the following
prototype:
        Type inner_product(InputIterator1 first1, InputIterator1 last1,
               InputIterator2 first2, Type init);

InputIterator1 first1 and InputIterator1 last1 define a pair of
input iterators on one range, while InputIterator2 first2 defines the
beginning of another range. Analogous notations may be used with other
iterator types.
    o  OutputIterators:
        OutputIterators can be used to write to a container. The
dereference operator is guaranteed to work as an lvalue in expressions,
but not necessarily as rvalue. Instead of an OutputIterator it is also
possible to use (see below) Forward-, Bidirectional- or
RandomAccessIterators.
    o  ForwardIterators:
        ForwardIterators combine InputIterators and
OutputIterators. They can be used to traverse containers in one direction,
for reading and/or writing. Instead of a ForwardIterator it is also possible
to use (see below) Bidirectional- or RandomAccessIterators.
    o  BidirectionalIterators:
        BidirectionalIterators can be used to traverse containers in
both directions, for reading and writing. Instead of a BidirectionalIterator
it is also possible to use (see below) a RandomAccessIterator.
    o  RandomAccessIterators:
        RandomAccessIterators provide random access to container
elements. An algorithm like sort requires a
RandomAccessIterator, and can therefore not be used to sort the elements
of lists or maps, which only provide BidirectionalIterators.
    o  ContiguousIterators:
        ContiguousIterators are like random access iterators, but in
addition guarantee that the elements these iterators point to are stored
contiguously in memory. A container like std::vector offers contiguous
iterators.

The example given with the RandomAccessIterator illustrates how to relate
iterators and generic algorithms: look for the iterator that's required by the
(generic) algorithm, and then see whether the datastructure supports the
required type of iterator. If not, the algorithm cannot be used with the
particular datastructure.


18.2.1: std::distance and std::size

    Earlier, in section 18.2 it was stated that iterators support
pointer arithmetic for containers storing their elements consecutively in
memory. This is not completely true: to determine the number of elements
between the elements to which two iterators refer the iterator must 
support the subtraction operator.  

Using pointer arithmetic to compute the number of elements between two
iterators in, e.g., a std::list or std::unordered_map is not possible,
as these containers do not store their elements consecutively in memory.

The function std::distance fills in that little gap:
std::distance expects two InputIterators and returns the number of
elements between them.

Before using distance the <iterator> header file must be included.

If the iterator specified as first argument exceeds the iterator specified as
its second argument then the number of elements is non-positive, otherwise it
is non-negative. If the number of elements cannot be determined (e.g., the
iterators do not refer to elements in the same container), then distance's
return value is undefined.

Example:
            #include <iostream>
    #include <unordered_map>
        
    using namespace std;
    
    int main()
    {
        unordered_map<int, int> myMap = {{1, 2}, {3, 5}, {-8, 12}};
    
        cout << distance(++myMap.begin(), myMap.end()) << '\n'; // shows: 2
    }

The iterator header file also defines the function std::size,
returning the number of elements in a containers (as returned by the
container's size member) or of an array whose dimension is known to the
compiler at the point of std::size's call. E.g., if the size of an array
data is known to the compiler, then to call a function handler
(expecting the address of the first element of an array and the address of the
location just beyond that array) the following statement can be used:
            handler(data, data + std::size(data));

As noted, the std::size function is defined in the iterator
header. However, it's also guaranteed available when including the header file
of a container supporting iterators (including the string header file).


18.2.2: Insert iterators

    Generic algorithms often require a target container into which the results of
the algorithm are deposited. For example, the copy generic
algorithm has three parameters. The first two define the range of visited
elements, the third defines the first position where the results
of the copy operation should be stored.

With the copy algorithm the number of elements to copy is usually
available beforehand, since that number can usually be provided by pointer
arithmetic. However, situations exist where pointer arithmetic cannot be
used. Analogously, the number of resulting elements sometimes differs from the
number of elements in the initial range. The generic algorithm
unique_copy is a case in point. Here the number of
elements that are copied to the destination container is normally not known
beforehand.

In situations like these an inserter  adapter
function can often be used to create elements in the destination container.
There are three types of inserter adapters:
    
    o back_inserter: calls the container's push_back member to add
new elements at the end of the container. E.g., to copy all elements of
source in reversed order to the back of destination, using the
copy generic algorithm:
        copy(source.rbegin(), source.rend(), back_inserter(destination));

o front_inserter calls the container's push_front member, adding
new elements at the beginning of the container. E.g., to copy all elements of
source to the front of the destination container (thereby also reversing
the order of the elements):
        copy(source.begin(), source.end(), front_inserter(destination));

o inserter calls the container's insert member adding new elements
starting at a specified starting point. E.g., to copy all elements of
source to the destination container, starting at the beginning of
destination, shifting up existing elements to beyond the newly inserted
elements:
        copy(source.begin(), source.end(), inserter(destination,
    destination.begin()));

The inserter adapters
  
 require the existence of two types:
    
    o using value_type = Data, where Data is the data type stored in
the class offering push_back, push_front or insert members (Example:
using value_type = std::string);
    o using const_reference = const &value_type

Concentrating on back_inserter, this iterator expects the name of a
container supporting a member push_back. The inserter's operator()
member calls the container's push_back member. Objects
  of any class supporting a push_back
member can be passed as arguments to back_inserter provided the class adds
            using const_reference = DataType const &;

to its interface (where DataType const & is the type of the parameter
of the class's member push_back). Example:
            #include <iostream>
    #include <algorithm>
    #include <iterator>
    using namespace std;

    class Insertable
    {
        public:
            using value_type = int;
            using const_reference = int const &;

            void push_back(int const &)
            {}
    };

    int main()
    {
        int arr[] = {1};
        Insertable insertable;

        copy(arr, arr + 1, back_inserter(insertable));
    }



18.2.3: Iterators for `istream' objects

    The istream_iterator<Type> can be used to define a set
of iterators for
  istream objects. The general form of the
istream_iterator iterator is:
            istream_iterator<Type> identifier(istream &in)

Here, Type is the type of the data elements read from the istream
stream. It is used as the `begin' iterator in an iterator range. Type may
be any type for which operator>> is defined in combination with istream
objects.

The default constructor is used as the end-iterator and corresponds to the
 end-of-stream. For example,
            istream_iterator<string> endOfStream;

The stream object that was specified when defining the
begin-iterator is not mentioned with the default constructor.

Using back_inserter and istream_iterator adapters, all strings
from a stream can easily be stored in a container. Example (using anonymous
istream_iterator adapters):
            #include <iostream>
    #include <iterator>
    #include <string>
    #include <vector>
    #include <algorithm>
    using namespace std;

    int main()
    {
        vector<string> vs;

        copy(istream_iterator<string>(cin), istream_iterator<string>(),
             back_inserter(vs));

        for
        (
            vector<string>::const_iterator begin = vs.begin(), end = vs.end();
                begin != end; ++begin
        )
            cout << *begin << ' ';
        cout << '\n';
    }



18.2.3.1: Iterators for `istreambuf' objects

        Input iterators are also available for streambuf objects.

To read from streambuf objects supporting input operations
 istreambuf_iterators can be used, supporting the
operations that are also available for istream_iterator. Different from
the latter iterator type istreambuf_iterators support three constructors:
    
    o istreambuf_iterator<Type>:
The end iterator of an iterator range is created using the default
istreambuf_iterator constructor. It represents the end-of-stream
condition when extracting values of type Type from the streambuf.

o istreambuf_iterator<Type>(streambuf *):
A pointer to a streambuf may be used when defining an
istreambuf_iterator. It represents the begin iterator of an iterator
range.
    o istreambuf_iterator<Type>(istream):
An istream may be also used when defining an
istreambuf_iterator. It accesses the istream's streambuf and it
also represents the begin iterator of an iterator range.

In section 18.2.4.1 an example is given using both
istreambuf_iterators and ostreambuf_iterators.


18.2.4: Iterators for `ostream' objects

    An ostream_iterator<Type> adapter can be used to pass
an ostream to algorithms expecting an OutputIterator. Two constructors are
available for defining ostream_iterators:
            ostream_iterator<Type> identifier(ostream &outStream);
    ostream_iterator<Type> identifier(ostream &outStream, char const *delim);

Type is the type of the data elements that should be inserted into an
ostream. It may be any type for which operator<< is defined in
combination with ostream objects.  The latter constructor can be used to
separate the individual Type data elements by delimiter strings. The
former constructor does not use any delimiters.

The example shows how istream_iterators and an
ostream_iterator may be used to copy information of a file to another
file. A subtlety here is that you probably want to use
in.unsetf(ios::skipws). It is used to clear the
 ios::skipws flag. As a consequence whitespace characters are
simply returned by the operator, and the file is copied character by
character. Here is the program:
            #include <iostream>
    #include <algorithm>
    #include <iterator>
    using namespace std;

    int main()
    {
        cin.unsetf(ios::skipws);
        copy(istream_iterator<char>(cin), istream_iterator<char>(),
             ostream_iterator<char>(cout));
    }



18.2.4.1: Iterators for `ostreambuf' objects

        Output iterators are also available for streambuf objects.

To write to  streambuf objects supporting output operations
 ostreambuf_iterators can be used, supporting the
operations that are also available for
ostream_iterator. Ostreambuf_iterators support two constructors:
    
    o ostreambuf_iterator<Type>(streambuf *):
A pointer to a streambuf may be used when defining an
ostreambuf_iterator. It can be used as an OutputIterator.
    o ostreambuf_iterator<Type>(ostream):
An ostream may be also used when defining an
ostreambuf_iterator. It accesses the ostream's streambuf and it
can also be used as an OutputIterator.

The next example illustrates the use of both
 istreambuf_iterators and ostreambuf_iterators
when copying a stream in yet another way. Since the stream's streambufs
are directly accessed the streams and stream flags are bypassed. Consequently
there is no need to clear ios::skipws as in the previous section, while
the next program's efficiency probably also exceeds the efficiency of the
program shown in the previous section.
            #include <iostream>
    #include <algorithm>
    #include <iterator>
    using namespace std;

    int main()
    {
        istreambuf_iterator<char> in(cin.rdbuf());
        istreambuf_iterator<char> eof;
        ostreambuf_iterator<char> out(cout.rdbuf());

        copy(in, eof, out);
    }



18.2.5: Moving elements to another container

    Sometimes we want to move elements from one container to another one. Instead
of retrieving elements in sequence and then moving them into the destination
container (e.g., when extracting words from streams) or explicitly calling
std::move on the source container's elements it's also possible to use
make_move_iterator(std::make_move_iterator(iterator iter)). The function's
argument is an iterator referring to a movable element, and the range of
elements to move from one container to another can be specified by calling
two make_move_iterators. One receives the source's begin iterator, and
other one the source's end iterator. The example illustrates how words
can be moved into a std::vector<std::string>:
            #include <iostream>
    #include <algorithm>
    #include <vector>
    #include <string>
    #include <iterator>

    using namespace std;

    size_t fstNonEmpty(vector<string> const &vs)
    {
        return
            find_if(vs.begin(), vs.end(),
                    [&](string const &str)
                    {
                        return str != "";
                    }
            ) - vs.begin();
    }

    int main()
    {
        vector<string> vs;

        copy(istream_iterator<string>(cin), istream_iterator<string>(),
                back_inserter(vs));

        cout << "vs contains " << vs.size() << " words\n"
                "first non-empty word at index " << fstNonEmpty(vs) << "\n"
                "moving the first half into vector v2\n";

        vector<string> v2{ make_move_iterator(vs.begin()),
                           make_move_iterator(vs.begin() + vs.size() / 2) };

        cout << "vs contains " << vs.size() << " words\n"
                "first non-empty word at index " << fstNonEmpty(vs) << "\n"
                "v2 contains " << v2.size() << " words\n";
    }

    Note that the elements in the source container have merely moved into the
destination container: the elements in the source container are empty after
moving. When moving other types of objects the results may be different,
although in all cases the source objects should keep their valid states.


18.3: The class 'unique_ptr'

Before using the unique_ptr class presented in this section the
<memory> header file must be included.

When pointers are used to access dynamically allocated memory strict
bookkeeping is required to prevent memory leaks. When a pointer variable
referring to dynamically allocated memory goes out of scope, the dynamically
allocated memory becomes inaccessible and the program suffers from a 
    memory leak.
    Consequently, the programmer has to make sure that the dynamically
allocated memory is returned to the common pool just before the pointer
variable goes out of scope.

When a pointer variable points to a dynamically allocated single value or
object, bookkeeping requirements are greatly simplified when the pointer
variable is defined as a std::unique_ptr object.

Unique_ptrs are objects masquerading as pointers. Since they are objects,
their destructors are called when they go out of scope.  Their destructors
automatically delete the dynamically allocated memory to which they
point. Unique_ptrs (and their cousins shared_ptrs (cf. section 18.4)
are also called
    smart pointers).

Unique_ptrs have several special characteristics:
    
    o  when assigning a unique_ptr to another move semantics is
used. If move semantics is not available compilation fails. On the other
hand, if compilation succeeds then the used containers or generic algorithms
support the use of unique_ptrs. Here is an example:
        std::unique_ptr<int> up1(new int);
std::unique_ptr<int> up2(up1);      // compilation error

The second definition fails to compile as unique_ptr's copy
constructor is private (the same holds true for the assignment operator). But
the unique_ptr class does offer facilities to initialize and assign
from rvalue references:
        class unique_ptr                        // interface partially shown
{
    public:
        unique_ptr(unique_ptr &&tmp);   // rvalues bind here
    private:
        unique_ptr(const unique_ptr &other);
};

In the next example move semantics is used and so it compiles correctly:
        unique_ptr<int> cp(unique_ptr<int>(new int));

o  a unique_ptr object should only point to memory that was made
available dynamically, as only dynamically allocated memory can be deleted.

o  multiple unique_ptr objects should not be allowed to point to the
same block of dynamically allocated memory. The unique_ptr's interface was
designed to prevent this from happening. Once a unique_ptr object goes out
of scope, it deletes the memory it points to, immediately changing any other
object also pointing to the allocated memory into a  wild
pointer.

o  When a class Derived is derived from Base, then a newly
allocated Derived class object can be assigned to a unique_ptr<Base>,
without having to define a virtual destructor for Base. The Base *
pointer that is returned by the unique_ptr object can simply be cast
statically to Derived, and Derived's destructor is automatically
called as well, if the unique_ptr definition is provided with a
deleter function address. This is illustrated in the next example:
            class Base
    { ... };
    class Derived: public Base
    {
        ...
        public:
            // assume Derived has a member void process()
   
            static void deleter(Base *bp);
    };
    void Derived::deleter(Base *bp)
    {
        delete static_cast<Derived *>(bp);
    }
    int main()
    {
        unique_ptr<Base, void (*)(Base *)> bp(new Derived, &Derived::deleter);
        static_cast<Derived *>(bp.get())->process(); // OK!

    } // here ~Derived is called: no polymorphism required.

The class unique_ptr offers several member functions to access the
pointer itself or to have a unique_ptr point to another block of
memory. These member functions (and unique_ptr constructors) are
introduced in the next few sections.

Unique_ptr can also be used with containers and (generic) algorithms. They
can properly destruct any type of object, as their constructors accept
customizable deleters. In addition, arrays can be handled by unique_ptrs.


18.3.1: Defining `unique_ptr' objects

    There are three ways to  define unique_ptr
objects. Each definition contains the usual <type> specifier between
angle brackets:
    
    o  The default constructor simply creates a unique_ptr object that
does not point to a particular block of memory. Its pointer is initialized to
0 (zero):
        unique_ptr<type> identifier;

This form is discussed in section 18.3.2.
    o  The move constructor  initializes an unique_ptr object.
Following the use of the move constructor its unique_ptr argument no
longer points to the dynamically allocated memory and its pointer data member
is turned into a zero-pointer:
        unique_ptr<type> identifier(another unique_ptr for type);

This form is discussed in section 18.3.3.
    o  The form that is used most often initializes a unique_ptr object
to the block of dynamically allocated memory that is passed to the object's
constructor. Optionally deleter can be provided. A (free) function (or
function object) receiving the unique_ptr's pointer as its argument can be
passed as deleter. It is supposed to return the dynamically allocated
memory to the common pool (doing nothing if the pointer equals zero).
        unique_ptr<type> identifier (new-expression [, deleter]);

This form is discussed in section 18.3.4.


18.3.2: Creating a plain `unique_ptr'

    Unique_ptr's default constructor defines a 
unique_ptr not pointing to a particular block of memory:
            unique_ptr<type> identifier;

The  pointer controlled by the unique_ptr
object is initialized to 0 (zero). Although the unique_ptr object
itself is not the pointer, its value can be compared to 0. Example:
            unique_ptr<int> ip;

    if (!ip)
        cout << "0-pointer with a unique_ptr object\n";

Alternatively, the member get can be used (cf. section 18.3.5).


18.3.3: Moving  another `unique_ptr'

        A unique_ptr  may be initialized
using an rvalue reference to a unique_ptr object for the same type:
            unique_ptr<type> identifier(other unique_ptr object);

The move constructor is used, e.g., in the following example:
            void mover(unique_ptr<string> &&param)
    {
        unique_ptr<string> tmp(move(param));
    }

Analogously, the assignment operator can  be
used. A unique_ptr object may be assigned to a temporary  unique_ptr
object of the same type (again move-semantics is used). For example:
            #include <iostream>
    #include <memory>
    #include <string>

    using namespace std;

    int main()
    {
        unique_ptr<string> hello1{ new string{ "Hello world" } };
        unique_ptr<string> hello2{ move(hello1) };
        unique_ptr<string> hello3;

        hello3 = move(hello2);
        cout << // *hello1 << '\n' <<   // would have segfaulted
                // *hello2 << '\n' <<   // same
                *hello3 << '\n';
    }
    // Displays:    Hello world

    The example illustrates that
    
    o  hello1 is initialized by a pointer to a dynamically allocated
string (see the next section).
    o  The unique_ptr hello2 grabs the pointer controlled by hello1
using a move constructor. This effectively changes hello1 into a
0-pointer.
    o  Then hello3 is defined as a default unique_ptr<string>. But
then it grabs its value using move-assignment from hello2 (which, as a
consequence, is changed into a 0-pointer).

If hello1 or hello2 had been inserted into cout a
segmentation fault would have resulted. The reason for this should now be
clear: it is caused by dereferencing 0-pointers. In the end, only hello3
actually points to the originally allocated string.


18.3.4: Pointing to a newly allocated object

    A unique_ptr  is most often initialized
using a pointer to dynamically allocated memory. The generic form is:
            unique_ptr<type [, deleter_type]> identifier(new-expression
            [, deleter = deleter_type{}]);

The second (template) argument (deleter(_type)) is optional and may
  refer to a free function, a function object handling the
destruction of the allocated memory, or a lambda function. A deleter is used,
e.g., in situations where a double pointer is allocated and the destruction
must visit each nested pointer to destroy the allocated memory (see below for
an illustration).

Here is an example initializing a unique_ptr pointing to a string
object:
            unique_ptr<string> strPtr{ new string{ "Hello world" } };

The argument that is passed to the constructor is the pointer returned by
operator new. Note that type does not mention the pointer.  The
 type that is used in the unique_ptr construction
is the same as the type that is used in new expressions.

Here is an example showing how an explicitly defined deleter may be used to
delete a dynamically allocated array of pointers to strings:
            #include <string>
    #include <memory>
    using namespace std;

    struct Deleter
    {
        size_t d_size;
        Deleter(size_t size = 0)
        :
            d_size(size)
        {}
        void operator()(string **ptr) const
        {
            for (size_t idx = 0; idx < d_size; ++idx)
                delete ptr[idx];
            delete[] ptr;
        }
    };
    int main()
    {
        unique_ptr<string *, Deleter> sp2{ new string *[10](),
                                           Deleter{ 10 } };

                                        // for illustration purposes:
                                        // retrieving the Deleter
        [[maybe_unused]] Deleter &obj = sp2.get_deleter();
    }


A unique_ptr can be used to reach the
  member functions that are available for
objects allocated by the new expression.  These members can be reached as
if the unique_ptr was a plain pointer to the dynamically allocated
object. For example, in the following program the text `C++' is inserted
behind the word `hello':
            #include <iostream>
    #include <memory>
    #include <cstring>
    using namespace std;

    int main()
    {
        unique_ptr<string> sp{ new string{ "Hello world" } };

        cout << *sp << '\n';
        sp->insert(strlen("Hello "), "C++ ");
        cout << *sp << '\n';
    }
    /*
        Displays:
            Hello world
            Hello C++ world
    */



18.3.5: Operators and members

    The class unique_ptr offers the  following
  operators:
    
    o unique_ptr<Type> &operator=(unique_ptr<Type> &&tmp):
This operator transfers the memory pointed to by the rvalue
        unique_ptr object to the lvalue unique_ptr object using
        move semantics. So, the rvalue object loses the memory it
        pointed at and turns into a 0-pointer. An existing unique_ptr may
        be assigned to another unique_ptr by converting it to an rvalue
        reference first using std::move. Example:
   unique_ptr<int> ip1(new int);
unique_ptr<int> ip2;

ip2 = std::move(ip1);

o operator bool() const:
This operator returns false if the unique_ptr does not point
        to memory (i.e., its get member, see below, returns 0). Otherwise,
        true is returned.
    o Type &operator*():
This operator returns a reference to the information accessible via
        a unique_ptr object . It acts like a normal pointer dereference
        operator.
    o Type *operator->():
This operator returns a pointer to the information accessible via a
        unique_ptr object. This operator allows you to select
        members of an object accessible via a unique_ptr object. Example:
   unique_ptr<string> sp{ new string{ "hello" } };
cout << sp->c_str();

The class unique_ptr supports the following 
member functions:
    
    o Type *get():
A pointer to the information controlled by the unique_ptr object
        is returned. It acts like operator->. The returned pointer can be
        inspected. If it is zero the unique_ptr object does not point to
        any memory.
    o Deleter &unique_ptr<Type>::get_deleter():
A reference to the deleter object used by the unique_ptr is
        returned.
    o Type *release():
A pointer to the information accessible via a unique_ptr object is
        returned. At the same time the object itself becomes a 0-pointer
        (i.e., its pointer data member is turned into a 0-pointer). This
        member can be used to transfer the information accessible via a
        unique_ptr object to a plain Type pointer. After calling this
        member the proper destruction of the dynamically allocated memory
        is the responsibility of the programmer.
    o void reset(Type *):
The dynamically allocated memory controlled by the unique_ptr
        object is returned to the common pool; the object thereupon controls
        the memory  to which the argument that is passed to the function
        points. It can also be called without argument, turning the object
        into a 0-pointer. This member function can be used to assign a new
        block of dynamically allocated memory to a unique_ptr object.
    o void swap(unique_ptr<Type> &):
Two identically typed unique_ptrs are swapped.


18.3.6: Using `unique_ptr' objects for arrays

    When a unique_ptr is used to store arrays the dereferencing operator makes
little sense but with arrays unique_ptr objects benefit from index
operators. The distinction between a single object unique_ptr and a
unique_ptr referring to a dynamically allocated array of objects is
realized through a template specialization.

With dynamically allocated arrays the following syntax is available:
    
    o  the index ([]) notation is used to specify that the smart
        pointer controls a dynamically allocated array. Example:
            unique_ptr<int[]> intArr(new int[3]);

o  the index operator can be used to access the array's
        elements. Example:
            intArr[2] = intArr[0];

In these cases the smart pointer's destructors call
delete[] rather than delete.


18.4: The class `shared_ptr'

In addition to the class unique_ptr the class
std::shared_ptr<Type> is available, which is a reference
counting smart pointer.

Before using shared_ptrs the <memory> header file must be included.

The shared pointer automatically destroys its content once its reference
count has decayed to zero. As with unique_ptr, when defining a
shared_ptr<Base> to store a newly allocated Derived class object, the
returned Base * may be cast to a Derived * using a static_cast:
polymorphism isn't required, and when resetting the shared_ptr or when the
shared_ptr goes out of scope, no slicing occurs, and Derived's
destructor (or, if configured: deleter) is called (cf. section
18.3).

Shared_ptrs support copy and move constructors as well as standard and
move overloaded assignment operators.

Like unique_ptrs, shared_ptrs may refer to dynamically allocated arrays.


18.4.1: Defining `shared_ptr' objects

    There are four ways to  define shared_ptr
objects. Each definition contains the usual <type> specifier between
angle brackets:
    
    o  The default constructor simply creates a shared_ptr object that
does not point to a particular block of memory. Its pointer is initialized to
0 (zero):
        shared_ptr<type> identifier;

This form is discussed in section 18.4.2.

o  The copy constructor initializes a shared_ptr so that both
objects share the memory pointed at by the existing object. The copy
constructor also increments the shared_ptr's reference count. Example:
        shared_ptr<string> org{ new string{ "hi there" } };
shared_ptr<string> copy(org);   // reference count now 2

o  The move constructor initializes a shared_ptr with the pointer
and reference count of a temporary shared_ptr. The temporary
shared_ptr is changed into a 0-pointer. An existing shared_ptr may
have its data moved to a newly defined shared_ptr (turning the existing
shared_ptr into a 0-pointer as well). In the next example a temporary,
anonymous shared_ptr object is constructed, which is then used to
construct grabber. Since grabber's constructor receives an anonymous
temporary object, the compiler uses shared_ptr's move constructor:
        shared_ptr<string> grabber{ shared_ptr<string>{ new string{ "hi there" } } };

o  The form that is used most often initializes a shared_ptr object
to the block of dynamically allocated memory that is passed to the object's
constructor. Optionally deleter can be provided. A (free) function (or
function object) receiving the shared_ptr's pointer as its argument can be
passed as deleter. It is supposed to return the dynamically allocated
memory to the common pool (doing nothing if the pointer equals zero).
        shared_ptr<type> identifier (new-expression [, deleter]);

This form is discussed in section 18.4.3.


18.4.2: Creating a plain `shared_ptr'

    Shared_ptr's default constructor defines a 
shared_ptr not pointing to a particular block of memory:
            shared_ptr<type> identifier;

The  pointer controlled by the shared_ptr
object is initialized to 0 (zero). Although the shared_ptr object
itself is not the pointer, its value can be compared to 0. Example:
            shared_ptr<int> ip;

    if (!ip)
        cout << "0-pointer with a shared_ptr object\n";

Alternatively, the member get can be used (cf. section 18.4.4).


18.4.3: Pointing to a newly allocated object

    Most often a shared_ptr  is initialized by a
dynamically allocated block of memory. The generic form is:
            shared_ptr<type> identifier(new-expression [, deleter]);

The second argument (deleter) is optional and
  refers to a function object or free function handling the
destruction of the allocated memory. A deleter is used, e.g., in situations
where a double pointer is allocated and the destruction must visit each nested
pointer to destroy the allocated memory (see below for an illustration).  It
is used in situations comparable to those encountered with unique_ptr
(cf. section 18.3.4).

Here is an example initializing a shared_ptr pointing to a string
object:
            shared_ptr<string> strPtr{ new string{ "Hello world" } };

The argument that is passed to the constructor is the pointer returned by
operator new. Note that type does not mention the pointer.  The
 type that is used in the shared_ptr construction
is the same as the type that is used in new expressions.

The next example illustrates that two shared_ptrs indeed share their
information. After modifying the information controlled by one of the
objects the information controlled by the other object is modified as well:
            #include <iostream>
    #include <memory>
    #include <cstring>
    using namespace std;

    int main()
    {
        shared_ptr<string> sp(new string{ "Hello world" });
        shared_ptr<string> sp2(sp);

        sp->insert(strlen("Hello "), "C++ ");
        cout << *sp << '\n' <<
                *sp2 << '\n';
    }
    /*
        Displays:
            Hello C++ world
            Hello C++ world
    */



18.4.4: Operators and members

    The class shared_ptr offers the following 
operators:
    
    o shared_ptr &operator=(shared_ptr<Type> const &other):
Copy assignment: the reference count of the operator's left hand side
        operand is reduced. If the reference count decays to zero the
        dynamically allocated memory controlled by the left hand side operand
        is deleted. Then it shares the information with the operator's
        right hand side operand, incrementing the information's reference
        count.
    o shared_ptr &operator=(shared_ptr<Type> &&tmp):
Move assignment: the reference count of the operator's left hand side
        operand is reduced. If the reference count decays to zero the
        dynamically allocated memory controlled by the left hand side operand
        is deleted. Then it grabs the information controlled by the
        operator's right hand side operand which is turned into a 0-pointer.
    o operator bool() const:
If the shared_ptr actually points to memory true is returned,
        otherwise, false is returned.
    o Type &operator*():
A reference to the information stored in the
        shared_ptr object is returned. It acts like a normal pointer.
    o Type *operator->():
A pointer to the information controlled by the shared_ptr object
        is returned. Example:
   shared_ptr<string> sp{ new string{ "hello" } };
cout << sp->c_str() << '\n';

The following member function member functions are supported:
    
    o Type *get():
A pointer to the information controlled by the shared_ptr object
        is returned. It acts like operator->. The returned pointer can be
        inspected. If it is zero the shared_ptr object does not point to
        any memory.
    o Deleter &get_deleter():
A reference to the shared_ptr's deleter (function or function
        object) is returned.
    o void reset(Type *):
The reference count of the information controlled by the shared_ptr
        object is reduced and if it decays to zero the memory it points to
        is deleted. Thereafter the
        object's information will refer to the argument that is passed to the
        function, setting its shared count to 1. It can also be called without
        argument, turning the object into a 0-pointer. This member function
        can be used to assign a new block of dynamically allocated memory to a
        shared_ptr object.
    o void reset(Type *, DeleterType &&):
This variant of the previous member accepts a specific Deleter
        type: if Type is a base-class and derived class objects are used,
        these derived class objects may require specific actions at
        destruction time. When the previous member is used, then eventually the
        newly assigned object's destructor is called without using an
        explicit deleter function. The current member ensures that by the time
        the shared counter has decayed to zero the provided deleter is used.
      o void shared_ptr<Type>::swap(shared_ptr<Type> &&):
Two identically typed shared_ptrs are swapped.
    o bool unique() const:
If the current object is the only object referring to the memory
        controlled by the object true is returned otherwise (including the
        situation where the object is a 0-pointer) false is returned.
    o size_t use_count() const:
The number of objects sharing the memory controlled by the object is
        returned.


18.4.5: Casting shared pointers

    Be cautious when using standard C++ style casts in combination with
shared_ptr objects. Consider the following two classes:
            struct Base
    {};
    struct Derived: public Base
    {};

As with unique_ptr, when defining a shared_ptr<Base> to store a
newly allocated Derived class object, the returned Base * may be cast
to a Derived * using a static_cast: polymorphism isn't required, and
when resetting the shared_ptr or when the shared_ptr goes out of
scope, no slicing occurs, and Derived's destructor is called (cf. section
18.3).

Of course, a shared_ptr<Derived> can easily be defined. Since a
Derived object is also a Base object, a pointer to Derived can
be considered a pointer to Base without using casts, but a static_cast
could be used to force the interpretation of a Derived * to a Base *:
            Derived d;
    static_cast<Base *>(&d);

However, a plain static_cast cannot be used when initializing a shared
pointer to a Base using the get member of a shared pointer to a
Derived object. The following code snipped eventually results in an
attempt to delete the dynamically allocated Base object twice:
            shared_ptr<Derived> sd{ new Derived };
    shared_ptr<Base> sb{ static_cast<Base *>(sd.get()) };

Since sd and sb point at the same object ~Base will be called
for the same object when sb goes out of scope and when sd goes out of
scope, resulting in premature termination of the program due to a
 double free error.

These errors can be prevented using casts that were specifically designed
for being used with shared_ptrs. These casts use specialized constructors
that create a shared_ptr pointing to memory but shares ownership (i.e.,
a reference count) with an existing shared_ptr. These special casts are:
    
    o std::static_pointer_cast<Base>(std::shared_ptr<Derived> ptr):
A shared_ptr to a Base class object is returned. The returned
shared_ptr refers to the base class portion of the Derived class to
which the shared_ptr<Derived> ptr refers. Example:
        shared_ptr<Derived> dp{ new Derived };
shared_ptr<Base> bp = static_pointer_cast<Base>(dp);

o std::const_pointer_cast<Class>(std::shared_ptr<Class const> ptr):
A shared_ptr to a Class class object is returned. The
returned shared_ptr refers to a non-const Class object whereas the
ptr argument refers to a Class const object. Example:
        shared_ptr<Derived const> cp{ new Derived };
shared_ptr<Derived> ncp = const_pointer_cast<Derived>(cp);

o std::dynamic_pointer_cast<Derived>(std::shared_ptr<Base> ptr):
A shared_ptr to a Derived class object is returned. The
Base class must have at least one virtual member function, and the class
Derived, inheriting from Base may have overridden Base's virtual
member(s). The returned shared_ptr refers to a Derived class object if
the dynamic cast from Base * to Derived * succeeded. If the dynamic
cast did not succeed the shared_ptr's get member returns 0.  Example
(assume Derived and Derived2 were derived from Base):
        shared_ptr<Base> bp(new Derived());
cout << dynamic_pointer_cast<Derived>(bp).get() << ' ' <<
        dynamic_pointer_cast<Derived2>(bp).get() << '\n';

The first get returns a non-0 pointer value, the second get
returns 0.


18.4.6: Using `shared_ptr' objects for arrays

    The shared_ptr class can also be used to handle dynamically allocated
arrays of objects. To use it on arrays simply use the square brackets when
specifying the shared_ptr's type. Here is an example:
        struct Type
    {
        ~Type()
        {
            cout << "destr\n";  // show the object's destruction
        }
    };

    int main()
    {
        shared_ptr<Type[]> sp{ new Type[3] };
        sp[0] = sp[1];
    }


When the shared_ptr itself is initialized with an array of pointers, then
a deleter must be used to delete the memory the pointers point at. In that
case the deleter is responsible for returning the memory to the common pool,
using delete[] when eventually deleting the array of pointers:
        struct Type
    {
        int value = 0;

        ~Type()
        {
            cout << "destr " << this << '\n';
        }
    };

    template<typename Tp>
    struct Deleter
    {
        size_t d_size;
        Deleter(size_t size)
        :
            d_size(size)
        {}
        void operator()(Tp **ptr)
        {
            for (; d_size--; )
                delete ptr[d_size];

            delete[] ptr;           // <-- NOTE !
        }
    };

    int main()
    {
        shared_ptr<Type *[]> sp{ new Type *[2] {new Type, new Type},
                                 Deleter<Type>{ 2 } };
    }



18.5: Smart `smart pointer' construction: `make_shared' and `make_unique'

Usually a shared_ptr is initialized at definition time
with a pointer to a newly allocated object. Here is an example:
            std::shared_ptr<string> sptr{ new std::string{ "hello world" } }

In such statements two memory allocation calls are used: one for the
allocation of the std::string and one used interally by
std::shared_ptr's constructor itself.

The two allocations can be combined into one single allocation (which is
also slightly more efficient than explicitly calling shared_ptr's
constructor) using the make_shared template. The function template
std::make_shared has the following prototype:
            template<typename Type, typename ...Args>
    std::shared_ptr<Type> std::make_shared(Args ...args);

Before using make_shared the <memory> header file must be included.

This function template allocates an object of type Type, passing
args to its constructor (using perfect forwarding, see section
22.5.2), and returns a shared_ptr initialized with the address of
the newly allocated Type object.

Here is how the above sptr object can be initialized
using std::make_shared. Notice the use of auto which frees us from
having to specify sptr's type explicitly:
            auto sptr(std::make_shared<std::string>("hello world"));

After this initialization std::shared_ptr<std::string> sptr has been
defined and initialized. It could be used as follows:
            std::cout << *sptr << '\n';

In addition to make_shared the function
    std::make_unique can be used. It can be used
make_shared but returns a std::unique_ptr rather than a
shared_ptr.


18.6: Classes having pointer data members

Classes having  pointer data members require special
attention. In particular at construction time one must be careful to prevent
  wild pointers and/or  memory leaks. Consider
the following class defining two pointer data members:
            class Filter
    {
        istream *d_in;
        ostream *d_out;
        public:
            Filter(char const *in, char const *out);
    };

Assume that Filter objects filter information read from *d_in and
write the filtered information to *d_out.  Using pointers to streams
allows us to have them point at any kind of stream like istreams,
ifstreams, fstreams or istringstreams.  The shown constructor could be
implemented like this:
            Filter::Filter(char const *in, char const *out)
    :
        d_in(new ifstream{ in }),
        d_out(new ofstream{ out })
    {
        if (!*d_in || !*d_out)
            throw "Input and/or output stream not available"s;
    }

Of course, the construction could fail. new could throw an exception;
the stream constructors could throw exceptions; or the streams could not be
opened in which case an exception is thrown from the constructor's body. Using
a function try block helps. Note that if d_in's initialization throws,
there's nothing to be worried about. The Filter object hasn't been
constructed, its destructor is not called and processing continues at the
point where the thrown exception is caught. But Filter's destructor is
also not called when d_out's initialization or the constructor's if
statement throws: no object, and hence no destructor is called. This may
result in memory leaks, as delete isn't called for d_in and/or
d_out. To prevent this, d_in and d_out must first be initialized
to 0 and only then the initialization can be performed:
            Filter::Filter(char const *in, char const *out)
    try
    :
        d_in(0),
        d_out(0)
    {
        d_in = new ifstream{ in };
        d_out = new ofstream{ out };

        if (!*d_in || !*d_out)
            throw "Input and/or output stream not available"s;
    }
    catch (...)
    {
        delete d_out;
        delete d_in;
    }

This quickly gets complicated, though. If Filter harbors yet another
data member of a class whose constructor needs two streams then that data
cannot be constructed or it must itself be converted into a pointer:
            Filter::Filter(char const *in, char const *out)
    try
    :
        d_in(0),
        d_out(0)
        d_filterImp(*d_in, *d_out)    // won't work
    { ... }

    // instead:

    Filter::Filter(char const *in, char const *out)
    try
    :
        d_in(0),
        d_out(0),
        d_filterImp(0)
    {
        d_in = new ifstream(in);
        d_out = new ofstream(out);
        d_filterImp = new FilterImp(*d_in, *d_out);
        ...
    }
    catch (...)
    {
        delete d_filterImp;
        delete d_out;
        delete d_in;
    }

Although the latter alternative works, it quickly gets hairy. In
situations like these smart pointers should be used to prevent the
hairiness. By defining the stream pointers as (smart pointer) objects they
will, once constructed, properly be destroyed even if the rest of the
constructor's code throws exceptions. Using a FilterImp and two
unique_ptr data members Filter's setup and its constructor becomes:
            class Filter
    {
        std::unique_ptr<std::ifstream> d_in;
        std::unique_ptr<std::ofstream> d_out;
        FilterImp d_filterImp;
        ...
    };

    Filter::Filter(char const *in, char const *out)
    try
    :
        d_in(new ifstream(in)),
        d_out(new ofstream(out)),
        d_filterImp(*d_in, *d_out)
    {
        if (!*d_in || !*d_out)
            throw "Input and/or output stream not available"s;
    }

We're back at the original implementation but this time without having to
worry about wild pointers and memory leaks. If one of the member initializers
throws the destructors of previously constructed data members (which are now
objects) are always called.

As a rule of thumb: when classes need to define pointer data members
they should define those pointer data members as smart pointers if there's any
chance that their constructors throw exceptions.


18.7: Comparison classes

With the introduction of the spaceship operator (<=>, cf. section
11.7.2) several comparison category classes 
     were added to the standard namespace.

Comparison classes are required when implementing the spaceship operator, and
to use them (or when declaring and implementing the spachip operator) the
<compare> header file must be included.

The weak class types do not support substitutability. Substitutability
means that if two objects one and two are equal (so: one == two is
true), then fun(one) == fun(two) is also true. Here fun is any
function that only uses public const members of its argument that return
value types (as compared to pointer types) which also support comparisons
(also called comparison-salient state).

The operators of comparison classes expect at least one argument of their own
class types. The other argument may either also be of their own class types or
it can be the value 0 (or any value that can be considered 0, like
nullptr_t).

There are five comparison classes, primarily used when implementing
spaceship operators:
    
    o weak_equality, used for classes that only support the == and
        != operators, but not substitutability;
    o strong_equality, used for classes that only support the == and
        != operators, as well as substitutability;
    o partial_ordering used for classes that support all comparison
        operators, do not support substitutability, and can be used when
        comparing incomparable arguments (i.e., all comparison operators
        return false);
    o weak_ordering used for classes that support all comparison
        operators and that do not support substitutability;
    o strong_ordering used for classes that support all comparison
        operators as well as substitutability;


18.7.1: The class `weak_equality'

    The class std::weak_equality is used when implementing
the spaceship operator for classes that only support (in)equality comparisons,
but not substitutability. The class provides free functions 
operator== and operator!= expecting weak_equality arguments (one
argument may be 0) and it defines two static objects:
    
    o weak_equality::equivalent, indicating equality;
    o weak_equality::nonequivalent, indicating non-equality;

Note: at the current release of the Gnu C++ compiler (10.0.0) this class
is not yet available in the <compare> header file.


18.7.2: The class `strong_equality'

    The class std::strong_equality is used when implementing
the spaceship operator for classes that only support (in)equality comparisons
as well as substitutability. The class provides free functions 
operator== and operator!= expecting strong_equality arguments (one
argument may be 0) and it defines four static objects:
    
    o strong_equality::equal, indicating equality;
    o strong_equality::equivalent, indicating equality;
    o strong_equality::nonequal, indicating non-equality;
    o strong_equality::nonequivalent, indicating non-equality;

Note: at the current release of the Gnu C++ compiler (10.0.0) this class
is not yet available in the <compare> header file.


18.7.3: The class `partial_ordering'

    The class std::partial_ordering is used when
implementing the spaceship operator for classes that support all comparison
operators (where one operand may be zero), that do not support
substitutability, and whose objects, using the spaceship operator itself, can
also be compared to any other type of object.

The class partial_ordering provides free functions for all comparison
operations (==, !=, <, <=, >, and >=) expecting partial_ordering
arguments (one argument may be 0). It also defines four static objects which
can be returned by the spaceship operator:
    
    o partial_ordering::less, returned when the lhs operand of the
        spaceship operator should be ordered before the rhs operand;
    o partial_ordering::equivalent, indicating equality: there is no
        ordering preference between the two operands of the spaceship operator;
    o partial_ordering::greater, returned when the lhs operand of the
        spaceship operator should be ordered after the rhs operand;
    o partial_ordering::unordered, returned when all comparison operators
        of a class implementing the spaceship operator should return false
        (so ==, !=, <, <=, >, and >= all return false).

As an example, consider road taxes. Trucks, cars, and motor cycles have to pay
road taxes, but there's no road tax for bicycles. For ordering road taxes a
class RoadTax may be used, defining the following spaceship operator
(assuming that all types of vehicles are derived from a class Vehicle, and
that Vehicle has a (virtual) member double roadTax() returning the
amount of road tax that is due for the various types of vehicles; the amount
is negative if no road tax is required):
        partial_ordering RoadTax::operator<=>(Vehicle const &lhs, 
                                          Vehicle const &rhs)
    {
        return
          lhs.roadTax() < 0 or rhs.roadTax() < 0 ?
                                          partial_ordering::unordered :
          lhs.roadTax() < rhs.roadTax() ? partial_ordering::less      :
          lhs.roadTax() > rhs.roadTax() ? partial_ordering::greater   :
                                          partial_ordering::equivalent;
    }


18.7.4: The class `weak_ordering'

    The class std::weak_ordering is used when
implementing the spaceship operator for classes that support all comparison
operators (where one operand may be zero), and that do not support
substitutability.

The class weak_ordering differs from the partial_ordering class in
that unordered cannot be used as a comparison result. Like the class
partial_ordering it provides free functions for all comparison
operations (==, !=, <, <=, >, and >=) expecting partial_ordering
arguments (one argument may be 0). It also defines three static objects which
can be returned by the spaceship operator:
    
    o weak_ordering::less, returned when the lhs operand of the
        spaceship operator should be ordered before the rhs operand;
    o weak_ordering::equal, indicating equality: there is no
        ordering preference between the two operands of the spaceship operator;
    o weak_ordering::greater, returned when the lhs operand of the
        spaceship operator should be ordered after the rhs operand;

The example in the previous section can easily adapted to the
weak_ordering comparison class: if the roadTax members of vehicles for
which no road tax is due return zero then RoadTax's spaceship operator can
be implemented this way:
        weak_ordering RoadTax::operator<=>(Vehicle const &lhs, 
                                          Vehicle const &rhs)
    {
        return
          lhs.roadTax() < rhs.roadTax() ? weak_ordering::less      :
          lhs.roadTax() > rhs.roadTax() ? weak_ordering::greater   :
                                          weak_ordering::equal;
    }


18.7.5: The class `strong_ordering'

    The class std::strong_ordering is used when
implementing the spaceship operator for classes that support all comparison
operators (where one operand may be zero), as well as substitutability.

The class strong_ordering provides free functions for all comparison
operations (==, !=, <, <=, >, and >=) expecting partial_ordering
arguments (one argument may be 0). It also defines three static objects which
can be returned by the spaceship operator:
    
    o strong_ordering::less, returned when the lhs operand of the
        spaceship operator should be ordered before the rhs operand;
    o strong_ordering::equal, indicating equality: there is no
        ordering preference between the two operands of the spaceship operator;
    o strong_ordering::greater, returned when the lhs operand of the
        spaceship operator should be ordered after the rhs operand;

An example where the class strong_ordering was used has already been
provided in section 11.7.2, where the spaceship operator itself was
introduced.


18.8: Regular Expressions

C++ itself provides facilities for handling regular expressions. Regular
expressions were already available in C++ via its C heritage (as C
has always offered functions like regcomp and regexec), but the
dedicated regular expression facilities have a richer interface than the
traditional C facilities, and can be used in code using templates.

Before using the specific C++ implementations of regular expressions the
header file <regex> must be included.

Regular expressions are extensively documented elsewhere (e.g., regex(7),
Friedl, J.E.F
  Mastering Regular Expressions, O'Reilly).
    The reader is referred to these sources for a refresher on the topic of
regular expressions. In essence, regular expressions define a small
meta-language recognizing textual units (like `numbers', `identifiers', etc.).
They are extensively used in the context of lexical scanners (cf. section
26.6.1) when defining the sequence of input characters associated with
tokens. But they are also intensively used in other situations.  Programs
like sed(1) and grep(1) use regular expressions to find pieces of text
in files having certain characteristics, and a program like perl(1) adds
some `sugar' to the regular expression language, simplifying the construction
of regular expressions. However, though extremely useful, it is also well
known that regular expressions tend to be very hard to read. Some even call
the regular expression language a write-only language: while specifying a
regular expression it's often clear why it's written in a particular way. But
the opposite, understanding what a regular expression is supposed to represent
if you lack the proper context, can be extremely difficult. That's why, from
the onset and as a rule of thumb, it is stressed that an appropriate
comment should be provided, with each regular expression, as to what it is
supposed to match.

In the upcoming sections first a short overview of the regular expression
language is provided, which is then followed by the facilities C++ is
currently offering for using regular expressions. These facilities mainly
consist of classes helping you to specify regular expression, matching them to
text, and determining which parts of the text (if any) match (parts of) the
text being analyzed.


18.8.1: The regular expression mini language

    Regular expressions are expressions consisting of elements resembling those of
numeric expressions. Regular expressions consist of basic elements and
operators, having various priorities and associations. Like numeric
expressions, parentheses can be used to group elements together to form a
unit on which operators operate. For an extensive discussion the reader is
referred to, e.g., section 15.10 of the ecma-international.org page, which
describes the characteristics of the regular expressions used by default by
C++'s regex classes.

C++'s default definition of regular expressions distinguishes the
following atoms:
    
    o x: the character `x';

o .: any character except for the newline character;

o [xyz]: a character class; in this case, either an `x', a `y', or a `z'
        matches the regular expression.  See also the paragraph about
        character classes below;

o [abj-oZ]: a character class containing a range of characters; this
        regular expression matches an `a', a `b', any letter from `j' through
        `o', or a `Z'.  See also the paragraph about character classes below;

o [^A-Z]: a negated character class: this regular expression matches
        any character but those in the class beyond ^.  In this case, any
        character except for an uppercase letter.  See also the paragraph
        about character classes below;

o [:predef:]: a predefined set of characters. See below for an
        overview. When used, it is interpreted as an element in a character
        class. It is therefore always embedded in a set of square brackets
        defining the character class (e.g., [[:alnum:]]);

o \X: if X is `a', `b', `f', `n', `r', `t', or `v', then the ANSI-C
        interpretation of `\x'. Otherwise, a literal `X' (used to escape
        operators such as *);

o (r): the regular expression r. It is used to override precedence
        (see below), but also to define r as a
       marked sub-expression whose matching characters may directly be
        retrieved from, e.g., a std::smatch object (cf. section
        18.8.3);

o (?:r): the regular expression r. It is used to override
        precedence (see below), but it is not regarded as a marked
        sub-expression;

In addition to these basic atoms, the following special atoms are available
(which can also be used in character classes):
    
    o \s: a whitespace character;

o \S: any character but a whitespace character;

o \d: a decimal digit character;

o \D: any character but a decimal digit character;

o \w: an alphanumeric character or an underscore (_) character;

o \W: any character but an alphanumeric character or an underscore
        (_) character.

Atoms may be concatenated. If r and s are atoms then the regular
expression rs matches a target text if the target text matches r
and s, in that order (without any intermediate characters 
inside the target text). E.g., the regular expression [ab][cd] matches the
target text ac, but not the target text a:c. 

Atoms may be combined using operators. Operators bind to the preceding
atom. If an operator should operate on multiple atoms the atoms must be
surrounded by parentheses (as in (r) used above: if r is one, then
use (one) when applying the operator to the word one, instead of just
the final e).  To use an operator character as an atom it can be
escaped. Eg., * represents an operator, \* the atom character
star. Note that character classes do not recognize escape sequences: [\*]
represents a character class consisting of two characters: a backslash and a
star.

The following operators are supported (r and s represent regular
expression atoms):
    
    o r*: zero or more rs;

o r+: one or more rs;

o r?: zero or one rs (that is, an optional r);

o r{m, n}: where 1 <= m <= n: matches `r' at least m, but at most n
        times;

o r{m,}: where 1 <= m: matches `r' at least m times;

o r{m}: where 1 <= m: matches `r' exactly m times;

o r|s: matches either an `r' or an `s'. This operator has a lower
        priority than any of the multiplication operators;

o ^r : ^ is a pseudo operator. This expression matches `r', if
        appearing at the beginning of the target text. If the ^-character
        is not the first character of a regular expression it is interpreted
        as a literal ^-character;

o r$: $ is a pseudo operator. This expression matches `r', if
        appearing at the end of the target text. If the $-character is not
        the last character of a regular expression it is interpreted as a
        literal $-character;

When a regular expression contains marked sub-expressions and multipliers, and
the marked sub-expressions are multiply matched, then the target's final
sub-string matching the marked sub-expression is reported as the text matching
the marked sub-expression. E.g, when using regex_search (cf. section
18.8.4.3), marked sub-expression (((a|b)+\s?)), and target text a a
b, then a a b is the fully matched text, while b is reported as the
sub-string matching the first and second marked sub-expressions.


18.8.1.1: Character classes

        Inside a character class all regular expression operators lose their special
meanings, except for the special atoms \s, \S, \d, \D, \w, and \W; the
character range operator -; the end of character class operator ];
and, at the beginning of the character class, ^. Except in combination
with the special atoms the escape character is interpreted as a literal
backslash character (to define a character class containing a backslash and a
d simply use [d\]).

To add a closing bracket to a character class use [] immediately following
the initial open-bracket, or start with [^] for a negated character class
not containing the closing bracket.  Minus characters are used to define
character ranges (e.g., [a-d], defining [abcd]) (be advised that the
actual range may depend on the locale being used). To add a literal minus
character to a character class put it at the very beginning ([-, or
[^-) or at the very end (-]) of a character class.

Once a character class has started, all subsequent characters are added to the
class's set of characters, until the final closing bracket (]) has been
reached.

In addition to characters and ranges of characters, character classes may also
contain predefined sets of character. They are:
             [:alnum:] [:alpha:] [:blank:]
         [:cntrl:] [:digit:] [:graph:]
         [:lower:] [:print:] [:punct:]
         [:space:] [:upper:] [:xdigit:]

These predefined sets designate sets of characters equivalent to the
corresponding standard C isXXX function. For example, [:alnum:]
defines all  characters for which isalnum(3) returns true.


18.8.2: Defining regular expressions: std::regex

    Before using the (w)regex class presented in this section the
<regex> header file must be included.

The types std::regex and std::wregex define regular
expression patterns. They define, respectively the types
basic_regex<char> and basic_regex<wchar_t>
types. Below, the class regex is used, but in the examples wregex
could also have been used.

Regular expression facilities were, to a large extent, implemented through
templates, using, e.g., the basic_string<char> type (which is equal to
std::string). Likewise, generic types like OutputIter (output
iterator) and BidirConstIter (bidirectional const iterator) are used with
several functions. Such functions are function templates. Function templates
determine the actual types from the arguments that are provided at
call-time.

These are the steps that are commonly taken when using regular expressions:
    
    o  First, a regular expression is defined. This involves
        defining or modifying a regex object.
    o  Then the regular expression is provided with a target text, which
        may result in sections of the target text matching the regular
        expression. 
    o  The sections of the target text matching (or not matching) the
        regular expression are retrieved to be processed elsewhere, or:
    o  The sections of the target text matching (or not matching) the
        regular expression are directly modified by existing regular
        expression facilities, after which the modified target text may be
        processed elsewhere.

The way regex objects handle regular expressions can be configured using a
bit_or combined set of std::regex_constants values,
defining a regex::flag_type value. These
regex_constants are:
    
    o std::regex_constants::awk:
awk(1)'s (POSIX) regular expression grammar is used to specify
        regular exressions (e.g., regular expressions are delimited by
        /-characters, like /\w+/; for further details and for details
        of other regular expression grammars the reader should consult the
        man-pages of the respective programs);

o std::regex_constants::basic:
the basic POSIX regular expression grammar is used to specify regular
        expressions; 

o std::regex_constants::collate:
the character range operator (-) used in character classes defines
        a locale sensitive range (e.g., [a-k]);

o std::regex_constants::ECMAScript:
this flag_type is used by default by regex
        constructors. The regular expression uses the Modified ECMAScript
        regular expression grammar;

o std::regex_constants::egrep:
egrep(1)'s (POSIX) regular expression grammar is used to specify
        regular expressions. This is the same grammar as used by
        regex_constants::extended, with the addition of the newline
        character ('\n') as an alternative for the '|'-operator;

o std::regex_constants::extended:
the extended POSIX regular expression grammar is used to specify
        regular expressions;

o std::regex_constants::grep:
grep(1)'s (POSIX) regular expression grammar is used to specify
        regular expressions. This is the same grammar as used by
        regex_constants::basic, with the addition of the newline character
        ('\n') as an alternative for the '|'-operator;

o std::regex_constants::icase:
letter casing in the target string is ignored. E.g., the regular
        expression A matches a and A;  

o std::regex_constants::nosubs:
When performing matches, all sub-expressions ((expr)) are
        treated as non-marked (?:expr);

o std::regex_constants::optimize:
optimizes the speed of matching regular expressions, at the cost of
        slowing down the construction of the regular expression somewhat. If
        the same regular expression object is frequently used then this flag
        may substantially improve the speed of matching target texts;

Constructors

The default, move and copy constructors are available. Actually, the
default constructor defines one parameter of type regex::flag_type, for
which the value regex_constants::ECMAScript is used by default.
    
    o regex():
the default constructor defines a regex object not containing a
        regular expression;

o explicit regex(char const *pattern):
defines a regex object containing the regular expression found at
        pattern;

o regex(char const *pattern, std::size_t count):
defines a regex object containing the regular expression found at
        the first count characters of pattern;

o explicit regex(std::string const &pattern):
defines a regex object containing the regular expression found at
        pattern. This constructor is defined as a member template,
        accepting a basic_string-type argument which may also use
        non-standard character traits and allocators;

o regex(ForwardIterator first, ForwardIterator last):
defines a regex object containing the regular expression found at
        the (forward) iterator range [first, last). This constructor is
        defined as a member template, accepting any forward iterator type
        (e.g., plain char pointers) which can be used to define the
        regular expression's pattern;

o regex(std::initializer_list<Char> init):
defines a regex object containing the regular expression from the
        characters in the initializer list init.

Here are some examples:
            std::regex re("\\w+");      // matches a sequence of alpha-numeric
                                // and/or underscore characters 

    std::regex re{'\\', 'w', '+'} ;     // idem

    std::regex re(R"(\w+xxx")", 3);     // idem

Member functions


    o regex &operator=(RHS):
The copy and move assignment operators are available. Otherwise, RHS
        may be:
        
        o  an NTBS (of type char const *);
        o  a std::string const & (or any compatible
            std::basic_string); 
        o  a std::initializer_list<char>;

o regex &assign(RHS):
This member accepts the same arguments as regex's constructors,
        including the (optional) regex_constants values;

o regex::flag_type flag() const:
Returns the regex_constants flags that are active for the current
        regex object. E.g.,
                int main()
    {
        regex re;

        regex::flag_type flags = re.flags();
    
        cout <<                                 // displays: 16 0 0
            (re.flags() & regex_constants::ECMAScript) << ' '  <<
            (re.flags() & regex_constants::icase) << ' '  <<
            (re.flags() & regex_constants::awk) << ' '  << '\n';
    }

Note that when a combination of flag_type values is specified at
        construction-time that only those flags that were specified are
        set. E.g., when re(regex_constants::icase) would have been
        specified the cout statement would have shown 0 1
        0. It's also possible to specify conflicting combinations of
        flag-values like regex_constants::awk | regex_constants::grep. The
        construction of such regex objects succeeds, but should be
        avoided.

o locale_type get_loc() const:
Returns the locale that is associated with the current regex
        object;

o locale_type imbue(locale_type locale):
Replaces the regex object's current locale setting with
        locale, returning the replaced locale;

o unsigned mark_count() const:
The number of marked sub-expressions in
        the regex object is returned. E.g., 
                int main()
    {
        regex re("(\\w+)([[:alpha:]]+)"); 
        cout << re.mark_count() << '\n';        // displays: 2
    }

o void swap(regex &other) noexcept:
Swaps the current regex object with other. Also available as a
        free function: void swap(regex &lhs, regex &rhs), swapping lhs
        and rhs.


18.8.3: Retrieving matches: std::match_results

    Once a regex object is available, it can be used to match some target text
against the regular expression. To match a target text against a regular
expression the following functions, described in the next section
(18.8.4), are available:
    
    o regex_match merely matches a target text against a regular
expression, informing the caller whether a match was found or not;
    o regex_search also  matches a target text against a regular
expression, but allows retrieval of matches of marked sub-expressions (i.e.,
parenthesized regular expressions);
    o regex_replace matches a target text against a regular
expression, and replaces pieces of matched sections of the target text by
another text.

These functions must be provided with a target text and a regex object
(which is not modified by these functions). Usually another argument, a
std::match_results object is also passed to these
functions, to contain the results of the regular expression matching
procedure.

Before using the match_results class the <regex> header file must be
included.

Examples of using match_results objects are provided in section
18.8.4. This and the next section are primarily for referential
purposes. 

Various specializations of the class match_results exist. The
specialization that is used should match the specializations of the used
regex class. E.g., if the regular expression was specified as a char
const * the match_results specialization should also operate on char
const * values. The various specializations of match_results have been
given names that can easily be remembered, so selecting the appropriate
specialization is simple.

The class match_results has the following specializations:
    
    o cmatch:
defines match_results<char const *>, using a char const * type
        of iterator. It should be used with a regex(char const *) regular
        expression specification;
    o wcmatch:
defines match_results<wchar_ const *>, using a wchar_t const *
        type of iterator. It should be used with a regex(wchar_t const *)
        regular expression specification;
    o smatch:
defines match_results<std::string::const_iterator>, using a
        std::string::const_iterator type of iterator. It should be used
        with a regex(std::string const &) regular expression
        specification;
    o wsmatch:
defines match_results<std::wstring::const_iterator>, using a
        std::wstring::const_iterator type of iterator. It should be used
        with a regex(wstring const &) regular expression specification.

Constructors

The default, copy, and move constructors are available. The default
constructor defines an Allocator const & parameter, which by default is
initialized to the default allocator. Normally, objects of the class
match_results receive their match-related information by passing them to
the above-mentioned functions, like regex_match. When returning from these
functions members of the class match_results can be used to retrieve
specific results of the matching process.

Member functions


    o match_results &operator=:
The copy and move assignment operators are available;

o std::string const &operator[](size_t idx) const:
Returns a (const) reference to sub-match idx. With idx value 0
        a reference to the full match is returned. If idx >= size() (see
        below) a reference to an empty sub-range of the target string is
        returned. The behavior of this member is undefined if the member
        ready() (see below) returns false;

o Iterator begin() const:
Returns an iterator to the first sub-match. Iterator is a
        const-iterator for const match_results objects;

o Iterator cbegin() const:
Returns an iterator to the first sub-match. Iterator is a
        const-iterator;

o Iterator cend() const:
Returns an iterator pointing beyond the last sub-match. Iterator
        is a const-iterator;

o Iterator end() const:
Returns an iterator pointing beyond the last sub-match. Iterator
        is a const-iterator for const match_results objects;

o ReturnType format(Parameters) const:
As this member requires a fairly extensive description, it would break
        the flow of the current overview. This member is used in combination
        with the regex_replace function, and it is therefore covered in
        detail in that function's section (18.8.4.5);

o allocator_type get_allocator() const:
Returns the object's allocator;

o bool empty() const:
Returns true if the match_results object contains
        no matches (which is also returned after merely using the default
        constructor). Otherwise it returns false;

o int length(size_t idx = 0) const:
Returns the length of sub-match idx. By default the length of
        the full match is returned. If idx >= size() (see below) 0 is
        returned;

o size_type max_size() const:
Returns the maximum number of sub-matches that can be contained in a
        match_results object. This is an implementation dependent constant
        value;

o int position(size_t idx = 0) const:
Returns the offset in the target text of the first character of
        sub-match idx. By default the position of the first character of
        the full match is returned. If idx >= size() (see below) -1 is
        returned;

o std::string const &prefix() const:
Returns a (const) reference to a sub-string of the target text that
        ends at the first character of the full match;

o bool ready() const:
No match results are available from a default constructed
        match_results object. It receives its match results from one of
        the mentioned matching functions. Returns true once match results
        are available, and false otherwise.

o size_type size() const:
Returns the number of sub-matches. E.g., with a regular expression
        (abc)|(def) and target defcon three submatches are reported:
        the total match (def); the empty text for (abc); and def for
        the (def) marked sub-expression. 

Note: when multipliers are used only the last match is counted and
        reported. E.g., for the pattern (a|b)+ and target aaab two
        sub-matches are reported: the total match aaab, and the last match
        (b);

o std::string str(size_t idx = 0) const:
Returns the characters defining sub-match idx. By default this is
        the full match. If idx >= size() (see below) an empty string
        returned;

o std::string const &suffix() const:
Returns a (const) reference to a sub-string of the target text that
        starts beyond the last character of the full match;

o void swap(match_results &other) noexcept:
Swaps the current match_results object with other. Also
        available as a free function: void swap(match_results &lhs,
        match_results &rhs), swapping lhs and rhs.


18.8.4: Regular expression matching functions

    Before using the functions presented in this section the <regex> header
file must be included.

There are three major families of functions that can be used to match a target
text against a regular expression. Each of these functions, as well as the
match_results::format member, has a final
std::regex_constants::match_flag_type parameter (see the next section),
which is given the default value regex_constants::match_default which can
be used to fine-tune the way the regular expression and the matching process
is being used. This
final parameter is not explicitly mentioned with the regular expression
matching functions or with the format member. The three families of
functions are:
    
    o bool std::regex_match(Parameters):
This family of functions is used to match a regular expression against
        a target text. Only if the regular expression matches the full target
        text true is returned; otherwise false is returned. Refer to
        section 18.8.4.2 for an overview of the available overloaded
        regex_match functions;

o bool std::regex_search(Parameters):
This family of functions is also used to match a regular expression
        against a target text. This function returns true once the regular
        expression matches a sub-string of the target text; otherwise
        false is returned. See below for an overview of the available
        overloaded regex_search functions;

o ReturnType std::regex_replace(Parameters):
This family of functions is used to produce modified texts, using the
        characters of a target string, a regex object and a format
        string. This member closely resembles the functionality of the 
        match_results::format member discussed in section 18.8.4.4.

The match_results::format member can be used after regex_replace
and is discussed after covering regex_replace (section 18.8.4.4).


18.8.4.1: The std::regex_constants::match_flag_type flags

        All overloaded format members and all regular expression matching
functions accept a final
regex_constants::match_flag_type argument, which is a
bit-masked type, for which the bit_or operator can be used. All format
members by default specify the argument match_default.

The match_flag_type enumeration defines the following values (below,
`[first, last)' refers to the character sequence being matched).


o format_default (not a bit-mask value, but a default value which is
	equal to 0). With just this specification ECMAScript rules are used to
	construct strings in std::regex_replace;

o format_first_only: std::regex_replace only replaces the first
	match;

o format_no_copy: non-matching strings are not passed to the output
	by std::regex_replace;

o format_sed:  POSIX sed(1) rules are used to construct strings in
	std::regex_replace;

o match_any: if multiple matches are possible, then any match is an
	acceptable result;

o match_continuous: sub-sequences are only matching if they start at
	first; 

o match_not_bol: the first character in [first, last) is treated
	as an ordinary character:  ^ does not match [first, first);

o match_not_bow: \b does not match [first, first);

o match_default (not a bit-mask value, but equal to 0): the default
    value of the final argument that's passed to the regular expression
    matching functions and match_results::format member. ECMAScript
	rules are used to construct strings in std::regex_replace;

o match_not_eol: the last character in [first, last) is treated
	as an ordinary character: $ does not match [last,last);

o match_not_eow: \b does  not match [last, last);

o match_not_null: empty sequences are not considered matches;

o match_prev_avail: --first refers to a valid character
	position. When specified match_not_bol and match_not_bow are
	ignored;


18.8.4.2: Matching full texts: std::regex_match

        The regular expression matching function std::regex_match
returns true if the regular expression defined in its provided
regex argument fully matches the provided target text. This means that
match_results::prefix and match_results::suffix must return empty
strings. But defining sub-expressions is OK.

The following overloaded variants of this function are available:
    
    o bool regex_match(BidirConstIter first, BidirConstIter last,
                  std::match_results &results,
                  std::regex const &re):
BidirConstIter is a bidirectional const iterator. The range
        [first, last) defines the target text. The match results are
        returned in results. The types of the iterators must match the
        type of the match_results that's used. E.g., a cmatch should
        be used if the iterators are of char const * types, and a
        smatch should be used if the iterators are of
        string::const_iterator types. Similar correspondence requirements
        hold true for the other overloaded versions of this function;

o bool regex_match(BidirConstIter first, BidirConstIter last,
        std::regex const &re):
this function behaves like the previous function, but does not
        return the results of the matching process in a match_results
        object;

o bool regex_match(char const *target,
                  std::match_results &results, std::regex const &re):
this function behaves like the first overloaded variant, using
        the characters in target as its target text;

o bool regex_match(char const *str, std::regex const &re):
this function behaves like the previous function but does not return
        the match results;

o bool regex_match(std::string const &target,
                  std::match_results &results, std::regex const &re):
this function behaves like the first overloaded variant, using
        the characters in target as its target text;

o bool regex_match(std::string const &str, std::regex const &re):
this function behaves like the previous function but does not return
        the match results;

o bool regex_match(std::string const &&, std::match_results &, 
                         std::regex &) = delete
       (the regex_match function does not accept temporary string
        objects as target strings, as this would result in invalid string
        iterators in the match_result argument.)

Here is a small example: the regular expression matches the matched text
(provided by argv[1]) if it starts with 5 digits and then 
merely contains  letters ([[:alpha:]]). The digits can be retrieved as
sub-expression 1:
            #include <iostream>
    #include <regex>
    
    using namespace std;
    
    int main(int argc, char const **argv)
    {
        regex re("(\\d{5})[[:alpha:]]+"); 

        cmatch results;

        if (not regex_match(argv[1], results, re))
            cout << "No match\n";
        else
            cout << "size: " << results.size() << ": " << 
                    results.str(1) << " -- " << results.str() << '\n';
    }


18.8.4.3: Partially matching text: std::regex_search

        Different from regex_match the regular expression matching function
std::regex_search returns true if the regular
expression defined in its regex argument partially matches the target
text.

The following overloaded variants of this function are available:
    
    o bool regex_search(BidirConstIter first, BidirConstIter last,
                  std::match_results &results,
                  std::regex const &re):
BidirConstIter is a bidirectional const iterator. The range 
        [first, last) defines the target text. The match results are
        returned in results. The types of the iterators must match the
        type of the match_results that's used. E.g., a cmatch should
        be used if the iterators are of char const * types, and a
        smatch should be used if the iterators are of
        string::const_iterator types. Similar correspondence requirements
        hold true for the other overloaded versions of this function;

o bool regex_search(BidirConstIter first, BidirConstIter last,
        std::regex const &re):
this function behaves like the previous function, but does not
        return the results of the matching process in a match_results
        object;

o bool regex_search(char const *target,
                  std::match_results &results, std::regex const &re):
this function behaves like the first overloaded variant, using
        the characters in target as its target text;

o bool regex_search(char const *str, std::regex const &re):
this function behaves like the previous function but does not return
        the match results;

o bool regex_search(std::string const &target,
                  std::match_results &results, std::regex const &re):
this function behaves like the first overloaded variant, using
        the characters in target as its target text;

o bool regex_search(std::string const &str, std::regex const &re):
this function behaves like the previous function but does not return
        the match results;

o bool regex_search(std::string const &&, std::match_results &, 
                         std::regex &) = delete:
the regex_search function does not accept temporary string
        objects as target strings, as this would result in invalid string
        iterators in the match_result argument.

The following example illustrates how regex_search could be used:
         1: #include <iostream>
     2: #include <string>
     3: #include <regex>
     4: 
     5: using namespace std;
     6: 
     7: int main()
     8: {
     9:     while (true)
    10:     {
    11:         cout << "Enter a pattern or plain Enter to stop: ";
    12: 
    13:         string pattern;
    14:         if (not getline(cin, pattern) or pattern.empty())
    15:             break;
    16: 
    17:         regex re(pattern);
    18:         while (true)
    19:         {
    20:             cout << "Enter a target text for `" << pattern << "'\n"
    21:                     "(plain Enter for the next pattern): ";
    22: 
    23:             string text;
    24:             if (not getline(cin, text) or text.empty())
    25:                 break;
    26: 
    27:             smatch results;
    28:             if (not regex_search(text, results, re))
    29:                 cout << "No match\n";
    30:             else
    31:             {
    32:                 cout << "Prefix: "  << results.prefix() << "\n"
    33:                         "Match:  "  << results.str()    << "\n"
    34:                         "Suffix: "  << results.suffix() << "\n";
    35:                 for (size_t idx = 1; idx != results.size(); ++idx)
    36:                     cout << "Match " << idx << " at offset " <<
    37:                                 results.position(idx) << ": " <<
    38:                                 results.str(idx) << '\n';
    39:             }
    40:         }
    41:     }
    42: }



18.8.4.4: The member std::match_results::format

        The  match_results::format member is a rather complex member
function of the class match_results, which can be used to modify text
which was previously matched against a regular expression, e.g., using the
function regex_search. Because of its complexity and because the
functionality of another regular expression processing function
(regex_replace) offers similar functionality it is discussed at this point
in the C++ Annotations, just before discussing the regex_replace function.

The format member operates on (sub-)matches contained in a
match_results object, using a format string, and producing text in
which  format specifiers (like $&) are replaced by
matching sections of the originally provided target text. In addition, the
format member recognizes all standard C escape sequences (like
\n). The format member is used to create text that is modified with
respect to the original target text.

As a preliminary illustration: if results is a match_results object
and match[0] (the fully matched text) equals `hello world', then
calling format with the format string this is [$&] produces the text
this is [hello world]. Note the specification $& in this format
string: this is an example of a format specifier. Here is an overview of all
supported format specifiers:
    
    o $`: corresponds to the text returned by the prefix member: all
        characters in the original target text up to the first character
        of the fully matched text;
    o $&: corresponds to the fully matched text (i.e., the text returned by
        the match_results::str member);
    o $n: (where n is an integral natural number): corresponds to the
        text returned bu operator[](n);
    o $': corresponds to the text returned by the suffix member: all
        characters in the original target string beyond the last character
        of the fully matched text;
    o $$: corresponds to the single $ character.

Four overloaded versions of the format members are available. All
overloaded versions define a final regex_constants::match_flag_type
parameter, which is by default initialized to match_default. This final
parameter is not explicitly mentioned in the following coverage of the
format members.

To further illustrate the way the format members can be used it is assumed
that the following code has been executed:
         1:     regex re("([[:alpha:]]+)\\s+(\\d+)");  // letters blanks digits
     2: 
     3:     smatch results;
     4:     string target("this value 1024 is interesting");
     5: 
     6:     if (not regex_search(target, results, re))
     7:         return 1;

  After calling regex_search (line 6) the results of the regular
expression matching process are available in the match_results results
object that is defined in line 3.

The first two overloaded format functions expect an output-iterator to
where the formatted text is written. These overloaded members return the 
final output iterator, pointing just beyond the character that was last
written.
    
    o OutputIter format(OutputIter out, char const *first, char const
        *last)  const:
the characters in the range [first, last) are applied to the 
        sub-expressions stored in the match_results object, and the
        resulting string is inserted at out. An illustration is provided
        with the next overloaded version;

o OutputIter format(OutputIter out, std::string const &fmt)  const:
the content of fmt is applied to the sub-expressions stored in
        the match_results object, and the resulting string is inserted at
        out. The next line of code inserts the value 1024 into cout
        (note that fmt must be a std::string, hence the explicit
        use of the string constructor):
         results.format(ostream_iterator<char>(cout, ""), "$2"s);

The remaining two overloaded format members expect a std::string or
an NTBS defining the format string. Both members return a std::string
containing the formatted text:
    
    o std::string format(std::string const &fmt) const
    o std::string format(char const *fmt) const

The next example shows how a string can be obtained
in which the order of the first and second marked sub-expressions contained in
the previously obtained match_results object have been
swapped: 
        string reverse(results.format("$2 and $1"));


18.8.4.5: Modifying target strings: std::regex_replace

        The family of std::regex_replace functions uses regular
expressions to perform substitution on sequences of characters. Their
functionality closely resembles the functionality of the
match_results::format member discussed in the previous section. The
following overloaded variants are available:
    
    o OutputIt regex_replace(OutputIter out, 
                        BidirConstIter first, BidirConstIter last,
                        std::regex const &re, std::string const &fmt):
OutputIter is an output iterator; BidirConstIter a
        bidirectional const iterator.

The function returns the possibly modified text in an iterator range
            [out, retvalue), where out is the output iterator
            passed as the first argument to regex_replace, and
            retvalue is the output iterator returned by
            regex_replace. 

The function matches the text at the range [first, last)
            against the regular expression stored in re. If the regular
            expression does not match the target text in the range
            [first, last) then the target text is literally copied to
            out. If the regular expression does match the target text
            then 
       
        o  first, the match result's prefix is copied to out. The prefix
            equals the initial characters of the target text up to the very
            first character of the fully matched text.
        o  next, the matched text is replaced by the content of the fmt
            format string, in which the format specifiers can be used that
            were described in the previous section (section 18.8.4.4), and
            the replaced text is copied to out;
        o  finally, the match result's suffix is copied to out. The
            suffix equals all characters of the target text beyond the last
            character of the matched text.

The workings of regex_replace is illustrated in the next example:
     1:     regex re("([[:alpha:]]+)\\s+(\\d+)");  // letters blanks digits
 2: 
 3:     string target("this value 1024 is interesting");
 4: 
 5:     regex_replace(ostream_iterator<char>(cout, ""), target.begin(),
 6:                     target.end(), re, "$2"s);
 
       In line 5 regex_replace is called. Its format string merely
        contains $2, matching 1024 in the target text. The prefix ends at
        the word value, the suffix starts beyond 1024, so the statement in
        line 5 inserts the text this 1024 is interesting

into the
        standard output stream.  

o OutputIt regex_replace( OutputIter out, BidirConstIter first,
        BidirConstIter last, std::regex const &re, char const *fmt):
This variant behaves like the first variant. When using, in the above
        example, "$2" instead of "$2"s, then this variant would
        have been used;

o std::string regex_replace(std::string const &str,
                   std::regex const &re, std::string const &fmt):
This variant returns a std::string containing the modified text,
        and expects a std::string containing the target text. Other than
        that, it behaves like the first variant. To use this overloaded
        variant in the above example the statement in line 5 could have been
        replaced by the following statement, initializing the string
        result:
       string result(regex_replace(target, re, "$2"s));

o std::string regex_replace(std::string const &str,
                   std::regex const &re, char const *fmt):
After changing, in the above statement, "$2"s into
        "$2", this variant is used, behaving exactly like the previous
        variant;

o std::string regex_replace(char const *str,
                   std::regex const &re, std::string const &fmt):
This variant uses a char const * to point to the target text, and
        behaves exactly like the previous but one variant;

o std::string regex_replace(char const *str,
                   std::regex const &re, char const *fmt):
This variant also uses a char const * to point to the target text,
        and also behaves exactly like the previous but one variant;


18.9: Randomization and Statistical Distributions

    Before the statistical distributions and accompanying random number
generators can be used the <random> header file must be included.

The STL offers several standard mathematical (statistical)
distributions. These distributions allow programmers to obtain randomly
selected values from a selected distribution.

These statistical distributions need to be provided with a random number
generating object. Several of such random number generating objects are
provided, extending the traditional rand function that is part of the
C standard library.

These random number generating objects produce pseudo-random numbers, which
are then processed by the statistical distribution to obtain values that are
randomly selected from the specified distribution.

Although the STL offers various statistical distributions their functionality
is fairly limited. The distributions allow us to obtain a random number from
these distributions, but
    probability density functions
 or
    cumulative distribution functions
 are currently not provided by the STL. These functions (distributions as well
as the density and the cumulative distribution functions) are, however,
available in other libraries, like the
    boost math library (specifically:

     http://www.boost.org/doc/libs/1_44_0/libs/math/doc/sf_and_dist/html/index.html).

It is beyond the scope of the C++ Annotations to discuss the mathematical
characteristics of the various statistical distributions. The interested
reader is referred to the pertinent mathematical textbooks (like Stuart and
Ord's (2009)
    
    Kendall's Advanced Theory of Statistics, Wiley) or to web-locations
like http://en.wikipedia.org/wiki/Bernoulli_distribution.


18.9.1: Random Number Generators

    The following generators are available:






 Class template  Integral/Floating point  Quality  Speed  Size of state 




 linear_congruential_engine  Integral  Medium  Medium  1 


 subtract_with_carry_engine  Both  Medium  Fast  25 


 mersenne_twister_engine  Integral  Good  Fast  624 








The linear_congruential_engine random number generator computes
    
value_i+1 = (+a * value_i +
                                                              c+) % m

    It expects template arguments for, respectively, the data type to contain
the generated random values; the multiplier a; the additive constant
c; and the modulo value m. Example:
            linear_congruential_engine<int, 10, 3, 13> lincon;

The linear_congruential generator may be seeded by providing its
constructor with a seeding-argument. E.g., lincon(time(0)).

The subtract_with_carry_engine random number generator computes
    
value_i = (value_i-s -
                                                            value_i-r - carry_i-1) % m

    It expects template arguments for, respectively, the data type to contain
the generated random values; the modulo value m; and the subtractive
constants s and r. Example:
            subtract_with_carry_engine<int, 13, 3, 13> subcar;

The subtract_with_carry_engine generator may be seeded by providing
its constructor with a seeding-argument. E.g., subcar(time(0)).

The predefined mersenne_twister_engine mt19937 (predefined as type in the
<random> header file) is used in the examples below. It can be constructed
using
    `mt19937 mt' or it can be seeded by providing its
constructor with an argument (e.g., mt19937 mt(time(0))). Its function
call operator returns a random unsigned integral value.

Other ways to initialize the mersenne_twister_engine are beyond the
scope of the C++ Annotations (but see Lewis et
al.\ (
    Lewis, P.A.W., Goodman, A.S., and Miller, J.M. (1969), A pseudorandom
number generator for the System/360, IBM Systems Journal, 8, 136-146.) (1969)).

The random number generators may also be seeded by calling their members
seed  accepting unsigned long values or generator functions (as in
lc.seed(time(0)), lc.seed(mt)).

The random number generators offer members min and max
returning, respectively, their minimum and maximum values (inclusive). If a
reduced range is required the generators can be nested in a function or class
adapting the range.

Here's a small example showing how the mersenne_twister_engine
mt19937 can be used to generate random numbers:
            #include <iostream>
    #include <ctime>
    #include <random>

    using namespace std;

        // arguments: 1st: number of random numbers to generate
        //            2nd: lowest positve random number,
        //            3rd: highest positive random number

    int main(int argc, char **argv)
    {
        mt19937 mt( time(0) );      // seed with the current time in secs.

        for (
            size_t nGenerate = stoul(argv[1]), lowest = stoul(argv[2]),
                                               mod = stoul(argv[3]) + 1 - lowest;
                nGenerate--;
        )
            cout << (lowest + mt() % mod) << ' ';

        cout << '\n';
    }


18.9.2: Statistical distributions

    In the following sections the various statistical distributions that are
supported by C++ are covered. The notation RNG is used to
indicate a Random Number Generator and URNG is used to indicate a
Uniform Random Number Generator. With each distribution a
struct param_type is defined containing the distribution's parameters. The
organization of these param_type structs depends on (and is described
at) the actual distribution.

All distributions offer the following members (result_type refers to
the type name of the values returned by the distribution):
    
    o result_type max() const

        returns the distribution's least upper bound;
    o result_type min() const

        returns the distribution's greatest lower bound;
    o param_type param() const

        returns the object's param_type struct;
    o void param(const param_type &param)
        redefines the parameters of the distribution;
    o void reset():
       clears all of its cached values;

All distributions support the following operators (distribution-name
        should be replaced by the name of the intended distribution, e.g.,
        normal_distribution):
    
    o template<typename URNG> result_type operator()(URNG &urng)

       returns the next random value from the statistical distribution, with
        the function object urng returning the next random number selected
        from a uniform random distribution;
    o template<typename URNG> result_type operator()

       (URNG &urng, param_type &param)
 returns the next random value
        from the statistical distribution initialized with the parameters
        provided by the param struct.  The function object urng
        returns the next random number selected from a uniform random
        distribution;
    o std::istream &operator>>(std::istream &in,
            distribution-name &object):
       The parameters of the distribution are extracted from an
        std::istream;
    o std::ostream &operator<<(std::ostream &out,
            distribution-name const &bd):
        The parameters of the distribution are inserted into an
        std::ostream

The following example shows how the distributions can be used. Replacing
the name of the distribution (normal_distribution) by another
distribution's name is all that is required to switch distributions. All
distributions have parameters, like the mean and standard deviation of the
normal distribution, and all parameters have default values. The names of the
parameters vary over distributions and are mentioned below at the individual
distributions. Distributions offer members returning or setting their
parameters.

Most distributions are defined as class templates, requiring the specification
of a data type that is used for the function's return type. If so, an empty
template parameter type specification (<>) will get you the default
type. The default types are either double (for real valued return types)
or int (for integral valued return types). The template parameter type
specification must be omitted with distributions that are not defined as
template classes.

Here is an example showing the use of the statistical distributions, applied
to the normal distribution:

#include <iostream>
#include <ctime>
#include <random>
using namespace std;

int main()
{
    std::mt19937 engine(time(0));
    std::normal_distribution<> dist;

    for (size_t idx = 0; idx < 10; ++idx)
        std::cout << "a random value: " << dist(engine) << "\n";

    cout << '\n' <<
        dist.min() << " " << dist.max() << '\n';
}



18.9.2.1: Bernoulli distribution

        The bernoulli_distribution is used to generate logical truth (boolean)
values with a certain probability p. It is equal to a binomial
distribution for one experiment (cf 18.9.2.2).

The bernoulli distribution is not defined as a class template.

Defined types:
                using result_type = bool;
    struct param_type
    {
      explicit param_type(double prob = 0.5);
      double p() const;                     // returns prob
    };

Constructor and members:
    
    o bernoulli_distribution(double prob = 0.5)

        constructs a bernoulli distribution with probability prob of
        returning true;
    o double p() const

        returns prob;
    o result_type min() const

        returns false;
    o result_type max() const

        returns true;


18.9.2.2: Binomial distribution

        The binomial_distribution<IntType = int> is used to determine the
probability of the number of successes in a sequence of n independent
success/failure experiments, each of which yields success with probability
p.

The template type parameter IntType defines the type of the generated
random value, which must be an integral type.

Defined types:
                using result_type =  IntType;
    struct param_type
    {
      explicit param_type(IntType trials, double prob = 0.5);
      IntType t() const;                    // returns trials
      double p() const;                     // returns prob
    };

Constructors and members and example:
    
    o binomial_distribution<>(IntType trials = 1, double prob = 0.5)
        constructs a binomial distribution for trials experiments, each
        having probability prob of success.
    o binomial_distribution<>(param_type const &param)
        constructs a binomial distribution according to the values stored in
        the param struct.
    o IntType t() const

        returns trials;
    o double p() const

        returns prob;
    o result_type min() const 

        returns 0;
    o result_type max() const

        returns trials;


18.9.2.3: Cauchy distribution

        The cauchy_distribution<RealType = double> looks similar to a normal
distribution. But cauchy distributions have heavier tails.  When studying
hypothesis tests that assume normality, seeing how the tests perform on data
from a Cauchy distribution is a good indicator of how sensitive the tests are
to heavy-tail departures from normality.

The mean and standard deviation of the Cauchy distribution are undefined.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType a = RealType(0),
                            RealType b = RealType(1));

        double a() const;
        double b() const;
    };

Constructors and members:
    
    o cauchy_distribution<>(RealType a = RealType(0),
                            RealType b = RealType(1))
        constructs a cauchy distribution with specified a and b
        parameters.
    o cauchy_distribution<>(param_type const &param)
        constructs a cauchy distribution according to the values stored in
        the param struct.
    o RealType a() const

        returns the distribution's a parameter;
    o RealType b() const

        returns the distribution's b parameter;
    o result_type min() const

        returns the smallest positive result_type value;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.4: Chi-squared distribution

        The chi_squared_distribution<RealType = double> with n degrees of
freedom is the distribution of a sum of the squares of n independent
standard normal random variables.

Note that even though the distribution's parameter n usually is an
integral value, it doesn't have to be integral, as the chi_squared
distribution is defined in terms of functions (exp and Gamma) that
take real arguments (see, e.g., the formula shown in the <bits/random.h>
header file, provided with the GNU g++ compiler distribution).

The chi-squared distribution is used, e.g., when testing the goodness of fit
of an observed distribution to a theoretical one.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType n = RealType(1));

        RealType n() const;
    };

Constructors and members:
    
    o chi_squared_distribution<>(RealType n = 1)
       constructs a chi_squared distribution with specified number of degrees
        of freedom.
    o chi_squared_distribution<>(param_type const &param)
       constructs a chi_squared distribution according to the value stored in
        the param struct;
    o IntType n() const

        returns the distribution's degrees of freedom;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.5: Extreme value distribution

        The extreme_value_distribution<RealType = double> is related to the
Weibull distribution and is used in statistical models where the variable of
interest is the minimum of many random factors, all of which can take positive
or negative values.

It has two parameters: a location parameter a and scale parameter b.
See also 

    http://www.itl.nist.gov/div898/handbook/apr/section1/apr163.htm

Defined types:
        using result_type = RealType;

struct param_type
{
    explicit param_type(RealType a = RealType(0),
                        RealType b = RealType(1));

    RealType a() const;     // the location parameter
    RealType b() const;     // the scale parameter
};

Constructors and members:
    
    o extreme_value_distribution<>(RealType a = 0, RealType b = 1)
        constructs an extreme value distribution with specified a and
        b parameters;
    o extreme_value_distribution<>(param_type const &param)
        constructs an extreme value distribution according to the values
        stored in the param struct.
    o RealType a() const

        returns the distribution's location parameter;
    o RealType stddev() const

        returns the distribution's scale parameter;
    o result_type min() const

        returns the smallest positive value of result_type;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.6: Exponential distribution

        The exponential_distribution<RealType = double> is used to describe the
lengths between events that can be modeled with a homogeneous Poisson
process. It can be interpreted as the continuous form of the
geometric distribution.

Its parameter prob defines the distribution's lambda parameter, called
its rate parameter. Its expected value and standard deviation are both
1 / lambda.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType lambda = RealType(1));

        RealType lambda() const;
    };

Constructors and members:
    
    o exponential_distribution<>(RealType lambda = 1)
       constructs an exponential distribution with specified lambda
        parameter.
    o exponential_distribution<>(param_type const &param)
       constructs an exponential distribution according to the value stored in
        the param struct.
    o RealType lambda() const

        returns the distribution's lambda parameter;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.7: Fisher F distribution

        The fisher_f_distribution<RealType = double> is intensively used in
statistical methods like the Analysis of Variance. It is the distribution
resulting from dividing two Chi-squared distributions.

It is characterized by two parameters, being the degrees of freedom of the two
chi-squared distributions.

Note that even though the distribution's parameter n usually is an
integral value, it doesn't have to be integral, as the Fisher F distribution
is constructed from Chi-squared distributions that accept a non-integral
parameter value (see also section 18.9.2.4).

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType m = RealType(1),
                            RealType n = RealType(1));

        RealType m() const; // The degrees of freedom of the nominator
        RealType n() const; // The degrees of freedom of the denominator
    };

Constructors and members:
    
    o fisher_f_distribution<>(RealType m = RealType(1),
                                RealType n = RealType(1))
        constructs a fisher_f distribution with specified degrees of freedom.
    o fisher_f_distribution<>(param_type const &param)
        constructs a fisher_f distribution according to the values stored in
        the param struct.
    o RealType m() const

        returns the degrees of freedom of the nominator;
    o RealType n() const

        returns the degrees of freedom of the denominator;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.8: Gamma distribution

        The gamma_distribution<RealType = double> is used when working with data
that are not distributed according to the normal distribution. It is often
used to model waiting times.

It has two parameters, alpha and beta. Its expected value is alpha
* beta and its standard deviation is alpha * beta^2.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType alpha = RealType(1),
                            RealType beta = RealType(1));

        RealType alpha() const;
        RealType beta() const;
    };

Constructors and members:
    
    o gamma_distribution<>(RealType alpha = 1, RealType beta = 1)
        constructs a gamma distribution with specified alpha and beta
        parameters.
    o gamma_distribution<>(param_type const &param)
        constructs a gamma distribution according to the values stored in
        the param struct.
    o RealType alpha() const

        returns the distribution's alpha parameter;
    o RealType beta() const

        returns the distribution's beta parameter;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.9: Geometric distribution

        The geometric_distribution<IntType = int> is used to model the number
of bernoulli trials (cf. 18.9.2.1) needed until the first success.

It has one parameter, prob, representing the probability of success in an
individual bernoulli trial.

Defined types:
            using result_type = IntType;

    struct param_type
    {
        explicit param_type(double prob = 0.5);
        double p() const;
    };

Constructors, members and example:
    
    o geometric_distribution<>(double prob = 0.5)
        constructs a geometric distribution for bernoulli trials each having
        probability prob of success.
    o geometric_distribution<>(param_type const &param)
        constructs a geometric distribution according to the values stored in
        the param struct.
    o double p() const

        returns the distribution's prob parameter;
    o param_type param() const

        returns the object's param_type structure;
    o void param(const param_type &param)
        redefines the parameters of the distribution;
    o result_type min() const

        returns the distribution's lower bound (= 0);
    o result_type max() const

        returns the distribution's upper bound;
    o template<typename URNG> result_type operator()(URNG &urng)

        returns the next random value from the geometric distribution
    o template<typename URNG> result_type operator()

        (URNG &urng, param_type &param)

        returns the next random value from a geometric distribution
        initialized by the provided param struct.
    o  The random number generator that is passed to the generating
       functions must return integral values. Here is an example showing how
        the geometric distribution can be used:
        #include <iostream>
#include <ctime>
#include <random>

int main()
{
    std::linear_congruential_engine<unsigned, 7, 3, 61> engine(0);

    std::geometric_distribution<> dist;

    for (size_t idx = 0; idx < 10; ++idx)
        std::cout << "a random value: " << dist(engine) << "\n";

    std::cout << '\n' <<
        dist.min() << " " << dist.max() << '\n';

}



18.9.2.10: Log-normal distribution

        The lognormal_distribution<RealType = double> is a probability
distribution of a random variable whose logarithm is normally distributed. If
a random variable X has a normal distribution, then Y = e^X has a
log-normal distribution.

It has two parameters, m and s representing, respectively, the mean
and standard deviation of ln(X).

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType m = RealType(0),
                            RealType s = RealType(1));

        RealType m() const;
        RealType s() const;
    };

Constructor and members:
    
    o lognormal_distribution<>(RealType m = 0, RealType s = 1)
       constructs a log-normal distribution for a random variable whose mean
        and standard deviation is, respectively, m and s.
    o lognormal_distribution<>(param_type const &param) constructs a
        log-normal distribution according to the values stored in the
        param struct.
    o RealType m() const

        returns the distribution's m parameter;
    o RealType stddev() const

        returns the distribution's s parameter;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.11: Normal distribution

        The normal_distribution<RealType = double> is commonly used in science to
describe complex phenomena. When predicting or measuring variables, errors are
commonly assumed to be normally distributed.

It has two parameters, mean and standard deviation.

Defined types:
           using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType mean = RealType(0),
                            RealType stddev = RealType(1));

        RealType mean() const;
        RealType stddev() const;
    };

Constructors and members:
    
    o normal_distribution<>(RealType mean = 0, RealType stddev = 1)
        constructs a normal distribution with specified mean and stddev
        parameters. The default parameter values define the
        standard normal distribution;
    o normal_distribution<>(param_type const &param)
        constructs a normal distribution according to the values stored in
        the param struct.
    o RealType mean() const

        returns the distribution's mean parameter;
    o RealType stddev() const

        returns the distribution's stddev parameter;
    o result_type min() const

        returns the lowest positive value of result_type;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.12: Negative binomial distribution

        The negative_binomial_distribution<IntType = int> probability distribution
describes the number of successes in a sequence of Bernoulli trials before a
specified number of failures occurs. For example, if one throws a die
repeatedly until the third time 1 appears, then the probability distribution
of the number of other faces that have appeared is a negative binomial
distribution.

It has two parameters: (IntType) k (> 0), being the number of failures
until the experiment is stopped and (double) p the probability of success
in each individual experiment.

Defined types:
            using result_type = IntType;

    struct param_type
    {
        explicit param_type(IntType k = IntType(1), double p = 0.5);

        IntType k() const;
        double p() const;
    };

Constructors and members:
    
    o negative_binomial_distribution<>(IntType k = IntType(1),
                                                            double p = 0.5)
       constructs a negative_binomial distribution with specified k and
        p parameters;
    o negative_binomial_distribution<>(param_type const &param)
       constructs a negative_binomial distribution according to the values
        stored in the param struct.
    o IntType k() const

        returns the distribution's k parameter;
    o double p() const

        returns the distribution's p parameter;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.13: Poisson distribution

        The poisson_distribution<IntType = int> is used to model the probability
of a number of events occurring in a fixed period of time if these events
occur with a known probability and independently of the time since the last
event.

It has one parameter, mean, specifying the expected number of events in
the interval under consideration. E.g., if on average 2 events are observed in
a one-minute interval and the duration of the interval under study is
10 minutes then mean = 20.

Defined types:
            using result_type = IntType;

    struct param_type
    {
        explicit param_type(double mean = 1.0);

        double mean() const;
    };

Constructors and members:
    
    o poisson_distribution<>(double mean = 1)
        constructs a poisson distribution with specified mean
        parameter.
    o poisson_distribution<>(param_type const &param)
        constructs a poisson distribution according to the values stored in
        the param struct.
    o double mean() const

        returns the distribution's mean parameter;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.14: Student t distribution

        The student_t_distribution<RealType = double> is a probability
distribution that is used when estimating the mean of a normally distributed
population from small sample sizes.

It is characterized by one parameter: the degrees of freedom, which is equal
to the sample size - 1.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType n = RealType(1));

        RealType n() const;    // The degrees of freedom
    };

Constructors and members:
    
    o student_t_distribution<>(RealType n = RealType(1))
        constructs a student_t distribution with indicated degrees of freedom.
    o student_t_distribution<>(param_type const &param)
        constructs a student_t distribution according to the values stored in
        the param struct.
    o RealType n() const

        returns the degrees of freedom;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.9.2.15: Uniform int distribution

        The uniform_int_distribution<IntType = int> can be used to select integral
values randomly from a range of uniformly distributed integral values.

It has two parameters, a and b, specifying, respectively, the lowest
value that can be returned and the highest value that can be returned.

Defined types:
            using result_type = IntType;

    struct param_type
    {
        explicit param_type(IntType a = 0, IntType b = max(IntType));

        IntType a() const;
        IntType b() const;
    };

Constructors and members:
    
    o uniform_int_distribution<>(IntType a = 0, IntType b = max(IntType))
        constructs a uniform_int distribution for the specified range of
        values.
    o uniform_int_distribution<>(param_type const &param)
        constructs a uniform_int distribution according to the values stored in
        the param struct.
    o IntType a() const

        returns the distribution's a parameter;
    o IntType b() const

        returns the distribution's b parameter;
    o result_type min() const

        returns the distribution's a parameter;
    o result_type max() const

        returns the distribution's b parameter;


18.9.2.16: Uniform real distribution

        The uniform_real_distribution<RealType = double> can be used to select
RealType values randomly from a range of uniformly distributed
RealType values.

It has two parameters, a and b, specifying, respectively, the
half-open range of values ([a, b)) that can be returned by the
distribution.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType a = 0, RealType b = max(RealType));

        RealType a() const;
        RealType b() const;
    };

Constructors and members:
    
    o uniform_real_distribution<>(RealType a = 0, RealType b = max(RealType))
        constructs a uniform_real distribution for the specified range of
        values.
    o uniform_real_distribution<>(param_type const &param)
        constructs a uniform_real distribution according to the values stored in
        the param struct.
    o RealType a() const

        returns the distribution's a parameter;
    o RealType b() const

        returns the distribution's b parameter;
    o result_type min() const

        returns the distribution's a parameter;
    o result_type max() const

        returns the distribution's b parameter;


18.9.2.17: Weibull distribution

        The weibull_distribution<RealType = double> is commonly used in
reliability engineering and in survival (life data) analysis.

It has two or three parameters and the two-parameter variant is offered by the
STL. The three parameter variant has a shape (or slope) parameter, a scale
parameter and a location parameter. The two parameter variant implicitly uses
the location parameter value 0. In the two parameter variant the shape
parameter (a) and the scale parameter (b) are provided. See 

    http://www.weibull.com/hotwire/issue14/relbasics14.htm for an
interesting coverage of the meaning of the Weibull distribution's parameters.

Defined types:
            using result_type = RealType;

    struct param_type
    {
        explicit param_type(RealType a = RealType{ 1 },
                            RealType b = RealType{ 1 });

        RealType a() const;     // the shape (slope) parameter
        RealType b() const;     // the scale parameter
    };

Constructors and members:
    
    o weibull_distribution<>(RealType a = 1, RealType b = 1)
        constructs a weibull distribution with specified a and b
        parameters;
    o weibull_distribution<>(param_type const &param)
        constructs a weibull distribution according to the values stored in
        the param struct.
    o RealType a() const

        returns the distribution's shape (or slope) parameter;
    o RealType stddev() const

        returns the distribution's scale parameter;
    o result_type min() const

        returns 0;
    o result_type max() const

        returns the maximum value of result_type;


18.10: tie

We already encountered structured bindings in section
3.3.7.1. Structured bindings allow us to access the fields of
structured types (like structs, std::pair or (cf. section 22.6)
tuples) as local variables inside functions. A basic example using
structured bindings is shown in the following code snippet:
        pair<int, int> factory()
    {
        return { 1, 2 };
    }
    
    void fun()
    {
        auto [one, two] = factory();
    
        cout << one << ' ' << two << '\n';
    }

Being able to use structured bindings is very useful in cases like these. 

But what if we want to assign the fields of a struct to variables that have
already been defined or that were passed to a function via its parameters? In
those situations structured bindings offer no help. E.g., in the following
code snippet a function retrieve is defined having an int & parameter
and an int local variable and we want to assign the values returned by
factory to those variables:
            void retrieve(int &one)
    {
        int two;
        // ...  = factory() ??
    }    

Structured bindings cannot be used here: the elements of structured bindings
cannot be references. Although it is possible to define a std::pair<int
&, int &> such an object cannot be initialized with the references of one
and two which are directly referring to the fields returned by
factory. These statements won't compile:
            pair<int &, int &> p{one, two} = factory();
    pair<int &, int &>{one, two} = factory();

While it is possible to first define a pair<int &, int &> object and then
assign factory's return value to it, that approach clearly is less elegant
than what's offered by structured bindings:
            pair<int &, int &> p{one, two};
    p = factory();

Fortunately, there is a better alternative. After including the <tuple>
header file (see also section 22.6) std::tie is available allowing
us to `tie' references to fields of structured data types. Using std::tie
it is very easy to associate the variables one and two of the function
retrieve with the fields of the pair returned by factory:
        void retrieve(int &one)
    {
        int two;
    
        tie(one, two) = factory();
    
        cout << one << ' ' << two << '\n';
    }

When Executing these statements:
        int one = 0;
    int two = 0;

    cout << one << ' ' << two << '\n';

    retrieve(one);

    cout << one << ' ' << two << '\n';

the following output is obtained:
        0 0
    1 2
    1 0

In addition to the above the std::tie function also supports ordering and
(in)equality comparisons. The struct Data in the next example defines
three fields: an int, a std::string and a double. Each of these
fields support ordering and (in)equality comparisons. In those cases, all
comparison operators can easily be implemented through the spaceship
operator (cf. section 11.7.2) using std::tie:
        struct Data
    {
        int d_int;
        string d_string;
        double d_double;
    };
    
    
    bool operator==(Data const &lhs, Data const &rhs)
    {
        return tie(lhs.d_int, lhs.d_string, lhs.d_double) ==
               tie(rhs.d_int, rhs.d_string, rhs.d_double);
    }
    
    partial_ordering operator<=>(Data const &lhs, Data const &rhs)
    {
        return tie(lhs.d_int, lhs.d_string, lhs.d_double) <=>
               tie(rhs.d_int, rhs.d_string, rhs.d_double);
    }

Note that struct Data's spaceship operator returns partial_ordering
values (cf. section 18.7.3). Although int and std::string's
spaceship operators return strong_ordering values, double's spaceship
operator doesn't. Instead it returns partial_ordering
values. Consequently, struct Data's spaceship operator also returns
partial_ordering values.


18.11: Optional return values

In order to use std::optional objects the <optional> header file must
be included.

Consider a function returning subsequent lines from a stream. That function
could be a member function reading from a stream which was opened by its
object. A first implementation of such a member function could be
            std::string Class::nextLine()
    {
        string line;
        getline(d_stream, line);
        return line;
    }
        
    Of course, this implementation is sub-optimal as getline may fail.

Common ways to handle failures in these situations are
    
    o  the functions return pointers to strings, which are 0 when
        getline fails, and point to strings containing the lines
        if getline succeeds;
    o  the functions return bools and define string pointers or reference
        parameters: the functions' return values indicate whether or not
        getline succeeded;
    o  the functions return std::pair or std::tuple objects of which
        one field is a bool and the other field is a std::string.

The standard template library offers yet another way to handle situations like
these: the template class 
            template <typename DataType>
    class optional;
        
    Here, DataType refers to the name of the data type that is handled by
the optional class. Instead of returning a std::string the function
nextLine may specify a std::optional<std::string> return type:
std::optional<std::string> Class::nextLine().

The interpretation of std::optional objects is simple: either it contains
an object of its DataType or it doesn't. If it does contain a
DataType object then that object is available as object instead of a
pointer to an object (which might have been dynamically allocated) of the
specified DataType. At the same type the optional object can be
interpreted as a bool. If the optional object contains a DataType
object the optional's bool value is true. If it doesn't contain a
DataType value, then its bool value is false. 

The class std::optional offers the following facilities:
    
    o  Constructors

       The default constructor (e.g., std::optional<std::string> opt;)
        does not contain a value;

       copy- and move constructors are available;

       objects may be constructed from values that are convertible to the
        optional's DataType (e.g., an optional<string> can be
        initialized from a NTBS). If the initializing value is an rvalue
        reference then the DataType object is move-constructed from the
        initializing value;

o operator=

       the assignment operator can be used to reassign the DataType values
        of optional objects or to reassign the optional objects from
        another optional object using the same DataType. Copy- and
        move-assignment operators are available;

o  Accessors

       explicit operator bool() and the has_value() members return
        true if the optional object contains a DataType value;

       value(), operator*(), and operator->() return references to the
        optional's DataType value. The reference is a const reference
        if called from an optional<DataType> const object; it is an
        rvalue-reference if called from an rvalue reference to an
        optional<DataType> object. Notes:
            
            o  the operator* and
               operator-> members act like value, but do not imply
                that the optional's DataType member itself is stored as a
                pointer;
            o value() checks whether the optional object actually
                contains a DataType object, and throws a
                std::bad_optional_access exception if not.

value_or(Type &&defaultValue) returns a copy of the optional
        object's DataType if the object contains a value or returns
        DataType{ defaultValue } if not. Note that DataType must be
        constructible from Type;

o  Modifiers

       swap(optional<DataType> &other): swaps the current and other
        optional objects' content.

       reset(): erases the optional's DataType member. Following
        reset() has_value() returns false.

       emplace(Args &&...args), emplace(initialize_list, Args &&...args):
        the first emplace forwards args to DataType's constructor;
        the second emplace forwards args to the initializer_list, and
        forwards that list to DataType's constructor;

o  Comparison operators

        all comparison operators (including operator<=>) are available (if
        defined for the optional's DataType) to compare the DataType
        objects of two optional objects;

o std::optional<DataType> make_optional(...)

       returns an optional object constructed from a DataType lvalue
        or rvalue reference, or constructed from the same arguments that are
        accepted by emplace.

Here is the implementation of a function nextLine, using
std::optional<std::string> and a simple main function illustrating its
workings: 
        #include <iostream>
    #include <sstream>
    #include <string>
    #include <optional>

    using namespace std;

    optional<string> nextLine(istream &in)
    {
        std::optional<std::string> opt;

        string line;
        if (getline(in, line))
            opt = move(line);

        cout << "internal: has value: " << opt.has_value() <<
                                    ", value = " << *opt << '\n';
        return opt;
    }

    int main()
    {
        istringstream in{ "hello world\n" };

        auto opt = nextLine(in);

        cout << "main:     has value: " << opt.has_value() <<
                                    ", value = " << *opt << '\n';
        opt = nextLine(in);

        cout << "main:     has value: " << opt.has_value() <<
                                    ", value = " << *opt << '\n';
    }

The ouput of this program is:
        internal: has value: 1, value = hello world
    main:     has value: 1, value = hello world
    internal: has value: 0, value = 
    main:     has value: 0, value = hello world
    
Note that after the 2nd call, when no value is returned, opt has kept the
value it received at the first call: optional's assignment operator
doesn't bother about values already present in their objects once it notices
that has_value will return false. So be sure to inspect has_value
or operator bool before calling value.



Chapter 19: The STL Generic Algorithms


19.1: The Generic Algorithms

Before using the generic algorithms presented in this chapter, except for
those in the Operators category (defined below), the <algorithm> header
file must be included. Before using a generic algorithm in the
Operators category the <numeric> header file must be included.

In the previous chapter the Standard Template Library (STL) was introduced. An
important element of the STL, the generic algorithms, was not covered in
that chapter as they form a fairly extensive part of the STL.  Over time the
STL has grown considerably, mainly as a result of a growing importance and
appreciation of templates. Covering generic algorithm in the STL chapter
itself would turn that chapter into an unwieldy one and so the generic
algorithms were moved to a chapter of their own.

Generic algorithms perform an amazing task. Due to the strength of templates,
algorithms could be developed that can be applied to a wide range of different
data types while maintaining type safety. The prototypical example of this is
the sort generic algorithm. To contrast: while C requires
programmers to write callback functions in which type-unsafe void const *
parameters have to be used, internally forcing the programmer to resort to
casts, STL's sort frequently allows the programmer merely to state
something akin to
            sort(first-element, last-element)

Generic algorithms should be used wherever possible. Avoid the urge to design
your own code for commonly encountered algorithms. Make it a habit to
first thoroughly search the generic algorithms for an available
candidate. The generic algorithms should become your weapon of choice when
writing code: acquire full familiarity with them and make their use your
`second nature'.

On the other hand, without downgrading the importance of the generic
algorithms, it's clear that over the years the number of available generic
algorithms have seen an almost unlimited growth. Many new algorithms were
added, even though for quite a few of them it's either very easy to provide
direct implementations, or which may hardly ever be used. An
example is the is_sorted generic algorithm, simply returning true if a
range of elements is considered sorted and false if not. How hard is it to
define such a function in the occasional situation you might need it? This
applies to quite a few other generic algorithms. It's of course nice to have
an extensive toolbox, but do you need, e.g., screwdrivers that
perfectly match the screws' heads? Most likely not.... To avoid an excessive
growth of this chapter, some sections contain comparable algorithms, and in
some sections links to cppreference are
provided when referring to comparable algorithms. 

Nevertheless, this chapter's sections cover many of the STL's
    generic algorithms (in alphabetical order). For each algorithm the
following information is provided:
    
    o  The required header file;
    o  The function prototype;
    o  A short description;
    o  A short example.

In the prototypes of the algorithms Type is used to specify a
generic data type. Furthermore, the particular type of iterator (see
section 18.2) that is required is mentioned as well as other generic
types that might be required (e.g., performing BinaryOperations, like
plus<Type>). Although iterators are commonly provided by abstract
containers and comparable pre-defined data structures, at some point you may
want to design your own iterators. Section 22.14 offers guidelines
for constructing your own iterator classes and provides an overview of of
operators that must be implemented for the various types of iterators.

Almost every generic algorithm expects an iterator range [first, last),
defining the series of elements on which the algorithm operates. The iterators
point to objects or values. When an iterator points to a Type value or
object, function objects used by the algorithms usually receive Type const
& objects or values. Usually function objects cannot modify the objects they
receive as their arguments. This does not hold true for modifying generic
algorithms, which are of course able to modify the objects they operate
upon.

 Generic algorithms may be categorized.  The
C++ Annotations distinguishes the following categories of generic algorithms:
    
    o  Comparators: comparing (ranges of) elements:
        
            all_of; any_of;
            equal;
            includes;
            lexicographical_compare;
            mismatch;
            none_of;

o  Copiers / movers: performing copy / move operations:
        
            copy;
            copy_backward;
            copy_if;
            move; move_backward;
            partition_copy;
            partial_sort_copy;
            remove_copy; remove_copy_if;
            replace_copy; replace_copy_if;
            reverse_copy;
            rotate_copy;
            sample;
            shift_left; shift_right;
            unique_copy;

o  Counters: performing count operations:
        
            count; count_if;

o  Heap operators: manipulating a max-heap:
        
            make_heap;
            pop_heap;
            push_heap;
            sort_heap;

o  Initializers: initializing data:
        
            fill; fill_n;
            generate; generate_n;
            iota;
            uninitialized (raw) memory;

o  Limiters: determining boundaries of data:
        
            begin; end;

o  Operators: performing some kind of (arithmetic?) operations:
        
            accumulate;
            adjacent_difference;
            exclusive_scan; inclusive_scan;
            inner_product;
            partial_sum;
            reduce;
            transform_reduce;

o  Searchers: performing search (and find) operations:
        
            adjacent_find;
            binary_search;
            equal_range;
            find;
            find_end;
            find_first_of;
            find_if; find_if_not;
            lower_bound;
            max;
            max_element; min_element;
            min;
            minmax;
            minmax_element;
            partition_point;
            search; search_n;
            set_difference;
            set_intersection;
            set_symmetric_difference;
            set_union;
            upper_bound;

o  Shufflers: performing reordering operations (sorting,
            merging, permuting, swapping):
        
            inplace_merge;
            iter_swap;
            merge;
            next_permutation;
            nth_element;
            partial_sort;
            partition;
            prev_permutation;
            remove; remove_copy; remove_copy_if; remove_if;
            reverse; reverse_copy;
            rotate; rotate_copy;
            shuffle;
            sort;
            stable_partition;
            stable_sort;
            swap; swap_ranges;
            unique;

o  Verifiers: checking for specific conditions:
        
            is_partitioned;
            is_permutation;
            is_sorted;
            is_sorted_until;

o  Visitors: visiting elements in a range:
        
            for_each;
            replace; replace_copy; replace_copy_if; replace_if;
            transform;
            unique_copy;


19.1.1: Execution policies

    Many of the following generic algorithms could very well be parallized. E.g.,
one of the following algorithms is generate (section 19.1.19), filling
the elements with values produced by a generating function. If those values
are randomly generated values then generated could very well be
parallized, where each parallel thread handles a separate section of the range
to be filled. Another example where parallel execution may be useful is when
sorting series of values. For this the the sort generic algorithm can be
used (section 19.1.54, which also contains an example of parallelized
sorting).

These (and many other) generic algorithms can be executed in parallel,
depending on the specified execution policy. When no execution policy is
specified then the algorithms operated in their standard, sequential, way. The
generic algorithms supporting execution policies have overloaded versions
where the first parameter specifies the execution policy to use, followed by
their remaining parameters. Function prototypes listed the following sections
showing a first parameter [ExecPol,] may specify one of the execution
policies introduced in this section as their first arguments. E.g., one of the
sort generic algorithm's prototypes is 
       void sort([ExecPol,] 
        RandomAccessIterator first, RandomAccessIterator last);
and to sort, e.g., a vector if strings (vs), it can be called as
       sort(vs.bgin(), vs.end());
or as (see below)
       sort(execution::par, vs.bgin(), vs.end());

In order to use execution policies the <execution> header file must be
included, and the linking option -ltbb must be specified with linking the
compiled object files(s).

There are four types of execution policies (all defined in the std
namespace):
    
    o <execution::sequenced_policy>, whose predefined object
        execution::seq is used to specify this execution policy when
        calling generic algorithms.

When calling a generic algorithm specifying this policy it will not 
        be using parallel execution.

o <execution::parallel_policy>, whose predefined object
        execution::par is used to specify this execution policy when
        calling generic algorithms.

When calling a generic algorithm specifying this policy it may 
        be using parallel execution: the generic algorithm may decide not to
        use parallel execution when it decides that the overhead of parallel
        execution is in fact reducing the efficiency of non-parallel
        execution. E.g., when sorting 100 elements sequential execution is
        faster than parallel execution and an algorithm like sort won't
        use parallel execution.

o <execution::parallel_unsequenced_policy>, whose predefined object
        execution::par_unseq is used to specify this execution policy when
        calling generic algorithms.

When calling a generic algorithm specifying this policy it may be
        using parallel execution, execution may be migrated across threads
        (using a so-called parent-stealing scheduler), or execution may be
        vectorized (i.e., a single thread is used
        accessing data items at completely different locations (like swapping
        the first and middle elements of vectors)). When using this
        policy the order in which processed elements are accessed and the
        threads from which these elements are accessed is undefined.

o <execution::unsequenced_policy>, whose predefined object
        execution::unseq is used to specify this execution policy when
        calling generic algorithms. 

When calling a generic algorithm specifying this policy the algorithm
        uses vectorized execution.

Whenever algorithms are called using the above policy specifications and
during the execution of these algorithms functions are called generating
uncaught exceptions std::terminate is called.

When using parallel execution the objects or functions passed to the generic
algorithms might access data defined elsewhere. If those data are modified
then it is possible that modifications are requested from different execution
threads, which could result in data races or 
deadlocks. The programmer should ensure that data races and/or
dadlocks cannot occur when using parallel execution.


19.1.2: accumulate

        
    
    o  Header file: <numeric>
    o  Function prototypes:
        
        o Type accumulate(InputIterator first, InputIterator last,
            Type init);
        o Type accumulate(InputIterator first, InputIterator
            last, Type init, BinaryOperation op);

o  Description:
        
        o  The first prototype: operator+ is applied to the initial
value init (lhs argument) and, in sequence, all elements reached from
the iterator range (rhs argument). The resulting value is returned.
        o  The second prototype: the binary operator op is applied to
the initial value init (lhs argument) and, in sequence, all elements
reached from the iterator range. The resulting value is returned.

At each step the initial value receives the result of that step's
        computation. E.g., if minus<int> is used with the intial value 1,
        and the iterators refer to 2 and 3, then at step 1: 1 - 2 = -1 is
        computed and at step 2: -1 - 3 = -4. So accumulate returns -4.

(See also the reduce algorithm (section 19.1.43)).

o  Example:
            #include <numeric>
    #include <vector>
    #include <iostream>
    using namespace std;

    int main()
    {
        int         ia[] = {1, 2, 3, 4};
        vector<int> iv(ia, ia + 4);

        cout <<
            "Sum: " << accumulate(iv.begin(), iv.end(), int()) << "\n"
            "Product: " << accumulate(iv.begin(), iv.end(), int(1),
                                            multiplies<int>{}) << '\n';
    }
    //  Displays:
    //      Sum: 10
    //      Product: 24



19.1.3: adjacent_difference

        
    
    o  Header file: <numeric>
    o  Function prototypes:
        
        o OutputIterator adjacent_difference([ExecPol,] InputIterator first,
InputIterator last, OutputIterator result);
        o OutputIterator adjacent_difference([ExecPol,] InputIterator first,
InputIterator last, OutputIterator result, BinaryOperation op);

o  Description:
            All operations are performed on the original values, all computed
values are returned values.
            
            o  The first prototype: the first returned element is equal to
the first element of the input range. The remaining returned elements are
equal to the difference of the corresponding element in the input range and
its previous element.
            o  The second prototype: the first returned element is equal to
    the first element of the input range. The remaining returned elements are
equal to the result of the binary operator op applied to the
corresponding element in the input range (left operand) and its previous
element (right operand).

o  Example:
            #include <numeric>
    #include <vector>
    #include <iterator>
    #include <iostream>
    using namespace std;

    int main()
    {
        int             ia[] = {1, 2, 5, 10};
        vector<int>     iv(ia, ia + 4);
        vector<int>     ov(iv.size());

        adjacent_difference(iv.begin(), iv.end(), ov.begin());

        copy(ov.begin(), ov.end(), ostream_iterator<int>(cout, " "));
        cout << '\n';

        adjacent_difference(iv.begin(), iv.end(), ov.begin(), minus<int>());

        copy(ov.begin(), ov.end(), ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      1 1 3 5
    //      1 1 3 5



19.1.4: adjacent_find

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator adjacent_find([ExecPol,] 
ForwardIterator first, ForwardIterator last);
            o OutputIterator adjacent_find([ExecPol,] ForwardIterator first,
ForwardIterator last, Predicate pred);

o  Description:
        
            o  The first prototype: the iterator pointing to the first
element of the first pair of two adjacent equal elements is returned. If no
such element exists, last is returned.
            o  The second prototype: the iterator pointing to the first
element of the first pair of two adjacent elements for which the binary
predicate pred returns true is returned. If no such element exists,
last is returned.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    using namespace std;

    bool squaresDiff10(size_t first, size_t second)
    {
        return second * second - first * first >= 10;
    }

    int main()
    {
        string sarr[] =
            {
                "Alpha", "bravo", "charley", "delta", "echo", "echo",
                "foxtrot", "golf"
            };
        auto last = end(sarr);          // see the 'begin / end' section
        string *result = adjacent_find(sarr, last);

        cout << *result << '\n';
        result = adjacent_find(++result, last);

        cout << "Second time, starting from the next position:\n" <<
            (
                result == last ?
                    "** No more adjacent equal elements **"
                :
                    "*result"
            ) << '\n';

        size_t iv[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        size_t *ires = adjacent_find(iv, end(iv), squaresDiff10);

        cout <<
            "The first numbers for which the squares differ at least 10: "
            << *ires << " and " << *(ires + 1) << '\n';
    }
    // Displays:
    //
    //   echo
    //   Second time, starting from the next position:
    //   ** No more adjacent equal elements **
    //   The first numbers for which the squares differ at least 10: 5 and 6



19.1.5: all_of / any_of / none_of

          
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool all_of([ExecPol,] InputIterator first,
                     InputIterator last, Predicate pred);
            o bool any_of([ExecPol,] InputIterator first,
                     InputIterator last, Predicate pred);
            o bool none_of([ExecPol,] InputIterator first,
                     InputIterator last, Predicate pred);

o  Description:
        
            o  The first prototype returns true if the unary predicate
pred returns true for all elements reached from the iterator range
[first, last); otherwise false is returned.
            o  The second prototype returns true if the unary predicate
pred returns true for at least on of the elements reached from the
iterator range [first, last); otherwise false is returned.
            o  The third prototype returns true if the unary predicate
pred returns true for none of the elements reached from the iterator
range [first, last); otherwise false is returned.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    using namespace std;

    bool contains_a(string const &str)
    {
        return str.find('a') != string::npos;
    }

    int main()
    {
        string sarr[] =
            { "Alpha", "Bravo", "Charley", "Delta", "Echo" };
        auto past = end(sarr);      // see the next section

        cout << "All elements contain 'a': " <<
            all_of(sarr, past, contains_a) << "\n"
            "At least one element contains 'a': " <<
            any_of(sarr, past, contains_a) << "\n"
            "None of the elements contains 'a': " <<
            none_of(sarr, past, contains_a) << '\n';
    }
    //  Displays:
    //      All elements contain 'a': 0
    //      At least one element contains 'a': 1
    //      None of the elements contains 'a': 0



19.1.6: begin / end
     
            
    
        o  Header file: <iterator> (also available when including
                <algorithm>)
        o  Function prototypes:
        
            o auto begin([const] &obj);
            o auto end([const] &obj);
            o Type [cons] *begin(Type [const] (&array)[N]);
            o Type [cons] *end(Type [const] (&array)[N]);
            o auto cbegin(const &obj);
            o auto cend(const &obj);

o  Description:
        The begin and end functions return, respectively, the begin-
and end-iterators or pointers of objects offering begin() and end()
members or of arrays of (compile-time) available sizes.
        
            o  The first two prototypes return, respectively,
                obj.begin() and obj.end() of (possibly const)
                references to obj
            o  Prototypes three and four return (possibly const)
                pointers to, respectively, the first and beyond the last
                element of the array argument, which must be an array of
                (compile-time) known size.
            o  The lastt two prototypes return, respectively,
                obj.begin() and obj.end() of const
                references to obj

o  Example:
            #include <iterator>
    #include <iostream>
    #include <string>
    using namespace std;

    int main()
    {
        string sarr[] =
            { "Alpha", "Bravo", "Charley", "Delta", "Echo" };

        // since sarr == begin(sarr), begin(...) is not used here
        auto next = end(sarr);

        cout << "sarr has " << size(sarr) <<
                " (== " << (next - sarr) << ") elements\n"
                "the last char. of the first element is " <<
                *(end(sarr[0]) - 1) << '\n';
    }
    //  Displays:
    //    sarr has 5 (== 5) elements
    //    the last char. of the first element is a



19.1.7: binary_search

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool binary_search(ForwardIterator first, ForwardIterator
last, Type const &value);
            o bool binary_search(ForwardIterator first, ForwardIterator
last, Type const &value, Comparator comp);

o  Description:
        
            o  The first prototype: value is searched for using binary
search in the elements reached from the iterator range [first,
last). The elements must have been sorted by the Type::operator<
function. True is returned if the element was found, false otherwise.
            o  The second prototype: value is searched for using binary
search in the elements reached from the iterator range [first,
last). The elements in the range must have been sorted by the Comparator
function object. True is returned if the element was found, false
otherwise. As illustrated by the following example, the function object
function's first parameter refers to an element in the iterator range, while
the function object's second parameter refers to value.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <functional>
    using namespace std;

    int main()
    {
        string sarr[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel"
            };
        auto past = end(sarr);
        bool result = binary_search(sarr, past, "foxtrot");
        cout << (result ? "found " : "didn't find ") << "foxtrot" << '\n';

        reverse(sarr, past);                // reverse the order of elements
                                            // binary search now fails:
        result = binary_search(sarr, past, "foxtrot");
        cout << (result ? "found " : "didn't find ") << "foxtrot" << '\n';
                                            // ok when using appropriate
                                            // comparator:
        result = binary_search(sarr, past, "foxtrot", greater<string>());
        cout << (result ? "found " : "didn't find ") << "foxtrot" << '\n';

        // alternatively, using a lambda expression showing the used 'sarr'
        // indices and the value of the second parameter:

        result = binary_search(sarr, past, "foxtrot",
            [&](string const &sarrEl, string const &value)
            {
                cout << "comparing element " << (&sarrEl - sarr) <<
                        " (" << sarrEl << ") to " << value << '\n';
                return sarrEl > value;
            }
        );
        cout << "found it: " << result << '\n';
    }
    //  Displays:
    //      found foxtrot
    //      didn't find foxtrot
    //      found foxtrot
    //      comparing element 4 (delta) to foxtrot
    //      comparing element 2 (foxtrot) to foxtrot
    //      comparing element 1 (golf) to foxtrot
    //      comparing element -3 (foxtrot) to foxtrot
    //      found it: 1


If value is in fact present in the range of values, then this generic
algorithm doesn't answer the question where value is located. If that
question must be answered the generic algorithms lower_bound
and upper_bound can be used. Refer to sections
19.1.30 and 19.1.61 for examples illustrating the use of these
latter two algorithms.


19.1.8: copy / copy_if

         
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator copy([ExecPol,] InputIterator first,
InputIterator last, OutputIterator destination);
            o OutputIterator copy_if([ExecPol,] InputIterator first,
InputIterator last, OutputIterator destination, Predicate pred);

o  Description:
        
            o  First prototype: the elements reached from the iterator range
[first, last) are copied (assigned) to the destination output
range. The return value is the OutputIterator pointing just beyond the last
element that was copied to the destination range (so, `last' in the
destination range is returned).
            o  Second prototype: same as the first prototype but the
function ends after copying n elements, returning an iterator pointing
just beyond the last element that was copied to the destination range.

o  Example:

    Note the second call to copy. It uses an ostream_iterator for
string objects. This iterator writes the string values to the
specified ostream (i.e., cout), separating the values by the specified
separation string (i.e., " ").
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <iterator>
    using namespace std;

    bool pred(std::string const &str)
    {
        return "aceg"s.find(str.front()) == string::npos;
    }

    int main()
    {
        string sarr[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel"
            };
        auto last = end(sarr);

        copy(sarr + 2, last, sarr); // move all elements two positions left

                     // copy to cout using an ostream_iterator for strings
        copy(sarr, last, ostream_iterator<string>(cout, " "));
        cout << '\n';

                    // using copy_if:
        copy_if(sarr, sarr + size(sarr), sarr, pred);
        copy(sarr, sarr + size(sarr), ostream_iterator<string>(cout, " "));
        cout << '\n';

    }
    //  Displays:
    //  charley delta echo foxtrot golf hotel golf hotel
    //  delta foxtrot hotel hotel golf hotel golf hotel

        o  See also:
            unique_copy


19.1.9: copy_backward

        
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o BidirectionalIterator copy_backward(InputIterator first,
InputIterator last, BidirectionalIterator last2);

o  Description:
        
            o  The series of elements reached from the iterator range
[first, last) are copied from the element at position last - 1
until (and including) the element at position first to the element range,
ending at position last2 - 1 using the assignment operator of the
underlying data type. The destination range is therefore [last2 - (last
- first), last2).

Note that this algorithm does not reverse the order of the elements when
copying them to the destination range.

The return value is the BidirectionalIterator pointing to the last element
that was copied to the destination range (so, `first' in the destination
range, pointed to by last2 - (last - first), is returned).

o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <iterator>
    using namespace std;

    int main()
    {
        string sarr[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel"
            };
        auto past = end(sarr);

        copy
        (
            copy_backward(sarr + 3, past, past - 3),
            past,
            ostream_iterator<string>(cout, " ")
        );
        cout << '\n';
    }
    // Displays:    golf hotel foxtrot golf hotel foxtrot golf hotel



19.1.10: count / count_if

             
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o size_t count([ExecPol,] InputIterator first, 
                InputIterator last, Type const &value);
            o size_t count_if([ExecPol,] InputIterator first,
                InputIterator last, Predicate predicate);

o  Description:
        
            o  The first prototype: returns the number of times value
occurs in the elements reached from the iterator range [first, last).
Uses Type::operator== to determine whether value is equal to an
element in the iterator range.
            o  The second prototype: returns the number of times
    unary predicate `predicate' returns true when applied to the
elements reached from the iterator range [first, last).

o  Example:
            #include <algorithm>
    #include <iostream>
    using namespace std;

    bool pred(int value)
    {
        return value & 1;
    }

    int main()
    {
        int ia[] = {1, 2, 3, 4, 3, 4, 2, 1, 3};

        cout << "Number of times the value 3 is available: " <<
                count(ia, ia + size(ia), 3) << "\n"
                "Number of odd values in the array is: " <<
                count_if(ia, ia + size(ia), pred) << '\n';

    }
    // Displays:    Number of times the value 3 is available: 3
    //              Number of odd values in the array is: 5



19.1.11: equal

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool equal([ExecPol,] InputIterator first, InputIterator
                last, InputIterator otherFirst);
            o bool equal([ExecPol,] InputIterator first, InputIterator last,
                InputIterator otherFirst, BinaryPredicate pred);

o  Description:
        
            o  The first prototype: the elements in the range [first,
last) are compared to a range of equal length starting at otherFirst. The
function returns true if the visited elements in both ranges are equal
pairwise. The ranges need not be of equal length, only the elements in the
indicated range are considered (and must be available).
            o  The second prototype: the elements in the range
[first, last) are compared to a range of equal length starting at
otherFirst. The function returns true if the binary predicate, applied
to all corresponding elements in both ranges returns true for every pair
of corresponding elements.  The ranges need not be of equal length, only the
elements in the indicated range are considered (and must be available).

o  Example:
            #include <algorithm>
    #include <string>
    #include <cstring>
    #include <iostream>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) == 0;
    }

    int main()
    {
        string first[] =
            {
                "Alpha", "bravo", "Charley", "delta", "Echo",
                "foxtrot", "Golf", "hotel"
            };
        string second[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel"
            };
        auto past = end(first);

        cout << "The elements of `first' and `second' are pairwise " <<
            (equal(first, past, second) ? "equal" : "not equal") <<
            '\n' <<
            "compared case-insensitively, they are " <<
            (
                equal(first, past, second, caseString) ?
                    "equal" : "not equal"
            ) << '\n';
    }
    //  Displays:
    //      The elements of `first' and `second' are pairwise not equal
    //      compared case-insensitively, they are equal



19.1.12: equal_range

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o pair<ForwardIterator, ForwardIterator>
            equal_range(ForwardIterator first, ForwardIterator last, Type
const &value);
            o pair<ForwardIterator, ForwardIterator>
            equal_range(ForwardIterator first, ForwardIterator last, Type
const &value, Compare comp);

o  Description (see also identically named member functions of,
e.g., the map (section 12.3.7) and multimap (section
12.3.8)):
        
            o  The first prototype: starting from a sorted sequence (where
the operator< of the data type to which the iterators point was used to
sort the elements in the provided range), a pair of iterators is returned
representing the return value of, respectively, lower_bound (returning
the first element that is not smaller than the provided reference value, see
section 19.1.30) and upper_bound (returning the first element
beyond the provided reference value, see section 19.1.61).
            o  The second prototype: starting from a sorted sequence (where
the comp function object was used to sort the elements in the provided
range), a pair of iterators is returned representing the return values of,
respectively, the functions lower_bound (section 19.1.30) and
upper_bound (section 19.1.61).

o  Example:
            #include <algorithm>
    #include <functional>
    #include <iterator>
    #include <iostream>
    using namespace std;

    int main()
    {
        int    range[] = {1, 3, 5, 7, 7, 9, 9, 9};
        auto past = end(range);

        pair<int *, int *>  pi;

        pi = equal_range(range, past, 6);

        cout << "Lower bound for 6: " << *pi.first << "\n"
                "Upper bound for 6: " << *pi.second << '\n';

        pi = equal_range(range, past, 7);

        cout << "Lower bound for 7: ";
        copy(pi.first, past, ostream_iterator<int>(cout, " "));
        cout << '\n';

        cout << "Upper bound for 7: ";
        copy(pi.second, past, ostream_iterator<int>(cout, " "));
        cout << '\n';

        sort(range, past, greater<int>());

        cout << "Sorted in descending order\n";

        copy(range, past, ostream_iterator<int>(cout, " "));
        cout << '\n';

        pi = equal_range(range, past, 7, greater<int>());

        cout << "Lower bound for 7: ";
        copy(pi.first, past, ostream_iterator<int>(cout, " "));
        cout << '\n';

        cout << "Upper bound for 7: ";
        copy(pi.second, past, ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      Lower bound for 6: 7
    //      Upper bound for 6: 7
    //      Lower bound for 7: 7 7 9 9 9
    //      Upper bound for 7: 9 9 9
    //      Sorted in descending order
    //      9 9 9 7 7 5 3 1
    //      Lower bound for 7: 7 7 5 3 1
    //      Upper bound for 7: 5 3 1



19.1.13: exchange

    

    o  Header file: <utility>
    o  Function prototype:
    
        o Type exchange(Type &object1, ValueType &&newValue);

o  Description:
    
        o  newValue is assigned to object1, and object1's
                previous value is returned.

o  Example:
        #include <utility>
    #include <iostream>
    using namespace std;

    int main(int argc, char **argv)
    {
        bool more = argc > 5;

        cout << "more than 5: " << exchange(more, argc > 2) <<
                ", more than 2: " << more << '\n';
    }
    //  With `a.out one two three' displays:
    //
    //  more than 5: 0, more than 2: 1



19.1.14: fill / fill_n

             
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void fill([ExecPol,] ForwardIterator first,
ForwardIterator last, Type const &value);
            o void fill_n([ExecPol,] ForwardIterator first, Size n, Type
                const &value);

o  Description:
        
            o  The first prototype: all elements reached from the iterator
range [first, last) are initialized to value, overwriting
previously stored values.
            o  The second prototype: n elements starting at the element
pointed to by first are initialized to value, overwriting previously
stored values.

o  Example:
            #include <algorithm>
    #include <vector>
    #include <iterator>
    #include <iostream>
    using namespace std;

    int main()
    {
        vector<int> iv(8);

        fill(iv.begin(), iv.end(), 8);
        copy(iv.begin(), iv.end(), ostream_iterator<int>(cout, " "));
        cout << '\n';

        fill_n(iv.begin() + 2, 4, 4);
        copy(iv.begin(), iv.end(), ostream_iterator<int>(cout, " "));
        cout << '\n';
     }
    // Displays:    8 8 8 8 8 8 8 8
    //              8 8 4 4 4 4 8 8



19.1.15: find / find_if / find_if_not

            
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o InputIterator find([ExecPol,] InputIterator first,
    InputIterator last, Type const &value);
            o InputIterator find_if([ExecPol,] InputIterator first,
    InputIterator last, Predicate pred);
            o InputIterator find_if_not([ExecPol,] InputIterator first,
    InputIterator last, Predicate pred);

o  Description:
        
            o  The first prototype: element value is searched for in the
elements reached from the iterator range [first, last). An iterator
pointing to the first element found is returned. If the element was not found,
last is returned. The operator== of the underlying data type is used
to compare the elements.
            o  The second prototype returns an iterator pointing to the
first element reached from the iterator range [first, last) for which
the (unary) predicate pred returns true. If the element was not found,
last is returned.
            o  The third prototype returns an iterator pointing to the first
element reached from the iterator range [first, last) for which the
(unary) predicate pred returns false. If the element was not found,
last is returned. Thus, pred defines what is considered acceptable, in
which case find_if_not returns an iterator to the first element that
isn't.

o  Example:
            #include <algorithm>
    #include <string>
    #include <cstring>
    #include <iterator>
    #include <iostream>
    using namespace std;

    class CaseName
    {
        std::string  d_string;

        public:
            CaseName(char const *str): d_string(str)
            {}
            bool operator()(std::string const &element) const
            {
                return strcasecmp(element.c_str(), d_string.c_str()) == 0;
            }
    };

    void show(string const *begin, string const *end)
    {
        if (begin == end)
            cout << "No elements were found";
        else
            copy(begin, end, ostream_iterator<string>{ cout, " " });
        cout << '\n';
    }

    int main()
    {
        string sarr[] =
            {
                "Alpha", "Bravo", "Charley", "Delta", "Echo"
            };
        auto past = end(sarr);

        show(find(sarr, past, "Delta"), past);
        show(find(sarr, past, "India"), past);

        show(find_if(sarr, sarr + size(sarr), CaseName{ "charley" }), past);

        if (find_if(sarr, sarr + size(sarr), CaseName{ "india" }) == past)
            cout << "`india' was not found in the range\n";

        show(find_if_not(sarr, sarr + size(sarr), CaseName{ "alpha" }), past);
    }
    //  Displays:
    //      Delta Echo
    //      No elements were found
    //      Charley Delta Echo
    //      `india' was not found in the range
    //      Bravo Charley Delta Echo



19.1.16: find_end

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator1 find_end([ExecPol,] ForwardIterator1 first1,
    ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2)
            o ForwardIterator1 find_end([ExecPol,] ForwardIterator1 first1,
    ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2,
                                            BinaryPredicate pred)

o  Description:
        
            o  The first prototype: the sequence of elements in the range
[first1, last1) is searched for the last occurrence of the sequence of
elements in the range [first2, last2). If the sequence
[first2, last2) is not found, last1 is returned, otherwise an
iterator pointing to the first element of the matching sequence is
returned. The operator== of the underlying data type is used to compare
the elements in the two sequences.
            o  The second prototype: the sequence of elements in the range
[first1, last1) is searched for the last occurrence of the sequence of
elements in the range [first2, last2). If the sequence [first2,
last2) is not found, last1 is returned, otherwise an iterator pointing to
the first element of the matching sequence is returned. The provided binary
predicate is used to compare the elements in the two sequences.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iterator>
    #include <iostream>
    using namespace std;

    bool twice(size_t first, size_t second)
    {
        return first == (second << 1);
    }

    int main()
    {
        string sarr[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel",
                "foxtrot", "golf", "hotel",
                "india", "juliet", "kilo"
            };
        string search[] =
            {
                "foxtrot",
                "golf",
                "hotel"
            };
        auto past = end(sarr);

        copy
        (
            find_end(sarr, past, search, search + 3),   // sequence starting
            past, ostream_iterator<string>{ cout, " " } // at 2nd 'foxtrot'
        );
        cout << '\n';

        size_t range[] = { 2, 4, 6, 8, 10, 4, 6, 8, 10 };
        size_t nrs[]   = { 2, 3, 4 };

        copy                // sequence of values starting at last sequence
        (                   // of range[] that are twice the values in nrs[]
            find_end(range, range + 9, nrs, nrs + 3, twice),
            range + 9, ostream_iterator<size_t>{ cout, " " }
        );
        cout << '\n';
    }
    //  Displays:
    //      foxtrot golf hotel india juliet kilo
    //      4 6 8 10



19.1.17: find_first_of

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator1 find_first_of([ExecPol,] 
ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, 
ForwardIterator2 last2)
            o ForwardIterator1 find_first_of([ExecPol,] 
ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2,
ForwardIterator2 last2, BinaryPredicate pred)

o  Description:
        
            o  The first prototype: the sequence of elements in the range
[first1, last1) is searched for the first occurrence of an element in
the sequence of elements in the range [first2, last2). If no
element in the sequence [first2, last2) is found, last1 is
returned, otherwise an iterator pointing to the first element in
[first1, last1) that is equal to an element in [first2, last2)
is returned. The operator== of the underlying data type is used to compare
the elements in the two sequences.
            o  The second prototype: the sequence of elements in the range
[first1, last1) is searched for the first occurrence of an element in
the sequence of elements in the range [first2, last2). Each element in
the range [first1, last1) is compared to each element in the range
[first2, last2), and an iterator to the first element in
[first1, last1) for which the binary predicate pred (receiving an
the element out of the range [first1, last1) and an element from the
range [first2, last2)) returns true is returned. Otherwise,
last1 is returned.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iterator>
    #include <iostream>
    using namespace std;

    bool twice(size_t first, size_t second)
    {
        return first == (second << 1);
    }
    int main()
    {
        string sarr[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel",
                "foxtrot", "golf", "hotel",
                "india", "juliet", "kilo"
            };
        string search[] =
            {
                "foxtrot",
                "golf",
                "hotel"
            };
        auto past = end(sarr);

        copy
        (                                                 // sequence starting
            find_first_of(sarr, past, search, search + 3),// at 1st 'foxtrot'
            past, ostream_iterator<string>{ cout, " " }
        );
        cout << '\n';

        size_t range[] = {2, 4, 6, 8, 10, 4, 6, 8, 10};
        size_t nrs[]   = {2, 3, 4};

            // copy the sequence of values in 'range', starting at the
            // first element in 'range' that is equal to twice one of the
            // values in 'nrs', and ending at the past element of 'range'
        copy
        (
            find_first_of(range, range + 9, nrs, nrs + 3, twice),
            range + 9, ostream_iterator<size_t>{ cout, " " }
        );
        cout << '\n';
    }
    //  Displays:
    //      foxtrot golf hotel foxtrot golf hotel india juliet kilo
    //      4 6 8 10 4 6 8 10



19.1.18: for_each

        
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o Function for_each([ExecPol,] ForwardIterator first,
                ForwardIterator last, Function func);

o  Description:
        
            o  The elements reached from the iterator range
[first, last) are passed in sequence as a reference to the function (or
function object) func. The function may modify the elements it receives
(as the used iterator is a forward iterator). Alternatively, if the elements
should be transformed, transform (see section 19.1.56) can be
used. The function itself or a copy of the provided function object is
returned: see the example below, in which an extra argument list is added to
the for_each call, which argument is eventually also passed to the
function given to for_each. Within for_each the return value of the
function that is passed to it is ignored. The for_each generic algorithm
looks a lot like the range-based for loop, but different from the range-based
for-loop the for_each algorithm can also be used with sub-ranges and with
reverse-iterators.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <cctype>
    using namespace std;

    void lowerCase(char &ch)                     // `ch' *is* modified
    {
        ch = tolower(static_cast<unsigned char>(ch));
    }

    void capitalizedOutput(string const &str)   // `str' is *not* modified
    {
        char *tmp = new char[str.size() + 1]();
        str.copy(tmp, str.size());

        for_each(tmp + 1, tmp + str.size(), lowerCase);

        tmp[0] = toupper(*tmp);
        cout << tmp << ' ';
        delete[] tmp;
    }

    int main()
    {
        string sarr[] =
            {
                "alpha", "BRAVO", "charley", "DELTA",  "echo",
                "FOXTROT", "golf", "HOTEL"
            };

        for_each(sarr, end(sarr), capitalizedOutput)("that's all, folks");
        cout << '\n';
    }
    // Displays:
    // Alpha Bravo Charley Delta Echo Foxtrot Golf Hotel That's all, folks

        o  Here is another example using a function object:
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <cctype>
    using namespace std;

    void lowerCase(char &c)
    {
        c = tolower(static_cast<unsigned char>(c));
    }
    class Show
    {
        int d_count;

        public:
            Show()
            :
                d_count(0)
            {}
            void operator()(std::string &str)
            {
                std::for_each(str.begin(), str.end(), lowerCase);
                str[0] = toupper(str[0]);   // assuming str is not empty
                std::cout << ++d_count << " " << str << "; ";
            }
            int count() const
            {
                return d_count;
            }
    };
    int main()
    {
        string sarr[] =
            {
                "alpha", "BRAVO", "charley", "DELTA", "echo",
                "FOXTROT", "golf", "HOTEL"
            };
        string *last = sarr + sizeof(sarr) / sizeof(string);

        cout << for_each(sarr, last, Show{}).count() << '\n';
    }
    /*
    Displays (all on one line):

    1 Alpha; 2 Bravo; 3 Charley; 4 Delta; 5 Echo; 6 Foxtrot;
                                              7 Golf; 8 Hotel; 8
    */


The example also shows that the for_each algorithm may be used with
functions defining const and non-const parameters. Also, see section
19.1.56 for differences between the for_each and transform
generic algorithms.

The for_each algorithm cannot directly be used (i.e., by passing
*this as the function object argument) inside a member function to modify
its own object as the for_each algorithm first creates its own copy of the
passed function object. A lambda function or a wrapper class whose
constructor accepts a pointer or reference to the current object and possibly
to one of its member functions solves this problem.


19.1.19: generate / generate_n

         
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o void generate([ExecPol,] ForwardIterator first,
                ForwardIterator last, Generator generator);
            o void generate_n([ExecPol,] ForwardIterator first, Size n,
                Generator generator);

o  Description:
        
            o  The first prototype: all elements reached from the iterator
range [first, last) are initialized by the return value of
generator, which can be a function or function
object. Generator::operator() does not receive any arguments. The example
uses a well-known fact from algebra: in order to obtain the square of n +
1, add 1 + 2 * n to n * n.
            o  The second prototype: n elements starting at the element
pointed to by iterator first are initialized by the return value of
generator.

o  Example:
            #include <algorithm>
    #include <vector>
    #include <iterator>
    #include <iostream>
    using namespace std;

    class NaturalSquares
    {
        size_t d_newsqr;
        size_t d_last;

        public:
            NaturalSquares(): d_newsqr(0), d_last(0)
            {}
            size_t operator()()
            {                   // using: (a + 1)^2 == a^2 + 2*a + 1
                return d_newsqr += (d_last++ << 1) + 1;
            }
    };
    int main()
    {
        vector<size_t> uv(10);

        generate(uv.begin(), uv.end(), NaturalSquares{});

        copy(uv.begin(), uv.end(), ostream_iterator<int>{ cout, " " });
        cout << '\n';

        uv = vector<size_t>(10);
        generate_n(uv.begin(), 5, NaturalSquares{});

        copy(uv.begin(), uv.end(), ostream_iterator<int>{ cout, " " });
        cout << '\n';

    }
    // Displays:   1 4 9 16 25 36 49 64 81 100
    //             1 4 9 16 25 0 0 0 0 0



19.1.20: includes

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool includes([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2);
            o bool includes([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2,
Compare comp);

o  Description:
        
            o  The first prototype: both sequences of elements in
the ranges [first1, last1) and [first2, last2) should have been
sorted using the operator< of the data type to which the iterators
point. The function returns true if every element in the second sequence
[first2, last2) is contained in the first sequence [first1,
last1) (the second range is a subset of the first range).
            o  The second prototype: both sequences of elements in
the ranges [first1, last1) and [first2, last2) should have been
sorted using the comp function object. The function returns true if
every element in the second sequence [first2, last2) is contained in
the first sequence [first1, last1) (the second range is a subset of
the first range).

o  Example:
            #include <algorithm>
    #include <string>
    #include <cstring>
    #include <iostream>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) == 0;
    }

    int main()
    {
        string first1[] =
            {
                "alpha", "bravo", "charley", "delta",  "echo",
                "foxtrot", "golf", "hotel"
            };
        auto past1 = end(first1);

        string first2[] =
            {
                "Alpha", "bravo", "Charley", "delta",  "Echo",
                "foxtrot", "Golf", "hotel"
            };
        auto past2 = end(first2);

        string second[] = { "charley", "foxtrot", "hotel" };

        cout << "The elements of `second' are " <<
            (includes(first1, past1, second, second + 3) ? "" : "not") <<
               " contained in the first sequence:\n"
               "second is a subset of first1\n"

            "The elements of `first1' are " <<
            (includes(second, second + 3, first1, past1) ? "" : "not") <<
            " contained in the second sequence\n"

            "The elements of `second' are " <<
            (includes(first2, past2, second, second + 3) ? "" : "not") <<
            " contained in the first2 sequence\n"

            "Using case-insensitive comparison,\n"
            "the elements of `second' are " <<
            (includes(first2, past2, second, second + 3, caseString) ?
                                                           "" : "not") <<
            " contained in the first2 sequence\n";
    }
    //  Displays:
    //     The elements of `second' are  contained in the first sequence:
    //     second is a subset of first1
    //     The elements of `first1' are not contained in the second sequence
    //     The elements of `second' are not contained in the first2 sequence
    //     Using case-insensitive comparison,
    //     the elements of `second' are  contained in the first2 sequence



19.1.21: inner_product

        
    
        o  Header file: <numeric>
        o  Function prototypes:
        
            o Type inner_product(InputIterator1 first1,
                InputIterator1 last1, InputIterator2 first2, Type init);
            o Type inner_product(InputIterator1 first1,
                InputIterator1 last1, InputIterator2 first2, Type init,
                BinaryOperator1 op1, BinaryOperator2 op2);

o  Description:
        
            o  The first prototype: the sum of all pairwise products of the
elements in the range [first1, last1) and the same number of
elements starting at the element pointed to by first2 are added to
init, and this sum is returned. The function uses the operator+ and
operator* of the data type to which the iterators point.
            o  The second prototype: binary operator op1 instead of the
default addition operator, and binary operator op2 instead of the default
multiplication operator are applied to all pairwise elements in the
range [first1, last1) and the same number of elements starting at the
element pointed to by first2. The results of the binary operator calls are
added to init and init's  final value is returned.

(See also section 19.1.57.)
        o  Example:
            #include <numeric>
    #include <algorithm>
    #include <iterator>
    #include <iostream>
    #include <string>
    using namespace std;

    class Cat
    {
        std::string  d_sep;
        public:
            Cat(string const &sep)
            :
                d_sep(sep)
            {}
            string operator()
                (string const &s1, string const &s2) const
            {
                return s1 + d_sep + s2;
            }
    };
    int main()
    {
        size_t ia1[] = { 1, 2, 3, 4, 5, 6, 7 };

        cout << "The sum of all squares in ";
        copy(ia1, ia1 + 7, ostream_iterator<size_t>{ cout, " " });
        cout << "is " << inner_product(ia1, ia1 + 7, ia1, 0) << '\n';

        size_t ia2[] = { 7, 6, 5, 4, 3, 2, 1 };

        cout << "The sum of all cross-products in ";
        copy(ia1, ia1 + 7, ostream_iterator<size_t>{ cout, " " });
        cout << "and ";
        copy(ia2, ia2 + 7, ostream_iterator<size_t>{ cout, " " });
        cout << "is " << inner_product(ia1, ia1 + 7, ia2, 0) << '\n';

        string names1[] = { "Frank", "Karel", "Piet" };
        string names2[] = { "Brokken", "Kubat", "Plomp"};

        cout << "All combined names of ";
        copy(names1, names1 + 3, ostream_iterator<string>{ cout, " " });
        cout << "and\n";
        copy(names2, names2 + 3, ostream_iterator<string>{ cout, " " });
        cout << "are:" <<
            inner_product(names1, names1 + 3, names2, string{ "\t" },
                Cat{ "\n\t"}, Cat{ " " }) << '\n';
    }
    // Displays:
    //  The sum of all squares in 1 2 3 4 5 6 7 is 140
    //  The sum of all cross-products in 1 2 3 4 5 6 7 and 7 6 5 4 3 2 1 is 84
    //  All combined names of Frank Karel Piet and Brokken Kubat Plomp are:
    //          Frank Brokken
    //          Karel Kubat
    //          Piet Plomp



19.1.22: inplace_merge

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void inplace_merge([ExecPol,] BidirectionalIterator first,
                BidirectionalIterator middle, BidirectionalIterator last);
            o void inplace_merge([ExecPol,] BidirectionalIterator first,
                BidirectionalIterator middle, BidirectionalIterator last,
                Compare comp);

o  Description:
        
            o  The first prototype: the two (sorted) ranges [first,
middle) and [middle, last) are merged, keeping a sorted list (using the
operator< of the data type to which the iterators point). The final
series is stored in the range [first, last).
            o  The second prototype: the two (sorted) ranges [first,
middle) and [middle, last) are merged, keeping a sorted list (using the
boolean result of the binary comparison operator comp). The final series
is stored in the range [first, last).

o  Example:
            #include <algorithm>
    #include <string>
    #include <cstring>
    #include <iterator>
    #include <iostream>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) == 0;
    }

    int main()
    {
        string range[] =
            {
                "alpha", "charley", "echo", "golf",
                "bravo", "delta", "foxtrot",
            };

        inplace_merge(range, range + 4, range + 7);
        copy(range, range + 7, ostream_iterator<string>{ cout, " " });
        cout << '\n';

        string range2[] =
            {
                "ALPHA", "CHARLEY", "DELTA", "foxtrot", "hotel",
                "bravo", "ECHO", "GOLF"
            };

        inplace_merge(range2, range2 + 5, range2 + 8, caseString);
        copy(range2, range2 + 8, ostream_iterator<string>{ cout, " " });
        cout << '\n';
    }
    //  Displays:
    //      alpha bravo charley delta echo foxtrot golf
    //      ALPHA bravo CHARLEY DELTA ECHO foxtrot GOLF hotel



19.1.23: iota

        
    
        o  Header file: <numeric>
        o  Function prototype:
        
            o void iota(ForwardIterator first,
                ForwardIterator last, Type value);

o  Description:
        
            o  All elements reached from the iterator range [first,
last) are assigned the values of the incremented sequence of values starting at
value. *first receives value, *(first + 1) receives
++value, etc.

o  Example:
            #include <numeric>
    #include <algorithm>
    #include <vector>
    #include <iostream>
    #include <iterator>

    using namespace std;

    int main()
    {
        vector<size_t> uv(10);

        iota(uv.begin(), uv.end(), 0);

        copy(uv.begin(), uv.end(), ostream_iterator<int>{ cout, " " });
        cout << '\n';
    }
    // Displays:   0 1 2 3 4 5 6 7 8 9



19.1.24: is_partitioned

        
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o bool is_partitioned([ExecPol,] InputIterator first,
                            InputIterator last, UnaryPred pred);

o  Description:
        
            o  Returns true if pred, receiving all elements reached
from the iterator range [first, last), returns true until it
returns false, and if pred, receiving the remaining elements, returns
false. It also returns true if the range is empty.

o  Example:
            #include <algorithm>
    #include <iterator>
    #include <vector>
    #include <iostream>
    using namespace std;

    bool pCheck(int value)
    {
        return value < 3;
    }

    int main()
    {
        vector<int> uv = { 1, -2, 3, -4, 5, -6, 7, -8, 9};

        cout << "partitioned before: " <<
                is_partitioned(uv.begin(), uv.end(), pCheck) << "\n"

                "first value returning 'false' when partitioning: " <<
                *partition(uv.begin(), uv.end(), pCheck) << '\n';

        copy(uv.begin(), uv.end(), ostream_iterator<int>{ cout, " " });
        cout << '\n';

        cout << "partitioned after: " <<
                is_partitioned(uv.begin(), uv.end(), pCheck) <<'\n';

    }
    // Displays:   1 4 9 16 25 36 49 64 81 100
    //             1 4 9 16 25 0 0 0 0 0



19.1.25: is_permutation

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool is_permutation(ForwardIterator first1,
                ForwardIterator last1, ForwardIterator first2);
            o bool is_permutation(ForwardIterator first1,
                ForwardIterator last1, ForwardIterator first2,
                ForwardIterator last2);
            o bool is_permutation(ForwardIterator first1,
                ForwardIterator last1, ForwardIterator firs2,
                BinaryPred pred);
            o bool is_permutation(ForwardIterator first1,
                ForwardIterator last1, ForwardIterator first2,
                ForwardIterator last2, BinaryPred pred);

o  Description:
        
            o  The first prototype: returns true if the elements in the
iterator range [first1, last1), are a permutation of the elements in
the identically sized range starting at first2.
            o  The second prototype: like the first prototype but the ending
iterator of the second range is provided as the fourth argument.
            o  The third prototype: like the first prototype but using the
result of pred to determine whether two elements are equal.
            o  The fourth prototype: like the second prototype but using the
result of pred to determine whether two elements are equal.

o  Example:
            #include <algorithm>
    #include <iostream>
    using namespace std;

    int main()
    {
        int one[] = { 1, -2, 3, -4, 5, -6, 7, -8, 9};
        int two[] = { -8, -2, -4, -6, 3, 1, 5, 9, 7};
        int three[] = { -8, -8, -4, -6, 3, 1, 5, 9, 7};

        cout << "one is a permutation of two: " <<
            is_permutation(one, end(one), two) << "\n"

            "one is a permutation of three: " <<
            is_permutation(one, end(one), three, end(three)) << '\n';
    }
    // Displays: one is a permutation of two: 1
    //           one is a permutation of three: 0



19.1.26: is_sorted
 
        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool is_sorted([ExecPol,] ForwardIterator first,
                ForwardIterator last);
            o bool is_sorted([ExecPol,] ForwardIterator first,
                ForwardIterator last, BinaryPredicate pred);

o  Description:
        
            o  The first prototype: returns true if the elements in the
iterator range [first, last), are sorted using the elements'
operator< comparison operator.
            o  The second prototype: returns true if the elements in the
iterator range [first, last), are sorted using the binary predicate
pred.

o  Example:
            #include <algorithm>
    #include <vector>
    #include <iostream>
    using namespace std;

    int main()
    {
        vector<int> uv = { 1, -2, 3, -4, 5, -6, 7, -8, 9};

        cout << "sorted before: " <<
                is_sorted(uv.begin(), uv.end()) << '\n';

        sort(uv.begin(), uv.end());

        cout << "sorted after " <<
                is_sorted(uv.begin(), uv.end()) << '\n';
    }
    // Displays:
    //    sorted before: 0
    //    sorted after 1



19.1.27: is_sorted_until

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator is_sorted_until([ExecPol,] 
                ForwardIterator first, ForwardIterator last);
            o ForwardIterator is_sorted_until([ExecPol,] 
                ForwardIterator first, ForwardIterator last, 
                BinaryPredicate pred);

o  Description:
        
            o  The first prototype: returns an iterator to the end of range,
starting at first whose elements are sorted using the elements'
operator< comparison operator.
            o  The second prototype: returns an iterator to the end of
range, starting at first whose elements using the binary predicate
pred.

The functions return at least first + 1.
        o  Example:
            #include <algorithm>
    #include <vector>
    #include <iostream>
    using namespace std;

    int main()
    {
        vector<int> uv = { 1, -2, 3, -4, 5, -6, 7, -8, 9};

        cout << "sorted before: " <<
                is_sorted(uv.begin(), uv.end()) << '\n';

        sort(uv.begin(), uv.end());

        cout << "sorted after " <<
                is_sorted(uv.begin(), uv.end()) << '\n';
    }
    // Displays:
    //    sorted before: 0
    //    sorted after 1



19.1.28: iter_swap
  
        
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o void iter_swap(ForwardIterator1 iter1,
                ForwardIterator2 iter2);

o  Description:
        
            o  The elements pointed to by iter1 and iter2 are
                swapped.

o  Example:
            #include <algorithm>
    #include <iterator>
    #include <iostream>
    #include <string>
    using namespace std;

    int main()
    {
        string first[] = { "alpha", "bravo", "charley" };
        string second[] = { "echo", "foxtrot", "golf" };

        cout << "Before:\n";
        copy(first, first + 3, ostream_iterator<string>(cout, " "));
        cout << '\n';
        copy(second, second + 3, ostream_iterator<string>(cout, " "));
        cout << '\n';

        for (size_t idx = 0; idx != 3; ++idx)
            iter_swap(first + idx, second + idx);

        cout << "After:\n";
        copy(first, first + 3, ostream_iterator<string>(cout, " "));
        cout << '\n';
        copy(second, second + 3, ostream_iterator<string>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      Before:
    //      alpha bravo charley
    //      echo foxtrot golf
    //      After:
    //      echo foxtrot golf
    //      alpha bravo charley



19.1.29: lexicographical_compare

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool lexicographical_compare([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2);
            o bool lexicographical_compare([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, Compare
comp);

o  Description:
        
            o  The first prototype: the corresponding pairs of elements in
the ranges pointed to by the ranges [first1, last1) and [first2,
last2) are compared. The function returns true
            
                o  at the first element in the first range which is less
than the corresponding element in the second range (using operator<
of the underlying data type),
                o  if last1 is reached, but last2 isn't reached yet.

False is returned in the other cases, which indicates that the first sequence
is not lexicographically less than the second sequence. So, false is
returned:
            
                o  at the first element in the first range which is greater
than the corresponding element in the second range (using operator<
of the data type to which the iterators point, reversing the  operands),
                o  if last2 is reached, but last1 isn't reached yet,
                o  if last1 and last2 are reached.

o  The second prototype: with this function the binary
comparison operation as defined by comp is used instead of
operator< of the data type to which the iterators point.

o  Example:
            #include <algorithm>
    #include <iterator>
    #include <iostream>
    #include <string>
    #include <cstring>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) < 0;
    }

    void compare(string const &word1, string const &word2)
    {
        cout << '`' << word1 << "' is " <<
            (
                lexicographical_compare(word1.begin(), word1.end(),
                                        word2.begin(), word2.end()) ?
                    "before `" : "beyond or at `"
            ) <<
            word2 << "' in the alphabet\n";
    }

    int main()
    {
        string word1 = "hello";
        string word2 = "help";

        compare(word1, word2);
        compare(word1, word1);
        compare(word2, word1);

        string one[] = {"alpha", "bravo", "charley"};
        string two[] = {"ALPHA", "BRAVO", "DELTA"};

        copy(one, one + 3, ostream_iterator<string>{ cout, " " });
        cout << " is ordered " <<
            (
                lexicographical_compare(one, one + 3,
                                        two, two + 3, caseString) ?
                    "before " : "beyond or at "
            );
        copy(two, two + 3, ostream_iterator<string>{ cout, " " });
        cout << "\n"
            "using case-insensitive comparisons.\n";
    }
    //   Displays:
    //      `hello' is before `help' in the alphabet
    //      `hello' is beyond or at `hello' in the alphabet
    //      `help' is beyond or at `hello' in the alphabet
    //      alpha bravo charley  is ordered before ALPHA BRAVO DELTA
    //      using case-insensitive comparisons.



19.1.30: lower_bound

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator lower_bound(ForwardIterator first,
                ForwardIterator last, const Type &value);
            o ForwardIterator lower_bound(ForwardIterator first,
                ForwardIterator last, const Type &value, BinaryPredicate 
                pred);

o  Description:
        
            o  The first prototype: the sorted elements (using ascending
sort) reached from the iterator range [first, last) are searched for
the first element that is not less than (i.e., greater than or equal to)
value. The returned iterator marks the location in the sequence where
value can be inserted without breaking the sorted order of the
elements. The operator< of the data type to which the iterators point is
used. If no such element is found, last is returned.
            o  The second prototype: the elements reached from the iterator
range [first, last) must have been sorted using the comp function
(-object). Each element in the range is compared to value using the
comp function, receiving an element from the iterator range as its first
argument and type as its second argument. An iterator to the first element
for which the binary predicate comp, applied to the elements of the range
and value, returns false is returned.  If no such element is found,
last is returned.

As illustrated by the following example, the function object
function's first parameter refers to an element in the iterator range, while
the function object's second parameter refers to value.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <vector>
    #include <functional>
    using namespace std;

    int main()
    {
        int     ia[] = { 10, 20, 30 };

        cout << "Sequence: ";
        copy(ia, ia + 3, ostream_iterator<int>(cout, " "));
        cout << "\n"
                "15 can be inserted before " <<
                *lower_bound(ia, ia + 3, 15) << "\n"
                "35 can be inserted after " <<
                (lower_bound(ia, ia + 3, 35) == ia + 3 ?
                                    "the last element" : "???") << '\n';

        cout << "Sequence: ";
        copy(ia, ia + 3, ostream_iterator<int>(cout, " "));
        cout << "\n"
                "15 can be inserted before " <<
                *lower_bound(ia, ia + 3, 15, less<int>()) << "\n"
                "35 can be inserted before " <<
                (lower_bound(ia, ia + 3, 35, less<int>()) == ia ?
                                    "the first element " : "???") << '\n';

        vector<int> array{ 5, 10, 20, 20, 20, 30 };
        auto iter = lower_bound(array.begin(), array.end(), 20,
                        [&](int &arrayEl, int value)
                        {
                            cout << "Comparing " << arrayEl <<
                                " (index: " << (&arrayEl - &array[0]) << ")"
                                " to " << value << '\n';
                            return arrayEl < value;
                        }
                    );
        cout << "New 20 to insert at idx " << (iter - array.begin()) << '\n';
    }
    //  Displays:
    //      Sequence: 10 20 30
    //      15 can be inserted before 20
    //      35 can be inserted after the last element
    //      Sequence: 10 20 30
    //      15 can be inserted before 20
    //      35 can be inserted before ???
    //      Comparing 20 (index: 3) to 20
    //      Comparing 10 (index: 1) to 20
    //      Comparing 20 (index: 2) to 20
    //      New 20 to insert at idx 2


The binary_search generic algorithm (cf. section 19.1.7)can be used
to determine whether or not value is present in the iterator range. The
upper_bound algorithm can be used to find the last element of a series of
values equal to value. The upper_bound section (19.1.61) also
contains an extensive example illustrating the use of lower_bound and as
upper_bound.


19.1.31: max / min
   
         
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o Type const &max(Type const &one, Type const &two);
            o Type const &max(Type const &one, Type const &two, Comparator
                comp);
            o Type const &min(Type const &one, Type const &two);
            o Type const &min(Type const &one, Type const &two, Comparator
                comp);

o  Description:
        
            o  The first prototype: the larger of the two elements one
and two is returned, using the operator> of the data type to which
the iterators point to determine which element is the larger one.
            o  The second prototype: one is returned if the binary
predicate comp(one, two) returns true, otherwise two is returned.
            o  Prototypes three and four act identically to, respectively,
prototypes one and two, using std::min instead of std::max,
returning the smaller of the two arguments.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cstring>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) <= 0;
    }

    string const &call(bool first, string const &lhs, string const &rhs)
    {
        return first? max(lhs, rhs) : min(lhs, rhs);
    }

    string const &call(bool first, string const &lhs, string const &rhs,
                bool (*cmp)(string const &, string const &))
    {
        return first? max(lhs, rhs, cmp) : min(lhs, rhs, cmp);
    }

    int main(int argc, char **argv) // no args: use max, else min
    {
        bool fun = argc == 1;
        char const *where = fun ? "last\n" : "first\n";

        cout << "Word '" << call(fun, "first", "second") <<
                            "' is lexicographically " << where <<

                "Word '" << call(fun, "first", "SECOND") <<
                            "' is lexicographically " << where <<

                "Word '" << call(fun, "first", "SECOND", caseString) <<
                            "' is lexicographically " << where;
    }
    //  Displays when calling without args:
    //      Word 'second' is lexicographically last
    //      Word 'first' is lexicographically last
    //      Word 'SECOND' is lexicographically last
    //  and with an argument:
    //      Word 'first' is lexicographically first
    //      Word 'SECOND' is lexicographically first
    //      Word 'first' is lexicographically first



19.1.32: max_element / min_element / minmax_element

          
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator max_element([ExecPol,] ForwardIterator first,
                            ForwardIterator last);
            o ForwardIterator max_element([ExecPol,] ForwardIterator first,
                            ForwardIterator last, BinaryPredicate pred);
            o ForwardIterator min_element([ExecPol,] ForwardIterator first,
                ForwardIterator last);
            o ForwardIterator min_element([ExecPol,] ForwardIterator first,
                ForwardIterator last, BinaryPredicate pred);

o pair<ForwardIterator, ForwardIterator> max_element([ExecPol,] 
                ForwardIterator first,ForwardIterator last);
            o pair<ForwardIterator, ForwardIterator> max_element([ExecPol,] 
                ForwardIterator first,ForwardIterator last, 
                BinaryPredicate pred);

o  Description:
        
            o  The first prototype: an iterator pointing to the largest
element reached from the iterator range [first, last) is returned. The
operator< of the data type to which the iterators point is used to decide
which of the elements is the largest.
            o  The second prototype: rather than using operator<, the
binary predicate pred is used to make the comparisons between the elements
reached from the iterator range [first, last). The element for which
pred returns most often true, compared with other elements, is
returned.
            o  Prototypes three and four act identically to, respectively,
prototypes one and two, using std::min_element instead of
std::max_element, returning iterators to the smallest elements in the 
iterator ranges [first, last).
            o  The final two prototypes act identically to, respectively,
prototypes one and two, but return std::pair objects whose first
members contain the iterators returned by the min generic algorithms and
whose second members contain the iterators returned by the max generic
algorithms.

o  Example:
            #include <algorithm>
    #include <iostream>
    using namespace std;

    bool absCompare(int first, int second)
    {
        return abs(first) < abs(second);
    };

    int *maxMin(bool high, int *begin, int *end)
    {
        return high ? max_element(begin, end) : min_element(begin, end);
    }

    int *maxMin(bool high, int *begin, int *end, bool (*comp)(int, int))
    {
        return high ? max_element(begin, end, comp)
                    : min_element(begin, end, comp);
    }

                // no args: calls max_element, else min_element
    int main(int argc, char **argv)
    {
        bool max = argc == 1;
        char const *type = max? "max" : "min";

        int     ia[] = {-4, 7, -2, 10, -12};

        cout << "The " << type << ". int value is " <<
                *maxMin(max, ia, ia + 5) << "\n"
                "The max. absolute int value is " <<
                *maxMin(max, ia, ia + 5, absCompare) << '\n';
    }
    //  Displays, when called without arguments:
    //      The max. int value is 10
    //      The max. absolute int value is -12
    //  otherwise:
    //      The minimum int value is -12
    //      The minimum absolute int value is -2



19.1.33: merge

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator merge([ExecPol,] InputIterator1 first1, 
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, 
OutputIterator result);
            o OutputIterator merge([ExecPol,] InputIterator1 first1, 
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, 
OutputIterator result, Compare comp);

o  Description:
        
            o  The first prototype: the two (sorted) ranges [first1,
last1) and [first2, last2) are merged, keeping a sorted list (using the
operator< of the data type to which the iterators point). The final
series is stored in the range starting at result and ending just before
the OutputIterator returned by the function.
            o  The second prototype: the two (sorted) ranges [first1,
last1) and [first2, last2) are merged, keeping a sorted list (using the
boolean result of the binary comparison operator comp). The final series
is stored in the range starting at result and ending just before the
OutputIterator returned by the function.

o  Example:
            #include <algorithm>
    #include <string>
    #include <cstring>
    #include <iterator>
    #include <iostream>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) < 0;
    }

    int main()
    {
        string range1[] =                           // 5 elements
                { "alpha", "bravo", "foxtrot", "hotel", "zulu" };
        string range2[] =                           // 4 elements
                { "delta", "echo", "golf", "romeo" };
        string result[5 + 4];

        copy(result,
            merge(range1, range1 + 5, range2, range2 + 4, result),
            ostream_iterator<string>{ cout, " " });
        cout << '\n';

        string range3[] =
                { "ALPHA", "bravo", "foxtrot", "HOTEL", "ZULU" };
        string range4[] = { "delta", "ECHO", "GOLF", "romeo" };

        copy(result,
            merge(range3, range3 + 5, range4, range4 + 4, result,
                                                    caseString),
            ostream_iterator<string>{ cout, " " });
        cout << '\n';
    }
    //  Displays:
    //      alpha bravo delta echo foxtrot golf hotel romeo zulu
    //      ALPHA bravo delta ECHO foxtrot GOLF HOTEL romeo ZULU



19.1.34: minmax
 
         
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o pair<Type const &, Type const &> minmax(
                                Type const &t1, Type const &t2);
            o pair<Type const &, Type const &> minmax(Type const &t1, 
                                Type const &t2, BinaryPred pred);
            o pair<Type const &, Type const &> minmax(
                                std:initializer<list<Type> values);
            o pair<Type const &, Type const &> minmax(
                        std:initializer<list<Type> values, BinaryPred pred);

o  Description:
        
            o  The first prototype: return a std::pair whose first
and second members contain, respectively, the smaller and the larger value
of the function's arguments, using Type's operator< to compare the two
values.
            o  The second prototype: identical to the first prototype, but
deciding that t1 is the smaller element if pred(t1, t2) returns
true. 
            o  Prototypes three and four act identically to, respectively,
prototypes one and two, but return, the smallest and largest values found in
the provided iterator_range objects.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <functional>

    using namespace std;

    int main()
    {
        vector<size_t> uv(10);

        auto values = minmax(5, 2, less<int>{});
        cout << values.first << " is smaler than " << values.second << '\n';
    }
    // Displays:   2 is smaler than 5



19.1.35: mismatch

        
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o pair<InputIterator1, InputIterator2>
                mismatch([ExecPol,] InputIterator1 first1, 
InputIterator1 last1, InputIterator2 first2);
            o pair<InputIterator1, InputIterator2>
                mismatch([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, Compare comp);

o  Description:
        
            o  The first prototype: the two sequences of elements starting
at first1 and first2 are compared using the equality operator of the
data type to which the iterators point. Comparison stops if the compared
elements differ (i.e., operator== returns false) or last1 is
reached. A pair containing iterators pointing to the final positions is
returned. The second sequence may contain more elements than the first
sequence. The behavior of the algorithm is undefined if the second sequence
contains fewer elements than the first sequence.
            o  The second prototype: the two sequences of elements starting
at first1 and first2 are compared using the binary comparison
operation as defined by comp, instead of
operator==. Comparison stops if the comp function returns false
or last1 is reached. A pair containing iterators pointing to the final
positions is returned. The second sequence may contain more elements than the
first sequence. The behavior of the algorithm is undefined if the second
sequence contains fewer elements than the first sequence.

o  Example:
            #include <algorithm>
    #include <string>
    #include <cstring>
    #include <iostream>
    #include <utility>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) == 0;
    }

    int main()
    {
        string range1[] = { "alpha", "bravo", "foxtrot", "hotel", "zulu" };
        string range2[] = { "alpha", "bravo", "foxtrot", "Hotel", "zulu" };
        pair<string *, string *> pss = mismatch(range1, range1 + 5, range2);

        cout << "The elements " << *pss.first << " and " << *pss.second <<
                " differ at index " << (pss.first - range1) << '\n';
        if
        (
            mismatch(range1, range1 + 5, range2, caseString).first
            ==
            range1 + 5
        )
            cout << "When compared case-insensitively they match\n";
    }
    /*
        Displays:
            The elements hotel and Hotel at offset 3 differ
            When compared case-insensitively they match
    */



19.1.36: move / move_backward

           
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o OutputIter move([ExecPol,] InputIter first, InputIter last, 
                OutputIter dest);
            o BidirIter move_backward(BidirIter first, BidirIter last, 
                BidirIter lastDest);

o  Description:
        
            o  The first prototype: all elements reached from the iterator
range [first, last) are moved to the range starting at dest,
returning the iterator pointing just past the last element in the range
starting at dest. 
            o  The Second prototype: all elements reached from the iterator
range [first, last) are moved in reverse order to the range starting,
iterating backward, at lastDest, returning the iterator pointing to the
beginning of the range ending at lastDest. The destination iterator
lastDest may not point to elements within the range [first, last).

Following move(_backward) the elements in the iterator range
[first, last) are still valid, but may have changed.

See also shift_left and shift_right:
            (cppreference).
        o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <iterator>
    using namespace std;

    void show(string const *begin, string const *end)
    {
        copy(begin, end, ostream_iterator<string>(cout, ", "));
        cout << '\n';
    }

    int main()
    {
        string sarr[] = { "alpha", "bravo", "charley", "delta" };
        string dest[4] = { "", };

        auto last = end(sarr);

        auto lastDest = move(sarr, last, dest); // move all elements to dest

        cout << "sarr after move:\n";
        show(sarr, last);

        cout << "dest after move:\n";
        show(dest, lastDest);

        move_backward(dest, lastDest, last); // move_backward to sarr
        cout << "sarr after move_backward:\n";
        show(sarr, last);

        cout << "dest after move_backward:\n";
        show(dest, lastDest);
    }
    //  Displays:
    //    sarr after move:
    //    , , , ,
    //    dest after move:
    //    alpha, bravo, charley, delta,



19.1.37: next_permutation / prev_permutation

         
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o bool next_permutation(BidirectionalIterator first,
                BidirectionalIterator last);
            o bool next_permutation(BidirectionalIterator first,
                BidirectionalIterator last, Comp comp);
            o bool prev_permutation(BidirectionalIterator first,
                BidirectionalIterator last);
            o bool prev_permutation(BidirectionalIterator first,
                BidirectionalIterator last, Comp comp);

o  Description:
        
            o  The first prototype: the next permutation, given the sequence
of elements in the range [first, last), is determined. For example, if
the elements 1, 2 and 3 are the range for which next_permutation
is called, then subsequent calls of next_permutation reorders the
following series:
                1 2 3
        1 3 2
        2 1 3
        2 3 1
        3 1 2
        3 2 1

This example shows that the elements are reordered such that each new
permutation represents the next bigger value (132 is bigger than 123, 213 is
bigger than 132, etc.) using operator< of the data type to which the
iterators point.  The value true is returned if a reordering took place,
the value false is returned if no reordering took place, which is the case
if the sequence represents the last (biggest) value. In that case, the
sequence is also sorted using operator<.
        o  The second prototype: the next permutation given the sequence of
elements in the range [first, last) is determined, using the binary
predicate comp to compare elements. The elements in the range are
reordered. The value true is returned if a reordering took place, the
value false is returned if no reordering took place, which is the case if
the resulting sequence would haven been ordered using the binary predicate
comp to compare elements.
        o  The final two prototypes act identically to the first two
prototypes, but instead of determining the next permutation the previous
permutation is computed. 

o  Example:
            #include <algorithm>
    #include <iterator>
    #include <iostream>
    #include <string>
    #include <cstring>
    using namespace std;

    bool caseString(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) < 0;
    }

    // to obtain previous permutations change 'next_' to 'prev_'

    int main()
    {
        string saints[] = {"Oh", "when", "the", "saints"};

        cout << "All permutations of 'Oh when the saints':\n"
                "Sequences:\n";
        do
        {
            copy(saints, saints + 4, ostream_iterator<string>{ cout, " " });
            cout << '\n';
        }
        while (next_permutation(saints, saints + 4, caseString));

        cout << "After first sorting the sequence:\n";
        sort(saints, saints + 4, caseString);
        cout << "Sequences:\n";
        do
        {
            copy(saints, saints + 4, ostream_iterator<string>{ cout, " " });
            cout << '\n';
        }
        while (next_permutation(saints, saints + 4, caseString));
    }
    //  Displays (partially):
    //      All permutations of 'Oh when the saints':
    //      Sequences:
    //      Oh when the saints
    //      saints Oh the when
    //      saints Oh when the
    //      saints the Oh when
    //      ...
    //      After first sorting the sequence:
    //      Sequences:
    //      Oh saints the when
    //      Oh saints when the
    //      Oh the saints when
    //      Oh the when saints
    //      ...



19.1.38: nth_element

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void nth_element([ExecPol,] RandomAccessIterator first,
                RandomAccessIterator nth, RandomAccessIterator last);
            o void nth_element([ExecPol,] RandomAccessIterator first,
                RandomAccessIterator nth, RandomAccessIterator last, Compare
                comp);

o  Description:
        
            o  The first prototype: all elements in the range [first,
last) are sorted relative to the element pointed to by nth: all elements
in the range [left, nth) are smaller than the element pointed to by
nth, and alle elements in the range [nth + 1, last) are greater
than the element pointed to by nth. The two subsets themselves are not
sorted. The operator< of the data type to which the iterators point is
used to compare the elements.
            o  The second prototype: all elements in the range [first,
last) are sorted relative to the element pointed to by nth: all elements
in the range [left, nth) are smaller than the element pointed to by
nth, and alle elements in the range [nth + 1, last) are greater
than the element pointed to by nth. The two subsets themselves are not
sorted. The comp function object is used to compare the elements.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <functional>
    using namespace std;

    int main()
    {
        int ia[] = {1, 3, 5, 7, 9, 2, 4, 6, 8, 10};

        nth_element(ia, ia + 3, ia + 10);

        cout << "sorting with respect to " << ia[3] << '\n';
        copy(ia, ia + 10, ostream_iterator<int>(cout, " "));
        cout << '\n';

        nth_element(ia, ia + 5, ia + 10, greater<int>());

        cout << "sorting with respect to " << ia[5] << '\n';
        copy(ia, ia + 10, ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      sorting with respect to 4
    //      1 2 3 4 9 7 5 6 8 10
    //      sorting with respect to 5
    //      10 8 7 9 6 5 3 4 2 1



19.1.39: partial_sort / partial_sort_copy

         
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void partial_sort([ExecPol,] RandomAccessIterator begin,
    RandomAccessIterator middle, RandomAccessIterator end);
            o void partial_sort([ExecPol,] RandomAccessIterator begin,
    RandomAccessIterator middle, RandomAccessIterator end, 
    BinaryPredicate pred);

o void partial_sort_copy([ExecPol,] InputIterator begin, 
    InputIterator end, 
    RandomAccessIterator dest_begin, RandomAccessIterator dest_end);
            o void partial_sort_copy([ExecPol,] InputIterator begin, 
    InputIterator end, 
    RandomAccessIterator dest_begin, RandomAccessIterator dest_end, 
    BinaryPredicate pred);

o  Description:
            
            o  The first prototype: the (middle - begin) smallest
elements are sorted and stored in the range [begin, middle) using the
operator< of the data type to which the iterators point to compare
elements. The remaining elements of the series remain unsorted, and are stored
in the range [middle, end).
            o  The second prototype: the (middle - begin) smallest
elements (according to the provided binary predicate pred) are sorted and
stored in the range [begin, middle). The remaining elements of the
series remain unsorted.
            o  The last two prototypes act like, respectively, the first two
prototypes, but instead of reordering the elements in the source iterator
range the selected elements are copied to the destination iterator range. The
number of elements that are copies is the smaller of the number of
source-iterators and the number of destination iterators.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <functional>
    #include <iterator>
    using namespace std;

    int main()
    {
        int ia[] = { 1, 9, 5, 3, 7, 2, 4, 6, 8, 10 };
        int ia2[6];

        partial_sort_copy(ia, ia + 10, ia2, ia2 + 6);
        cout << "the 6 smallest elements: ";
        copy(ia2, ia2 + 6, ostream_iterator<int>(cout, " "));
        cout << '\n';

        partial_sort(ia, ia + 3, ia + 10);

        cout << "find the 3 smallest elements:\n";
        copy(ia, ia + 3, ostream_iterator<int>(cout, " "));
        cout << '\n';

        cout << "find the 5 largest elements:\n";
        partial_sort(ia, ia + 5, ia + 10, greater<int>());
        copy(ia, ia + 5, ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      the 6 smallest elements: 1 2 3 4 5 6
    //      find the 3 smallest elements:
    //      1 2 3
    //      find the 5 biggest elements:
    //      10 9 8 7 6



19.1.40: partial_sum
 
          
    
        o  Header file: <numeric>
        o  Function prototypes:
        
            o OutputIterator partial_sum(InputIterator first,
InputIterator last, OutputIterator result);
            o OutputIterator partial_sum(InputIterator first, InputIterator
last, OutputIterator result, BinaryOperation op);

o  Description:
        
        o  The first prototype: each element in the range
[result, <returned OutputIterator>) receives a value which is obtained
by adding the elements in the corresponding range of the range [first,
last). The first element in the resulting range will be equal to the element
pointed to by first.
        o  The second prototype: the value of each element in the range
[result, <returned OutputIterator>) is obtained by applying the binary
operator op to the previous element in the resulting range and the
corresponding element in the range [first, last). The first
element in the resulting range will be equal to the element pointed to by
first.

See also inclusive_scan and exclusive_scan, supporting
        execution policies:
            (cppreference).
        o  Example:
            #include <numeric>
    #include <algorithm>
    #include <iostream>
    #include <functional>
    #include <iterator>
    using namespace std;

    int main()
    {
        int ia[] = {1, 2, 3, 4, 5};
        int ia2[5];

        copy(ia2,
            partial_sum(ia, ia + 5, ia2),
            ostream_iterator<int>(cout, " "));
        cout << '\n';

        copy(ia2,
            partial_sum(ia, ia + 5, ia2, multiplies<int>()),
            ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    /*
        Displays:
            1 3 6 10 15
            1 2 6 24 120
    */



19.1.41: partition / partition_point / stable_partition

          
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o BidirectionalIterator partition([ExecPol,] 
BidirectionalIterator first, BidirectionalIterator last, 
UnaryPredicate pred);
            o BidirectionalIterator stable_partition([ExecPol,]
BidirectionalIterator first, BidirectionalIterator last, UnaryPredicate
pred);
            o ForwardIterator partition_point( ForwardIterator first, 
ForwardIterator last, UnaryPredicate pred );

o  Description:
        
            o  The first prototype: all elements in the range [first,
last) for which the unary predicate pred evaluates as true are placed
before the elements which evaluate as false.
            o  The second prototype: identical to the first prototype, but
the relative order of all elements for which the predicate evaluates to
false and the relative order of all elements for which the predicate
evaluates to true is kept.  
            o  The third prototype: does not reorder the elements in the
[first, last) range, but returns an iterator to the first element for
which pred returns false. If pred returns true for all
elements then last is returned.

The return value points just beyond the last element in the
partitioned range for which pred evaluates as true.
        o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <iterator>
    using namespace std;

    void show(int *begin, int *end)
    {
        copy(begin, end, ostream_iterator<int>{ cout, " " });
        cout << '\n';
    }

    int main()
    {
        int org[] = { 1, 3, 5, 7, 9, 10, 2, 8, 6, 4 };
        int ia[10];

        copy(org, org + 10, ia);

        auto lessThan4 = [=](int value)
                        {
                            return value <= 4;
                        };

        int *split = partition(ia, ia + 10, lessThan4);

        cout << "Last ia[]- element <= 4 is ia[" << split - ia - 1 << "]\n";
        show(ia, end(ia));

        copy(org, org + 10, ia);
        split = stable_partition(ia, ia + 10, lessThan4);
        cout << "Last org[]-element <= 4 is ia[" << split - ia - 1 << "]\n";
        show(ia, end(ia));

        cout << "org[]-elements up to the partition point 4 are:\n";
        show(org, partition_point(org, org + 10, lessThan4));
    }
    //  Displays:
    //    Last ia[]- element <= 4 is ia[3]
    //    1 3 4 2 9 10 7 8 6 5
    //    Last org[]-element <= 4 is ia[3]
    //    1 3 2 4 5 7 9 10 8 6
    //    org[]-elements up to the partition point 4 are:
    //    1 3



19.1.42: partition_copy

        
    
        o  Header file: <algorithm>
        o  Function prototype:
            
            o std::pair<ForwardIter2, ForwardIter3>
    partition_copy([ExecPol,] ForwardIter1 first, ForwardIter1 last,
                    ForwardIter2 trueDest, ForwardIter3 falseDest,
                    UnaryPredicate pred );

o  Description:
        
            o  All elements reached from the iterator range [first,
last) for which pred returns true are copied to the range starting at
trueDest, while the remaining elements are copied to the range starting at
falseDest. the range  [first, last) may not overlap with the ranges
starting at trueDest or falseDest.

o  Example:
            #include <algorithm>
    #include <string>
    #include <iostream>
    #include <iterator>
    using namespace std;

    bool pred(std::string const &str)
    {
        return "aeiou"s.find(str.front()) != string::npos;
    }

    void show(string const *begin, string const *end)
    {
        copy(begin, end, ostream_iterator<string>(cout, ", "));
        cout << '\n';
    }

    int main()
    {
        string sarr[] =
            {
                "alpha", "bravo", "charley", "delta", "echo",
                "foxtrot", "golf", "hotel"
            };
        string trueDest[size(sarr)];
        string falseDest[size(sarr)];

        pair<string *, string *> lastTF =   // lastTrue, lastFalse
            partition_copy(sarr, end(sarr), trueDest, falseDest, pred);

        cout << "pred() == true elements:\n";
        show(trueDest, lastTF.first);

        cout << "pred() == false elements:\n";
        show(falseDest, lastTF.second);

    }
    //  Displays:
    //    pred() == true elements:
    //    alpha, echo,
    //    pred() == false elements:
    //    bravo, charley, delta, foxtrot, golf, hotel,



19.1.43: reduce

        
    
    o  Header file: <numeric>
    o  Function prototypes:
        
        o Type reduce([ExecPol,] InputIterator first, InputIterator last);
        o Type reduce([ExecPol,] InputIterator first, InputIterator last, 
                    Type init);
        o Type reduce([ExecPol,] InputIterator first, InputIterator last,
                BinaryOperation op); 
        o Type reduce([ExecPol,] InputIterator first, InputIterator last,
                Type init, BinaryOperation op);

o  Description:
        This algorithm acts like accumulate
(cf. accumulate), but the algorithm requires that the used
operator is both associative and commutative: regrouping and rearranging the
elements in any order may not affect the final outcome. E.g., the numeric
addition operator satisfies both requirements.
        
        o  The first prototype: operator+ is applied to the elements in
the range [first, last), returning the resulting sum.
        o  The second prototype: the binary operator op is applied to
the initial value init (lhs argument) and, respectively, the elements from
the iterator range [first, last). The resulting sum value is returned.
        o  The last two prototypes act like, respectively, the first two
prototypes but instead of using operator+ the binary operator op is
used, receiving the variable that's eventually returned from the function as
its lhs argument and the elements in the iterator range as its rhs argument,
assigning the operator's returned values to the variable that's eventually
returned. 

o  Example:
            // compile as: g++ -O2 reduce.cc -ltbb
    #include <numeric>
    #include <vector>
    #include <execution>
    #include <iostream>
    using namespace std;

    int main()
    {
        int         ia[] = {1, 2, 3, 4};
        vector<int> iv(ia, ia + 4);

                            // for demonstration purpose: it's unlikely that
        cout <<             // for 4 values parallel execution is used
            "Sum: " << reduce(execution::par,
                        iv.begin(), iv.end(), int()) << "\n"
            "Product: " << reduce(iv.begin(), iv.end(), int(1),
                                            multiplies<int>{}) << '\n';
    }
    //  Displays:
    //      Sum: 10
    //      Product: 24



19.1.44: remove / remove_if / remove_copy / remove_copy_if

        
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o ForwardIterator remove([ExecPol,] ForwardIterator first, 
                ForwardIterator last, Type const &value);

o OutputIterator remove_copy([ExecPol,] InputIterator first, 
                InputIterator last, OutputIterator dest, Type const &value);

o OutputIterator remove_copy_if([ExecPol,] 
                InputIterator first, InputIterator last, 
                OutputIterator dest, 
                UnaryPredicate pred);

o ForwardIterator remove_if([ExecPol,] ForwardIterator first,
                ForwardIterator last, UnaryPredicate pred);

o  Description:
        
            o  The first prototype: elements in the range pointed to by
[first, last) are reordered such that all values unequal to value
are placed at the beginning of the range. The returned forward iterator points
to the first element that can be removed after reordering. The range
[returnvalue, last) is called the leftover of the algorithm. Note
that the leftover may contain elements different from value, but these
elements can be removed safely, as such elements are also present in the range
[first, returnvalue). The function uses operator== of the data type
to which the iterators point to determine which elements to remove.
            o  The second prototype: elements in the range pointed to by
[first, last) not matching value are copied to the range
[dest, returnvalue), where returnvalue is the value returned by
the function. The range [first, last) is not modified.  The function
uses operator== of the data type to which the iterators point to determine
which elements not to copy.
            o  The third prototype: the elements in the range pointed to by
[first, last) for which the unary predicate pred returns true
are not inserted into the result iterator. All other elements are copied
to the range [dest, returnvalue), where returnvalue is the value
returned by the function. The range [first, last) is not modified.
            o  The fourth prototype: the elements in the range pointed to by
[first, last) are reordered such that all values for which the unary
predicate pred returns false are placed at the beginning of the
range, keeping their relative order. The returned forward iterator
points to the first element, after reordering, for which pred returned
true. The range [returnvalue, last) is called the leftover of
the algorithm. The leftover may contain elements for which the predicate
pred returns false, but these can safely be removed, as such elements
are also present in the range [first, returnvalue). Such duplication is
the result of the fact that the algorithm copies, rather than moves
elements into new locations.

Note that the copy-overloads expect output iterators. If the
kept elements are to be stored in, e.g., a vector kept then
kept.begin() could be passed as the function's dest argument. However,
that requires that size(kept) is large enough to contain all the kept
elements, which must therefore be known before the function is
called. Alternatively, back_insert_iterator could be used ensuring that
kept merely contains the kept elements once the function has
returned. This approach is used in the example program.
        o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <iterator>
    #include <vector>
    using namespace std;
    using StrVect = vector<string>;

    bool judge(string const &word)
    {
        return count(word.begin(), word.end(), 'a') > 1;
    }

    void show(auto const &begin, auto const &end)
    {
        copy(begin, end, ostream_iterator<string>(cout, ", "));
        cout << "\n";
    }

    int main()
    {
        StrVect words =
            {
                "kilo", "alpha", "lima", "mike", "alpha", "november",
                "alpha", "alpha", "alpha", "papa", "quebec"
            };

        auto src{ words };

        cout << "Removing all \"alpha\"s:\n";
        auto end = remove(src.begin(), src.end(), "alpha");
        show(src.begin(), end);
        cout << "Leftover elements are:\n";
        show(end, src.end());

        src = words;
        cout << "Remove_copy_if removes words having > 1 'a' chars:\n";
        StrVect kept;
        remove_copy_if(src.begin(), src.end(), back_inserter(kept), judge);
        show(kept.begin(), kept.end());

    }
    //  Displays:
    //    Removing all "alpha"s:
    //    kilo, lima, mike, november, papa, quebec,
    //    Leftover elements are:
    //    alpha, alpha, alpha, , ,
    //    Remove_copy_if removes words having > 1 'a' chars:
    //    kilo, lima, mike, november, quebec,



19.1.45: replace / replace_if / replace_copy / replace_copy_if

       
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o void replace([ExecPol,] ForwardIterator first, 
    ForwardIterator last, Type const &oldvalue, Type const &newvalue);
            o ForwardIterator replace_if([ExecPol,] ForwardIterator first,
    ForwardIterator last, UnaryPredicate pred, Type const &value);
            o OutputIterator replace_copy([ExecPol,] InputIterator first, 
InputIterator last, OutputIterator result, Type const &oldvalue, 
Type const &newvalue);
            o OutputIterator replace_copy_if([ExecPol,] 
ForwardIterator first, ForwardIterator last, OutputIterator result, 
UnaryPredicate pred, Type const &value);

o  Description:
        
            o  The first predicate: all elements equal to oldvalue in
the range pointed to by [first, last) are replaced by a copy of
newvalue. The algorithm uses operator== of the data type to which the
iterators point.
            o  The second prototype: elements in the range pointed to by
[first, last) for which the unary predicate pred evaluates as
true are replaced by value.
            o  The third prototype: all elements equal to oldvalue in
the range pointed to by [first, last) are sent to the result output
iterator, replacing elements equal to oldValue by newValue, using
operator== of the data type to which the iterators point.
            o  The fourth prototype: the elements in the range pointed to by
[first, last) for which the unary predicate pred returns false
are sent to the result output iterator, and value is sent to
result if pred returns true. The range [first, last) is not
modified.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <vector>
    #include <iterator>
    using namespace std;
    using StrVect = vector<string>;

    void show(StrVect const &vect)
    {
        copy(vect.begin(), vect.end(), ostream_iterator<string>(cout, " "));
        cout.put('\n');
    }

    bool isAlpha(string const &str)
    {
        return str == "alpha";
    }

    int main()
    {
        StrVect words =
            {
                "kilo", "alpha", "lima", "mike", "alpha", "november",
                "alpha", "oscar", "alpha", "alpha", "papa"
            };

        // replace(words.begin(), words.end(), "alpha"s, "ALPHA"s);
        // show(words);

        // or, using replace_if:
        //
        // replace_if(words.begin(), words.end(), isAlpha, "ALPHA"s);
        // show(words);

        // or, using replace_copy:
        //
        // StrVect result;
        // replace_copy(words.begin(), words.end(), result.begin(),
        //              "alpha"s, "ALPHA"s);
        // show(result);

        // or, using replace_copy_if:
        //
        //StrVect result;
        replace_copy_if(words.begin(), words.end(), back_inserter(result),
                     isAlpha, "ALPHA"s);
         show(result);
    }
    /*
        Displays
            kilo ALPHA lima mike ALPHA november ALPHA oscar ALPHA ALPHA papa
    */



19.1.46: reverse / reverse_copy

     

    o  Header file:
    <algorithm>
    o  Function prototype:
    
        o void reverse([ExecPol,] BidirectionalIterator first,
            BidirectionalIterator last);
        o OutputIterator reverse_copy([ExecPol,] BidirectionalIterator first,
            BidirectionalIterator last, OutputIterator dest);

o  Description:
    
        o  The first prototype: the order of the elements in the range
pointed to by [first, last) are reversed.
        o  The second prototype: elements in the range pointed to by
[first, last) are inserted in reversed order into dest, returning
the output iterator following the last insertion.

o  Example:
        #include <algorithm>
    #include <iostream>
    #include <string>
    #include <vector>
    #include <iterator>
    using namespace std;
    using StrVect = vector<string>;

    void show(StrVect const &vect)
    {
        copy(vect.begin(), vect.end(), ostream_iterator<string>(cout, " "));
        cout.put('\n');
    }

    int main(int argc, char **argv)
    {
        StrVect words =
            {
                "alpha", "kilo", "lima", "mike",
                "november", "oscar", "papa"
            };

        if (argc == 1)          // no args: plain reverse
        {
            reverse(words.begin(), words.end());
            show(words);
            return 0;
        }

        using StrVect = vector<string>;
        StrVect dest;

        reverse_copy(words.begin(), words.end(),
                     back_inserter(dest));
        show(dest);
    }
    // Displays:
    //    papa oscar november mike lima kilo alpha



19.1.47: rotate / rotate_copy
                
     
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o void rotate([ExecPol,] ForwardIterator first, ForwardIterator
                middle, ForwardIterator last);
            o OutputIterator rotate_copy([ExecPol,] ForwardIterator first,
ForwardIterator middle, ForwardIterator last, OutputIterator result);

o  Description:
        
            o  The first prototype: elements in the range [first,
middle) are moved to the end of the container, the elements in the range
[middle, last) are moved to the beginning of the container, keeping the
order of the elements in the two sub-ranges intact.
            o  The elements in the range [middle, last) and
then the elements in the range [first, middle) are inserted into
result, returning the output iterator following the last insertion.
The order of the elements in the source ranges is not altered.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <iterator>
    #include <vector>
    using namespace std;
    using StrVect = vector<string>;

    void show(StrVect const &vect)
    {
        copy(vect.begin(), vect.end(), ostream_iterator<string>(cout, " "));
        cout.put('\n');
    }

    int main(int argc, char **argv)
    {
        StrVect words =
            {
                "kilo", "lima", "mike", "november", "oscar",
                "foxtrot", "golf", "hotel", "india", "juliet"
            };

        if (argc == 1)          // no args: plain rotate
        {
            rotate(words.begin(), words.begin() + words.size() / 2,
                   words.end());
            show(words);
            return 0;
        }

        using StrVect = vector<string>;
        StrVect dest;

        rotate_copy(words.begin(), words.begin() + words.size() / 2,
                    words.end(), back_inserter(dest));
        show(dest);
    }
    //  Displays:
    //      foxtrot golf hotel india juliet kilo lima mike november oscar



19.1.48: sample

         
    
        o  Header file: <algorithm>
        o  Function prototype:
        
            o OutputIterator sample(InputIterator first, InputIterator last,
                       OutputIterator out, size_t sampleSize,
                       Generator &&generator);

o  Description:
        
            o  sampleSize elements are randomly selected (without
replacement) from the input range [first, last), and copied to the
destination range starting at out and ending at the function's return
value. If sampleSize >= (last - first) all elements are selected. The
generator is a (uniform) random number generator, like the mt19937
mercenne twister.

See also shuffle:
            (cppreference).

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <random>
    #include <string>

    using namespace std;

    int main()
    {
        string src{ "abcdefghijklmnopqrstuvwxyz" };
        string dest;

        sample(src.begin(), src.end(), back_inserter(dest), 7, mt19937{});

        std::cout << "Seven random letters out of " << src << " : " <<
                                                       dest << '\n';
    }
    //  Could display:
    //    Seven random letters out of abcdefghijklmnopqrstuvwxyz : bciorux



19.1.49: search / search_n

     
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator search([ExecPol,] ForwardIterator first1,
ForwardIterator last1, ForwardIterator first2, ForwardIterator last2);
            o ForwardIterator1 search([ExecPol,] ForwardIterator first1,
ForwardIterator last1, ForwardIterator first2, ForwardIterator last2,
BinaryPredicate pred);
            o constexpr ForwardIterator1 search([ExecPol,] 
ForwardIterator first, ForwardIterator last, Searcher const &searcher);

o ForwardIterator search_n([ExecPol,] ForwardIterator first,
ForwardIterator last, Size count, Type const &value);
            o ForwardIterator search_n([ExecPol,] ForwardIterator first1,
ForwardIterator last1, Size count, Type const &value, 
BinaryPredicate pred);

o  Description:
        
            o  The first prototype: an iterator into the first range
[first1, last1) is returned where the elements in the range
[first2, last2) are found using operator== of the data
type to which the iterators point. If no such location exists, last1 is
returned.
            o  The second prototype: an iterator into the first range
[first1, last1) is returned where the elements in the range
[first2, last2) are found using the provided binary predicate pred
to compare the elements in the two ranges. If no such location exists,
last1 is returned.
            o  The third prototype: returns searcher(first, last).first:
searcher receives the range of elements, and returns an iterator into this
range (or last if searcher found no match) as searcher(first,
last).first.
            o  The final two prototypes inspect the elements in the range
[first, last) for the first series of count elements equal to
value, returning the first iterator to those count elements. If no
such series exists then last is returned. The last protottype calls
pred to determine whether an element is equal to value.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <iterator>
    using namespace std;

    bool absEq(int i1, int i2)
    {
        return abs(i1) == abs(i2);
    }

    int main()
    {
        int range1[] =  {-2, -4, -6, -8, 2, 4, 4, 6, 8};
        int range2[] =  {6, 8};

        copy
        (
            search(range1, end(range1), range2, range2 + 2),
            end(range1), ostream_iterator<int>(cout, " ")
        );
        cout << '\n';

        copy
        (
            search(range1, end(range1), range2, range2 + 2, absEq),
            end(range1), ostream_iterator<int>(cout, " ")
        );
        cout << '\n';

        copy
        (
            search_n(range1, end(range1), 2, 4, absEq),
            end(range1), ostream_iterator<int>(cout, " ")
        );
        cout << '\n';
    }
    //  Displays:
    //      6 8
    //      -6 -8 2 4 4 6 8
    //      4 4 6 8



19.1.50: set_difference

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator set_difference([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
            o OutputIterator set_difference([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);

o  Description:
        
            o  The first prototype: a sorted sequence of the elements
pointed to by the range [first1, last1) that are not present in the
range [first2, last2) is returned, starting at result, and ending
at the OutputIterator returned by the function. The elements in the two
ranges must have been sorted using operator< of the data type to which
the iterators point.
            o  The second prototype: a sorted sequence of the elements
pointed to by the range [first1, last1) that are not present in the
range [first2, last2) is returned, starting at result, and ending
at the OutputIterator returned by the function. The elements in the two
ranges must have been sorted using the comp function object.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cstring>
    #include <iterator>
    using namespace std;

    bool caseless(string const &left, string const &right)
    {
        return strcasecmp(left.c_str(), right.c_str()) < 0;
    }
    int main()
    {
        string set1[] = { "kilo", "lima", "mike", "november",
                          "oscar", "papa", "quebec" };
        string set2[] = { "papa", "quebec", "romeo"};
        string result[7];

        copy(result,
            set_difference(set1, set1 + 7, set2, set2 + 3, result),
            ostream_iterator<string>(cout, " "));
        cout << '\n';

        string set3[] = { "PAPA", "QUEBEC", "ROMEO"};

        copy(result,
            set_difference(set1, set1 + 7, set3, set3 + 3, result,
            caseless),
            ostream_iterator<string>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      kilo lima mike november oscar
    //      kilo lima mike november oscar



19.1.51: set_intersection

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator set_intersection([ExecPol,] 
InputIterator1 first1, InputIterator1) linebreak() tt(last1, InputIterator2
first2, InputIterator2 last2, OutputIterator result); 
            o OutputIterator set_intersection([ExecPol,] InputIterator1
first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp); 

o  Description:
        
            o  The first prototype: a sorted sequence of the elements
pointed to by the range [first1, last1) that are also present in the
range [first2, last2) is returned, starting at result, and ending
at the OutputIterator returned by the function. The elements in the two
ranges must have been sorted using operator< of the data type to which
the iterators point.
            o  The second prototype: a sorted sequence of the elements
pointed to by the range [first1, last1) that are also present in the
range [first2, last2) is returned, starting at result, and ending
at the OutputIterator returned by the function. The elements in the two
ranges must have been sorted using the comp function object.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cstring>
    #include <iterator>
    using namespace std;

    bool caseless(string const &left, string const &right)
    {
        return strcasecmp(left.c_str(), right.c_str()) < 0;
    }
    int main()
    {
        string set1[] = { "kilo", "lima", "mike", "november",
                          "oscar", "papa", "quebec" };
        string set2[] = { "papa", "quebec", "romeo"};
        string result[7];

        copy(result,
            set_intersection(set1, set1 + 7, set2, set2 + 3, result),
            ostream_iterator<string>(cout, " "));
        cout << '\n';

        string set3[] = { "PAPA", "QUEBEC", "ROMEO"};

        copy(result,
            set_intersection(set1, set1 + 7, set3, set3 + 3, result,
                                                             caseless),
            ostream_iterator<string>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      papa quebec
    //      papa quebec



19.1.52: set_symmetric_difference

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator set_symmetric_difference([ExecPol,]
InputIterator1 first1, InputIterator1 last1, InputIterator2 first2,
InputIterator2 last2, OutputIterator result);
            o OutputIterator set_symmetric_difference([ExecPol,]
InputIterator1 first1, InputIterator1 last1, InputIterator2 first2,
InputIterator2 last2, OutputIterator result, Compare comp);

o  Description:
        
            o  The first prototype: a sorted sequence of the elements
pointed to by the range [first1, last1) that are not present in the
range [first2, last2) and those in the range [first2, last2)
that are not present in the range [first1, last1) is returned, starting
at result, and ending at the OutputIterator returned by the
function. The elements in the two ranges must have been sorted using
operator< of the data type to which the iterators point.
            o  The second prototype: a sorted sequence of the elements
pointed to by the range [first1, last1) that are not present in the
range [first2, last2) and those in the range [first2, last2)
that are not present in the range [first1, last1) is returned, starting
at result, and ending at the OutputIterator returned by the
function. The elements in the two ranges must have been sorted using the
comp function object.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cstring>
    #include <iterator>
    using namespace std;

    bool caseless(string const &left, string const &right)
    {
        return strcasecmp(left.c_str(), right.c_str()) < 0;
    }
    int main()
    {
        string set1[] = { "kilo", "lima", "mike", "november",
                          "oscar", "papa", "quebec" };
        string set2[] = { "papa", "quebec", "romeo"};
        string result[7];

        copy(result,
            set_symmetric_difference(set1, set1 + 7, set2, set2 + 3,
                                                           result),
            ostream_iterator<string>(cout, " "));
        cout << '\n';

        string set3[] = { "PAPA", "QUEBEC", "ROMEO"};

        copy(result,
            set_symmetric_difference(set1, set1 + 7, set3, set3 + 3,
                                                     result, caseless),
            ostream_iterator<string>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      kilo lima mike november oscar romeo
    //      kilo lima mike november oscar ROMEO



19.1.53: set_union

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator set_union([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
            o OutputIterator set_union([ExecPol,] InputIterator1 first1,
InputIterator1 last1, InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);

o  Description:
        
            o  The first prototype: a sorted sequence of the elements that
are present in either the range [first1, last1) or the range
[first2, last2) or in both ranges is returned, starting at result,
and ending at the OutputIterator returned by the function. The elements in
the two ranges must have been sorted using operator< of the data type to
which the iterators point;
            o  The second prototype: a sorted sequence of the elements that
are present in either the range [first1, last1) or the range
[first2, last2) or in both ranges is returned, starting at result,
and ending at the OutputIterator returned by the function. The elements in
the two ranges must have been sorted using comp function object.  

Note that in the final range each element appears only once, unless
        there are already duplicate elements in the sources, in which case they
        will also appear multiple times in the result;
        o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cstring>
    #include <iterator>
    #include <vector>
    using namespace std;

    bool caseless(string const &left, string const &right)
    {
        return strcasecmp(left.c_str(), right.c_str()) < 0;
    }

    int main()
    {
        string set1[] =
            {
                "kilo", "lima", "mike", "november",
                "oscar", "papa", "quebec"
            };
        string set2[] = { "papa", "quebec", "romeo"};
        string result[8];

        copy(result,
            set_union(set1, set1 + 7, set2, set2 + 3, result),
            ostream_iterator<string>(cout, " "));
        cout << '\n';

        string set3[] = { "PAPA", "QUEBEC", "ROMEO"};

        copy(result,
            set_union(set1, set1 + 7, set3, set3 + 3, result, caseless),
            ostream_iterator<string>(cout, " "));
        cout << '\n';

        std::vector<int> v1 = {1, 2, 3,        4, 5, 5, 5};
        std::vector<int> v2 = {      3, 3, 3,  4, 5,       6, 7};

        set_union(v1.begin(), v1.end(), v2.begin(), v2.end(),
                   ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    //  Displays:
    //      kilo lima mike november oscar papa quebec romeo
    //      kilo lima mike november oscar papa quebec ROMEO
    //      1 2 3 3 3 4 5 5 5 6 7



19.1.54: sort / stable_sort 

     
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void sort([ExecPol,] RandomAccessIterator first,
                RandomAccessIterator last);
            o void sort([ExecPol,] RandomAccessIterator first,
                RandomAccessIterator last, Compare comp);
            o void stable_sort([ExecPol,] RandomAccessIterator first,
                RandomAccessIterator last);
            o void stable_sort([ExecPol,] RandomAccessIterator first,
                RandomAccessIterator last, Compare comp);

o  Description:
        
           o  The first prototype: the elements in the range [first,
last) are sorted in ascending order using operator< of the data type to
which the iterators point.
            o  The second prototype: the elements in the range
[first, last) are sorted in ascending order using the comp
function object to compare the elements. The binary predicate comp
should return true if its first argument should be placed earlier in the
sorted sequence than its second argument.
           o  The last two prototypes are used identically to the first two
prototypes, but use stable-sorting, keeping the initial order of equal
elements. 

o  Example:
            // compile as: g++ -O2 sort.cc -ltbb
    #include <algorithm>
    #include <cstdlib>
    #include <execution>
    #include <functional>
    #include <iostream>
    #include <iterator>
    #include <string>
    using namespace std;

    int main()
    {
        string words[] =
                {
                    "november", "kilo", "mike", "lima",
                    "oscar", "quebec", "papa"
                };

        sort(words, words + 7);
        copy(words, words + 7, ostream_iterator<string>(cout, " "));
        cout << '\n';

        sort(words, words + 7, greater<string>());
        copy(words, words + 7, ostream_iterator<string>(cout, " "));
        cout << '\n';

        int *vect = new int[10'000];
        generate(execution::par, vect, vect + 10'000, random);
        cout << "sorted: " << is_sorted(vect, vect + 10'000) << '\n';

        sort(execution::par, vect, vect + 10'000);
        cout << "sorted: " << is_sorted(vect, vect + 10'000) << '\n';

        delete[] vect;

        // stable-sorting:

        using Pair = pair<size_t, string>;  // days & nrs of the months
        vector<Pair> months =
        {
            { 31, "Jan." }, { 28, "Feb." }, { 31, "Mar." }, { 30, "Apr." },
            { 31, "May." }, { 30, "Jun." }, { 31, "Jul." }, { 31, "Aug." },
            { 30, "Sep." }, { 31, "Oct." }, { 30, "Nov." }, { 31, "Dec." },
        };

        stable_sort(months.begin(), months.end(),
                    [&](Pair const &lhs, Pair const &rhs)
                    {
                        return lhs.first > rhs.first;
                    }
        );

        for (size_t idx = 0; auto const &month: months)
            cout << month.first << ": " << month.second <<
                (++idx % 4 == 0 ? "\n" : "  ");

    }
    //  Displays:
    //      kilo lima mike november oscar papa quebec
    //      quebec papa oscar november mike lima kilo
    //      unordered sequence
    //      sorted sequence
    //      31: Jan.  31: Mar.  31: May.  31: Jul.
    //      31: Aug.  31: Oct.  31: Dec.  30: Apr.
    //      30: Jun.  30: Sep.  30: Nov.  28: Feb.

        The example also shows two generic algorithms using the
execution::par execution policy: first generate is used to fill an
array with randomly determined int-values, then sort is used to sort
those values.


19.1.55: swap / swap_ranges

     

    o  Header file: <algorithm>
    o  Function prototypes:
    
        o void swap(Type &object1, Type &object2) noexcept;
        o void swap(Type (&object1)[N], Type (&object2))[N] noexcept;
        o ForwardIterator2 swap_ranges([ExecPol,] ForwardIterator1 first1,
            ForwardIterator1 last1, ForwardIterator2 result);

o  Description:
    
        o  The first prototype: the elements object1 and object2
exchange their values.  They do so by either cyclic copy assignment or cyclic
move assignment (if available).
        o  The second prototype: the elements of the (compile-time) known
sizes of the object1 and object2 arrays exchange their values.
        o  The third prototype: elements in the range pointed to by
[first1, last1) are swapped with that number of elements in the range
[result, returnvalue), where returnvalue is the value returned by
the function. The two ranges must be disjoint.

o  Example:
        #include <algorithm>
    #include <iostream>
    #include <string>
    #include <iterator>
    using namespace std;

    void show(string const *begin, string const *end)
    {
        copy(begin, end, ostream_iterator<string>(cout, " "));
        cout << '\n';
    }

    int main()
    {
        string first[] = { "alpha", "bravo", "charley" };
        string second[] = { "echo", "foxtrot", "golf" };

        cout << "Before:\n";
        show(first, end(first));
        show(second, end(second));

        for (size_t idx = 0; idx < size(first); ++idx)
            swap(first[idx], second[idx]);

        cout << "After:\n";
        show(first, end(first));
        show(second, end(second));

        swap_ranges(first, end(first), second);

        cout << "After swap_ranges:\n";
        show(first, end(first));
        show(second, end(second));

    }
    //  Displays:
    //    Before:
    //    alpha bravo charley
    //    echo foxtrot golf
    //    After:
    //    echo foxtrot golf
    //    alpha bravo charley
    //    After swap_ranges:
    //    alpha bravo charley
    //    echo foxtrot golf



19.1.56: transform

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator transform([ExecPol,] InputIterator first, 
InputIterator last, OutputIterator result, UnaryOperator op);
            o OutputIterator transform([ExecPol,] InputIterator1 first1, 
InputIterator1 last1, InputIterator2 first2, OutputIterator result,
BinaryOperator op);

o  Description:
        
            o  The first prototype: the unary operator op is applied to
each of the elements in the range [first, last), and the resulting
values are stored in the range starting at result. The return value points
just beyond the last generated element.
            o  The second prototype: the binary operator op is applied
to each of the elements in the range [first1, last1) and the
corresponding element in the second range starting at first2. The
resulting values are stored in the range starting at result. The return
value points just beyond the last generated element.

o  Example:
            #include <functional>
    #include <vector>
    #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cctype>
    #include <iterator>
    using namespace std;

    string caps(string const &src)
    {
        string tmp;
        tmp.resize(src.length());

        transform(src.begin(), src.end(), tmp.begin(), ::toupper);
        return tmp;
    }
    int main()
    {
        string words[] = {"alpha", "bravo", "charley"};

        copy(words, transform(words, words + 3, words, caps),
                                ostream_iterator<string>(cout, " "));
        cout << '\n';

        int         values[] = {1, 2, 3, 4, 5};
        vector<int> squares;

        transform(values, values + 5, values,
                                back_inserter(squares), multiplies<int>());

        copy(squares.begin(), squares.end(),
                                ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    /*
        Displays:
            ALPHA BRAVO CHARLEY
            1 4 9 16 25
    */


the following differences between the for_each (section
19.1.18) and transform generic algorithms should be noted:
    
    
        o  With transform the return value of the function
object's operator() member is used; the argument that is passed to the
operator() member itself is not changed.
        o  With for_each the function object's operator()
receives a reference to an argument, which itself may be changed by the
function object's operator().

Also note that the range-based for loop can often be used instead of the
transform generic algorithm. However, but different from the
range-based for-loop the transform algorithm can also be used width
sub-ranges and with reverse-iterators.


19.1.57: transform_reduce

    
    
        o  Header file: <numeric>
        o  Function prototypes:
        
            o Type transform_reduce([ExecPol,] 
                InputIterator first1, InputIterator last1, 
                InputIterator first2,
                Type value);

o Type transform_reduce([ExecPol,] 
                InputIterator first1, InputIterator last1, 
                InputIterator first2,
                Type value,
                BinaryOperation reduce,
                BinaryOperation transform);

o Type transform_reduce([ExecPol,] 
                InputIterator first1, InputIterator last1, 
                Type value,
                BinaryOperation reduce,
                UnaryOperation transform);

o  Description:
        
            o  The first prototype: equals the second prototype using
std::plus<> for the reduce binary operator and std::mutiplies<>
for the transform binary operator. It's also equivalent to a parallelized
version of inner_product (cf. section 19.1.21).
            o  The second prototype: applies the transform binary
operation to, as left-hand side operand, each element in the [first1,
end1) range and as right-hand side operand the corresponding element of the
range starting at first2. Each thus computed value is passed as right-hand
side operand, using value as left-hand side operand to the reduce
binary operation, returning the final value returned by reduce.
            o  The third prototype: applies transform to each element of
the range [first1, end1), and then passes value and each of the
values returned by transform to reduce, returning the final value
returned by reduce.

o  Example:
            #include <numeric>
    #include <algorithm>
    #include <iterator>
    #include <iostream>
    #include <string>
    using namespace std;

    class Cat
    {
        std::string  d_sep;
        public:
            Cat(string const &sep)
            :
                d_sep(sep)
            {}
            string operator()
                (string const &s1, string const &s2) const
            {
                return s1 + d_sep + s2;
            }
    };
    int main()
    {
        size_t ia1[] = { 1, 2, 3, 4, 5, 6, 7 };

                    // instead of inner_product:
        cout << "The sum of all squares in ";
        copy(ia1, ia1 + 7, ostream_iterator<size_t>{ cout, " " });
        cout << "is " << transform_reduce(ia1, ia1 + 7, ia1, 0) << '\n';

        size_t ia2[] = { 7, 6, 5, 4, 3, 2, 1 };

        cout << "The sum of all cross-products in ";
        copy(ia1, ia1 + 7, ostream_iterator<size_t>{ cout, " " });
        cout << "and ";
        copy(ia2, ia2 + 7, ostream_iterator<size_t>{ cout, " " });
        cout << "is " << transform_reduce(ia1, ia1 + 7, ia2, 0) << '\n';

        string names1[] = { "Frank", "Karel", "Piet" };
        string names2[] = { "Brokken", "Kubat", "Plomp"};

        cout << "All combined names of ";
        copy(names1, names1 + 3, ostream_iterator<string>{ cout, " " });
        cout << "and ";
        copy(names2, names2 + 3, ostream_iterator<string>{ cout, " " });
        cout << "are:" <<
            transform_reduce(names1, names1 + 3, names2, "\t"s,
                Cat{ "\n\t"}, Cat{ " " }) << '\n';
    }
    // Displays:
    //  The sum of all squares in 1 2 3 4 5 6 7 is 140
    //  The sum of all cross-products in 1 2 3 4 5 6 7 and 7 6 5 4 3 2 1 is 84
    //  All combined names of Frank Karel Piet and Brokken Kubat Plomp are:
    //          Frank Brokken
    //          Karel Kubat
    //          Piet Plomp



19.1.58: handling uninitialized memory

    
Section 9.1.5 covers the placement new operator. The placement new
operator is used to install values or objects in 'raw memory', i.e., memory
that is already available, but hasn't yet been initialized for the intended
object types.

As covered before, when calling something like auto ptr = new string{
"hello" } the string is constructed in memory specifically allocated to
contain the object, and the object type's constructor initializes the object
in that memory. Likewise, when calling delete ptr the string's destructor
is called, followed by returning the memory allocated by new to the common
pool. 

When using placement new the memory to contain the object is already
available, and the construction auto ptr = new (storageAddress) string{
"hello" } is used to merely construct the string at the location specified by
storageAddress. That string can then (as usual) be accessed via ptr,
but delete ptr cannot be used, since the memory at storageAddress was
already available before invoking the placement new operator. Therefore, in
these cases the remarkable situation is encountered where the object's
destructor must explicitly be called (using ptr->~string()) and using
delete ptr is completely wrong, causing a memory error which aborts the
program. 

Several generic algorithms, all supporting execution policies, are available
simplifying the use of placement new. To use these algorithm the
<memory> header file must be included. 

Facilities are available to copy, fill, initialize, and move objects to/in
uninitialized (raw) memory, as well as facilities to delete the objects stored
in raw memory. Here is an overvieuw of the available facilities (cf.
cppreference for more details
about the algorithms handling uninitialized memory):
    
    o uninitialized_copy([ExecPol,] 
            ForwardIterator first, ForwardIterator last,
            ForwardIterator dest);

        copies the elements in the [first, last) to the raw memory
        starting at dest, returning the location beyond the last copied
        element.
    o uninitialized_copy_n([ExecPol,] 
            ForwardIterator first, size_t nObjects,
            ForwardIterator dest);

       same as the previous algorithm, but copies nObjects.
    o uninitialized_default_construct([ExecPol,] 
            ForwardIterator first, ForwardIterator last);

       installs default constructed values at the raw memory locations reached
        by the iterator range [first, last). The algorithm requires
        that the types referred to by the iterators are either trivial types
        (like built-in types) or define value_type returning their type
        names. When using trivial types the installed do not assume that the
        installed values are 0-initialized.
    o uninitialized_default_construct_n([ExecPol,] 
            ForwardIterator first, size_t nObjects);

       same as the previous algorithm, but installs nObjects in the
        uninitialized memory.
    o uninitialized_fill([ExecPol,] 
            ForwardIterator first, ForwardIterator last, 
            Type const &value);

      like the first algorithm, but installing copies of value in the
        uninitialized memory.
    o uninitialized_fill([ExecPol,] 
            ForwardIterator first, size_t nObjects,Type const &value);

       same as the previous algorithm, but copies value to the
        nObjects subsequent locations in the uninitialized memory.
    o  uninitialized_move([ExecPol,] 
            ForwardIterator first, ForwardIterator last,
            ForwardIterator dest);

        same as the first algorithm, but the elements in the [first,
        last) are moved to the raw memory.
    o uninitialized_move_n([ExecPol,] 
            ForwardIterator first, size_t nObjects, 
            ForwardIterator dest);

        same as the previous algorithm, but nObjects are moved.
    o uninitialized_value_construct([ExecPol,] 
            ForwardIterator first, ForwardIterator last);

       same as uninitialized_default_construct, but requires that the
        types referred to by the iterators define value_type returning
        their type names.
    o uninitialized_value_construct_n([ExecPol,] 
            ForwardIterator first, size_t nObjects);

       same as the previous algorithm, but installs nObjects in the
        uninitialized memory.

The algorithm  Type *construct_at(Type *raw, Args
&&...args) constructs an object of type Type in the raw memory at
raw, passing args... to Type's constructor.

To delete the objects installed in raw memory the following facilities are
available:
    
    o void destroy([ExecPol,] ForwardIterator first, 
        ForwardIterator last);

       assuming the the types to which first refers: it calls
        iterator->~Type() for all elements in the range [first,
        last). 
    o void destroy([ExecPol,] ForwardIterator first, size_t nObjects);

       same as the previous algorithm, but calls the destructors of
        nObjects objects.
    o void destroy_at(Type *raw);

       calls the destructor of the object installed at raw using placement
        new. If the raw pointer points to an array of placement new
        allocated objects then the destructors of the elements of the array
        are called.

Here is an example:
        #include <memory>
    #include <vector>
    #include <iostream>
    #include <string>
    using namespace std;

    int main()
    {
        char raw[4 * sizeof(string)];       // raw memory to receive strings
        string *ptr = reinterpret_cast<string *>(raw);  // pointer to strings

                                            // construct 4 strings in raw
        uninitialized_default_construct_n(ptr, 4);
        destroy(ptr, ptr + 4);              // call the strings' destructors

        vector<string> vs(4, "string");     // move 4 strings to raw memory
        uninitialized_move(vs.begin(), vs.end(), ptr);

        cout << vs.front() << ", " << vs.back() << '\n' <<
                ptr[0] << ", " << ptr[3] << '\n';

        destroy(ptr, ptr + 4);              // call the strings' destructors
    }
    //  Displays:
    //      ,
    //      string, string



19.1.59: unique

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator unique([ExecPol,] ForwardIterator first,
ForwardIterator last);
            o ForwardIterator unique([ExecPol,] ForwardIterator first,
ForwardIterator last, BinaryPredicate pred);
            o OutputIterator unique_copy([ExecPol,] InputIterator first,
InputIterator last, OutputIterator result);
            o OutputIterator unique_copy([ExecPol,] InputIterator first,
InputIterator last, OutputIterator result, BinaryPredicate pred);

o  Description:

    The std::unique generic algorithm assumes that the elements in the
range have previously been sorted (cf. section 19.1.54).
        
            o  The first prototype: using operator== of the data type to
which the iterators point, all but the first of consecutively equal elements
in the range pointed to by [first, last) are relocated to the end of
the range. The returned forward iterator marks the beginning of the
leftover. All elements in the range [first, return-value) are
unique, all elements in the range [return-value, last) have
undetermined (but valid) values.
            o  The second prototype: all but the first of consecutive
elements in the range pointed to by [first, last) for which the binary
predicate pred returns true are relocated to the end of the range. The
predicate pred expects two arguments of the data type to which the
iterators point. The returned forward iterator marks the beginning of the
leftover. For all pairs of elements in the range [first,
return-value) pred returns false (i.e., they are unique).  All
elements in the leftover (i.e., the range [return-value, last))
have undetermined (but valid) values.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <cstring>
    #include <iterator>
    using namespace std;

    bool casestring(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) == 0;
    }

    int main()
    {
        string words[] = {"alpha", "alpha", "Alpha", "papa", "quebec" };
        size_t const size = sizeof(words) / sizeof(string);

        string *removed = unique(words, words + size);
        copy(words, removed, ostream_iterator<string>(cout, " "));
        cout << '\n'
             << "Trailing elements are:\n";
        copy(removed, words + size, ostream_iterator<string>(cout, " "));
        cout << '\n';

        removed = unique(words, words + size, casestring);
        copy(words, removed, ostream_iterator<string>(cout, " "));
        cout << '\n'
             << "Trailing elements are:\n";
        copy(removed, words + size, ostream_iterator<string>(cout, " "));
        cout << '\n';
    }
    /*
        Displays:
            alpha Alpha papa quebec
            Trailing elements are:
            quebec
            alpha papa quebec
            Trailing elements are:
            quebec quebec
    */



19.1.60: unique_copy

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o OutputIterator unique_copy([ExecPol,] InputIterator first,
InputIterator last, OutputIterator result);
            o OutputIterator unique_copy([ExecPol,] InputIterator first,
InputIterator last, OutputIterator result, BinaryPredicate pred);

o  Description:
        
            o  The first prototype: the elements in the range [first,
last) are copied to the resulting container, starting at result.
Consecutively equal elements (using operator== of the data type to which
the iterators point) are copied only once (keeping the first of a series of
equal elements). The returned output iterator points
just beyond the last copied element.
            o  The second prototype: the elements in the range
[first, last) are copied to the resulting container, starting at
result.  Consecutive elements in the range pointed to by [first,
last) for which the binary predicate pred returns true are copied only
once (keeping the first of a series of equal elements). The returned output
iterator points just beyond the last copied element.

o  Example:
            #include <algorithm>
    #include <iostream>
    #include <string>
    #include <vector>
    #include <iterator>
    #include <cstring>
    using namespace std;

    bool casestring(string const &first, string const &second)
    {
        return strcasecmp(first.c_str(), second.c_str()) == 0;
    }
    int main()
    {
        string words[] = {"oscar", "Alpha", "alpha", "alpha",
                                                        "papa", "quebec" };
        size_t const size = sizeof(words) / sizeof(string);
        vector<string> remaining;

        unique_copy(words, words + size, back_inserter(remaining));

        copy(remaining.begin(), remaining.end(),
                ostream_iterator<string>(cout, " "));
        cout << '\n';

        vector<string> remaining2;

        unique_copy(words, words + size,
                                back_inserter(remaining2), casestring);

        copy(remaining2.begin(), remaining2.end(),
                ostream_iterator<string>(cout, " "));
        cout << '\n';
    }
    /*
        Displays:
            oscar Alpha alpha papa quebec
            oscar Alpha papa quebec
    */



19.1.61: upper_bound

    
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o ForwardIterator upper_bound(ForwardIterator first,
ForwardIterator last, Type const &value);
            o ForwardIterator upper_bound(ForwardIterator first,
ForwardIterator last, Type const &value, Compare comp);

o  Description:
        

o  The first prototype: the sorted elements (using ascending sort)
reached from the iterator range [first, last) are searched for the first
element that is greater than value. The returned iterator marks the first
location in the sequence where value can be inserted without breaking the
sorted order of the elements using operator< of the data type to which the
iterators point. If no such element is found, last is returned.

o  The second prototype: the elements reached from the iterator
range [first, last) must have been sorted using the comp function
or function object. Each element in the range is compared to value using
the comp function. An iterator is returned pointing to the first element
for which the binary predicate comp, applied to the elements of the range
and value, returns true.  The comp function object function's
first parameter refers to value and the function object's second parameter
refers to an element in the iterator range.

Caveat: note that the comp object's parameters when using
upper_bound are swapped compared to the parameters expected by
lower_bound. 

o  When the values in the iterator range were sorted in ascending
order (i.e., using operator<) then upper_bound returns an iterator
pointing beyond the last of a series of values equal to value, while
lower_bound returns an iterator pointing to the first of such a series of
equal values.

When the iterator range contains a series of values which are, according to
comp, equal to value then upper_bound returns an iterator to the
first element beyond that series, while lower_bound returns an iterator to
the first element of that series.

The following program illustrates the various possibilities. Th program
illustrates both lower_bound and upper_bound and also illustrates the
situation where value' Type is unequal to the types of the values in the
iterator range. Specific comment is provided below the program's code.

o  Example:
         1: #include <algorithm>
 2: #include <iostream>
 3: using namespace std;
 4: 
 5: int main()
 6: {
 7:     using pic =  pair<int, char>;
 8: 
 9:     pic picArr[] =
10:         { {1, 'f'}, {5, 'r'}, {5, 'a'}, {7, 'n'}, {8, 'k'} };
11:     pic *picArrEnd = picArr + size(picArr);
12: 
13:     cout << "Sequence: ";
14:     for (auto &pair: picArr)
15:         cout << '{' << pair.first << ',' << pair.second << "}, ";
16:     cout << '\n';
17: 
18:     auto iter = lower_bound(picArr, picArrEnd, 5,
19:                     [&](pic const &range, int value)
20:                     {
21:                         return range.first < value;
22:                     }
23:                 );
24:     cout << "  lower bound, <,  {5,?} can be inserted before {" <<
25:                 iter->first << ',' << iter->second << "}\n";
26: 
27:     iter = upper_bound(picArr, picArrEnd, 5,
28:                     [&](int value, pic const &range)
29:                     {
30:                         return value < range.first;
31:                     }
32:                 );
33:     cout << "  upper_bound, <,  {5,?} can be inserted before {" <<
34:                 iter->first << ',' << iter->second << "}\n";
35: 
36:     iter = upper_bound(picArr, picArrEnd, 9,
37:                     [&](int value, pic const &range)
38:                     {
39:                         return value < range.first;
40:                     }
41:                 );
42:     cout << "  upper_bound, <,  {9,?} can be inserted " <<
43:                 ( &*iter == picArrEnd ? "at the end" : "???") << '\n';
44: 
45:     sort(picArr, picArrEnd,
46:         [](pic const &lhs, pic const &rhs)
47:         {
48:             return lhs.first > rhs.first;
49:         }
50:     );
51: 
52:     cout << "\nSequence: ";
53:     for (auto &pair: picArr)
54:         cout << '{' << pair.first << ',' << pair.second << "}, ";
55:     cout << '\n';
56: 
57:     iter = lower_bound(picArr, picArrEnd, 5,
58:                     [&](pic const &range, int value)
59:                     {
60:                         return range.first > value;
61:                     }
62:                 );
63:     cout << "  lower_bound, >,  {5,?} can be inserted before {" <<
64:                 iter->first << ',' << iter->second << "}\n";
65: 
66:     iter = upper_bound(picArr, picArrEnd, 5,
67:                     [&](int value, pic const &range)
68:                     {
69:                         return value > range.first;
70:                     }
71:                 );
72:     cout << "  upper_bound, >,  {5,?} can be inserted before {" <<
73:                 iter->first << ',' << iter->second << "}\n";
74: 
75:     iter = upper_bound(picArr, picArrEnd, 0,
76:                     [&](int value, pic const &range)
77:                     {
78:                         return value > range.first;
79:                     }
80:                 );
81:     cout << "  upper_bound, >,  {0,?} can be inserted " <<
82:                 ( &*iter == picArrEnd ? "at the end" : "???") << '\n';
83: }
84: // Displays:
85: // Sequence: {1,f}, {5,r}, {5,a}, {7,n}, {8,k},
86: //   lower bound, <,  {5,?} can be inserted before {5,r}
87: //   upper_bound, <,  {5,?} can be inserted before {7,n}
88: //   upper_bound, <,  {9,?} can be inserted at the end
89: //
90: // Sequence: {8,k}, {7,n}, {5,r}, {5,a}, {1,f},
91: //   lower_bound, >,  {5,?} can be inserted before {5,r}
92: //   upper_bound, >,  {5,?} can be inserted before {1,f}
93: //   upper_bound, >,  {0,?} can be inserted at the end



        o  Lines 7 thru 12: the iterator range consists of a series of
            pairs, sorted by their first members.
        o  Lines 18 thru 23: lower_bound is called using a lambda
            expression to define the Compare function object. Note (line
            19) that a reference to a value in the iterator range is the
            lambda expression's first parameter, while the target value is its
            second parameter. 
        o  Lines 27 thru 32: here upper_bound is called, also using a
            lambda expression. With upper_bound the target value is the
            lambda expression's first parameter, while a reference to a value
            in the iterator range is its second parameter. 
        o  Lines 57 thru 62, 66 thru 71, and 75 thru 80: after sorting the
            values in the picArr array in descending order lower_bound
            and upper_bound are again used. This time instead of using the
            < operator the > operator should be used.

The binary_search generic algorithm can be used to simply
determine whether or nog value is present in the iterator range. The
lower_bound generic algorithm can be used to find the first
element of a series of values equal to value.


19.1.62: Heap algorithms

    A heap is a kind of binary tree which can be represented by an array. In
the standard heap, the key of an element is not smaller than the key of its
children. This kind of heap is called a max heap.  A tree in which
numbers are keys could be organized as shown in figure 25.
    ------------------------------------------------------------------Insert Figure 25(A binary tree representation of a heap)about here (file: stl/heap)------------------------------------------------------------------
    Such a tree may also be organized in an array:
                12, 11, 10, 8, 9, 7, 6, 1, 2, 4, 3, 5

In the following description, keep two pointers into this array in mind:
a  pointer node indicates the location of the next node of the tree, a
pointer child points to the next element which is a child of the node
pointer. Initially, node points to the first element, and child points
to the second element.
    
    o *node++ (== 12). 12 is the top node. its children are *child++
(11) and *child++ (10), both less than 12.
    o  The next node (*node++ (== 11)), in turn, has *child++ (8)
and *child++ (9) as its children.
    o  The next node (*node++ (== 10)) has *child++ (7)
and *child++ (6) as its children.
    o  The next node (*node++ (== 8)) has *child++ (1)
and *child++ (2) as its children.
    o  Then, node (*node++ (== 9)) has children *child++ (4)
and *child++ (3).
    o  Finally (as far as children are concerned) (*node++ (== 7)) has
one child *child++ (5)

Since child now points beyond the array, the remaining nodes have no
children. So, nodes 6, 1, 2, 4, 3 and 5 don't have children.

Note that the left and right branches are not ordered: 8 is less than 9, but 7
is larger than 6.

A heap is created by traversing a binary tree level-wise, starting from the
top node. The top node is 12, at the zeroth level. At the first level we find
11 and 10. At the second level 8, 9, 7 and 6 are found, etc.

Heaps can be constructed in containers supporting random access. So, a list is
not an appropriate data structure for a heap. Heaps can be constructed from an
(unsorted) array (using make_heap). The top-element can
be pruned from a heap, followed by reordering the heap (using
pop_heap), a new element can be added to the heap,
followed by reordering the heap (using push_heap), and
the elements in a heap can be sorted (using sort_heap,
which, of course, invalidates the heap).


19.1.62.1: The `make_heap' function

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void make_heap(RandomAccessIterator first,
RandomAccessIterator last);
            o void make_heap(RandomAccessIterator first,
RandomAccessIterator last, Compare comp);

o  Description:
        
            o  The first prototype: the elements in the range [first,
last) are reordered to form a max-heap using operator< of the data
type to which the iterators point.
            o  The second prototype: the elements in the range [first,
last) are reordered to form a max-heap using the binary comparison function
object comp to compare elements.


19.1.62.2: The `pop_heap' function

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void pop_heap(RandomAccessIterator first,
RandomAccessIterator last);
            o void pop_heap(RandomAccessIterator first,
RandomAccessIterator last, Compare comp);

o  Description:
        
            o  The first prototype: the first element in the range
[first, last) is moved to last - 1. Then, the elements in the range
[first, last - 1) are reordered to form a max-heap using the
operator< of the data type to which the iterators point.
            o  The second prototype: the first element in the range
[first, last) is moved to last - 1. Then, the elements in the range
[first, last - 1) are reordered to form a max-heap using the binary
comparison function object comp to compare elements.


19.1.62.3: The `push_heap' function

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void push_heap(RandomAccessIterator first,
RandomAccessIterator last);
            o void push_heap(RandomAccessIterator first,
RandomAccessIterator last, Compare comp);

o  Description:
        
            o  The first prototype: assuming that the range [first,
last - 1) contains a valid heap, and the element at last - 1 contains an
element to be added to the heap, the elements in the range [first, last
- 1) are reordered to form a max-heap using the operator< of the data
type to which the iterators point.
            o  The second prototype: assuming that the range [first,
last - 1) contains a valid heap, and the element at last - 1 contains an
element to be added to the heap, the elements in the range [first, last
- 1) are reordered to form a max-heap using the binary comparison function
object comp to compare elements.


19.1.62.4: The `sort_heap' function

        
    
        o  Header file: <algorithm>
        o  Function prototypes:
        
            o void sort_heap(RandomAccessIterator first,
RandomAccessIterator last);
            o void sort_heap(RandomAccessIterator first,
RandomAccessIterator last, Compare comp);

o  Description:
        
            o  The first prototype: assuming the elements in the range
[first, last) form a valid max-heap, the elements in the range
[first, last) are sorted using operator< of the data type to
which the iterators point.
            o  The second prototype: assuming the elements in the range
[first, last) form a valid heap, the elements in the range
[first, last) are sorted using the binary comparison function
object comp to compare elements.


19.1.62.5: An example using the heap functions

        Here is an example showing the various generic algorithms manipulating heaps:
            #include <algorithm>
    #include <iostream>
    #include <functional>
    #include <iterator>
    using namespace std;

    void show(int *ia, char const *header)
    {
        cout << header << ":\n";
        copy(ia, ia + 20, ostream_iterator<int>(cout, " "));
        cout << '\n';
    }
    int main()
    {
        int ia[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
                    11, 12, 13, 14, 15, 16, 17, 18, 19, 20};

        make_heap(ia, ia + 20);
        show(ia, "The values 1-20 in a max-heap");

        pop_heap(ia, ia + 20);
        show(ia, "Removing the first element (now at the end)");

        push_heap(ia, ia + 20);
        show(ia, "Adding 20 (at the end) to the heap again");

        sort_heap(ia, ia + 20);
        show(ia, "Sorting the elements in the heap");


        make_heap(ia, ia + 20, greater<int>());
        show(ia, "The values 1-20 in a heap, using > (and beyond too)");

        pop_heap(ia, ia + 20, greater<int>());
        show(ia, "Removing the first element (now at the end)");

        push_heap(ia, ia + 20, greater<int>());
        show(ia, "Re-adding the removed element");

        sort_heap(ia, ia + 20, greater<int>());
        show(ia, "Sorting the elements in the heap");
    }
    /*
        Displays:
            The values 1-20 in a max-heap:
            20 19 15 18 11 13 14 17 9 10 2 12 6 3 7 16 8 4 1 5
            Removing the first element (now at the end):
            19 18 15 17 11 13 14 16 9 10 2 12 6 3 7 5 8 4 1 20
            Adding 20 (at the end) to the heap again:
            20 19 15 17 18 13 14 16 9 11 2 12 6 3 7 5 8 4 1 10
            Sorting the elements in the heap:
            1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
            The values 1-20 in a heap, using > (and beyond too):
            1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
            Removing the first element (now at the end):
            2 4 3 8 5 6 7 16 9 10 11 12 13 14 15 20 17 18 19 1
            Re-adding the removed element:
            1 2 3 8 4 6 7 16 9 5 11 12 13 14 15 20 17 18 19 10
            Sorting the elements in the heap:
            20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
    */




Chapter 20: Multi Threading

The 98 C++ standard did not acknowledge the existence of
multi-threading. Between then and the release of the current C++ standard
computers have evolved to multi-core machines, and using multi-threading by
now is a real option to consider when developing software.


Multi-threading is an extensive and complex subject, and many
good reference texts on the subject exist. The C++ multi-threading is
built upon the facilities offered by the pthreads library (cf. 
    Nichols, B, et al.'s
        Pthreads Programming, O'Reilly
        ). However,
in line with C++'s current-day philosophy the multi-threading
implementation offered by the language offers a high level interface to
multi-threading, and using the raw pthread building blocks is hardly ever
necessary (cf. Williams, A. (2019): C++ Concurrency in action).  

This chapter covers the facilities for multi-threading as supported by
C++. Although the coverage aims at providing the tools and examples
allowing you to create your own multi-threaded programs, coverage necessarily
is far from complete. The topic of multi threading is too extensive for
that. The mentioned reference texts provide a good starting point for any
further study of multi threading.

A thread of execution (commonly abbreviated to a thread) is a single
flow of control within a program. It differs from a separately executed
program, as created by the fork(1) system call in the sense that threads
all run inside one program, while fork(1) creates independent copies of a
running program. Multi-threading means that multiple tasks are being executed
in parallel inside one program, and no assumptions can be made as to which
thread is running first or last, or at what moment in time. Especially when
the number of threads does not exceed the number of cores, each thread may be
active at the same time. If the number of threads exceed the number of cores,
the operating system will resort to task switching, offering each thread
time slices in which it can perform its tasks. Task switching takes time, and
the law of diminishing returns applies here as well: if the number of threads
greatly exceeds the number of available cores (also called
overpopulation), then the overhead incurred may exceed the benefit of
being able to run multiple tasks in parallel.

Since all threads are running inside one single program, all threads share the
program's data and code. When the same data are accessed by multiple threads,
and at least one of the threads is modifying these data, access must be
synchronized to avoid that threads read data while these data are being
modified by other threads, and to avoid that multiple threads modify the same
data at the same time.

So how do we run a multi-threaded program in C++? Let's look at hello
world, the multi-threaded way:
             1: #include <iostream>
     2: #include <thread>
     3: 
     4: void hello()
     5: {
     6:     std::cout << "hello world!\n";
     7: }
     8: 
     9: int main()
    10: {
    11:     std::thread hi(hello);
    12:     hi.join();
    13: }

    
    o  At line 2 the header thread is included, informing the compiler
about the existence of the class std::thread (cf. section 20.1.2);

o  At line 11 the std::thread hi object is created. It is provided
with the name of a function (hello) which will be called in a separate
thread. Actually, the second thread, running hello, is immediately started
when a std::thread is defined this way;

o  The main function itself also represents a thread: the program's
first thread. It should wait until the second thread has finished. This
is realized in line 12, where hi.join() waits until the thread hi
has finished its job. Since there are no further statements in main, the
program itself ends immediately thereafter.

o  The function hello itself, defined in lines 4 through 7, is
trivial: it simply inserts the text `hello world' into cout, and 
terminates, thus ending the second thread.


20.1: Multi Threading

In C++ multi threading may be implemented at various levels of abstraction. In
general the highest level of abstraction which is available to implement a
multi-threaded problem should be used. Not so much because it's often simpler
than using lower levels of abstraction, but because higher levels of
abstraction are usually semantically closer to the original problem
description, resulting in code which is easier to understand and therefore
easier to maintain.  Also, high-abstraction classes also provide exception
safety and prevent the occurrence of memory leaks.

C++'s main tool for creating multi-threaded programs is the class
std::thread, and some examples of its use have already been shown at the
beginning of this chapter. 

Characteristics of individual threads can be queried from the
std::this_thread namespace. Also, std::this_thread offers some control
over the behavior of an individual thread.

To synchronize access to shared data C++ offers mutexes (implemented
by the class std::mutex) and condition variables (implemented by the
class std::condition_variable).

Members of these classes may throw system_error objects (cf. section
10.9) when encountering a low-level error condition.


20.1.1: The namespace std::this_thread

    The  namespace std::this_thread
contains functions that are uniquely associated with the currently running
thread.

Before using the namespace this_thread the <thread> header file must
be included.

Inside the std::this_thread namespace several free functions are defined,
providing information about the current thread or that can be used to control
its behavior:
    
    o thread::id this_thread::get_id() noexcept:
returns an object of type thread::id that identifies the currently
active thread of execution. For an active thread the returned id is unique
in the sense that it maps 1:1 to the currently active thread, and is not
returned by any other thread. If the thread is currently not running then the
default thread::id object is returned by the std::thread
object's get_id member.

o void yield() noexcept:
when a thread calls this_thread::yield() the current thread is
briefly suspended, allowing other (waiting) threads to start.

o void sleep_for(chrono::duration<Rep, Period> const &relTime)
            noexcept:
when a thread calls this_thread::sleep_for(...) it is suspended
for the amount of time that's specified in its argument. E.g.,
   std::this_thread::sleep_for(std::chrono::seconds(5));

o void sleep_until(chrono::time_point<Clock, Duration> const &absTime)
            noexcept:
when a thread calls this member it is suspended until the specified
absTime is in the past. The next example has the same effect as the
previous example:
   // assume using namespace std
this_thread::sleep_until(chrono::system_clock().now() + chrono::seconds(5));

Conversely, the sleep_until call in the next example immediately
returns:
   this_thread::sleep_until(chrono::system_clock().now() - chrono::seconds(5));


20.1.2: The class std::thread

    Multi threading in C++ starts off with objects of the class
std::thread. Each object of this class handles a separate
thread.

Before using Thread objects the <thread> header file must be included.

Thread objects can be constructed in various ways:
    
    o thread() noexcept:
The default constructor creates a thread object. As it receives no
function to execute, it does not start a separate thread of execution. It is
used, e.g., as a data member of a class, allowing class objects to start a
separate thread at some later point in time;

o thread(thread &&tmp) noexcept:
The move constructor takes ownership of the thread controlled by
tmp, while tmp, if it runs a thread, loses control over its
thread. Following this, tmp is in its default state, and the newly created
thread is responsible for calling, e.g., join.

o explicit thread(Fun &&fun, Args &&...args):
This member template (cf. section 22.1.3) expects a function
(or functor) as its first argument. The function is immediately started as a
separate thread. If the function (or functor) expects arguments, then these
arguments can be passed to the thread's constructor immediately following
its first (function) argument. Additional arguments are passed with their
proper types and values to fun. Following the thread object's
construction, a separately running thread of execution is started.

The notation Arg &&...args indicates that any additional arguments are
passed as is to the function. The types of the arguments that are passed to
the thread constructor and that are expected by the called function must
match: values must be values, references must be reference, r-value references
must be r-value references (or move construction must be supported). The
following example illustrates this requirement:
         1: #include <iostream>
     2: #include <thread>
     3: 
     4: using namespace std;
     5: 
     6: struct NoMove
     7: {
     8:     NoMove() = default;
     9:     NoMove(NoMove &&tmp) = delete;
    10: };
    11: 
    12: struct MoveOK
    13: {
    14:     int d_value = 10;
    15: 
    16:     MoveOK() = default;
    17:     MoveOK(MoveOK const &) = default;
    18: 
    19:     MoveOK(MoveOK &&tmp)
    20:     {
    21:         d_value = 0;
    22:         cout << "MoveOK move cons.\n";
    23:     }
    24: };
    25: 
    26: void valueArg(int value)
    27: {}
    28: void refArg(int &ref)
    29: {}
    30: void r_refArg(int &&tmp)
    31: {
    32:     tmp = 100;
    33: }
    34: void r_refNoMove(NoMove &&tmp)
    35: {}
    36: void r_refMoveOK(MoveOK &&tmp)
    37: {}
    38: 
    39: int main()
    40: {
    41:     int value = 0;
    42: 
    43:     std::thread(valueArg,   value).join();
    44:     std::thread(refArg,     ref(value)).join();
    45:     std::thread(r_refArg,   move(value)).join();
    46: 
    47: //  std::thread(refArg,     value);
    48: 
    49:     std::thread(r_refArg,   value).join();
    50:     cout << "value after r_refArg: " << value << '\n';
    51: 
    52: //  std::thread(r_refNoMove, NoMove());
    53: 
    54:     NoMove noMove;
    55: //  std::thread(r_refNoMove, noMove).join();
    56: 
    57:     MoveOK moveOK;
    58:     std::thread(r_refMoveOK, moveOK).join();
    59:     cout << moveOK.d_value << '\n';
    60: }

    
    o  At lines 43 through 45 we see a value, reference, and and r-value
reference being passed to a std::thread: with 
the functions running the threads expecting matching argument types.

o  Line 47 fails to compile, as a value argument doesn't match the
reference expected by refArg. Note that this problem was solved in line 43
by using the std::ref function.

o  On the other hand lines 49 and 58 compile OK, as int values and
class-types supporting move operations can be passed as values to functions
expecting r-value references. In this case notice that the functions expecting
the r-value references do not access the provided arguments (except for the
actions performed by their move constructors), but use move construction to
create temporary values or objects on which the functions operate.

o  Lines 52 and 55 won't compile as the NoMove struct doesn't offer
a move constructor.

o  Member functions of classes can also be used as thread functions. In
those cases the constructor's first argument must be the address of the member
function, the second argument must be a pointer (or reference, or object) for
which the member function is called as thread function, while subsequent
arguments are passed as arguments to the member function. Here is a simple
example illustrating these options:
    struct Demo
{
    int d_value = 0;

    void fun(int value)
    {
        d_value = value;
        cout << "fun sets value to " << value << "\n";
    }
};

int main()
{
    Demo demo;

    thread thr{&Demo::fun, ref(demo), 12 };
    thr.join();
    cout << "demo's value: " << demo.d_value << '\n';   // 12

    thr = thread{&Demo::fun, &demo, 42 };
    thr.join();
    cout << "demo's value: " << demo.d_value << '\n';   // 42

    thr = thread{&Demo::fun, demo, 77 };
    thr.join();
    cout << "demo's value: " << demo.d_value << '\n';   // 42: the thread
                                                        // copied demo
}

Be careful when passing local variables as arguments to thread objects: if
the thread continues to run when the function whose local variables are used
terminates, then the thread suddenly uses wild pointers or wild references, as
the local variables no longer exist. To prevent this from happening
(illustrated by the next example) do as follows:
        
        o  pass an anonymous copy of the local variable as argument to the
thread constructor, or

o  call join on the thread object to ensure that the thread has
finished within the local variable's lifetime.

     1: #include <iostream>
     2: #include <thread>
     3: #include <string>
     4: #include <chrono>
     5: 
     6: void threadFun(std::string const &text)
     7: {
     8:     for (size_t iter = 1; iter != 6; ++iter)
     9:     {
    10:         std::cout << text << '\n';
    11:         std::this_thread::sleep_for(std::chrono::seconds(1));
    12:     }
    13: }
    14: 
    15: std::thread safeLocal()
    16: {
    17:     std::string text = "hello world";
    18:     return std::thread(threadFun, std::string{ text });
    19: }
    20: 
    21: int main()
    22: {
    23:     std::thread local(safeLocal());
    24:     local.join();
    25:     std::cout << "safeLocal has ended\n";
    26: }

    In line 18 be sure not to call std::ref(text) instead of
std::string{ text }.

If the thread cannot be created a std::system_error exception is
thrown.

Since this constructor not only accepts functions but also function objects as
its first argument, a local context may be passed to the function
object's constructor. Here is an example of a thread receiving a function
object using a local context:
            #include <iostream>
    #include <thread>
    #include <array>

    using namespace std;

    class Functor
    {
        array<int, 30> &d_data;
        int d_value;

        public:
            Functor(array<int, 30> &data, int value)
            :
                d_data(data),
                d_value(value)
            {}
            void operator()(ostream &out)
            {
                for (auto &value: d_data)
                {
                    value = d_value++;
                    out << value << ' ';
                }
                out << '\n';
            }
    };

    int main()
    {
        array<int, 30> data;
        Functor functor{ data, 5 };
        thread funThread{ functor, ref(cout) };
        funThread.join();
    };


The class std::thread does not provide a copy constructor.

The following members are available:
    
    o thread &operator=(thread &&tmp) noexcept:
       If the operator's left-hand side operand (lhs) is a joinable
thread, then terminate is called. Otherwise, tmp is assigned to the
operator's lhs and tmp's state is changed to the thread's default state
(i.e., thread()).

o void detach():
Requires joinable (see below) to return true.  The thread for
which detach is called continues to run. The (e.g., parent) thread calling
detach continues immediately beyond the detach-call.  After calling
object.detach(), `object' no longer represents the (possibly still
continuing but now detached) thread of execution. It is the detached thread's
implementation's responsibility to release its resources when its execution
ends.

Since detach disconnects a thread from the running program, e.g.,
main no longer can wait for the thread's completion.  As a program
ends when main ends, its still running detached threads also stop, and
a program may not properly finish all its threads, as demonstrated by
the following example: 
                #include <thread>
    #include <iostream>
    #include <chrono>

    void fun(size_t count, char const *txt)
    {
        for (; count--; )
        {
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
            std::cout << count << ": " << txt << std::endl;
        }
    }

    int main()
    {
        std::thread first(fun, 5, "hello world");
        first.detach();

        std::thread second(fun, 5, "a second thread");
        second.detach();

        std::this_thread::sleep_for(std::chrono::milliseconds(400));
        std::cout << "leaving" << std::endl;
    }


A detached thread may very well continue to run after the function that
launched it has finished. Here, too, you should be very careful not to pass
local variables to the detached thread, as their references or pointers will
be undefined once the function defining the local variables terminates:
            #include <iostream>
#include <thread>
#include <chrono>

using namespace std;
using namespace chrono;

void add(int const &p1, int const &p2)
{
    this_thread::sleep_for(milliseconds(200));
    cerr << p1 << " + " << p2 << " = " << (p1 + p2) << '\n';
}

void run()
{
    int v1 = 10;
    int v2 = 20;

//  thread(add, ref(v1), ref(v2)).detach();     // DON'T DO THIS
    thread(add, int(v1), int(v2)).detach();     // this is OK: own copies
}

int main()
{
    run();
    this_thread::sleep_for(seconds(1));
}


o id get_id() const noexcept:
If the current object does not represent a running thread 
thread::id() is returned. Otherwise, the thread's unique ID
(also obtainable from within the thread via this_thread::get_id()) is
returned.

o unsigned thread::hardware_concurrency() noexecpt:
This static member returns the number of threads that can run at the
same time on the current computer. On a stand-alone multi-core computer it
(probably) returns the number of cores.

o void join():
Requires joinable to return true.  If the thread for which
join is called hasn't finished yet then the thread calling join will
be suspended (also called blocked) until the thread for which join is
called has completed. Following its completion the object whose join
member was called no longer represents a running thread, and its get_id
member will return std::thread::id().

This member was used in several examples shown so far. As noted: when
main ends while a joinable thread is still running, terminate is
called, aborting the program.

o bool joinable() const noexcept:
returns object.get_id() != id(), where object is the
thread object for which joinable was called.

o native_handle_type  native_handle():
returns the thread's handle (implementation defined). This handle
can be passed to functions like pthread_getschedparam and
pthread_setschedparam to get/set the thread's scheduling policy and
parameters.

o void swap(thread &other) noexcept:
The states of the thread object for which swap was called and
other are swapped. Note that threads may always be swapped, even when
their thread functions are currently being executed.

Things to note:
    
    o  When intending to define an anonymous thread it may appear not to
start, unless you immediately also call join. E.g.,
            void doSomething();
int main()
{
    thread(doSomething);        // nothing happens??
    thread(doSomething).join(); // doSomething is executed??
}

This similar to the situation we encountered in section 7.5:
the first statement doesn't define an anonymous thread object at all. It
simply defines the thread object doSomething. Consequently,
compilation of the second statement fails, as there is no thread(thread &)
constructor. When the first statement is omitted, the doSomething function
is executed by the second statement. If the second statement is omitted, a
default constructed thread object by the name of doSomething is
defined.

o  A thread only starts after its construction has completed. This
includes move constructions or move assignments. E.g., in a statement like
            thread object(thread(doSomething));

the move constructor is used to transfer control from an anonymous thread
executing doSomething to the thread object. Only after object's
construction has completed doSomething is started in the separate thread.

o  Exceptions thrown from the thread (e.g., by the function defining the
thread's actions) are local to the executed thread. Either they must be caught
by the executing thread (as each running thread has its own execution stack),
or they can be passed to the starting thread using a packaged_task and a
future (cf., respectively, sections 20.11 and 20.8).

A thread ends when the function executing a thread finishes. When a
thread object is destroyed while its thread function is still running,
terminate is called, aborting the program's end. Bad news: the destructors
of existing objects aren't called and exceptions that are thrown are left
uncaught. This happens in the following program as the thread is still active
when main ends:
            #include <iostream>
    #include <thread>

    void hello()
    {
        while (true)
            std::cout << "hello world!\n";
    }

    int main()
    {
        std::thread hi(hello);
    }

    There are several ways to solve this problem. One of them is discussed in
the next section.


20.1.2.1: Static data and threads: thread_local

        With multi-threaded programs the well-known distinction between global and
local data is somewhat too coarse. For single- and multi-threaded programs
alike, global data are available to all of the program's code, and local data
are available to the function (or compound statement) in which the local data
are defined. But multi-threaded programs may feel the need for an intermediate
type of data, uniquely available to the different threads.

The thread_local keyword provides this intermediate data level. Global
variables declared as thread_local are global within each individual
thread. Each thread owns a copy of the thread_local variables, and may
modify them at will. A thread_local variable in one thread is completely
separated from that variable in another thread. Here is an example:
             1: #include <iostream>
     2: #include <thread>
     3: 
     4: using namespace std;
     5: 
     6: thread_local int t_value = 100;
     7: 
     8: void modify(char const *label, int newValue)
     9: {
    10:     cout << label << " before: " << t_value << ". Address: " <<
    11:                                                     &t_value << '\n';
    12:     t_value = newValue;
    13:     cout << label << " after: " << t_value << '\n';
    14: }
    15: 
    16: int main()
    17: {
    18:     thread(modify, "first", 50).join();
    19:     thread(modify, "second", 20).join();
    20:     modify("main", 0);
    21: }

    
    o  At line 6 the thread_local variable t_value is defined. It is
initialized to 100, and that becomes the initial value for each separately
running thread;

o  In lines 8 through 14 the function modify is defined. It assigns
a new value to t_value;

o  At lines 18 and 19 two threads are started, which are immediately
joining the main thread again. 

o  The main thread itself is also a thread, and it directly calls
modify. 

Running this program shows that each separate thread starts with
t_value being 100, and then modifies it without affecting the values of
t_value used by other threads. 

Note that, although the t_value variables are unique to each thread,
identical addresses may be shown for them. Since each thread uses its own
stack, these variables may occupy the same relative locations within their
respective stacks, giving the illusion that their physical addresses are
identical.


20.1.2.2: Exceptions and join()

        Once a thread starts and it isn't detached it must eventually join
its starting (parent) thread, or the program aborts. Usually, once a thread
has started the parent thread continues to do some work by itself:
            void childActions();
    void doSomeWork();

    void parent()
    {
        thread child(childActions);
        doSomeWork();
        child.join();
    }

However, maybe doSomeWork can't complete its work, and throws an
exception, to be caught outside of parent. This, unfortunately, ends
parent, and child.join() is missed. Consequently, the program aborts
because of a thread that hasn't been joined.

Clearly, all exceptions must be caught, join must be called, and the
exception must be rethrown. But parent cannot use a function try-block, as
the thread object is already out of scope once execution reaches the matching
catch-clause. So we get:
            void childActions();
    void doSomeWork();

    void parent()
    {
        thread child(childActions);
        try
        {
            doSomeWork();
            child.join();
        }
        catch (...)
        {
            child.join();
            throw;
        }
    }

This is ugly: suddenly the function's code is clobbered with a
try-catch clause, as well as some unwelcome code-duplication.

This situation can be avoided using object based programming. Like, e.g.,
unique pointers, which use their destructors to encapsulate the destruction of
dynamically allocated memory, we can use a comparable technique to encapsulate
thread joining in an object's destructor.

By defining the thread object inside a class we're sure that by the time
the our object goes out of scope, even if the childActions function
throws an exception, the thread's join member is called. Here are the bare
essentials of our JoinGuard class, providing the join-guarantee (using
in-line member implementations for brevity):
             1: #include <thread>
     2: 
     3: class JoinGuard
     4: {
     5:     std::thread d_thread;
     6: 
     7:     public:
     8:         JoinGuard(std::thread &&threadObj)
     9:         :
    10:             d_thread(std::move(threadObj))
    11:         {}
    12:         ~JoinGuard()
    13:         {
    14:             if (d_thread.joinable())
    15:                 d_thread.join();
    16:         }
    17: };

    
    o  At line 8 its only constructor starts: it receives a temporary
thread object, which is moved, in line 10, to JoinGuard's d_thread
data member. 

o  When the JoinGuard object ceases to exist, its destructor (line
12) makes sure the thread is joined if it's still joinable (lines 14 and 15).

Here is an example how JoinGuard could be used:
             1: #include <iostream>
     2: #include "joinguard.h"
     3: 
     4: void childActions();
     5: 
     6: void doSomeWork()
     7: {
     8:     throw std::runtime_error("doSomeWork throws");
     9: }
    10: 
    11: void parent()
    12: {
    13:     JoinGuard{std::thread{childActions}};
    14:     doSomeWork();
    15: }
    16: 
    17: int main()
    18: try
    19: {
    20:     parent();
    21: }
    22: catch (std::exception const &exc)
    23: {
    24:     std::cout << exc.what() << '\n';
    25: }

    
    o  At line 4 childActions is declared. Its implementation (not
provided here) defines the child thread's actions.

o  The main function (lines 17 through 25) provides the function 
try-block to catch the exception thrown by parent;

o  The parent function defines (line 13) an anonymous JoinGuard,
receiving  an anonymous thread object. Anonymous objects are used, as the
parent function doesn't need to access them anymore.

o  In line 14 doSomeWork is called, which throws an exception. This
ends parent, but just before that JoinGuard's destructor makes sure
that the child-thread has been joined.


20.1.3: The class std::jthread

     In addition to std::thread the class std::jthread can be
used. 

Before using jthread objects the <thread> header file must be
included.

Objects of the class jthread act like thread objects, but a
jthread thread automatically joins the thread that activated
jthread. Moreover, in some situations jthread threads can directly 
be ended.

Once a jthread object receiving a function defining the thread's actions
has been constructed that function immediately starts as a separate thread.
If that function ends by returning a value then that value is ignored. If the
function throws an exception the program ends by calling
std::terminate. Alternatively, if the function should communicate a return
value or an exception to, e.g., the function starting the jthread a
std::promise (cf. section (20.12)) can be used or it can modify
variables which are shared with other threads (see also sections 20.2
and 20.5).

The class jthread offers these constructors:
    
    o jthread() noexcept:
The default constructor creates a jthread object that doesn't
start a thread. It could be used as a data member of a class, allowing class
objects to start the jthread at some later point in time;

o explicit jthread(Function &&function, Args &&...args):
This constructor (which is a member template, cf. section
22.1.3) expects a function (or functor) as its first argument, starting
the thread defined by function. The function receives as its first
argument the return value of jthread's member get_stop_token (see
below), followed by the args parameters (if present). If function's
first argument is not a std::stop_token then function, merely
receiving the args parameter values as its arguments. Arguments are passed
to function with their proper types and values (see the example shown
below at the description of the jthread member request_stop;)

o  The class jthread supports move construction and move assignment,
but does not offer copy construction and copy assignment.

The following members are available and operate like the identically named
std::thread members. Refer to section 20.1.2 for their descriptions:
    
    o void detach();
    o id get_id() const noexcept;
    o unsigned thread::hardware_concurrency() noexecpt
    o void join();
    o bool joinable() const noexcept;
    o native_handle_type  native_handle();
    o void swap(thread &other) noexcept.

The following members are specific to jthread, allowing other threads to
end the thread started by jthread:


    o std::stop_source get_stop_source() noexcept:
returns the  jthread's std::stop_source.

o std::get_stop_token get_stop_token() const noexcept:
returns the  jthread's std::stop_token.

o bool request_stop() noexcept:
attempts to end the thread started by the jthread object. The
        function operates atomically: it can be called from multiple threads
        without causing race conditions. It returns true if the stop
        request was successfully issued. It returns false if a stop
        request has already been issued, which may also happen if
        request_stop was issued by different threads, and another thread
        is still in the process of ending jthread's thread.
 When
        issuing request_stop then std::stop_callback functions (see
        the next section) that were registered for the thread's stop state are
        synchroneously called. If those callback functions throw exceptions
        then std::terminate is called. Also, any waiting condition
        variables that are associated with the jthread's stop state end
        their waiting states.

Here is a short program illustrating request_stop:
             1: #include <iostream>
     2: #include <thread>
     3: #include <chrono>
     4: using namespace std;
     5: 
     6: void fun(std::stop_token stop)
     7: {
     8:     while (not stop.stop_requested())
     9:     {
    10:         cout << "next\n";
    11:         this_thread::sleep_for(1s);
    12:     }
    13: }
    14: 
    15: int main()
    16: {
    17:     jthread thr(fun);
    18: 
    19:     this_thread::sleep_for(3s);
    20: 
    21:     thr.request_stop();
    22: 
    23:     // thr.join() not required.
    24: }


    o  at line 17 the jthread thread starts, receiving function fun
        as its argument;
    o  as fun defines a std::stop_token parameter, jthread will
        start that function. It performs (line 8) a while loop that
        continues until stop's stop_requested returns true. The loop
        itself shows a brief output line (line 10) followed by a one-second
        sleep (line 11);
    o  the main function, having started the thread, sleeps for three
        seconds (line 19), and then (line 21) issues a stop-request, 
        ending the thread.

When running the program three lines containing next are displayed.


20.1.3.1: std::stop_callback

        Before using std::stop_callback objects the
<stop_token> header file must be included.

In addition to merely ending thread functions via jthread's request_stop
member function it's also possible to associate  callback
functions with request_stop, which are executed when request_stop is
called. In situations where callback functions are registered when the
thread function has already been stopped the callback functions are
immediately called when they are being registered (registering callback
functions is covered below). 

Note that multiple callback functions can be registered. However, the order in
which these callback functions are run once the thread is stopped is not
defined. Moreover, exceptions may not leave callback functions or the program
ends by calling std::terminate.

Callback functions are registered by objects of the class
std::stop_callback. The class stop_callback offers the following
constructors:
    
    o explicit stop_callback(std::stop_token const &st, 
                               Function &&cb) noexcept;
    o explicit stop_callback(std::stop_token &&st, Function &&cb) noexcept;

Notes:
    
    o Function can be the name of a (void) function without parameters or
        it can be an (anonymous or existing) object offering a parameter-less
        (void) function call operator. The functions do not necessarily have
        to be void functions, but their return values are ignored;
    o  The noexcept is only used if Function is also declared as
        noexcept (if Function is the name of a functor-class then
        noexcept is used if its constructor is declared with
        noexcept);
    o  The class stop_callback does not offer copy/move construction and
        assignment. 

Here is the example used in the previous section, this time defining a
callback function. When running this program its output is
            next
    next
    next
    stopFun called via stop_callback

     1: void fun(std::stop_token stop)
     2: {
     3:     while (not stop.stop_requested())
     4:     {
     5:         cout << "next\n";
     6:         this_thread::sleep_for(1s);
     7:     }
     8: }
     9: 
    10: void stopFun()
    11: {
    12:     cout << "stopFun called via stop_callback\n";
    13: }
    14: 
    15: int main()
    16: {
    17:     jthread thr(fun);
    18: 
    19:     stop_callback sc{ thr.get_stop_token(), stopFun };
    20: 
    21:     this_thread::sleep_for(3s);
    22: 
    23:     thr.request_stop();
    24:     thr.join();
    25: }

The function fun is identical to the one shown in the previous
section, but main defines (line 19) the stop_callback object sc,
passing it thr's get_stop_token's return value and the address of the
function stopFun, defined in lines 10 thru 13. In this case once
request_stop is called (line 23) the callback function stopFun is
called as well.


20.2: Synchronization (mutexes)

Objects of mutex classes are used to protect shared data.

Before using mutexes the <mutex> header file must be included.

One of the key characteristics of multi-threaded programs is that threads may
share data. Functions running as separate threads have access to all global
data, and may also share the local data of their parent threads. However,
unless proper measures are taken, this may easily result in data corruption,
as illustrated by the following simulation of some steps that could be
encountered in a multi-threaded program:
        ---------------------------------------------------------------------------
Time step:    Thread 1:     var        Thread 2:       description
---------------------------------------------------------------------------
    0                        5
    1           starts                                  T1 active
    2           writes var                              T1 commences writing
    3           stopped                                 Context switch
    4                                   starts          T2 active
    5                                   writes var      T2 commences writing
    6                       10          assigns 10      T2 writes 10
    7                                   stopped         Context switch
    8           assigns 12                              T1 writes 12
    9                       12
----------------------------------------------------------------------------

In this example, threads 1 and 2 share variable var, initially having
the value 5. At step 1 thread 1 starts, and starts to write a value into
var. However, it is interrupted by a context switch, and thread 2 is
started (step 4). Thread 2 also wants to write a value into var, and
succeeds until time step 7, when another context switch takes place. By now
var is 10. However, thread 1 was also in the process of writing a value
into var, and it is given a chance to complete its work: it assigns 12
to var in time step 8. Once time step 9 is reached, thread 2 proceeds on
the (erroneous) assumption that var must be equal to 10. Clearly, from the
point of view of thread 2 its data have been corrupted.

In this case data corruption was caused by multiple threads accessing the same
data in an uncontrolled way. To prevent this from happening, access to shared
data should be protected in such a way that only one thread at a time may
access the shared data.

Mutexes are used to prevent the abovementioned kinds of problems by
offering a guarantee that data are only accessed by the thread that could lock
the mutex that is used to synchronize access to those data.

Exclusive data access completely depends on cooperation between the
threads. If thread 1 uses mutexes, but thread 2 doesn't, then thread 2 may
freely access the common data. Of course that's bad practice, which should be
avoided.

It is stressed that although using mutexes is the programmer's
responsibility, their implementation isn't: mutexes offer the necessary
atomic calls. When requesting a mutex-lock the thread is blocked (i.e., the
mutex statement does not return) until the lock has been obtained by the
requesting thread.

Apart from the class std::mutex the class 
std::recursive_mutex is available.  When a recursive_mutex is called
multiple times by the same thread it increases its lock-count. Before other
threads may access the protected data the recursive mutex must be unlocked
again that number of times. Moreover, the classes 
        std::timed_mutex 
    and 
        std::recursive_timed_mutex 
    are available. Their locks expire when released, but also after a certain
amount of time.

The members of the mutex classes perform atomic actions: no context
switch occurs while they are active. So when two threads are trying to
lock a mutex only one can succeed. In the above example: if both threads
would use a mutex to control access to var thread 2 would not have been
able to assign 12 to var, with thread 1 assuming that its value was 10. We
could even have two threads running purely parallel (e.g., on two separate
cores). E.g.:
        -------------------------------------------------------------------------
Time step:    Thread 1:        Thread 2:        description
-------------------------------------------------------------------------
    1         starts           starts           T1 and T2 active
    2         locks            locks            Both threads try to 
                                                lock the mutex
    3         blocks...        obtains lock     T2 obtains the lock,
                                                and T1 must wait
    4         (blocked)        processes var    T2 processes var,
                                                T1 still blocked
    5         obtains lock     releases lock    T2 releases the lock,
                                                and T1 immediately 
                                                obtains the lock
    6         processes var                     now T1 processes var
    7         releases lock                     T1 also releases the lock
-------------------------------------------------------------------------

Although mutexes can directly be used in programs, this rarely happens. It is
more common to embed mutex handling in locking classes that make sure that the
mutex is automatically unlocked again when the mutex lock is no longer
needed. Therefore, this section merely offers an overview of the interfaces of
the mutex classes. Examples of their use will be given in the upcoming
sections (e.g., section 20.3).

All mutex classes offer the following constructors and members:
    
    o mutex() constexpr:
The default constexpr constructor is the only available
constructor; 

o ~mutex():
The destructor does not unlock a locked mutex. If locked it must
explicitly be unlocked using the mutex's unlock member;

o void lock():
The calling thread blocks until it owns the mutex. Unless lock is
called for a recursive mutex a system_error is thrown if the thread
already owns the lock. Recursive mutexes increment their internal 
    lock count;

o bool try_lock() noexcept:
The calling thread tries to obtain ownership of the mutex. If
ownership is obtained, true is returned, otherwise false. If the
calling thread already owns the lock true is also returned, and in this
case a recursive mutex also increments its internal lock count;

o void unlock() noexcept:
The calling thread releases ownership of the mutex.  A
system_error is thrown if the thread does not own the
lock. A recursive mutex decrements its interal lock count, releasing
ownership of the mutex once the lock count has decayed to zero;

The timed-mutex classes (timed_mutex, recursive_timed_mutex) also offer
these members:
    
    o bool try_lock_for(chrono::duration<Rep, Period> const 
            &relTime) noexcept:
The calling thread tries to obtain ownership of the mutex within the
specified time interval. If ownership is obtained, true is returned,
otherwise false. If the calling thread already owns the lock true is
also returned, and in this case a recursive timed mutex also increments its
internal lock count. The Rep and Duration types are inferred from
the actual relTime argument. E.g.,
       std::timed_mutex timedMutex;
timedMutex.try_lock_for(chrono::seconds(5));

o bool try_lock_until(chrono::time_point<Clock,
            Duration> const &absTime) noexcept:
The calling thread tries to obtain ownership of the mutex until
absTime has passed. If ownership is obtained, true is returned,
otherwise false. If the calling thread already owns the lock true is
also returned, and in this case a recursive timed mutex also increments its
internal lock count. The Clock and Duration types are inferred
from the actual absTime argument. E.g.,
       std::timed_mutex timedMutex;
timedMutex.try_lock_until(chrono::system_clock::now() + chrono::seconds(5));


20.2.1: Initialization in multi-threaded programs

    Before using the std::once_flag and the
std::call_once function, introduced in this section, the
<mutex> header file must be included.

In single threaded programs the initialization of global data does not
necessarily occur at one point in code. An example is the initialization of
the object of a singleton class (cf. Gamma et al. (1995), Design Patterns,
Addison-Wesley). Singleton classes may define a single static pointer data
member Singleton *s_object, pointing to the singleton's object, and may
offer a static member instance, implemented something like this:
            Singleton &Singleton::instance()
    {
        return s_object ? 
                    s_object 
                : 
                    (s_object = new Singleton);
    }

With multi-threaded programs this approach immediately gets complex. For
example, if two threads call instance at the same time, while s_object
still equals 0, then both may call new Singleton, resulting in one
dynamically allocated Singleton object becoming unreachable. Other
threads, called after s_object was initialized for the first time, may
either return a reference to that object, or may return a reference to the
object initialized by the second thread. Not exactly the expected behavior of
a singleton.

Mutexes (cf. section 20.2) can be used to solve these kinds of problems,
but they result in some overhead and inefficiency, as the mutex must be
inspected at each call of Singleton::instance. 

When variables must dynamically be initialized, and the initialization should
take place only once the std::once_flag type and the std::call_once
function should be used.

The call_once function expects two or three arguments:
    
    o   The first argument is a once_flag variable, keeping track of the
actual initialization status. The call_once function simply returns if
the once_flag indicates that initialization already took place;

o  The second argument is the address of a function which must be called
only once. This function may be a free function or it may be the address of a
class member function;

o  If the second argument is the address of a class member function,
then the object for which the member function should be called must be
provided as call_once's third argument.

A thread-safe implementation of the singleton's instance function can
now easily be designed (using in-class implementations for brevity):
            class Singleton
    { 
        static std::once_flag s_once;
        static Singleton *s_singleton;
        ...
        public:
            static Singleton *instance()
            {
                std::call_once(s_once, []{s_singleton = new Singleton;} );
                return s_singleton;
            }
        ...
    };

However, there are additional ways to initialize data, even for multi-threaded programs:
    
    o  First, suppose a constructor is declared with the constexpr
keyword (cf. section 8.1.4.1), satisfying the requirements for constant
initialization. In this case, a static object, initialized using that
constructor, is guaranteed to be initialized before any code is run as part of
the static initialization phase. This is used by std::mutex, as it
eliminates the possibility of race conditions when global mutexes are
initialized.

o  Second, a static variable defined within a compound statement may be
used (e.g., a static local variable within a function body).  Static
variables  defined within a compound
statement are initialized the first time the function is called at the point
in the code where the static variable is defined. Here is an example:
                #include <iostream>

        struct Cons
        {
            Cons()
            {
                std::cout << "Cons called\n";
            }
        };
        void called(char const *time)
        {
            std::cout << time << "time called() activated\n";
            static Cons cons;
        }
        int main()
        {
            std::cout << "Pre-1\n";
            called("first");
            called("second");
            std::cout << "Pre-2\n";
            Cons cons;
        }
    /*
        Displays:
            Pre-1
            firsttime called() activated
            Cons called
            secondtime called() activated
            Pre-2
            Cons called
    */

    This feature causes a thread to wait automatically if another thread is
still initializing the static data (note that non-static data never cause
problems, as non-static local variables only exist within their own thread of
execution).


20.2.2: Shared mutexes

    Shared mutexes (via the type std::shared_mutex) are available after
including the <shared_mutex> header file.  Shared mutex types behave like
timed_mutex types and optionally have the characteristics described below.

The class shared_mutex provides a non-recursive mutex with shared
ownership semantics, comparable to, e.g., the shared_ptr type.
A program using shared_mutexes is undefined if:
    
    o  it destroys a shared_mutex object owned by any thread;
    o  a thread recursively attempts to gain ownership of a
        shared_mutex;   
    o  a thread terminates while owning a shared_mutex.

Shared mutex types provide a shared lock ownership mode. Multiple threads can
simultaneously hold a shared lock ownership of a shared_mutex type of
object. But no thread can hold a shared lock while another thread holds an
exclusive lock on the same shared_mutex object, and vice-versa.

Shared mutexes are useful in situations where multiple threads (consumers)
want to access information for reading: the consumers don't want to change the
data, but merely want to retrieve them. At some point another thread (the
producer) wants to modify the data. At that point the producer requests
exclusive access to the data, and is forced to wait until all consumers have
released their locks. While the producer waits for the exclusive lock, new
consumers' requests for shared locks remain pending until the producer has
released the exclusive lock. Thus, reading is possible for many threads, but
for writing the exclusive lock guarantees that no other threads can access the
data.

The type shared_mutex offers the following members providing shared lock
ownership. To obtain exclusive ownership omit the _shared from the
following member functions:
    
    o void lock_shared():
Blocks the calling thread until shared ownership of the mutex can be
obtained by the calling thread. An exception is thrown if the current thread
already owns the lock, if it is not allowed to lock the mutex, or if the mutex
is already locked and blocking is not possible;

o void unlock_shared():
Releases a shared lock on the mutex held by the calling thread. Nothing
happens if the current thread does not already own the lock;

o bool try_lock_shared():
The current thread attempts to obtain shared ownership of the mutex
without blocking. If shared ownership is not obtained, there is no effect and
try_lock_shared immediately returns.  Returns true if the shared
ownership lock was acquired, false otherwise.  An implementation may fail
to obtain the lock even if it is not held by any other thread. Initially the
calling thread may not yet own the mutex;

o bool try_lock_shared_for(rel_time):
Attempts to obtain shared lock ownership for the calling thread within
the relative time period specified by rel_time. If the time specified by
rel_time is less than or equal to rel_time.zero(), the member attempts
to obtain ownership without blocking (as if by calling
try_lock_shared()). The member shall return within the time interval
specified by rel_time only if it has obtained shared ownership of the mutex
object.  Returns true if the shared ownership lock was acquired, false
otherwise. Initially the calling thread may not yet own the mutex;

o bool try_lock_shared_until(abs_time):
Attempts to obtain shared lock ownership for the calling thread until the
time specified by abs_time has passed. If the time specified by
abs_time has already passed then the member attempts to obtain ownership
without blocking (as if by calling try_lock_shared()).  Returns true
if the shared ownership lock was acquired, false otherwise. Initially the
calling thread may not yet own the mutex;


20.3: Locks and lock handling

Locks are used to simplify the use of mutexes. Before locks can be used the
<mutex> header file must be included.

Whenever threads share data, and at least one of the threads may change common
data, mutexes should be used to prevent threads from using the same data
synchronously.

Usually locks are released at the end of action blocks. This requires explicit
calls to the mutexes' unlock function, which introduces comparable
problems as we've seen with the thread's join member.

To simplify locking and unlocking two mutex wrapper classes are available:
    
    o std::lock_guard:
objects of this class offer the basic unlock-guarantee: their
destructors call the member unlock of the mutexes they control;

o std::unique_lock:
objects of this class offer a more extensive interface, allowing
explicit unlocking and locking of the mutexes they control, while their
destructors preserve the unlock-guarantee also offered by lock_guard;

The class lock_guard offers a limited, but useful interface:
    

o lock_guard<Mutex>(Mutex &mutex):
when defining a lock_guard object the mutex type (e.g.,
std::mutex, std::timed_mutex, std::shared_mutex) is specified, and a mutex
of the indicated type is provided as its argument. The construction blocks
until the lock_guard object owns the lock. The lock_guard's destructor
automatically releases the mutex lock.

o lock_guard<Mutex>(Mutex &mutex, std::adopt_lock_t):
this constructor is used to transfer control over the mutex from the
calling thread to the lock_guard.  The mutex lock is released again by the
lock_guard's destructor. At construction time the mutex must already be
owned by the calling thread. Here is an illustration of how it can be used:
         1: void threadAction(std::mutex &mut, int &sharedInt)
 2: {
 3:     std::lock_guard<std::mutex> lg{mut, std::adopt_lock_t()};
 4:     // do something with sharedInt
 5: }

            
            o  At line 1 threadAction receives a reference to a
mutex. Assume the mutex owns the lock;

o  At line 3 control is transferred to the lock_guard. Even
though we don't explicitly use the lock_guard object, an object should be
defined to prevent the compiler from destroying an anonymous object before the
function ends;

o  When the function ends, at line 5, the mutex's lock is
released by the lock_guard's destructor.

o mutex_type:
in  addition to the constructors and destructor,
lock_guard<Mutex> types also define the type mutex_type: it is a
synonym of the Mutex type that is passed to the lock_guard's
constructor.

Here is a simple example of a multi-threaded program using lock_guards
to prevent information inserted into cout from getting mixed.
            bool oneLine(istream &in, mutex &mut, int nr)
    {
       lock_guard<mutex> lg(mut);
    
        string line;
        if (not getline(in, line))
            return false;
    
        cout << nr << ": " << line << endl;
    
        return true;
    }
    
    void io(istream &in, mutex &mut, int nr)
    {
        while (oneLine(in, mut, nr))
            this_thread::yield();
    }
    
    int main(int argc, char **argv)
    {
        ifstream in(argv[1]);
        mutex ioMutex;
    
        thread t1(io, ref(in), ref(ioMutex), 1);
        thread t2(io, ref(in), ref(ioMutex), 2);
        thread t3(io, ref(in), ref(ioMutex), 3);
    
        t1.join();
        t2.join();
        t3.join();
    }


As with lock_guard, a mutex-type must be specified when defining
objects of the class std::unique_lock.  The class
unique_lock is much more elaborate than the basic lock_guard class
template.  Its interface does not define a copy constructor or overloaded
assignment operator, but it does define a move constructor and a move
assignment operator. In the following overview of unique_lock's interface
Mutex refers to the mutex-type that is specified when defining a
unique_lock:
    
    o unique_lock() noexcept:
the default constructor is not yet associated with a mutex
object. It must be assigned a mutex (e.g., using move-assignment) before
it can do anything useful;

o explicit unique_lock(Mutex &mutex):
initializes a unique_lock with an existing Mutex object, and
calls mutex.lock();

o unique_lock(Mutex &mutex, defer_lock_t) noexcept:
initializes a unique_lock with an existing Mutex object, but
does not call mutex.lock(). Call it by passing a defer_lock_t object
as the constructor's second argument, e.g.,
            unique_lock<mutex> ul(mutexObj, defer_lock_t())

o unique_lock(Mutex &mutex, try_to_lock_t) noexcept:
initializes a unique_lock with an existing Mutex object, and
calls mutex.try_lock(): the constructor won't block if the mutex cannot be
locked;

o unique_lock(Mutex &mutex, adopt_lock_t) noexcept:
initializes a unique_lock with an existing Mutex object,
and assumes that the current thread has already locked the mutex;

o unique_lock(Mutex &mutex, chrono::duration<Rep, Period> const
        &relTime) noexcept:
this constructor tries to obtain ownership of the Mutex object by
calling mutex.try_lock_for(relTime). The specified mutex type must
therefore support this member (e.g., it is a std::timed_mutex). It could
be called like this:
           std::unique_lock<std::timed_mutex> ulock(timedMutex, 
                                         std::chrono::seconds(5));

o unique_lock(Mutex &mutex, chrono::time_point<Clock, Duration> const
        &absTime) noexcept:
this constructor tries to obtain ownership of the Mutex object by
calling mutex.try_lock_until(absTime). The specified mutex type must
therefore support this member (e.g., it is a std::timed_mutex). 
This constructor could be called like this:
            std::unique_lock<std::timed_mutex> ulock(
                timedMutex, 
                std::chrono::system_clock::now() + std::chrono::seconds(5)
            );

o void lock():
blocks the current thread until ownership of the mutex that is managed
by the unique_lock is obtained. If no mutex is currently managed, then a
system_error exception is thrown.

o Mutex *mutex() const noexcept:
returns a pointer to the mutex object stored inside the
unique_lock (a nullptr is returned if no mutex object is currently
associated with the unique_lock object.)

o explicit operator bool() const noexcept:
returns true if the unique_lock owns a locked mutex, otherwise
false is returned;

o unique_lock& operator=(unique_lock &&tmp) noexcept:
if the left-hand operand owns a lock, it will call its mutex's
unlock member, whereafter tmp's state is transferred to the left-hand
operand;

o bool owns_lock() const noexcept:
returns true if the unique_lock owns the mutex, otherwise
false is returned;

o Mutex *release() noexcept:
returns a pointer to the mutex object that is associated with the
unique_lock object, discarding that association;

o void swap(unique_lock& other) noexcept:
swaps the states of the current unique_lock and other;

o bool try_lock():
tries to obtain ownership of the mutex that is associated with the
unique_lock, returning true if this succeeds, and false
otherwise. If no mutex is currently associated with the unique_lock
object, then a system_error exception is thrown;

o bool try_lock_for(chrono::duration<Rep, Period> const
        &relTime):
this member function tries to obtain ownership of the Mutex object
managed by the unique_lock object by calling the mutex's
try_lock_for(relTime) member. The specified mutex type must therefore
support this member (e.g., it is a std::timed_mutex);

o bool try_lock_until(chrono::time_point<Clock,
        Duration> const &absTime):
this member function tries to obtain ownership of the Mutex object
managed by the unique_lock object by calling the mutex's
mutex.try_lock_until(absTime) member. The specified mutex type must
therefore support this member (e.g., it is a std::timed_mutex);

o void unlock():
releases ownership of the mutex (or reduces the mutex's lock count). A
system_error exception is thrown if the unique_lock object does not
own the mutex.

In addition to the members of the classes std::lock_guard and
std::unique_lock the functions std::lock and
std::try_lock are available. These functions can be
used to prevent deadlocks, the topic of the next section.


20.3.1: Name-independent declarations

    A common error when using lock_guards is defining it as an anonymous
object:
            void Class::notLocked()
    {
        lock_guard<mutex>{d_mutex};

        // using data available to multiple threads
    }
    In cases like these, since the lock_guard is defined as an anonymous
object, it's immediately destroyed after its construction offering no guard
against multiple threads using the shared data.

Traditionally this situation is solved by explicitly defining an object. The
object's name is irrelevant, because it's used nowhere else, resulting in
constructions like
            void Class::lockedOK()
    {
        lock_guard<mutex> guard{d_mutex};

        // using data available to multiple threads
    }
    But in this context the name's irrelevant and not used nowhere elsew in
the function.

Since the C++26 standard, however, a generalized alternative approach
is available. It's called 
        name-independent declaration
    Very simple (and broadly applicable), requiring --std=c++26 or
beyond, which is supported since g++-14.

The 'name' _ (a single underscore) results in a name-independent
declaration. It's definitely not a name we would use for 'common'
variables, but starting with the C++26 standard a variable named `_' implies a
name-independent declaration. So in 'lockedOK' we can now do:
            void Class::lockedOK()
    {
        lock_guard<mutex> _{d_mutex};

        // using data available to multiple threads
    }
    and we never have to think again about how to name a required (but not
used by us) variable or object. As long as there's no ambiguity it's even
possible to define multiple `_' variables or objects. As long as they're not
being used by name it's possible to define, e.g.,
            void neverUsed()
    {
        int _{12};
        int _{43};      // a different int because of the initialization
        auto _ = 42;            // 'auto' also works fine
        auto _("hello world");  // a string...
    }
    But in practice, use name independent declarations as illustrated in the
above lockedOK function.


20.3.2: Deadlocks

    A deadlock occurs when two locks are required to process data, but one thread
obtains the first lock and another thread obtains the second lock. C++
defines the generic
    std::lock and std::try_lock functions that can
be used to help preventing such situations.

Before these functions  can be used the <mutex> header file must be
included

In the following overview L1 &l1, ... represents one or more
references to objects of lockable types:
    
    o void std::lock(L1 &l1, ...):
When the function returns locks were obtained on all li
        objects. If a lock could not be obtained for at least one of the
        objects, then all locks obtained so far are relased, even if the
        object for which no lock could be obtained threw an exception;
    o int std::try_lock(L1 &l1, ...):
This function calls the lockable objects' try_lock members. If all
        locks could be obtained, then -1 is returned. Otherwise the (0-based)
        index of the first argument which could not be locked is returned,
        releasing all previously obtained locks.

As an example consider the following little multi-threaded program: The
threads use mutexes to obtain unique access to cout and to an int
value. However, fun1 first locks cout (line 7), and then value
(line 10); fun2 first locks value (line 16) and then cout (line
19). Clearly, if fun1 has locked cout fun2 can't obtain the lock
until fun1 has released it. Unfortunately, fun2 has locked value,
and the functions only release their locks when returning. But in order to
access the information in value fun1 it must have obtained a lock on
value, which it can't, as fun2 has already locked value: the
threads are waiting for each other, and neither thread gives in.
             1: 
     2: int value;
     3: mutex valueMutex;
     4: mutex coutMutex;
     5: 
     6: void fun1()
     7: {
     8:     lock_guard<mutex> lg1(coutMutex);
     9:     cout << "fun 1 locks cout\n";
    10: 
    11:     lock_guard<mutex> lg2(valueMutex);
    12:     cout << "fun 1 locks value\n";
    13: }
    14: 
    15: void fun2()
    16: {
    17:     lock_guard<mutex> lg1(valueMutex);
    18:     cerr << "fun 2 locks value\n";
    19: 
    20:     lock_guard<mutex> lg2(coutMutex);
    21:     cout << "fun 2 locks cout\n";
    22: }
    23: 
    24: int main()
    25: {
    26:     thread t1(fun1);
    27:     fun2();
    28:     t1.join();
    29: }
    30: 


A good recipe for avoiding deadlocks is to prevent nested (or multiple) mutex
lock calls. But if multiple mutexes must be used, always obtain the locks in
the same order. Rather than doing this yourself, std::lock and
std::try_lock should be used whenever possible to obtain multiple mutex
locks. These functions accept multiple arguments, which must be lockable types
like lock_guard, unique_lock, or even a plain mutex. The previous
deadlocking program, can be modified to call std::lock to lock both
mutexes. In this example using one single mutex would also work, but the
modified program now looks as similar as possible to the previous
program. Note how in lines 10 and 21 a different
ordering of the unique_locks arguments was used: it is not necessary to
use an identical argument order when calling std::lock or
std::try_lock. 
             1: int value;
     2: mutex valueMutex;
     3: mutex coutMutex;
     4: 
     5: void fun1()
     6: {
     7:     scoped_lock sl{ coutMutex, valueMutex };
     8:     cout << "fun 1 locks cout\n";
     9:     sleep(1);
    10:     cout << "fun 1 locks value\n";
    11: }
    12: 
    13: void fun2()
    14: {
    15:     scoped_lock sl{ valueMutex, coutMutex };
    16:     cout << "fun 2 locks value\n";
    17:     sleep(1);
    18:     cout << "fun 2 locks cout\n";
    19: }
    20: 
    21: int main()
    22: {
    23:     thread t1(fun1);
    24:     fun2();
    25:     t1.join();
    26: }
    27: //  Displays:
    28: //    fun 2 locks value
    29: //    fun 2 locks cout
    30: //    fun 1 locks cout
    31: //    fun 1 locks value



20.3.3: Shared locks

    Shared locks are available through the type
    std::shared_lock, after including the
    <shared_mutex> header file.

An object of the type std::shared_lock controls the shared ownership of a
lockable object within a scope. Shared ownership of the lockable object may be
acquired at construction time or thereafter, and once acquired, it may be
transferred to another shared_lock object. Objects of type shared_lock
cannot be copied, but move construction and assignment is supported. 

The behavior of a program is undefined if the contained pointer to a mutex
(pm) has a non-zero value and the lockable object pointed to by pm does
not exist for the entire remaining lifetime of the shared_lock
object. The supplied mutex type must be a shared_mutex or a type having
the same characteristics.

The type shared_lock offers the following constructors, destructor and
operators:
    
    o shared_lock() noexcept:
The default constructor creates a shared_lock which is not owned
by a thread and for which pm == 0;

o explicit shared_lock(mutex_type &mut):
This constructor locks the mutex, calling mut.lock_shared(). The
calling thread may not already own the lock. Following the construction pm
== &mut, and the lock is owned by the current thread;

o shared_lock(mutex_type &mut, defer_lock_t) noexcept:
This constructor assigns pm to &mut, but the calling thread 
does not own the lock;

o shared_lock(mutex_type &mut, try_to_lock_t):
This constructor tries to locks the mutex, calling
mut.try_lock_shared(). The calling thread may not already own the
lock. Following the construction pm == &mut, and the lock may or may not
be owned by current thread, depending on the return value of
try_lock_shared;

o shared_lock(mutex_type &mut, adopt_lock_t):
This constructor can be called if the calling thread has shared
ownership of the mutex. Following the construction pm == &mut, and the
lock is owned by the current thread;

o shared_lock(mutex_type &mut,
                chrono::time_point<Clock, Duration> const &abs_time):
This constructor is a member template, where Clock and
Duration are types specifying a clock and absolute time (cf. section
4.2). It can be called if the calling thread does not already own
the mutex. It calls mut.try_lock_shared_until(abs_time). Following the
construction pm == &mut, and the lock may or may not be owned by current
thread, depending on the return value of try_lock_shared_until;

o shared_lock(mutex_type &mut, 
                chrono::duration<Rep, Period> const &rel_time):
This constructor is a member template, where Clock and
Period are types specifying a clock and relative time (cf. section
4.2). It can be called if the calling thread does not already own
the mutex. It calls mut.try_lock_shared_for(abs_time). Following the
construction pm == &mut, and the lock may or may not be owned by current
thread, depending on the return value of try_lock_shared_for;

o shared_lock(shared_lock &&tmp) noexcept:
The move constructor transfers the information in tmp to the
newly constructed shared_lock. Following the construction tmp.pm == 0
and tmp no longer owns the lock;

o ~shared_lock():
If the lock is owned by the current thread, 
pm->unlock_shared() is called;

o shared_lock &operator=(shared_lock &&tmp) noexcept
        (The move assignment operator calls pm->unlock_shared and then
transfers the information in tmp to the
current shared_lock object. Following this tmp.pm == 0
and tmp no longer owns the lock;)

o explicit operator bool () const noexcept:
Returns whether or not the shared_lock object owns the lock.

The following members are provided:


    o void lock():
Calls pm->lock_shared(), after which the current tread owns the
shared lock. Exceptions may be thrown from lock_shared, and otherwise if
pm == 0 or if the current thread already owns the lock;

o mutex_type *mutex() const noexcept:
Returns pm;

o mutex_type *release() noexcept:
Returns the previous value of pm, which is equal to zero after
calling this member. Also, the current object no longer owns the lock;

o void swap(shared_lock &other) noexcept:
Swaps the data members of the current and the other shared_lock
objects. There is also a free member swap, a function template, swapping
two shared_lock<Mutex> objects, where Mutex represents the mutex type
for which the shared lock objects were instantiated: void
swap(shared_lock<Mutex> &one, shared_lock<Mutex> &two) noexcept;

o bool try_lock():
Calls pm->try_lock_shared(), returning this call's return value.
Exceptions may be thrown from try_lock_shared, and
otherwise if pm == 0 or if the current thread already owns the lock;

o bool try_lock_for(const chrono::duration<Rep, Period>&
rel_time):
A member template, where Clock and Period are types specifying a
clock and relative time (cf. section 4.2). It calls
mut.try_lock_shared_for(abs_time). Following the call the lock may or may
not be owned by current thread, depending on the return value of
try_lock_shared_until.  Exceptions may be thrown from
try_lock_shared_for, and otherwise if pm == 0 or if the current thread
already owns the lock;

o bool try_lock_until(const chrono::time_point<Clock,
Duration>& abs_time):
A member template, where Clock and Duration are types specifying
a clock and absolute time (cf. section 4.2). It calls
mut.try_lock_shared_until(abs_time), returning its return value. Following
the call the lock may or may not be owned by current thread, depending on the
return value of try_lock_shared_until.  Exceptions may be thrown from
try_lock_shared_until, and otherwise if pm == 0 or if the current
thread already owns the lock;

o void unlock():
Unlocks the shared mutex lock, releasing its ownership. Throws an
exception if the shared mutex was not owned by the current thread.


20.3.4: Scoped locks

    Deadlocks can be avoided using the principles described in the previous
section. However, instead of placing the responsibility for avoiding deadlocks
on the shoulders of the software engineer, an alternative approach is
available: a scoped_lock can be used to lock multiple semaphores at once,
where the scoped_lock ensures that deadlocks are avoided.

The scoped_lock also has a default constructor, performing no actions, so
it's up to the software engineer to define scoped_lock objects with at
least one mutex. Before using scoped_lock objects the <mutex>
header file must be included. Adapting the example from section
20.3.2: both functions define a scoped_lock (note that the order
in which the mutexes are specified isn't relevant), and deadlocks are do not
occur: 
             1: 
     2: int value;
     3: mutex valueMutex;
     4: mutex coutMutex;
     5: 
     6: void fun1()
     7: {
     8:     unique_lock<mutex> lg1(coutMutex, defer_lock);
     9:     unique_lock<mutex> lg2(valueMutex, defer_lock);
    10: 
    11:     lock(lg1, lg2);
    12: 
    13:     cout << "fun 1 locks cout\n";
    14:     cout << "fun 1 locks value\n";
    15: }
    16: 
    17: void fun2()
    18: {
    19:     unique_lock<mutex> lg1(coutMutex, defer_lock);
    20:     unique_lock<mutex> lg2(valueMutex, defer_lock);
    21: 
    22:     lock(lg2, lg1);
    23: 
    24:     cout << "fun 2 locks cout\n";
    25:     cout << "fun 2 locks value\n";
    26: }
    27: 
    28: int main()
    29: {
    30:     thread t1(fun1);
    31:     thread t2(fun2);
    32:     t1.join();
    33:     t2.join();
    34: }
    35: 


Thus, instead of using lock_guard objects, scoped_lock objects can be
used. It's a matter of taste whether lock_guards or scoped_locks
should be preferred when only one mutex is used. Maybe scoped_lock should
be preferred, since it always works....


20.4: Event handling (condition variables)

This section introduces condition variables. Condition variables allow
programs to synchronize threads using the states of data, rather than
simply locking the access to data (which is realized using mutexes).

Before condition variables can be used the <condition_variable> header
file must be included.

To start our discussion, consider a classic producer-consumer scenario: the
producer generates items which are consumed by a consumer. The producer can
only produce a certain number of items before its storage capacity has filled
up and the client cannot consume more items than the producer has produced.

At some point the producer's storage capacity has filled to the brim, and the
producer has to wait until the client has at least consumed some items,
thereby creating space in the producer's storage. Similarly, the consumer
cannot start consuming until the producer has at least produced some items.

Implementing this scenario only using mutexes (data locking) is not an
attractive option, as merely using mutexes forces a program to implement the
scenario using polling: processes must continuously (re)acquire the
mutex's lock, determine whether they can perform some action, followed by the
release of the lock. Often there's no action to perform, and the process is
busy acquiring and releasing the mutex's lock. Polling forces threads to wait
until they can lock the mutex, even though continuation might already be
possible. The polling interval could be reduced, but that too isn't an
attractive option, as that increases the overhead associated with handling the
mutexes (also called `busy waiting').

Condition variables can be used to prevent polling. Threads can use condition
variables to notify waiting threads that there is something for them to
do. This way threads can synchronize on data values (states).

As data values may be modified by multiple threads, threads still need to use
mutexes, but only for controlling access to the data.  In addition, condition
variables allow threads to release ownership of mutexes until a certain
value has been obtained, until a preset amount of time has been passed, or
until a preset point in time has been reached.

The prototypical setup of threads using condition variables looks like this:
    
    o  consumer thread(s) act like this:
            lock the mutex
    while the required condition has not yet been attained (i.e., is false):
        wait until being notified 
         (this automatically releasing the mutex's lock).
    once the mutex's lock has been reacquired, and the required condition
    has been attained:
        process the data
    release the mutex's lock.

o  producer thread(s) act similarly:
            lock the mutex
    while the required condition has not yet been attained:
        do something to attain the required condition
    notify waiting threads (that the required condition has been attained)
    release the mutex's lock.

No matter which thread starts, the thread holding the mutex's lock will at
some point release the lock, allowing the other process to (re)acquire it. If
the consumer starts it immediately releases the lock once it enters its
waiting state; if the producer starts it releases the lock once the condition
is true. 

This protocol hides a subtle initial synchronization requirement. The consumer
will miss the producer's notification if it (i.e., the consumer) hasn't yet
entered its waiting state. So waiting (consumer) threads should start
before notifying (producer) threads. Once threads have started, no
assumptions can be made anymore about the order in which any of the condition
variable's members (notify_one, notify_all, wait, wait_for, and
wait_until) are called.

Condition variables come in two flavors: objects of the class 
    std::condition_variable are used in combination
with objects of type unique_lock<mutex>. Because of 
optimizations which are available for this specific combination using 
condition_variables is somewhat more efficient than using the more
generally applicable class
    std::condition_variable_any, which may be
used with any (e.g., user supplied) lock type.

Condition variable classes (covered in detail in the next two sections) offer
members like wait, wait_for, wait_until, notify_one and notify_all
that may concurrently be called.  The notifying members are always atomically
executed. Execution of the wait members consists of three atomic parts:
    
    o  the mutex is released, and the thread is suspended until its
        notification; 
    o  Once the notification has been received, the lock is reacquired
    o  The wait state ends (and processing continues beyond the wait
        call). 

So, returning from wait-members the previously waiting thread 
has reacquired the mutex's lock.

In addition to the condition variable classes the following free function and
enum type is provided:
    
    o void
        std::notify_all_at_thread_exit(condition_variable &cond,
        unique_lock<mutex> lockObject):
once the current thread has ended, all other threads waiting on
        cond are notified. It is good practice to exit the thread as
        soon as possible after calling notify_all_at_thread_exit.

Waiting threads must verify that the thread they were waiting for has
        indeed ended. This is usually realized  by first obtaining the lock on
        lockObject, followed by verifying that the condition
        they were waiting for is true and that the lock was not
        reacquired before notify_all_at_thread_exit was called.

o std::cv_status:
the cv_status enum is used by several member functions of the
        condition variable classes (cf. sections 20.4.1 and
        20.4.2): 
       namespace std
{
    enum class cv_status 
    { 
        no_timeout, 
        timeout 
    };
}


20.4.1: The class std::condition_variable

    The class std::condition_variable merely offers a
default constructor. No copy constructor or overloaded assignment operator is
provided.

Before using the class condition_variable the <condition_variable>
header file must be included.

The class's destructor requires that no thread is blocked by the thread
destroying the condition_variable. So all threads waiting on a
condition_variable must be notified before a condition_variable
object's lifetime ends. Calling notify_all (see below) before a
condition_variable's lifetime ends takes care of that, as the
condition_variable's thread releases its lock of the mutex variable,
allowing one of the notified threads to lock the mutex.

In the following member-descriptions a type Predicate indicates that a
provided Predicate argument can be called as a function without arguments,
returning a bool. Also, other member functions are frequently referred
to. It is tacitly assumed that all member referred to below were called using
the same condition variable object.

The class condition_variable supports several wait members, which
block the thread until notified by another thread (or after a configurable
waiting time). However, wait members may also spuriously unblock, without
having reacquired the lock. Therefore, returning from wait members threads
should verify that the required condition is actually true. If not,
again calling wait may be appropriate.  The next piece of pseudo code
illustrates this scheme:
           while (conditionNotTrue())
        condVariable.wait(&uniqueLock);

The class condition_variable's members are:
    
    o void notify_one() noexcept:
one wait member called by other threads returns. Which one
        actually returns cannot be predicted.

o void notify_all() noexcept:
all wait members called by other threads unblock their wait
        states. Of course, only one of them will subsequently succeed in
        reacquiring the condition variable's lock object.

o void wait(unique_lock<mutex>& uniqueLock):
before calling wait the current thread must have acquired the lock
        of uniqueLock. Calling wait releases the lock, and the current
        thread is blocked until it has received a notification from another
        thread, and has reacquired the lock.

o void wait(unique_lock<mutex>& uniqueLock, Predicate pred):
this is a member template, using the template header template
        <typename Predicate>. 
       The template's type is automatically derived from the function's
        argument type and does not have to be specified explicitly.

Before calling wait the current thread must have acquired the lock
        of uniqueLock. As long as `pred' returns false
        wait(lock) is called.

o cv_status wait_for(unique_lock<mutex> &uniqueLock,
                      std::chrono::duration<Rep, Period> const &relTime):
this member is defined as a member template, using the template header
        template <typename Rep, typename Period>.  
       The template's types are automatically derived from the types of the
        function's arguments and do not have to be specified explicitly.
       E.g., to wait for at most 5 seconds wait_for can be called like
        this:
       cond.wait_for(&unique_lock, std::chrono::seconds(5));

This member returns when being notified or when the time interval
        specified by relTime has passed.

When returning due to a timeout, std::cv_status::timeout is
        returned, otherwise std::cv_status::no_timeout is
        returned.

Threads should verify that the required data condition has been met
        after wait_for has returned.

o bool wait_for(unique_lock<mutex> &uniqueLock,
                      chrono::duration<Rep, Period> const &relTime, Predicate
                       pred):
this member is defined as a member template, using the template
        header template <typename Rep, typename Period, typename
        Predicate>. 
       The template's types are automatically derived from the types of the
        function's arguments and do not have to be specified explicitly.

As long as pred returns false, the previous wait_for member is
        called. If the previous member returns cv_status::timeout, then
        pred is returned, otherwise true.

o cv_status wait_until(unique_lock<mutex>& uniqueLock,
                        chrono::time_point<Clock, Duration> const &absTime):
this member is defined as a member template, using the template
        header template <typename Clock, typename Duration>. 
       The template's types are automatically derived from the types of the
        function's arguments and do not have to be specified explicitly.
       E.g., to wait until 5 minutes after the current time wait_until can
        be called like this:
       cond.wait_until(&unique_lock, chrono::system_clock::now() +
                              std::chrono::minutes(5));

This function acts identically to the wait_for(unique_lock<mutex>
        &uniqueLock, chrono::duration<Rep, Period> const &relTime) member
        described earlier, but uses an absolute point in time, rather than a
        relative time specification. 

This member returns when being notified or when the time interval
        specified by relTime has passed.
       When returning due to a timeout, std::cv_status::timeout is
        returned, otherwise std::cv_status::no_timeout is
        returned.

o bool wait_until(unique_lock<mutex> &lock,
                  chrono::time_point<Clock, Duration> const &absTime,
                  Predicate pred):
this member is defined as a member template, using the template header
        template <typename Clock, typename Duration, typename Predicate>.
       The template's types are automatically derived from the types of the
        function's arguments and do not have to be specified explicitly.

As long as pred returns false, the previous wait_until member
        is called. If the previous member returns cv_status::timeout, then
        pred is returned, otherwise true.

Threads should verify that the required condition is true when
wait-members of condition variables return.


20.4.2: The class std::condition_variable_any

    Different from the class condition_variable the class
    std::condition_variable_any can be used with
any (e.g., user supplied) lock type, and not just with the stl-provided
unique_lock<mutex>.

Before using the class condition_variable_any the <condition_variable>
header file must be included.

The functionality that is offered by condition_variable_any is identical
to the functionality offered by the class condition_variable, albeit that
the lock-type that is used by condition_variable_any is not
predefined. The class condition_variable_any therefore requires the
specification of the lock-type that must be used by its objects.

In the interface shown below this lock-type is referred to as
Lock. Most of condition_variable_any's members are defined as
member templates, defining a Lock type as one of its parameters. The
requirements of these lock-types are identical to those of the stl-provided
unique_lock, and user-defined lock-type implementations should provide at
least the interface and semantics that is also provided by unique_lock.

This section merely presents the interface of the class
condition_variable_any. As its interface offers the same members as
condition_variable (allowing, where applicable, passing any lock-type
instead of just unique_lock to corresponding members), the reader is
referred to the previous section for a description of the semantics of the
class members.

Like condition_variable, the class condition_variable_any only
offers a default constructor. No copy constructor or overloaded assignment
operator is provided.

Also, like condition_variable, the class's destructor requires that no
thread is blocked by the current thread. This implies that all other (waiting)
threads must have been notified; those threads may, however, subsequently
block on the lock specified in their wait calls.

Note that, in addition to Lock, the types Clock, Duration, Period,
Predicate, and Rep are template types, defined just like the identically
named types mentioned in the previous section.

Assuming that MyMutex is a user defined mutex type, and that MyLock is
a user defined lock-type (cf. section 20.3 for details about
lock-types), then a condition_variable_any object can be defined and used
like this:
            MyMutex mut;
    MyLock<MyMutex> ul(mut);
    condition_variable_any cva;

    cva.wait(ul);

These are the class condition_variable_any's members:
    
    o void notify_one() noexcept;
    o void notify_all() noexcept;
    o void wait(Lock& lock);
    o void wait(Lock& lock, Predicate pred);
    o cv_status wait_until(Lock& lock, const
        chrono::time_point<Clock, Duration>& absTime);
    o bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>&
        absTime, Predicate pred); 
    o cv_status wait_for(Lock& lock, const chrono::duration<Rep,
        Period>& relTime); 
    o bool wait_for(Lock& lock, const chrono::duration<Rep, Period>&
        relTime,) Predicate pred; 


20.4.3: An example using condition variables

    Condition variables are used to synchronize threads on the values of data,
rather than on the mere access to data (for which plain mutex-objects can be
used). Using condition variables, a thread simply sleeps until it is notified
by another thread. In a producer-consumer type of program this is usually
accomplished like this:
            consumer loop:
        - wait until there's an item in store,
            then reduce the number of stored items
        - remove the item from the store
        - increment the number of available storage locations
        - do something with the retrieved item

    producer loop:
        - produce the next item
        - wait until there's room to store the item,
            then reduce the number of available storage locations
        - store the item
        - increment the number of stored items

It is important that the two storage administrative tasks (registering the
number of available items and available storage locations) are either
performed by the client or by the producer. For the consumer `waiting' means:
    
    o  Get a lock on the variable containing the actual count
    o  As long as the count is zero: wait, releasing the lock until another
        thread has increased the count, then re-acquire the lock.
    o  Reduce the count
    o  Release the lock.

This scheme is implemented in a class Semaphore, offering members
wait and notify_all. For a more extensive discussion of semaphores see
Tanenbaum, A.S. (2016)
    Structured Computer Organization, Pearson Prentice-Hall.

As a brief summary: semaphores restrict the number of threads that can access
a resource of limited size. It ensures that the number of threads that add
items to the resource (the producers) can never exceed the resource's maximum
size, or it ensures that the number of threads that retrieve items from the
resource (the consumers) can never exceed the resource's current size. Thus,
in a producer/consumer design two semaphores are used: one to control access
to the resource by the producers, and one to control access to the resource by
the consumers.

For example, say we have ten producing threads, as well as ten consumers, and
a lockable queue that must not grow bigger than 1000 items.
Producers try to push one item at a time; consumers try to pop one.

The data member containing the actual count is called d_available. It is
protected by mutex d_mutex.  In addition a condition_variable
d_condition is defined:
            mutable std::mutex d_mutex;     // mutable because of its use in
                                    // 'size_t size() const'
    std::condition_variable d_condition;
    size_t d_available;


The waiting process is implemented through its member function wait:
             1: void Semaphore::wait()
     2: {
     3:     std::unique_lock<std::mutex> lk(d_mutex);   // get the lock
     4:     while (d_available == 0)
     5:         d_condition.wait(lk);   // internally releases the lock
     6:                                 // and waits, on exit
     7:                                 // acquires the lock again
     8:     --d_available;              // dec. available
     9: }   // the lock is released

    In line 5 d_condition.wait releases the lock. It waits until receiving
a notification, and re-acquires the lock just before returning. Consequently,
wait's code always has complete and unique control over d_available.

What about notifying a waiting thread? This is handled in lines 4 and
5 of the  member function notify_all:
             1: void Semaphore::notify_all()
     2: {
     3:     std::lock_guard<std::mutex> lk(d_mutex);    // get the lock
     4:     if (d_available++ == 0)
     5:         d_condition.notify_all();   // use notify_one to notify one other
     6:                                     // thread
     7: }   // the lock is released

    At line 4 d_available is always incremented; by using a postfix
increment it can simultaneously be tested for being zero. If it was initially
zero then d_available is now one. A thread waiting until d_available
exceeds zero may now continue. A waiting thread is notified by calling
d_condition.notify_one. In situations where multiple threads are waiting
`notify_all' can also be used.

Using the facilities of the class Semaphore whose constructor expects
an initial value of its d_available data member, the classic
consumer-producer paradigm can now be implemented using
multi-threading\ (A more elaborate example of the producer-consumer
program is found in the yo/threading/examples/events.cc file in the
C++ Annotations's source archive):
            Semaphore available(10);
    Semaphore filled(0);
    std::queue<size_t> itemQueue;
    std::mutex qMutex;

    void consumer()
    {
        while (true)
        {
            filled.wait();
            // mutex lock the queue:
            {
                std::lock_guard lg(qMutex);
                size_t item = itemQueue.front();
                itemQueue.pop();
            }
            available.notify_all();
            process(item);      // not implemented here
        }
    }

    void producer()
    {
        size_t item = 0;
        while (true)
        {
            ++item;
            available.wait();
            // mutex lock the queue with multiple consumers
            {
                std::lock_guard lg(qMutex);
                itemQueue.push(item);
            }
            filled.notify_all();
        }
    }
    int main()
    {
        thread consume(consumer);
        thread produce(producer);

        consume.join();
        produce.join();
    }
    Note that a mutex is used to avoid simultaneous access to the queue by
multiple threads. Consider the situation where the queue contains 5 items: in
that situation the semaphores allow the consumer and the producer to access
the queue, but to avoid currupting the queue only one of them may modify the
queue at a time. This is realized by both threads obtaining the std::mutex
qMutex lock before modifying the queue.


20.5: Atomic actions: mutexes not required

Before using the facilities introduced in this section the <atomic> header
file must be included.

When data are shared among multiple threads, data corruption is usually
prevented using mutexes. To increment a simple int using this strategy
code as shown below is commonly used:
            {
        lock_guard<mutex> lk{ intVarMutex };
        ++intVar;
    }

The compound statement is used to limit the lock_guard's lifetime, so
that intVar is only locked for a short little while.

This scheme is not complex, but at the end of the day having to define a
lock_guard for every single use of a simple variable, and having to define
a matching mutex for each simple variable is a bit annoying and cumbersome.

C++ offers a way out through the use of atomic data types.
Atomic data types are available for all basic types, and also for (trivial)
user defined types. Trivial types are (see also section 23.6.2) all
scalar types, arrays of elements of a trivial type, and classes whose
constructors, copy constructors, and destructors all have default
implementations, and their non-static data members are themselves of trivial
types.

The class template std::atomic<Type> is available for all
built-in types, including pointer types. E.g., std::atomic<bool> defines
an atomic bool type. For many types alternative somewhat shorter
type names are available. E.g, instead of std::atomic<unsigned short> the
type std::atomic_ushort can be used. Refer to the atomic header file
for a complete list of alternate names.

If Trivial is a user-defined trivial type then
        std::atomic<Trivial>
defines an atomic variant of Trivial: such a type does not require 
a separate mutex to synchronize access by multiple threads.

Objects of the class template std::atomic<Type> cannot directly be copied
or assigned to each other. However, they can be initialized by values of type
Type, and values of type Type can also directly be assigned to
std::atomic<Type> objects. Moreover, since atomic<Type> types offer
conversion operators returning their Type values, an atomic<Type>
objects can also be assigned to or initialized by another atomic<Type>
object using a static_cast:
            atomic<int> a1 = 5;
    atomic<int> a2{ static_cast<int>(a1) };

The class std::atomic<Type> provides several public members, shown
below. Non-member (free) functions operating on atomic<Type> objects are
also available.

The std::memory_order enumeration defines the following symbolic
constants, which are used to specify ordering constraints of atomic operations:
    
    o memory_order_acq_rel: the operation must be a read-modify-write
        operation, combining memory_order_acquire and
        memory_order_release; 
    o memory_order_acquire: the operation is an acquire operation. It
        synchronizes with a release operation that wrote the same memory
        location; 
    o memory_order_consume: the operation is a consume operation on the
        involved memory location;
    o memory_order_relaxed: no ordering constraints are provided by the
        operation;
    o memory_order_release: the operation is a release operation. It
        synchronizes with acquire operations on the same location;
    o memory_order_sec_cst: the default memory order specification for all
        operations. Memory storing operations use memory_order_release,
        memory load operations use memory_order_acquire, and
        read-modify-write operations use memory_order_acq_rel.

The memory order cannot be specified for the overloaded operators provided by
atomic<Type>. Otherwise, most atomic member functions may also be
given a final memory_order argument. Where this is not available it is
explictly mentioned at the function's description.

Here are the standard available std::atomic<Type> member functions:
    
    o bool compare_exchange_strong(Type
        &currentValue, Type newValue) noexcept:
The value in the atomic object is compared to newValue using
        byte-wise comparisons. If equal (and true is returned) then
        newValue is stored in the atomic object; if unequal (and false
        is returned) the object's current value is stored in
        currentValue;

o bool compare_exchange_weak(Type &oldValue,
        Type newValue) noexcept:
The value in the atomic object is compared to newValue using
        byte-wise comparisons. If equal (and true is returned),
        then newValue is stored in the atomic object; if unequal, or
        newValue cannot be atomically assigned to the current object
        false is returned and the object's current value is stored in
        currentValue;

o Type exchange(Type newValue) noexcept:
The object's current value is returned, and newValue is assigned
        to the current object;

o bool is_lock_free() const noexept:
If the operations on the current object can be performed lock-free
        true is returned, otherwise false.
        This member has no memory_order parameter;

o Type load() const noexcept:
The object's value is returned;

o operator Type() const noexcept:
The object's value is returned;

o void store(Type newValue) noexcept:
NewValue is assigned to the current object. Note that the standard
        assignment operator can also be used.

In addition to the above members, integral atomic types `Integral'
(essentially the atomic variants of all built-in integral types) also offer
the following member functions:
    
    o Integral fetch_add(Integral value) noexcept:
Value is added to the object's value, and the object's
        value at the time of the call is returned;

o Integral fetch_sub(Integral value) noexcept:
Value is subtracted from the object's value, and the object's
        value at the time of the call is returned;

o Integral fetch_and(Integral mask) noexcept:
The bit-and operator is applied to the object's value and
        mask, assigning the resulting value to the current object. The
        object's value at the time of the call is returned;

o Integral fetch_|=(Integral mask) noexcept:
The bit-or operator is applied to the object's value and mask,
        assigning the resulting value to the current object. The object's
        value at the time of the call is returned;

o Integral fetch_^=(Integral mask) noexcept:
The bit-xor operator is applied to the object's value and
        mask, assigning the resulting value to the current object. The
        object's value at the time of the call is returned;

o Integral operator++() noexcept:
The prefix increment operator, returning object's new value;

o Integral operator++(int) noexcept:
The postfix increment operator, returning the object's value before it
        was incremented;

o Integral operator--() noexceptThe prefix decrement operator, returning object's new value;

o Integral operator--(int) noexceptThe postfix decrement operator, returning the object's value
            before it was decremented;

o Integral operator+=(Integral value) noexcept:
Value is added to the object's current value and the object's
        new value is returned;

o Integral operator-=(Integral value) noexcept:
Value is subtracted from the object's current value and the
        object's new value is returned;

o Integral operator&=(Integral mask) noexcept:
The bit-and operator is applied to the object's current value and
        mask, assigning the resulting value to the current object. The
        object's new value is returned;

o Integral operator|=(Integral mask) noexcept:
The bit-or operator is applied to the object's current value and
        mask, assigning the resulting value to the current object. The
        object's new value is returned;

o Integral operator^=(Integral mask) noexcept:
The bit-xor operator is applied to the object's current value and
        mask, assigning the resulting value to the current object. The
        object's new value is returned;

Some of the free member functions have names ending in _explicit. The
_explicit functions define an additional parameter `memory_order
order', which is not available for the non-_explicit functions (e.g.,
atomic_load(atomic<Type> *ptr) and atomic_load_explicit(atomic<Type>
*ptr, memory_order order))

Here are the free functions that are available for all atomic types:
    
    o bool
        std::atomic_compare_exchange_strong(_explicit)(std::atomic<Type> *ptr,
        Type *oldValue, Type newValue) noexept:
returns ptr->compare_exchange_strong(*oldValue, newValue);

o bool
        std::atomic_compare_exchange_weak(_explicit)(std::atomic<Type> *ptr,
        Type *oldValue, Type newValue) noexept:
returns ptr->compare_exchange_weak(*oldValue, newValue);

o Type
        std::atomic_exchange(_explicit)(std::atomic<Type> *ptr, Type newValue)
        noexept:
returns ptr->exchange(newValue);

o void std::atomic_init(std::atomic<Type> *ptr, Type
        init) noexept:
Stores init non-atomically in *ptr. The object pointed to
        by ptr must have been default constructed, and as yet no member
        functions must have been called for it.
        This function has no memory_order parameter;

o bool std::atomic_is_lock_free(std::atomic<Type>
        const *ptr) noexept:
returns ptr->is_lock_free().
        This function has no memory_order parameter;

o Type
        std::atomic_load(_explicit)(std::atomic<Type> *ptr) noexept:
returns ptr->load();

o void
        std::atomic_store(_explicit)(std::atomic<Type> *ptr, Type value)
        noexept:
calls ptr->store(value).

In addition to the abovementioned free functions atomic<Integral> types
also offer the following free member functions:
    
    o Integral
        std::atomic_fetch_add(_explicit)(std::atomic<Integral> *ptr, Integral
        value) noexcept:
returns ptr->fetch_add(value);

o Integral
        std::atomic_fetch_sub(_explicit)(std::atomic<Integral> *ptr, Integral
        value) noexcept:
returns ptr->fetch_sub(value);

o Integral
        std::atomic_fetch_and(_explicit)(std::atomic<Integral> *ptr, Integral
        mask) noexcept:
returns ptr->fetch_and(value);

o Integral
        std::atomic_fetch_or(_explicit)(std::atomic<Integral> *ptr, Integral
        mask) noexcept:
returns ptr->fetch_or(value);

o Integral
        std::atomic_fetch_xor(_explicit)(std::atomic<Integral> *ptr, Integral
        mask) noexcept:
returns ptr->fetch_xor(mask).


20.6: An example: threaded quicksort

The quicksort sorting algorithm (Hoare, 1962) is a well-known sorting
algorithm. Given an array of n elements, it works like this:
    
    o  Pick an element from the array, and partition the array with respect
        to this element (call it the pivot element) (in the example below,
        assume a function partition performing the partition is
        available). This leaves us with two (possibly empty) sub-arrays: one
        to the left of the pivot element, and one to the right of the pivot
        element;
    o  Recursively perform quicksort on the left-hand sub-array;
    o  Recursively perform quicksort on the right-hand sub-array.

To convert this algorithm to a multi-threaded algorithm appears to be be a
simple task: 
            void quicksort(Iterator begin, Iterator end)
    {
        if (end - begin < 2)            // less than 2 elements are left
            return;                     // and we're done

        Iter pivot = partition(begin, end); // determine an iterator pointing
                                            // to the pivot element

        thread lhs(quicksort, begin, pivot);// start threads on the left-hand
                                            // side sub-arrays
        thread rhs(quicksort, pivot + 1, end);  // and on the right-hand side
                                                // sub-arrays
        lhs.join();
        rhs.join();                         // and we're done
    }

Unfortunately, this translation to a multi-threaded approach won't work for 
reasonably large arrays because of a phenomenon called overpopulation:
more threads are started than the operating system is prepared to give us. In
those cases a Resource temporarily unavailable exception is thrown, and
the program ends.

Overpopulation can be avoided by using a pool of workers, where each
`worker' is a thread, which in this case is responsible for handling one (sub)
array, but not for the nested calls. The pool of workers is controlled by a
scheduler, receiving the requests to sort sub-arrays, and passing these
requests on to the next available worker. 

The main data structure of the example program developed in this section is a
queue of std::pairs containing iterators of the array to be sorted
(cf. Figure 26, the sources of the program are found in the C++ Annotations's
yo/threading/examples/multisort directory). Two queues are being used: one
queue is a task-queue, receiving the iterators of sub-arrays to be
partitioned. Instead of immediately launching new threads (the lhs and
rhs threads in the above example), the ranges to be sorted are pushed on
the task-queue. The other queue is the work-queue: elements are moved from the
task-queue to the work-queue, where they will be processed by one of the
worker threads.

------------------------------------------------------------------Insert Figure 26(Data structure used for multi-threading quicksort)about here (file: threading/sorting)------------------------------------------------------------------

The program's main function starts the workforce, reads the data, pushes
the arrays begin and end iterators on the task queue and then starts
the scheduler. Once the scheduler ends the sorted array is displayed:
            int main()
    {
        workForce();            // start the worker threads
        readData();             // read the data into vector<int> g_data
        g_taskQ.push(           // prepare the main task
                    Pair(g_data.begin(), g_data.end())
                ); 
        scheduler();            // sort g_data
        display();              // show the sorted elements
    }

The workforce consists of a bunch of detached threads. Each thread represents
a worker, implemented in the function void worker. Since the number of
worker threads is fixed, overpopulation doesn't occur. Once the array has been
sorted and the program stops these detached threads simply end:
            for (size_t idx = 0; idx != g_sizeofWorkforce; ++idx)
        thread(worker).detach();

The scheduler continues for as long as there are sub-arrays to sort. When this
is the case the task queue's front element is moved to the work queue. This
reduces the work queue's size, and prepares an assignment for the next
available worker. The scheduler now waits until a worker is available. Once 
workers are available one of them is informed of the waiting assignment, and
the scheduler waits for the next task:
            void scheduler()
    {
        while (newTask())
        {
            g_workQ.rawPushFront(g_taskQ);
    
            g_workforce.wait();           // wait for a worker to be available
            g_worker.notify_all();            // activate a worker
        }
    }


The function newTask simply checks whether the task queue is empty. If so,
and none of the workers is currently busy sorting a sub-array then the array
has been sorted, and newTask can return false.  When the task queue is
empty but a worker is still busy, it may be that new sub-array dimensions are
going to be placed on the task queue by an active worker. Whenever a worker is
active the Semaphore g_workforce's size is less than the size of the work
force:
            bool wip()
    {
        return g_workforce.size() != g_sizeofWorkforce;
    }

            bool newTask()
    {
        bool done;
    
        unique_lock<mutex> lk(g_taskMutex);
        while ((done = g_taskQ.empty()) && wip())
            g_taskCondition.wait(lk);
    
        return not done;
    }


Each detached worker thread performs a continuous loop. In the loop it waits
for a notification by the scheduler. Once it receives a notification it
retrieves its assignment from the work queue, and partitions the sub-array
specified in its assignment. Partitioning may result in new tasks. Once this
has been completed the worker has completed its assignment: it increments the
available workforce and notifies the scheduler that it should check whether
all tasks have been performed:
            void worker()
    {
        while (true)
        {
            g_worker.wait();      // wait for action
    
            partition(g_workQ.popFront());
            g_workforce.notify_all();
    
            lock_guard<mutex> lk(g_taskMutex);
            g_taskCondition.notify_one();
        }
    }


Sub-arrays smaller than two elements need no partitioning. All larger
sub-arrays are partitioned relative to their first element. The
std::partition generic algorithm does this well, but if the pivot is
itself an element of the array to partition then the pivot's eventual location
is undetermined: it may be found anywhere in the series of elements which are
at least equal to the pivot. The two required sub-arrays, however, can easily
be constructed:
    
    o  First call std::partition relative to an array's first element, 
        partitioning the array's remaining elements, returning mid,
        pointing to the first element of the series of elements that are at
        least as large as the array's first element;
    o  Then swap the array's first element with element to which mid - 1
        points;
    o  The two sub-arrays range from, respectively, array.begin() to
        mid - 1 (elements all smaller than the pivot), and from mid to
        array.end() (elements all at least as large as the pivot).

The two iterator pairs defining these two sub-arrays are thereupon added
to the task queue, creating two new tasks to be dealt with by the scheduler:
            void partition(Pair const &range)
    {
        if (range.second - range.first < 2)
            return;
    
        auto rhsBegin = partition(range.first + 1, range.second,
                                      [=](int value)
                                      {
                                          return value < *range.first;
                                      }
                                  );
        auto lhsEnd = rhsBegin - 1;
    
        swap(*range.first, *lhsEnd);
    
        pushTask(range.first, lhsEnd);
        pushTask(rhsBegin, range.second);
    }



20.7: Shared States

Just before a thread ends it may have produced some results. These results may
have to to be communicated to other threads. In multi threaded programs
several classes and functions can be used that produce 
    shared states, making it easy to communicate results
to other threads. Results could be values, objects or exceptions.

Objects that contain such shared states are called 
    asynchronous return objects. However,
due to the nature of multi threading, a thread may request the results of an
asynchronous return object before these result are actually available. In
those cases the requesting thread blocks, waiting for the results to become
available. Asynchronous return objects offer wait and get members
which, respectively, wait until the results have become available, and
produce the asynchronous results once they are available. The phrase that
is used to indicate that the results are available is `the shared state has
been made ready'.

Shared states are made ready by 
    asynchronous providers. Asynchronous
providers are simply objects or functions providing results to shared
states. Making a shared state ready means that an asynchronous provider
    
    o  marks its shared state as being ready, and
    o  unblocks any waiting threads (e.g., by allowing blocking members,
        like wait, to return).

Once a shared state has been made ready it contains a value, object, or
exception which can be retrieved by objects having access to the shared
state. While code is waiting for a shared state to become ready the value or
exception that is going to be stored in the shared state may be computed. When
multiple threads try to access the same shared state they must use
synchronizing mechanisms (like mutexes, cf. section 20.2) to prevent
access-conflicts.

Shared states use reference counting to keep track of the number of
asynchronous return objects or asynchronous providers that hold references to
them. These return objects and providers may release their references to these
shared states  (which is called `releasing the
shared state). This happens when a return object or provider holds the last
reference to the shared state, and
     the shared state is destroyed.

On the other hand, an asynchronous provider may also 
    abandon its shared state. In that case the
provider, in sequence,
    
    o  stores an exception object of type 
            std::future_error, holding the error condition
            std::broken_promise in its shared state;
    o  makes its shared data ready; and
    o  releases its shared data.

Objects of the class std::future (see the next section) are asynchronous
return objects. They can be produced by the std::async (section
20.10) family of functions, and by objects of the classes
std::packaged_task (section 20.11), and std::promise (section
20.12).


20.8: Asynchronous return objects: std::future

Condition variables allow threads to wait until data have obtained certain
values. A thread may also have to wait until a sub-thread has finished when
calling a sub-thread's join member. 

Waiting may be unwelcome: instead of just waiting our thread might also be
doing something useful. It might as well pick up the results produced by a
sub-thread at some point in the future. 

In fact, exchanging data among threads always poses some difficulties, as it
requires shared variables, and the use of locks and mutexes to prevent data
corruption. Rather than waiting and using locks it would be nice if some
asynchronous task could be started, allowing the initiating thread (or even
other threads) to pick up the result at some point in the future, when the
results are needed, without having to worry about data locks or waiting times.
For situations like these C++ provides the class std::future.

Before using the class  std::future the <future> header file
must be included.

Objects of the class template std::future harbor the results
produced by asynchronously executed tasks. The class std::future is a
class template. Its template type parameter specifies the type of the result
returned by the asynchronously executed task. This type may be void.

On the other hand, the asynchronously executed task may throw an exception
(ending the task). In that case the future object catches the exception,
and rethrows it once its return value (i.e., the value returned by the
asynchronously executed task) is requested.

In this section the members of the class template future are
described. Future objects are commonly initialized through anonymous
future objects returned by the factory function std::async or by the
get_future members of the classes std::promise, and
std::packaged_task (introduced in upcoming sections). Examples of the use
of std::future objects are provided in those sections.

Some of future's members return a value of the strongly typed
enumeration std::future_status. This enumeration defines
three symbolic constants: future_status::ready, future_status::timeout,
and future_status::deferred. 

Error conditions are returned through std::future_error
exceptions. These error conditions are represented by the values of the
strongly typed enumeration std::future_errc (covered in the next section).

The class future itself provides the following constructors:
    
    o future():
The default constructor constructs an future object that does not
        refer to shared results. Its valid member returns false.

o future(future &&tmp) noexcept:
The move constructor is available. Its valid member returns what
        tmp.valid() would haved returned prior to the constructor
        invocation. After calling the move constructor tmp.valid() returns
        false.

The class future does not offer a copy constructor or an overloaded
assignment operator. 

Here are the members of the class std::future:
    
    o future &operator=(future &&tmp):
The move assignment operator grabs the information from the tmp
        object; following this, tmp.valid() returns false.

o std::shared_future<ResultType> share() &&:
Returns a std::shared_future<ResultType> (see section
        20.9). After calling this function, the future's
        valid member returns false.

o ResultType get():
First wait (see below) is called. Once wait has returned the
        results produced by the associated asynchronous task
        are returned. With future<Type> specifications the returned value
        is the moved shared value if Type supports move assignment,
        otherwise a copy is returned.  With future<Type &> specifications
        a Type & is returned, with future<void> specifications nothing
        is returned. If the shared value is an exception, it is thrown instead
        of returned. After calling this member the future object's
        valid member returns false.

o bool valid() const:
Returns true if the (future) object for which valid is
        called refers to an object returned by an asynchronous task.  If
        valid returns false, the future object exists, but in
        addition to valid only its destructor and move constructor can
        safely be called. When other members are called while valid
        returns false a std::future_error exception is
        thrown (having the value future_errc::no_state).

o void wait() const:
The thread is blocked until the results produced by the associated
        asynchronous task are available.

o std::future_status wait_for(chrono::duration<Rep, Period> const
        &rel_time) const:
This member template derives the template types Rep and Period
        from the actually specified duration (cf. section 4.2.2). If
        the results contain a deferred function nothing happens. Otherwise
        wait_for blocks
 
        until the results are available or until the amount of time
        specified by rel_time has expired. Possible return values are:
            
            o future_status::deferred if the results contains a
                deferred function;
            o future_status::ready if the results are available;
            o future_status::timeout if the function is returning because
                the amount of time specified by rel_time has expired. 

o future_status wait_until(chrono::time_point<Clock, Duration> const
                                                        &abs_time) const:
This member template derives the template types Clock and
        Duration from the actually specified abs_time (cf. section
        4.2.4). If the results contain a deferred function nothing
        happens. Otherwise wait_until blocks until the results are
        available or until the point in time specified by abs_time has
        expired. Possible return values are:
           
            o future_status::deferred if the results contain a
                deferred function;
            o future_status::ready if the results are available;
            o future_status::timeout if the function is returning because
                the point in time specified by abs_time has expired. 

The class std::future<ResultType> declares the following friends:
            std::promise<ResultType>

(sf. section 20.12), and
            template<typename Function, typename... Args>
        std::future<typename result_of<Function(Args...)>::type> 
        std::async(std::launch, Function &&fun, Args &&...args);

(cf. section 20.10).


20.8.1: The std::future_error exception and the std::future_errc enum

    Members of the class std::future may return errors by throwing
std::future_error exceptions. These error conditions are represented by
the values of the strongly typed enumeration
    std::future_errc which defines the following symbolic
constants:
    
    o broken_promise
         
   Broken_promise is thrown when a future object was received whose
    value was never assigned by a promise or packaged_task. For
    example, an object of the class promise<int> should set the value of
    the future<int> object returned by its get_future member
    (cf. section 20.12), but if it doesn't do so, then a
    broken_promise exception is thrown, as illustrated by the following
    program:
    1: std::future<int> fun()
 2: {
 3:     return std::promise<int>().get_future();
 4: }
 5: 
 6: int main()
 7: try
 8: {
 9:     fun().get();
10: }
11: catch (std::exception const &exc)
12: {
13:     std::cerr << exc.what() << '\n';
14: }

   At line 3 a promise object is created, but its value is never
    set. Consequently, it `breaks its promise' to produce a value: when
    main tries to retrieve its value (in line 9) a std::futue_error
    exception is thrown containing the future_errc::broken_promise
    value

o future_already_retrieved 
        
   Future_already_retrieved is thrown when multiple attempts are made to
    retrieve the future object from, e.g., a promise or
    packaged_task object that (eventually) should be ready. For example:
    1: int main()
 2: {
 3:     std::promise<int> promise;
 4:     promise.get_future();
 5:     promise.get_future();
 6: }

   Note that after defining the std::promise object in line 3 it has
    merely been defined: no value is ever assigned to its future. Even
    though no value is assigned to the future object, it is a valid
    object. I.e., after some time the future should be ready, and the
    future's get member should produce a value. Hence, line 4 succeeds,
    but then, in line 5, the exception is thrown as `the future has already
    been retrieved'.

o promise_already_satisfied 
        
   Promise_already_satisfied is thrown when multiple attempts are made to
    assign a value to a promise object. Assigning a value or
    exception_ptr to the future of a promise object may happen
    only once.  For example:
    1: int main()
 2: {
 3:     std::promise<int> promise;
 4:     promise.set_value(15);
 5:     promise.set_value(155);
 6: }


o no_state 
        
   No_state is thrown when a member function (other than valid, see
    below) of a future object is called when its valid member returns
    false. This happens, e.g., when calling members of a default
    constructed future object. No_state is not thrown for future
    objects returned by the async factory function or returned by the
    get_future members of promise or packaged_task type
    of objects. Here is an example:
    1: int main()
 2: {
 3:     std::future<int> fut;
 4:     fut.get();
 5: }


The class   std::future_error is derived from the class
std::exception, and offers, in addition to the char const *what()
const member also the member std::error_code const &code() const,
returning an std::error_code object associated
with the thrown exception. 


20.9: Shared asynchronous return objects: std::shared_future

When a thread activates an asynchronous provider (e.g., a std::async) then
the return value of the asynchronously called function becomes available in
its activating thread through a std::future object. The
future object cannot be used by another thread. If this is required (e.g.,
see this chapter's final section) the future object must be converted to a
    std::shared_future object. 

Before using the class std::shared_future the <future> header file
must be included.

Once a shared_future object is available, its get member (see below)
can repeatedly be called to retrieve the results of the original future
object. This is illustrated by the next small example:
         1: int main()
     2: {
     3:     std::promise<int> promise;
     4:     promise.set_value(15);
     5: 
     6:     auto fut = promise.get_future();
     7:     auto shared1 = fut.share();
     8: 
     9:     std::cerr << "Result: " << shared1.get() << "\n"
    10:                  "Result: " << shared1.get() << "\n"
    11:                  "Valid: " << fut.valid() << '\n';
    12: 
    13:     auto shared2 = fut.share();
    14: 
    15:     std::cerr << "Result: " << shared2.get() << "\n"
    16:                  "Result: " << shared2.get() << '\n';
    17: }

    In lines 9 and 10 the promise's results are retrieved multiple times,
but having obtained the shared_future in line 7, the original future
object no longer has an associated shared state. Therefore, when another
attempt is made (in line 13) to obtain the shared_future, a no
associated state exception is thrown and the program aborts.

However, multiple copies of shared_future objects may co-exist.  When
multiple copies of shared_future objects exist (e.g. in different
threads), the results of the associated asynchronous task are made ready
(become available) at exactly the same moment in time.

The relationship between the classes future and shared_future
resembles the relationship between the classes unique_ptr and
shared_ptr: there can only be one instance of a unique_pointer,
pointing to data, whereas there can be many instances of a shared_pointer,
each pointing to the same data. 

The effect of calling any member of a shared_future object for which
valid() == false other than the destructor, the move-assignment operator,
or valid is undefined.

The class shared_future supports the following constructors:
    
    o shared_future() noexcept
        an empty shared_future object is constructed that does not
            refer to shared results. After using this constructor the object's
            valid member returns false.

o shared_future(shared_future const &other)
        a shared_future object is constructed that refers to the
            same results as other (if any). After using this constructor
            the object's valid member returns the same value as
            other.valid().

o shared_future(shared_future<Result> &&tmp) noexcept
         Effects: move constructs a shared_future object that refers to
            the results that were originally referred to by tmp (if
            any). After using this constructor the object's valid member
            returns the same value as tmp.valid() returned prior to the
            constructor invocation, and tmp.valid() returns false.

o shared_future(future<Result> &&tmp) noexcept
         Effects: move constructs a shared_future object that refers to
            the results that were originally referred to by tmp (if
            any). After using this constructor the object's valid member
            returns the same value as tmp.valid() returned prior to the
            constructor invocation, and tmp.valid() returns false.

The class's destructor destroys the shared_future object for which it is
called. If the object for which the destructor is called is the last
shared_future object, and no std::promise or
std::packaged_task is associated with the results associated
with the current object, then the results are also destroyed.

Here are the members of the class std::shared_future:
    
    o shared_future& operator=(shared_future &&tmp):
The move assignment operator releases the current object's shared
        results, and move assigns tmp's results to the current
        object. After calling the move assignment operator the current
        object's valid member returns the same value as tmp.valid()
        returned prior to the invocation of the move assignment operator, and
        tmp.valid() returns false;

o shared_future& operator=(shared_future const &rhs):
The assignment operator releases the current object's shared results,
        and rhs's results are shared with the current object. After
        calling the assignment operator the current object's valid member
        returns the same value as tmp.valid();

o Result const &shared_future::get() const:
(Specializations for shared_future<Result &> and
        shared_future<void> are also available). This member waits until
        the shared results are available, and subsequently returns Result
        const &. Note that access to the data stored in Results, accessed
        through get is not synchronized. It is the responsibility of the
        programmer to avoid race conditions when accessing Result's
        data. If Result holds an exception, it is thrown when get is
        called;

o bool valid() const:
Returns true if the current object refers to shared
            results;

o void wait() const:
Blocks until shared results are available (i.e., the associated
        asynchronous task has produced results);

o future_status wait_for(const chrono::duration<Rep, Period>& rel_time)
            const:
(The template types Rep and Period normally are derived by the
        compiler from the actual rel_time specification.) If the shared
        results contain a deferred function (cf. section 20.10) nothing
        happens. Otherwise wait_for blocks until the results of the
        associated asynchronous task has produced results, or until the
        relative time specified by rel_time has expired. The member
        returns 
            
           o future_status::deferred if the shared results contain a
            deferred function; o future_status::ready if the shared results
            are available;
           o future_status::timeout if the function is returning because
            the amount of time specified by rel_time has expired;

o future_status wait_until(const chrono::time_point<Clock, Duration>&
            abs_time) const:
(The template types Clock and Duration normally are derived by
        the compiler from the actual abs_time specification.) If the
        shared results contain a deferred function nothing happens. Otherwise
        wait_until blocks until the shared results are available or until
        the point in time specified by abs_time has expired. Possible
        return values are:
           
            o future_status::deferred if the shared results contain a
                deferred function;
            o future_status::ready if the shared results are available;
            o future_status::timeout if the function is returning because
                the point in time specified by abs_time has expired. 


20.10: Starting a new thread: std::async

In this section the function template std::async is
covered. Async is used to start asynchronous tasks, returning values (or
void) to the calling thread, which is hard to realize merely using the
std::thread class.

Before using the function async the <future> header file must be
included. 

When starting a thread using the facilities of the class std::thread the
initiating thread at some point commonly calls the thread's join
method. At that point the thread must have finished or execution blocks until
join returns. While this often is a sensible course of action, it may not
always be: maybe the function implementing the thread has a return value, or
it could throw an exception.

In those cases join cannot be used: if an exception leaves a thread, then
your program ends. Here is an example:
         1: void thrower()
     2: {
     3:     throw std::exception();
     4: }
     5: 
     6: int main()
     7: try
     8: {
     9:    std::thread subThread(thrower);
    10: }
    11: catch (...)
    12: {
    13:     std::cerr << "Caught exception\n";
    14: }

    In line 3 thrower throws an exception, leaving the thread. This
exception is not caught by main's try-block (as it is defined in another
thread). As a consequence, the program terminates.

This scenario doesn't occur when std::async is used. Async may start a
new asynchronous task, and the activating thread may retrieve the return value
of the function implementing the asynchronous task or any exception leaving
that function from a std::future object returned by the async
function. Basically, async is called similarly to the way a thread is
started using std::thread: it is passed a function and optionally
arguments which are forwarded to the function.

Although the function implementing the asynchronous task may be passed as
first argument, async's first argument may also be a value of the strongly
typed enumeration std::launch:
            enum class launch
    {
        async,
        deferred
    };

When passing launch::async the asynchronous task immediately starts;
when passing launch::deferred the asynchronous task is deferred. When 
std::launch is not specified the default value launch::async |
launch::deferred is used, giving the implementation freedom of choice, 
usually resulting in deferring execution of the asynchronous task.

So, here is the first example again, this time using async to start the
sub-thread:
         1: bool fun()
     2: {
     3:     return std::cerr << "    hello from fun\n";
     4: }
     5: int exceptionalFun()
     6: {
     7:     throw std::exception();
     8: }
     9: 
    10: int main()
    11: try
    12: {
    13:     auto fut1 = std::async(std::launch::async, fun);
    14:     auto fut2 = std::async(std::launch::async, exceptionalFun);
    15: 
    16:     std::cerr << "fun returned " << std::boolalpha << fut1.get() << '\n';
    17:     std::cerr << "exceptionalFun did not return " << fut2.get() << '\n';
    18: }
    19: catch (...)
    20: {
    21:     std::cerr << "caught exception thrown by exceptionalFun\n";
    22: }

    Now the threads immediately start, but although the results are
available around line 13, the thrown exception isn't terminating the
program. The first thread's return value is made available in line 16, the
exception thrown by the second thread is simply caught by main's try-block
(line 19).

The function template async has several overloaded versions:
    
    o  The basic form expects a function or functor as its first argument,
returning a std::future holding the function's return value or exception
thrown by the function (invoke_result is declared in the header file
<functional>):
            template <typename Function, class ...Args>
    std::future< std::invoke_result< Function, Args ...>::type > 
        std::async(Function &&fun, Args &&...args);

o  Alternatively, the first argument may be the address of a member
function. In that case the (required) second argument is an object (or a
pointer to an object) of that member function's class. Any remaining arguments
are passed to the member function (see also the remarks below).

o  The first argument may also be a combination (using the bit_or
operator) of the enumeration values of the std::launch enumeration:
            template <class Function, class ...Args>
    std::future<std::invoke_result<Function, Args ...>::type> 
        std::async(std::launch policy, Function &&fun, Args &&...args);

o  If the first argument specifies std::launch values, the second
argument may also be the address of a member function.  In that case the
(required) third argument is an object (or a pointer to an object) of that
member function's class. Any remaining arguments are passed to the member
function (see also the remarks below).

When calling async all arguments except for the std::launch
argument must be references, pointers or move-constructible objects:
   
   o  When a
    member function is specified, then the object for which the member
    function is called must be a named object, an anonymous object, or a
    pointer to a named object. 
   o  When a named object is passed to the async function template then
    copy construction is used to construct a copy of the argument which is
    then forwarded to the thread-launcher. 
   o  When an anonymous object is passed to the async function template
    then move construction is used to forward the anonymous object to the
    thread launcher. 

Once the thread itself starts another move construction is used to construct
an object for the duration of the thread. When a pointer to an object is
passed, the sub-thread uses the object referred to by the pointer, and neither
copy- nor move-construction is required. However, when using a pointer to an
object the programmer should make sure that the object's lifetime exceeds the
duration of the thread (note that this is not automatically guaranteed, as the
asynchronous task may not actually start before the future's get member is
called).

Because of the default std::launch::deferred | std::launch::async argument
used by the basic async call it is likely that the function which is
passed to async doesn't immediately start. The launch::deferred policy
allows the implementor to defer its execution until the program explicitly
asks for the function's results. Consider the following program:
             1: void fun()
     2: {
     3:     std::cerr << "    hello from fun\n";
     4: }
     5: 
     6: std::future<void> asyncCall(char const *label)
     7: {
     8:     std::cerr << label << " async call starts\n";
     9:     auto ret = std::async(fun);
    10:     std::cerr << label << " async call ends\n";
    11:     return ret;
    12: }
    13: 
    14: int main()
    15: {
    16:     asyncCall("First");
    17:     asyncCall("Second");
    18: }

    Although async is called in line 9, the program's output may not show
fun's output line when it is run.  This is a result of the (default) use
of lauch::deferred: the system simply defers fun's execution until
requested, which doesn't happen. But the future object that's returned by
async has a member wait. Once wait returns the shared state must
be available. In other words: fun must have finished. Here is what happens
when after line 9 the line ret.wait() is inserted:
            First async call starts
        hello from fun
    First async call ends
    Second async call starts
        hello from fun
    Second async call ends

Actually, evaluation of fun can be requested at the point where we
need its results, maybe even after calling asyncCall, as shown in the next
example:
             1: int main()
     2: {
     3:     auto ret1 = asyncCall("First");
     4:     auto ret2 = asyncCall("Second");
     5: 
     6:     ret1.get();
     7:     ret2.get();
     8: }

    Here the ret1 and ret2 std::future objects are created, but their
fun functions aren't evaluated yet. Evaluation occurs at lines 6 and 7,
resulting in the following output:
            First async call starts
    First async call ends
    Second async call starts
    Second async call ends
        hello from fun
        hello from fun

The std::async function template is used to start a thread, making its
results available to the calling thread. On the other hand, we may only be
able to prepare (package) a task (a thread), but may have to leave the
completion of the task to another thread. Scenarios like this are realized
through objects of the class std::packaged_task, which is the topic of the
next section.


20.11: Preparing a task for execution: std::packaged_task

The class template std::packaged_task allows a program to
`package' a function or functor and pass the package to a thread for further
processing. The processing thread then calls the packaged function, passing it
its arguments (if any). After completing the function the packaged_task's
future is ready, allowing the program to retrieve the results produced by
the function. Thus, functions and the results of function calls can be
transferred between threads.

Before using the class template packaged_task the <future> header file
must be included.

Before describing the class's interface, let's first look at an example to get
an idea about how a packaged_task can be used. Remember that the essence
of packaged_task is that part of your program prepares (packages) a task
for another thread to complete, and that the program at some point needs the
result of the completed task. 

To clarify what's happening here, let's first look at a real-life
analogon. Every now and then I make an appointment with my garage to have my
car serviced. The `package' in this case are the details about my car: its
make and type determine the kind of actions my garage performs when servicing
it. My neighbor also has a car, which also needs to be serviced every now and
then. This also results in a `package' for the garage. At the appropriate time
me and my neighbor take our cars to the garage (i.e., the packages are passed
to another thread). The garage services the cars (i.e., calls the functions
stored in the packaged_tasks [note that the tasks differ, depending on the
types of the cars]), and performs some actions that are associated with it
(e.g., registering that my or my neighbor's car has been serviced, or order
replacement parts). In the meantime my neighbor and I perform our own
businesses (the program continues while a separate thread runs as well). But
by the end of the day we'd like to use our cars again (i.e., get the results
associated with the packaged_task). A common result in this example is the
garage's bill, which we have to pay (the program obtains the
packaged_task's results).

Here is a little C++ program illustrating the use of a packaged_task
(assuming the required headers and using namespace std have been
specified):
         1: mutex carDetailsMutex;
     2: condition_variable condition;
     3: string carDetails;
     4: packaged_task<size_t (std::string const &)> serviceTask;
     5: 
     6: size_t volkswagen(string const &type)
     7: {
     8:     cout << "performing maintenance by the book for a " << type << '\n';
     9:     return type.size() * 75;            // the size of the bill
    10: }
    11: 
    12: size_t peugeot(string const &type)
    13: {
    14:     cout << "performing quick and dirty maintenance for a " << type << '\n';
    15:     return type.size() * 50;             // the size of the bill
    16: }
    17: 
    18: void garage()
    19: {
    20:     while (true)
    21:     {
    22:         unique_lock<mutex> lk(carDetailsMutex);
    23:         while (carDetails.empty())
    24:             condition.wait(lk);
    25: 
    26:         cout << "servicing a " << carDetails << '\n';
    27:         serviceTask(carDetails);
    28:         carDetails.clear();
    29:     }
    30: }
    31: 
    32: int main()
    33: {
    34:     thread(garage).detach();
    35: 
    36:     while (true)
    37:     {
    38:         string car;
    39:         if (not getline(cin, car) || car.empty())
    40:             break;
    41:         {
    42:             lock_guard<mutex> lk(carDetailsMutex);
    43:             carDetails = car;
    44:         }
    45:         serviceTask =  packaged_task<size_t (string const &)>(
    46:                     car[0] == 'v' ? volkswagen : peugeot
    47:                 );
    48:         auto bill = serviceTask.get_future();
    49:         condition.notify_one();
    50:         cout << "Bill for servicing a " << car <<
    51:                                 ": EUR " << bill.get() << '\n';
    52:     }
    53: }

    
    o  Lines 1-3 define the variables used for synchronization;
    o  Line 4 defines a packaged_task: serviceTask is initialized with a
        function (or functor) expecting a string, returning a size_t;
    o  Lines 6-10 and 12-16 define such functions: volkswagen and
        peugeot represent the tasks to perform when cars of the provided
        types come in for service; presumably they return the bill.
    o  Lines 18-30 define the function void garage, defining the actions
        performed by the garage when cars come in for service. These actions
        are performed by a separate detached thread, starting in line 34. In a
        continuous loop it waits until it obtains a lock on the
        carDetailsMutex and carDetails is no longer empty. Then, at
        line 27, it passes carDetails to the packaged_task
        `serviceTask'. By itself this is not identical to calling the
        packaged_task's function, but eventually its function will be
        called. At this point the packaged_task receives its function's
        arguments, which it eventually will forward to its configured
        function. Finally, at line 28 it clears carDetails, thus preparing
        itself for the next request.
    o  Lines 32-53 define main:
        
        o  First, at line 34 the anonymous detached thread running
            garage is started.

Then the program's main loop starts (lines 36-52):
        
        o  The main thread reads commands from the standard input until an
            empty or no line is received (lines 38-40).
        o  By convention the line's first letter starts the car's brand
            (volkswagen or peugeot), and the packaged_task, provided with
            the right servicing function, is constructed
            next (line 45).
        o  Then, at line 48 the results, stored in a future, are
            retrieved. Although at this point the future might not be
            ready, the future object itself is, and it is simply
            returned as the bill.
        o  Now we're ready to inform the garage that it can service a car:
            the garage is notified in line 49.

Anything may happen next: the program may perform any actions,
        but eventually it requests the results produced by the garage.
       
        o  The main thread obtains the results by calling bill.get()
            in line 51. If, by this time, the car is still being serviced, the
            bill isn't ready yet, and bill.get() blocks until it is, and
            the bill for servicing a car is shown.

Now that we've seen an example of a program using a packaged_task,
let's have a look at its interface. Note that the class packaged_task is a
class template: its template type parameter specifies the prototype of a
function or function object implementing the task performed by the
packaged_task object.

Constructors and destructor:
    
    o packaged_task() noexcept:
The default constructor constructs a packaged_task object which is
        not associated with a function or shared state;

o explicit packaged_task<ReturnType(Args...)> task(fun):
A packaged_task is constructed for a function or functor fun
        expecting arguments of types Args..., and returning a value of
        type ReturnType. The packaged_task class template specifies
        ReturnType (Args...) as its template type parameter. The
        constructed object contains a shared state, and a (move constructed)
        copy of function.

Optionally an Allocator may be specified as second template type
        parameter, in which case the first two arguments are
        std::allocator_arg_t, Allocator const
        &alloc. The type std::allocator_arg_t is a type introduced to
        disambiguate constructor selections, and can simply be specified as
        std::allocator_arg_t().

This constructor may throw a std::bad_alloc exception or
        exceptions thrown by function's copy or move constructors.

o packaged_task(packaged_task &&tmp) noexcept:
The move constructor moves  any existing shared state from tmp to
        the newly constructed object, removing the shared state from tmp.

o ~packaged_task():
The object's shared state (if any) is abandoned

Member functions:
    
    o future<ReturnType> get_future():
A std::future object is returned holding the results of the
        separately executed thread. When get_future is incorrectly called
        a future_error exception is thrown, containing one of the
        following values:
       
       o future_already_retrieved if get_future was already called on a
            packaged_task object containing the same shared state as the
            current object;
        o no_state if the current object has no shared state.

Note: Any futures that share the object's shared state may access
        the result returned by the object's task.

o void make_ready_at_thread_exit(Args... args):
Calls void operator()(Args... args) (see below) when the current
        thread exits, once all objects of thread storage duration associated
        with the current thread have been destroyed.

o packaged_task &operator=(packaged_task &&tmp):
The move assignment operator first releases the current object's
        shared state (if available), after which the current object and
        tmp are swapped;

o void operator()(Args... args):
The args arguments are forwarded to the current object's
        stored task. When the stored task returns its return value is stored
        in the current object's shared state. Otherwise any exception thrown
        by the task is stored in the object's shared state. Following this the
        object's shared state is made ready, and any threads blocked in a
        function waiting for the object's shared state to become ready are
        unblocked. A future_error exception is thrown upon error,
        containing 
       
       o promise_already_satisfied if the shared state has already been
            made ready;
        o no_state if the current object does not have any shared state.

Calling this member synchronizes with calling any member function of a
        (shared_)future object that provides access to the
        packaged_task's results.

o void reset():
Abandons any available shared state, initializing the current object
        to packaged_task(std::move(funct)), where funct is the
        object's stored task. This member may throw the following exceptions:
       
       o bad_alloc if memory for the new shared state could not be
            allocated;
        o  any exception thrown by the move constructor of the task stored in
                the shared state;   
        o future_error with a no_state error condition if the current
            object contains no shared state.

o void swap(packaged_task &other) noexcept:
The shared states and stored tasks of the current object  and other
        are swapped. 

o bool valid() const noexcept:
Returns true if the current object contains a shared state,
        otherwise false is returned;

The following non-member (free) function operating on packaged_task
objects is available:
    
    o  void swap(packaged_task<ReturnType(Args...)> &lhs,
                                                            
            packaged_task<ReturnType(Args...)> &rhs) noexcept
       Calls lhs.swap(rhs)


20.12: The class `std::promise'

In addition to std::packaged_task and std::async the class template
    std::promise can be used to obtain the results from a
separate thread.

Before using the class template promise the <future> header file
must be included.

A promise is used to obtain the results from another thread without
further synchronization requirements. Consider the following program:
            void compute(int *ret)
    {
        *ret = 9;
    }
    
    int main()
    {
        int ret = 0;
        std::thread(compute, &ret).detach();
        cout << ret << '\n';
    }
 
 Chances are that this program shows the value 0: the cout statement has
already been executed before the detached thread has had a chance to complete
its work. In this example that problem can easily be solved by using a
non-detached thread, and using the thread's join member, but when multiple
threads are used that requires named threads and as many join
calls. Instead, using a promise might be preferred:
             1: void compute(promise<int> &ref)
     2: {
     3:     ref.set_value(9);
     4: }
     5: 
     6: int main()
     7: {
     8:     std::promise<int> prom;
     9:     std::thread(compute, ref(prom)).detach();
    10: 
    11:     cout << prom.get_future().get() << '\n';
    12: }

    This example also uses a detached thread, but its results are kept for
future reference in a promise object, instead of directly being assigned
to a final destination variable. The promise object contains a future
object holding the computed value. The future's get member blocks until
the future has been made ready, at which point the result becomes
available. By then the detached thread may or may not yet have been
completed. If it already completed its work then get immediately returns,
otherwise there will be a slight delay.

Promises are useful when implementing a multi threaded version of some
algorithm without having to use additional synchronization statements. As an
example consider matrix multiplications.  Each element of the resulting
product matrix is computed as the inner product of two vectors: the inner
product of a row of the left-hand matrix operand and a column of the
right-hand matrix operand becomes element [row][column] of the resulting
matrix. Since each element of the resulting matrix can independently be
computed from the other elements, a multi threaded implementation is well
possible. In the following example the function innerProduct (lines 4..11)
leaves its result in a promise object:
             1: int m1[2][2] = {{1, 2}, {3, 4}};
     2: int m2[2][2] = {{3, 4}, {5, 6}};
     3: 
     4: void innerProduct(promise<int> &ref, int row, int col)
     5: {
     6:     int sum = 0;
     7:     for (int idx = 0; idx != 2; ++idx)
     8:         sum += m1[row][idx] * m2[idx][col];
     9: 
    10:     ref.set_value(sum);
    11: }
    12: 
    13: int main()
    14: {
    15:     promise<int> result[2][2];
    16: 
    17:     for (int row = 0; row != 2; ++row)
    18:     {
    19:         for (int col = 0; col != 2; ++col)
    20:             thread(innerProduct, ref(result[row][col]), row, col).detach();
    21:     }
    22: 
    23:     for (int row = 0; row != 2; ++row)
    24:     {
    25:         for (int col = 0; col != 2; ++col)
    26:             cout << setw(3) << result[row][col].get_future().get();
    27:         cout << '\n';
    28:     }
    29: }

    Each inner product is computed by a separate (anonymous and detached)
thread (lines 17..21), which starts as soon as the run-time system allows it
to start. By the time the threads have finished the resulting inner products
can be retrieved from the promises' futures. Since futures' get members
block until their results are actually available, the resulting matrix can
simply be displayed by calling those members in sequence (lines 23..28).

So, a promise allows us to use a thread to compute a value (or
exception, see below), which value may then be collected by another thread at
some future point in time. The promise remains available, and as a consequence
further synchronization of the threads and the program starting the threads is
not necessary. When the promise object contains an exception, rather than a
value, its future's get member rethrows the stored exception.

Here is the class promise's interface. Note that the class promise is
a class template: its template type parameter ReturnType specifies the
template type parameter of the std::future that can be retrieved from it.

Constructors and destructor:
    
    o promise():
The default constructor constructs a promise object containing a
        shared state. The shared state may be returned by the member
        get_future (see below), but that future has not yet been made
        ready;

o promise(promise &&tmp) noexcept:
The move constructor constructs a promise object, transferring
        the ownership of tmp's shared state to the newly constructed
        object. After the object has been constructed, tmp no longer
        contains a shared state;

o ~promise():
The object's shared state (if any) is abandoned;

Member functions:
    
    o std::future<ReturnType> get_future():
A std::future object sharing the current object's shared state is
        returned. A future_error exception is thrown upon error,
        containing 
       
       o future_already_retrieved if get_future was already called on a
            packaged_task object containing the same shared state as the
            current object;
        o no_state if the current object has no shared state.

Note: Any futures that share the object's shared state may
        access the result returned by the object's task;

o promise &operator=(promise &&rhs) noexcept:
The move assignment operator first releases the current object's
        shared state (if available), after which the current object and
        tmp are swapped;

o void promise<void>::set_value():
See below, at the last set_value member's description;

o void set_value(ReturnType &&value):
See below, at the last set_value member's description;

o void set_value(ReturnType const &value):
See the next member function's description;

o void set_value(ReturnType &value):
The argument (value) is atomically stored in the shared state,
        which is then also made ready. A future_error exception is thrown
        upon error, containing
       
       o promise_already_satisfied if the shared state has already been
            made ready;
        o no_state if the current object does not have any shared state.

Alternatively, any exception thrown by value's move or copy
        constructor may be thrown;
     

o void set_exception(std::exception_ptr obj):
Exception_ptr obj (cf. section 10.9.4) is atomically stored
        in the shared state, making that state ready. A future_error
        exception is thrown upon error, containing
       
       o promise_already_satisfied if the shared state has already been
            made ready;
        o no_state if the current object does not have any shared state;

o void set_exception_at_thread_exit(exception_ptr ptr):
The exception pointer ptr is stored in the shared state without
        immediately making that state ready. The state becomes ready when the
        current thread exits, once all objects of thread storage duration
        which are associated with the ending thread have been destroyed.  A
        future_error exception is thrown upon error, containing
       
       o promise_already_satisfied if the shared state has already been
            made ready;
        o no_state if the current object does not have any shared state;

o void set_value_at_thread_exit():
See below, at the last set_value_at_thread_exit member's
        description;

o void set_value_at_thread_exit(ReturnType &&value):
See below, at the last set_value_at_thread_exit member's
        description;

o void set_value_at_thread_exit(ReturnType const &value):
See the next set_value_at_thread_exit member's
        description;

o void set_value_at_thread_exit(ReturnType &value):
Stores value in the shared state without immediately making that
        state ready.  The state becomes ready when the current thread exits,
        once all objects of thread storage duration which are associated with
        the ending thread have been destroyed.  A future_error exception
        is thrown upon error, containing
       
       o promise_already_satisfied if the shared state has already been
            made ready;
        o no_state if the current object does not have any shared state;

o void swap(promise& other) noexcept:
The shared states (if any) of the current object and other are
        exchanged.

The following non-member (free) function operating on promise objects is
available:
    
    o void swap(promise<ReturnType> &lhs, promise<ReturnType> &rhs)
        noexcept:
Calls lhs.swap(rhs)


20.13: An example: multi-threaded compilations

In this section another program is developed. This section's  example program
illustrates the use of packaged_tasks. 

Like the multi-threaded quicksort example a worker pool is used. However, in
this example the workers in fact do not know what their task is. In the
current example the tasks happens to be identical, but different tasks might
as well have been used, without having to update the workers.

The program uses a class Task containing a command-specification
(d_command), and a task specification (d_task) (cf. Figure 27), the
sources of the program are found in the 
yo/threading/examples/multicompile directory of the C++ Annotations.

------------------------------------------------------------------Insert Figure 27(Data structure used for the multi-threading compilation)about here (file: threading/compile)------------------------------------------------------------------

In this program main starts by firing up its workforce in a series of
threads. Following this, the compilation jobs are prepared and pushed on a
task-queue by jobs, where they're retrieved from by the workers. Once
the compilations have been completed (i.e., after the worker threads have
joined the main thread), the results of the compilation jobs are handled by
results:
            int main()
    {
        workforce();                    // start the worker threads
        jobs();                         // prepare the jobs: push all tasks on the
                                        // taskQ
    
        for (thread &thr: g_thread)     // wait for the workers to end
            thr.join();
    
        results();                      // show the results
    
    }


The jobs function receives the names of the files to compile from the
nextCommand function, which ignores empty lines and returns non-empty
lines. Eventually nextCommand returns an empty line once all lines of the
standard input stream have been read:
            string nextCommand()
    {
        string ret;
        while (true)
        {
            if (not getline(cin, ret))    // no more lines
                break;
    
            if (not ret.empty())          // ready once there's line content.
                break;
        }
        return ret;
    }


With non-empty lines jobs waits for an available worker using (line 12)
the g_dispatcher semaphore. Initialized to the size of the work force, it
is reduced by an active worker, and incremented by workers who have completed
their tasks. If a compilation fails, then g_done is set to true and no
additional compilations are performed (lines 14, 15). While jobs receives
the names of the files to compile, workers may detect compilation errors. If
so, the workers set variable g_done to true. Once the job
function's while loop ends the workers are notified once again (line 24),
who will then, because there's no task to perform anymore, end their threads
             1: void jobs()
     2: {
     3:     while (true)
     4:     {
     5:         string line = nextCommand();
     6:         if (line.empty())                   // no command? jobs() done.
     7:         {
     8:             g_done = true;
     9:             break;
    10:         }
    11: 
    12:         g_dispatcher.wait();                // wait for an available worker
    13: 
    14:         if (g_done.load())                  // if a worker found an error
    15:             break;                          // then quit anyway
    16: 
    17:         newTask(line);                      // push a new task (and its
    18:                                             // results)
    19: 
    20:         g_worker.notify_all();              // inform the workers: job is
    21:                                             // available
    22:     }
    23: 
    24:     g_worker.notify_all();                  // end the workers at an empty Q
    25: }


The function newTask prepares the program for the next task. First a
Task object is constructed. Task contains the name of the file to
compile, and a packaged_task. It encapsulates all activities that are
associated with a packaged_task. Here is its (in-class) definition:
             1: using PackagedTask = packaged_task<Result (string const &fname)>;
     2: 
     3: class Task
     4: {
     5:     string d_command;
     6:     PackagedTask d_task;
     7: 
     8:     public:
     9:         Task()  = default;
    10: 
    11:         Task(string const &command, PackagedTask &&tmp)
    12:         :
    13:             d_command(command),
    14:             d_task(move(tmp))
    15:         {}
    16: 
    17:         void operator()()
    18:         {
    19:             d_task(d_command);
    20:         }
    21: 
    22:         shared_future<Result> result()
    23:         {
    24:             return d_task.get_future().share();
    25:         }
    26: };


Note (lines 22-25) that result returns a shared_future. Since the
dispatcher runs in a different thread than the one processing the results, the
futures created by the dispatcher must be shared with the futures
required by the function processing the results. Hence the shared_futures
returned by Task::result. 

Once a Task object has been constructed its shared_future object is
pushed on the result queue. Although the actual results aren't available by
this time, the result function is eventually called to process the results
that were pushed on the result-queue. Additionally, the Task itself is
pushed on a task queue, and it will be retrieved by a worker:
            class Task
    {
        string d_command;
        PackagedTask d_task;

        public:
            Task()  = default;

            Task(string const &command, PackagedTask &&tmp);
            void operator()();
            shared_future<Result> result();
    };


    void pushResultQ(shared_future<Result> const &sharedResult)
    {
        lock_guard<mutex> lk(g_resultQMutex);
        g_resultQ.push(sharedResult);
    }


The workers have a simple task: wait for the next task, then retrieve it from
the task queue, and complete that task. Whatever happens inside the tasks
themselves is of no concern to the worker. Also, when notified (normally by
the jobs function) that there's a task waiting it'll execute that
task. However, at the end, once all tasks have been pushed on the task queue,
jobs once again notifies the workers. In that case the task queue is
empty, and the worker function ends. But just before that it notifies its
fellow workers, which in turn end, thus ending all worker threads, allowing
them to join the main-thread:
            void worker()
    {
        Task task;
        while (true)
        {
            g_worker.wait();                    // wait for an available task
    
            if (g_taskQ.empty())                // no task? then done
                break;
    
            g_taskQ.popFront(task);
            g_dispatcher.notify_all();          // notify the dispatcher that
                                                // another task can be pushed
            task();
        }
        g_worker.notify_all();                  // no more tasks: notify the other
    }                                           // workers.


This completes the description of how tasks are handled. 

The task itself are now described. In the current program C++ source files
are compiled. The compilation command is passed to the constructor of a
CmdFork object, which starts the compiler as a child process. The result
of the compilation is retrieved via its childExit member (returning the
compiler's exit code) and childOutput member (returning any textual output
produced by the compiler). If compilation fails, the exit value won't be
zero. In this case no further compilation tasks will be issued as g_done
is set to true (lines 11 and 12; the implementation of the class
CmdFork is available from the C++ Annotations'
yo/threading/examples/cmdfork directory). Here is the function
compile:
             1: Result compile(string const &line)
     2: {
     3:     string command("/usr/bin/g++ -Wall -c " + line);
     4: 
     5:     CmdFork cmdFork(command);
     6:     cmdFork.fork();
     7: 
     8:     Result ret {cmdFork.childExit() == 0,
     9:                 line + "\n" + cmdFork.childOutput()};
    10: 
    11:     if (not ret.ok)
    12:         g_done = true;
    13: 
    14:     return ret;
    15: }


The results function continues for as long as newResults indicates
that results are available. By design the program will show all available
successfully completed compilations, and (if several workers encountered
compilation errors) only the compiler's output of the first 
compilation error is displayed. All available successfully completed
compilations meaning that, in case of a compilation error, the source files
that were successfully compiled by the currently active work force are listed,
but remaining source files are not processed anymore:
            void results()
    {
        Result result;
    
        string errorDisplay;
    
        while (newResult(result))
        {
            if (result.ok)
                cerr << result.display;
            else if (errorDisplay.empty())
                errorDisplay = result.display;  // remember the output of
        }                                       // the first compilation error
        if (not errorDisplay.empty())           // which is displayed at the end
            cerr << errorDisplay;
    }


The function newResult controls results' while-loop. It returns
true when as long as the result queue isn't empty, in which case the
queue's front element is stored at the external Result object, and the
queue's front element is removed from the queue:
            bool newResult(Result &result)
    {
        if (g_resultQ.empty())
            return false;
    
        result = g_resultQ.front().get();
        g_resultQ.pop();
    
        return true;
    }



20.14: Transactional Memory



Transactional memory is used to simplify shared data access in multithreaded
programs. The benefits of transactional memory is best illustrated by a
small program. Consider a situation where threads need to write information to
a file. A plain example of such a program would be:
            void fun(int value)
    {
        for (size_t rept = 0; rept != 10; ++rept)
        {
            this_thread::sleep_for(chrono::seconds(1));
            cout << "fun " << value << '\n';
        }
    }
    
    int main()
    {
        thread thr{ fun, 1 };
        fun(2);
        thr.join();
    }

When this program is run the fun 1 and fun 2 messages are
intermixed. To prevent this we traditionally define a mutex, lock it,
write the message, and release the lock:
            void fun(int value)
    {
        static mutex guard;

        for (size_t rept = 0; rept != 10; ++rept)
        {
            this_thread::sleep_for(chrono::seconds(1));
            guard.lock();
            cout << "fun " << value << '\n';
            guard.unlock();
        }
    };

Transactional memory handles the locking for us. Transactional memory is used
when statements are embedded in a synchronized block. The function
fun, using transactional memory, looks like this:
            void fun(int value)
    {
        for (size_t rept = 0; rept != 10; ++rept)
        {
            this_thread::sleep_for(chrono::seconds(1));
            synchronized
            {
                cout << "fun " << value << '\n';
            }
        }
    };


    To compile source files using transactional memory the g++ compiler
option -fgnu-tm must be specified.

The code inside a synchronized block is executed as a single, as if the
block was protected by a mutex. Different from using mutexes transactional
memory is implemented in software instead of using hardware-facilities.

Considering how easy it is to use transactional memory compared to using
the mutex-based locking mechanism using transactional memory appears
too good to be true. And in a sense it is. When encountering a synchronized
block the thread unconditionally executes the block's statements. At the same
time it keeps a detailed log of all its actions. Once the statements have been
completed the thread checks whether another thread didn't start executing the
block just before it. If so, it reverses its actions, using the synchronized
block's log. The implication of this should be clear: there's at least the
overhead of maintaining the log, and if another thread started executing 
the synchronized block before the current thread then there's the additional
overhead of reverting its actions and to try again. 

The advantages of transactional memory should also be clear: the
programmers no longer is responsible for correctly controlling access to
shared memory; risks of encountering deadlocks have disappeared as has all
adminstrative overhead of defining mutexes, locking and unlocking. Especially
for inherently slow operations like writing to files transactional memory can
greatly simplify parts of your code. Consider a std::stack. Its
top-element can be inspected but its pop member does not return the
topmost element. To retrieve the top element and then maybe remove it
traditionally requires a mutex lock surrounding determining the stack's size,
and if empty, release the lock and wait. If not empty then retrieve its
topmost element, followed by removing it from the stack. Using a transactional
memory we get something as simple as:
                bool retrieve(stack<Item> &itemStack, Item &item)
    {
        synchronized
        {
            if (itemStack.empty())
                return false;
            item = std::move(itemStack.top());
            itemStack.pop();
            return true;
        }
    }

Variants of synchronized are:
    
    o atomic_noexcept: the statements inside its compound statement may not
        throw exceptions. If they do, std::abort is called. If the earlier
        fun function specifies atomic_noexcept instead of
        synchronized the compiler generates and error about the use of the
        insertion operator, from which an exception may be thrown.
    o atomic_cancel: not yet supported by g++. If an
        exception other than (std::) bad_alloc, bad_array_new_length,
        bad_cast, bad_typeid, bad_exception, exception, tx_exception<Type> is
        thrown std::abort is called. If an acceptable exception is thrown,
        then the statements executed so far are undone.
    o atomic_commit: if an exception is thrown from its compound statement
        all thus far executed statements are kept (i.e., not undone).


20.15: Synchronizing output to streams

Consider the situation where different threads of a multi-threaded program
must write to the same file. The information written by each thread should
show up as a single block in that file. There are several ways to solve this
problem: each thread could write to a global file that's associated with just
one thread, and by the time the threads have stopped all these files are
copied to the destination file. Alternatively, the destination file could be
passed to the threads, while each thread defines its own local file, writing
its information to that file. Then, by the time the thread is about to end it
locks access to the destination file, and copies its local file to the
destination file. 

Recently the class std::osyncstream was added to the language, allowing
multi threaded programs allowing threads to write information block-wise to a
common stream without having to define separate streams receiving the
thread-specific information, eventually copying those streams to the
destination stream. Before using osyncstream objects the <syncstream>
header file must be included.

The osyncstream class publicly inherits from std::ostream,
initializing the ostream base class with a std::syncbuf stream buffer
(described in the next section), which performs the actual synchronization.

Information written to osyncstream objects can explicitly be copied to a
destination ostream, or is automatically copied to the destination
ostream by the osyncstream's destructor. Each thread may construct its
own osyncstream object, handling the block-wise copying of the
information it receives to the destination stream.

Constructors


    o osyncstream{ostream &out} constructs an osyncstream object
        eventually writing the information it receives to out. Below,
        out is called the destination stream;
    o osyncstream{osyncstream &&tmp} the move constructor is available;

The default- and copy-constructors are not available.

Member functions

In addition to the members inherited from std::ostream (like the
rdbuf member returing a pointer to the object's syncbuf (described in
the next section)) the class osyncstream offers these members:
    
    o get_wrapped, returning a pointer to the destination stream's stream
        buffer;
    o emit, copies the received information as a block to the destination
        stream.

The following program illustrates how osyncstream objects can be used.
     1: #include <iostream>
 2: #include <syncstream>
 3: #include <string>
 4: #include <thread>
 5: 
 6: using namespace std;
 7: 
 8: void fun(char const *label, size_t count)
 9: {
10:     osyncstream out(cout);
11: 
12:     for (size_t idx = 0; idx != count; ++idx)
13:     {
14:         this_thread::sleep_for(1s);
15:         out << label << ": " << idx << " running...\n";
16:     }
17:     out << label << " ends\n";
18: }
19: 
20: int main(int argc, char **argv)
21: {
22:     cout << "the 1st arg specifies the #iterators "
23:             "using 3 iterations by default\n";
24: 
25:     size_t count = argc > 1 ? stoul(argv[1]) : 3;
26: 
27:     thread thr1{ fun, "first", count };
28:     thread thr2{ fun, "second", count };
29: 
30:     thr1.join();
31:     thr2.join();
32: }


    o  The function fun (line 8) is called by main from two threads
        (lines 27, 28);
    o  It defines an osyncstream out and, using short one-second pauses,
        writes some lines of text to out (lines 14, 15);
    o  Just before leaving fun the local out content is written
        as a block to cout (line 18). Writing out's content to
        cout can also explicitly be requested by calling out.emit().


20.15.1: The `std::syncbuf' streambuf

    The osyncstream stream in fact is only a wrapper of ostream, using a
syncbuf as its stream buffer. The std::syncbuf handles the actual
synchronization. In order to use the syncbuf stream buffer the
<syncstream> header file must be included.

A syncbuf stream buffer collects the information it receives from an
ostream in an internal buffer, and its destructor and emit member 
flush its buffer as a block to its destination stream.

Constructors


    o syncbuf(), the default constructor, constructs a syncbuf object
        with its emit-on-sync policy (see below) set to false;

o explicit syncbuf(streambuf *destbuf) constructs a std::syncbuf
        with its emit-on-sync policy set to false, using destbuf as
        the destination stream's streambuf;

o syncbuf(syncbuf &&rhs), the move constructor, moves the content of
        rhs to the constructed syncbuf.

Member functions

In addition to the members inherited from std::streambuf the class
syncbuf offers these members: 
    
    o get_wrapped, returning a pointer to the destination stream's stream
        buffer;
    o emit, copies the received information as a block to the destination
        stream;
    o void set_emit_on_sync(bool how) changes the
        current emit-on-sync policy. By default how == false flushing its
        internal buffer to the destination's stream buffer. When how ==
        true the internal buffer is always immediately flushed;


20.15.2: Multi-threaded compilations using `osyncstream'

    Section 20.13 describes the construction of a multi-threaded program
performing compilations. In that program separate threads were used for the 
workers, who push their results on a results-queue. At the end of the
program the function results processes the queued results by showing the
names of the successfully compiled source files, and (if a compilation failed)
the name and error messages of the first source whose compilation failed.

The results-queue was used to store the results in a retrievable data
structure, using a mutex to ensure that the workers cannot simultaneously
push results on the results-queue.

Using osyncstream objects the results-queue and its mutexed protection
scheme is no longer required (the sources of the modified program are
available in the C++ Annotations' directory
yo/threading/examples/osyncmulticompile). 

Instead of using a results-queue the program uses a single destination stream
fstream g_out{ "/tmp/out", ios::trunc | ios::in | ios::out }, and its
compile function defines a local a osyncstream object, ensuring that
its output is sent as a block to g_out:
             1: void compile(string const &line)
     2: {
     3:     if (g_done.load())
     4:         return;
     5: 
     6:     string command("/usr/bin/g++ -Wall -c " + line);
     7: 
     8:     CmdFork cmdFork(command);
     9:     cmdFork.fork();
    10: 
    11:     int exitValue = cmdFork.childExit();
    12: 
    13:     osyncstream out(g_out);
    14:     out << exitValue << ' ' << line << '\n';
    15: 
    16:     if (exitValue != 0)
    17:     {
    18:         out << cmdFork.childOutput() << '\n' << g_marker << '\n';
    19:         g_done = true;
    20:     }
    21:     // out.emit();          // handled by out's destructor
    22: }


    o  at line 13 the osyncstream out object is defined, and the results
        of the compilation are written to out at lines 14 and 18;
    o  at line 14 the result of the compilation followed by the name of the
        source file is inserted into out;
    o  if a compilation fails then, at line 18, the compiler's error
        messages are inserted into out terminated by a marker,
        used by results (see below), to recognize the end of the error
        messages.

Since the results of the compilation are no longer transferred to another
thread, there's no need for defining a shared_future<Result>. In fact,
since compile handles the results of a compilation itself, it defines
return type void and the packaged_task itself doesn't return anything
either. Therefore the class Task doesn't need a result() member
anymore. Instead, its function-call operator, having completed its task, calls
the task's get_future so exceptions that might have been generated by
the packaged_tasks are properly retrieved. Here's the simplified class
Task:
            using PackagedTask = packaged_task<void (string const &fname)>;
    
    class Task
    {
        string d_command;
        PackagedTask d_task;
    
        public:
            Task()  = default;
    
            Task(string const &command, PackagedTask &&tmp)
            :
                d_command(command),
                d_task(move(tmp))
            {}
    
            void operator()()
            {
                d_task(d_command);
                d_task.get_future();    // handles potential exceptions
            }
    };

At the end of main the function results is called:
             1: void results()
     2: {
     3:     g_out.seekg(0);
     4: 
     5:     int value;
     6:     string line;
     7:     string errorDisplay;
     8: 
     9:     while (g_out >> value >> line)      // process g_out's content
    10:     {
    11:         g_out.ignore(100, '\n');
    12: 
    13:         if (value == 0)                 // no error: show the source file
    14:         {
    15:             cerr << line << '\n';
    16:             continue;
    17:         }
    18:                                         // at compilation errors:
    19:         if (not errorDisplay.empty())   // after the 1st error: skip
    20:         {
    21:             do
    22:             {
    23:                 getline(g_out, line);
    24:             }
    25:             while (line != g_marker);
    26: 
    27:             continue;
    28:         }
    29:                                         // first compilation error:
    30:         errorDisplay = line + '\n';     // keep the the name of the source
    31:         while (true)                    // and its error messages
    32:         {
    33:             getline(g_out, line);
    34: 
    35:             if (line == g_marker)
    36:                 break;
    37: 
    38:             errorDisplay += line + '\n';
    39:         }
    40:     }
    41: 
    42:     cerr << errorDisplay;               // eventually insert the error-info
    43: }                                       // (if any)


    o  Each compilation starts with a compilation result and a source
        name. These are extracted in the while condition at line 9.
    o  If the compilation was successful (line 13) the source's name is
        displayed.
    o  If not, only the info of the first failed compilation is displayed
        (all failed compilation messages could of course also be
        displayed, but this program only shows the messages of the first
        encountered failing compilation). If a compilation has already been
        encountered then the next error messages are ignored (lines 19 thru
        28).
    o  The info of the first encountered compilation error is collected in
        errorDisplay (lines 30 thru 39).
    o  Once g_out has completely been read errorDisplay is displayed
        (line 42), which is either empty or contains the error messages of the
        first encountered compilation failure.



Chapter 21: Function and Variable Templates


C++ supports syntactic constructs allowing programmers to define and use
completely general (or abstract) functions or classes, based on generic types
and/or (possibly inferred) constant values. In the chapters on abstract
containers (chapter 12) and the STL (chapter 18) we've
already used these constructs, commonly known as the
    template mechanism.

The template mechanism allows us to specify classes and algorithms, fairly
independently of the actual types for which the templates are eventually going
to be used. Whenever the template is used, the compiler generates code that is
tailored to the particular data type(s) used with the template. This code is
generated at compile-time from the template's definition. The piece of
generated code is called an instantiation of the template.

In this chapter the syntactic peculiarities of templates are covered.  The
notions of template type parameter, template non-type parameter, and
function template are introduced and several examples of templates are
provided (both in this chapter and in chapter 26).  Template
classes are covered in chapter 22. For good reasons variadic
functions 
     
 are deprecated in C++. However, variadic templates tell us a
completely different story, and variadic templates are perfectly
acceptable. Both function- and class-templates can be defined as variadic
templates. Both forms are covered in section 22.5.

Templates already offered by the language include the abstract containers
(cf. chapter 12); the string (cf. chapter 5);
streams (cf. chapter 6); and the generic algorithms (cf. chapter
19). So, templates play a central role in present-day C++, and
should not be considered an esoteric feature of the language.

Templates should be approached somewhat similarly as generic algorithms:
they're a way of life; a C++ software engineer should actively look
for opportunities to use them.  Initially, templates may appear to be rather
complex and you might be tempted to turn your back on them. However, over time
their strengths and benefits are more and more appreciated. Eventually you'll
be able to recognize opportunities for using templates. That's the time where
your efforts should no longer focus on constructing ordinary functions
  and classes  (i.e., functions or
classes that are not templates), but on constructing templates.

This chapter starts by introducing function templates. The emphasis is on
the required syntax. This chapter lays the foundation upon which the other
chapters about templates are built.


21.1: Defining function templates

A function template's definition is very similar to the definition of
a normal function. A function template has a function head, a function body, a
return type, possibly overloaded definitions, etc.. However, different from
ordinary functions, function templates always use one or more
 formal types: types for which almost any existing
(class or primitive) type could be used. Let's have a look at a simple
example. The following function add expects two Type arguments and
returns their sum:
            Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }

Note how closely the above function's definition follows its description.
It receives two arguments, and returns its sum. Now consider what would happen
if we defined this function for, e.g., int values. We would write:
            int add(int const &lhs, int const &rhs)
    {
        return lhs + rhs;
    }

So far, so good. However, were we to add two doubles, we would overload
this function:
            double add(double const &lhs, double const &rhs)
    {
        return lhs + rhs;
    }

There is no end to the number of overloaded versions we might be forced to
construct: an overloaded version for string, for size_t, for .... In
general, we would need an overloaded version for every type supporting
operator+ and a copy constructor. All these overloaded versions of
basically the same function are required because of the strongly typed nature
of C++. Because of this, a truly generic function cannot be constructed
without resorting to the template mechanism.

Fortunately, we've already seen an important part of a template
function. Our initial function add actually is an implementation of such a
function although it isn't a full template definition yet. If we gave the
first add function to the compiler, it would produce an error message
like:
            error: `Type' was not declared in this scope
    error: parse error before `const'

And rightly so, as we failed to define Type. The error is prevented
when we change add into a full template definition. To do this, we look
at the function's implementation and decide that Type is actually a
formal typename. Comparing it to the alternate implementations, it is
clear that we could have changed Type into int to get the first
implementation, and into double to get the second.

The full template definition allows for this formal nature of the
Type typename. Using the keyword template, we prefix one line to
our initial definition, obtaining the following function template
definition:
            template <typename Type>
    Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }

    In this definition we distinguish:
    
    o  The keyword template, starting a template definition or
declaration.
    o  The angle bracket enclosed list following template. This is a
list containing one or more comma-separated elements. This angle bracket
enclosed list is called the
 template parameter list. Template parameter lists using multiple
elements could look like this:
            typename Type1, typename Type2

o  Inside the template parameter list we find the formal type name
Type. It is a formal type name, comparable to a formal parameter name in a
function's definition. Up to now we've only encountered formal variable names
with functions. The types of the parameters were always known by the time
the function was defined. Templates escalate the notion of formal names one
step further up the ladder. Templates allow type names to be formalized,
rather than just the variable names themselves. The fact that Type is a
formal type name is indicated by the keyword typename, prefixed to
Type in the template parameter list. A formal type name like Type is
also called a template type parameter. Template non-type parameters also
exist, and are shortly introduced.

Other texts on C++ sometimes use the keyword class where we use
 typename. So, in other texts template definitions
might start with a line like:
            template <class Type>

In the C++ Annotations the use of typename over class is preferred,
reasoning that a template type parameter is, after all, a type name (some
authors prefer class over typename; in the end it's a matter of
taste).
    o  The template keyword and the template parameter list is called
the template header.
    o  The function head: it is like a normal function head, albeit that the
template's type parameters must be used in its parameter list. When the
function is eventually called using actual arguments having actual types,
these actual types are used by the compiler to infer which version (i.e.,
overload to fit the actual argument types) of the function template must be
used. At the point where the function is called the compiler creates the
function that is called, a process called instantiation. The function
head may also use a formal type to specify its return value. This feature was
actually used in the add template definition.
    o  The function parameters are specified as Type const &
parameters. This has the usual meaning: the parameters are references to
Type objects or values that will not be modified by the function.
    o  The function body is like a normal function body. In the body the
formal type names may be used to define or declare variables, which may then
be used as any other local variable. But some restrictions apply. Looking at
add's body, it is clear that operator+ is used, as well as a copy
constructor, as the function returns a value. This allows us to formulate the
following restrictions for the formal type Type as used by our add
function template:
        
        o  Type should support operator+
        o  Type should support a copy constructor

Consequently, while Type could be a string, it could never be
an ostream, as neither operator+ nor the copy constructor are
available for streams.

Normal scope rules and identifier visibility rules apply to templates.
Within the template definition's scope formal type names overrule identically
named identifiers of broader scopes.


21.1.1: Considerations regarding template parameters

    We've managed to design our first function template:
            template <typename Type>
    Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }

    Look again at add's parameters.  By specifying Type const &
rather than Type superfluous copying is prevented, at the same time
allowing values of primitive types to be passed as arguments to the
function. So, when add(3, 4) is called, int{4} is assigned to
Type const &rhs. In general, function parameters should be defined as
Type const & to prevent unnecessary copying. The compiler is smart enough
to handle `references to references' in this case, which is something the
language normally does not support. For example, consider the following
main function (here and in the following simple examples it is assumed
that the template and the required headers and namespace declarations have
been provided):
            int main()
    {
        size_t const &var = size_t{ 4 };
        cout << add(var, var) << '\n';
    }

Here var is a reference to a constant size_t. It is passed as
argument to add, thereby initializing lhs and rhs as Type
const & to size_t const & values. The compiler interprets Type as
size_t. Alternatively, the parameters might have been specified using
Type &, rather than Type const &. The disadvantage of this (non-const)
specification being that temporary values cannot be passed to the function
anymore. The following therefore fails to compile:
            int main()
    {
        cout << add(string{ "a" }, string{ "b" }) << '\n';
    }

Here, a string const & cannot be used to initialize a string &.
Had add defined Type && parameters then the above program would have
compiled just fine. In addition the following example correctly compiles as
the compiler decides that Type apparently is a string const:
            int main()
    {
        string const &s = string{ "a" };
        cout << add(s, s) << '\n';
    }

What can we deduce from these examples?
    
    o  In general, function parameters should be specified as Type const
& parameters to prevent unnecessary copying.
    o  The template mechanism is fairly flexible.  Formal types are
interpreted as plain types, const types, pointer types, etc., depending on the
actually provided types. The rule of thumb is that the formal type is used
as a generic mask for the actual type, with the formal type name covering
whatever part of the actual type must be covered. Some examples, assuming the
parameter is defined as Type const &:



        
        

        

        
             Provided argument:   Actually used Type: 

        

        

        
             size_t const   size_t 

        

        
             size_t   size_t 

        

        
             size_t *   size_t * 

        

        
             size_t const *   size_t const * 

        

        

        

            


As a second example of a function template, consider the following
function template:
            template <typename Type, size_t Size>
    Type sum(Type const (&array)[Size])
    {
        Type tp{};  // note: the default constructor must exist.

        for (size_t idx = 0; idx < Size; idx++)
            tp += array[idx];

        return tp;
    }

This template definition introduces the following new concepts and
features:
    
    o  The template parameter list. This template parameter list has
two elements. The first element is a well-known template type parameter, but
the second element has a very specific type: a size_t. Template parameters
of specific (i.e., non-formal) types used in template parameter lists are
called
 template non-type parameters.
    A template non-type parameter defines the type of a
 constant expression, which must be known by the time the template is
instantiated and which is specified in terms of existing types, such as a
size_t.
        o  Looking at the function's head, we see one parameter:
        Type const (&array)[Size]

This parameter defines array as a reference to an array having
Size elements of type Type that may not be modified.
    o  In the parameter definition, both Type and Size are
used. Type is of course the template's type parameter Type, but
Size is also a template parameter. It is a size_t, whose value must be
inferable by the compiler when it compiles an actual call of the sum
function template. Consequently, Size must be a const value. Such a
constant expression is called a template non-type parameter, and its type
is named in the template's parameter list.
    o  When the function template is called, the compiler must be able to
infer not only Type's concrete value, but also Size's value. Since the
function sum only has one parameter, the compiler is only able to infer
Size's value from the function's actual argument. It can do so if the
provided argument is an array (of known and fixed size) rather than a pointer
to Type elements. So, in the following main function the first
statement will compile correctly but the second statement will not:
    int main()
{
    int values[5];
    int *ip = values;

    cout << sum(values) << '\n';    // compiles OK
    cout << sum(ip) << '\n';        // won't compile
}

o  Inside the function's body the definition Type tp{} is used
to define and initialize tp to a default value. Note here that no fixed
value (like 0) is used. Also, be careful not to use Type tp(), as this is
a declaration of a function tp, expecting no arguments, and returning
a Type. Generally, when it is required to explicitly initialize a value
the empty curly braces should be used. The advantage of explicitly calling the
type's constructor is primarily encountered when Type is a basic
type. E.g., if Type is an int then Type tp{} initializes tp to
zero, whereas Type tp results in tp having an undefined value.
 
    But all types, even the primitive types, support default constructors
(some classes may choose not to implement a default constructor, or to make it
inaccessible; but most do offer default constructors). The default constructor
     of primitive types initializes their
variables to 0 (or false). Furthermore, the statement Type tp = Type()
is a true initialization: tp is initialized by Type's default
constructor, rather than using Type's copy constructor to assign
Type's copy to tp.

It's interesting to note (although not directly related to the
current topic) that the syntactic construction Type tp(Type()) cannot
be used, even though it also looks like a proper initialization. Usually an
initializing argument can be provided to an object's definition, like
string s("hello"). Why, then, is Type tp = Type() accepted, whereas
Type tp(Type()) isn't?  When Type tp(Type()) is used it won't result
in an error message. So we don't immediately detect that it's not a
Type object's default initialization. Instead, the compiler starts
generating error messages once tp is used. This is caused by the fact that
in C++ (and in C alike) the compiler does its best to recognize a
function or
  function pointer whenever possible: the
function prevalence rule.  According to this rule Type() is (because
of the pair of parentheses) interpreted as a pointer to a function
expecting no arguments; returning a Type. The compiler will do so unless
it clearly isn't possible to do so. In the initialization Type tp = Type()
it can't see a pointer to a function as a Type object cannot be given
the value of a function pointer (remember: Type() is interpreted as
Type (*)() whenever possible). But in Type tp(Type()) it can use
the pointer interpretation: tp is now declared as a function
expecting a pointer to a function returning a Type, with tp itself
also returning a Type. E.g., tp could have been defined as:
    Type tp(Type (*funPtr)())
{
    return (*funPtr)();
}

o  Comparable to the first function template, sum also assumes the
existence of certain public members in Type's class. This time
operator+= and Type's copy constructor.

Like class definitions, template definitions should not contain using
        
    directives or declarations: the template might be used in a situation
where such a directive overrides the programmer's intentions: ambiguities or
other conflicts may result from the template's author and the programmer using
different using directives (E.g, a cout variable defined in the
std namespace and in the programmer's own namespace). Instead, within
template definitions only 
fully qualified names, including all required namespace specifications should
be used.


21.1.2: Auto and decltype

        In section 3.3.7 the auto keyword was introduced. The keyword
decltype, related to auto, shows somewhat different behavior.  This
section concentrates on decltype. Different from auto, which requires
no further specifications, decltype is always followed by an
expression between parentheses (e.g., decltype(variable)).

As an initial illustration, assume we have a function defining a parameter
std::string const &text. Inside the function we may encounter the
following two definitions:
            auto scratch1{text};
    decltype(text) scratch2 = text;

With auto the compiler deduces a plain type, so scratch1 is a
string, and copy construction is used to initialize it from
`text'. 

Now consider decltype: decltype determines text's type:
string const &, which is thereupon used as scratch2's type: string
const &scratch2, referring to whatever string text refers to. This is
decltype's standard behavior: when provided with a variable's name,
it is replaced by that variable's type.

Alternatively, an expression can be specified when using decltype. Of
course, a variable is an expression by itself, but in the context of
decltype we define an `expression' as any expression that is more complex
than just a plain variable specification. But it may be as simple as
(variable): the name of a variable between parentheses.

When an expression is used, the compiler determines whether a reference could
be appended to the expression's type. If so, decltype(expression) is
replaced by the type of such an lvalue reference (so you get
expression-type &). If not, decltype(expression) is replaced by the
expression's plain type.

Here are some examples:
        int *ptr;
    decltype(ptr) ref = ptr;
        // decltype's argument is a plain variable, and so
        // ptr's type is used: int *ref = ptr.
        // decltype(ptr) is replaced by int *.
        // (resulting in two warnings about not-initialized/used variables).

    int *ptr;
    decltype( (ptr) ) ref = ptr;
        // decltype's argument is an expression, and so
        // int *&ref = ptr is used.
        // decltype( (ptr) )  is replaced by int *&.

    int value;
    decltype(value + value) var = value + value;
        // decltype's argument is an expression, and so the compiler tries
        // to replace decltype(...) by int & (int &var = value + value)
        // since value + value is a temporary, var's type cannot be int &
        // and so decltype(...) is replaced by int 
        // (i.e., value + value's type) 
    
    string lines[20];
    decltype(lines[0]) ref = lines[6];
        // decltype's argument is an expression, so
        // string &ref = lines[6] is used.
        // decltype(...) is replaced by string &

    string &&strRef = string{};
    decltype(strRef) ref = std::move(strRef);
        // decltype's argument is a plain variable so the variable's
        // type is used: string &&ref = std::move(strRef).
        // decltype(...) is replaced by string &&

    string &&strRef2 = string{}
    decltype((strRef2)) ref2 = strRef2;
        // decltype's argument is an expression, so
        // string && &ref = strRef is used. This automatically becomes
        // string &ref = strRef which is OK
        // decltype is replaced by string &.

In addition to this, decltype(auto) specifications can be used, in
which case decltype's rules are applied to auto. So, auto is used
to determine the type of the initializing expression. Then, if the
initializing expression is a mere variable, then the expression's type is
used. Otherwise, if a reference can be added to the expression's type then
decltype(auto) is replaced by a reference to the expression's type. Here
are some examples:
        int *ptr;
    decltype(auto) ptr2 = ptr;
        // auto produces ptr's type: int *, ptr is a plain variable, so
        // decltype(auto) is replaced by int *

    int value;
    decltype(auto) ret = value + value;
        // auto produces int, value + value is an expression, so int & is
        // attempted. However, value + value cannot be assigned to a 
        // reference so the expression's type is used:
        // decltype(auto) is replaced by int
    
    string lines[20];
    decltype(auto) line = lines[0];
        // auto produces string, lines[0] is an expression, so string & is
        // attempted. string &line = lines[0] is OK, so
        // decltype(auto) is replaced by string &

    decltype(auto) ref = string{} 
        // auto produces string, string{} is an expression, so string & is
        // attempted. However, string &ref = string{} is not a valid
        // initialization, so string itself is used:
        // decltype(auto) is replaced by string

In practice, the decltype(auto) form is most often encountered with
function templates to define return types. Have a look at the following
struct definition (not using function templates, but illustrating the workings
of decltype(auto)):
            struct Data
    {
        vector<string> d_vs;
        string *d_val = new string[10];
    
        Data()
        :
            d_vs(1)
        {}
    
        auto autoFun() const
        {
            return d_val[0];
        }

        decltype(auto) declArr() const       
        {
            return d_val[0];
        }

        decltype(auto) declVect() const
        {
            return d_vs[0];
        }
    };


o  The member autoFun returns auto. Since d_val[0] is passed to
    auto, auto is deducing as string, and the function's return
    type is string;

o  The member declArr returns decltype(auto). Since d_val[0]
    is an expression, representing a string, decltype(auto) is deduced
    as string &, which becomes the function's return type.

o  The member declVect returns decltype(auto). Since d_vs[0]
    is an expression, representing string, decltype(auto) is deduced
    as string &. However, since declVect is also
    a const member, this reference should  be a string const &. This is
    recognized by decltype(auto), and so the function's return type
    becomes string const &.

If you're wondering why there's no const in declArr's return type
while there is one in declVect's return type then have a look at d_vs
and d_val: both are constant in the context of their functions, but
d_val, so a const *, points to non-const string objects. So,
declArr does not have to return a string const &, whereas
declVect should return a string const &.


21.1.2.1: declval

        The keyword decltype is a tool for determining the type of an
expression. To use it an expression to which decltype is applied must be
available.  But what if a function template defines a typename Class
template parameter and the function template should use the return type of the
function Class::fun()? Since two classes may define members fun having
different return types, the return type to use is not immediately available.

These kinds of problems are solved by using the function template
    std::declval, defined in the <utility> header
file. This function template defines one template type parameter, and returns
an rvalue reference to an object of the template type parameter's class,
without actually creating a temporary object. But since an rvalue reference is
available, its fun function can be called, and the return type of that
function can then be produced by decltype. There are no specific
requirements for the constructors of the class type that's passed to
declval. Specifically: it doesn't have to have a default or public
constructor (but access rights are used). Consider this 
function template:
            template <typename Type>
    decltype(std::declval<Type>().fun()) value()
    {
        return 12.5;
    }

The function value's return type is defined as the as yet unknown 
Type::fun's return type. 

By defining two structs, both having fun member functions value's
actual return type can now be returned. This is used in main where
respectively an int and a double is returned, resulting in the output
12 12.5:
            struct Integral
    {
        int fun() const;            // implementation not required
    };
    
    struct Real
    {
        double fun() const;         // same.
    };
    
    int main()
    {
        std::cout << value<Integral>() << ' ' << value<Real>() << '\n';
    }


21.1.3: Late-specified return type

    Traditional  C++ requires function templates to specify their return type
or to specify the return type as a template type parameter. Consider the
following function:
            int add(int lhs, int rhs)
    {
        return lhs + rhs;
    }

The above function may be converted to a function template:
            template <typename Lhs, typename Rhs>
    Lhs add(Lhs lhs, Rhs rhs)
    {
        return lhs + rhs;
    }

Unfortunately, when the function template is called as
            add(3, 3.4)

the intended return type is probably a double rather than an
int. This can be solved by adding an additional template type parameter
specifying the return type but then  that type must explicitly be specified:
            add<double>(3, 3.4);

Using decltype (cf. section 3.3.7) to define the return type won't
work as lhs and rhs aren't known to the compiler by the time
decltype is used. Thus the next attempt to get rid of the additional
template type parameter fails to compile:
            template <typename Lhs, typename Rhs>
    decltype(lhs + rhs) add(Lhs lhs, Rhs rhs)
    {
        return lhs + rhs;
    }

The decltype-based definition of a function's return type may become
fairly complex. This complexity can be reduced by using the 
 late-specified return type syntax that does allow the use of
decltype to define a function's return type. It is primarily used with
function templates but it may also be used for ordinary (non-template)
functions:
            template <typename Lhs, typename Rhs>
    auto add(Lhs lhs, Rhs rhs) -> decltype(lhs + rhs)
    {
        return lhs + rhs;
    }

When this function is used in a statement like cout << add(3, 3.4) the
resulting value will be 6.4, which is most likely the intended result, rather
than 6. As an example how a late-specified return type may reduce the
complexity of a function's return type definition consider the following:
            template <typename T, typename U>
    decltype((*(T*)0)+(*(U*)0)) add(T t, U u);

Kind of hard to read? A term like  (*(T*)0)  defines 0, using a C
    cast, as a pointer to type T and then dereferences the pointer,
    producing a value of type T (even though that value itself doesn't
    exist as a variable). Likewise for the second term that's used in the
    decltype expression. The resulting type is thereupon used as add's
    return type. 
    Using a late-specified return type we get the equivalent:
            template <typename T, typename U>
    auto add(T t, U u) -> decltype(t+u);

which most people consider easier to understand.

The expression specified with decltype does not necessarily use the
parameters lhs and rhs themselves. In the next function
definition lhs.length is used instead of lhs itself:
            template <typename Class, typename Rhs>
    auto  add(Class lhs, Rhs rhs) -> decltype(lhs.length() + rhs)
    {
        return lhs.length() + rhs;
    }

Any variable visible at the time decltype is compiled can be used
in the decltype expression. It is also possible to
handle member selection through pointers to members. The following code aims
at specifying the address of a member function as add's first argument and
then use its return value type to determine the function template's return
type. Here is an example:
            std::string global{"hello world"};

    template <typename MEMBER, typename RHS>
    auto  add(MEMBER mem, RHS rhs) -> decltype((global.*mem)() + rhs)
    {
        return (global.*mem)() + rhs;
    }

    int main()
    {
        std::cout << add(&std::string::length, 3.4) << '\n'; // shows: 14.4
    }


21.2: Passing arguments by reference (reference wrappers)

Before using the reference wrappers discussed in this section the
<functional> header file must be included.

Situations exist where the compiler is unable to infer that a reference rather
than a value is passed to a function template. In the following example the
function template  outer receives int x as its argument and the
compiler dutifully infers that Type is int:
            template <typename Type>
    void outer(Type t)
    {
        t.x();
    }
    void useInt()
    {
        int arg;
        outer(arg);
    }

    Compilation will of course fail (as int values don't have x
members) and the compiler nicely reports the inferred type, e.g.:
            In function 'void outer(Type) [with Type = int]': ...

Another type of error results from using call in the next example. Here,
call is a function template expecting a function-type argument. The
function that's passed to call is sqrtArg, defining a reference to a
double: the variable that's passed to sqrtArg is modified by
sqrtArg. 
            void sqrtArg(double &arg)
    {
        arg = sqrt(arg);
    }
    template<typename Fun, typename Arg>
    void call(Fun fun, Arg arg)
    {
        fun(arg);
        cout << "In call: arg = " << arg << '\n';
    }

    The first time call is used, call(sqrtArg, value) will not modify
value: the compiler infers Arg to be a double value, and hence
passes value by value to call, thus preventing sqrtArg to modify
main's variable.

To change main's variable value the compiler must be informed that
value must be passed by reference. Note that we do not want to 
define call's template parameter as a reference parameter, 
as passing arguments by value might be appropriate in other situations.

In these situations the ref(arg) and cref(arg)
 reference wrappers should be used. They accept an
argument and return their argument as a (const) reference-typed argument. To
actually change value it can be passed to call using ref(value) as
shown in the following main function:
            int main()
    {
        double value = 3;
        call(sqrtArg, value);
        cout << "Passed value, returns: " << value << '\n';

        call(sqrtArg, ref(value));
        cout << "Passed ref(value), returns: " << value << '\n';
    }
    /*
        Displays:
            In call: arg = 1.73205
            Passed value, returns: 3
            In call: arg = 1.73205
            Passed ref(value), returns: 1.73205
    */



21.3: Using local and unnamed types as template arguments

Usually, types have names. But an anonymous type may also be defined:
            enum
    {
        V1,
        V2,
        V3
    };

Here, the enum defines an  unnamed or
        
    anonymous type.

When defining a function template, the compiler normally
deduces the types of its template type parameters from its arguments:
            template <typename T>
    void fun(T &&t);

    fun(3);     // T is int
    fun('c');   // T is char

The following, however, can also be used:
            fun(V1);    // T is a value of the above enum type

Within fun a T variable may be defined, even if it's an anonymous
type:
            template <typename T>
    void fun(T &&t)
    {
        T var(t);
    }

Values or objects of
        
    locally defined types may also be passed as arguments to function
templates. E.g.,
            void definer()
    {
        struct Local
        {
            double  dVar;
            int     iVar;
        };
        Local local;            // using a local type

        fun(local);             // OK: T is 'Local'
    }


21.4: Template parameter deduction

    In this section we concentrate on the process by which the compiler
deduces the actual types of the template type parameters. These types are
deduced when a function template is called using a process called
 template parameter deduction. As we've already seen, the compiler is
able to substitute a wide range of actual types for a single formal template
type parameter. Even so, not every thinkable conversion is possible. In
particular when a function has multiple parameters of the same template type
parameter, the compiler is very restrictive when determining what argument
types are actually accepted.

When the compiler deduces the actual types for template type parameters it
only considers the types of the arguments that are actually
used. Neither local variables nor the function's return value is considered in
this process. This is understandable. When a function is called the compiler
is only certain about the types of the function template's arguments. At the
point of the call it definitely does not see the types of the function's local
variables. Also, the function's return value might not actually be used or may
be assigned to a variable of a subrange (or super-range) type of a deduced
template type parameter. So, in the following example, the compiler won't ever
be able to call fun(), as it won't be able to deduce the actual type for
the Type template type parameter.
            template <typename Type>
    Type fun()              // can never be called as `fun()'
    {
        return Type{};
    }

Although the compiler won't be able to handle a call to `fun()', it
is possible to call fun() using an
 explicit type specification. E.g.,
fun<int>() calls fun, instantiated for int. This is of course
not the same as compiler argument deduction.

In general, when a function has multiple parameters of identical template
type parameters, the actual types must be exactly the same. So, whereas
            void binarg(double x, double y);

may be called using an int and a double, with the int argument
silently being converted to a double, a similar function template cannot
be called using an int and double argument: the compiler won't by
itself promote int to double deciding that Type should be
double:
            template <typename Type>
    void binarg(Type const &p1, Type const &p2)
    {}

    int main()
    {
        binarg(4, 4.5); // ?? won't compile: different actual types
    }

What, then, are the transformations the compiler  applies when deducing
the actual types of template type parameters? It performs but three types
of
        
    parameter type transformations and a fourth one to function template
non-type parameters. If it cannot deduce the actual types using these
transformations, the function template will not be considered. The
transformations performed by the compiler are:
    
    o  lvalue transformations, creating an rvalue from an
lvalue;
    o  qualification transformations, inserting a const modifier to
a non-constant argument type;
    o  transformation to a base class instantiated from a class template,
using a template base class when an argument of a template derived class type
was provided in the call.
    o  Standard transformations for function template non-type
parameters. This isn't a template type parameter transformation, but it refers
to any remaining template non-type parameter of function templates. For these
function parameters the compiler performs any standard conversion it has
available (e.g., int to size_t, int to double, etc.).

The purpose of the various template parameter type deduction
transformations is not to match function arguments to function parameters,
but rather, having matched arguments to parameters, to determine the actual
types of the various template type parameters.


21.4.1: Lvalue transformations

    There are three types of lvalue transformations:
    
    o  lvalue-to-rvalue transformations.
        An lvalue-to-rvalue transformation is applied when an
rvalue is required, but an lvalue is provided.  This happens when a
variable is used as argument to a function specifying a value
parameter. For example,
    template<typename Type>
Type negate(Type value)
{
    return -value;
}
int main()
{
    int x = 5;
    x = negate(x);  // lvalue (x) to rvalue (copies x)
}

o  array-to-pointer transformations.
        An array-to-pointer transformation is applied when the name
of an array is assigned to a pointer variable. This is frequently used with
functions defining pointer parameters.  Such functions frequently receive
arrays as their arguments. The array's address is then assigned to the
pointer-parameter and its type is used to deduce the corresponding template
parameter's type. For example:
    template<typename Type>
Type sum(Type *tp, size_t n)
{
    return accumulate(tp, tp + n, Type());
}
int main()
{
    int x[10];
    sum(x, 10);
}

In this example, the location of the array x is passed to sum,
expecting a pointer to some type. Using the array-to-pointer transformation,
x's address is considered a pointer value which is assigned to tp,
deducing that Type is int in the process.

o  function-to-pointer transformations.
        This transformation is most frequently used with function
templates defining a parameter which is a pointer to a function.  When calling
such a function the name of a function may be specified as its argument. The
address of the function is then assigned to the pointer-parameter, deducing
the template type parameter in the process. This is called a
    function-to-pointer transformation. For example:
        #include <cmath>

template<typename Type>
void call(Type (*fp)(Type), Type const &value)
{
    (*fp)(value);
}
int main()
{
    call(sqrt, 2.0);
}

    In this example, the address of the sqrt function is passed to
call, expecting a pointer to a function returning a Type and expecting
a Type for its argument. Using the function-to-pointer transformation,
sqrt's address is assigned to fp, deducing that Type is double
in the process (note that sqrt is the address of a function, not a
variable that is a pointer to a function, hence the lvalue
transformation).

The argument 2.0 could not have been specified as 2 as there is no
int sqrt(int) prototype. Furthermore, the function's first parameter
specifies Type (*fp)(Type), rather than Type (*fp)(Type const &) as
might have been expected from our previous discussion about how to specify the
types of function template's parameters, preferring references over values.
However, fp's argument Type is not a function template parameter, but
a parameter of the function fp points to. Since sqrt has prototype
double sqrt(double), rather than double sqrt(double const &),
call's parameter fp must be specified as Type
(*fp)(Type). It's that strict.


21.4.2: Qualification transformations

    A qualification transformation adds const or volatile
qualifications to pointers. This transformation is applied when the
function template's type parameter explicitly specifies const (or
volatile) but the function's argument isn't a const or volatile
entity. In that case const or volatile is provided by the compiler.
Subsequently the compiler deduces the template's type parameter. For example:
            template<typename Type>
    Type negate(Type const &value)
    {
        return -value;
    }
    int main()
    {
        int x = 5;
        x = negate(x);
    }

Here we see the function template's Type const &value parameter: a
reference to a const Type. However, the argument isn't a const int,
but an int that can be modified. Applying a qualification transformation,
the compiler adds const to x's type, and so it matches int const
x.  This is then matched against Type const &value allowing the compiler
to deduce that Type must be int.


21.4.3: Transformation to a base class

    Although the construction of class templates is the topic of chapter
22, we've already extensively used class templates before. For
example, abstract containers (cf. chapter 12) are defined as
class templates.  Class templates can, like
 
    ordinary classes, participate in the construction of class hierarchies.

In section 22.11 it is shown how a class template can
be derived from another class template.

As class template derivation remains to be covered, the following
discussion is necessarily somewhat premature. The reader may of course skip
briefly to section 22.11 returning back to this section
thereafter.

In this section it should be assumed, for the sake of argument, that a
class template Vector has somehow been derived from a std::vector.
Furthermore, assume that the following function template has been
constructed to sort a vector using some function object obj:
            template <typename Type, typename Object>
    void sortVector(std::vector<Type> vect, Object const &obj)
    {
        sort(vect.begin(), vect.end(), obj);
    }

To sort std::vector<string> objects case-insensitively, a class
Caseless could be constructed as follows:
            class CaseLess
    {
        public:
            bool operator()(std::string const &before,
                            std::string const &after) const
            {
                return strcasecmp(before.c_str(), after.c_str()) < 0;
            }
    };

Now various vectors may be sorted using sortVector():
            int main()
    {
        std::vector<string> vs;
        std::vector<int> vi;

        sortVector(vs, CaseLess());
        sortVector(vi, less<int>());
    }

Applying the transformation
        
        
    transformation to a base class instantiated from a class template, the
function template sortVector may now also be used to sort Vector
objects. For example:
            int main()
    {
        Vector<string> vs;      // `Vector' instead of `std::vector'
        Vector<int> vi;

        sortVector(vs, CaseLess());
        sortVector(vi, less<int>());
    }

In this example, Vectors were passed as argument to
sortVector. Applying the transformation to a base class instantiated from
a class template, the compiler considers Vector to be a std::vector
enabling it to deduce the template's type parameter. A std::string for the
Vector vs, an int for Vector vi.


21.4.4: The template parameter deduction algorithm

        The compiler uses the following algorithm to deduce the actual types of
         its template type parameters:
    
    o  In turn, the function template's parameters are identified using the
arguments of the called function.
    o  For each template parameter used in the function template's parameter
list, the template type parameter is associated with the corresponding
argument's type (e.g., Type is int if the argument is int x, and
the function's parameter is Type &value).
    o  While matching the argument types to the template type parameters,
the three allowed transformations (see section 21.4) for template
type parameters are applied where necessary.
    o  If identical template type parameters
        
    are used with multiple function parameters, the deduced template types
must exactly match. So, the next function template cannot be called with an
int and a double argument:
    template <typename Type>
Type add(Type const &lhs, Type const &rhs)
{
    return lhs + rhs;
}

When calling this function template, two identical types must be used
(albeit that the three standard transformations are of course allowed). If the
template deduction mechanism does not come up with identical actual types for
identical template types, then the function template is not going to be
instantiated.


21.4.5: Template type contractions

    With function templates the combination of the types of template arguments and
template parameters shows some interesting contractions. What happens, for
example if a template type parameter is specified as an rvalue reference but
an lvalue reference argument type is provided?

In such cases the compiler performs type contractions. Doubling identical
reference types results in a simple contraction: the type is deduced to be a
single reference type. Example: if the template parameter type is specified as
a Type && and the actual parameter is an int && then Type is
deduced to be an int, rather than an int &&.

This is fairly intuitive. But what happens if the actual type is int &?
There is no such thing as an int & &&param and so the compiler contracts
the double reference by removing the rvalue reference, keeping the lvalue
reference. Here the following rules are applied:


    1. A function template parameter defined as an lvalue reference to
    a template's type parameter (e.g., Type &) receiving an lvalue
    reference argument results in a single lvalue reference.

2. A function template parameter defined as an rvalue reference to a
    template's type parameter (e.g., Type &&) receiving any kind of
    reference argument uses the reference type of the argument.  
Examples:
    
    o  When providing an Actual & argument then Type & becomes an
    Actual & and Type is inferred as Actual;

o  When providing an Actual & then Type && becomes an
    Actual & and Type is inferred as Actual;

o  When providing an Actual && then Type & also becomes
    Actual &  and Type is inferred as Actual;

o  When providing an Actual && then Type &&  becomes Actual
    && and Type is inferred as Actual;

Let's look at a concrete example where contraction occurs. Consider the
following function template where a function parameter is defined as an rvalue
references to some template type parameter:
            template <typename Type>
    void function(Type &&param)
    {
        callee(static_cast<Type &&>(param));
    }

In this situation, when function is called with an (lvalue) argument of
type TP & the template type parameter Type is deduced to be Tp
&. Therefore, Type &&param is instantiated as Tp &param, Type
becomes Tp and the rvalue reference is replaced by an lvalue reference.

Likewise, when callee is called using the static_cast the same
contraction occurs, so Type &&param operates on Tp &param. Therefore
(using contraction) the static cast also uses type Tp &param. If
param happened to be of type Tp && then the static cast uses type
Tp &&param.

This characteristic allows us to pass a function argument to a nested function
without changing its type: lvalues remain lvalues, rvalues remain
rvalues. This characteristic is therefore also known as
 perfect forwarding which is discussed in greater detail in section
22.5.2. Perfect forwarding prevents the template author from having to
define multiple overloaded versions of a function template.


21.5: Declaring function templates

    Up to now, we've only defined function templates. There are various
  consequences of including function
template definitions in multiple source files, none of them serious, but worth
knowing.
    
    o  Like class interfaces, template definitions are usually included in
header files. Every time a header file containing a template definition is
read by the compiler it must process the full definition. It must do so even
if it does not actually use the template. This somewhat  slows-down the
compilation. For example, compiling a template header file like algorithm
on my old laptop takes about four times the amount of time it takes to compile
a plain header file like cmath. The header file iostream is even
harder to process, requiring almost 15 times the amount of time it takes to
process cmath. Clearly, processing templates is serious business for the
compiler. On the other hand this drawback shouldn't be taken too seriously.
Compilers are continuously improving their template processing capacity and
computers keep getting faster and faster. What was a nuisance a few years ago
is hardly noticeable today.
    o  Every time a function template is instantiated, its code appears in
the resulting object module. However, if multiple instantiations of a template
using the same actual types for its template parameters exist in multiple
object files the one definition rule is lifted.  The linker weeds out
superfluous instantiations (i.e., identical definitions of instantiated
templates). In the final program only one instantiation for a particular set
of actual template type parameters remain available (see section
21.6 for an illustration). Therefore, the linker has an
additional task to perform (viz. weeding out multiple instantiations),
which somewhat slows down the linking process.
    o  Sometimes the definitions themselves are not required, but
references or pointers to the templates are. Requiring the compiler to
process the full template definitions in those cases needlessly slows down the
compilation process.
    o  In the context of template meta programming (see chapter
23) it is sometimes not even required to provide a template
implementation. Instead, only specializations (cf. section
21.9) are created which are based upon the mere declaration.

So in some contexts template definitions may not be required.  Instead
the software engineer may opt to declare a template rather than to
include the template's definition time and again in various source files.

When templates are declared, the compiler does not have to process the
template's definitions again and again; and no instantiations are created
on the basis of template declarations alone. Any actually required
instantiation must then be available elsewhere (of course, this holds true for
declarations in general). Unlike the situation we encounter with ordinary
functions, which are usually stored in libraries, it is currently not possible
to store templates in libraries (although the compiler may construct
precompiled header files). Consequently, using template declarations
puts a burden on the shoulders of the software engineer, who has to make sure
that the required instantiations exist. Below a simple way to accomplish that
is introduced.

To create a function template declaration simply replace the function's
body by a semicolon. Note that this is exactly identical to the way ordinary
function declarations are constructed. So, the previously defined function
template add can simply be declared as
            template <typename Type>
    Type add(Type const &lhs, Type const &rhs);

We've already encountered template declarations. The header file
iosfwd may be included in sources not requiring instantiations of elements
from the class ios and its derived classes. For example, to compile the
declaration
            std::string getCsvLine(std::istream &in, char const *delim);

it is not necessary to include the string and istream header
files. Rather, a single
            #include <iosfwd>

is sufficient. Processing iosfwd requires only a fraction of the time
it takes to process the string and istream header files.


21.5.1: Instantiation declarations

        If declaring function templates speeds up the compilation and the linking
phases of a program, how can we make sure that the required instantiations of
the function templates are available when the program is eventually linked
together?

For this a variant of a template declaration is available, a so-called
 explicit instantiation declaration.
    An explicit instantiation declaration consists of the following elements:
    
    o  It starts with the keyword template, omitting the template
parameter list.
    o  Next the function template's return type and name are specified.
    o  The function template's name is followed by a type specification
list. A type specification list is an angle brackets enclosed list of type
names.  Each type specifies the actual type of the corresponding template type
parameter in the template's parameter list.
    o  Finally the function template's parameter list is specified,
terminated by a semicolon.

Although this is a declaration, it is understood by the compiler as a
request to instantiate that particular variant of the function template.

Using explicit instantiation declarations all instantiations of template
functions required by a program can be collected in one file. This file, which
should be a normal source file, should include the template definition
header file and should subsequently specify the required explicit
instantiation declarations. Since it's a source file, it is not included
by other sources. So namespace using directives and declarations may
safely be used once the required headers have been included.  Here is an
example showing the required instantiations for our earlier add function
template, instantiated for double, int, and std::string types:
            #include "add.h"
    #include <string>
    using namespace std;

    template int add<int>(int const &lhs, int const &rhs);
    template double add<double>(double const &lhs, double const &rhs);
    template string add<string>(string const &lhs, string const &rhs);

If we're sloppy and forget to mention an instantiation required by our
program then the repair is easily made by adding the missing instantiation
declaration to the above list. After recompiling the file and relinking the
program we're done.


21.6: Instantiating function templates

    Different from an ordinary function that results in code once the compiler
reads its definition a template is not instantiated when its definition is
read. A template is merely a recipe telling the compiler how to create
particular code once it's time to do so. It's indeed very much like a recipe
in a cooking book. You reading how to bake a cake doesn't mean you have
actually baked that cake by the time you've read the recipe.

So, when is a function template actually instantiated? There are two
situations where the compiler decides to instantiate templates:
    
    o  They are instantiated when they are used (e.g., the function
add is called with a pair of size_t values);
    o  When addresses of function templates are taken they are instantiated.
Example:
        char (*addptr)(char const &, char const &) = add;

The location of statements causing the compiler to instantiate a template
is called the template's
        
    point of instantiation. The point of instantiation has serious
implications for the function template's code. These implications are
discussed in section 21.13.

The compiler is not always able to deduce the template's type parameters
unambiguously. When the compiler reports an ambiguity it must be solved by the
software engineer. Consider the following code:
            #include <iostream>
    #include "add.h"

    size_t fun(int (*f)(int *p, size_t n));
    double fun(double (*f)(double *p, size_t n));

    int main()
    {
        std::cout << fun(add);
    }

    When this little program is compiled, the compiler reports an ambiguity it
cannot resolve. It has two candidate functions as for each overloaded version
of fun an add function can be instantiated:
            error: call of overloaded 'fun(<unknown type>)' is ambiguous
    note: candidates are: int fun(size_t (*)(int*, size_t))
    note:                 double fun(double (*)(double*, size_t))

Such situations should of course be avoided. Function templates can only
be instantiated if there's no ambiguity. Ambiguities arise when multiple
functions emerge from the compiler's function selection mechanism (see section
21.14). It is up to us to resolve the ambiguities. They
could be resolved using a blunt static_cast (by which we select among
alternatives, all of them possible and available):
            #include <iostream>
    #include "add.h"

    int fun(int (*f)(int const &lhs, int const &rhs));
    double fun(double (*f)(double const &lhs, double const &rhs));

    int main()
    {
        std::cout << fun(
                        static_cast<int (*)(int const &, int const &)>(add)
                    );
    }

    But it's good practice to avoid type casts wherever possible. How to do
this is explained in the next section (21.7).


21.6.1: Instantiations: no `code bloat'

        As mentioned in section 21.5, the linker removes
  identical
instantiations of a template from the final program, leaving only one
instantiation for each unique set of actual template type parameters. To
illustrate the linker's behavior we do as follows:
    
    o  First we construct several source files:
            
            o source1.cc defines a function fun, instantiating
add for int-type arguments, including add's template
definition. It displays add's address using union PointerUnion:
            union PointerUnion
    {
        int (*fp)(int const &, int const &);
        void *vp;
    };


Here is a program using PointerUnion:
            #include <iostream>
    #include "add.h"
    #include "pointerunion.h"

    void fun()
    {
        PointerUnion pu = { add };

        std::cout << pu.vp << '\n';
    }

            o source2.cc defines the same function, but merely declares the
proper add template using a template declaration (not an instantiation
declaration). Here is source2.cc:
            #include <iostream>
    #include "pointerunion.h"

    template<typename Type>
    Type add(Type const &, Type const &);

    void fun()
    {
        PointerUnion pu = { add };

        std::cout << pu.vp << '\n';
    }

            o main.cc again includes add's template definition,
declares the function fun and defines main, defining add
for int-type arguments as well and displaying add's function
address. It also calls the function fun. Here is main.cc:
            #include <iostream>
    #include "add.h"
    #include "pointerunion.h"

    void fun();

    int main()
    {
        PointerUnion pu = { add };

        fun();
        std::cout << pu.vp << '\n';
    }


o  All sources are compiled to object modules. Note the different sizes
of source1.o (1912 bytes using g++ version 4.3.4 (sizes of object
modules reported in this section may differ for different compilers and/or
run-time libraries)) and source2.o (1740 bytes). Since source1.o
contains the instantiation of add, it is somewhat larger than
source2.o, containing only the template's declaration. Now we're ready to
start our little experiment.
    o  Linking main.o and source1.o, we obviously link together two
object modules, each containing its own instantiation of the same template
function. The resulting program produces the following output:
            0x80486d8
    0x80486d8

Furthermore, the size of the resulting program is 6352 bytes.
    o  Linking main.o and source2.o, we now link together an object
module containing the instantiation of the add template, and another
object module containing the mere declaration of the same template
function. So, the resulting program cannot but contain a single instantiation
of the required function template. This program has exactly the same size, and
produces exactly the same output as the first program.

From our little experiment we conclude that the linker indeed removes
identical template instantiations from a final program. Furthermore we
conclude that using mere template declarations does not result in template
instantiations.


21.7: Using explicit template types

    In the previous section we saw that the compiler may encounter ambiguities
when attempting to instantiate a template. In an example overloaded versions
of a function (fun) existed, expecting different types of arguments. The
ambiguity resulted from the fact that both arguments could have been provided
by an instantiation of a function template. The intuitive way to solve such an
ambiguity is to use a static_cast. But casts should be avoided wherever
possible.

With function templates static casts may indeed be avoided using
 explicit template type arguments. Explicit template type arguments can
be used to inform the compiler about the actual types it should use when
instantiating a template. To use explicit type arguments the function's name
is followed by an actual template type argument list which may again be
followed by the function's argument list. The actual types mentioned in the
actual template argument list are used by the compiler to `deduce' what types
to use when instantiating the template. Here is the example from the previous
section, now using explicit template type arguments:
            #include <iostream>
    #include "add.h"

    int fun(int (*f)(int const &lhs, int const &rhs));
    double fun(double (*f)(double const &lhs, double const &rhs));

    int main()
    {
        std::cout << fun(add<int>) << '\n';
    }


Explicit template type arguments  can be used in situations where the
compiler has no way to detect which types should actually be used. E.g., in
section 21.4 the function template Type fun() was defined. To
instantiate this function for the double type, we can call
fun<double>().


21.8: Overloading function templates

Let's once again look at our add template. That template was designed to
return the sum of two entities. If we would want to compute the sum of three
entities, we could write:
            int main()
    {
        add(add(2, 3), 4);
    }

This is an acceptable solution for the occasional situation. However, if
we would have to add three entities regularly, an overloaded version of
the add function expecting three arguments might be a useful function to
have. There's a simple solution to this problem: function templates may be
        
        
overloaded.

To define an overloaded function template, merely put multiple definitions
of the template in its header file. For the add function this would boil
down to:
            template <typename Type>
    Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }
    template <typename Type>
    Type add(Type const &lhs, Type const &mid, Type const &rhs)
    {
        return lhs + mid + rhs;
    }

The overloaded function does not have to be defined in terms of simple
values. Like all overloaded functions, a unique set of function parameters is
enough to define an overloaded function template. For example, here's an
overloaded version that can be used to compute the sum of the elements of a
vector:
            template <typename Type>
    Type add(std::vector<Type> const &vect)
    {
        return accumulate(vect.begin(), vect.end(), Type());
    }

When overloading function templates we do not have to restrict ourselves
to the function's parameter list. The template's type parameter list itself
may also be
        
    overloaded. The last definition of the add template allows us to
specify a vector as its first argument, but no deque or
map. Overloaded versions for those types of containers could of course be
constructed, but how far should we go? A better approach seems to be to look
for common characteristics of these containers. If found we may be able to
define an overloaded function template based on these common
characteristics. One common characteristic of the mentioned containers is that
they all support begin and end members, returning iterators. Using
this, we could define a template type parameter representing containers that
must support these members. But mentioning a plain `container type' doesn't
tell us for what type of data it was instantiated. So we need a second
template type parameter representing the container's data type, thus
overloading the template's type parameter list. Here is the resulting
overloaded version of the add template:
            template <typename Container, typename Type>
    Type add(Container const &cont, Type const &init)
    {
        return std::accumulate(cont.begin(), cont.end(), init);
    }

One may wonder whether the init parameter could not be left out of the
parameter list as init often has a default initialization value. The
answer is `yes', but there are complications. It is possible to define the
add function as follows:
            template <typename Type, typename Container>
    Type add(Container const &cont)
    {
        return std::accumulate(cont.begin(), cont.end(), Type());
    }

Note, however, that the template's type parameters were reordered, which
is necessary because the compiler won't be able to determine Type in a
call like:
            int x = add(vectorOfInts);

After reordering the template type parameters, putting Type first, an
explicit template type argument can be provided for the first template type
parameter:
            int x = add<int>(vectorOfInts);

In this example we provided a vector<int> argument. One might wonder
why we have to specify int explicitly to allow the compiler to determine
the template type parameter Type. In fact, we don't. A third kind of
template parameter exists, a template template parameter, allowing the
compiler to determine Type directly from the actual container
argument. Template template parameters are discussed in section
23.4.


21.8.1: An example using overloaded function templates

        With all these overloaded versions in place, we may now start the compiler
to compile the following function:
            using namespace std;

    int main()
    {
        vector<int> v;

        add(3, 4);          // 1 (see text)
        add(v);             // 2
        add(v, 0);          // 3
    }


    o  In statement 1 the compiler recognizes two identical types, both
int. It therefore instantiates add<int>, our very first definition
of the add template.
    o  In statement 2 a single argument is used. Consequently, the compiler
looks for an overloaded version of add requiring but one argument. It
finds the overloaded function template expecting a std::vector, deducing
that the template's type parameter must be int. It instantiates
            add<int>(std::vector<int> const &)

o  In statement 3 the compiler again encounters an argument list having
  two arguments. However, this time the types of the arguments aren't equal,
so add template's first definition can't be used. But it can use the
last definition, expecting entities having different types. As a
std::vector supports begin and end, the compiler is now able to
instantiate the function template
        add<std::vector<int>, int>(std::vector<int> const &, int const &)

Having defined the add function template for two equal and two
different template type parameters we've exhausted the possibilities for using
an add function template having two template type parameters.


21.8.2: Ambiguities when overloading function templates

    Although it is possible to define another function template add this
introduces an ambiguity as the compiler won't be able to choose which of
the two overloaded versions defining two differently typed function parameters
should be used. For example when defining:
            #include "add.h"

    template <typename T1, typename T2>
    T1 add(T1 const &lhs, T2 const &rhs)
    {
        return lhs + rhs;
    }
    int main()
    {
        add(3, 4.5);
    }

the compiler reports an ambiguity like the following:
                error: call of overloaded `add(int, double)' is ambiguous
        error: candidates are: Type add(const Container&, const Type&)
                                    [with Container = int, Type = double]
        error:                 T1 add(const T1&, const T2&)
                                    [with T1 = int, T2 = double]

Now recall the overloaded function template accepting three arguments:
            template <typename Type>
    Type add(Type const &lhs, Type const &mvalue, Type const &rhs)
    {
        return lhs + mvalue + rhs;
    }

It may be considered as a disadvantage that only equally typed arguments
are accepted by this function (three ints, three doubles, etc.). To
remedy this we define yet another overloaded function template, this time
accepting arguments of any type.  This function template can only be used if
operator+ is defined between the function's actually used types, but apart
from that there appears to be no problem. Here is the overloaded version
accepting arguments of any type:
            template <typename Type1, typename Type2, typename Type3>
    Type1 add(Type1 const &lhs, Type2 const &mid, Type3 const &rhs)
    {
        return lhs + mid + rhs;
    }

Now that we've defined the above two overloaded function templates
expecting three arguments let's call add as follows:
            add(1, 2, 3);

Should we expect an ambiguity here? After all, the compiler might select
the former function, deducing that Type == int, but it might also select
the latter function, deducing that Type1 == int, Type2 == int and Type3
== int. Remarkably, the compiler reports no ambiguity.

No ambiguity is reported because of the following. If overloaded template
functions are defined using less and more specialized template type
parameters (e.g., less specialized: all types different vs. more specialized:
         all types equal)
then the compiler selects the more specialized function whenever possible.

As a rule of thumb: overloaded function templates must allow a unique
combination of template type arguments to be specified to prevent ambiguities
when selecting which overloaded function template to instantiate.  The
ordering of template type parameters in the function template's type
parameter list is not important. E.g., trying to instantiate one of the
following function templates results in an ambiguity:
            template <typename T1, typename T2>
    void binarg(T1 const &first, T2 const &second)
    {}
    template <typename T1, typename T2>
    void binarg(T2 const &first, T1 const &second)
    {}

This should not come as a surprise. After all, template type parameters
are just formal names. Their names (T1, T2 or Whatever) have no
concrete meanings.


21.8.3: Declaring overloaded function templates

        Like any function, overloaded functions may be declared, either using
plain declarations or instantiation declarations. Explicit template argument
types may also be used. Example:
    
    o  To declare a function template add accepting certain containers:
    template <typename Container, typename Type>
Type add(Container const &container, Type const &init);

o  to use an instantiation declaration (in which case the compiler must
already have seen the template's definition):
    template int add<std::vector<int>, int>
                (std::vector<int> const &vect, int const &init);

o  to use explicit template type arguments:
    std::vector<int> vi;
int sum = add<std::vector<int>, int>(vi, 0);


21.9: Specializing templates for deviating types

    The initial add template, defining two identically typed parameters
works fine for all types supporting operator+ and a copy
constructor. However, these assumptions are not always met. For example, with
char *s, using operator+ or a `copy constructor' does not make
sense. The compiler tries to instantiate the function template, but
compilation fails as  operator+ is not defined for pointers.

In such situations the compiler may be able to resolve the template type
parameters but it (or we ...) may then detect that the standard implementation
is pointless or produces errors.

To solve this problem a template explicit specialization may be
defined.  A template explicit specialization defines the function template for
which a generic definition already exists using specific actual template type
parameters. As we saw in the previous section the compiler always prefers
a more specialized function over a less specialized one. So the template
explicit specialization is selected whenever possible.

A template explicit specialization offers a specialization for its template
type parameter(s). The special type is consistently substituted for
the template type parameter in the function template's code. For
example if the explicitly specialized type is char const * then in the
template definition
            template <typename Type>
    Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }

Type must be replaced by char const *, resulting in a function
having prototype
            char const *add(char const *const &lhs, char const *const &rhs);

Now we try to use this function:
            int main(int argc, char **argv)
    {
        add(argv[0], argv[1]);
    }

However, the compiler ignores our specialization and tries to instantiate
the initial function template. This fails, leaving us wondering why it didn't
select the explicit specialization....

To see what happened here we replay, step by step, the compiler's actions:
    
    o  add is called with char * arguments.
    o  Both types are equal, so the compiler deduces that Type equals
char *.
    o  Now it inspects the specialization. Can a char * template type
argument match a char const *const & template parameter? Here
opportunities for the allowable transformations from section 21.4
may arise. A qualification transformation seems to be the only viable one,
allowing the compiler to bind a const-parameter to a non-const argument.
    o  So, in terms of Type the compiler can match an argument of some
Type or an argument of some Type const to a Type const &.
    o  Type itself is not modified, and so Type is a char *.
    o  Next the compiler inspects the available explicit specializations. It
finds one, specializing for char const *.
    o  Since a char const * is not a char * it rejects the explicit
specialization and uses the generic form, resulting in a compilation error.

If our add function template should also be able to handle char *
template type arguments another explicit specialization for char * may be
required, resulting in the prototype
            char *add(char *const &lhs, char *const &rhs);

Instead of defining another explicit specialization an overloaded
function template could be designed expecting pointers. The following function
template definition expects two pointers to constant Type values and
returns a pointer to a non-constant Type:
            template <typename Type>
    Type *add(Type const *t1, Type const *t2)
    {
        std::cout << "Pointers\n";
        return new Type;
    }

What actual types may be bound to the above function parameters? In this
case only a Type const *, allowing char const *'s to be passed as
arguments. There's no opportunity for a qualification transformation here.
The qualification transformation allows the compiler to add a const to a
non-const argument if the parameter itself (and not  Type) is
specified in terms of a const or const &. Looking at, e.g., t1 we
see that it's defined as a Type const *. There's nothing const here
that's referring to the parameter (in which case it would have been Type
const *const t1 or Type const *const &t1). Consequently a qualification
transformation cannot be applied here.

As the above overloaded function template only accepts char const *
arguments, it will not accept (without a reinterpret cast) char *
arguments. So main's argv elements cannot be passed to our overloaded
function template.


21.9.1: Avoiding too many specializations

        So do we have to define yet another overloaded function template, this
time expecting Type * arguments? It is possible, but at some point it
should become clear that our approach doesn't scale. Like ordinary functions
and classes, function templates should have one conceptually clear
purpose. Trying to add overloaded function templates to overloaded function
templates quickly turns the template into a kludge. Don't use this approach. A
better approach is to construct the template so that it fits its original
purpose, to make allowances for the occasional specific case and to describe
its purpose clearly in its documentation.

In some situations constructing template explicit specializations may of
course be defensible. Two specializations for const and non-const
pointers to characters might be appropriate for our add function
template.  Here's how they are
constructed:
    
    o  Start with the keyword template.
    o  Next, an empty set of angle brackets is written. This indicates to
the compiler that there must be an existing template whose prototype
matches the one we're about to define. If we err and there is no such template
        
then the compiler reports an error like:
            error: template-id `add<char*>' for `char* add(char* const&, char*
           const&)' does not match any template declaration

o  Now the function's head is defined. It must match the prototype of
the initial function template or the form of a template explicit instantiation
declaration (see section 21.5.1) if its specialized type cannot be
determined from the function's arguments. It must specify the correct
returntype, function name, maybe explicit template type arguments, as well as
the function's parameter list.
    o  Finally the function's body is defined, providing the special
implementation that is required for the specialization.

Here are two explicit specializations for the function template add,
expecting char * and char const * arguments:
            template <> char *add<char *>(char *const &p1,
                                        char *const &p2)
    {
        std::string str(p1);
        str += p2;
        return strcpy(new char[str.length() + 1], str.c_str());
    }

    template <> char const *add<char const *>(char const *const &p1,
                                        char const *const &p2)
    {
        static std::string str;
        str = p1;
        str += p2;
        return str.c_str();
    }

Template explicit specializations are normally included in the file
containing the other function template's implementations.


21.9.2: Declaring specializations

        Template explicit specializations can be declared in the usual way. I.e.,
by replacing its body with a semicolon.

When declaring a template explicit specialization the pair of
 angle brackets following the template keyword are essential. If
omitted, we would have constructed a
        template instantiation declaration.
    The compiler would silently process it, at the expense of a somewhat
longer compilation time.

When declaring a template explicit specialization (or when using an
instantiation declaration) the
        
    explicit specification of the template type parameters can be omitted if
the compiler is able to deduce these types from the function's arguments.  As
this is the case with the char (const) * specializations, they could also
be declared as follows:
            template <> char *add(char *const &p1, char *const &p2);
    template <> char const *add(char const *const &p1,
                                char const *const &p2);

If in addition template <> would be omitted the function declaration would
no longer be a function template declaration but an ordinary function
declaration. This is not an error: function templates and ordinary
(non-template) functions may mutually overload each other. Ordinary functions
are not as restrictive as function templates with respect to allowed type
conversions. This could be a reason to overload a template with an ordinary
function every once in a while.

A function template explicit specialization is not just another overloaded
         version of
the function template. Whereas an overloaded version may define a completely
different set of template parameters, a specialization must use the same set
of template parameters as its non-specialized variant. The compiler uses the
specialization in situations where the actual template arguments match the
types defined by the specialization (following the rule that the most
specialized set of parameters matching a set of arguments will be
used). For different sets of parameters  overloaded versions of functions (or
function templates) must be used.


21.9.3: Complications when using the insertion operator

        Now that we've covered explicit specializations and overloading let's
consider what happens when a class defines a std::string conversion
operator (cf. section 11.3).

A conversion operator is guaranteed to be used as an rvalue. This means that
objects of a class defining a string conversion operator can be assigned
to, e.g., string objects. But when trying to insert objects defining
string conversion operators into streams then the compiler complains that
we're attempting to insert an inappropriate type into an ostream.

On the other hand, when this class defines an int conversion operator
insertion is performed flawlessly.

The reason for this distinction is that operator<< is defined as a plain
(free) function when inserting a basic type (like int) but it is defined
as a function template when inserting a string. Hence, when trying to
insert an object of our class defining a string conversion operator the
compiler visits all overloaded versions of insertion operators inserting into
ostream objects.

Since no basic type conversion is available the basic type insertion operators
can't be used. Since the available conversions for template arguments do not
allow the compiler to look for conversion operators our class defining the
string conversion operator cannot be inserted into an ostream.

If it should be possible to insert objects of such a class into ostream
objects the class must define its own overloaded insertion operator (in
addition to the string conversion operator that was required to use the
class's objects as rvalue in string assignments).


21.10: Static assertions

The 
            static_assert(constant expression, error message)

utility is available to allow assertions to be made from inside template
definitions. Here are two examples of its use:
            static_assert(BUFSIZE1 == BUFSIZE2,
                                "BUFSIZE1 and BUFSIZE2 must be equal");

    template <typename Type1, typename Type2>
    void rawswap(Type1 &type1, Type2 &type2)
    {
        static_assert(sizeof(Type1) == sizeof(Type2),
                        "rawswap: Type1 and Type2 must have equal sizes");
        // ...
    }

The first example shows how to avoid yet another preprocessor directive
        
 (in this case the #error directive).

The second example shows how static_assert can be used to ensure that
a template operates under the right condition(s).

The string defined in static_assert's second argument is displayed and
compilation stops if the condition specified in static_assert's first
argument is false.

Like the #error preprocessor directive static_assert is a
compile-time matter that doesn't have any effect on the run-time
efficiency of the code in which it is used.


21.11: Numeric limits

The header file <climits> defines constants for various types, e.g.,
INT_MAX defines the maximum value that can be stored in an int.

The disadvantage of the limits defined in climits is that they are fixed
limits. Let's assume you write a function template that receives an argument
of a certain type. E.g,
            template<typename Type>
    Type operation(Type &&type);

Assume this function should return the largest negative value for Type
if type is a negative value and the largest positive value if type is
a positive value. However, 0 should be returned if the type is not an integral
value.

How to proceed?

Since the constants in climits can only be used if the type to use is
already known, the only approach seems to be to create function template
specializations for the various integral types, like:
            template<>
    int operation<int>(int &&type)
    {
        return type < 0 ? INT_MIN : INT_MAX;
    }

The facilities provided by numeric_limits provide an alternative. To use
these facilities the header file <limits> header file must be included.

The class template numeric_limits offers various members answering all
kinds of questions that could be asked of numeric types. Before introducing
these members, let's have a look at how we could implement the operation
function template as just one single function template:
            template<typename Type>
    Type operation(Type &&type)
    {
        return
            not numeric_limits<Type>::is_integer ? 0 :
            type < 0 ? numeric_limits<Type>::min() :
                       numeric_limits<Type>::max();
    }

Now operation can be used for all the language's primitive types.

Here is an overview of the facilities offered by numeric_limits. Note
that the member functions defined by numeric_limits return constexpr
values. A member `member' defined by numeric_limits for type Type
can be used as follows:
            numeric_limits<Type>::member    // data members
    numeric_limits<Type>::member()  // member functions


    o Type denorm_min():

        if available for Type: its minimum positive denormalized value;
        otherwise it returns numeric_limits<Type>::min().

o int digits:

        the number of non-sign bits used by Type values, or (floating
        point types) the number of digits in the mantissa are returned.

o int digits10:

        the number of digits that are required to represent a Type value
        without  changing it.

o constexpr Type epsilon():

        The difference for Type between the smallest value exceeding 1 and
        1 itself.

o float_denorm_style has_denorm:

        
        
        denormalized floating point value representations use a variable
        number of exponent bits. The has_denorm member returns information
        about denormalized values for type Type:
        
        o denorm_absent: Type does not allow denormalized values;
        o denorm_indeterminate: Type may or may not use denormalized
            values; the compiler cannot determine this at compile-time;
        o denorm_present: Type uses denormalized values;

o bool has_denorm_loss:

        true if a loss of accuracy was detected as a result of using
        denormalization (rather than being an inexact result).

o bool has_infinity:

        true if Type has a representation for positive infinity.

o bool has_quiet_NaN:

        true if Type has a representation for a non-signaling
        `Not-a-Number' value.

o bool has_signaling_NaN:

        true if Type has a representation for a signaling
        `Not-a-Number' value.

o constexpr Type infinity():

        if available for Type: its positive infinity value.

o bool is_bounded:

        true if Type contains a finite set of values.

o bool is_exact:

        true if Type uses an exact representation.

o bool is_iec559:

        true if Type uses the IEC-559 (IEEE-754) standard. Such
        types always return true for has_infinity, has_quiet_NaN and
        has_signaling_NaN, while infinity(), quiet_NaN() and
        signaling_NaN() return non-zero values.

o bool is_integer:

        true if Type is an integral type.

o bool is_modulo:

        true if Type is a `modulo' type. Values of modulo types can
        always be added, but the addition may `wrap around' producing a
        smaller Type result than either of the addition's two operands.

o bool is_signed:

        true if Type is signed.

o bool is_specialized:

        true for specializations of Type.

o constexpr Type lowest():

        Type's lowest finite value representable by Type: 
        no other finite value smaller than the value returned by lowest
        exists. This value equals the value returned by min except for
        floating-point types. 

o constexpr Type max():

        Type's maximum value.

o constexpr Type min():

        Type's minimum value. For denormalized floating point types the
        minimum positive normalized value.

o int max_exponent:

        maximum positive integral value for the exponent of a floating point
        type Type producing a valid Type value.

o int max_exponent10:

        maximum integral value for the exponent that can be used for base 10,
        producing a valid Type value.

o int min_exponent:

        minimum negative integral value for the exponent of a floating point
        type Type producing a valid Type value.

o int min_exponent10:

        minimum negative integral value for the exponent that can be used for
        base 10, producing a valid Type value.

o constexpr Type quiet_NaN():

        if available for Type: its a non-signaling `Not-a-Number' value.

o int radix:

        if Type is an integral type: base of the representation; if
        Type is a floating point type: the base of the exponent of the
        representation.

o constexpr Type round_error():

        the maximum rounding error for Type.

o float_round_style round_syle:

        the rounding style used by Type. It has one of the following
        enum float_round_style
                 values:
        
        o round_toward_zero: values are rounded towards zero;
        o round_to_nearest: values are rounded to the nearest representable
            value;
        o round_toward_infinity:values are rounded towards infinity;
        o round_toward_neg_infinity: if it rounds towards negative
            infinity; 
        o round_indeterminate: if the rounding style is indeterminable at
            compile-time.

o constexpr Type signaling_NaN():

        if available for Type: its a signaling `Not-a-Number' value.

o bool tinyness_before:

        true if Type allows tinyness to be detected before rounding.

o bool traps:

        true if Type implements trapping.


21.12: Polymorphous wrappers for function objects

In C++ pointers to (member) functions have fairly strict rvalues. They can
only point to functions matching their types.  This becomes a problem when
defining templates where the type of a function pointer may depend on the
template's parameters.

To solve this problem 
        
        
    polymorphous (function object) wrappers can be used.  Polymorphous
wrappers refer to function pointers, member functions or function objects, as
long as their parameters match in type and number.

Before using polymorphic function wrappers the <functional> header file
must be included.

Polymorphic function wrappers are made available through the
std::function  class template. Its template argument
is the prototype of the function to create a wrapper for. Here is an example
of the definition of a polymorphic function wrapper that can be used to point
to a function expecting two int values and returning an int:
            std::function<int (int, int)> ptr2fun;

Here, the template's parameter is int (int, int), indicating a
function expecting two int arguments, and returning and int. Other
prototypes return other, matching, function wrappers.

Such a function wrapper can now be used to point to any function the wrapper
was created for. E.g., `plus<int> add' creates a functor defining an
int operator()(int, int) function call member. As this qualifies as a
function having prototype int (int, int), our ptr2fun may point to
add:
            ptr2fun = add;

If ptr2fun does not yet point to a function (e.g., it is merely
defined) and an attempt is made to call a function through it a
        `std::bad_function_call'
    exception is thrown. Also, a polymorphic function wrapper that hasn't been
assigned to a function's address represents the value false in logical
expressions (as if it had been a pointer having value zero):
            std::function<int(int)> ptr2int;

    if (not ptr2int)
        cout << "ptr2int is not yet pointing to a function\n";

Polymorphous function wrappers can also be used to refer to functions,
functors or other polymorphous function wrappers having prototypes for which
standard conversions exist for either parameters or return values. E.g.,
            bool predicate(long long value);

    void demo()
    {
        std::function<int(int)> ptr2int;

        ptr2int = predicate;    // OK, convertible param. and return type

        struct Local
        {
            short operator()(char ch);
        };

        Local object;

        std::function<short(char)> ptr2char(object);

        ptr2int = object;       // OK, object is a functor whose function
                                //  operator has a convertible param. and
                                // return type.
        ptr2int = ptr2char;     // OK, now using a polym. funct. wrapper
    }


21.13: Compiling template definitions and instantiations

    Consider this definition of the add function template:
            template <typename Container, typename Type>
    Type add(Container const &container, Type init)
    {
        return std::accumulate(container.begin(), container.end(), init);
    }

Here std::accumulate is called using container's begin and
end members.

The calls container.begin() and container.end() are said to
        
    depend on template type parameters. The compiler, not having seen
container's interface, cannot check whether container actually
has members begin and end returning input iterators.

On the other hand, std::accumulate itself is independent of any
template type parameter. Its arguments depend on template parameters, but
the function call itself isn't. Statements in a template's body that are
independent of template type parameters are said not to depend on template
type parameters.

When the compiler encounters a template definition, it verifies the
syntactic correctness of all statements not depending on template
parameters. I.e., it must have seen all class definitions, all type
definitions, all function declarations etc. that are used in those statements.
If the compiler hasn't seen the required definitions and declarations then it
will reject the template's definition. Therefore, when submitting the above
template to the compiler the numeric header file must first have been
included as this header file declares std::accumulate.

With statements depending on template parameters the compiler cannot
perform those extensive syntactic checks. It has no way to verify the
existence of a member begin for the as yet unspecified type
Container. In these cases the compiler performs superficial checks,
assuming that the required members, operators and types eventually become
available.

The location in the program's source where the template is instantiated is
          called
its point of instantiation. At the point of instantiation the compiler
deduces the actual types of the template's parameters. At that point it checks
the syntactic correctness of the template's statements that depend on template
type parameters. This implies that the compiler must have seen the required
declarations only at the point of instantiation.  As a
        rule of thumb,
    you should make sure that all required declarations (usually: header
files) have been read by the compiler at every point of instantiation of the
template. For the template's definition itself a more relaxed requirement can
be formulated. When the definition is read only the declarations required for
statements not depending on the template's type parameters must have been
provided.

On the other hand: at the point of instantiation the compiler must
also have access to the template's definition. Therefore, template
definitions are commonly provided in header files: if a template must be
instantiated in some source file then the template definition, provided in a
header file (e.g., template.h) is included by the source file, so the
compiler can verify whether the template can be instantiated for the actually
used types (the same holds true for class templates, covered in the next
chapter). Here is a short example:
    
    o  The function template add is defined in add.h:
            template <typename Type>
    Type add(Type const &fst, Type const &snd)
    {
        return fst + snd;
    }
        
    o  To use it fun.h includes add.h:
            #include <iostream>
    #include "add.h"
    vouid fun(size_t lhs, size_t rhs)
    {
        std::cout << add(lhs, rhs) << '\n';
    }
        

Function template definitions can be defined as inline functions
by prefixing their definitions with the keyword inline. E.g,
            template <typename Type>
    inline Type add(Type const &fst, Type const &snd)
    {
        return fst + snd;
    }
       
    As always, inline should only be used for short, one-liner function
definitions. 


21.14: The function selection mechanism

When the compiler encounters a function call, it must decide which function to
call when overloaded functions are available. Earlier we've encountered
principles like `the most specific function is selected'. This is a
fairly intuitive description of the compiler's function selection mechanism.
In this section we'll have a closer look at this mechanism.

Assume we ask the compiler to compile the following main function:
            int main()
    {
        process(3, 3);
    }

Furthermore assume that the compiler has encountered the following
function declarations when it's about to compile main:
            template <typename T>
    void process(T &t1, int i);                 // 1

    template <typename T1, typename T2>
    void process(T1 const &t1, T2 const &t2);   // 2

    template <typename T>
    void process(T const &t, double d);         // 3

    template <typename T>
    void process(T const &t, int i);            // 4

    template <>
    void process<int, int>(int i1, int i2);     // 5

    void process(int i1, int i2);               // 6

    void process(int i, double d);              // 7

    void process(double d, int i);              // 8

    void process(double d1, double d2);         // 9

    void process(std::string s, int i)          // 10

    int add(int, int);                          // 11

The compiler, having read main's statement, must now decide which
function must actually be called. It proceeds as follows:
    
    o  First, a set of
  candidate functions is constructed. This set
contains all functions that:
        
        o  are visible at the point of the call;
        o  have the same names as the called function.

As function 11 has a different name, it is removed from the set. The
compiler is left with a set of 10 candidate functions.

o  Second, the set of
  viable functions is constructed. Viable functions
are functions for which type conversions exist that can be applied so as to
match the types of the function's parameters to the types of the actual
arguments.

This implies that at least the number of arguments must match the number of
parameters of the viable functions. Function 10's first argument is a
string. As a string cannot be initialized by an int value no
appropriate conversion exists and function 10 is removed from the list of
candidate functions.  double parameters can be retained. Standard
conversions do exists for ints to doubles, so all functions having
ordinary double parameters can be retained. Therefore, the set of viable
functions consists of functions 1 through 9.

At this point the compiler tries to determine the types of the template
type parameters. This step is outlined in the following subsection.


21.14.1: Determining the template type parameters

    Having determined the set of candidate functions and from that set the set of
viable functions the compiler must now determine the actual types of the
template type parameters.

It may use any of the three standard template parameter transformation
procedures (cf. section 21.4) when trying to match actual types to
template type parameters. In this process it concludes that no type can be
determined for the T in function 1's T &t1 parameter as the argument
3 is a constant int value. Thus function 1 is removed from the list of
viable functions. The compiler is now confronted with the following set of
potentially instantiated function templates and ordinary functions:
            void process(T1 [= int] const &t1, T2 [= int] const &t2);   // 2
    void process(T [= int] const &t, double d);                 // 3
    void process(T [= int] const &t, int i);                    // 4
    void process<int, int>(int i1, int i2);                     // 5
    void process(int i1, int i2);                               // 6
    void process(int i, double d);                              // 7
    void process(double d, int i);                              // 8
    void process(double d1, double d2);                         // 9

The compiler associates a direct match count value to each of the
viable functions. The direct match count counts the number of arguments that
can be matched to function parameters without an (automatic) type
conversion. E.g., for function 2 this count equals 2, for function 7 it is 1
and for function 9 it is 0. The functions are now (decrementally) sorted by
their direct match count values:
                                                                     match
                                                             count
    void process(T1 [= int] const &t1, T2 [= int] const &t2);  2 // 2
    void process(T [= int] const &t, int i);                   2 // 4
    void process<int, int>(int i1, int i2);                    2 // 5
    void process(int i1, int i2);                              2 // 6
    void process(T [= int] const &t, double d);                1 // 3
    void process(int i, double d);                             1 // 7
    void process(double d, int i);                             1 // 8
    void process(double d1, double d2);                        0 // 9

If there is no draw for the top value the corresponding function is
selected and the function selection process is completed.

When multiple functions appear at the top the compiler verifies that no
ambiguity has been encountered. An ambiguity is encountered if the
sequences of parameters for which type conversions were (not) required
differ. As an example consider functions 3 and 8. Using D for `direct match'
and C for `conversion' the arguments match function 3 as D,C and function 8 as
C,D. Assuming that 2, 4, 5 and 6 were not available, then the compiler would
have reported an ambiguity as the sequences of argument/parameter matching
procedures differ for functions 3 and 8. The same difference is encountered
comparing functions 7 and 8, but no such difference is encountered comparing
functions 3 and 7.

At this point there is a draw for the top value and the compiler
proceeds with the subset of associated functions (functions 2, 4, 5 and
6). With each of these functions an `ordinary parameter count' is associated
counting the number of non-template parameters of the functions. The functions
are decrementally sorted by this count, resulting in:
                                                                 ordin. param.
                                                             count
    void process(int i1, int i2);                              2 // 6
    void process(T [= int] const &t, int i);                   1 // 4
    void process(T1 [= int] const &t1, T2 [= int] const &t2);  0 // 2
    void process<int, int>(int i1, int i2);                    0 // 5

Now there is no draw for the top value. The corresponding function
(process(int, int), function 6) is selected and the function selection
process is completed. Function 6 is used in main's function call
statement.

Had function 6 not been defined, function 4 would have been used. Assuming
that neither function 4 nor function 6 had been defined, the selection
process would continue with functions 2 and 5:
                                                                 ordin. param.
                                                             count
    void process(T1 [= int] const &t1, T2 [= int] const &t2);  0 // 2
    void process<int, int>(int i1, int i2);                    0 // 5

In this situation a draw is encountered once again and the selection
process continues. A `type of function' value is associated with each of the
functions having the highest ordinary parameter count and these functions are
decrementally sorted by their type of function values. Value 2 is associated
to ordinary functions, value 1 to template explicit specializations and value
0 to plain function templates.

If there is no draw for the top value the corresponding function is
selected and the function selection process is completed. If there is a draw
the compiler reports an ambiguity and cannot determine which function to
call.  Assuming only functions 2 and 5 existed then this selection step
would have resulted in the following ordering:
                                                                   function
                                                             type
    void process<int, int>(int i1, int i2);                    1 // 5
    void process(T1 [= int] const &t1, T2 [= int] const &t2);  0 // 2

Function 5, the template explicit specialization, would have been
selected.
    ------------------------------------------------------------------Insert Figure 28(The function template selection mechanism)about here (file: functiontemplates/selection)------------------------------------------------------------------
 Here is a summary of the function template selection mechanism (cf. figure
Figure 28):
    
    o  The set of candidate functions is constructed: identical names;
    o  The set of viable functions is constructed: correct number of
        parameters and available type conversions;
    o  Template type determination, dropping templates whose type parameters
        cannot be determined;
    o  Decrementally sort the functions by their direct match count
        values. If there is no draw for the top value the associated function
        is selected, completing the selection process.
    o  Inspect the functions associated with the top value for ambiguities
        in automatic type conversion sequences. If different sequences are
        encountered report an ambiguity and terminate the selection process.
    o  Decrementally sort the functions associated with the top value by
        their ordinary parameter count values.  If there is no draw for the
        top value the associated function is selected, completing the
        selection process.
    o  Decrementally sort the functions associated with the top value by
        their function type values using 2 for ordinary functions, 1 for
        template explicit specializations and 0 for plain function templates.
        If there is no draw for the top value the associated function is
        selected, completing the selection process.
    o  Report an ambiguity and terminate the selection process.


21.15: SFINAE: Substitution Failure Is Not An Error

Consider the following struct definition:
            struct Int
    {
        using type = int;
    };

Although at this point it may seem strange to embed a using declaration in a
struct, in chapter 23 we will encounter situations where this
is actually very useful.  It allows us to define a variable of a type that is
required by the template. E.g., (ignore the use of typename in the
following function parameter list, but see section 22.2.1 for
details):
            template <typename Type>
    void func(typename Type::type value)
    {
    }

When calling func(10) Int has to be specified explicitly since
there may be many structs that define type: the compiler needs some
assistance. The correct call is func<Int>(10). Now that it's clear that
Int is meant, and the compiler correctly deduces that value is an
int.

But templates may be overloaded and our next definition is:
            template <typename Type>
    void func(Type value)
    {}

Now, to call this function we specify func<int>(10) and
again this flawlessly compiles.

But as we've seen in the previous section when the compiler determines
which template to instantiate it creates a list of viable functions and
selects the function to instantiate by matching the parameter types of viable
functions with the provided actual argument types. To do so it has to
determine the types of the parameters and herein lies a problem.

When evaluating Type = int the compiler encounters the prototypes
func(int::type) (first template definition) and func(int) (second
template definition). But there is no int::type, and so in a way this
generates an error. The error results from matching the provided template type
argument with the types used in the various template definitions.

A type-problem caused by substituting a type in a template definition is,
however, not considered an error, but merely an indication that that
particular type cannot be used in that particular template. The template is
therefore removed from the list of candidate functions.

This principle is known as substitution failure is not an error
(SFINAE) and it is often used by the compiler to select not only a
simple overloaded function (as shown here) but also to choose among available
template specializations (see also sections 23.6.1 and 23.9.3).


21.16: Conditional function definitions using `if constexpr'

In addition to the common if (cond) selection statement the if
constexpr (cond) syntax is supported by the language. Although it can be used
in all situations where a standard if selection statement are used, its
specific use is encountered inside function templates: if constexpr allows
the compiler to (conditionally) instantiate elements of a template function,
depending on the compile-time evaluation of the if constexpr's (cond)
clause.

Here is an example:
         1: void positive();
     2: void negative();
     3: 
     4: template <int value>
     5: void fun()
     6: {
     7:     if constexpr (value > 0)
     8:         positive();
     9:     else if constexpr (value < 0)
    10:         negative();
    11: }
    12: 
    13: int main()
    14: {
    15:     fun<4>();
    16: }


    o  At lines 7 and 9 if constexpr statements start. Since value
        is a template non-type parameter its value is compile-time available,
        and so are the values of the condition sections.
    o  In line 15 fun<4>() is called: the condition in line 7 is
        therefore true, and the condition in line 9 is false.
    o  The compiler therefore instantiates fun<4>() this way:
            void fun<4>()
    {
        positive();
    }

Note that the if constexpr statements themselves do not result in
executable code: it is used by the compiler to select which part (or
parts) it should instantiate. In this case only positive, which must be
available before the program's linking phase can properly complete.


21.17: Summary of the template declaration syntax

In this section the basic syntactic constructions for declaring templates
are summarized. When defining templates, the terminating
semicolon should be replaced by a function body.

Not every template declaration may be converted into a template definition. If
a definition may be provided it is explicitly mentioned.
    
    o  A plain template declaration (a definition may be provided):
    template <typename Type1, typename Type2>
void function(Type1 const &t1, Type2 const &t2);

o  A template instantiation declaration (no definition may be provided):
    template
void function<int, double>(int const &t1, double const &t2);

o  A template using explicit types (no definition may be provided):
    void (*fp)(double, double) = function<double, double>;
void (*fp)(int, int) = function<int, int>;

o  A template explicit specialization (a definition may be provided):
    template <>
void function<char *, char *>(char *const &t1, char *const &t2);

o  A template declaration declaring friend function templates within
         class templates (covered
in section 22.10, no definition may be provided):
    friend void function<Type1, Type2>(parameters);


21.18: Variables as templates (template variables)

In addition to function templates and class templates (cf. chapter
22) C++ supports variable templates. Variable templates
might come in handy when defining (function or class) templates defining
variables of types specified by template type parameters.

A variable template starts with a familiar template header, followed by
the definition of the variable itself. The template header specifies a type,
for which a default type may also be specified. E.g.,
            template<typename T = long double>
    constexpr T pi = T(3.1415926535897932385);

To use this variable a type must be specified, and as long as the initialized
value can be converted to the specified type the conversion is silently 
performed by the compiler:
            cout << pi<> << ' ' << pi<int>;

At the second insertion the long double initialization value is
converted to int, and 3 is displayed.

Specializations are also supported. E.g., to show the text `pi' a
specialization for a char const * type can be defined:
            template<>
    constexpr char const *pi<char const *> = "pi";

With this specialization we can do cout << pi<char const *> to show
pi.



Chapter 22: Class Templates

Templates can not only be constructed for functions but also for complete
classes. Consider constructing a class template when a class should be able
to handle different types of data. Class templates are frequently used in
C++: chapter 12 discusses data structures like vector,
stack and queue, that are implemented as class templates. With class
templates, the algorithms and the data on which the algorithms operate are
completely separated from each other. To use a particular data structure in
combination with a particular data type only the data type needs to be
specified when defining or declaring a class template object (as in
stack<int> iStack).

In this chapter constructing and using class templates is discussed. In a
sense, class templates compete with object oriented programming (cf. chapter
14), that uses a mechanism resembling that of
templates. Polymorphism allows the programmer to postpone the implementation
of algorithms by deriving classes from base classes in which algorithms are
only partially implemented. The actual definition and processing of the data
upon which the algorithms operate may be postponed until derived classes are
defined. Likewise, templates allow the programmer to postpone the
specification of the data upon which the algorithms operate. This is most
clearly seen with abstract containers, which completely specify the algorithms
and at the same time leave the data type on which the algorithms operate
completely unspecified. 

There exists an intriguing correspondence between the kind of polymorphism
we've encountered in chapter 14 and certain uses of class
templates.  In their book C++ Coding Standards (Addison-Wesley, 2005)
        
    Sutter and Alexandrescu refer to
        static polymorphism and
         dynamic polymorphism.
    Dynamic polymorphism is what we use when overriding virtual members.
Using vtables the function that is actually called depends on the type of
object a (base) class pointer points at. Static polymorphism is
encountered in the context of templates, and is discussed and compared to
dynamic polymorphism in section 22.12.

Generally, class templates are easier to use than polymorphism. It is
certainly easier to write stack<int> istack to create a stack of ints
than to derive a new class Istack: public stack and to implement all
necessary member functions defining a similar stack of ints using object
oriented programming. On the other hand, for each different type for which an
object of a class template is defined another, possibly complete class must be
reinstantiated.  This is not required in the context of object oriented
programming where derived classes use, rather than copy, the functions
that are already available in their base classes (but see also section
22.11).

Previously we've already used class templates. Objects like vector<int> vi
and vector<string> vs are commonly used. The data types for which these
templates are defined and instantiated are an inherent part of such container
types. It is stressed that it is the combination of a class template type
and its template parameter(s), rather than the mere class template's type
that defines or generates a
  type. So vector<int> is a type as is
vector<string>. Such types could very well be specified through
using-declarations: 
            using IntVector =  std::vector<int>;
    using StringVector = std::vector<std::string>;

    IntVector vi;
    StringVector vs;
        

Like function templates, class templates, both the class interface and the
implementations of the class member functions, must be available at their
points of instantiation. Therefore the interfaces and implementations of class
templates are commonly provided in a header file, which is then included by
source files using those classes (see also section 21.13).


22.0.1: Template Argument Deduction

    One important difference between function templates and class templates is
that the template arguments of function templates can normally be deduced by
the compiler, whereas template arguments of class templates may have to be 
specified by the user. That's not always required, but since the template
types of class templates are an inherent part of the class template's type it
must be completely clear to the compiler what the class template's types are.

So, in case we have a 
        template <typename T>
    void fun(T const &param);

we can do
        vector<int> vi;
    fun(vi)

and the compiler deduces T == vector<int>. 

On the other hand,  if we have
        template <typename T>
    struct Fun
    {
        T d_data;
        Fun();
    };

we cannot do
        
    Fun fun;

since Fun is not a type, and the compiler cannot deduce what the intended
type is.

Sometimes the compiler can deduce the intended types. Consider this:
    
    vector vi{ 1, 2, 3, 4 };

In this case the compiler deduces int from the provided values. The
compiler is smart enought to select the most general type, as in this example,
where double is deduced:
    
    vector vi{ 1, 2.5 };

The compiler is doing its utmost to deduce types when they're not
explicitly specified, and it will stay on the safe side. So the vector is a
vector<int> in the first example and a vector<double> in the second. 

Although it's nice that the compiler is willing and able to deduce types in
many situations it's at the same time a potential source of confusion. In the
first example only non-negative values are used, so why not define a
vector<unsigned> or a vector<size_t>? In the second example the
compiler deduces vector<double>, but vector<float> could very well
also have been used. 

To avoid confusion, as a rule of thumb, it might be a good idea to specify
the types of class templates when defining class type objects. It requires
hardly any effort and it completely clarifies what you want and mean. So
prefer
    
    vector<int>     vi{ 1, 2, 3 };
    vector<double>  vd{ 1, 2.5 };

over the definitions implicitly using types.


22.0.1.1: Simple Definitions

        Here are some examples of how simple definitions where the compiler deduces
template arguments can (and cannot) be specified. 

Starting point:
        template <class ...Types>           // any set of types
    class Deduce
    {
        public:
            Deduce(Types ...params);    // constructors
            void fun();                 // a member function
    };

Some definitions:
                                            // deduce:     full definition:
                                    // --------------------------------
    Deduce first{1};                // 1: int   -> Deduce<int> first{1} 
    Deduce second;                  // no Types -> Deduce<> second;  
    Deduce &&ref = Deduce<int>{ 1 };// int      -> Deduce<int> &&ref

    template <class Type>
    Deduce third{ static_cast<Type *>(0) };

The template third is a recipe for constructing Deduce
objects from a type that's specified for third. The pointer's type simply 
is a pointer to the specified type (so, specifying third<int> implies an
int *). Now that the type of third's argument is available (i.e., an
int *) the compiler deduces that third{0} is a Deduce<int *>.

This Deduce<int *> object could, e.g., be used to initialize a named
Deduce<int *> object:
            auto x = third<int>;        // x's full definition: Deduce<int *> x{0}

Deduce's member functions can be used by anonymous and named objects:
            x.fun();                    // OK: fun called by named object
    third<int>.fun();           // OK: fun called by anonymous object

Here are some examples that won't compile:
            extern Deduce object;       // not an object definition
    Deduce *pointer = 0;        // any set of types could be intended
    Deduce function();          // same.

When defining objects, either using function-type or curly brace definitions
template argument deduction is realized as follows:
    
    o  First, a list of available constructors is formed. The list contains
all available ordinary constructors and constructor templates (i.e,
constructors defined as member templates);
    o  For each element of that list a parallel imaginary function
is formed by the compiler (forming function templates for constructor
templates, and ordinary functions for ordinary constructors);
    o  The return types of these imaginary functions are the class types of
the constructors, using the template parameters of the original class
template. For example, for the Deduce::Deduce constructor the imaginary
function 
            template <class ...Types>
    Deduce<Types ...> imaginary(Types ...params);

is formed.
    o  Next, ordinary argument deduction and overload resolution is applied
to the set of imaginary functions. If this results in a best match for 
one of these imaginary functions then that function's class type (or
specialization) will be used. Otherwise the program is ill-formed.

Let's apply this process to the class Deduce. The set of imaginary
functions matching Deduce looks like this:
                                                // already encountered: matching
    template <class ...Types>           // Deduce(Types ...params)
    Deduce<Types ...> imaginary(Types ...params);

                                        // the copy constructor: matching
    template <class ...Types>           // Deduce(Deduce<Types ...> const &)
    Deduce<Types ...> imaginary(Deduce<Types ...> const &other);
                                    
                                        // the move constructor, matching
    template <class ...Types>           // Deduce(Deduce<Types ...> &&)
    Deduce<Types ...> imaginary(Deduce<Types ...> &&tmp);

For the construction Deduce first{1} the first imaginary function wins
the overload contest, resulting in the template argument deduction int for
class ...Types, and hence Deduce<int> first{1} is defined.

Note that when a class template is nested under a class template, the nested
class template's name depends on the outer class type. The outer class then
provides the name qualifier for the inner class template. In such cases
template argument deduction is used for the nested class, but (as it is not
used for name qualifiers) is not used for the outer class. Here is an example:
adding a nested class template to Outer:

    template <class OuterType>
    class Outer
    {
        public:
            template <class InnerType>
            struct Inner
            {
                Inner(OuterType);
                Inner(OuterType, InnerType);
                template <typename ExtraType>
                Inner(ExtraType, InnerType);
            };
    };
    // defining:
    Outer<int>::Inner inner{2.0, 1};

In this case, the compiler uses these imaginary functions:
        template <typename InnerType>
    Outer<int>::Inner<InnerType>            // copy constructor
        imaginary(Outer<int>::Inner<InnerType> const &);

    template <typename InnerType>       
    Outer<int>::Inner<InnerType>            // move constructor
        imaginary(Outer<int>::Inner<InnerType> &&);

    template <typename InnerType>           // first declared constructor
    Outer<int>::Inner<InnerType> imaginary(int);

    template <typename InnerType>           // second declared constructor
    Outer<int>::Inner<InnerType> imaginary(int, InnerType);

    template <typename InnerType>           // third declared constructor
    template <typename ExtraType>       
    Outer<int>::Inner<InnerType> imaginary(ExtraType, InnerType);

Template argument deduction for calling  imaginary(2.0, 1) results in
double for its first argument and int for its second. Overload
resolution then favors the last imaginary function, and so ExtraType:
double and InnerType: int. Consequently, 
        Outer<int>::Inner inner{ 2.0, 1 };

is defined as:
        Outer<int>::Inner<int> Inner{ 2.0, 1 };


22.0.1.2: Explicit Conversions

        Have a look at the following class interface:
            template <class T>
    struct Class
    {
        std::vector<T> d_data;

        struct Iterator
        {
            using type = T;
            bool operator!=(Iterator const &rhs);
            Iterator operator++(int);
            type const &operator*() const;
            // ... 
        };
    
        Class();

        Class(T t);

        Class(Iterator begin, Iterator end);
    
        template <class Tp>
        Class(Tp first, Tp second)
        {}

        Iterator begin();
        Iterator end();
    };

The implementation of the Class constructor expecting two
Class::Iterator arguments would probably be somewhat similar to the
following: 
            template <class T>
    Class<T>::Class(Iterator begin, Iterator end)
    {
        while (begin != end)
            d_data.push_back(*begin++);
    }

where d_data is some container storing T values. A
Class object can now constructed from a pair of Class::Iterators:
            Class<int> source;      
    ...
    Class<int> dest{source.begin(), source.end()};

Here, the simple template argument deduction procedure fails to deduce the
int template argument. This fails:
            Class dest{source.begin(), source.end()};

When attempting to instantiate a Class object by passing it
Class::Iterators the compiler cannot directly deduce from the provided
arguments that a Class<Class::Iterator::type> is to be used: type
isn't directly available. Compare this to Class's third constructor,
where
        Class intObject{12};

allows the compiler to create an imaginary function 
            template <class Type>
    Class <Type> imaginary(Type param)

in which case Type clearly is an int, and so a Class<int>
object is constructed.

When we try to do this for Class(Iterator, Iterator) we get
            template <class Iterator>
    Class<???> imaginary(Iterator, Iterator);

and here Class's template argument isn't directly related to
Iterator: the compiler cannot deduce its type and consequently compilation
fails. 

A similar argument applies to the fourth constructor, which receives two Tp
arguments, which are both independent from Class's template type. 

In cases like these simple type template argument deduction procedure
fails. Still, not everything is lost: explicit conversions, defined as
explicitly specified deduction rules which are added (below) to the
class's interface can be used.

An explicitly specified deduction rule connects a class template constructor
signature to a class template type. It specifies the template arguments for
the class template object that is constructed using the constructor whose
signature is specified. The generic syntactical form of an explicitly
specified deduction rule looks like this:
            class template constructor signature -> resulting class type ;

Let's apply this to Class(Iterator begin, Iterator end). Its signature is
            template <class Iterator>
    Class(Iterator begin, Iterator end)

Requiring that Iterator defines a typename type, we can now formulate
a resulting class type:
            Class<typename Iterator::type>

Now we can combine both in an explicitly specified deduction rule (which is
added as a separate line below Class's interface):
            template <class Iterator>
    Class(Iterator begin, Iterator end) -> Class<typename Iterator::type>;

After adding this deduction rule to Class's interface the following
constructor calls successfully compile:
            Class src{12};      // already OK

    Class dest1{src.begin(), src.end()};
                        // begin() and end() return Class<int>::Iterator
                        // objects. Typename Class<int>::Iterator::type
                        // is defined as int, so Class<int> dest1 is
                        // defined.

    struct Double       // used at the next construction
    {
        using type = double;
        // ... members ...
    };

    Class dest2{Double{}, Double{}};
                        // Here the struct Double defines 
                        // typename double type, so
                        // Class<double> dest2 is defined.

Within classes the compiler uses (as before) the class itself when merely
referring to the class name: when referring to Class in the class
Class, the compiler considers Class to be Class<T>. So the headers
of the declaration and definition of Class's copy constructor look like
this:
            Class(Class const &other);      // declaration

    template <class T>              // definition
    Class<T>::Class(Class const &other)
    { 
        // ... 
    }

Sometimes the default type is not what you want, in which case the required
type must explicitly be specified. Consider what happens if add a
member dup to Class:
        template <typename T>
    template <typename Tp>
    auto Class<T>::dup(Tp first, Tp second)
    {
        return Class{ first, second };  // probably not what you want
    }                                   // (see the following text)

Here, because we're inside Class the compiler deduces that
Class<T> must be returned. But in the previous section we decided that,
when initializing Class from iterators, Class<typename Tp::type>
should be constructed and returned. To accomplish that, the required type is
explicitly specified:
        template <typename T>
    template <typename Tp>
    auto Class<T>::dup(Tp first, Tp second)
    {                               // OK: explicitly specified Class tyoe.
        return Class<typename Tp::type>{ first, second };
    }

As shown in this example, simple (implicit) or explicit deduction rules do not
have to be used: they can be used in many standard situations where
explicitly specifying the class's template arguments appears to be
superfluous. Template argument deduction was added to the language to simplify
object construction when using class templates. But in the end you don't
have to use these deduction rules: it's always still possible to
explicitly specify template arguments.


22.1: Defining class templates

Having covered the construction of function templates, we're now ready for the
next step: constructing class templates. Many useful class templates already
exist. Rather than illustrating the construction of a class template by
looking at an already existing class template the construction of another
potentially useful new class template will be undertaken.

The new class implements a circular queue.  A circular queue has a fixed
number of max_size elements. New elements are inserted at its back and
only its head and tail elements can be accessed. Only the head element can be
removed from a circular queue. Once n elements have been appended the next
element is inserted again at the queue's (physical) first position. The
circular queue allows insertions until it holds max_size elements. As long
as a circular queue contains at least one element elements may be removed from
it. Trying to remove an element from an empty circular queue or to add another
element to a full circular queue results in exceptions being thrown. In
addition to other constructors a circular queue must offer a constructor
initializing its objects for max_size elements. This constructor must make
available the memory for the max_size elements but must not call those
elements default constructors (hinting at the use of the placement new
operator). A circular queue should offer value semantics as well as a move
constructor.

Please note that in the above description the actual data type that is used
for the circular queue is nowhere mentioned. This is a clear indication that
our class could very well be defined as a
 class template. Alternatively, the class could be
defined for some concrete data type which is then abstracted when converting
the class to a class template.

The actual construction of a class template is provided in the next section,
where the class template CirQue (circular queue) is developed.


22.1.1: Constructing the circular queue: CirQue

    The construction of a class template is illustrated in this section. Here,
we'll develop the class template CirQue (circular queue). This class
template has one template type parameter, Data, representing the data type
that is stored in the circular queue. The outline of the interface of this
class template looks like this:
    
            template<typename Data>
    class CirQue
    {
        // member declarations
    };

A class template's definition starts like a function template's
definition:
    
    o  The keyword template, starting a template definition or
declaration.
    o  The angle bracket enclosed list following template: a
list containing one or more comma-separated elements is called the
 template parameter list. Template parameter lists may have multiple
elements, like this:
            typename Type1, typename Type2, typename Type3

When a class template defines multiple template type parameters they are
matched in sequence with the list of template type arguments provided when
defining objects of such a class template. Example:
        template <typename Type1, typename Type2, typename Type3>
class MultiTypes
{
    ...
};

MultiTypes<int, double, std::string> multiType;
    // Type1 is int, Type2 is double, Type3 is std::string

o  Inside the template parameter list we find the formal type name
(Data for CirQue). It is a formal (type) name, like the formal types
used in function template parameter lists.
    o  Following the template header the class interface is defined. It may
use the formal type names defined in the template header as type names.

Once the CirQue class template has been defined it can be used to
create all kinds of circular queues. As one of its constructors expects a
size_t argument defining the maximum number of elements that can be stored
in the circular queue, circular queues could be defined like this:
            CirQue<int> cqi(10);            // max 10 ints
    CirQue<std::string> cqstr(30);  // max 30 strings

As noted in the introductory section of this chapter the combination of
name of the class template and the data type for which it is instantiated
defines a data type. Also note the similarity between defining a
std::vector (of some data type) and a CirQue (of some data type).

Like std::map containers class templates may be defined with multiple
template type parameters.

Back to CirQue. A CirQue must be capable of storing max_size
Data elements. These elements are eventually stored in memory pointed at
by a pointer Data *d_data, initially pointing to raw memory. New elements
are added at the backside of the CirQue. A pointer Data *d_back is
used to point to the location where the next element is going to be
stored. Likewise, Data *d_front points to the location of the CirQue's
first element. Two size_t data members are used to monitor the filling
state of the CirQue: d_size represents the number of elements
currently stored in the CirQue, d_maxSize represents the maximum
number of elements that the CirQue can contain. Thus, the CirQue's
data members are:
        size_t d_size;
    size_t d_maxSize;
    Data *d_data;
    Data *d_front;
    Data *d_back;



22.1.2: Non-type parameters

    Function template parameters are either template type parameters or template
non-type parameters (actually, a third type of template parameter exists, the
template template parameter, which is discussed in chapter
23 (section 23.4)).

Class templates  may also define
non-type parameters. Like the function template non-type parameters they must
be (integral) constants whose values must be known at object instantiation
time.

Different from function template non-type parameters the values of class
template non-type parameters are not deduced by the compiler using
arguments passed to class template members.

Assume we extend our design of the class template CirQue so that it
defines a second (non-type) parameter size_t Size. Our intent is to use
this Size parameter in the constructor defining an array parameter of
Size elements of type Data.

The  CirQue class template now becomes (only showing the relevant
constructor):
            template <typename Data, size_t Size>
    class CirQue
    {
        // ... data members
        public:
            CirQue(Data const (&arr)[Size]);
            ...
    };

    template <typename Data, size_t Size>
    CirQue<Data, Size>::CirQue(Data const (&arr)[Size])
    :
        d_maxSize(Size),
        d_size(0),
        d_data(operator new(Size * sizeof(Data))),
        d_front(d_data),
        d_back(d_data),
    {
        std::copy(arr, arr + Size, back_inserter(*this));
    }

Unfortunately, this setup doesn't satisfy our needs as  the values of
template non-type parameters are not deduced by the compiler. When the
compiler is asked to compile the following main function it reports a
mismatch between the required and actual number of template parameters:
            int main()
    {
        int arr[30];

        CirQue<int> ap{ arr };
    }
    /*
        Error reported by the compiler:

        In function `int main()':
            error: wrong number of template arguments (1, should be 2)
            error: provided for `template<class Data, size_t Size>
                   class CirQue'
    */

Defining Size as a non-type parameter having a default value doesn't
work either. The compiler always uses the default unless its value is
explicitly specified. Reasoning that Size can be 0 unless we need another
value, we might be tempted to specify size_t Size = 0 in the template's
parameter type list.  Doing so we create  a mismatch between the default
value 0 and the actual size of the array arr as defined in the above
main function. The compiler, using the default value, reports:
            In instantiation of `CirQue<int, 0>':
    ...
    error: creating array with size zero (`0')

So, although class templates may use non-type parameters they must always
be specified like type parameters when an object of that class is
defined. Default values can be specified for those non-type parameters causing
the compiler to use the default when the non-type parameter is left
unspecified.

 Default template parameter values
(either type or non-type template parameters) may not be specified when
defining template member functions. In general: function template definitions
(and thus: class template member functions) may not be given default template
(non) type arguments. If default template arguments are to be used for class
template members, they have to be specified by the class interface.

Similar to non-type parameters of function templates default argument
values for non-type class template parameters may only be specified as
constants:
    
    o  Global  variables have constant addresses, which
can be used as arguments for non-type parameters.
    o  Local  and dynamically allocated variables have
addresses that are not known by the compiler when the source file is
compiled. These addresses can therefore not be used as arguments for
non-type parameters.
    o  Lvalue  transformations are allowed: if a
pointer is defined as a non-type parameter, an array name may be specified.
    o   Qualification 
conversions are allowed: a pointer to a non-const object may be used with a
non-type parameter defined as a const pointer.
    o  Promotions  are allowed: a constant of a `narrower'
data type may be used when specifying a default argument for a non-type
parameter of a `wider' type (e.g., a short can be used when an int is
called for, a long when a double is called for).
    o  Integral  conversions are allowed: if a
size_t parameter is specified, an int may be used as well.
    o  Variables cannot be used to specify template non-type parameters as
their values are not constant expressions. Variables defined using the
const modifier, however, may be used as their values never change.

Although our attempts to define a constructor of the class CirQue
accepting an array as its argument has failed so far, we're not yet out of
options. In the next section a method is described that does allow us
to reach our goal.


22.1.3: Member templates

    Our previous attempt to define a template non-type parameter that is
initialized by the compiler to the number of elements of an array failed
because the template's parameters are not implicitly deduced when a
constructor is called. Instead, they must explicitly be specified when an
object of the class template is defined. As the template arguments are
specified just before the template's constructor is called the compiler
doesn't have to deduce anything and it can simply use the explicitly specified
template arguments.

In contrast, when function templates are used, the actual template
parameters are deduced from the arguments used when calling the function. This
opens up an alley leading to the solution of our problem. If the constructor
itself is turned into a function template (having its own template header),
then the compiler will be able to deduce the non-type parameter's value
and there is no need anymore to specify it explicitly using a class template
non-type parameter.

Members (functions or nested classes) of class templates that are themselves
templates are called
        
        
 member templates.

Member templates are defined as any other template, including its own template
header.

When converting our earlier CirQue(Data const (&array)[Size]) constructor
into a member template the class template's Data type parameter can still
be used, but we must provide the member template with a non-type parameter of
its own. Its declaration in the (partially shown) class interface looks like
this:
            template <typename Data>
    class CirQue
    {
        public:
            template <size_t Size>
            explicit CirQue(Data const (&arr)[Size]);
    };

Its implementation becomes:
        template <typename Data>
    template <size_t Size>
    CirQue<Data>::CirQue(Data const (&arr)[Size])
    :
        d_size(0),
        d_maxSize(Size),
        d_data(static_cast<Data *>(operator new(sizeof(arr)))),
        d_front(d_data),
        d_back(d_data)
    {
        std::copy(arr, arr + Size, std::back_inserter(*this));
    }

    The implementation uses the STL's copy algorithm and a
back_inserter adapter to insert the array's elements into the
CirQue. To use the back_inserter CirQue must provide two (public)
using-declarations (cf. section 18.2.2):
        using value_type = Data;
    using const_reference = value_type const &;


Member templates have the following characteristics:
    
    o  Two template
            
     headers must be used: the class template's template
header is specified first followed by the member template's template header;
    o  Normal access rules apply: the member template can be
used by programs to construct an CirQue object of a given data
type. As usual for class templates, the data type must be specified when the
object is constructed. To construct a CirQue object from the array
int array[30] we define:
        CirQue<int> object(array);

o  Any member can be defined as a member template, not just a
constructor;
    o  When a template member is implemented below its class interface, the
template class header must precede the function template header of the member
template;
    o  The implementation of the member template must specify its proper
scope.  The member template is defined as a member of the class CirQue,
instantiated for the formal template parameter type Data;
    o  The template parameter names in the declaration and
implementation must be identical;
    o  The member template should be defined inside its proper namespace
environment. The organization of files defining class templates within a
namespace should therefore be:
        namespace SomeName
{
    template <typename Type, ...>   // class template definition
    class ClassName
    {
        ...
    };

    template <typename Type, ...>   // non-inline member definition(s)
    ClassName<Type, ...>::member(...)
    {
        ...
    }
}                                   // namespace closes

A potentially occurring problem remains. Assume that in addition to the
above member template a CirQue<Data>::CirQue(Data const *data) has been
defined. Some (here not further elaborated) protocol could be defined allowing
the constructor to determine the number of elements that should be stored in
the CirQue object. When we now define
            CirQue<int> object{ array };

it is this latter constructor, rather than the member template, that the
compiler will use.

The compiler selects this latter constructor as it is a more specialized
version of a constructor of the class CirQue than the member template
(cf. section 21.14). Problems like these can be solved by
defining the constructor CirQue(Data const *data) into a member template
as well or by defining a constructor using two parameters, the second
parameter defining the number of elements to copy.

When using the former constructor (i.e., the member template) it must define a
template type parameter Data2. Here `Data' cannot be used as template
parameters of a member template may not shadow
     template parameters of its
class. Using Data2 instead of Data takes care of this subtlety. The
following declaration of the constructor CirQue(Data2 const *) could
appear in CirQue's header file:
            template <typename Data>
    class CirQue
    {
        template <typename Data2>
        explicit CirQue(Data2 const *data);
    }

Here is how the two constructors are selected in code defining two
CirQue objects:
            int main()
    {
        int array[30];
        int *iPtr = array;

        CirQue<int> ac{ array };    // calls CirQue(Data const (&arr)[Size])
        CirQue<int> acPtr{ iPtr };  // calls CirQue(Data2 const *)
    }


22.1.4: CirQue's constructors and member functions

        It's time to return to Cirque's design and construction again.

The class CirQue offers various member functions.  Normal design
principles should be adhered to when constructing class template
members. Class template type parameters should preferably be defined as
Type const &, rather than Type, to prevent unnecessary copying of
large data structures. Template class constructors should use member
initializers rather than member assignment within the body of the
constructors. Member function definitions should preferably not be provided
in-class but below the class interface. Since class template member functions
are function templates their  definitions should be provided in the header
file offering the class interface. They may be given the inline
attribute.

CirQue declares several constructors and (public) members (their
definitions are provided as well; all definitions are provided below the class
interface).

Here are the constructors and the destructor:


    o explicit CirQue(size_t maxSize = 0):
Constructor initializing a
        CirQue capable of storing max_size Data elements. As the
constructor's parameter is given a default argument value this constructor can
also be used as a default constructor, allowing us to define, e.g., vectors of
CirQues.  The constructor initializes the Cirque object's d_data
member to a block of raw memory and d_front and d_back are initialized
to d_data. As class template member functions are themselves function
templates their implementations outside of the class template's interface must
start with the class template's template header. Here is the implementation of
the CirQue(size_t) constructor:
            template<typename Data>
    CirQue<Data>::CirQue(size_t maxSize)
    :
        d_size(0),
        d_maxSize(maxSize),
        d_data(
            maxSize == 0 ?
                0
            :
                static_cast<Data *>(
                            operator new(maxSize * sizeof(Data)))
        ),
        d_front(d_data),
        d_back(d_data)
    {}


o CirQue(CirQue<Data> const &other):
The copy constructor has no
special features. It uses a private support member inc to increment
d_back (see below) and placement new to copy the other's Data elements
to the current object. The implementation of the copy constructor is
straightforward:
            template<typename Data>
    CirQue<Data>::CirQue(CirQue<Data> const &other)
    :
        d_size(other.d_size),
        d_maxSize(other.d_maxSize),
        d_data(
            d_maxSize == 0 ?
                0
            :
                static_cast<Data *>(
                    operator new(d_maxSize * sizeof(Data)))
        ),
        d_front(d_data + (other.d_front - other.d_data))
    {
        Data const *src = other.d_front;
        d_back = d_front;
        for (size_t count = 0; count != d_size; ++count)
        {
            new(d_back) Data(*src);
            d_back = inc(d_back);
            if (++src == other.d_data + d_maxSize)
                src = other.d_data;
        }
    }


o CirQue(CirQue<Data> &&tmp):
The move constructor merely
initializes the current object's d_data pointer to 0 and swaps (see the
member swap, below) the temporary object with the current
object. CirQue's destructor inspects d_data and immediately
returns when it's zero. Implementation:
            template<typename Data>
    CirQue<Data>::CirQue(CirQue<Data> &&tmp)
    :
        d_data(0)
    {
        swap(tmp);
    }


o CirQue(Data const (&arr)[Size]):
This constructor is declared
as a member template, providing the Size non-type parameter. It allocates
room for Size data elements and copies arr's content to the newly
allocated memory.Implementation:
            template <typename Data>
    template <size_t Size>
    CirQue<Data>::CirQue(Data const (&arr)[Size])
    :
        d_size(0),
        d_maxSize(Size),
        d_data(static_cast<Data *>(operator new(sizeof(arr)))),
        d_front(d_data),
        d_back(d_data)
    {
        std::copy(arr, arr + Size, std::back_inserter(*this));
    }


o CirQue(Data const *data, size_t size):
This constructor acts
very much like the previous one, but is provided with a pointer to the first
Data element and with a size_t providing the number of elements to
copy. In our current design the member template variant of this constructor is
left out of the design. As the implementation of this constructor is very
similar to that of the previous constructor, it is left as an exercise to the
reader.

o ~CirQue():
The destructor inspects the d_data member. If it is
zero then nothing has been allocated and the destructor immediately
returns. This may occur in two situations: the circular queue contains no
elements or the information was grabbed from a temporary object by some move
operation, setting the temporary's d_data member to zero. Otherwise
d_size elements are destroyed by explicitly calling their destructors
followed by returning the element's raw memory to the common
pool. Implementation:
            template<typename Data>
    CirQue<Data>::~CirQue()
    {
        if (d_data == 0)
            return;
        for (; d_size--; )
        {
            d_front->~Data();
            d_front = inc(d_front);
        }
        operator delete(d_data);
    }


Here are Cirque's members:
    
    o CirQue &operator=(CirQue<Data> const &other):
The copy assignment
        operator has a  standard implementation:
            template<typename Data>
    CirQue<Data> &CirQue<Data>::operator=(CirQue<Data> const &rhs)
    {
        CirQue<Data> tmp(rhs);
        swap(tmp);
        return *this;
    }


o CirQue &operator=(CirQue<Data> &&tmp):
The move assignment
        operator also has a standard implementation. As its implementation
        merely calls swap it is defined as an inline function template:
            template<typename Data>
    inline CirQue<Data> &CirQue<Data>::operator=(CirQue<Data> &&tmp)
    {
        swap(tmp);
        return *this;
    }


o void pop_front():
removes the element pointed at by d_front from
        the CirQue.  Throws an exception if the CirQue is empty. The
        exception is thrown as a CirQue<Data>::EMPTY value, defined by the
        enum CirQue<Data>::Exception (see push_back). The
        implementation is straightforward (explicitly calling the destructor
        of the element that is removed):
           template<typename Data>
    void CirQue<Data>::pop_front()
    {
        if (d_size == 0)
            throw EMPTY;

        d_front->~Data();
        d_front = inc(d_front);
        --d_size;
    }


o void push_back(Data const &object):
adds another element to the
        CirQue.  Throws a CirQue<Data>::FULL exception if the
        CirQue is full. The exceptions that can be thrown by a CirQue
        are defined in its Exception enum:
           enum Exception
    {
        EMPTY,
        FULL
    };

        A copy of object is installed in the CirQue's raw memory using
        placement new and its d_size is incremented.
           template<typename Data>
    void CirQue<Data>::push_back(Data const &object)
    {
        if (d_size == d_maxSize)
            throw FULL;

        new (d_back) Data(object);
        d_back = inc(d_back);
        ++d_size;
    }


o void swap(CirQue<Data> &other):
swaps the current CirQue object
        with another CirQue<Data> object;
           template<typename Data>
    void CirQue<Data>::swap(CirQue<Data> &other)
    {
        static size_t const size = sizeof(CirQue<Data>);

        char tmp[size];
        memcpy(tmp, &other, size);
        memcpy(reinterpret_cast<char *>(&other), this, size);
        memcpy(reinterpret_cast<char *>(this), tmp, size);
    }


The remaining public members all consist of one-liners and were
implemented as inline function templates:
    
    o Data &back():
returns a reference to the element pointed at by
        d_back (undefined result if the CirQue is empty):
           template<typename Data>
    inline Data &CirQue<Data>::back()
    {
        return d_back == d_data ? d_data[d_maxSize - 1] : d_back[-1];
    }


o Data &front():
returns reference to the element pointed at by
        d_front (undefined result if the CirQue is empty);
           template<typename Data>
    inline Data &CirQue<Data>::front()
    {
        return *d_front;
    }


o bool empty() const:
returns true if the CirQue is empty;
           template<typename Data>
    inline bool CirQue<Data>::empty() const
    {
        return d_size == 0;
    }


o bool full() const:
returns true if the CirQue is full;
           template<typename Data>
    inline bool CirQue<Data>::full() const
    {
        return d_size == d_maxSize;
    }


o size_t size() const:
returns the number of elements currently stored
        in the  CirQue;
           template<typename Data>
    inline size_t CirQue<Data>::size() const
    {
        return d_size;
    }


o size_t maxSize() const:
returns the maximum number of elements that
        can be stored in the CirQue;
           template<typename Data>
    inline size_t CirQue<Data>::maxSize() const
    {
        return d_maxSize;
    }


Finally, the class has one private member, inc, returning a
cyclically incremented pointer into CirQue's raw memory:
           template<typename Data>
    Data *CirQue<Data>::inc(Data *ptr)
    {
        ++ptr;
        return ptr == d_data + d_maxSize ? d_data : ptr;
    }



22.1.5: Using CirQue objects

        When objects of a class template are instantiated, only the
definitions of all the template's member functions that are actually used must
have been seen by the compiler.

That characteristic of templates could be refined to the point where each
definition is stored in a separate function template definition file. In that
case only the definitions of the function templates that are actually needed
would have to be included. However, it is hardly ever done that way. Instead,
the usual way to define class templates is to define the interface and to
define the remaining function templates immediately below the class template's
interface (defining some functions inline).

Now that the class CirQue has been defined, it can be used. To use the
class its object must be instantiated for a particular data type. In the
following example it is initialized for data type std::string:
        #include "cirque.h"
#include <iostream>
#include <string>
using namespace std;

int main()
{
    CirQue<string> ci(4);
    ci.push_back("1");
    ci.push_back("2");
    cout << ci.size() << ' ' << ci.front() << ' ' << ci.back() << '\n';

    ci.push_back("3");
    ci.pop_front();
    ci.push_back("4");
    ci.pop_front();
    ci.push_back("5");
    cout << ci.size() << ' ' << ci.front() << ' ' << ci.back() << '\n';

    CirQue<string> copy(ci);
    copy.pop_front();
    cout << copy.size() << ' ' << copy.front() << ' ' << copy.back() << '\n';

    int arr[] = {1, 3, 5, 7, 9};
    CirQue<int> ca(arr);
    cout << ca.size() << ' ' << ca.front() << ' ' << ca.back() << '\n';

//    int *ap = arr;
//    CirQue<int> cap(ap);
}

    This program produces the following output:
            2 1 2
    3 3 5
    2 4 5
    5 1 9


22.1.6: Default class template parameters

    Different from function templates, template parameters of template classes may
be given default argument
         values.
    This holds true for both template type- and template non-type
parameters. If default template arguments were defined and if a class template
object is instantiated without specifying arguments for its template
parameters then the template parameter's defaults are used.

When defining defaults keep in mind that they should be suitable for the
majority of instantiations of the class. E.g., for the class template
CirQue the template's type parameter list could have been altered
by specifying int as its default type:
            template <typename Data = int>

Even though default arguments can be specified, the compiler must still be
informed that object definitions refer to templates. When instantiating class
template objects using default template arguments the type specifications may
be omitted but the angle brackets must be retained. Assuming a default
type for the CirQue class, an object of that class may be defined
as:
            CirQue<> intCirQue(10);

Default template arguments cannot be specified when defining template
members. So, the definition of, e.g., the push_back member must always
begin with the same template specification:
            template <typename Data>

When a class template uses multiple template parameters, all may be given
default values. Like default function arguments, once a default value is used
all remaining template parameters must also use their default values. A
template type specification list may not start with a comma, nor may it
contain multiple consecutive commas.


22.1.7: Declaring class templates

        Class templates may also be declared. This may be useful in
situations where forward class declarations are required. To declare a
        
    class template, simply remove its interface (the part between the curly
braces):
            template <typename Data>
    class CirQue;

Default  template arguments may also
be specified when declaring class templates. However, default template
arguments cannot be specified for both the declaration and the definition of a
class template. As a rule of thumb default template arguments should be
omitted from declarations, as class template declarations are never used
when instantiating objects but are only occasionally used as forward
references. Note that this differs from default parameter value specifications
for member functions in ordinary classes. Such defaults are always specified
when declaring the member functions in the class interface.


22.1.8: Preventing template instantiations

    In C++ templates
    
 are instantiated when the address of a function template or class template
object is taken or when a function template or class template is used. As
described in section 22.1.7 it is possible to (forward) declare a
class template to allow the definition of a pointer or reference to
that template class or to allow it being used as a return type.

In other situations templates are instantiated when they are being used.  If
this happens many times (i.e., in many different source files) then this may
slow down the compilation process considerably. Fortunately, C++ allows
programmers to prevent templates
    
 from being instantiated, using the 
extern template syntax. Example:
            extern template class std::vector<int>;

Having declared the class template it can be used in its translation
unit. E.g., the following function  properly compiles:
        #include <vector>
#include <iostream>
using namespace std;

extern template class vector<int>;

void vectorUser()
{
    vector<int> vi;
    cout << vi.size() << '\n';
}

    But be careful:
    
    o  The declaration by itself does not make the class definition
available. The vector header file still needs to be included to make
the features of the class vector known to the compiler. But due to the
extern template declaration none of the used members will be instantiated
for the current compilation unit;
    o  The compiler assumes (as it always does) that what is declared
has been implemented elsewhere. In this case the compiler encounters an
implicit declaration: the features of the vector class that are actually
used by the above program are not individually declared but they are declared
as a group, using the extern template declaration. This not only holds
true for explicitly used members but hidden members (copy constructors, move
constructors, conversion operators, constructors called during promotions, to
name a few): all are assumed by the compiler to have been instantiated
elsewhere;
    o  Although the above source file compiles, the instantiations
of the templates must be available before the linker can build the final
program. To accomplish this one or more source files may be constructed in
which all required instantiations are made available.

In a stand-alone program one might postpone defining the required members
and wait for the linker to complain about unresolved external
references. These may then be used to create a series of instantiation
declarations which are then linked to the program to satisfy the linker. Not
a very simple task, though, as the declarations must strictly match the way
the members are declared in the class interface. An easier approach is to
define an instantiation source file in which all facilities that
are used by the program are actually instantiated in a function that is never
called by the program. By adding this instantiation function to the source
file containing main we can be sure that all required members are
instantiated as well. Here is an example of how this can be done:
        #include <vector>
#include <iostream>

extern void vectorUser();

int main()
{
    vectorUser();
}

// this part is never called. It is added to make sure all required
// features of declared templates will also be instantiated.

namespace
{
    void instantiator()
    {
        std::vector<int> vi;
        vi.size();
    }
}


o  Last, but certainly not least: a fully matching
        instantiation declaration of a class template (e.g., for
std::vector<int>) looks like this:
            template class std::vector<int>;

Adding this to a source file, however, will instantiate the full
class, i.e., all its members are now instantiated. This may not what you
want, as it might needlessly inflate your final executable.
    o  On the other hand, if it is known that the required template members
have already been instantiated elsewhere, then an extern template
declaration can be used to prevent member instantiations in the current
compilation unit, which may speed up compilation. E.g.,
                // the compiler assumes that required members of
        // vector<int> have already been instantiated elsewhere
extern template class std::vector<int>;

int main()
{
    std::vector<int> vi(5);     // constructor and operator[]
    ++vi[0];                    // are NOT instantiated
}


22.1.9: Template parameter initialization

    Class templates commonly specify their parameter types, which types are then
used to define, e.g., the parameters of their members. But template parameter
types can also, somewhat comparable to the way template function parameter
types are determined, be derived from the actually used template arguments,
which is called template parameter initialization. Template parameter
types which are derived from the actually used template argument values start
with auto as illustrated in the following example (the examples in this
section use in-class implementations for brevity and assume #include
<iostream> and using namespace std):
        template <auto val> struct Demo
    {
        Demo()
        {
            cout << val << '\n';
        }
    };
    
    int main()
    {
        Demo<12> d1;
        Demo<'a'> d2;
    }

This program shows, respectively, 12 (an int) and a (a
char). Those types are determined from the actually used template
parameter arguments, and the argument name(s) can be used by the template
class's members.

Those members are available inside the class template as constexpr
data members. Since they are constexpr data members the compiler must be
able to construct those arguments compile-time. Consequently, when
specifying a template argument using a class object only the object's
const members can be used, and that object's class cannot declare
mutable data members, illustrated by the next example:
        struct Accepted
    {
        int d_val = 10;
        void unchanged() const
        {
            cout << d_val << '\n';
        }
        void changed()              // OK: not used by Demo below
        {
            cout << ++d_val << '\n';
        }
    };
    
    struct Rejected
    {
        mutable int d_val = 10;     // 'Rejected' can't be used by
                                    // Demo because of 'mutable'
        void unchanged() const
        {
            cout << d_val << '\n';
        }
        void changed()              // OK: not used by Demo below
        {
            cout << ++d_val << '\n';
        }
    };
    
    template <auto val> struct Demo
    {
        Demo()
        {
            val.unchanged();
        }
    };
    
    int main()
    {
        Demo<Accepted{}> sa;
        // Demo<Rejected{}> sr;     // won't compile
    }


Assuming the class's template parameter is defined as used above as auto
val then val's type can be obtained with a using declaration. E.g.,
        template <auto val> struct Demo
    {
        using Type = decltype(val);
        ...
    }
    and Type can be used to define such data members or to refer to such
types when specifying member function parameters. Such Type variables
are not by default immutable:
        template <auto val> struct Demo
    {
        using Type = decltype(val);
        Type d_val = val;
        Demo()
        {
            cout << ++d_val << '\n';
        }
    }

Initialized template parameters become (immutable) data members which can be
used in the class template's members. In the next example the class template's
Adder defines an initialized template parameter auto add whose value
is added to the elements of a std::vector. Also, all the initial elements
of the vector are summed, which value is then retrieved by the member
total:
        #include <vector>
    #include <string>
    
    template <auto val> class Adder
    {
        using Type = decltype(val);
    
        Type d_sum = Type();            // initialize d_sum
    
        public:
            Adder()
            {
                cout << "adding " << val << "\n\n";
            }
                                        // append 'val' to vect's elements
            void append(std::vector<Type> &vect)
            {
                for (auto &elem: vect)
                {
                    d_sum += elem;
                    elem += val;
                }
            }
                                        // show vect's elements
            void show(std::vector<Type> const &vect) const
            {
                for (auto &elem: vect)
                    cout << elem << '\n';
                cout.put('\n');
            }
    
            Type const &total() const   // show vect elements' original sum
            {
                return d_sum;
            }
    };
    
    template <auto val>
    void process(vector<decltype(val)> &vect)
    {
        Adder<val> adder;
        adder.show(vect);
        adder.append(vect);
        adder.show(vect);
        cout << "original sum: " << adder.total() << "\n\n";
    }
    
    int main()
    {
        vector<int> vi{1, 2, 3, 4};
        process<3>(vi);                     // add 3 to the vi's elements
    
        vector<double> vd{1.1, 2.2, 3.3, 4.4};
        process<3.5>(vd);                   // add 3.5 to the vd's elements
    }



22.1.10: Generic lambda expressions

    Like template parameter initializations generic lambda expressions
 may use auto to define their parameters. When used, a
lambda expression is instantiated by looking at the actual types of
arguments. Since auto is generic, different parameters defined using
auto can be instantiated to different types. Here is an example (assuming
all required headers and namespace declaration were specified):
         1: int main()
     2: {
     3:     auto lambda = [](auto lhs, auto rhs)
     4:                 {
     5:                     return lhs + rhs;
     6:                 };
     7: 
     8:     vector<int> values {1, 2, 3, 4, 5};
     9:     vector<string> text {"a", "b", "c", "d", "e"};
    10: 
    11:     cout <<
    12:         accumulate(values.begin(), values.end(), 0, lambda) << '\n' <<
    13:         accumulate(text.begin(), text.end(), string{}, lambda) << '\n';
    14: }

    The generic lambda function is defined in lines 3 through 6, and is
assigned to the lambda identifier. Then, lambda is passed to
accumulate in lines 12 and 13. In line 12 it is instantiated to add
int values, in line 13 to add std::string values: the same lambda
is instantiated to two completely different functors, which are only locally
available in main.

Generic lambda expressions are in fact class templates. To illustrate: the
above example of a generic lambda expression could also have been implemented
using the following class template:
            struct Lambda
    {
        template <typename LHS, typename RHS>
        auto operator()(LHS const &lhs, RHS const &rhs) const 
        {
            return lhs + rhs;
        }
    };
    auto lambda = Lambda{};

This identity implies that using auto in the lambda expression's
parameter list obeys the rules of template argument deduction (cf. section
21.4), which are somewhat different from the way auto normally
operates.

Another extension is the way generic lambda expressions capture outer scope
variables.  Previously variables could only be captured by value or by
reference. As a consequence an outer scope variable of a type that only
supports move construction could not be passed by value to a lambda
function. This restriction was dropped, allowing variables to be initialized
from arbitrary expressions. This not only allows move-initialization of
variables in the lambda introducer, but with generic lambdas variables may
also be initialized if they do not have a correspondingly named variable in
the lambda expression's outer scope. In this case initializer expressions can
be used as follows:
            auto fun = [value = 1] 
               {
                   return value;
               };

This lambda function (of course) returns 1: the declared capture deduces
the type from the initializer expression as if auto had been used.

To use move-initialization std::move should be used. E.g., a
unique_ptr only supports move-assignment. Here it is used to return one of
its values via a lambda function:
            std::unique_ptr<int> ptr(new int(10));
    auto fun = [value = std::move(ptr)] 
               {
                   return *value;
               };

In generic lambda expressions the keyword auto indicates that the compiler
determines which types to use when the lambda function is instantiated. A
generic lambda expression therefore is a class template, even though it
doesn't look like one. As an example, the following lambda expression defines
a generic class template, which can be used as shown:
            char const *target = "hello";

    auto lambda =         
       [target](auto const &str)
       {
           return str == target;
       };
       
    vector<string> vs{stringVectorFactory()};

    find_if(vs.begin(), vs.end(), lambda);

This works fine, but if you define lambda this way then you should be
prepared for complex error messages if the types of the derefenced iterators
and lambda's (silently assumed) str type don't match.

Here is a little program illustrating how generic lambda expressions can be
used in other generic lambda expressions: class templates could also have been
used. In lines 1 through 9 a generic lambda expression accumulate is
defined, defining a second paramater which is used as a function: its argument
therefore should be usable as a function. A functor definitely is, an the
second generic lambda expression lambda, defined in lines 11 through 14
provides it. In line 21 accumulate and lambda are instantiated so that
they operate on (a vector of) ints, in line 22 they are instantiated for
processing (a vector of) strings:
         1: auto accumulate(auto const &container, auto function)
     2: {
     3:     auto accu = decltype(container[0]){};
     4: 
     5:     for (auto &value: container)
     6:         accu = function(accu, value);
     7: 
     8:     return accu;
     9: }
    10: 
    11: auto lambda = [](auto lhs, auto rhs)
    12:             {
    13:                 return lhs + rhs;
    14:             };
    15: 
    16: int main()
    17: {
    18:     vector<int> values  = {1, 2, 3, 4, 5};
    19:     vector<string> text = {"a", "b", "c", "d", "e"};
    20: 
    21:     cout << accumulate(values,  lambda) << '\n' <<
    22:             accumulate(text,    lambda) << '\n';
    23: }

In some situations generic lambdas are a bit too generic, resulting in verbose
implementations which are not required by templates in general. Consider a
generic lambda that specifies an auto &it parameter, but in addition it
should specify a parameter value of type ValueType that should be
defined by it's class. Such a parameter requires the use of a decltype
(and maybe also the use of std::decay_t) to retrieve it's actual
type. Inside the lambda's body a using declaration can be specified to
make the type available, but that again requires a verbose specification using
std::decay_t and decltype. Here is an example:
            auto generic = 
        [](auto &it, 
           typename std::decay_t<decltype(it)>::ValueType value)
        {
            using Type = std::decay_t<decltype(it)>;
            typename Type::ValueType val2{ value };
            Type::staticMember();
        };

To avoid this kind of verbosity generic lambda functions can also be defined
like ordinary templates, in which case the template header immediately follows
the lambda-introducer. Using this alternative form the definition of the
generic generic lambda simply and straightforwardly becomes:
            auto generic = 
        []<typename Type>(Type &it, typename Type::ValueType value)
        {
            typename Type::ValueType val2{ value };
            Type::staticMember();
        };


22.2: Static data members

    When static members are defined in class templates, they
        
    are defined for every new type for which the class template is
instantiated. As they are static
members, there will only be one member per type for which the class template
is instantiated. For example, in a class like:
            template <typename Type>
    class TheClass
    {
        static int s_objectCounter;
    };

There will be one TheClass<Type>::objectCounter for each different
Type specification. The following object definitions result in the
instantiation of just one single static variable, shared among the two
objects:
            TheClass<int> theClassOne;
    TheClass<int> theClassTwo;

Mentioning static members in interfaces does not mean these members are
actually defined. They are only declared and must be defined
separately.  With static members of class templates this is no different. The
definitions of static members
        
    are usually provided immediately following (i.e., below) the template
class interface. For example, the static member
s_objectCounter's definition, positioned just below its class interface,
looks like this:
            template <typename Type>                    // definition, following
    int TheClass<Type>::s_objectCounter = 0;    // the interface

Here s_objectCounter is an int and is thus independent of the
template type parameter Type. Multiple instantiations of
s_objectCounter for identical Types cause no problem, as the linker
will remove all but one instantiation from the final executable (cf. section
21.5).


    In list-like constructions, where a pointer to objects of the class
itself is required, the template type parameter Type must be used when
defining the static variable. Example:
            template <typename Type>
    class TheClass
    {
        static TheClass *s_objectPtr;
    };

    template <typename Type>
    TheClass<Type> *TheClass<Type>::s_objectPtr = 0;

    As usual, the definition can be read from the variable name back to the
beginning of the definition: s_objectPtr of the class TheClass<Type>
is a pointer to an object of TheClass<Type>.

When a static variable of a template's type parameter's type is defined,
it should of course not be given the initial value 0. The default constructor
(e.g., Type()) is usually more appropriate. Example:
            template <typename Type>                    // s_type's definition
    Type TheClass<Type>::s_type = Type();


22.2.1: Extended use of the keyword `typename'

Until now the keyword typename has been used to indicate a template type
parameter. However, it is also used to
    
 disambiguate code inside templates. Consider the following function template:
            template <typename Type>
    Type function(Type t)
    {
        Type::Ambiguous *ptr;

        return t + *ptr;
    }

When this code is processed by the compiler, it complains with an -at
first sight puzzling- error message like:
            4: error: 'ptr' was not declared in this scope

The error message is puzzling as it was the programmer's intention to
declare a pointer to a type Ambiguous defined within the class template
Type. But the compiler, confronted with Type::Ambiguous may interpret
the statement in various ways. Clearly it cannot inspect Type itself
trying to uncover Type's true nature as Type is a template
type. Because of this Type's actual definition isn't available yet.

The compiler is confronted with two possibilities: either
Type::Ambiguous is a static member of the as yet mysterious template
Type, or it is a subtype of Type. As the standard
specifies that the compiler must assume the former, the statement
            Type::Ambiguous *ptr;

is interpreted as a multiplication of the static member
Type::Ambiguous and the (now undeclared) entity ptr. The reason for
the error message should now be clear: in this context ptr is unknown.

To disambiguate code in which an identifier refers to a
        
        
        
 subtype of a template type parameter the keyword typename must be
used. Accordingly, the above code is altered into:
            template <typename Type>
    Type function(Type t)
    {
        typename Type::Ambiguous *ptr;

        return t + *ptr;
    }

Classes fairly often define subtypes. When such subtypes appear inside
template definitions as subtypes of template type parameters the typename
keyword must be used to identify them as subtypes. Example: a class
template Handler defines a typename Container as its template type
parameter. It also defines a data member storing the iterator returned by the
container's begin member. In addition Handler offers a constructor
accepting any container supporting a begin member.  Handler's class
interface could then look like this:
            template <typename Container>
    class Handler
    {
        Container::const_iterator d_it;

        public:
            Handler(Container const &container)
            :
                d_it(container.begin())
            {}
    };

What did  we have in mind when designing this class?
    
    o  The typename Container represents any container supporting
iterators.
    o  The container presumably supports a member begin. The
initialization d_it(container.begin()) clearly depends on the
template's type parameter, so it's only checked for basic syntactic
correctness.
    o  Likewise, the container presumably supports a subtype
const_iterator, defined in the class Container.

The final consideration is an indication that typename is required. If
this is omitted and a Handler is instantiated the compiler produces a
peculiar compilation error:
            #include "handler.h"
    #include <vector>
    using namespace std;

    int main()
    {
        vector<int> vi;
        Handler<vector<int> > ph{ vi };
    }
    /*
        Reported error:

    handler.h:4: error: syntax error before `;' token
    */

Clearly the line
            Container::const_iterator d_it;

in the class Handler causes a problem. It is interpreted by the
compiler as a static member instead of a subtype. The problem is
solved  using typename:
            template <typename Container>
    class Handler
    {
        typename Container::const_iterator d_it;
        ...
    };

An interesting illustration that the compiler indeed assumes X::a to
be a member a of the class X is provided by the error message we get
when we try to compile main using the following implementation of
Handler's constructor:
            Handler(Container const &container)
    :
        d_it(container.begin())
    {
        size_t x = Container::ios_end;
    }
    /*
        Reported error:

        error: `ios_end' is not a member of type `std::vector<int,
                std::allocator<int> >'
    */

Now consider what happens if the function template introduced at the
beginning of this section doesn't return a Type value, but a
Type::Ambiguous value. Again, a subtype of a template type is referred to,
and typename must be used:
            template <typename Type>
    typename Type::Ambiguous function(Type t)
    {
        return t.ambiguous();
    }

Typenames can be embedded in using declarations. As is often the case,
this
        
reduces the complexities of declarations and definitions appearing
elsewhere. In the next example the type Iterator is defined as a subtype
of the template type Container. Iterator may now be used without
requiring the use of the keyword typename:
            template <typename Container>
    class Handler
    {
        using Iterator = Container::const_iterator;

        Iterator d_it;
        ...
    };


22.3: Specializing class templates for deviating types

The class CirQue can be used for many different types. Their common
characteristic is that they can simply be pointed at by the class's d_data
member. But this is not always as simple as it looks. What if Data turns
out to be a vector<int>? For such data types the vanilla CirQue
implementation cannot be used and a specialization could be considered. When
considering a specialization one should also consider inheritance. Often a
class derived from the class template accepting the incompatible data
structure as its argument but otherwise equal to the original class template
can easily be designed. The developmental advantage of inheritance over
specialization is clear: the inherited class inherits the members of its base
class while the specialization inherits nothing. All members defined by the
original class template must be implemented again by the class template's
specialization.

The specialization considered here is a true specialization in that the
data members and representation used by the specialization greatly differ from
the original CirQue class template. Therefore all members defined by the
orginal class template must be modified to fit the specialization's data
organization.

Like function template specializations class template specializations
start with a template header that may or may not have an empty template
parameter list. If the template parameters are directly specialized by the
specialization it remains empty (e.g., CirQue's template type parameter
Data is specialized for char * data). But the template parameter list
may show typename Data when specializing for a vector<Data>, i.e., a
vector storing any type of data. This leads to the following principle:
     A template specialization is recognized
by the template argument list following a function or class template's
name and not by an empty template parameter list. Class template
specializations may have non-empty template parameter lists. If so, a
partial class template specialization is defined.

A completely specialized class has the following characteristics:
    
    o  The class template specialization must be provided after the generic
class template definition. As it is a specialization the compiler
must first have seen the original class template;
    o  The completely specialized class template's template parameter list
is empty;
    o  All of the class's template parameters are given explicit type names
or (for the non-type parameters) explicit values. These explicitations are
provided in a template parameter specification list (surrounded by angle
brackets) that is inserted immediately after the specialized template's class
name;
    o  All members of the specialized class template use specialized types
and values where original template parameters are used in the original
template definition;
    o  All original template's members (maybe with the exception of some
constructors) should be redefined by the specialization. If a member is
left out of the specialization, it cannot be used for a specialized class
template object;
    o  The specialization may define additional members (but maybe shouldn't
as it breaks the one-to-one correspondence between the original and
specialized class template);
    o  Member functions of specialized class templates may be declared by
the specializing class and implemented below their class interface. If their
implementations follow the class interface they may not begin with a
template <> header, but must immediately start with the member function's
header.


22.3.1: Example of a class specialization

    Here is an example of a completely specialized CirQue class, specialized
for a vector<int>. All members of the specialized class are declared, but
only non-trivial implementations of its members are provided. The specialized
class uses a copy of the vector passed to the constructor and implements a
circular queue using its vector data member:

#ifndef INCLUDED_CIRQUEVECTOR_H_
#define INCLUDED_CIRQUEVECTOR_H_

#include <vector>
#include "cirque.h"

template<>
class CirQue<std::vector<int>>
{
    using IntVect =  std::vector<int>;

    IntVect d_data;
    size_t d_size;

    using iterator = IntVect::iterator;
    iterator d_front;
    iterator d_back;

    public:
        using value_type = int;
        using const_reference = value_type const &;

        enum Exception
        {
            EMPTY,
            FULL
        };

        CirQue();
        CirQue(IntVect const &iv);
        CirQue(CirQue<IntVect> const &other);

        CirQue &operator=(CirQue<IntVect> const &other);

        int &back();
        int &front();
        bool empty() const;
        bool full() const;
        size_t maxSize() const;
        size_t size() const;
        void pop_front();
        void push_back(int const &object);
        void swap(CirQue<IntVect> &other);

    private:
        iterator inc(iterator const &iter);
};

CirQue<std::vector<int>>::CirQue()
:
    d_size(0)
{}
CirQue<std::vector<int>>::CirQue(IntVect const &iv)
:
    d_data(iv),
    d_size(iv.size()),
    d_front(d_data.begin()),
    d_back(d_data.begin())
{}
CirQue<std::vector<int>>::CirQue(CirQue<IntVect> const &other)
:
    d_data(other.d_data),
    d_size(other.d_size),
    d_front(d_data.begin() + (other.d_front - other.d_data.begin())),
    d_back(d_data.begin() + (other.d_back - other.d_data.begin()))
{}
CirQue<std::vector<int>> &CirQue<std::vector<int>>::operator=(
                                        CirQue<IntVect> const &rhs)
{
    CirQue<IntVect> tmp(rhs);
    swap(tmp);
}
void CirQue<std::vector<int>>::swap(CirQue<IntVect> &other)
{
    char tmp[sizeof(CirQue<IntVect>)];
    memcpy(tmp, &other, sizeof(CirQue<IntVect>));
    memcpy(reinterpret_cast<char *>(&other), this, sizeof(CirQue<IntVect>));
    memcpy(reinterpret_cast<char *>(this), tmp, sizeof(CirQue<IntVect>));
}
void CirQue<std::vector<int>>::pop_front()
{
    if (d_size == 0)
        throw EMPTY;

    d_front = inc(d_front);
    --d_size;
}
void CirQue<std::vector<int>>::push_back(int const &object)
{
    if (d_size == d_data.size())
        throw FULL;

    *d_back = object;
    d_back = inc(d_back);
    ++d_size;
}
inline int &CirQue<std::vector<int>>::back()
{
    return d_back == d_data.begin() ? d_data.back() : d_back[-1];
}
inline int &CirQue<std::vector<int>>::front()
{
    return *d_front;
}
CirQue<std::vector<int>>::iterator CirQue<std::vector<int>>::inc(
    CirQue<std::vector<int>>::iterator const &iter)
{
    iterator tmp(iter + 1);
    tmp =  tmp == d_data.end() ? d_data.begin() : tmp;
    return tmp;
}

#endif


The next example shows how to use the specialized CirQue class:
    static int iv[] = {1, 2, 3, 4, 5};

int main()
{
    vector<int> vi(iv, iv + 5);
    CirQue<vector<int>> ci(vi);

    cout << ci.size() << ' ' << ci.front() << ' ' << ci.back() << '\n';
    ci.pop_front();
    ci.pop_front();

    CirQue<vector<int>> cp;

    cp = ci;
    cout << cp.size() << ' ' << cp.front() << ' ' << cp.back() << '\n';
    cp.push_back(6);
    cout << cp.size() << ' ' << cp.front() << ' ' << cp.back() << '\n';
}

/*
    Displays:
        5 1 5
        3 3 5
        4 3 6
*/



22.4: Partial specializations

In the previous section class template specializations were introduced.  In
this section we'll introduce a variant of this specialization, both in
number and type of template parameters that are specialized.  Partial
specializations may be defined for class templates having
        
        
    multiple template parameters. 

Function templates cannot be partially specialized: there is no need for
        
    that, as a `partially specialized function template' merely is a function
template that is tailored to particular types of some of its parameters. Since
function templates can be overloaded, `partially specializing' a function
template simply means that overloads have to be defined for those specialized
parameter types.

With partial specializations a subset (any subset) of template type parameters
are given specific values. It is also possible to use a class template partial
specialization when the intent is to specialize the class template, but to
parameterize the data type that is processed by the specialization.

To start our discussion with an example of the latter use of a partial class
template specialization consider the class CirQue<vector<int>>
developed in the previous section. When designing CirQue<vector<int>>
you may have asked yourself how many specializations you'd have to
implement. One for vector<int>, one for vector<string>, one for
vector<double>?  As long as the data types handled by the vector used
by the class CirQue<vector<...>> behaves like an int (i.e., is a
value-type of class) the answer is: zero. Instead of defining full
specializations for each new data type the data type itself can be
parameterized, resulting in a partial specialization:
            template <typename Data>
    class CirQue<std::vector<Data>>
    {
        ...
    };

The above class is a specialization as a template argument list is
appended to the CirQue class name. But as the class template itself has a
non-empty template parameter list it is in fact recognized as a partial
specialization. There is one characteristic that distinguishes the
implementation (subsequent to the class template's interface) of a class
template member function of a partial specialization from the implementation
of a member function of a full specialization. Implementations of partially
specialized class template member functions receive a template header. No
template headers are used when implementing fully specialized class template
members.

Implementing the partial specialization for CirQue is not difficult and is
left as an exercise for the reader (hints: simply change int into Data
in the CirQue<vector<int>> specialization of the previous section).
Remember to prefix the type iterator by typename (as in using
iterator = DataVect::iterator) (as discussed in section 22.2.1).

In the next subsections we'll concentrate on specializing class template
non-type template parameters. These partial specializations are now
illustrated using some simple concepts defined in matrix algebra, a branch of
linear algebra.


22.4.1: Intermezzo: some simple matrix algebraic concepts

    In this section some simple matrix algebraic terms are introduced. These terms
are used in the next sections to illustrate and discuss partial
specializations of class templates.  Readers proficient in matrix algebra may
skip this section without loss of continuity.

A matrix is commonly thought of as a table of some rows and columns,
filled with numbers. Immediately we recognize an opening for using templates:
the numbers might be plain double values, but they could also be complex
numbers, for which our complex container (cf. section 12.4) might
prove useful. Our class template is therefore provided with a DataType
template type parameter. It is  specified when a
matrix is constructed. Some simple matrices using double values, are:
            1   0   0           An identity matrix,
    0   1   0           (a 3 x 3 matrix).
    0   0   1

    1.2  0    0    0    A rectangular matrix,
    0.5  3.5  18  23    (a 2 x 4 matrix).

    1   2   4   8       A matrix of one row
                        (a 1 x 4 matrix), also known as a
                        `row vector' of 4 elements.
                        (column vectors are analogously defined)

Various operations are defined on matrices. They may, for example be
added, subtracted or multiplied. Here we will not focus on these
operations. Rather, we concentrate on some simple operations: computing
marginals and sums.

Marginals are the sums of row elements or the sums of column elements of a
matrix. These two kinds of marginals are also known as, respectively,
row marginals and
column marginals.


    o  Row marginals are obtained by adding, for each row,
all the row's elements and putting these (Rows) sums in corresponding
elements of a (column) vector of Rows elements.
    o  Column marginals are
obtained by adding, for each column, all the column's elements and putting
these (Columns) sums in corresponding elements of a (row) vector of
Columns elements.
    o  The sum of all elements of a matrix can of course be computed as the
sum of the elements of one of its marginals.

The following example shows a matrix, its marginals, and the sum of its
values:
            -------------------------------------
                                row
                matrix          marginals
                ---------
                1   2   3        6
                4   5   6       15
                ---------
    column      5   7   9       21  (sum)
    marginals
    -------------------------------------


22.4.2: The Matrix class template

    We'll start out by introducing a class template defining a matrix. Having
defined this class template we'll continue with defining several
specializations.

Since matrices consist of well defined numbers of rows and columns (the
dimensions of the matrix), that normally do not change when matrices are
used, we might consider specifying their values as template non-type
parameters. The DataType = double will be used in the majority of
cases. Therefore, double can be selected as the template's default type
argument. Since it's a sensible default, the DataType template type
parameter is used last in the template type parameter list.

Our template class Matrix begins its life as:
            template <size_t Rows, size_t Columns, typename DataType = double>
    class Matrix
    ...

What do we want our class template to offer?
    
    o  It needs a place to store its matrix elements. This can be defined as
an array of `Rows' rows each containing `Columns' elements of type
DataType. It can be an array, rather than a pointer, since the matrix'
dimensions are known a priori. Since a vector of Columns elements (a
row of the matrix), as well as a vector of Row elements (a column
of the matrix) is often used, the class could specify using-declarations to
represent them. The class interface's initial section thus contains:
                using MatrixRow = Matrix<1, Columns, DataType>;
        using MatrixColumn = Matrix<Rows, 1, DataType>;

        MatrixRow d_matrix[Rows];

    o  It should offer constructors: a default constructor and (e.g.,) a
constructor initializing the matrix from a stream. A copy or move constructor
is not required as the class does not use pointers. Likewise, no overloaded
assignment operator or destructor is required. Implementations:
            template <size_t Rows, size_t Columns, typename DataType>
    Matrix<Rows, Columns, DataType>::Matrix()
    {
        std::fill(d_matrix, d_matrix + Rows, MatrixRow());
    }
    template <size_t Rows, size_t Columns, typename DataType>
    Matrix<Rows, Columns, DataType>::Matrix(std::istream &str)
    {
        for (size_t row = 0; row < Rows; row++)
            for (size_t col = 0; col < Columns; col++)
                str >> d_matrix[row][col];
    }

    o  The class's operator[] member (and its const variant) only
handles the first index, returning a reference to a complete
MatrixRow. How elements in a MatrixRow can be retrieved is shortly
covered. To keep the example simple, no array bound check has been
implemented:
            template <size_t Rows, size_t Columns, typename DataType>
    Matrix<1, Columns, DataType>
    &Matrix<Rows, Columns, DataType>::operator[](size_t idx)
    {
        return d_matrix[idx];
    }

    o  Now we get to the interesting parts: computing marginals and the sum
of all elements in a Matrix. We'll define the type MatrixColumn as the
type containing the row marginals of a matrix, and the type MatrixRow as
the type containing the column marginals of a matrix.

There is also the sum of all the elements of a matrix. This sum of all the
elements of a matrix is a number that itself can be thought of as a 1 x 1
matrix.

Marginals can be considered as special forms of matrices. To represent these
marginals we can construct partial specializations defining the class
templates MatrixRow and MatrixColumn objects; and we construct a
partial specialization handling 1 x 1 matrices.  These partial
specializations are used to compute marginals and the sum of all the elements
of a matrix.

Before concentrating on these partial specializations themselves we'll use
them here to implement the members computing the marginals and the sum of all
elements of a matrix:
            template <size_t Rows, size_t Columns, typename DataType>
    Matrix<1, Columns, DataType>
    Matrix<Rows, Columns, DataType>::columnMarginals() const
    {
        return MatrixRow(*this);
    }

    template <size_t Rows, size_t Columns, typename DataType>
    Matrix<Rows, 1, DataType>
    Matrix<Rows, Columns, DataType>::rowMarginals() const
    {
        return MatrixColumn(*this);
    }

    template <size_t Rows, size_t Columns, typename DataType>
    DataType Matrix<Rows, Columns, DataType>::sum()  const
    {
        return rowMarginals().sum();
    }



22.4.3: The MatrixRow partial specialization

        Class template partial specializations can be defined for any (subset)
        
    of template parameters. They can be defined for template type parameters
and for template non-type parameters alike. Our first partial specialization
defines a row of a generic Matrix, mainly (but not only) used for the
construction of column marginals. Here is how such a partial specialization is
designed:
    
    o  The partial specialization starts with a template header defining all
template parameters that are not specialized in the partial
specialization. This template header cannot specify any defaults (like
DataType = double) since defaults were already specified by the generic
class template definition.  The specialization must follow the definition
of the generic class template's definition, or the compiler complains that it
doesn't know what class is being specialized. Following the template header,
the class's interface starts. It's a class template (partial) specialization
so the class name must be followed by a template argument list specifying the
template arguments used by the partial specialization. The arguments specify
explicit types or values for some of the template's parameters. Remaining
types are simply copied from the class template partial specialization's
template parameter list. E.g., the MatrixRow specialization specifies 1
for the generic class template's Rows non-type parameter (as we're talking
here about a single row). Both Columns and DataType remain to be
specified. The MatrixRow partial specialization therefore starts as
follows:
            template <size_t Columns, typename DataType>  // no default allowed
    class Matrix<1, Columns, DataType>

    o  A MatrixRow holds the data of a single row. So it needs a
data member storing Columns values of type DataType. Since Columns
is a constant value, the d_row data member can be defined as an array:
                DataType d_column[Columns];

    o  The class template partial specialization's constructors require some
attention. The default constructor is simple. It merely initializes the
MatrixRow's data elements using DataType's default constructor:
            template <size_t Columns, typename DataType>
    Matrix<1, Columns, DataType>::Matrix()
    {
        std::fill(d_column, d_column + Columns, DataType());
    }

    Another constructor is needed initializing a MatrixRow object
with the column marginals of a generic Matrix object. This requires us to
provide the constructor with a non-specialized Matrix parameter.

The rule of thumb here is to define a member template that allows us to
keep the general nature of the parameter. The generic Matrix
template requires three template parameters. Two of these were already
provided by the template specialization. The third parameter is mentioned in
the member template's template header. Since this parameter refers to the
number of rows of the generic matrix it is simply called Rows.

Here then is the implementation of the second constructor, initializing the
MatrixRow's data with the column marginals of a generic Matrix object:
            template <size_t Columns, typename DataType>
    template <size_t Rows>
    Matrix<1, Columns, DataType>::Matrix(
                            Matrix<Rows, Columns, DataType> const &matrix)
    {
        std::fill(d_column, d_column + Columns, DataType());

        for (size_t col = 0; col < Columns; col++)
            for (size_t row = 0; row < Rows; row++)
                d_column[col] += matrix[row][col];
    }

    The constructor's parameter is a reference to a Matrix template using
the additional Row template parameter as well as the template parameters
of the partial specialization.
    o  We don't really require additional members to satisfy our current
needs. To access the data elements of the MatrixRow an overloaded
operator[]() is of course useful. Again, the const variant can be
implemented like the non-const variant. Here is its implementation:
            template <size_t Columns, typename DataType>
    DataType &Matrix<1, Columns, DataType>::operator[](size_t idx)
    {
        return d_column[idx];
    }


Now that we have defined the generic Matrix class and the
partial specialization defining a single row the compiler selects the
row's specialization whenever a Matrix is defined using Row = 1. For
example:
            Matrix<4, 6> matrix;        // generic Matrix template is used
    Matrix<1, 6> row;           // partial specialization is used


22.4.4: The MatrixColumn partial specialization

        The partial specialization for a MatrixColumn is constructed
similarly. Let's present its highlights (the full Matrix class template
definition as well as all its specializations are provided in the
cplusplus.yo.zip archive (that can be obtained from the C++ Annotations' 
Gitlab website) in the
file yo/classtemplates/examples/matrix.h):
    
    o  The class template partial specialization once again starts with a
template header. Now the class interface specifies a fixed value for the
second template parameter of the generic class template. This illustrates that
we can construct partial specializations for every single template parameter;
not just for the first or the last:
            template <size_t Rows, typename DataType>
    class Matrix<Rows, 1, DataType>

    o  Its constructors are implemented completely analogously to the way
the MatrixRow constructors were implemented. Their implementations are
left as an exercise to the reader (and they can be found in matrix.h).
    o  An additional member sum is defined to compute the sum of the
elements of a MatrixColumn vector. It's simply implemented
using the accumulate generic algorithm:
            template <size_t Rows, typename DataType>
    DataType Matrix<Rows, 1, DataType>::sum()
    {
        return std::accumulate(d_row, d_row + Rows, DataType());
    }



22.4.5: The 1x1 matrix: avoid ambiguity

        The reader might wonder what happens if we define the following matrix:
            Matrix<1, 1> cell;

Is this a MatrixRow or a MatrixColumn specialization? The answer
is: neither.

It's ambiguous, precisely because both the columns and the rows could
be used with a (different) template partial specialization. If such a
Matrix is actually required, yet another specialized template must be
designed.

Since this template specialization can be useful to obtain the sum of the
elements of a Matrix, it's covered here as well.
    
    o  This class template partial specialization also needs a template
header, this time only specifying DataType. The class definition specifies
two fixed values: 1 for the number of rows and 1 for the number of columns:
            template <typename DataType>
    class Matrix<1, 1, DataType>

    o  The specialization defines the usual batch of constructors.
Constructors expecting a more generic Matrix type are again implemented as
member templates. For example:
            template <typename DataType>
    template <size_t Rows, size_t Columns>
    Matrix<1, 1, DataType>::Matrix(
                            Matrix<Rows, Columns, DataType> const &matrix)
    :
        d_cell(matrix.rowMarginals().sum())
    {}

    template <typename DataType>
    template <size_t Rows>
    Matrix<1, 1, DataType>::Matrix(Matrix<Rows, 1, DataType> const &matrix)
    :
        d_cell(matrix.sum())
    {}

    o  Since Matrix<1, 1> is basically a wrapper around a DataType
value, we need members to access that latter value. A type conversion
operator might be useful, but we also defined a get member to obtain
the value if the conversion operator isn't used by the compiler (which
happens when the compiler is given a choice, see section
11.3). Here are the accessors (leaving out their const
variants):
            template <typename DataType>
    Matrix<1, 1, DataType>::operator DataType &()
    {
        return d_cell;
    }

    template <typename DataType>
    DataType &Matrix<1, 1, DataType>::get()
    {
        return d_cell;
    }


Finally, the main function shown below illustrates how the Matrix
class template and its partial specializations can be used:
        #include <iostream>
    #include "matrix.h"
    using namespace std;

    int main(int argc, char **argv)
    {
        Matrix<3, 2> matrix(cin);

        Matrix<1, 2> colMargins(matrix);
        cout << "Column marginals:\n";
        cout << colMargins[0] << " " << colMargins[1] << '\n';

        Matrix<3, 1> rowMargins(matrix);
        cout << "Row marginals:\n";
        for (size_t idx = 0; idx < 3; idx++)
            cout << rowMargins[idx] << '\n';

        cout << "Sum total: " << Matrix<1, 1>(matrix) << '\n';
    }
    /*
        Generated output from input: 1 2 3 4 5 6

        Column marginals:
        9 12
        Row marginals:
        3
        7
        11
        Sum total: 21
    */



22.5: Variadic templates

Up to this point we've encountered templates defining a fixed number of
template parameters. However, templates may also defined as
    variadic templates, allowing any number of
arguments to be passed to their instantiations.

Variadic templates are defined for function templates and for class
templates. Variadic templates allow us to specify an arbitrary number of
template arguments of any type.

Variadic templates were added to the language to prevent us from having to
define many overloaded templates and to be able to create type safe
variadic functions.

Although C (and C++) support variadic functions, their use has
always been deprecated in C++ as those functions are notoriously
type-unsafe. Variadic function templates can be used to process objects
that until now couldn't be processed properly by C-style variadic
functions.

Template headers of variadic templates use the phrase
 typename ...Params (Params being a formal
name). A variadic class template Variadic could be declared as follows:
            template<typename ...Params> class Variadic;

Assuming the class template's definition is available then this template
can be instantiated using any number of template arguments. Example:
            class Variadic<
            int,
            std::vector<int>,
            std::map<std::string, std::vector<int>>
    > v1;

The template argument list of a variadic template can also be
empty. Example:
            class Variadic<> empty;

If this is considered undesirable using an empty template argument list
can be prevented by providing one or more fixed parameters. Example:
            template<typename First, typename ...Rest>
    class tuple;

C's function printf is a well-known example of a type-unsafe
function. It is turned into a type-safe function when it is implemented as a
variadic function template. Not only does this turn the function into a
type-safe function but it is also automatically extended to accept any type
that can be defined by C++. Here is a possible declaration of a variadic
function template printcpp:
            template<typename ...Params>
    void printcpp(std::string const &strFormat, Params ...parameters);

The ellipsis (...) used in the declaration serves two purposes:
    
    o  In the template header it is written to the left of a template
        parameter name where it declares a parameter pack. A parameter
        pack allows us to specify any number of template arguments when
        instantiating the template. Parameter packs can be used to
        bind type and non-type template arguments to template parameters.
    o  In a template implementation it appears to the right of the
        template pack's parameter name. In that case it represents a series of
        template arguments that are subsequently matched with a function
        parameter that in turn is provided to the right of the ellipsis. Here
        the ellipsis is known as the unpack operator as it `unpacks' a
        series of arguments in a function's argument list thereby implicitly
        defining its parameters.

C++ offers no syntax to access the individual template arguments
directly. However, the arguments can be visited recursively. An example is
provided in the next section. The number
  of arguments is determined using a
new invocation of the sizeof operator:
            template<typename ...Params>
    struct StructName
    {
        enum: size_t { s_size = sizeof ...(Params) };
    };

    // StructName<int, char>::s_size          -- initialized to 2


22.5.1: Defining and using variadic templates

    The arguments associated with a variadic template parameter are not directly
available to the implementation of a function or class template. We have to
resort to other means to obtain them.

By defining a partial specialization of a variadic template, explicitly
defining an additional template type parameter, we can associate the first
template argument of a parameter pack with this additional (first) type
parameter. The setup of such a variadic function template (e.g., printcpp,
see the previous section) is as follows:
    
    o  The printcpp function receives at least a format
string. Following the format string any number of additional arguments may be
specified.
    o  If there are no arguments trailing the format string then there is no
need to use a function template. An overloaded (non-template) function
is defined to handle this situation.
    o  A variadic function template handles all remaining situations. In
this case there is always at least one  argument trailing the format
string. That argument's type is matched with the variadic template function's
first (ordinary) template type parameter First. The types of any remaining
arguments are bound to the template function's second template parameter,
which is a parameter pack.
    o  The variadic function template processes the argument trailing the
format string. Then it recursively calls itself passing the format string and
the parameter pack to the recursive call
    o  If the recursive call merely receives the format string the
overloaded (non-template) function is called (cf. section
21.14) ending the recursion. Otherwise the parameter pack's
first argument is matched with the recursive call's First parameter. As
this reduces the size of the recursive call's parameter pack the recursion
eventually stops.

The overloaded non-template function prints the remainder of the format
string, en passant checking for any left-over format specifications:
            void printcpp(string const &format)
    {
        size_t left = 0;
        size_t right = 0;

        while (true)
        {
            if ((right = format.find('%', right)) == string::npos)
                break;
            if (format.find("%%", right) != right)
                throw std::runtime_error(
                            "printcpp: missing arguments");
            ++right;
            cout << format.substr(left, right - left);
            left = ++right;
        }
        cout << format.substr(left);
    }

Here is the variadic function template's implementation:
            template<typename First, typename ...Params>
    void printcpp(std::string const &format, First value, Params ...params)
    {
        size_t left = 0;
        size_t right = 0;
        while (true)
        {
            if ((right = format.find('%', right)) == string::npos)      // 1
                throw std::runtime_error("printcpp: too many arguments");

            if (format.find("%%", right) != right)                      // 2
                break;

            ++right;
            cout << format.substr(left, right - left);
            left = ++right;
        }
        cout << format.substr(left, right - left) << value;
        printcpp(format.substr(right + 1), params...);
   }


    o  At 1 the format string is searched for a parameter specification
%. If none is found then the function is called with too many arguments
and it throws an exception;
    o  At 2 it verifies that it has not encountered %%. If only a single
% has been seen the while-loop ends, the format string is inserted
into cout up to the % followed by value, and the recursive call
receives the remaing part of the format string as well as the remaining
parameter pack;
    o  If %% was seen the format string is inserted up to the second
%, which is ignored, and processing of the format string continues beyond
the %%.

Make sure that the overloaded function is at least declared before the
compiler processes the function template's definition or it won't
call the overloaded function printcpp when compiling the function
template.

Different from C's printf function printcpp only recognizes
% and %% as format specifiers. The above implementation does not
recognize, e.g., field widths. Type specifiers like %c and %x are of
course not needed as ostream's insertion operator is aware of the types of
the arguments that are inserted into the ostream.  Extending the format
specifiers so that field widths etc. are recognized by this printcpp
implementation is left as an exercise to the reader. Here is an example
showing how printcpp can be called:
            printcpp("Hello % with %%main%% called with % args"
                                            " and a string showing %\n",
        "world", argc, "A String"s);


22.5.2: Perfect forwarding

    Consider string's member insert. String::insert has several
overloaded implementations. It can be used to insert text (completely or
partially) provided by a string or by a char const * argument; to
insert single characters a specified number of times; iterators can be used to
specify the range of characters to be inserted; etc., etc.. All in all,
string offers as many as five overloaded insert members.

Assume the existence of a class Inserter that is used to insert
information into all kinds of objects. Such a class could have a string
data member into which information can be inserted. Inserter's interface
only partially has to copy string's interface to realize this: only
string::insert's interfaces must be duplicated. The members duplicating
interfaces often contain one statement (calling the appropriate member
function of the object's data member) and are for this reason often
implemented in-line. Such wrapper functions merely forward their
parameters to the matching member functions of the object's data member.

Another example is found in factory functions that also frequently forward
their parameters to the constructors of objects that they return.

C++ simplifies and generalizes forwarding of parameters by
offering perfect forwarding, implemented through rvalue references and
variadic templates. With perfect forwarding the arguments passed to functions
are `perfectly forwarded' to nested functions. Forwarding is called
perfect as the arguments are forwarded in a type-safe way. To use perfect
forwarding nested functions must define parameter lists matching the
forwarding parameters both in types and number.

Perfect forwarding is easily implemented:
    
    o  The forwarding function is defined as a template

        (usually a variadic template, but single argument forwarding is
        also possible. To define and forward a single argument omit the
        ellipsis from the following code);
    o  The forwarding function's parameter list is an
        
        rvalue reference parameter pack (e.g., Params &&...params);
    o std::forward is used to forward the forwarding function's
        arguments to the nested function, keeping track of their types and
        number. Before forward can be used the <utility> header file
        must be included.
    o  The nested function is called using this stanza to specify its
        arguments: 

        std::forward<Params>(params)....

In the next example perfect forwarding is used to implement one member
Inserter::insert that can be used to call any of the five overloaded
string::insert members. The insert function that's actually called now
simply depends on the types and number of arguments that are passed to
Inserter::insert:
            class Inserter
    {
        std::string d_str;  // somehow initialized
        public:
                            // constructors not implemented,
                            // but see below
            Inserter();
            Inserter(std::string const &str);
            Inserter(Inserter const &other);
            Inserter(Inserter &&other);

            template<typename ...Params>
            void insert(Params &&...params)
            {
                d_str.insert(std::forward<Params>(params)...);
            }
    };

A factory function returning an Inserter can also easily be implemented
using perfect forwarding. Rather than defining four overloaded factory
functions a single one now suffices. By providing the factory function with an
additional template type parameter specifying the class of the object to
construct the factory function is turned into a completely general factory
function:
            template <typename Class, typename ...Params>
    Class factory(Params &&...params)
    {
        return Class(std::forward<Params>(params)...);
    }

Here are some examples showing its use:
            Inserter inserter(factory<Inserter>("hello"));
    string delimiter(factory<string>(10, '='));
    Inserter copy(factory<Inserter>(inserter));

The function std::forward is provided by the  standard library. It
performs no magic, but merely returns params as a nameless object. That
way it acts like std::move that also removes the name from an object,
returning it as a nameless object. The unpack operator has nothing to do with
the use of forward but merely tells the compiler to apply forward to
each of the arguments in turn. Thus it behaves similarly to C's ellipsis
operator used by variadic functions.

Perfect forwarding was introduced in section 21.4.5: a template
function defining a Type &&param, with Type being a template type
parameter converts Type && to Tp & if the function is called with
an argument of type Tp &. Otherwise it binds Type to Tp,
with param being defined as  Tp &&param. As a result an lvalue
argument binds to an lvalue-type (Tp &), while an rvalue argument
binds to an rvalue-type (Tp &&).

The function std::forward merely passes the argument (and its type) on to
the called function or object. Here is its simplified implementation:
            typedef <type T>
    T &&forward(T &&a)
    {
        return a;
    }

Since T && turns into an lvalue reference when forward is called
with an lvalue (or lvalue reference) and remains an rvalue reference if
forward is called with an rvalue reference, and since forward (like
std::move) anonymizes the variable passed as argument to forward, the
argument value is forwarded while passing its type from the function's
parameter to the called function's argument.

This is called perfect forwarding as the nested function can only be
called if the types of the arguments that were used when calling the `outer'
function (e.g., factory) exactly match the types of the parameters of the
nested function (e.g., Class's constructor). Perfect forwarding therefore
is a tool to realize type safety.

A cosmetic improvement to forward requires users of forward to
specify the type to use rather than to have the compiler deduce the type as a
result of the function template parameter type deduction's process. This is
realized by a small support struct template:
            template <typename T>
    struct identity
    {
        using type = T;
    };

This struct merely defines identity::type as T, but as it is a
struct it must be specified explicitly. It cannot be determined from the
function's argument itself. The subtle modification to the above
implementation of forward thus becomes (cf. section 22.2.1 for
an explanation of the use of typename):
            typedef <type T>
    T &&forward(typename identity<T>::type &&arg)
    {
        return arg;
    }

Now forward must explicitly state arg's type, as in:
            std::forward<Params>(params)

Using the std::forward function and the rvalue reference specification
is not restricted to the context of parameter packs. Because of the special
way rvalue references to template type parameters are treated (cf. section
21.4.5) they can profitably be used to forward individual function
    
 parameters as well. Here is an example showing how an argument to a function
can be forwarded from a template to a function that is itself passed to the
template as a pointer to an (unspecified) function:
            template<typename Fun, typename ArgType>
    void caller(Fun fun, ArgType &&arg)
    {
        fun(std::forward<ArgType>(arg));
    }

A function display(ostream &out) and increment(int &x) may now
both be called through caller. Example:
            caller(display, cout);
    int x = 0;
    caller(increment, x);


22.5.3: The unpack operator

        The unpack operator is used to obtain template arguments in many
situations. No mechanism other than recursion (as shown in section
22.5.1) is available to obtain the individual types and values of a
variadic template.

The unpack operator can also be used to define template classes that are
derived from any number of base classes.
   Here is how it's done:
            template <typename ...BaseClasses>
    class Combi: public BaseClasses...          // derive from base classes
    {
        public:
                                                // specify base class objects
                                                // to its constructor using
                                                // perfect forwarding
            Combi(BaseClasses &&...baseClasses)
            :
                BaseClasses(baseClasses)...     // use base class initializers
            {}                                  // for each of the base
    };                                          // classes

This allows us to define classes that combine the features of any number
of other classes. If the class Combi is derived of classes A, B, and
C then Combi is-an A, B, and C. It should of course be given a
virtual destructor. A Combi object can be passed to functions expecting
pointers or references to any of its base class type objects. Here is an
example defining Combi as a class derived from a vector of complex
numbers, a string and a pair of ints and doubles (using uniform intializers in
a sequence matching the sequence of the types specified for the used Combi
type):
            using MultiTypes = Combi<
        vector<complex<double>>, string, pair<int, double> >;

    MultiTypes mt = { {3.5, 4}, "mt", {1950, 1.72} };

The same construction can also be used to define template data members
supporting variadic type lists such as tuples (cf. section
22.6). Such a class could be designed along these lines:
            template <typename ...Types>
    struct Multi
    {
        std::tuple<Types...> d_tup;        // define tuple for Types types

        Multi(Types ...types)
        :                                   // initialize d_tup from Multi's
            d_tup(std::forward<Types>(types)...)   //             arguments
        {}
    };

The ellipses that are used when forwarding parameter packs are
essential. The compiler considers their omission an error. In the following
struct definition it was the intent of the programmer to pass a parameter
pack on to a nested object construction but ellipses were omitted while
specifying the template parameters, resulting in a
    
 parameter packs not expanded with `...' error message:
            template <int size, typename ...List>
    struct Call
    {
        Call(List &&...list)
        {
            Call<size - 1, List &&> call(std::forward<List>(list)...);
        }
    };

Instead of the above definition of the call object the programmer
should have used:
            Call<size - 1, List &&...> call(std::forward<List>(list)...);


22.5.4: Non-type variadic templates

    Variadic templates not necessarily define template types. Non-types can also
be used with
    
        variadic templates. The following function template accepts any series
of int values, forwarding these values to a class template. The class
template defines an enum value result which is returned by the
function, unless no int values were specified, in which case 0 is returned.
            template <int ...Ints>
    int forwarder()
    {
        return computer<Ints ...>::result;  // forwarding the Ints
    }

    template <>     // specialization if no ints are provided
    int forwarder<>()
    {
        return 0;
    }

    // use as:
    cout << forwarder<1, 2, 3>() << ' ' << forwarder<>() << '\n';

The sizeof operator can be used for variadic non-type parameters as
well: sizeof...(Ints) would return 3 when used in the first function
template for forwarder<1, 2, 3>().

Variadic non-type parameters are used to define variadic literal
operators, introduced in section 23.3.


22.5.5: Folding expressions

    Functions accepting variable numbers of arguments (of possibly varying types)
are commonly handled using variadic templates.  Implementations usually
process the first argument, passing the remaining arguments to an overloaded
function, which is defined by the compiler for these remaining argument
types. One of the overloaded versions, accepting zero or one argument, ends
the compiler's recursive implementations.

Sometimes the arguments must be combined using binary operators (like arg1
+ arg2 + ...). In those cases a folding expression can be used instead
of combining the arguments using a traditional variadic template.

All binary operators (including the assignment, compound assignment and comma
operators) can be used in folding expressions. 


    o  A unary left fold is a fold expression that looks like this:
            (... op expr)

where op is the binary operator that is used in the fold expression,
and expr is an expression formulated in terms of the function parameter
representing the variadic arguments. Here is an example:
            template <typename ReturnType, typename ...Params>
    ReturnType sum(Params ...params)
    {
        return (... + params);
    }

If a more elaborate expression than just the designator of the variadic
arguments is used then the expression must be clearly demarcated, e.g., by 
surrounding it with parentheses:
            return (... + (2 * params));

In a unary fold expression all the function's arguments matching the types
of the parameter pack are combined using specified operator. E.g.,
            sum<int>(1, 2, 3);

returns 6.

There are no special restrictions that apply to function templates using
fold expressions. Directly returning the result of the fold expression is OK,
but the result could also be used in another expression (maybe inserting its
value into an ostream), or when initializing a variable or object. Also,
the types of the arguments do not have to be identical: the only requirement
is that the fully expanded expression (in the example: 1 + 2 + 3 is a
valid expression).


    o  A unary right fold is a fold expression that results in the same
expression as its unary left fold alternative, but swaps the ellipses and the
params identifier:
            (expr op ...)

Together, unary left folds and unary right folds are called unary folds.


    o  A binary fold is a fold expression of the following form:
            (expr1 op ... op expr2)

Here, either expr1 is the identifier representing the variable
arguments or expr2 must be that identifier. The other one can be any other
valid expression (as with unary folds, both expressions must be clearly
demarcated). In addition, both operators must be identical.

If a binary operator has been overloaded, it will be used where applicable. A
well-known example is of course operator<< as defined for std::ostream
objects. A binary fold defined for operator<< will not only do shifts, but
can also be used to insert a series of arguments into cout:
            template <class ...Pack>
    ostream &out2(ostream &out, Pack &&...args)
    {
        return (out << ... << args);
    };

Here is another interesting example: a fold expression for the comma operator:
        
    template <class ...Pack>
    void call(Pack &&...args)
    {
        ... , args();
    };

This unary fold calls each of its arguments in sequence. Arguments could
be function addresses, function object or lambda functions. Note the use of
the rvalue reference when defining the variadic parameter list which prevents
copying of function objects that might be passed to call. Assuming that
struct Functor defines a functor, and that void function() has been
defined, then call could be called this way:
            Functor functor;
    call(functor, function, functor, 
        []()
        {
            // ...    
        }
    );

Finally: don't forget the 
        
    parentheses surrounding fold expressions: they are required!


22.6: Tuples

C++ offers a generalized pair container: the tuple, covered in
this section.  Before tuples can be used the header file <tuple> must be
included.

Whereas std::pair containers have limited functionality and only support
two members, tuples have slightly more functionality and may contain an
unlimited number of different data types. In that respect a tuple can be
considered the `template's answer to C's struct'.

A tuple's generic declaration (and definition) uses the variadic template
notation:
            template <class ...Types>
    class tuple;

Here is an example of its use:
            using tuple_idsc = std::tuple<int, double &, std::string, char const *>;

    double pi = 3.14;
    tuple_idsc idsc(59, pi, "hello", "fixed");

    // access a field:
    std::get<2>(idsc) = "hello world";

The std::get<idx>(tupleObject) function template returns a
reference to the idx^th (zero based) field of the tuple
tupleObject. The index is specified as the function template's non-type
template argument.

Type-based tuple addressing () can be
used for tuple types used once in a tuple definition (if the same type is used
repeatedly referring to that type introduces an ambiguity). The next example
shows how to refer to the elements in the above example by type:
            get<int>(idsc)              // 59
    get<double &>(idsc)         // 3.14
    get<string>(idsc)           // "hello"s
    get<char const *>(idsc)     // "fixed"

Tuples may be constructed without specifying initial values. Primitive
types are initialized to zeroes; class type fields are initialized by their
default constructors. Be aware that in some situations the construction of a
tuple may succeed but its use may fail. Consider:
            tuple<int &> empty;
    cout << get<0>(empty);

Here the tuple empty cannot be used as its int & field is an
undefined reference. However, empty's construction succeeds.

Tuples may be assigned to each other if their type lists are identical; if
supported by their constituent types copy constructors are available as
well. Copy construction and assignment is also available if a right-hand type
can be converted to its matching left-hand type or if the left-hand type can
be constructed from the matching right-hand type. Tuples
(matching in number and (convertible) types) can be compared using relational
operators as long as their constituent types support comparisons. In this
respect tuples are like pairs.

Tuples offer the following static elements (using compile-time
initialization):
    
    o std::tuple_size<Tuple>::value returns the number of
types defined for the tuple type Tuple. Example:
        cout << tuple_size<tuple_idsc>::value << '\n';  // displays: 4

o std::tuple_element<idx, Tuple>::type
        returns the type of element idx of Tuple. Example:
        tuple_element<2, tuple_idsc>::type text;  // defines std::string text

The unpack operator can also be used to forward the arguments of a
constructor to a tuple data member. Consider a class Wrapper that is
defined as a variadic template:
            template <typename ...Params>
    class Wrapper
    {
        ...
        public:
            Wrapper(Params &&...params);
    };

This class may be given a tuple data member which should be initialized by
the types and values that are used when initializing an object of the class
  Wrapper using perfect
forwarding. Comparable to the way a class may inherit from its template types
(cf. section 22.5.3) it may forward its types and constructor arguments
to its tuple data member:
            template <typename ...Params>
    class Wrapper
    {
        std::tuple<Params...> d_tuple;     // same types as used for
                                            // Wrapper itself
        public:
            Wrapper(Params &&...params)
            :                               // initialize d_tuple with
                                            // Wrapper's arguments
                d_tuple(std::forward<Params>(params)...)
            {}
    };


22.6.1: Tuples and structured bindings

    Structured bindings were introduced in section 3.3.7.1. That
section concentrated on associating structured binding declarations with
data members of structs, returned as POD values. 

Structured bindings, however, can also be used in a much more generic way, by
associating them with tuples. By doing so structured bindings don't have to be
associated with data members, but they may hold values returned by class
members.

The association between structured binding declarations and tuples is very
strong. In fact it is so strong that the standard explicitly allows users to
define tuple specializations, even though tuple specializations live in the
standard namespace, which otherwise is off-limits to programmers (except, of
course, when using its features).

In order to allow structured bindings to be associated with class members
the following steps must be performed:


    o  The class must be provided with overloaded get member templates,
        using int (or another integral type) specializations, where each
        specialization returns a class element (e.g., a member function).

The availability of if constexpr clauses makes it easy to combine
        all these specializations into a single member template.

Alternatively, a function template defined outside of the class can be
        defined, which allows associating structured bindings with class
        members even if you're not the class's author. In this case the
        function defines a ClassType [cv] &object parameter.

o  A specialization of std::tuple_size<Type> is provided, defining
        static size_t const value as the number of index values that can
        be specified with the get<idx> function. Although defining
        entities in the standard namespace is normally
        off limits to ordinary programs, in
        this special case such specializations are allowed by the C++
        standard.

o  A specialization of std::tuple_element<idx, Type> is provided,
        defining type as the type matching the return type of
        get<idx>. 

The flexibility that is achieved by being able to use structured bindings this
way is enormous. As long as a class offers members returning values those
members can be associated with structured binding variables. The member
functions don't even have to return values that are immediately available
(e.g, as data members accessors) but their return values could also be
computed on the spot, by simply referring to the corresponding structured
binding variable.

To illustrate the abovementioned steps for associating structured bindings
with class members we use these classes:
        class Euclid
    {
        size_t d_x;
        size_t d_y;
    
        public:
            Euclid(size_t x = 0, size_t y = 0);
            double distance() const;        // sqrt(d_x * d_x + d_y * d_y)
    };
    class Data
    {
        std::string d_text{ "hello from Data" };
        Euclid d_euclid;
    
        public:
            void setXY(size_t x, size_t y);
    
            Euclid factory() const;
            double distance() const;
            std::string const &text() const;
    };

The first step towards allowing structured bindings for Data consists of
defining a (member) termplate get.  If Data is our own class we can
add a member template get. Alternatively, if we're only interested in
accessing Data's public members we could derive a class DataGet from
Data, and provide that class with the get member template. Yet another
possibility is to define a free get function template. The get
function must return whatever we're interested in. To make the appropriate
selection we use an integral type (int, size_t, ...) selector value, and
the function template thus only has a non-type template parameter. Rather than
defining several specializations using the if constexpr clause is advised,
as it greatly simplifies the function's definition. 

Our get function defines selector 0 for factory, selector 1 for
distance and selector 2 for text. The distance member simply
returns d_euclid.distance(), and Euclid::distance is run-time
evaluated using its d_x and d_y values. Thus, distance is an
example of a function that will be run-time evaluated when we're referring,
later on, to the third structured bindings variable.

Here's the definition of the member template get:
        template <size_t Nr>
    auto get() const
    {
        if constexpr (Nr == 0)
            return factory();
    
        if constexpr (Nr == 1)
            return distance();
    
        if constexpr (Nr == 2)
            return text();
    
        static_assert(Nr >= 0 and Nr < 3);
    }

This function is still suboptimal. Consider its specialization for value 2: it
returns Data::text(). As auto merely inspects the returned data type,
get<2>() returns a std::string, rather than (text's) return type,
which is a std::string const &. To use the return types that are actually
returned by Data's member functions, get's return type should be
defined as decltype(auto) rather than just auto:
            template <size_t Nr>
    decltype(auto) get() const
    ...

When defining get as a free function template it must be provided with a
parameter Data const &data (or Data &data if members are used that may
modify data's data members), returning the parameter's member
functions. E.g.,
                // get as a free function template:
    template <size_t Nr>
    decltype(auto) get(Data const &data)
    {
        if constexpr (Nr == 0)
            return data.factory();
    
        if constexpr (Nr == 1)
            return data.distance();
    
        ... // (etc)
    }

Now that step 1 has been completed, we turn our attention to the
std::tuple specializations. These specializations 
     are defined inside the std namespace
(using namespace std { ... } or std::tuple...).

The specialization of std::tuple_size for Data defines 
static size_t const value as the number of index values
that can be specified with the get<idx> function:
        template<>
    struct std::tuple_size<Data>
    {
        static size_t const value = 3;
    };

The specialization of std::tuple_element for Data returns the types
matching the various return types of the get member template. Its
implementation also provides a nice example where declval is used: the
return type of the get<Nr> specialization must be determined. But to
obtain that return type a Data object is required, which isn't available
by just mentioning Data in tupe_element's specialization. However,
declval<Data>() defines an rvalue reference, which can be passed to
get<Nr>. But the function's return value isn't required, so the object
doesn't have to be constructed. Only its return type is needed. Hence, by
surrounding the get<Nr> call by decltype no object is constructed, and
merely its return type is used:
        template<size_t Nr>
    struct std::tuple_element<Nr, Data>
    {
        using type = decltype( declval<Data>().get<Nr>() );
        // if get<Nr> is a free function use:
        //           decltype( get<Nr>( declval<Data>() ) );
    
    };

As tuple_size and tuple_element are directly associated with the class
Data their definitions should be provided in Data's header file, below
Data's class interface.

Here's how this could be used in a main function, showing single object
access and access using a range-based for-loop:
        int main()
    {
        Data data;
        auto &[ ef, dist, txt ] = data;
        // or maybe:
        // auto &&[ ef, dist, txt ] = Data{};
    
        cout << dist << ' ' << txt << '\n';
    
        Data array[5];
        for (auto &[ ef, dist, txt]: array)
            cout << "for: " << dist << ' ' << txt << '\n';
    }


22.7: Computing the return type of function objects

As amply illustrated in chapter 19 function objects play an
important role with generic algorithms. Like generic algorithms themselves,
function objects can be generically defined as members of class templates. If
the function call operators (operator()) of such classes define parameters
then the types of those parameters may also be abstracted by defining the
function call operators themselves as member templates.  Example:
            template <typename Class>
    class Filter
    {
        Class obj;
        public:
            template <typename Param>
            Param operator()(Param const &param) const
            {
                return obj(param);
            }
    };

The template class Filter is a wrapper around Class, filtering
Class's function call operator through its own function call operator. In
the above example the return value of Class's function call operator is
simply passed on, but any other manipulation is of course also possible.

A type that is specified as Filter's template type argument may of
course have multiple function call operators:
            struct Math
    {
        int operator()(int x);
        double operator()(double x);
    };

Math objects can now be filtered using Filter<Math> fm using
Math's first or second function call operator, depending on the actual
argument type. With fm(5) the int-version is used, with fm(12.5)
the double-version is used.

However, this scheme doesn't work if the function call operators have
different return and argument types. Because of this the following class
Convert cannot be used with Filter:
            struct Convert
    {
        double operator()(int x);           // int-to-double
        std::string operator()(double x);   // double-to-string
    };

This problem can be tackled successfully by the class template
 
 std::invoke_result<Functor, Types...>. Before using
std::invoke_result the header file <functional> must be included.

The result_of class template offers a using declaration (type),
representing the type that is returned by Functor<TypeList>. It can be
used as follows to improve the implementation of Filter:
            template <typename Class>
    class Filter
    {
        Class obj;
        public:
            template <typename Arg>
            std::invoke_result<Class, Arg>::type 
                operator()(Arg const &arg) const
                {
                    return obj(arg);
                }
    };

Using this definition, Filter<Convert> fc can be constructed. Now
fc(5) returns a double, while fc(4.5) returns a
std::string.

The class Convert must define the relationships between its function call
operators and their return types. Predefined function objects (like those in
the standard template library) already do so, but self-defined function
objects must do this explicitly.

If a function object class defines only one function call operator it can
specify its return type through a using-declaration. If the above class
Convert would only define the first of its two function call operators
then the using declaration (in the class's public section) should be:
            using type = double;

If multiple function call operators are defined, each with its own signature
and return type, then the association between signature and return type is set
up as follows (all in the class's public section):
    
    o  define a generic struct result like this:
        template <typename Signature>
struct result;

o  For each function call signature define a specialization of
struct result. Convert's first function call operator gives rise to:
        template <typename Class>
struct result<Class(int)>
{
    using type = double;
};

and Convert's second function call operator to:
        template <typename Class>
struct result<Class(double)>
{
    using type = std::string;
};

o  In cases where function call operators have multiple arguments the
specifications should again provide for the correct signatures. A function
call operator called with an int and a double, returning a size_t
gets:
        template <typename Class>
struct result<Class(int, double)>
{
    using type = size_t;
};


22.8: Instantiating class templates

Class templates  are instantiated when an
object of a class template is defined. When a class template object is
defined or declared its template parameters must explicitly be specified.

Template parameters are also specified when default template parameter
values are specified albeit that in that case the compiler provides the
defaults (cf. section 22.4 where double is used as the default
type to use for the template's DataType parameter).  The actual values or
types of template parameters are
    
 never deduced from arguments as is done with function template
parameters. So to define a Matrix of complex-valued elements, the
following syntax is used:
            Matrix<3, 5, std::complex> complexMatrix;

Since the class template Matrix uses a default data type
a matrix of double-valued elements can be defined like this:
            Matrix<3, 5> doubleMatrix;

A class template object may be declared using the keyword extern.
        
    For example, to declare the matrix complexMatrix use:
            extern Matrix<3, 5, std::complex> complexMatrix;


        
        
        
    A class template declaration suffices to compile return values or
parameters that are of class template types. Example: the following source
file may be compiled, although the compiler hasn't seen the definition of the
Matrix class template.  Generic classes and (partial) specializations may
all be declared. A function expecting or returning a class template object,
reference, or parameter automatically becomes a function template itself. This
is necessary to allow the compiler to tailor the function to the types of
various actual arguments that may be passed to the function:
            #include <cstddef>

    template <size_t Rows, size_t Columns, typename DataType = double>
    class Matrix;

    template <size_t Columns, typename DataType>
    class Matrix<1, Columns, DataType>;

    Matrix<1, 12> *function(Matrix<2, 18, size_t> &mat);


When class templates are used the compiler must first have seen their
implementations. So, template member functions must be known to the compiler
when the template is instantiated.  This does not mean that all members of
a template class are instantiated or must have been seen when a class template
object is defined.
        
    The compiler only instantiates those members that are actually used. This
is illustrated by the following simple class Demo that has two
constructors and two members. When we use one constructor and call one member
in main note the sizes of the resulting object file and executable
program. Next the class definition is modified in that the unused constructor
and member are commented out. Again we compile and link the program. Now
observe that these latter sizes are identical to the former sizes.  There are
other ways to illustrate that only used members are instantiated. The nm
program could be used. It shows the symbolic content of object files.  Using
nm we'll reach the same conclusion: only template member functions that
are actually used are instantiated.  Here is the class template Demo that
was used for our little experiment. In main only the first constructor
and the first member function are called and thus only these members were
instantiated:
            #include <iostream>

    template <typename Type>
    class Demo
    {
        Type d_data;
        public:
            Demo();
            Demo(Type const &value);
            void member1();
            void member2(Type const &value);
    };
    template <typename Type>
    Demo<Type>::Demo()
    :
        d_data(Type())
    {}
    template <typename Type>
    void Demo<Type>::member1()
    {
        d_data += d_data;
    }

    // the following members should be commented out before
    // compiling for the 2nd time:

    template <typename Type>
    Demo<Type>::Demo(Type const &value)
    :
        d_data(value)
    {}
    template <typename Type>
    void Demo<Type>::member2(Type const &value)
    {
        d_data += value;
    }

    int main()
    {
        Demo<int> demo;
        demo.member1();
    }



22.9: Processing class templates and instantiations

In section 21.13 the distinction between code depending on template
parameters and code not depending on template parameters was introduced.
The same distinction also holds true when class templates are defined and
used.

Code not depending on template parameters is verified by the compiler when
the template is defined. If a member function in a class template uses a
qsort function, then qsort does not depend on a template
parameter. Consequently, qsort must be known to the compiler when it
encounters qsort's function call. In practice this implies that the
<cstdlib> header file must have been read by the compiler before it
is able to compile the class template definition.

On the other hand, if a template defines a <typename Ret> template
type parameter to parameterize the return type of some template member
function as in:
            Ret member();

then the compiler may encounter member or the class to which
member belongs in the following locations:
    
    o  the location where a class template object is defined. This is called
the
        point of instantiation of the class template object.
    The compiler must have read the class template's implementation and has
performed a basic check for syntactic correctness of member functions like
member. It won't accept a definition or declaration like Ret &&
*member, because C++ does not support functions returning pointers to
rvalue references. Furthermore, it checks whether the actual type name
that is used for instantiating the object is valid. This type name must be
known to the compiler at the object's point of instantiation.
    o  the location where the template member function is used. This is
called the template member function's point of instantiation. Here
the Ret parameter must have been specified (or deduced) and at this point
member's
statements that depend on the Ret template parameter are checked for
syntactic correctness. For example, if member contains a statement
like
            Ret tmp(Ret(), 15);

then this is in principle a syntactically valid statement. However, when
Ret = int the statement fails to compile as int does not have a
constructor expecting two int arguments. Note that this is not a
problem when the compiler instantiates an object of member's class.  At
the point of instantiation of the object its member function `member' is
not instantiated and so the invalid int construction remains undetected.


22.10: Declaring friends

    Friend  functions are normally constructed as support (free)
functions of a class that cannot be implemented and declared as class
members. The insertion operator for output streams is a well-known
example. Friend classes are most often seen in the context of nested classes.
Here the inner class declares the outer class as its friend (or the other way
around). Again we see a support mechanism: the inner class is constructed to
support the outer class.

Like ordinary classes, class templates may declare other functions and
classes as their friends. Conversely, ordinary classes may declare template
classes as their friends. Here too, the friend is constructed as a special
function or class augmenting or supporting the functionality of the declaring
class. Although the friend keyword can be used by any type of class
        
        
    (ordinary or template) when using class templates the following cases
should be distinguished:
    
    o  A class template may declare an ordinary function or class as its
friend. This is a common friend declaration, such as the insertion operator
for ostream objects.
    o  A class template may declare another function template or class
template as its friend. In this case, the friend's template parameters may
have to be specified.

If the actual values of the friend's template parameters must be equal
to the template parameters of the class declaring the friend, the friend is
said to be a bound friend class or function template. In
this case the template parameters of the template specifying the friend
declaration determine (bind) the values of the template parameters of the
friend class or function. Bound friends result in a one-to-one correspondence
between the template's parameters and the friend's template parameters.
    o  In the most general case a class template may declare another
function template or class template to be its friend, irrespective of the
friend's actual template arguments.

In this case an unbound friend class or function template is
declared. The template parameters of the friend class or function template
remain to be specified and are not related in some predefined way to the
template parameters of the class declaring the friend. If a class template has
data members of various types, specified by its template parameters and
another class should be allowed direct access to these data members we want to
specify any of the current template arguments when specifying such a
friend. Rather than specifying multiple bound friends, a single generic
(unbound) friend may be declared, specifying the friend's actual template
parameters only when this is required.
    o  The above cases, in which a template is declared as a friend, may
also be encountered when ordinary classes are used:
        
        o  The ordinary class declaring ordinary friends has already been
covered (chapter 15).
        o  The equivalent of bound friends occurs if an ordinary class
specifies specific actual template parameters when declaring its friend.
        o  The equivalent of unbound friends occurs if an ordinary class
declares a generic template as its friend.


22.10.1: Non-templates used as friends in templates

        A class template may declare an ordinary function, ordinary member
function or ordinary class as its friend.  Such a friend may access the
class template's private members.

Concrete classes and ordinary functions can be declared as friends, but
before a single member function of a class can be declared as a friend, the
compiler must have seen the class interface declaring that member. Let's
consider the various possibilities:
    
    o  A class template may declare an ordinary function to be its
friend. It is not completely clear why we would like to declare an
ordinary function as a friend. Usually we pass an object of the class
declaring the friend to such a function. With class templates this requires us
to provide the (friend) function with a template parameter without specifying
its types. As the language does not support constructions like
        void function(std::vector<Type> &vector)

unless function itself is a template, it is not immediately clear how
and why such a friend should be constructed. One reason could be to allow the
function access to the class's private static members. In addition such
friends could instantiate objects of classes that declare them as their
friends. This would allow the friend functions direct access to such object's
private members. For example:
            template <typename Type>
    class Storage
    {
        friend void basic();
        static size_t s_time;
        std::vector<Type> d_data;
        public:
            Storage();
    };
    template <typename Type>
    size_t Storage<Type>::s_time = 0;
    template <typename Type>
    Storage<Type>::Storage()
    {}

    void basic()
    {
        Storage<int>::s_time = time(0);
        Storage<double> storage;
        std::sort(storage.d_data.begin(), storage.d_data.end());
    }

    o  Declaring an ordinary class to be a class template's friend probably
finds more applications. Here the ordinary (friend) class may instantiate any
kind of object of the class template. The friend class may then access all
private members of the instantiated class template:
            template <typename Type>
    class Composer
    {
        friend class Friend;
        std::vector<Type> d_data;
        public:
            Composer();
    };

    class Friend
    {
        Composer<int> d_ints;
        public:
            Friend(std::istream &input);
    };

    inline::Friend::Friend(std::istream &input)
    {
        std::copy(std::istream_iterator<int>(input),
                  std::istream_iterator<int>(),
                  back_inserter(d_ints.d_data));
    }

    o  Alternatively, just a single member function of an ordinary class may
be declared as a friend. This requires that the compiler has read the friend
class's interface before declaring the friend. Omitting the required
destructor and overloaded assignment operators, the following shows an example
of a class whose member sorter is declared as a friend of the class
Composer:
            template <typename Type>
    class Composer;

    class Friend
    {
        Composer<int> *d_ints;
        public:
            Friend(std::istream &input);
            void sorter();
    };

    template <typename Type>
    class Composer
    {
        friend void Friend::sorter();
        std::vector<Type> d_data;
        public:
            Composer(std::istream &input)
            {
                std::copy(std::istream_iterator<int>(input),
                          std::istream_iterator<int>(),
                          back_inserter(d_data));
            }
    };

    inline Friend::Friend(std::istream &input)
    :
        d_ints(new Composer<int>{ input })
    {}

    inline void Friend::sorter()
    {
        std::sort(d_ints->d_data.begin(), d_ints->d_data.end());
    }

    In this example Friend::d_ints is a pointer member. It
cannot be a Composer<int> object as the Composer class interface
hasn't yet been seen by the compiler when it reads Friend's class
interface. Disregarding this and defining a data member Composer<int>
d_ints results in the compiler generating the error
        
        error: field `d_ints' has incomplete type

`Incomplete type', as the compiler at this points knows of the existence
of the class Composer, but as it hasn't seen Composer's interface it
doesn't know what size the d_ints data member has.


22.10.2: Templates instantiated for specific types as friends

        With bound friend class or function templates there
is a one-to-one mapping between the template arguments of the friend templates
and the template arguments of the class templates declaring them as
friends. In this case, the friends themselves are templates too. Here are the
various possibilities:
    
    o  A function template is a friend of a class template. In this case we
don't experience the problems we encountered with ordinary functions declared
as friends of class templates. Since the friend function template itself is a
template it may be provided with the required template arguments allowing it
to become the declaring class's friend. The various declarations are organized
like this:
    
    o  The class template declaring the bound template friend function is
defined;
    o  The (friend) function template is defined, now having access to all
the class template's (private) members.

The bound template friend declaration specifies the required template
arguments immediately following the template's function name. Without the
template argument list affixed to the function name it would remain an
ordinary friend function. Here is an example showing a bound friend to create
a subset of the entries of a dictionary. For real life examples, a dedicated
function object returning !key1.find(key2) is probably more useful.  For
the current example, operator== is acceptable:
            template <typename Key, typename Value>
    class Dictionary
    {
        friend Dictionary<Key, Value>
            subset<Key, Value>(Key const &key,
                               Dictionary<Key, Value> const &dict);

        std::map<Key, Value> d_dict;
        public:
            Dictionary();
    };

    template <typename Key, typename Value>
    Dictionary<Key, Value>
               subset(Key const &key, Dictionary<Key, Value> const &dict)
    {
        Dictionary<Key, Value> ret;

        std::remove_copy_if(dict.d_dict.begin(), dict.d_dict.end(),
                            std::inserter(ret.d_dict, ret.d_dict.begin()),
                            std::bind2nd(std::equal_to<Key>(), key));
        return ret;
    }


o  By declaring a full class template as a class template's friend, all
members of the friend class may access all private members of the class
declaring the friend. As the friend class only needs to be declared, the
organization of the declaration is much easier than when function templates
are declared as friends. In the following example a class Iterator is
declared as a friend of a class Dictionary. Thus, the Iterator is able
to access Dictionary's private data. There are some interesting points to
note here:
        
        o  To declare a class template as a friend, that class only needs to
be declared as a class template before it is declared as a friend:
                template <typename Key, typename Value>
    class Iterator;

    template <typename Key, typename Value>
    class Dictionary
    {
        friend class Iterator<Key, Value>;

        o  However, members of the friend class may already be used, even
though the compiler hasn't seen  the friend class's interface yet:
                template <typename Key, typename Value>
    template <typename Key2, typename Value2>
    Iterator<Key2, Value2> Dictionary<Key, Value>::begin()
    {
        return Iterator<Key, Value>(*this);
    }
    template <typename Key, typename Value>
    template <typename Key2, typename Value2>
    Iterator<Key2, Value2> Dictionary<Key, Value>::subset(Key const &key)
    {
        return Iterator<Key, Value>(*this).subset(key);
    }

        o  Of course, the friend class's interface must eventually be seen
by the compiler. Since it's a support class for Dictionary it can safely
define a std::map data member that is initialized by the friend class's
constructor. The constructor may then access the Dictionary's private data
member d_dict:
                template <typename Key, typename Value>
    class Iterator
    {
        std::map<Key, Value> &d_dict;

        public:
            Iterator(Dictionary<Key, Value> &dict)
            :
                d_dict(dict.d_dict)
            {}

        o  The Iterator member begin may return a map
iterator. Since the compiler does not know what the instantiation of the map
looks like, a map<Key, Value>::iterator is a template subtype. So it
cannot be used as-is, but it must be prefixed by
typename (see the function begin's return type in the next example):
                template <typename Key, typename Value>
    typename std::map<Key, Value>::iterator Iterator<Key, Value>::begin()
    {
        return d_dict.begin();
    }


o  In the previous example we might decide that only a Dictionary
should be able to construct an Iterator (maybe because we conceptually
consider Iterator to be a sub-type of Dictionary). This is easily
accomplished by defining Iterator's constructor in its private section,
and by declaring Dictionary to be a friend of Iterator. Consequently,
only a Dictionary can create an Iterator. By declaring the constructor
of a specific Dictionary type as a friend of Iterator's we declare
a bound friend. This ensures that only that particular type of
Dictionary can create Iterators using template parameters identical to
its own. Here is how it's done:
            template <typename Key, typename Value>
    class Iterator
    {
        friend Dictionary<Key, Value>::Dictionary();
        std::map<Key, Value> &d_dict;
        Iterator(Dictionary<Key, Value> &dict);
        public:

    In this example, Dictionary's constructor is Iterator's
friend. The friend is a template member. Other members can be declared as
a class's friend as well. In those cases their prototypes must be used, also
specifying the types of their return values. Assuming that
        std::vector<Value> sortValues()

is a member of Dictionary then the matching bound friend declaration
is:
        friend std::vector<Value> Dictionary<Key, Value>::sortValues();

o  A class template may define free members, which should have access to
the data of instantiations of the class template, but only for matching
types. An example is a class template for which the free member operator==
must be available. If this is required for a template ClassTemplate,
requiring a typename Type template type argument, then
a free member 
            template<typename Type>
    bool operator==(ClassTemplate<Type> const &lhs, 
                    ClassTemplate<Type> const &rhs);

must have been declared prior to ClassTemplate's interface
itself. Within the class interface operator== may then be declared as a
friend, specifying operator== as a specialized function template (note the
<> following the function name) like this:
            template <typename Type>
    class ClassTemplate
    {
        friend  bool operator==<>(ClassTemplate<Type> const &lhs, 
                                ClassTemplate<Type> const &rhs);
        ...
    };

Now that the class has been declared, operator=='s implementation may
follow. 

Finally, the following example can be used as a prototype for
situations where bound friends are useful:
            template <typename T>   // a function template
    void fun(T *t)
    {
        t->not_public();
    };
    template<typename X>    // a free member function template
    bool operator==(A<X> const &lhs, A<X> const &rhs);
    template <typename X>   // a class template
    class A
    {                       // fun() is used as friend bound to A,
                            // instantiated for X, whatever X may be
        friend void fun<A<X>>(A<X> *);
                            // operator== is a friend for A<X> only
        friend  bool operator==<>(A<X> const &lhs, A<X> const &rhs);

        int d_data = 10;

        public:
            A();

        private:
            void not_public();
    };
    template <typename X>
    A<X>::A()
    {
        fun(this);
    }
    template <typename X>
    void A<X>::not_public()
    {}
    template<typename X>    // may access lhs/rhs's private data
    bool operator==(A<X> const &lhs, A<X> const &rhs)
    {
        return lhs.d_data == rhs.d_data;
    }

    int main()
    {
        A<int> a;

        fun(&a);            // fun instantiated for A<int>.
    }


22.10.2.1: Free operators as friends of nested classes

        When nesting a class under a class template it may be desirable to provide
that nested class with free operators, which are bound to the surrounding
class. This situation is encountered when defining a nested iterator for which
a free operator==(iterator const &lhs, iterator const &rhs) operator is
preferred over a member operator==(iterator const &rhs) const, as the
member does not allow promotions of the lhs operand.

It is possible to define the free operator== as friend in the nested
class, which then automatically becomes a bound friend. E.g.,
            #include <string>

    template <typename Type>
    struct String
    {
        struct iterator
        {
            std::string::iterator d_iter;

            friend bool operator==(iterator const &lhs, iterator const &rhs)
            {
                return lhs.d_iter == rhs.d_iter;
            }
        };

        iterator begin()
        {
            return iterator{};
        }
    };

    int main()
    {
        String<int> str;
        return  str.begin() == str.begin();
    }

    However, this requires an in-class implementation, which should be avoided
as it combines interfaces and implementations which reduces clarity.

But when moving the implementation out of the interface we run into the
problem that operator== is a function template, which must be declared as
such in the class interface. The compiler suggests to make sure the
function template has already been declared and add '<>' after the function
name. But declaring
            template <typename Type>
    bool operator==(Type const &lhs, Type const &rhs);

before the struct String class template, and then implementing
operator== as 
            template <typename Type>
    inline bool operator==(String<Type>::iterator const &lhs,
                           String<Type>::iterator const &rhs)
    {
        return lhs.d_iter == rhs.d_iter;
    }

    the compiler still complains, reporting declaration of 'operator==' as
non-function.

So how to solve this issue? There are two known ways to solve this
problem. One was suggested by Radu Cosma (teaching assistant of our C++
course in the 2021-2022 academic year), the other solution is provided in 
section 23.13.7.2.

Radu proposed using an application of SFINAE: by defining a free operator
provided with a type uniquely defined by the nested class, which is then
provided with a default value in the free function's implementation, the
compiler automatically selects the appropriate overloaded function. This works
for multiple classes declaring nested classes and for classes defining
multiple nested classes alike. Here is an example of two classes each
declaring a nested class iterator:
            template <typename Type>
    struct String
    {
        struct iterator
        {
            using StringType_iterator = int;
            friend bool operator==<>(iterator const &lhs, iterator const &rhs);
    
            std::string::iterator d_iter;
        };
        iterator begin()
        {
            return iterator{};
        }
    };
    
    template <typename Type>
    struct Container
    {
        struct iterator
        {
            using ContainerType_iterator = int;
            friend bool operator==<>(iterator const &lhs, iterator const &rhs);
    
            int *d_ptr;
        };
        iterator begin()
        {
            return iterator{};
        }
    };

    Note that the nested classes declare the free functions as function
template specializations.

Then, for each nested class an implementation of the free operator is
provided. These implementations are function templates 
provided with a template type Type and a second type which is the uniquely
named int type of the nested class for which the free operator is
implemented:
            template <typename Type, Type::StringType_iterator = 0>
    inline bool operator==(Type const &lhs, Type const &rhs)
    {
        return lhs.d_iter == rhs.d_iter;
    }
    
    template <typename Type, Type::ContainerType_iterator = 0>
    inline bool operator==(Type const &lhs, Type const &rhs)
    {
        return lhs.d_ptr == rhs.d_ptr;
    }

    The main function illustrates the use of both free operators:
            int main()
    {
        String<int> str;
        Container<int> cnt;
        return str.begin() == str.begin() and cnt.begin() == cnt.begin();
    }



22.10.3: Unbound templates as friends

        When a friend is declared as an
 unbound friend it merely declares an
existing template to be its friend (no matter how it is instantiated). This
may be useful in situations where the friend should be able to instantiate
objects of class templates declaring the friend, allowing the friend to access
the instantiated object's private members.  Functions, classes, and member
functions may all be declared as unbound friends.

Here are  the syntactic conventions  declaring unbound friends:
    
        o  Declaring a function template as an unbound friend: any
instantiation of the function template may instantiate objects of the template
class and may access its private members. Assume the following
function template has been defined
        template <typename Iterator, typename Class, typename Data>
Class &ForEach(Iterator begin, Iterator end, Class &object,
                    void (Class::*member)(Data &));

This function template can be declared as an unbound friend in the
following class template Vector2:
        template <typename Type>
class Vector2: public std::vector<std::vector<Type> >
{
    template <typename Iterator, typename Class, typename Data>
    friend Class &ForEach(Iterator begin, Iterator end, Class &object,
                void (Class::*member)(Data &));
    ...
};

If the function template is defined inside some namespace this namespace
must be mentioned as well. Assuming that ForEach is defined in the
namespace FBB its friend declaration becomes:
        template <typename Iterator, typename Class, typename Data>
friend Class &FBB::ForEach(Iterator begin, Iterator end, Class &object,
                void (Class::*member)(Data &));

The following example illustrates the use of an unbound friend. The class
Vector2 stores vectors of elements of template type parameter
Type. Its process member allows ForEach to call its private
rows member. The rows member, in turn, uses another ForEach to
call its private columns member. Consequently, Vector2 uses two
instantiations of ForEach which is a clear hint for using an unbound
friend. It is assumed that Type class objects can be inserted into
ostream objects (the definition of the ForEach function template can
be found in the cplusplus.yo.zip archive that can be retrieved from
https://fbb-git.gitlab.io/cppannotations/). Here is the program:
            template <typename Type>
    class Vector2: public std::vector<std::vector<Type> >
    {
        using iterator = typename Vector2<Type>::iterator;

        template <typename Iterator, typename Class, typename Data>
        friend Class &ForEach(Iterator begin, Iterator end, Class &object,
                    void (Class::*member)(Data &));
        public:
            void process();

        private:
            void rows(std::vector<Type> &row);
            void columns(Type &str);
    };

    template <typename Type>
    void Vector2<Type>::process()
    {
        ForEach<iterator, Vector2<Type>, std::vector<Type> >
                (this->begin(), this->end(), *this, &Vector2<Type>::rows);
    }

    template <typename Type>
    void Vector2<Type>::rows(std::vector<Type> &row)
    {
        ForEach(row.begin(), row.end(), *this,
                                        &Vector2<Type>::columns);
        std::cout << '\n';
    }

    template <typename Type>
    void Vector2<Type>::columns(Type &str)
    {
        std::cout << str << " ";
    }

    using namespace std;

    int main()
    {
        Vector2<string> c;
        c.push_back(vector<string>(3, "Hello"));
        c.push_back(vector<string>(2, "World"));

        c.process();
    }
    /*
        Generated output:

        Hello Hello Hello
        World World
    */

    o  Analogously, a full class template may be declared as friend. This
allows all instantiations of the friend's member functions to instantiate
objects of the class template declaring the friend class. In this case, the
class declaring the friend should offer functionality that is useful to
different instantiations of its friend class (i.e., instantiations using
different template arguments). The syntactic convention is comparable to the
convention used when declaring an unbound friend function template:
        template <typename Type>
class PtrVector
{
    template <typename Iterator, typename Class>
    friend class Wrapper;      // unbound friend class
};

All members of the class template Wrapper may now instantiate
PtrVectors using any actual type for its Type parameter. This allows
the Wrapper instantiation to access all of PtrVector's private
members.
    o  When only some members of a class template need access to private
members of another class template (e.g., the other class template has private
constructors and only some members of the first class template need to
instantiate objects of the second class template), then the latter class
template may declare only those members of the former class template requiring
access to its private members as its friends. Again, the friend class's
interface may be left unspecified. However, the compiler must be informed that
the friend member's class is indeed a class. A forward declaration of that
class must therefore be provided. In the next example PtrVector
declares Wrapper::begin as its friend. Note the forward declaration of
the class Wrapper:
        template <typename Iterator>
class Wrapper;

template <typename Type>
class PtrVector
{
    template <typename Iterator> friend
        PtrVector<Type> Wrapper<Iterator>::begin(Iterator const &t1);
    ...
};


22.10.4: Extended friend declarations

    Through extended friend declarations, which are also available for
class templates, template type parameters can be declared as friends. A
template type argument, however, does not necessarily have to be a type for
which the keyword friend makes sense, like int. In those cases the
friend declaration is simply ignored.

Consider the following class template, declaring Friend as a friend:
            template <typename Friend>
    class Class
    {
        friend Friend;
        void msg();             // private, displays some message
    };

Now, an actual Friend class may access all of Class's members
            class Concrete
    {
        Class<Concrete> d_class;
        Class<std::string> d_string;

       public:
            void msg()
            {
                d_class.msg();    // OK: calls private Class<Concrete>::msg()
                //d_string.msg(); // fails to compile: msg() is private
            }
    };

A declaration like Class<int> intClass is also OK, but here the friend
declaration is simply ignored. After all, there are no `int members' to access
Class<int>'s private members.


22.11: Class template derivation

Class templates can be used for inheritance purposes as well. When a
class template is used in class derivation, the following situations should be
distinguished:
    
    o  An existing class template is used as base class when deriving a
ordinary class. The derived class itself will partially be a class template,
but this is somewhat hidden from view when an object of the derived class is
defined.
    o  An existing class template is used as the base class when deriving
another class template. Here the class template characteristics remain clearly
visible.
    o  An ordinary class is used as the base class when deriving a template
class. This interesting hybrid allows us to construct class templates that
        
are partially compiled.

These three variants of  class template
derivation are elaborated in this and the upcoming sections.

Consider the following base class:
            template<typename T>
    class Base
    {
        T const &t;

        public:
            Base(T const &t);
    };

The above class is a class template that can be used as a base class for
the following derived class template Derived:
            template<typename T>
    class Derived: public Base<T>
    {
        public:
            Derived(T const &t);
    };
    template<typename T>
    Derived<T>::Derived(T const &t)
    :
        Base(t)
    {}

Other combinations are also possible. The base class may be instantiated
by specifying template arguments, turning the derived class into an ordinary
class (showing a class object's definition as well):
            class Ordinary: public Base<int>
    {
        public:
            Ordinary(int x);
    };
    inline Ordinary::Ordinary(int x)
    :
        Base(x)
    {}

    Ordinary ordinary{ 5 };

This approach allows us to
 
    add functionality to a class template, without the need for constructing a
derived class template.

Class template derivation pretty much follows the same rules as ordinary
class derivation, not involving class templates. Some subtleties that are
specific for class template derivation may easily cause confusion like the use
of this when members of a template base class are called from a derived
class. The reasons for using this are discussed in section 23.1. In
the upcoming sections the focus will be on the class derivation proper.


22.11.1: Deriving ordinary classes from class templates

    When an existing class template is used as a base class for deriving an
ordinary class, the class template parameters are specified when defining the
derived class's interface.  If in a certain context an existing class template
lacks a particular functionality, then it may be useful to derive an ordinary
class from a class template. For example, although the class map can
easily be used in combination with the find_if() generic algorithm
(section 19.1.15), it requires the construction of a class and at least
two additional function objects of that class. If this is considered too much
overhead then extending a class template with tailor-made functionality
might be considered.

Example: a program executing commands entered at the keyboard might accept all
unique initial abbreviations of the commands it defines. E.g., the command
list might be entered as l, li, lis or list. By deriving a class
Handler from
            map<string, void (Handler::*)(string const &cmd)>

and by defining a member function process(string const &cmd) to do the
actual command processing a program might simply execute the following
main() function:
            int main()
    {
        string line;
        Handler cmd;

        while (getline(cin, line))
            cmd.process(line);
    }

    The class Handler itself is derived from a std::map, in which
the map's values are pointers to Handler's member functions, expecting the
command line entered by the user. Here are Handler's characteristics:
    
    o  The class is derived from a std::map, expecting the command
associated with each command-processing member as its keys. Since
Handler uses the map merely to define associations between
         the commands and the processing
member functions and to make map's types available, private derivation is
used:
            class Handler: private std::map<std::string,
                            void (Handler::*)(std::string const &cmd)>

    o  The actual association can be defined using static private data
members: s_cmds is an array of Handler::value_type values, and
s_cmds_end is a constant pointer pointing beyond the array's last element:
                static value_type s_cmds[];
        static value_type *const s_cmds_end;

    o  The constructor simply initializes the map from these two static data
members. It could be implemented inline:
            inline Handler::Handler()
    :
        std::map<std::string,
                    void (Handler::*)(std::string const &cmd)>
        (s_cmds, s_cmds_end)
    {}

    o  The member process iterates along the map's elements. Once the
first word on the command line matches the initial characters of the command,
the corresponding command is executed. If no such command is found, an error
message is issued:
            void Handler::process(std::string const &line)
    {
        istringstream istr(line);
        string cmd;
        istr >> cmd;
        for (iterator it = begin(); it != end(); it++)
        {
            if (it->first.find(cmd) == 0)
            {
                (this->*it->second)(line);
                return;
            }
        }
        cout << "Unknown command: " << line << '\n';
    }



22.11.2: Deriving class templates from class templates

        Although it's perfectly acceptable to derive an ordinary class from a
class template, the resulting class of course has limited generality compared
to its template base class. If generality is important, it's probably a better
idea to derive a class template from a class template. This allows us to
extend an existing class template with new functionality or to override the
functionality of the existing class template.

The class template SortVector presented below is derived from the
existing class template std::vector. It allows us to perform a
hierarchic sort of its elements using any ordering of any data members
its data elements may contain. To accomplish this there is but one
requirement. SortVector's data type must offer dedicated member
functions comparing its members.

For example, if SortVector's data type is an object of class
MultiData, then MultiData should implement member functions having the
following prototypes for each of its data members which can be compared:
            bool (MultiData::*)(MultiData const &rhv)

So, if MultiData has two data members, int d_value and
std::string d_text and both may be used by a hierarchic sort, then
MultiData should offer the following two members:
            bool intCmp(MultiData const &rhv);  // returns d_value < rhv.d_value
    bool textCmp(MultiData const &rhv); // returns d_text  < rhv.d_text

Furthermore, as a convenience it is assumed that operator<< and
operator>> have been defined for MultiData objects.

The class template SortVector is directly derived from the template
class std::vector. Our implementation inherits all members from that base
class. It also offers two simple constructors:
            template <typename Type>
    class SortVector: public std::vector<Type>
    {
        public:
            SortVector()
            {}
            SortVector(Type const *begin, Type const *end)
            :
                std::vector<Type>(begin, end)
            {}


Its member hierarchicSort is the true raison d'etre for the
class. It defines the hierarchic sort criteria. It expects
        
    a pointer to a series of pointers to member functions of the class
Type as well as a size_t representing the size of the array.

The array's first element indicates Type's most significant sort
criterion, the array's last element indicates the class's least significant
sort criterion. Since the stable_sort generic algorithm was designed
explicitly to support hierarchic sorting, the member uses this generic
algorithm to sort SortVector's elements. With hierarchic sorting, the
least significant criterion should be sorted first. hierarchicSort's
implementation is therefore easy. It requires a support class SortWith
whose objects are initialized by the addresses of the member functions passed
to the hierarchicSort() member:
            template <typename Type>
    void SortVector<Type>::hierarchicSort(
                bool (Type::**arr)(Type const &rhv) const,
                size_t n)
    {
        while (n--)
            stable_sort(this->begin(), this->end(), SortWith<Type>(arr[n]));
    }


The class SortWith is a simple wrapper class around a pointer to
a predicate function. Since it depends on SortVector's actual data
type the class SortWith must also be a class template:
            template <typename Type>
    class SortWith
    {
       bool (Type::*d_ptr)(Type const &rhv) const;


SortWith's constructor receives a pointer to a predicate function and
initializes the class's d_ptr data member:
            template <typename Type>
    SortWith<Type>::SortWith(bool (Type::*ptr)(Type const &rhv) const)
    :
        d_ptr(ptr)
    {}


Its binary predicate member (operator()) must return true if its
first argument should eventually be placed ahead of its second argument:
            template <typename Type>
    bool SortWith<Type>::operator()(Type const &lhv, Type const &rhv) const
    {
        return (lhv.*d_ptr)(rhv);
    }


The following examples, which can be embedded in a main
function, provides an illustration:
    
    o  First, A SortVector object is created for MultiData objects.
It uses the copy generic algorithm to fill the SortVector object from
information appearing at the program's standard input stream. Having
initialized the object its elements are displayed to the standard output
stream:
                SortVector<MultiData> sv;

        copy(istream_iterator<MultiData>(cin),
                istream_iterator<MultiData>(),
                back_inserter(sv));

    o  An array of pointers to members is initialized with the addresses of
two member functions. The text comparison is the most significant sort
criterion:
                bool (MultiData::*arr[])(MultiData const &rhv) const =
        {
            &MultiData::textCmp,
            &MultiData::intCmp,
        };

    o  Next, the array's elements are sorted and displayed to the standard
output stream:
                sv.hierarchicSort(arr, 2);

    o  Then the two elements of the array of pointers to MultiData's
member functions are swapped, and the previous step is repeated:
                swap(arr[0], arr[1]);
        sv.hierarchicSort(arr, 2);


After compiling the program the following command can be given:
            echo a 1 b 2 a 2 b 1 | a.out

This results in the following output:
            a 1 b 2 a 2 b 1
    ====
    a 1 a 2 b 1 b 2
    ====
    a 1 b 1 a 2 b 2
    ====


22.11.3: Deriving class templates from ordinary classes

        An ordinary class may be used as the base class for deriving a template
         class. The advantage
of such an inheritance tree is that the base class's members may all be
compiled beforehand. When objects of the class template are now instantiated
only the actually used members of the derived class template must be
instantiated.

This approach may be used for all class templates having member functions
whose implementations do not depend on template parameters. These members may
be defined in a separate class which is then used as a base class of the
class template derived from it.

As an illustration of this approach we'll develop such a class template
below. We'll develop a class Table derived from an ordinary class
TableType. The class Table displays elements of some type in a
table having a configurable number of columns. The elements are either
displayed horizontally (the first k elements occupy the first row) or
vertically (the first r elements occupy a first column).

When displaying the table's elements they are inserted into a stream.  The
table is handled by a separate class (TableType), implementing the table's
presentation. Since the table's elements are inserted into a stream, the
conversion to text (or string) is implemented in Table, but the
handling of the strings themselves is left to TableType. We'll cover some
characteristics of TableType shortly, concentrating on Table's
interface first:


    o  The class Table is a class template, requiring only one template
type parameter: Iterator referring to an iterator to some data type:
        template <typename Iterator>
class Table: public TableType
{

    o  Table doesn't need any data members. All data manipulations are
performed by TableType.
    o  Table has two constructors. The constructor's first two
parameters are Iterators used to iterate over the elements that must be
entered into the table. The constructors require us to specify the number of
columns we would like our table to have as well as a
FillDirection. FillDirection is an enum, defined by TableType,
having values HORIZONTAL and VERTICAL. To allow Table's users to
exercise control over headers, footers, captions, horizontal and vertical
separators, one constructor has a TableSupport reference parameter. The
class TableSupport is developed at a later stage as a virtual class
allowing clients to exercise this control. Here are the class's constructors:
                Table(Iterator const &begin, Iterator const &end,
                size_t nColumns, FillDirection direction);
        Table(Iterator const &begin, Iterator const &end,
                TableSupport &tableSupport,
                size_t nColumns, FillDirection direction);

    o  The constructors are Table's only two public members. Both
constructors use a base class initializer to initialize their TableType
base class and then call the class's private member fill to insert data
into the TableType base class object. Here are the constructor's
implementations:
        template <typename Iterator>
Table<Iterator>::Table(Iterator const &begin, Iterator const &end,
                TableSupport &tableSupport,
                size_t nColumns, FillDirection direction)
:
    TableType(tableSupport, nColumns, direction)
{
    fill(begin, end);
}

template <typename Iterator>
Table<Iterator>::Table(Iterator const &begin, Iterator const &end,
                size_t nColumns, FillDirection direction)
:
    TableType(nColumns, direction)
{
    fill(begin, end);
}

    o  The class's fill member iterates over the range of elements
[begin, end), as defined by the constructor's first two parameters.
As we will see shortly, TableType defines a protected data member
std::vector<std::string> d_string. One of the requirements of the data
type to which the iterators point is that this data type can be inserted into
streams. So, fill uses an ostringstream object to obtain the textual
representation of the data, which is then appended to d_string:
        template <typename Iterator>
void Table<Iterator>::fill(Iterator it, Iterator const &end)
{
    while (it != end)
    {
        std::ostringstream str;
        str << *it++;
        d_string.push_back(str.str());
    }
    init();
}


This completes the implementation of the class Table. Note that this
class template only has three members, two of them being
constructors. Therefore, in most cases only two function templates must be
instantiated: a constructor and the class's fill member. For example, the
following defines a table having four columns, vertically filled by
strings extracted from the standard input stream:
            Table<istream_iterator<string> >
        table(istream_iterator<string>(cin), istream_iterator<string>(),
              4, TableType::VERTICAL);

The fill-direction is specified as
TableType::VERTICAL. It could also have been specified using Table,
but since Table is a class template its specification would have been
slightly more complex: Table<istream_iterator<string> >::VERTICAL.

Now that the Table derived class has been designed, let's turn our
attention to the class TableType. Here are its essential characteristics:
    
    o  It is an ordinary class, designed to operate as Table's base
class.
    o  It uses various private data members, among which d_colWidth, a
vector storing the width of the widest element per column and d_indexFun,
pointing to the class's member function returning the element in
table[row][column], conditional to the table's fill
direction. TableType also uses a TableSupport pointer and a
reference. The constructor not requiring a TableSupport object uses the
TableSupport * to allocate a (default) TableSupport object and then
uses the TableSupport & as the object's alias. The other constructor
initializes the pointer to 0 and uses the reference data member to refer to
the TableSupport object provided by its parameter. Alternatively, a static
TableSupport object could have been used to initialize the reference data
member in the former constructor. The remaining private data members are
probably self-explanatory:
                TableSupport           *d_tableSupportPtr;
        TableSupport           &d_tableSupport;
        size_t                d_maxWidth;
        size_t                d_nRows;
        size_t                d_nColumns;
        WidthType               d_widthType;
        std::vector<size_t>   d_colWidth;
        size_t               (TableType::*d_widthFun)
                                        (size_t col) const;
        std::string const     &(TableType::*d_indexFun)
                                        (size_t row, size_t col) const;

    o  The actual string objects populating the table are stored in a
protected data member:
                std::vector<std::string> d_string;

    o  The (protected) constructors perform basic tasks: they initialize the
object's data members. Here is the constructor expecting a reference to a
TableSupport object:
        #include "tabletype.ih"

TableType::TableType(TableSupport &tableSupport, size_t nColumns,
                        FillDirection direction)
:
    d_tableSupportPtr(0),
    d_tableSupport(tableSupport),
    d_maxWidth(0),
    d_nRows(0),
    d_nColumns(nColumns),
    d_widthType(COLUMNWIDTH),
    d_colWidth(nColumns),
    d_widthFun(&TableType::columnWidth),
    d_indexFun(direction == HORIZONTAL ?
                    &TableType::hIndex
                :
                    &TableType::vIndex)
{}

    o  Once d_string has been filled, the table is initialized by
Table::fill. The init protected member resizes d_string so
that its size is exactly rows x columns, and it determines the maximum
width of the elements per column. Its implementation is straightforward:
        #include "tabletype.ih"

void TableType::init()
{
    if (!d_string.size())                   // no elements
        return;                             // then do nothing

    d_nRows = (d_string.size() + d_nColumns - 1) / d_nColumns;
    d_string.resize(d_nRows * d_nColumns);  // enforce complete table

                                            // determine max width per column,
                                            // and max column width
    for (size_t col = 0; col < d_nColumns; col++)
    {
        size_t width = 0;
        for (size_t row = 0; row < d_nRows; row++)
        {
            size_t len = stringAt(row, col).length();
            if (width < len)
                width = len;
        }
        d_colWidth[col] = width;

        if (d_maxWidth < width)             // max. width so far.
            d_maxWidth = width;
    }
}

    o  The public member insert is used by the insertion operator
(operator<<) to insert a Table into a stream. First it informs the
TableSupport object about the table's dimensions. Next it displays the
table, allowing the TableSupport object to write headers, footers and
separators:
        #include "tabletype.ih"

ostream &TableType::insert(ostream &ostr) const
{
    if (!d_nRows)
        return ostr;

    d_tableSupport.setParam(ostr, d_nRows, d_colWidth,
                            d_widthType == EQUALWIDTH ? d_maxWidth : 0);

    for (size_t row = 0; row < d_nRows; row++)
    {
        d_tableSupport.hline(row);

        for (size_t col = 0; col < d_nColumns; col++)
        {
            size_t colwidth = width(col);

            d_tableSupport.vline(col);
            ostr << setw(colwidth) << stringAt(row, col);
        }

        d_tableSupport.vline();
    }
    d_tableSupport.hline();

    return ostr;
}

    o  The cplusplus.yo.zip archive contains TableSupport's full
implementation. This implementation is found in the directory
yo/classtemplates/examples/table. Most of its remaining members are
private. Among those, these two members return table element
[row][column] for, respectively, a horizontally filled table and a
vertically filled table:
          inline std::string const &TableType::hIndex(size_t row, size_t col) const
    {
        return d_string[row * d_nColumns + col];
    }
    inline std::string const &TableType::vIndex(size_t row, size_t col) const
    {
        return d_string[col * d_nRows + row];
    }


The support class TableSupport is used to display headers, footers,
captions and separators. It has four virtual members to perform those tasks
(and, of course, a virtual constructor):
    
    o hline(size_t rowIndex): called just before displaying
the elements in row
rowIndex.
    o hline(): called immediately after displaying the final row.
    o vline(size_t colIndex): called just before displaying the element in
column colIndex.
    o vline(): called immediately after displaying all elements in a row.

The reader is referrred to the cplusplus.yo.zip archive for the full
implementation of the classes Table, TableType and TableSupport.
Here is a little program showing their use:
        /*
                                  table.cc
    */

    #include <fstream>
    #include <iostream>
    #include <string>
    #include <iterator>
    #include <sstream>

    #include "tablesupport/tablesupport.h"
    #include "table/table.h"

    using namespace std;
    using namespace FBB;

    int main(int argc, char **argv)
    {
        size_t nCols = 5;
        if (argc > 1)
        {
            istringstream iss(argv[1]);
            iss >> nCols;
        }

        istream_iterator<string>   iter(cin);   // first iterator isn't const

        Table<istream_iterator<string> >
            table(iter, istream_iterator<string>(), nCols,
                  argc == 2 ? TableType::VERTICAL : TableType::HORIZONTAL);

        cout << table << '\n';
    }
    /*
        Example of generated output:
        After: echo a b c d e f g h i j | demo 3
            a e i
            b f j
            c g
            d h
        After: echo a b c d e f g h i j | demo 3 h
            a b c
            d e f
            g h i
            j
    */



22.12: Static Polymorphism

Chapter 14 introduced polymorphism. Polymorphism allows us to
use a base class's interface to call implementations which are defined in
derived classes. Traditionally this involves defining Vtables for
polymorphic classes, containing pointers to functions that can be overridden
in derived classes.  Objects of polymorphic classes harbor hidden pointers,
pointing to their class's Vtables. This type of polymorphism is called
dynamic polymorphism, and it uses late binding as the function to
call is determined run-time, rather than compile-time.

In many cases, however, dynamic polymorphism isn't really required. Usually
the derived class objects that are passed to functions expecting base class
references are invariants: at fixed locations in programs fixed class types
are used to create objects. The polymorphic nature of these objects is used
inside the functions that receive these objects, expecting references to their
base classes. As an example consider reading information from a network
socket. A class SocketBuffer is derived from std::streambuf, and the
std::stream receiving a pointer to the SocketBuffer merely uses
std::streambuf's interface. However, the implementation, by using
polymorphism, in fact communicates with functions defined in SocketBuffer.

The disadvantages of this scheme are that, firstly, inside the functions
expecting references to polymorphic base classes execution is somewhat slowed
down precisely because of late-binding. Member functions aren't
directly called, but are called indirectly via the object's vpointer and
their derived class's Vtable. Secondly, programs using dynamic
polymorphism tend to become somewhat bloated compared to programs using static
binding. The code bloat is caused by the requirement to satisfy at
link-time all the references that are mentioned in all the object files
comprising the final program. This requirement forces the linker to link all
the functions whose addresses are stored in the Vtables of all polymorphic
classes, even if these functions are never actually called.

Static polymorphism  allows us to avoid these
disadvantages. It can be used instead of dynamic polymorphism in cases where
the abovementioned invariant holds. Static polymorphism, also called the
    curiously recurring template pattern (CRTP), is an example of 
    template meta programming (see also chapter 23 for
additional examples of template meta programming).

Whereas dynamic polymorphism is based on the concepts of vpointers,
Vtables, and function overriding, static polymorphism capitalizes on the
fact that function templates (c.q., member templates) are only compiled into
executable code when they are actually called. This allows us to write code in
which functions are called which are completely non-existent at the time we
write our code. This, however, shouldn't worry us too much. After all, we use
a comparable approach when calling a purely virtual function of an abstract
base class. The function is really called, but which function is eventually
called is determined later in time. With dynamic polymorphism it is determined
run-time, with static polymorphism it is determined compile-time.

There's no need to consider static and dynamic polymorphism as mutually
exclusive variants of polymorphism. Rather, both can be used together,
combining their strengths. 

This section contains several subsections.
    
    o  First, the syntax used with static polymorphism is introduced and
        illustrated;
    o  Next, it is shown how  classes that use dynamic polymorphism can be
        converted to classes that use static polymorphism;
    o  Finally it is illustrated how static polymorphism can be used to
        reduce implementation effort. Static polymorphism allows us to
        implement only once what would have to be implemented repeatedly when
        only dynamic polymorphism is used.


22.12.1: An example of static polymorphism

    With static polymorphism a class template takes the role of a base class in
dynamic polymorphism. This class template declares several members, which are
comparable to members of a polymorphic base class: they are either support
members or they call members that are overridden in derived classes.

In the context of dynamic polymorphism these overridable members are the base
class's virtual members. In the context of static polymorphism there are no
virtual members. Instead, the statically polymorphic base class (referred to
as the `base class' below) declares a template type parameter (referred to
as the `derived class type' below). Next, the base class's interfacing members
call members of the derived class type.

Here is a simple example: a class template acting as a base class. Its public
interface consists of one member. But different from dynamic polymorphism
there's no reference in the class's interface to any member showing
polymorphic behavior (i.e, no `virtual' members are declared):
            template <class Derived>
    struct Base
    {
        void interface();
    }

Let's have a closer look at the member `interface'. This member is called
by functions receiving a reference or pointer to the base class, but it may
call members that must be available in the derived class type at the point
where interface is called. Before we can call members of the derived class
type an object of the derived class type must be available. This object is
obtained through inheritance. The derived class type is going to be derived
from the base class. Thus Base's this pointer is also Derived's this
pointer.

Forget about polymorphism for a second: when we have a class Derived:
public Base then (because of inheritance) a static_cast<Derived *> can be
used to cast a Base * to a Derived object. A dynamic_cast of
course doesn't apply, as we don't use dynamic polymorphism. But a
static_cast is appropriate since our Base * does in fact point to
a Derived class object.

So, to call a Derived class member from inside interface we
can use the following implementation (remember that Base is a base class
of Derived):
            template<class Derived>
    void Base<Derived>::interface()
    {
        static_cast<Derived *>(this)->polymorphic();
    }

It's remarkable that, when the compiler is given this implementation it
cannot determine whether Derived is really derived from
Base. Neither can it determine whether the class Derived indeed offers
a member polymorphic. The compiler simply assumes this to be true. If
so, then the provided implementation is syntactically correct. One of the
key characteristics of using templates is that the implementation's viability
is eventually determined at the function's point of instantiation (cf. section
21.6). At that point the compiler will verify that, e.g., the
function polymorphic really is available.

Thus, in order to use the above scheme we must ensure that 
        
        o  derived class type is actually derived from the base class and
        o  that the derived class type defines a member
            `polymorphic'. 

The first requirement is satisfied by using the 
        curiously recurring template pattern:
            class First: public Base<First>

In this curious pattern the class First is derived from Base,
which itself is instantiated for First. This is acceptable, as the
compiler already has determined that the type First exists. At this point
that is all it needs.

The second requirement is simply satisfied by defining the member
polymorphic. In chapter 14 we saw that virtual and
overriding members belong to the class's private interface. We can apply the
same philosophy here, by placing polymorphic in First's private
interface, allowing it to be accessed from the base class by declaring
            friend void Base<First>::interface();

First's complete class interface can now be designed, followed by
polymorphic's implementation:
            class First: public Base<First>
    {
        friend void Base<First>::interface();

        private:
            void polymorphic();
    };
    void First::polymorphic()
    {
        std::cout << "polymorphic from First\n";
    }

Note that the class First itself is not a class template: its members
can be separately compiled and stored in, e.g., a library. Also, as is the
case with dynamic polymorphism, the member polymorphic has full access to
all of First's data members and member functions.

Multiple classes can now be designed like First, each offering their
own implementation of polymorphic. E.g., the member
Second::polymorphic of the class Second, designed like First,
could be implemented like this:
            void Second::polymorphic()
    {
        std::cout << "polymorphic from Second\n";
    }

The polymorphic nature of Base becomes apparent once a function
template is defined in which Base::interface is called. Again, the
compiler simply assumes a member interface exists when it reads the
definition of the following function template:
            template <class Class>
    void fun(Class &object)
    {
        object.interface();
    }

Only where this function is actually called will the compiler verify the
viability of the generated code. In the following main function a
First object is passed to fun: First declares interface
through its base class, and First::polymorphic is called by
interface. The compiler will at this point (i.e., where fun is called)
check whether first indeed has a member polymorphic. Next a Second
object is passed to fun, and here again the compiler checks whether
Second has a member Second::polymorphic:
            int main()
    {
        First first;
        fun(first);

        Second second;
        fun(second);
    }

There are also downsides to 
         
    using static polymorphism:
        
        o  First, the sentence `a Second object is passed to fun'
formally isn't correct, since fun is a function template the functions
fun called as fun(first) and fun(second) are
different functions, not just calls of one function with different
arguments. With static polymorphism every instantiation using its own template
parameters results in completely new code which is generated when the template
(e.g., fun) is instantiated. This is something to consider when creating
statically polymorphic base classes. If the base class defines data members
and member functions, and if these additional members are used by derived
class types, then each member has its own instantiation for each derived class
type. This also results in code bloat, albeit of a different kind than
observed with dynamic polymorphism. This kind of code bloat can often be
somewhat reduced by deriving the base class from its own (ordinary,
non-template) base class, encapsulating all elements of the statically
polymorphic base class that do not depend on its template type parameters.
        o  Second, if different types of statically polymorphic objects are
dynamically created (using the new operator) then the types of the
returned pointers are all different. In addition, the types of the pointers to
their statically polymorphic base classes differ from each other. These
latter pointers are different because they are pointers to Base<Derived>,
representing different types for different Derived types. Consequently,
and different from dynamic polymorphism, these pointers cannot be collected
in, e.g., a vector of shared pointers to base class pointers. There simply
isn't one base class pointer type. Thus, because of the different base class
types, there's no direct statically polymorphic equivalent to virtual
destructors.
        o  Third, as illustrated in the next section, designing static
polymorphic classes using multiple levels of inheritance is not a trivial
task.

Summarizing, static polymorphism is best used in situations where a small
number of different derived class types are used, where a fixed number of
derived class objects are used, and where the statically polymorphic base
classes themselves are lean (possibly encapsulating some of their code in
ordinary base classes of their own).


22.12.2: Converting dynamic polymorphic classes to static
            polymorphic classes

    So you've decided that you want to convert some of your dynamically
polymorphic classes into statically polymorphic classes. Now what?

In chapter 14 the base class Vehicle and some derived
classes were introduced. Vehicle, Car and Truck's interfaces look like
this (regarding the members that are involved in their polymorphic behaviors):
            class Vehicle
    {
        public:
            int mass() const;

        private:
            virtual int vmass() const;
    };

    class Car: public Vehicle
    {
        private:
            int vmass() const override;
    };
        
    class Truck: public Car
    {
        private:
            int vmass() const override;
    };

When converting dynamically polymorphic classes to statically polymorphic
classes we must realize that polymorphic classes show two important
characteristics:
    
    o  they define facilities (data members, member functions) that are
inherited by derived classes (e.g., Vecicle::mass) (i.e., the inheritable
interface), and
    o  derived classes implement the redefinable interface in a way that
suits their purpose (e.g., Truck::vmass).

With statically polymorphic classes these two characteristics should
completely be separated.  As we've seen in the previous section, a
statically polymorphic derived class derives from its base class by using its
own class type as argument to the base class's type parameter. This works fine
if there's only one level of inheritance (i.e., one base class, and one or
more classes that are directly derived from that base class).

With multiple levels of inheritance (e.g., Truck -> Car -> Vehicle)
Truck's inheritance specification becomes a problem. Here's an intial
attempt to use static polymorphism and multiple levels of inheritance:
            template <class Derived>
    class Vehicle
    {
        public:
            void mass() 
            {
                static_cast<Derived *>(this)->vmass();
            }
    };
    
    class Car: public Vehicle<Car>
    {
        friend void Vehicle<Car>::mass();
        void vmass();
    };

    class Truck: public Car
    {
        void vmass();
    };


        o  If Truck inherits from Car, then Truck implicitly
derives from Vehicle<Car>, as Car derives from
Vehicle<Car>. Consequently, when Truck{}.mass() is called it is not
Truck::vmass that's activated, but Car's vmass function. But
Truck must derive from Car in order to use Car's protected
features and to add Car's public features to its own public
interface. 
        o  Multiple inheritance also doesn't solve the issue: when
inheriting Truck from Vehicle<Truck> and from Car 
results in a class Truck that also inherits from
Vehicle<Car> (through Truck's Car base class), and compilation fails
as the compiler encounters an ambiguity when instantiating Vehicle::mass:
should it call Class::vmass or should it call Truck::vmass?
    

To solve this problem (i.e., to ensure that Truck{}.mass() calls
Truck::vmass) the redefinable interface must be separated 
from the inheritable interface.

In derived classes the protected and public interfaces of (direct or indirect)
base classes are made available using standard inheritance. This is shown in
the left-hand side of figure 29.  

------------------------------------------------------------------Insert Figure 29(Vehicle Static Polymorphic Class Design)about here (file: classtemplates/converting)------------------------------------------------------------------

The left-hand side classes are used as base classes for the next level of
inheritance (except for TruckBase, but TruckBase could be used as base
class for yet another level of class inheritance). This line of inheritance
declares the inheritable interface of the classes.

Each of the classes to the left is a base class of a single class to the
right: VehicleBase is a base class for Vehicle, TruckBase for
Truck. The classes to the left contain all members that are completely
independent of the elements that are involved in realizing the static
polymorphism. As that's a mere design principle to realize multiple levels of
static polymorphism the common data hiding principles are relaxed, and the
left-hand side classes declare their matching right-hand side derived class
templates as friend, to allow full access to all members of a left-hand side
class, including the private members, from the matching derived class template
to the right. E.g., VehicleBase declares Vechicle as its friend:
            class VehicleBase
    {
        template <class Derived>
        friend class Vehicle;

        // all members originally in Vehicle, but not involved 
        // in realizing static polymorphism are declared here. E.g.,
        size_t d_massFactor = 1;
    };

The top level class to the left (VehicleBase) lays the foundation of
static polymorphism, by defining that part of the interface that uses the
statically redefinable functions. E.g, using the curiously recurring template
pattern it defines a class member mass that calls the function vmass
of its derived class (in addition it can use all members of its non-class
template base class). E.g,:
            template <class Derived>
    class Vehicle: public VehicleBase
    {
        public:
            int mass() const
            {
                return d_massFactor *
                        static_cast<Derived const *>(this)->vmass();
            }
    };

Which function actually is called when vmass is called depends on the
implementation in the class Derived, which is handled by the remaining
classes, shown below Vehicle, which are all derived from Vehicle (as
well as from their own ...Base class). These classes use the curiously
recurring template pattern. E.g.,
            class Car: public CarBase, public Vehicle<Car>

So, if Car now implements its own vmass function, which can use
any of its own (i.e., CarBase's members), then that function is called
when calling a Vehicle's mass function. E.g.,
            template <class Vehicle>
    void fun(Vehicle &vehicle)
    {
        cout << vehicle.mass() << '\n';
    }
    
    int main()
    {
        Car car;
        fun(car);       // Car's vmass is called
    
        Truck truck;
        fun(truck);     // Truck's vmass is called
    }

Now that we've analyzed the design of statically polymorphic classes using
multiple levels of inheritance let's summarize the steps we took to
implement static polymorphism
    
    o  First, starting at the base class,
the class Vehicle. Vehicle's non-redefinable interface is moved
to a class VehicleBase, and Vehicle itself is turned into a statically
polymorphic base class. In general all members of the original 
polymorphic base class that do not use or implement virtual members should be
moved to the XBase class. 

o  VehicleBase declares Vehicle to be a friend, to allow
Vehicle full access to its former members, that are now in
VehicleBase.

o  Vehicle's members refer to the redefinable interface. I.e., its
members call members of its Derived template type parameter.  In this
implementation Vehicle does not implement its own vmass member. We
cannot define Vehicle<Vehicle>, and with static polymorphism the base
class is essentially comparable to an abstract base class. If this is
inconvenient then a default class can be specified for Vehicle's Derived
class, implementing the redefinable interface of the original 
polymorphic base class (allowing for definitions like Vehicle<> vehicle).

o  Likewise the remaining classes have their members not involved in
static polymorphism moved to a base class. E.g., 
class Car moves these members to  CarBase and
Truck moves those members to  TruckBase.  

o  A standard linear line of inheritance is used from VehicleBase to
CarBase and from there to TruckBase.

o  Each of the remaining classes (here: Car and Truck) is a
class template that derives from its base classes, and also, using the
curiously recurrent template pattern, from Vehicle.

o  Each of these remaining classes can now implement its own version of
the redefinable interface, as used by the members of Vehicle.

This design pattern can be extended to any level of inheritance: for each
new level a base class is constructed, deriving from the most deeply nested
XXXBase class so far, and deriving from Vehicle<XXX>, implementing its
own ideas about the redefinable interface.

Functions that are used in combination with statically polymorphic classes
themselves must be function templates. E.g.,
            template <class Vehicle>
    void fun(Vehicle &vehicle)
    {
        cout << vehicle.mass() << '\n';
    }

Here, Vehicle is just a formal name. When an object is passed to
fun it must offer a member mass, or compilation will fail. If the
object is in fact a Car or Truck, then their Vehicle<Type> static
base class member mass is called, which in turn uses static polymorphism
to call the member vmass as implemented by the actually passed class
type. The following main function displays, respectively, 1000 and 15000:
            int main()
    {
        Car car;
        fun(car);
    
        Truck truck;
        fun(truck);
    }

Note that this program implements fun twice, rather than once in the
case of dynamic polymorphism. The same holds true for the Vehicle class
template: two implementations, one for the Car type, and one for the
Truck type. The statically polymorphic program will be slightly faster,
though.

(A compilable example using static polymorphism is found in the
C++ Annotations's source distribution's file
yo/classtemplates/examples/staticpolymorphism/polymorph.cc.)


22.12.3: Using static polymorphism to avoid reimplementations

    Static polymorphism may profitably be used to avoid reimplementing code in an
otherwise dynamic polymorphic environment.

Consider the situation where we have a class containing a container of
pointers to some polymorphic base class type (like the class Vehicle from
chapter 14). How to copy such a container to another container?
We're not hinting at using shared pointers here, but would like to make a full
copy. Clearly, we'll need to duplicate the objects the pointers point at, and
assign these new pointers to the copied object's container.

The prototype design patttern is commonly used to create copies of objects of
polymorphic classes, given pointers to their base classes. 

To apply the prototype design pattern we have to implement newCopy in all
derived classes. Not by itself a big deal, but static polymorphism can nicely
be used here to avoid having to  reimplement this function for each derived
class. 

We start off with an abstract base class VehicleBase declaring a pure
virtual member newCopy:
            struct VehicleBase
    {
        virtual ~VehicleBase();
        VehicleBase *clone() const; // calls newCopy

        // declare any additional members defining the 
        // public user interface here

        private:
            VehicleBase *newCopy() const = 0;   
    };

Next we define a static polymorphic class CloningVehicle which is derived
from VehicleBase (note that we thus combine dynamic and static
polymorphism). This class provides the generic implementation of
newCopy. This is possible because all derived classes can use this
implementation. Also, CloningVehicle will be re-implemented for each
concrete type of vehicle that is eventually used: a Car, a Truck, an
AmphibiousVehicle, etc, etc. CloningVehicle thus isn't shared (like
VehicleBase), but instantiated for each new type of vehicle. 

The core characteristic of a statically polymorphic class is that it can use
its class template type parameter via a static_cast of its own type. A
member function like newCopy is implemented always the same way, viz.,
by using the derived class's copy constructor. Here is the class
CloningVehicle:
            template <class Derived>
    class CloningVehicle: public VehicleBase
    {
        VehicleBase *newCopy() const
        {
            return new Derived(*static_cast<Derived const *>(this));
        }
    };

And we're done. All types of vehicles should now be derived from
CloningVehicle, which automatically provides them with their own
implementation of newCopy. E.g., a class Car looks like this:
            class Car: public CloningVehicle<Car>
    {
        // Car's interface, no need to either
        // declare or implement newCopy,
        // but a copy constructor is required.
    };

Having defined a std::vector<VehicleBase *> original we could create a
copy of original like this:
                for(auto pointer: original)
        duplicate.push_back(pointer->clone());

Irrespective of the actual type of vehicle to which the pointers point,
their clone members will return pointers to newly allocated copies of
objects of their own types.


22.13: Class templates and nesting

When a class is nested within a class template, it automatically becomes a
class template itself.
        
    The nested class may use the template parameters of the surrounding
class, as shown by the following skeleton program. Within a class
PtrVector, a class iterator is defined. The nested class receives its
information from its surrounding class, a PtrVector<Type> class. Since
this surrounding class should be the only class constructing its iterators,
iterator's constructor is made private and the surrounding class is
given access to the private members of iterator using a
 bound friend declaration.
    Here is the initial section of PtrVector's class interface:
            template <typename Type>
    class PtrVector: public std::vector<Type *>

    This shows that the std::vector base class stores pointers to Type
values, rather than the values themselves. Of course, a destructor is now
required as the (externally allocated) memory for the Type objects must
eventually be freed. Alternatively, the allocation might be part of
PtrVector's tasks, when it stores new elements. Here it is assumed that
PtrVector's clients do the required allocations and that the destructor is
implemented later on.

The nested class defines its constructor as a private member, and allows
PtrVector<Type> objects access to its private members. Therefore only
objects of the surrounding PtrVector<Type> class type are allowed to
construct their iterator objects. However, PtrVector<Type>'s clients
may construct copies of the PtrVector<Type>::iterator objects they
use.

Here is the nested class iterator, using a (required) friend
declaration. Note the use of the typename keyword: since
std::vector<Type *>::iterator depends on a template parameter, it is not
yet an instantiated class. Therefore iterator becomes an implicit
typename.  The compiler issues a warning if typename has been
omitted. Here is the class interface:
                    class iterator
            {
                friend class PtrVector<Type>;
                typename std::vector<Type *>::iterator d_begin;

                iterator(PtrVector<Type> &vector);

                public:
                    Type &operator*();
            };

    The implementation of the members shows that the base class's begin
member is called to initialize d_begin.  PtrVector<Type>::begin's
return type must again be preceded by typename:
            template <typename Type>
    PtrVector<Type>::iterator::iterator(PtrVector<Type> &vector)
    :
        d_begin(vector.std::vector<Type *>::begin())
    {}

    template <typename Type>
    Type &PtrVector<Type>::iterator::operator*()
    {
        return **d_begin;
    }

    The remainder of the class is simple. Omitting all other functions that
might be implemented, the function begin returns a newly constructed
PtrVector<Type>::iterator object. It may call the constructor since the
class iterator declared its surrounding class as its friend:
            template <typename Type>
    typename PtrVector<Type>::iterator PtrVector<Type>::begin()
    {
        return iterator(*this);
    }

    Here is a simple skeleton program, showing how the nested class
iterator might be used:
            int main()
    {
        PtrVector<int> vi;

        vi.push_back(new int(1234));

        PtrVector<int>::iterator begin = vi.begin();

        std::cout << *begin << '\n';
    }


Nested  enumerations and 
  nested typedefs and using declarations can also be defined
by class templates. The class Table, mentioned before (section
22.11.3) inherited the enumeration TableType::FillDirection. Had
Table been implemented as a full class template, then this enumeration
would have been defined in Table itself as:
            template <typename Iterator>
    class Table: public TableType
    {
        public:
            enum FillDirection
            {
                HORIZONTAL,
                VERTICAL
            };
        ...
    };

In this case, the actual value of the template type parameter must be
specified when referring to a FillDirection value or to its type. For
example (assuming iter and nCols are defined as in section
22.11.3):
            Table<istream_iterator<string> >::FillDirection direction =
                argc == 2 ?
                    Table<istream_iterator<string> >::VERTICAL
                :
                    Table<istream_iterator<string> >::HORIZONTAL;

    Table<istream_iterator<string> >
        table(iter, istream_iterator<string>(), nCols, direction);


22.14: Constructing iterators

    Section 18.2 iterators as used by generic algorithms were
introduced. We've seen that several types of iterators were distinguished:
InputIterators, ForwardIterators, OutputIterators, BidirectionalIterators,
RandomAccessIterators, and ContiguousIterators.

To ensure that an object of a class is interpreted as a particular type of
iterator, the class must be designed so that it satisfies the requirements of
that type of iterator. The interface of a class implementing an iterator uses
     iterator_tags, which are available when including the
<iterator> header file.

Iterator classes should provide the following using declarations:
    
    o iterator_category, specifying the iterator's tag, as in
          using iterator_category = std::random_access_iterator_tag;
    o differenct_type, defining the type representing differences between
        pointers, commonly:
          using difference_type = std::ptrdiff_t;
    o value_type, defining the data type of a dereferenced iterator. E.g.,
          using value_type = std::string;
    o pointer, defining the iterator's pointer type which can also be
        defined as a const pointer. E.g.,
          using pointer = value_type *; // or maybe:
    using pointer = value_type const *;
    o reference, defining the iterator's pointer type which can also be
        defined as a const pointer. E.g.,
          using reference = value_type &; // or maybe:
    using reference = value_type const &;

These using declarations are commonly encountered in the public
sections of iterators, which therefore might be defined as structs.
  Here's an
example of how an iterator's interface could be designed:
        struct Iterator
    {
        using iterator_category = std::random_access_iterator_tag;
        using difference_type   = std::ptrdiff_t;
        using value_type        = std::string;
        using pointer           = value_type *;
        using reference         = value_type &;
 
        friend bool operator==(iterator const &lhs, iterator const &rhs);
        friend auto operator<=>(Iterator const &lhs, Iterator const &rhs);
        friend int operator-(Iterator const &lhs, Iterator const &rhs);
        friend Iterator operator+(Iterator const &lhs, int step);
        friend Iterator operator-(Iterator const &lhs, int step);
 
    private:
        // private data members
 
    public:
        // public members

    private:
        // private support members
    }
    In section 18.2 the characteristics of iterators were
discussed. All iterators
        
        
    should support (using Iterator as the generic name of the designed
iterator class and Type to represent the (possibly const, in which
case the associated operator should be a const member as well) data type to
which Iterator objects refer):
    
    o  a prefix increment operator (Iterator &operator++());
    o  a dereference operator (Type &operator*());
    o  a 'pointer to' operator (Type *operator->());
    o  comparison operators testing the (in)equality of two iterator objects
(preferably free operators, like bool operator==(Iterator const &lhs,
        Iterator const &rhs), bool operator!=((Iterator const &lhs,
        Iterator const &rhs))). Instead of operator!= the `spaceship
        operator (operator<=>) can also be used, allowing ordering of the
        iterators.

When
        
    iterators are used in the context of generic algorithms they must meet
additional requirements. These are required because generic algorithms perform
checks on the types of the iterators they receive. Simple pointers are usually
accepted, but if an iterator-object is used the generic algorithm might
require the iterator to specify what type of iterator it represents.

The type of iterator that is implemented is indicated by the class's
iterator_category. The six standard iterator categories are:
    
    o std::input_iterator_tag. This tag is used when
the iterator class defines an InputIterator.  Iterators of this type allow
reading operations, iterating from the first to the last element of the series
to which the iterator refers.

The InputIterator  dereference operator could be declared as follows:
            value_type const &operator*() const;
    or, in a concrete implementation the data type specified at using
value_type = ... could be used instead to value_type.  Except for the
standard operators there are no further requirements for InputIterators.

o std::output_iterator_tag. This tag is used when
the iterator class defines an OutputIterator.  Iterators of this type allow
assignments to their dereferenced iterators. Therefore, the OutputIterator
dereference operator is usually declared as:
            value_type &operator*();
Except for the standard operators there are no further requirements for
OutputIterators.

o std::forward_iterator_tag. This tag is used
when the iterator class defines a ForwardIterator.  Iterators of this type
allow reading and assignment operations, iterating from the first to the
last element of the series to which the iterator refers. Therefore, the
ForwardIterator dereference operator should be declared as follows:
            value_type &operator*();
    and it could also declare an overloaded operator*() const:    
            value_type const &operator*() const;
Except for the standard operators there are no further requirements for
ForwardIterators.

o std::bidirectional_iterator_tag. This
tag is used when the iterator class defines a
BidirectionalIterator. Iterators of this type allow reading and
assignment operations, iterating step by step, possibly in either direction,
over all elements of the series to which the iterator refers.

The Bidirectional dereference operator should allow assignment to the data
its dereference operator refers to and it should allow stepping
backward. BidirectionalIterator should therefore, in addition to the standard
operators required for iterators, offer the following operators:
            value_type &operator*(); // any maybe its const-overload
    Iterator &operator--();

o std::random_access_iterator_tag. This tag is
used when the iterator class defines a RandomAccessIterator.  These
iterators are like Bidirectional iterators, but in addition allow stepping to
the next iterator using int step sizes (the resulting iterator may point
to a location outside of the valid iterator range. Like other iterators,
RandomAccessIterators do not check the validity of the obtained iterator).

In addition to the members offered by the BidirectionalIterator, 
RandomAccessIterators provide the following operators:
    
    o Type operator-(Iterator const &rhs) const, returning the number of
data elements between the current and rhs iterator (returning a negative
value if rhs refers to a data element beyond the data element this
iterator refers to);
    o Iterator operator+(int step) const, returning an iterator referring
to a data element step data elements beyond the data element this
iterator refers to;
    o Iterator operator-(int step) const, returning an iterator referring
to a data element step data elements before the data element this
iterator refers to;
    o bool operator<(Iterator const &rhs) const, returning true if
the data element this iterator refers to is located before the data
element the rhs iterator refers to.

o std::contiguous_iterator_tag. This tag is
used when the iterator class defines a ContiguousIterator.  These iterators
offer the same facilities as the RandomAccessIterators, but it is the
responsibility of the software engineer to ensure that the elements to which
the iterators refer are stored contiguously in memory.

Each iterator tag assumes that a certain set of operators is
available. The RandomAccessIterators ant ContiguousIterators are the
most complex, as other iterators only offer subsets of their facilities.

Note that iterators are always defined over a certain range
([begin, end)). Increment and decrement operations may result in
undefined behavior of the iterator if the resulting iterator value would refer
to a location outside of this range.

Normally iterators of classes merely accesses the data elements of those
classes. Internally, an iterator might use an ordinary pointer but it is
hardly ever necessary for the iterator to allocate memory. Therefore,
iterators normally don't need to define copy constructors and assignment
operators. For the same reason iterators usually don't require destructors.

Most classes offering members returning iterators do so by having members
construct the required iterators that are thereupon returned as objects. As
the caller of these member functions only has to use or sometimes
copy the returned iterator objects, there is usually no need to provide
publicly available constructors, except for the copy constructor (which is
available by default). Therefore additional constructors are usually defined as
private or protected members. To allow an outer class to create
iterator objects, the iterator class usually declares the outer class as its
friend.

In the following sections a RandomAccessIterator, and a reverse
RandomAccessIterator is covered. The container class for which a random
access iterator must be developed may actually store its data elements in many
different ways (e.g., using containers or pointers to pointers).  Section
26.5 contains an example of a self-made template iterator class.

The random access iterator developed in the next sections reaches data
elements that are only accessible through pointers. The iterator class is
designed as an inner class of a class derived from a vector of string
pointers.


22.14.1: Implementing a `RandomAccessIterator'

    In the chapter about containers (chapter 12) it was noted that
containers own the information they contain. If they contain objects, then
those objects are destroyed once the containers are destroyed. As pointers are
not objects their use in containers is discouraged (STL's unique_ptr and
shared_ptr type objects may be used, though).  Although discouraged, we
might be able to use pointer data types in specific contexts. In the following
class StringPtr, an ordinary class is derived from the std::vector
container that uses std::string * as its data type:
            #ifndef INCLUDED_STRINGPTR_H_
    #define INCLUDED_STRINGPTR_H_

    #include <string>
    #include <vector>

    class StringPtr: public std::vector<std::string *>
    {
        public:
            StringPtr(StringPtr const &other);
            ~StringPtr();

            StringPtr &operator=(StringPtr const &other);
    };

    #endif

    This class needs a destructor: as the object stores string pointers, a
destructor is required to destroy the strings once the StringPtr object
itself is destroyed.  Similarly, a copy constructor and an overloaded
assignment operator are required. Other members (in particular: constructors)
are not explicitly declared here as they are not relevant to this section's
topic.

Assume that we want to use the sort generic algorithm with
StringPtr objects. This algorithm (see section 19.1.54) requires two
RandomAccessIterators. Although these iterators are available (via
std::vector's begin and end members), they return iterators to
std::string *s. When comparing string * values the pointer values
instead of the content of the strings are compared, which is not what we want.

To remedy this, an internal type StringPtr::iterator is defined,
not returning iterators to pointers, but iterators to the objects these
pointers point to. Once this iterator type is available, we can add the
following members to our StringPtr class interface, hiding the identically
named, but useless members of its base class:
            StringPtr::iterator begin();    // returns iterator to the first element
    StringPtr::iterator end();      // returns iterator beyond the last
                                    // element

Since these two members return the (proper) iterators, the elements in a
StringPtr object can easily be sorted:
            int main()
    {
        StringPtr sp;               // assume sp is somehow filled

        sort(sp.begin(), sp.end()); // sp is now sorted
    }

To make this work, a type StringPtr::iterator is defined. As suggested by
its type name, iterator is a nested type of StringPtr. To use a
StringPtr::iterator in combination with the sort generic algorithm it
must be a RandomAccessIterator, whose value_type is a
std::string. Therefore, the iterator specifies:
            using iterator_category = std::random_access_iterator_tag;
    using difference_type   = std::ptrdiff_t;
    using value_type        = std::string;
    using pointer           = value_type *;
    using reference         = value_type &;


Now we're ready to redesign StringPtr's class interface. It offers
members returning (reverse) iterators, and a nested iterator class. Here
is its interface:
        struct StringPtr: public std::vector<std::string *>
{
    class iterator;
    using reverse_iterator = std::reverse_iterator<iterator>;

    iterator begin();
    iterator end();
    reverse_iterator rbegin();
    reverse_iterator rend();
};

struct StringPtr::iterator
{
//USING
    using iterator_category = std::random_access_iterator_tag;
    using difference_type   = std::ptrdiff_t;
    using value_type        = std::string;
    using pointer           = value_type *;
    using reference         = value_type &;


Next we have a look at StringPtr::iterator's characteristics:
    
    o iterator defines StringPtr as its friend, so iterator's
constructor can be private. Only the StringPtr class itself should be able
to construct iterators. Copy construction and iterator-assignment should be
possible, but that's possible by default and needs no specific declaration or
implementation. Furthermore, since an iterator is already provided by
StringPtr's base class, we can use that iterator to access the information
stored in the StringPtr object.

o  StringPtr::begin and StringPtr::end may simply return
iterator objects. They are implemented like this:
        inline StringPtr::iterator StringPtr::begin()
{
    return iterator(std::vector<std::string *>::begin());
}
inline StringPtr::iterator StringPtr::end()
{
    return iterator(std::vector<std::string *>::end());
}


o  All of iterator's remaining members are public. It's very easy to
implement them, mainly manipulating and dereferencing the available iterator
d_current. A RandomAccessIterator requires a series of operators. They
usually have very simple implementations, and can often very well be
implemented inline:
        
        o iterator &operator++(); the pre-increment operator:
            inline StringPtr::iterator &StringPtr::iterator::operator++()
{
    ++d_current;
    return *this;
}

        o  iterator operator++(int); the post-increment operator:
            inline StringPtr::iterator StringPtr::iterator::operator++(int)
{
    return iterator(d_current++);
}

        o iterator &operator--(); the pre-decrement operator:
            inline StringPtr::iterator &StringPtr::iterator::operator--()
{
    --d_current;
    return *this;
}

        o  iterator operator--(int); the post-decrement operator:
            inline StringPtr::iterator StringPtr::iterator::operator--(int)
{
    return iterator(d_current--);
}

        o iterator &operator=(iterator const &other); the overloaded
            assignment operator. Since iterator objects do not allocate
            any memory themselves, the default assignment operator can be
            used.
        o bool operator==(iterator const &rhv) const; testing the equality
            of two iterator objects:
                inline bool operator==(StringPtr::iterator const &lhs,
                       StringPtr::iterator const &rhs)
{
    return lhs.d_current == rhs.d_current;
}

        o auto operator<=>(iterator const &rhv) const; testing the
            ordering of two iterator objects: 
                inline auto operator<=>(StringPtr::iterator const &lhs,
                        StringPtr::iterator const &rhs)
{
    return lhs.d_current <=> rhs.d_current;
}

        o int operator-(iterator const &rhv) const; returning the number of
            elements between the element pointed to by the left-hand side
            iterator and the right-hand side iterator (i.e., the value to add
            to the left-hand side iterator to make it equal to the value of
            the right-hand side iterator):
                inline int operator-(StringPtr::iterator const &lhs,
                     StringPtr::iterator const &rhs)
{
    return lhs.d_current - rhs.d_current;
}

        o Type &operator*() const; returning a reference to the object to
            which the current iterator points. With an InputIterator and
            with all const_iterators, the return type of this overloaded
            operator should be Type const &. This operator returns a
            reference to a string. This string is obtained by dereferencing
            the dereferenced d_current value. As d_current is an
            iterator to string * elements, two dereference operations are
            required to reach the string itself:
                inline std::string &StringPtr::iterator::operator*() const
{
    return **d_current;
}

        o iterator operator+(int stepsize) const; this operator
            advances the current iterator by stepsize:
                inline StringPtr::iterator operator+(StringPtr::iterator const &lhs,
                                                                int step)
{
    StringPtr::iterator ret{ lhs };
    ret.d_current += step;          // avoids ambiguity
    return ret;
}

        o iterator operator-(int stepsize) const; this operator
            decreases the current iterator by stepsize:
                inline StringPtr::iterator operator-(StringPtr::iterator const &lhs,
                                                                int step)
{
    StringPtr::iterator ret{ lhs };
    ret.d_current -= step;          // avoids ambiguity
    return ret;
}

        o std::string *operator->() const is an additionally added
            operator. Here only one dereference operation is required,
            returning a pointer to the string, allowing us to access the
            members of a string via its pointer.
                inline std::string *StringPtr::iterator::operator->() const
{
    return *d_current;
}

        o  Two more additionally added operators are operator+= and
            operator-=. They are not formally required by
            RandomAccessIterators, but they come in handy anyway:
                inline StringPtr::iterator &StringPtr::iterator::operator+=(int step)
{
    d_current += step;
    return *this;
}
inline StringPtr::iterator &StringPtr::iterator::operator-=(int step)
{
    d_current -= step;
    return *this;
}


The interfaces required for other iterator types are simpler, requiring
only a subset of the interface required by a random access iterator. E.g.,
the forward iterator is never decremented and never incremented over arbitrary
step sizes. Consequently, in that case all decrement operators and
operator+(int step) can be omitted from the interface. Of course, the tag
to use would then be std::forward_iterator_tag. The tags (and the set of
required operators) vary accordingly for the other iterator types.


22.14.2: Implementing a `reverse_iterator'

        Once we've implemented an iterator, the matching reverse iterator can
be implemented in a jiffy. To implement a reverse_iterator
std::reverse_iterator  is used, which nicely
implements the reverse iterator once we have defined an iterator. Its
constructor merely requires an object of the iterator type for which we want
to construct a reverse iterator.

To implement a reverse iterator for StringPtr we only need to define
the reverse_iterator type in its interface. This requires us to specify
only one line of code, which must be inserted after the interface of the class
iterator:
            using reverse_iterator = std::reverse_iterator<iterator>;

Also, the well-known members rbegin and rend are added to
StringPtr's interface. Again, they can easily be implemented inline:
        inline StringPtr::reverse_iterator StringPtr::rbegin()
{
    return reverse_iterator(end());
}
inline StringPtr::reverse_iterator StringPtr::rend()
{
    return reverse_iterator(begin());
}

    Note the arguments the reverse_iterator constructors receive: the
        
    begin point of the reversed iterator is obtained by providing
reverse_iterator's constructor with the value returned by the member
end: the endpoint of the normal iterator range; the endpoint of
the reversed iterator is obtained by providing reverse_iterator's
constructor with the value returned by the member begin: the begin
point of the normal iterator range.

The following program illustrates the use of StringPtr's
RandomAccessIterator:
            #include <iostream>
    #include <algorithm>
    #include "stringptr.h"
    using namespace std;

    int main(int argc, char **argv)
    {
        StringPtr sp;

        while (*argv)
            sp.push_back(new string{ *argv++ });

        sort(sp.begin(), sp.end());
        copy(sp.begin(), sp.end(), ostream_iterator<string>(cout, " "));

        cout << "\n======\n";

        sort(sp.rbegin(), sp.rend());
        copy(sp.begin(), sp.end(), ostream_iterator<string>(cout, " "));

        cout << '\n';
    }
    /*
            when called as:
        a.out bravo mike charlie zulu quebec

            generated output:
        a.out bravo charlie mike quebec zulu
        ======
        zulu quebec mike charlie bravo a.out
    */

    Although it is thus possible to construct a reverse iterator from a normal
iterator, the opposite does not hold true: it is not possible to
initialize a normal iterator from a reverse iterator.

Assume we would like to process all lines stored in vector<string>
lines up to any trailing empty lines (or lines only containing blanks) it
might contain. How should we proceed? One approach is to start the processing
from the first line in the vector, continuing until the first of the trailing
empty lines. However, once we encounter an empty line it does of course not
have to be the first line of the set of trailing empty lines. In that case,
we'd better use the following algorithm:
    
    o  First, use
            rit = find_if(lines.rbegin(), lines.rend(), NonEmpty());

to obtain a reverse_iterator rit pointing to the last non-empty
        line.
    o  Next, use
            for_each(lines.begin(), --rit, Process());

to process all lines up to the first empty line.

However, we can't mix iterators and reverse iterators when using generic
algorithms. So how can we initialize the second iterator using the available
reverse_iterator? The solution is not very difficult, as an iterator may
be initialized from a pointer. Although the reverse iterator rit is not a
pointer, &*(rit - 1) or &*--rit is. So we use
            for_each(lines.begin(), &*--rit, Process());
    to process all the lines up to the first of the set of trailing empty
lines. In general, if rit is a reverse_iterator pointing to some
element and we need an iterator to point to that element, we may use
&*rit to initialize the iterator. Here, the dereference operator is
applied to reach the element the reverse iterator refers to. Then the address
operator is applied to obtain its address with which we can initialize the
iterator.

When defining a const_reverse_iterator (e.g., matching a
const_iterator class), then the const_iterator's operator* member
should be a member returning a non-modifiable value or object. Since a
const_reverse_iterator uses the iterator's operator-- member,
we're running against a small conceptual conflict. On the one hand, a
std::input_iterator_tag is inappropriate, since we must allow decrementing
the iterator. On the other hand, a std::bidirectional_iterator is
inappropriate, since we don't allow modification of the data.

Iterator tags are primarily conceptual. If const_iterators and
const_reverse_iterators only allow increment operations, then an
input_iterator_tag is the best match for the iterator's intended
use. Hence this tag is used below.

Furthermore, in line with the nature of a input_iterator_tag our
const_iterator should not offer an operator--. This, of course,
causes problems: a reverse iterator must be able to use the iterator's
operator-- member. This can easily be solved by stashing the iterator's
operator-- in the iterator's private section, and declaring
std::reverse_iterator<(const_)iterator> its friend (note that declaring a
(const_)reverse_iterator that is derived from std::reverse_iterator
doesn't solve the issue: it is std::reverse_iterator that calls the
iterator's operator--, not a class that is derived from it).

To keep the const-ness of dereferencing const_iterator iterators
using specification can added to the const_iterator class. The
using specifications provided by Data::iterator are inherited by
Data::const_iterator, but Data::const_iterator may also specify:
            using const_pointer   = value_type const *;
    using const_reference = value_type const &;
    whereafter its operator *() can return a const_reference.

The elements involved in defining an iterator, const_iterator,
reverse_iterator and const_reverse_iterator for a class Data is
illustrated in the following example, which only requires the definitions of
the iterator and const_iterator classes, as these types can be passed
to the reverse_iterator template to obtain the corresponding
reverse-iterators:
        // compile only: members only declared, not implemented

#include <string>
#include <iterator>
#include <iostream>

struct Data
{
    class iterator;
    class const_iterator;

    using reverse_iterator = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    private:
        std::string *d_data;
        size_t d_n;

    public:
        // ...
};

struct Data::iterator
{
    using iterator_category = std::bidirectional_iterator_tag;
    using difference_type   = std::ptrdiff_t;
    using value_type        = std::string;
    using pointer           = value_type *;
    using reference         = value_type &;

    friend class Data;

    private:
        pointer d_current;

    public:
        iterator() = default;

        iterator &operator++();
        iterator &operator--();
        std::string &operator*();

    private:
        iterator(std::string *data, size_t idx);

        friend class std::reverse_iterator<iterator>;
};

bool operator==(Data::iterator const &lhs, Data::iterator const &rhs);

struct Data::const_iterator: public Data::iterator
{
    using const_pointer   = value_type const *;
    using const_reference = value_type const &;

    friend class Data;
    friend class std::reverse_iterator<const_iterator>;

    const_iterator() = default;

    const_iterator &operator++();
    const_iterator &operator--();

    const_reference operator*() const;
    // or, in this case: std::string const &operator*() const;

    private:
        const_iterator(std::string const *data, size_t idx);
};

bool operator==(Data::const_iterator const &lhs,
                Data::const_iterator const &rhs);


void demo()
{
    Data::iterator iter;
    Data::reverse_iterator riter(iter);

    std::cout << *riter << '\n';

    Data::const_iterator citer;
    Data::const_reverse_iterator criter(citer);

    std::cout << *criter << '\n';
};




Chapter 23: Advanced Template Use


The main purpose of templates is to provide a generic definition of classes
and functions that may then be tailored to specific types.

But templates allow us to do more than that. If not for compiler
implementation limitations, templates could be used to program, at
compile-time, just about anything we use computers for. This remarkable feat,
offered by no other current-day computer language, stems from the fact that
templates allow us to do three things at compile-time:
    
    o  Templates allow us to do integer arithmetic (and to save computed
values symbolically);
    o  Templates allow us to make compile-time decisions;
    o  Templates allow us to do things repeatedly.

Of course, asking the compiler to compute, e.g., prime numbers, is one
thing. But it's a completely different thing to do so in an award winning
way. Don't expect speed records to be broken when the compiler performs
complex calculations for us. But that's all beside the point. In the end we
can ask the compiler to compute virtually anything using C++'s
template language, including prime numbers....

In this chapter these remarkable features of templates are
discussed. Following a short overview of subtleties related to templates the
main characteristics of template meta programming are introduced.

In addition to template type and template non-type parameters there is a third
kind of template parameter, the
    template template parameter. This kind of template parameter is
introduced shortly, laying the groundwork for the discussion of trait
classes and policy classes.

This chapter ends with the discussion of several additional and interesting
applications of templates: adapting compiler error messages, conversions to
class types and an elaborate example discussing compile-time list processing.

Much of the inspiration for this chapter came from two highly recommended
books: Andrei Alexandrescu's 2001 book
    Modern C++ design (Addison-Wesley) and Nicolai
Josutis and David Vandevoorde's 2003 book
    Templates (Addison-Wesley).


23.1: Subtleties

    In section 22.2.1 a special application of the keyword typename
was discussed. There we learned that it is not only used to define a name for
a (complex) type, but also to distinguish types defined by class templates
from members defined by class templates.  In this section another application
is introduced In section 23.1.1 we cover the problem of how to refer to
base class templates from derived class templates.

In addition to the special applications of typename section
23.1.2 introduces some new syntax that is related to the extended use of
the keyword typename: ::template, .template and ->template are
used to inform the compiler that a name used inside a template is itself a
class template.


23.1.1: Type resolution for base class members

    Below we see two class templates. Base and Derived, Base being
Derived's base class:
            #include <iostream>

    template <typename T>
    class Base
    {
        public:
            void member();
    };
    template <typename T>
    void Base<T>::member()
    {
        std::cout << "This is Base<T>::member()\n";
    }
    template <typename T>
    class Derived: public Base<T>
    {
        public:
            Derived();
    };
    template <typename T>
    Derived<T>::Derived()
    {
        member();
    }

    This example won't compile, and the compiler tells us something like:
        
            error: there are no arguments to 'member' that depend on a template
           parameter, so a declaration of 'member' must be available

This error causes some confusion as ordinary (non-template) base classes
readily make their public and protected members available to classes that are
derived from them. This is no different for class templates, but only if the
compiler can figure out what we mean. In the above example the compiler
can't as it doesn't know for what type T the member function
member must be initialized when called from Derived<T>::Derived.

To appreciate why this is true, consider the situation where we have
defined a specialization:
            template <>
    Base<int>::member()
    {
        std::cout << "This is the int-specialization\n";
    }

Since the compiler, when Derived<SomeType>::Derived is called, does
not know whether a specialization of member is in effect, it can't
decide (when compiling Derived<T>::Derived) for what type to instantiate
member. It can't decide this when compiling Derived<T>::Derived as
member's call in Derived::Derived doesn't require a template type
parameter.

In cases like these, where no template type parameter is available to
determine which type to use, the compiler must be told that it should postpone
its decision about the template type parameter to use (and therefore about the
particular (here: member) function to call)
    
until instantiation time.

This may be implemented in two ways: either by using this or by
explicitly mentioning the base class, instantiated for the derived class's
template type(s). When this is used the compiler is informed that we're
referring to the type T for which the template was instantiated. Any
confusion about which member function to use (the derived class or base class
member) is resolved in favor of the derived class member. Alternatively, the
base or derived class can explicitly be mentioned (using Base<T> or
Derived<T>) as shown in the next example. Note that with the int
template type the int specialization is used.
            #include <iostream>

    template <typename T>
    class Base
    {
        public:
            virtual void member();
    };
    template <typename T>
    void Base<T>::member()
    {
        std::cout << "This is Base<T>::member()\n";
    }
    template <>
    void Base<int>::member()
    {
        std::cout << "This is the int-specialization\n";
    }
    template <typename T>
    class Derived: public Base<T>
    {
        public:
            Derived();
            void member() override;
    };
    template <typename T>
    void Derived<T>::member()
    {
        std::cout << "This is Derived<T>::member()\n";
    }
    template <typename T>
    Derived<T>::Derived()
    {
        this->member();         // Using `this' implies using the
                                // type for which T was instantiated
        Derived<T>::member();   // Same: calls the Derived member
        Base<T>::member();      // Same: calls the Base member
        std::cout << "Derived<T>::Derived() completed\n";
    }

    int main()
    {
        Derived<double> d;
        Derived<int> i;
    }

    /*
        Generated output:
    This is Derived<T>::member()
    This is Derived<T>::member()
    This is Base<T>::member()
    Derived<T>::Derived() completed
    This is Derived<T>::member()
    This is Derived<T>::member()
    This is the int-specialization
    Derived<T>::Derived() completed
    */

    The above example also illustrates the use of virtual member templates
(although virtual member templates aren't often used). In the example Base
declares a virtual void member and Derived defines its overriding
function member. In that case this->member() in Derived::Derived
calls, due to member's virtual nature, Derived::member. The statement
Base<T>::member(), however, always calls Base's member function
and can be used to
     bypass dynamic polymorphism.


23.1.2: ::template, .template and ->template

    In general, the compiler is able to determine the true nature of a name. As
discussed, this is not always the case and sometimes we have to advise the
compiler. The typename keyword is often used for that purpose.

While parsing a source the compiler receives a series of tokens,
representing meaningful units of text encountered in the program's source. A
token could represent, e.g., an identifier or a number. Other tokens represent
operators, like =, + or <. It is precisely the last token that may
cause problems as it may have very different meanings. The correct meaning
cannot always be determined from the context in which the compiler encounters
<. In some situations the compiler does know that < does not
represent the less than operator, as when a template parameter list
follows the keyword template, e.g.,
            template <typename T, int N>

Clearly, in this case < does not represent a `less than' operator.

The special meaning of < when it is preceded by template forms the
basis for the syntactic constructs discussed in this section.

Assume the following class has been defined:
            template <typename Type>
    class Outer
    {
        public:
            template <typename InType>
            class Inner
            {
                public:
                    template <typename X>
                    void nested();
            };
    };

The class template Outer defines a nested class template
Inner. Inner in turn defines a member template function.

Next a class template Usage is defined, offering a member function
caller expecting an object of the above Inner type.  An initial setup
for Usage looks like this:
            template <typename T1, typename T2>
    class Usage
    {
        public:
            void caller(Outer<T1>::Inner<T2> &obj);
       ...
    };

The compiler won't accept this as it interprets Outer<T1>::Inner as a
class type. But there is no class Outer<T1>::Inner. Here the compiler
generates an error like:
            error: 'class Outer<T1>::Inner' is not a type

There are two ways to inform the compiler that Inner
 
 
    itself is a template, using the template type parameter <T2>. One way
is to prefix caller's parameter specification with typename:
        void caller(typename Outer<T1>::Inner<T2> &obj)
    Although this works, specifying that Outer<T1>::Inner<T2> is a
typename might looks a bit strange. Therefore. alternatively, the 
::template construction can be used:
        void caller(Outer<T1>::template Inner<T2> &obj)
    ::template tells the compiler that the next < must not be
interpreted as a `less than' token, but rather as a template type
argument.

Of course, caller itself is not just declared, it must also be
implemented. Assume that its implementation should call Inner's member
nested, instantiated for yet another type X. The class template
Usage therefore receives a third template type parameter, called
T3. Assuming it has been defined by Usage, then to implement
caller, we write:
            template <typename T1, typename T2, typename T3>
    void caller(typename Outer<T1>::template Inner<T2> &obj)
    {
        obj.nested<T3>();
    }
    Here again we run into a problem. In the function's body the compiler 
again interprets < as `less than', seeing a logical expression having as
its right-hand side a primary expression instead of a function call specifying
a template type T3.

To tell the compiler that is should interpret <T3> as a type to
instantiate, the template keyword is used, as mentioned by the compiler:
        expected `template' keyword before dependent template name
    Although issued as a warning, this warning is then followed by error
messages caused by the compiler interpreting < as `less than'. To remove
the warning and the error messages we write .template informing
the compiler that what follows is not a `less than' operator, but a
type specification. The function's final implementation thus becomes:
            template <typename T1, typename T2, typename T3>
    void caller(typename Outer<T1>::template Inner<T2> &obj)
    {
        obj.template nested<T3>();
    }

Instead of defining value or reference parameters functions may also
define pointer parameters. Had obj been defined as a pointer parameter the
implementation would have had to use the ->template construction, rather
than the .template construction. E.g.,
            template <typename T1, typename T2, typename T3>
    void caller(typename Outer<T1>::template Inner<T2> *ptr)
    {
        ptr->template nested<T3>();
    }

As we've seen class templates can be derived from base class templates. The
base class template can declare a static member template, which is
available to a class that is derived from this base class. Such a base class
might look like this:
            template <typename Type>
    struct Base
    {
        template <typename Tp>
        static void fun();
    };

Normally, when a base class defines a static member we can just call that
member by prefixing its name by its class name. E.g.,
            int main()
    {
        Base<int>::fun<double>();
    }

This also works fine if a class Derived is derived from Base,
instantiated for a specific type:
            struct Der: public Base<int>
    {
        static void call()
        {
            Base<int>::fun<int>();      // OK
            fun<int>();                 // also OK
        };
    };
    But when the derived class itself is a class template this way to
call fun won't compile anymore, as it interprets Base<Type>::fun in
Base<Type>::fun<int> as a type, to be instantiated for int. This
interpretation can be overruled by indicating that fun itself is a
template. For this the ::template prefix is used:
            template <typename Type>
    struct Der: public Base<Type>
    {
        //template <typename Tp>    // 'call' may be a member template
        //static                    // 'call' may be a static member
        void call()
        {
            // fun<int>();                      // won't compile
            // Base<Type>::fun<int>();          // won't compile
            Base<Type>::template fun<int>();    // OK
            Base<Type>::template fun<Tp>();     // OK if call is a 
                                                //    member template
        };                                      // (activate typename Tp)
    };


23.2: Template Meta Programming



23.2.1: Values according to templates

    In template programming values are preferably represented by enum
values. Enums are preferred over, e.g., int const values since enums never
require any linkage. They are pure symbolic values with no memory
representation whatsoever.

Consider the situation where a programmer must use a cast, say a
reinterpret_cast. A problem with a reinterpret_cast is that it is the
ultimate way to turn off all compiler checks. All bets are off, and we can
write extreme but absolutely pointless reinterpret_cast statements, like
            int intVar = 12;
    ostream &ostr = reinterpret_cast<ostream &>(intVar);

Wouldn't it be nice if the compiler would warn us against such oddities by
generating an error message?

If that's what we'd like the compiler to do, there must be some way to
distinguish madness from weirdness. Let's assume we agree on the following
distinction: reinterpret casts are never acceptable if the target type
represents a larger type than the expression (source) type, since that would
immediately result in exceeding the amount of memory that's actually available
to the target type. For this reason it's clearly silly to
reinterpret_cast<double *>(&intVar), but reinterpret_cast<char
*>(&intVar) could be defensible.

The intent is now to create a new kind of cast, let's call it
reinterpret_to_smaller_cast. It should only be allowed to perform a
reinterpret_to_smaller_cast if the target type occupies less memory than
the source type (note that this exactly the opposite reasoning as used by
Alexandrescu   (2001), section 2.1).

To start, we construct the following template:
            template<typename Target, typename Source>
    Target &reinterpret_to_smaller_cast(Source &source)
    {
        // determine whether Target is smaller than source
        return reinterpret_cast<Target &>(source);
    }

At the comment an enum-definition is inserted defining a symbol having a
suggestive name.  A compile-time error results if the required condition is
not met and the error message displays the name of the symbol. A division by
zero is clearly not allowed, and noting that a false value represents a
zero value, the condition could be:
            1 / (sizeof(Target) <= sizeof(Source));

The interesting part is that this condition doesn't result in any code at
all. The enum's value is a plain value that's computed by the compiler while
evaluating the expression:
            template<typename Target, typename Source>
    Target &reinterpret_to_smaller_cast(Source &source)
    {
        enum
        {
            the_Target_size_exceeds_the_Source_size =
                1 / (sizeof(Target) <= sizeof(Source))
        };
        return reinterpret_cast<Target &>(source);
    }

When reinterpret_to_smaller_cast is used to cast from int to
double an error is produced by the compiler, like this:
            error: enumerator value for 'the_Target_size_exceeds_the_Source_size'
        is not an integer constant

whereas no error is reported if, e.g.,
reinterpret_to_smaller_cast<int>(doubleVar) is requested with
doubleVar defined as a double.

In the above example an enum was used to compute (at compile-time) a
value that is illegal if an assumption is not met. The creative part is
finding an appropriate expression.

Enum values are well suited for these situations as they do not consume
any memory and their evaluation does not produce any executable code. They can
be used to accumulate values too: the resulting enum value then contains a
final value, computed by the compiler rather than by executable code as the
next sections illustrate. In general, programs shouldn't do run-time what they
can do
        
    at compile-time and performing complex calculations resulting in constant
values is a clear example of this principle.


23.2.1.1: Converting integral types to types

        Another use of values buried inside templates is to `templatize' simple scalar
int values. This is useful in situations where a scalar value (often a
bool value) is available to select a specialization but a type is required
to base the selection on. This situation is shortly encountered (section
23.2.2).

Templatizing
        
    integral values is based on the fact that a
        
    class template together with its template arguments defines a type. E.g.,
vector<int> and vector<double> are different types.

Turning integral values into templates is easily done. Define a template (it
does not have to have any content at all) and store the integral value in an
enum:
            template <int x>
    struct IntType
    {
        enum { value = x };
    };

As IntType does not have any members the `class IntType' can be
defined as `struct IntType', saving us from having to type public:.

Defining the enum value `value' allows us to retrieve the value
used at the instantiation at no cost in storage. Enum values are neither
variables nor data members and thus have no address. They are mere values.

It's easy to use the struct IntType. An anonymous or named
object can be defined by specifying a value for its int non-type
parameter. Example:
            int main()
    {
        IntType<1> it;
        cout << "IntType<1> objects have value: " << it.value << "\n" <<
                "IntType<2> objects are of a different type "
                        "and have values " << IntType<2>().value << '\n';
    }

Actually, neither the named object nor the anonymous object is
    
required. As the enum is defined as a plain value, associated with the
struct IntType we merely have to specify the specific int for which
the struct IntType is defined to retrieve its `value', like this:
            int main()
    {
        cout << "IntType<100>, no object, defines `value': " <<
                IntType<100>::value << "\n";
    }


23.2.2: Selecting alternatives using templates

    An essential characteristic of programming languages is that they allow the
conditional execution of code. For this C++ offers the if and
switch statements. If we want to be able to `program the compiler'
this feature must also be offered by templates.

Like templates storing values templates making choices do not require any code
to be executed at run-time. The selection is purely made by the compiler, at
compile-time. The essence of template meta programming is that we are not
using or relying on any executable code.  The result of a template meta
program often is executable code, but that code is a function of decisions
merely made by the compiler.

Template (member) functions are only instantiated when they are actually
used. Consequently we can define specializations of functions that are
mutually exclusive. Thus it is possible to define a specialization that can be
compiled in situation one, but not in situation two and to define another
specialization that can be compiled in situation two, but not in situation
one. Using specializations code can be generated that is tailored to the
demands of a particular situation.

A feature like this cannot be implemented in run-time executable code. For
example, when designing a generic storage class the software engineer may
intend to store value class type objects as well as objects of
polymorphic class types in the final storage class. Thus the software
engineer may conclude that the storage class should contain pointers to
objects, rather than the objects themselves. The initial implementation
attempt could look like this:
            template <typename Type>
    void Storage::add(Type const &obj)
    {
        d_data.push_back(
            d_ispolymorphic ?
                obj.clone()
            :
                new Type{obj}
        );
    }

The intent is to use the clone member function of the class Type
if Type is a polymorphic class and the standard copy constructor if
Type is a value class.

Unfortunately, this scheme usually fails as value classes do not define
clone member functions and polymorphic base classes should delete
their copy constructors (cf. section 7.6). It doesn't matter to the
compiler that clone is never called for value classes and that the copy
constructor is only available in value classes and not in polymorphic
classes. It merely has some code to compile, and can't do that because of
missing members. It's as simple as that.


23.2.2.1: Defining overloading members

            Template meta programming comes to the rescue. Knowing that class template
member functions are only instantiated when used, our plan is to design
overloaded add member functions of which only one is going to be called
(and thus instantiated). Our selection will be based on an additional (in
addition to Type itself) template non-type parameter that indicates
whether we'll use Storage for polymorphic or non-polymorphic classes. Our
class Storage starts like this:
            template <typename Type, bool isPolymorphic>
    class Storage

Initially two overloaded versions of our add member are defined:
one used with Storage objects storing polymorphic objects (using true
as its template non-type argument) and one storing value
class objects (using false as its template non-type argument).

Unfortunately we run into a small problem: functions cannot be overloaded
by their argument values but only by their argument types. But the
small problem may be solved. Realizing that types are defined by the
combination of template names and their template arguments we may convert the
values true and false into types using the knowledge from section
23.2.1.1 about how to convert integral values to types.

We'll provide one (private) add member with an IntType<true> parameter
(implementing the polymorphic class) and another (private) add member with
an IntType<false> parameter (implementing the non-polymorphic class).

In addition to these two private members a third (public) member add is
defined calling the appropriate private add member by providing an
IntType argument, constructed from Storage's template non-type
parameter.

Here are the implementations of the three add members:
            // declared in Storage's private section:

    template <typename Type, bool isPolymorphic>
    void Storage<Type, isPolymorphic>::add(Type const &obj, IntType<true>)
    {
        d_data.push_back(obj.clone());
    }

    template <typename Type, bool isPolymorphic>
    void Storage<Type, isPolymorphic>::add(Type const &obj, IntType<false>)
    {
        d_data.push_back(new Type(obj));
    }

    // declared in Storage's public section:

    template <typename Type, bool isPolymorphic>
    void Storage<Type, isPolymorphic>::add(Type const &obj)
    {
        add(obj, IntType<isPolymorphic>());
    }

The appropriate add member is instantiated and called because a
primitive value can be converted to a type. Each of the possible template
non-type values is thus used to define an overloaded class template member
function.

Since class template members are only instantiated when used only one of
the overloaded private add members is instantiated. Since the other one is
never called (and thus never instantiated) compilation errors are prevented.


23.2.2.2: Class structure as a function of template parameters

            Some software engineers have reservations when thinking about the
Storage class that uses pointers to store copies of value class
objects. Their argument is that value class objects can very well be stored by
value, rather than by pointer. They'd rather store value class objects by
value and polymorphic class objects by pointer.

Such distinctions  frequently occur in template meta programming and
the following struct IfElse
        
        
    may be used to obtain one of two types, depending on a bool selector
value.

First define the generic form of the template:
            template<bool selector, typename FirstType, typename SecondType>
    struct IfElse
    {
        using type = FirstType;
    };

Then define a partial specialization. The specialization represents
a specific selector value (e.g., false) and leaves the remaining types
open to further specification:
            template<typename FirstType, typename SecondType>
    struct IfElse<false, FirstType, SecondType>
    {
        using type = SecondType;
    };

The former (generic) definition associates FirstType with the
IfElse::type type definition, the latter definition (partially specialized
for the logical value false) associates SecondType with the
IfElse::type type definition.

The IfElse template allows us to define class templates whose data
organization is conditional to the template's parameters.
    
 Using IfElse the Storage class may define
pointers to store copies of polymorphic class type objects and values
to store value class type objects:
            template <typename Type, bool isPolymorphic>
    class Storage
    {
        using DataType = typename IfElse<isPolymorphic, Type *, Type>::type;

        std::vector<DataType> d_data;

        private:
            void add(Type const &obj, IntType<true>);
            void add(Type const &obj, IntType<false>);
        public:
            void add(Type const &obj);
    }

    template <typename Type, bool isPolymorphic>
    void Storage<Type, isPolymorphic>::add(Type const &obj, IntType<true>)
    {
        d_data.push_back(obj.clone());
    }

    template <typename Type, bool isPolymorphic>
    void Storage<Type, isPolymorphic>::add(Type const &obj, IntType<false>)
    {
        d_data.push_back(obj);
    }

    template <typename Type, bool isPolymorphic>
    void Storage<Type, isPolymorphic>::add(Type const &obj)
    {
        add(obj, IntType<isPolymorphic>());
    }

The above example uses IfElse's type, defined by IfElse as
either FirstType or SecondType. IfElse's type defines the
actual data type to use for Storage's vector data type.

The remarkable result in this example is that the data organization of
the Storage class now depends on its template arguments. Since the
isPolymorphic == true situation uses different data types than the
isPolymorphic == false situation, the overloaded private add members
can utilize this difference immediately. E.g., add(Type const &obj,
IntType<false>) uses direct copy construction to store a copy of obj
in d_vector.

It is also possible to make a selection from multiple types as IfElse
structs can be nested. Realize that using IfElse never has any effect on
the size or execution time of the final executable program. The final program
simply contains the appropriate type, conditional to the type that's
eventually selected.


23.2.2.3: An illustrative example

            The next example, defining MapType as a map having plain types or
pointers for either its key or value types, illustrates this approach:
            template <typename Key, typename Value, int selector>
    class Storage
    {
        using MapType =  
                typename IfElse<
                    selector == 1,              // if selector == 1:
                    map<Key, Value>,            // use map<Key, Value>

                    typename IfElse<
                        selector == 2,          // if selector == 2:
                        map<Key, Value *>,      // use map<Key, Value *>

                        typename IfElse<
                            selector == 3,      // if selector == 3:
                            map<Key *, Value>,  // use map<Key *, Value>
                                                // otherwise:
                            map<Key *, Value *> // use map<Key *, Value *>

                        >::type
                    >::type
                >::type;

        MapType d_map;

        public:
            void add(Key const &key, Value const &value);
        private:
            void add(Key const &key, Value const &value, IntType<1>);
           ...
    };
    template <typename Key, typename Value, int selector>
    inline void Storage<selector, Key, Value>::add(Key const &key,
                                                   Value const &value)
    {
        add(key, value, IntType<selector>());
    }

The principle used in the above examples is: if class templates may use
data types that depend on template non-type parameters, an IfElse struct
can be used to select the appropriate data types. Knowledge about the various
data types may also be used to define overloaded member functions. The
implementations of these overloaded members may then be optimized to the
various data types. In programs only one of these alternate functions (the one
that is optimized to the actually used data types) will then be instantiated.

The private add functions define the same parameters as the public
add wrapper function, but add a specific IntType type, allowing the
compiler to select the appropriate overloaded version based on the template's
non-type selector parameter.


23.2.3: Templates: Iterations by Recursion

        As there are no variables in template meta programming, there is no
template equivalent to a for or while statement. However, iterations
can always be rewritten as recursions.  Recursions are supported
by templates and so iterations can always be implemented as (tail) recursions.
        

To implement iterations by (tail) recursion do as follows:
    
    o  define a specialization implementing the end-condition;
    o  define all other steps using recursion.
    o  store intermediate values as enum values.

The compiler selects a more specialized template implementation over a
more generic one. By the time the compiler reaches the end-condition the
recursion stops since the specialization does not use recursion.

Most readers are probably familiar with the recursive implementation of
the mathematical `factorial' operator, commonly represented by the
exclamation mark (!): n factorial (so: n!) returns the successive
products n * (n - 1) * (n - 2) * ... * 1, representing the number of ways
n objects can be permuted. Interestingly, the factorial operator is itself
usually defined by a recursive definition:
            n! = (n == 0) ?
            1
        :
            n * (n - 1)!

To compute n! from a template, a template Factorial can be defined
using an int n template non-type parameter. A specialization is defined
for the case n == 0. The generic implementation uses recursion according
to the factorial definition. Furthermore, the Factorial template defines
an enum value `value' containing its factorial value. Here is the
generic definition:
            template <int n>
    struct Factorial
    {
        enum { value = n * Factorial<n - 1>::value };
    };

Note how the expression assigning a value to `value' uses constant
values that can be determined by the compiler. The value n is provided, and
Factorial<n - 1> is computed using template meta
programming. Factorial<n-1> in turn results in value that can be
determined by the compiler (viz.
Factorial<n-1>::value). Factorial<n-1>::value represents the value
defined by the type Factorial<n - 1>. It is not the value returned
by an object of that type. There are no objects here but merely values
defined by types.

The recursion ends in  a specialization. The compiler selects the
specialization (provided for the terminating value 0) instead of  the generic
implementation whenever possible. Here is the specialization's implementation:
            template <>
    struct Factorial<0>
    {
        enum { value = 1 };
    };

The Factorial template can be used to determine, compile time, the
number of permutations of a fixed number of objects. E.g.,
            int main()
    {
        cout << "The number of permutations of 5 objects = " <<
                Factorial<5>::value << "\n";
    }

Once again, Factorial<5>::value is not evaluated at run-time, but
at compile-time. The run-time equivalent of the above cout statement is,
therefore:
            int main()
    {
        cout << "The number of permutations of 5 objects = " <<
                120 << "\n";
    }


23.3: User-defined literals

In addition to the literal operators discussed in section 11.14 
C++ also offers a function template literal operator, matching the
prototype
            template <char ...Chars>
    Type operator "" _identifier()

This variadic non-type parameter function template defines no parameters,
but merely a variadic non-type parameter list.

Its argument must be an int constant, as is also expected by the literal
operator defining an unsigned long long int parameter. All the characters
of the int constant are passed as individual char non-type template
arguments to the literal operator.

For example, if _NM2km is a literal operator function template, it can
be called as 80_NM2km. The function template is then actually called as
_NM2km<'8', '0'>(). If this function template merely uses template meta
programming techniques and only processes integral data then its actions can
be performed completely at compile-time. To illustrate this, let's assume
NM2km only processes and returns unsigned values.

The function template _NM2km can forward its argument to a class template,
defining an enum constant value, and that performs the required
computations. Here is the implementation of the variadic literal operator
function template _NM2km:
                template <char ... Chars>
    constexpr size_t operator "" _NM2km()
    {
        return static_cast<size_t>(             // forward Chars to NM2km
                NM2km<0, Chars ...>::value * 1.852);
    }

The class template NM2km defines three non-type parameters: acc
accumulates the value, c is the first character of the variadic non-type
parameters, while ...Chars represents the remaining non-type parameters,
contained in a non-type parameter pack. Since c is, at each recursive
call, the next character from the original non-type parameter pack, the value
so far multiplied by 10 plus the value of the next character is passed as the
next accumulated value to its recursive call, together with the remaining
elements of the parameter pack, represented by Chars ...:
                template <size_t acc, char c, char  ...Chars>
    struct NM2km
    {
        enum
        {
            value = NM2km<10 * acc + c - '0', Chars ...>::value
        };
    };

Eventually, the parameter pack is empty. For this case a partial
specialization of NM2km is available:
            template <size_t acc, char c>   // empty parameter pack
    struct NM2km<acc, c>
    {
        enum
        {
            value = 10 * acc + c - '0'
        };
    };

This works fine, but of course not in cases where binary, octal, or
hexadecimal values must also be interpreted. In that case we must first
determine whether the first character(s) indicate a special number
system. This can be determined by the class template NM2kmBase, that is
now called from the _NM2km literal operator:
                template <char ... Chars>
    constexpr size_t operator "" _NM2km()
    {
        return static_cast<size_t>(         // forward Chars to NM2kmBase
                NM2kmBase<Chars ...>::value * 1.852);
    }

The NM2kmBase class template normally assumes the decimal number
system, passing base value 10 and initial sum 0 to NM2km. The NM2km
class template is provided with an additional (first) non-type parameter
representing the base value of the number system to use. Here is
NM2kmBase:
            template <char ...Chars>
    struct NM2kmBase
    {
        enum
        {
            value = NM2km<10, 0, Chars ...>::value
        };
    };

Partial specializations handle the different number systems, by inspecting
the first (one or two) characters:
            template <char ...Chars>
    struct NM2kmBase<'0', Chars ...>        // "0..."
    {
        enum
        {                                   // octal value: base 8
            value = NM2km<8, 0, Chars ...>::value
        };
    };

    template <char ...Chars>
    struct NM2kmBase<'0', 'b', Chars ...>   // "0b..."
    {
        enum
        {                                   // binary value: base 2
            value = NM2km<2, 0, Chars ...>::value
        };
    };

    template <char  ...Chars>
    struct NM2kmBase<'0', 'x', Chars ...>   // "0x..."
    {
        enum
        {                                   // hex value: base 16
            value = NM2km<16, 0, Chars ...>::value
        };
    };

NM2km is implemented as before, albeit that it can now handle
various number systems. The conversion from character to numeric value is left
to a small support function template, cVal:
            template <char c>
    constexpr int cVal()
    {
        return '0' <= c <= '9' ? c - '0' : 10 + c - 'a';
    }

    template <size_t base, size_t acc, char c, char  ...Chars>
    struct NM2km
    {
        enum
        {
            value = NM2km<base, base * acc + cVal<c>(),
                                        Chars ...>::value
        };
    };

    template <size_t base, size_t acc, char c>
    struct NM2km<base, acc, c>
    {
        enum { value = base * acc + cVal<c>() };
    };


23.4: Template template parameters

Consider the following situation: a software engineer is asked to design a
storage class Storage. Data stored in Storage objects may either make
and store copies of the data or store the data as received. Storage
objects may also either use a vector or a linked list as its underlying
storage medium. How should the engineer tackle this request? Should four
different Storage classes be designed?

The engineer's first reaction could be to develop an all-in Storage class.
It could have two data members, a list and a vector, and its constructor could
be provided with maybe an enum value indicating whether the data itself or new
copies should be stored. The enum value can be used to initialize a series of
pointers to member functions performing the requested tasks (e.g., using a
vector to store the data or a list to store copies).

Complex, but doable. Then the engineer is asked to modify the class: in the
case of new copies a custom-made allocation scheme should be used rather than
the standard new operator.  He's also asked to allow the use of yet
another type of container, in addition to the vector and the list that were
already part of the design. Maybe a deque would be preferred or maybe even
a stack.

It's clear that the approach aiming at implementing all functionality and all
possible combinations in one class doesn't scale. The class Storage soon
becomes a monolithic giant which is hard to understand, maintain, test, and
deploy.

One of the reasons why the big, all-encompassing class is hard to deploy and
understand is that a well-designed class should
        
    enforce constraints: the design of the class should, by itself,
disallow certain operations, violations of which should be detected by the
compiler, rather than by a program that might terminate in a fatal error.

Think about the above request. If the class offers both an interface to access
the vector data storage and an interface to access the list data storage,
then it's likely that the class offers an overloaded operator[] member
to access elements in the vector. This member, however, will be syntactically
present, but semantically invalid when the list data storage is selected,
which doesn't support operator[].

Sooner or later, users of the monolithic all-encompassing class
Storage will fall into the trap of using operator[] even though
they've selected the list as the underlying data storage. The compiler won't
be able to detect the error, which only appears once the program is
running, confusing its users.

The question remains: how should the engineer proceed, when confronted with
the above questions? It's time to introduce  policies.


23.4.1: Policy classes - I

    A policy defines (in some contexts: prescribes) a particular kind of
behavior. In C++ a
    
 policy class defines a certain part of the class interface. It may also
define inner types, member functions, and data members.

In the previous section the problem of creating a class that might use any of
a series of allocation schemes was introduced. These allocation schemes all
depend on the actual data type to use, and so the `template reflex' should
kick in.

Allocation schemes should probably be defined as template classes, applying
the appropriate allocation procedures to the data type at hand. When such
allocation schemes are used by the familiar STL containers (like
std::vector, std::stack, etc.), then such home-made allocation schemes
should probably be derived from std::allocator, to provide
for the requirements made by these containers. The class template
std::allocator is declared by the <memory> header file and the three
allocation schemes developed here were all derived from std::allocator.

Using in-class implementations for brevity the following allocation classes
could be defined:
    
    o  No special allocation takes place, Data is used `as is':
    template <typename Data>
class PlainAlloc: public std::allocator<Data>
{
    template<typename IData>
    friend std::ostream &operator<<(std::ostream &out,
                                    PlainAlloc<IData> const &alloc);
    Data d_data;

    public:
        PlainAlloc()
        {}
        PlainAlloc(Data const &data)
        :
            d_data(data)
        {}
        PlainAlloc(PlainAlloc<Data> const &other)
        :
            d_data(other.d_data)
        {}
};

o  The second allocation scheme uses the standard new operator to
allocate a new copy of the data:
    template <typename Data>
class NewAlloc: public std::allocator<Data>
{
    template<typename IData>
    friend std::ostream &operator<<(std::ostream &out,
                                    NewAlloc<IData> const &alloc);
    Data *d_data;

    public:
        NewAlloc()
        :
            d_data(0)
        {}
        NewAlloc(Data const &data)
        :
            d_data(new Data(data))
        {}
        NewAlloc(NewAlloc<Data> const &other)
        :
            d_data(new Data(*other.d_data))
        {}
        ~NewAlloc()
        {
            delete d_data;
        }
};

o  The third allocation scheme uses the placement new operator (see
        section 9.1.5), requesting memory from a
        common pool (the implementation of the member request,
        obtaining the required amount of memory, is left as an exercise to the
        reader):
    template<typename Data>
class PlacementAlloc: public std::allocator<Data>
{
    template<typename IData>
    friend std::ostream &operator<<(std::ostream &out,
                                    PlacementAlloc<IData> const &alloc);
    Data *d_data;

    static char s_commonPool[];
    static char *s_free;

    public:
        PlacementAlloc()
        :
            d_data(0)
        {}
        PlacementAlloc(Data const &data)
        :
            d_data(new(request()) Data(data))
        {}
        PlacementAlloc(PlacementAlloc<Data> const &other)
        :
            d_data(new(request()) Data(*other.d_data))
        {}
        ~PlacementAlloc()
        {
            d_data->~Data();
        }
    private:
        static char *request();
};

The above three classes define policies that may be selected by the
user of the class Storage introduced in the previous section. In addition
to these classes, additional allocation schemes could be implemented by the
user as well.

To apply the proper allocation scheme to the class Storage,
Storage should be designed as a class template itself. The class also
needs a template type parameter allowing users to specify the data type.

The data type to be used by a particular allocation scheme could of course
be specified when specifying the allocation scheme to use. The class
Storage would then have two template type parameters, one for the data
type, one for the allocation scheme:
            template <typename Data, typename Scheme>
    class Storage ...

To use the class Storage we would then write, e.g.:
            Storage<string, NewAlloc<string>> storage;

Using Storage this way is fairly complex and potentially error-prone,
as it requires the user to specify the data type twice. Instead, the
allocation scheme should be specified using a new type of template parameter,
not requiring the user to specify the data type required by the allocation
scheme. This new kind of template parameter (in addition to the well-known
template type parameter and template non-type parameter) is called the
 template template parameter.

Starting with the C++14 standard the keyword class 
         
    in the syntactical form of template template parameters (template
<parameter specifications> class Name) is no longer required. From that
standard onward, the keyword typename can also be used (e.g., template
<parameter specifications> typename Name).


23.4.2: Policy classes - II: template template parameters

    Template  template parameters allow us to
specify a class template as a template parameter. By specifying a class
template, it is possible to add a certain kind of behavior (called a
policy) to an existing class template.

To specify an allocation policy, rather than an allocation type
for the class Storage we rephrase its class template header:
definition starts as follows:
            template <typename Data, template <typename> class Policy>
    class Storage...

The second template parameter is new. It is a template template
parameter. It has the following elements:
    
    o  The keyword template, starting the template template parameter;
    o  The keyword template is followed (between angle brackets) by a
        list of template parameters that must be specified for the template
        template parameter. These parameters may be given names, but names
        are usually omitted as those names cannot be used in subsequent
        template definitions. On the other hand, providing formal names may
        help the reader of the template to understand the kind of templates
        that must be specified with the template template parameter.
    o  Template template parameters must match, in numbers and types (i.e.,
        template type parameters, template non-type parameters, template
        template parameters) the template parameters that must be specified
        for the policy. This can be tricky, as some templates use default
        parameters that are hardly ever changed (like the allocation schemes
        for containers). When specifying template template parameters those
        default types are used. If they should be specifiable then they must
        be added to the template template parameter's types. In the following
        example fun can only use the default allocator, gun must
        specify the allocator when defining vd, and hun defines a
        default allocator type, offering the option to either specify the
        allocator type or use the default type:
                #include <vector>
    using namespace std;

    template<typename Data, template <typename> class Vect>
    void fun()
    {
        // Vect<Data, std::allocator<Data>> vd; // fails
        Vect<Data> vd;                          // OK, uses std::allocator
    }

    template<typename Data,
             template <typename, typename> class Vect>
    void gun()
    {
        Vect<Data, std::allocator<Data>> vd;    // allocator type is required
    }

    template<typename Data,
             template <typename, typename = std::allocator<Data>> class Vect>
    void hun()
    {
        Vect<Data> vd;                          // uses default allocator type
    }

    int main()
    {
        fun<int, vector>();
        gun<int, vector>();
        hun<int, vector>();
    }

Alternatively, alias templates, introduced in section
        23.5, can be used to define names for templates using
        predefined parameter types.
    o  Following the bracketed list the keyword class or typename 
        must be specified. 
    o  All parameters may be provided with default arguments. This is shown
        in the next example of a hypothetical class template:
        template <
    template <
        typename = std::string,
        int = 12,
        template <typename = int> class Inner = std::vector
    >
    class Policy
>
class Demo
{
   ...
};

Here, the class template Demo expects a template template parameter
        named Policy, expecting three template parameters: a template type
        parameter (by default std::string); a template non-type parameter
        (by default having value 12); and Policy itself expects a template
        template parameter, called Inner, by default using an int as
        its template type parameter.

Policy classes are often an integral part of the class under
consideration. Because of this they are often deployed as base classes. In the
example the class Policy could be used as a base class of the class
Storage.

The policy operates on the class Storage's data type. Therefore the
policy is informed about that data type as well. Our class Storage now
begins like this:
            template <typename Data, template <typename> class Policy>
    class Storage: public Policy<Data>

This automatically allows us to use Policy's members when implementing
the members of the class Storage.

Our home-made allocation classes do not really provide us with many useful
members. Except for the extraction operator they offer no immediate access to
the data. This can easily be repaired by adding more members. E.g., the class
NewAlloc could be augmented with operators allowing access to and
modification of stored data:
                    operator Data &()   // optionally add a `const' member too
        {
            return *d_data;
        }
        NewAlloc &operator=(Data const &data)
        {
            *d_data = data;
        }

The other allocation classes could be given comparable members.

Let's use the allocation schemes in some real code. The next
example shows how Storage can be defined using some data type and an
allocation scheme. We start out again with a class Storage:
            template <typename Data, template <typename> class Allocate>
    class Storage: public std::vector<Data, Allocate<Data>>
    {};

That's all we have to do. Note that std::vector formally has two
template parameters. The first one is the vector's data type, which is always
specified; the second one is the allocator used by the vector. Usually the
allocator is left unspecified (in which case the default STL allocator is
used), but here it is mentioned explicitly, allowing us to pass our own
allocation policy to Storage.

All required functionality is inherited from the vector base class, while
the policy is `factored into the equation' using a template template
parameter. Here's an example showing how this is done:
            Storage<std::string, NewAlloc> storage;

    copy(istream_iterator<std::string>(cin), istream_iterator<std::string>(),
            back_inserter(storage));

    cout << "Element index 1 is " << storage[1] << '\n';
    storage[1] = "hello";

    copy(storage.begin(), storage.end(),
         ostream_iterator<NewAlloc<std::string> >(cout, "\n"));

Since Storage objects are also std::vector objects the STL copy
function can be used in combination with the back_inserter iterator to add
some data to the storage object. Its elements can be accessed and modified
using the index operator. Then NewAlloc<std::string> objects are inserted
into cout (also using the copy function).

Interestingly, this is not the end of the story. Remember that our
intention was to create a class allowing us to specify the storage type as
well. What if we don't want to use a vector, but instead would like to use
a list?

It's easy to change Storage's setup so that a completely different
storage type can be used on request, like a deque. To implement this, the
storage class is parameterized as well, using yet another template template
parameter:
            template <typename Data, template <typename> class AllocationPolicy,
              template <typename, typename> class Container = std::vector>
    class Storage: public Container<Data, AllocationPolicy<Data>>
    {};

The earlier example using a Storage object can be used again without
requiring any modifications at all (except for the above redefinition). It
clearly can't be used with a list container, as the list lacks
operator[]. Trying to do so is immediately recognized by the compiler,
producing an error if an attempt is made to use operator[] on, e.g., a
    list.\ (A complete example showing the definition of the
allocation classes and the class Storage as well as its use is provided in
the C++ Annotations's distribution in the file
yo/advancedtemplates/examples/storage.cc.) A list container, however
can still be specified as the container to use. In that case a Storage is
implemented as a list, offering list's interface, rather than
vector's interface, to its users.


23.4.2.1: The destructor of Policy classes

        In the previous section policy classes were used as base classes of template
classes. This resulted in the interesting observation that a policy class
may serve as a base class
        
    of a derived class.
    As a policy class may act as a base class, a pointer or reference to such
a policy class can be used to point or refer to the derived class using the
policy.

This situation, although legal, should be avoided for various reasons:
        
    
    o  Destruction of a derived class object using the base class's
destructor requires the implementation of a virtual destructor;
    o  A virtual destructor introduces overhead to a class that normally has
no data members, but merely defines behavior: suddenly a vtable is
required as well as a data member pointing to the vtable;
    o  Virtual member functions somewhat reduce the efficiency of code;
virtual member functions use dynamic polymorphism, which in principle is
undoing the advantages of static polymorphism as offered by templates;
    o  Virtual member functions in templates may result in code bloat:
once an instantiation of a class's member is required, the class's vtable and
all its virtual members must be implemented too.

To avoid these drawbacks, it is good practice to prevent the use of
references or pointers to policy classes to refer or point to derived
class objects. This is accomplished by providing policy classes with
        
    non-virtual protected destructors. With a non-virtual destructor there
is no performance penalty and since its destructor is protected users cannot
refer to classes derived from the policy class using a pointer or reference to
the policy class.


23.4.3: Structure by Policy

    Policy classes usually define behavior, not structure. Policy classes
        
 are normally used to parameterize some aspect of the behavior of classes that
are derived from them. However, different policies may require different data
members. These data members may also be defined by the policy classes. Policy
classes may therefore be used to define both behavior and structure.

By providing a well-defined interface a class derived from a policy class may
define member specializations using the different structures of policy classes
to their advantage. For example, a plain pointer-based policy class could
offer its functionality by resorting to C-style pointer juggling,
whereas a vector-based policy class could use the vector's members
directly.

In this example a generic class template Size could be designed
expecting a container-like policy using features commonly found in
containers, defining the data (and hence the structure) of the container
specified in the policy. E.g.:
            template <typename Data, template <typename> class Container>
    struct Size: public Container<Data>
    {
        size_t size()
        {                           // relies on the container's `size()'
                                    // note: can't use `this->size()'
            return Container<Data>::size();
        }
    };

A specialization can now be defined for a much simpler storage class
using, e.g., plain pointers (the implementation capitalizes on first and
second, data members of std::pair.  Cf. the example at the end of this
section):
            template <typename Data>
    struct Size<Data, Plain>: public Plain<Data>
    {
        size_t size()
        {                           // relies on pointer data members
            return this->second - this->first;
        }
    };

Depending on the intentions of the template's author other members could
be implemented as well.

To simplify the real use of the above templates, a generic wrapper class
can be constructed: it uses the Size template matching the actually used
storage type (e.g., a std::vector or some plain storage class) to define
its structure:
            template <typename Data, template <typename> class Store>
    class Wrapper: public Size<Data, Store>
    {};

The above classes could now be used as follows (en passant showing an
extremely basic Plain class):
            #include <iostream>
    #include <vector>

    template <typename Data>
    struct Plain: public std::pair<Data *, Data *>
    {};

    int main()
    {
        Wrapper<int, std::vector> wiv;
        std::cout << wiv.size() << "\n";

        Wrapper<int, Plain> wis;
        std::cout << wis.size() << "\n";
    }

The wiv object now defines vector-data, the wis object merely
defines a std::pair object's data members.


23.5: Alias Templates

In addition to function and class templates, C++ also uses templates to
define an alias for a set of types. This is called an
    alias template. Alias templates can be specialized. The name of an
alias template is a type name.

Alias templates can be used as arguments to template template parameters. This
allows us to avoid the `unexpected default parameters' you may encounter
(e.g., when using C++ standards before the c++23 standard) when using
template template parameters.  Defining a template specifying a template
<typename> class Container is fine, but (before c++23) it was impossible
to specify a container like vector or set as template template
argument, as vector and set containers also define second template
parameters, specifying their allocation policies.

Alias templates are defined like using declarations, specifying an alias
for an existing (maybe partially or fully specialized) template type. In the
following example Vector is defined as an alias for vector:
            template <typename Type>
    using Vector = std::vector<Type>;

    Vector<int> vi;             // same as std::vector<int>
    std::vector<int> vi2(vi);   // copy construction: OK

So, what's the point of doing this?
    Looking at the vector container, we see that it defines two, rather
than one, template parameters, the second parameter being the allocation
policy _Alloc, by default set to std::allocator<_Tp>:
            template<typename _Tp, typename _Alloc = std::allocator<_Tp> >
    class vector: ...

Now define a class template Generic defining a template template
parameter:
            template <typename Type, template <typename> class Container>
    class Generic: public Container<Type>
    {
        ...
    };

Most likely, Generic offers members made available by the container
that is actually used to create the Generic object, and adds to those some
members of it own. However, a simple container like std::vector cannot be
used, as std::vector doesn't match a template <typename> class
Container> parameter; it requires a template <typename, typename> class
Container> template template parameter.

The Vector alias template, however, is defined as a template
having one template type parameter, and it uses the vector's default
allocator. Consequently, passing a Vector to Generic works fine:
            Generic<int, Vector> giv;       // OK
    Generic<int, std::vector> err;  // won't compile: 2nd argument mismatch

With the aid of a small alias template it is also possible to use a
completely different kind of container, like a map, with Generic:
            template <typename Type>
    using MapString = std::map<Type, std::string>;

    Generic<int, MapString> gim;    // uses map<int, string>


23.6: Trait classes

Scattered over the std namespace
    
    trait classes are found. E.g., most C++ programmers have seen
the compiler mentioning `std::char_traits<char>' when performing an
illegal operation on std::string objects, as in std::string s(1).

Trait classes are used to make compile-time decisions about types. Trait
classes allow us to apply the proper code to the proper data type, be it a
pointer, a reference, or a plain value, all this maybe in combination with
const. The particular type of data to use can be inferred from the actual
type that is specified (or implied) when the template is used. This can be
fully automated, not requiring the template writer to make any decision.

Trait classes allow us to develop a template <typename Type1, typename
Type2, ...> without the need to specify many specializations covering all
combinations of, e.g., values, (const) pointers, or (const) references, which
would soon result in an unmaintainable exponential explosion of template
specializations (e.g., allowing these five different types for each template
parameter already results in 25 combinations when two template type parameters
are used: each must be covered by potentially different specializations).

Having available a trait class, the actual type can be inferred compile
time, allowing the compiler to deduce whether or not the actual type is a
pointer, a pointer to a member, or a const pointer, and to make comparable
deductions in case the actual type is, e.g., an lvalue or rvalue reference
type. This in turn allows us to write templates that define types like
    
 argument_type, first_argument_type, second_argument_type and
result_type, which are required by several generic algorithms (e.g.,
count_if()).

A trait class usually performs no behavior. I.e., it has no constructor
and no members that can be called. Instead, it defines a series of types and
enum values that have certain values depending on the actual type that is
passed to the trait class template.  The compiler uses one of a set of
available specializations to select the one appropriate for an actual template
type parameter.

The point of departure when defining a trait template is a plain
vanilla struct, defining the characteristics of a plain value type like
an int.  This sets the stage for specific specializations, modifying the
characteristics for any other type that could be specified for the template.

To make matters concrete, assume the intent is to create a trait class
BasicTraits telling us whether a type is a plain value type, a pointer
type, an lvalue reference type or an rvalue reference type (all of which may
or may not be const types).

Whatever the actually provided type, we want to be able to determine the
`plain' type (i.e., the type without any modifiers, pointers or references),
the `pointer type' and the `reference type', allowing us to define in all
cases, e.g., an rvalue reference to its built-in type, even though we passed a
const pointer to that type.

Our point of departure, as mentioned, is a plain struct defining the
required parameter. Maybe something like this:
                template <typename T>
        struct Basic
        {
            using Type = T;
            enum
            {
                isPointer = false,
                isConst = false,
                isRef = false,
                isRRef = false
            };
        };


Although some conclusions can be drawn by combining various enum
values (e.g., a plain type is not a pointer or a reference or an rvalue
reference or a const), it is good practice to provide a full implementation of
trait classes, not requiring its users to construct these logical expressions
themselves. Therefore, the basic decisions in a trait class are usually made
by a
        nested trait class,
    leaving the task of creating appropriate logical expressions to a
surrounding trait class.

So, the struct Basic defines the generic form of our inner trait
class. Specializations handle specific details. E.g., a pointer type is
recognized by the following specialization:
                template <typename T>
        struct Basic<T *>
        {
            using Type = T;
            enum
            {
                isPointer = true,
                isConst = false,
                isRef = false,
                isRRef = false
            };
        };


whereas a pointer to a const type is matched with the next specialization:
                template <typename T>
        struct Basic<T const *>
        {
            using Type = T;
            enum
            {
                isPointer = true,
                isConst = true,
                isRef = false,
                isRRef = false
            };
        };


Several other specializations should be defined: e.g., recognizing
const value types or (rvalue) reference types. Eventually all these
specializations are implemented as nested structs of an outer class
BasicTraits, offering the public traits class interface. The outline of
the outer trait class is:
            template <typename TypeParam>
    class BasicTraits
    {
        // Define specializations of the template `Basic' here

        public:
            BasicTraits(BasicTraits const &other) = delete;

            using ValueType = Basic<TypeParam>::Type;
            using PtrType = ValueType *;
            using RefType = ValueType &;
            using RvalueRefType = ValueType &&;

            enum
            {
                isPointerType = Basic<TypeParam>::isPointer,
                isReferenceType = Basic<TypeParam>::isRef,
                isRvalueReferenceType = Basic<TypeParam>::isRRef,
                isConst = Basic<TypeParam>::isConst,
                isPlainType = not (isPointerType or isReferenceType or
                                   isRvalueReferenceType or isConst)
            };
    };

The trait class's public interface explicitly deletes its copy
constructor. As it therefore defines no constructor at all and as it has no
static members it does not offer any run-time executable code.
All the trait class's facilities must therefore be used compile time.

A trait class template can be used to obtain the proper type, irrespective
of the template type argument provided. It can also be used to select
a proper specialization that depends on, e.g., the const-ness of a
template type. Example:
             cout <<
      "int: plain type? "     << BasicTraits<int>::isPlainType << "\n"
      "int: ptr? "            << BasicTraits<int>::isPointerType << "\n"
      "int: const? "          << BasicTraits<int>::isConst << "\n"
      "int *: ptr? "          << BasicTraits<int *>::isPointerType << "\n"
      "int const *: ptr? "    << BasicTraits<int const *>::isPointerType <<
                                                                      "\n"
      "int const: const? "    << BasicTraits<int const>::isConst << "\n"
      "int: reference? "      << BasicTraits<int>::isReferenceType << "\n"
      "int &: reference? "    << BasicTraits<int &>::isReferenceType << "\n"
      "int const &: ref ? "   << BasicTraits<int const &>::isReferenceType <<
                                                                        "\n"
      "int const &: const ? " << BasicTraits<int const &>::isConst << "\n"
      "int &&: r-reference? " << BasicTraits<int &&>::isRvalueReferenceType <<
                                                                        "\n"
      "int &&: const? " << BasicTraits<int &&>::isConst << "\n"
      "int const &&: r-ref ? "<< BasicTraits<int const &&>::
                                                isRvalueReferenceType << "\n"
      "int const &&: const ? "<< BasicTraits<int const &&>::isConst << "\n"
        "\n";

     BasicTraits<int *>::ValueType           value = 12;
     BasicTraits<int const *>::RvalueRefType rvalue = int(10);
     BasicTraits<int const &&>::PtrType      ptr = new int(14);
     cout << value << ' ' << rvalue << ' ' << *ptr << '\n';



23.6.1: Distinguishing class from non-class types

    In the previous section the BasicTraits trait class was developed. Using
specialized versions of a nested struct Type modifiers, pointers,
references and values could be distinguished.

Knowing whether a type is a class type or not (e.g., the type represents a
primitive type) could also be a useful bit of knowledge to a template
developer. The class template developer might want to define a
specialization when the template's type parameter represents a class
type (maybe using some member function that should be available)
and another specialization for non-class types.

This section addresses the question how a trait class can distinguish class
        
    types from non-class types.

In order to distinguish classes from non-class types a distinguishing feature
that can be used at compile-time must be found. 
     It may require some thinking to
find such a distinguishing characteristic, but a good candidate eventually is
found in the pointer to members syntactic construct. Pointers to members
are only available for classes. Using the pointer to member construct as the
distinguishing characteristic, a specialization can be developed that uses the
pointer to member if available. Another specialization (or the generic
template) does something else if the pointer to member construction is not
available.

How can we distinguish a pointer to a member from `a generic situation',
not being a pointer to a member? Fortunately, such a distinction is
possible. A function template specialization can be defined having a parameter
which is a pointer to a member function. The generic function template
then accepts any other argument. The compiler selects the former
(specialized) function when the provided type is a class type as class types
may support a pointer to a member. The interesting verb here is `may':
the class does not have to define a pointer to member.

Furthermore, the compiler does not actually call any function: we're
talking compile-time here. All the compiler does is to select the
appropriate function by evaluating a constant expression.

So, our intended function template now looks like this:
            template <typename ClassType>
    static `some returntype'  fun(void (ClassType::*)());

The function's return type (`(some returntype)') will be defined
shortly. Let's first have a closer look at the function's parameter.  The
function's parameter defines a pointer to a member returning void. Such a
function does not have to exist for the concrete class-type that's
specified when the function is used.  In fact, no implementation is
provided. The function fun is only declared as a static member in the
trait class. It's not implemented and no trait class object is required to
call it. What, then, is its use?

To answer the question we now have a look at the generic function
template that should be used when the template's argument is not a class
type. The language offers a `worst case' parameter in its
        
    ellipsis parameter list. The ellipsis is a final resort the compiler
may turn to if everything else fails. The generic function template specifies
a plain ellipsis in its parameter list:
            template <typename NonClassType>
    static `some returntype' fun(...);

It would be an error to define the generic alternative as a function
expecting an int. The compiler, when confronted with alternatives,
favors the simplest, most specified alternative over a more complex, generic
one. So, when providing fun with an argument it selects int
whenever possible and it won't select fun(void (ClassType::*)()). When
given the choice between fun(void (ClassType::*)()) and fun(...) it
selects the former unless it can't do that.

The question now becomes: what argument can be used for both a pointer to
a member and for the ellipsis? Actually, there is such a `one size fits all'
argument: 0. The value 0 can be passed as argument value to functions defining
an ellipsis parameter and to functions defining a pointers-to-member
parameter.

But 0 does not specify a particular class. Therefore, fun must specify
an explicit template argument, appearing in our code as fun<Type>(0), with
Type being the template type parameter of the trait class.

Now for the return type. The function's return type cannot be a simple
value (like true or false). Our eventual intent is to provide the
trait class with an enum telling us whether the trait class's template
argument represents a class type or not. That enum becomes something like
this:
            enum { isClass = some class/non-class distinguishing expression } ;

The distinguishing expression cannot be
            enum { isClass = fun<Type>(0) } ;

as fun<Type>(0) is not a constant expression and enum values must
        
 be defined by constant expressions so they can be determined at compile-time.

To determine isClass's value we must find an expression allowing for
compile-time discriminations between fun<Type>(...)  and fun<Type>(void
(Type::*)()).

In situations like these the sizeof operator often is our tool of
choice as it is evaluated at compile-time. By defining different sized return
types for the two fun declarations we are able to distinguish (at
compile-time) which of the two fun alternatives is selected by the
compiler.

The char type is by definition a type having size 1. By defining
another type containing two consecutive char values a larger type is
obtained. A char [2] is of course not a type, but a char[2] can be
defined as a data member of a struct, and a struct does define a
type. That struct then has a size exceeding 1. E.g.,
            struct Char2
    {
        char data[2];
    };

Char2 can be defined as a nested type of our traits class. The
two fun function template declarations become:
            template <typename ClassType>
    static Char2 fun(void (ClassType::*)());

    template <typename NonClassType>
    static char fun(...);

Since sizeof expressions are evaluated at compile-time we can now
determine isClass's value:
            enum { isClass = sizeof(fun<Type>(0)) == sizeof(Char2) };

This expression has several interesting implications:
    
    o  no fun function template is ever instantiated;
    o  the compiler considers Type and selects
fun's function template specialization if Type is a class type and the
generic function template if not;
    o  From the selected function it determines the return type and thus the
return type's size;
    o  Resulting in the proper evaluation of isClass.

Without requiring any instantiation the trait class can now provide an
answer to the question whether a template type argument represents a class
type or not. Marvelous!


23.6.2: Available type traits

    C++ offers many facilities to identify and modifiy characteristics of
types. Before using these facilities the <type_traits> header file must be
included.

All facilities offered by type_traits are defined in the std
namespace (omitted from the examples given below), allowing programmers to
determine various characteristics of types and values.

In the description of available type traits the following concepts are
encountered: 
    
    o  arithmetic type:
       any integral or floating point type;

o  class type:
       not a union type, without non-static data members, without virtual
        members, without virtual or non-empty base classes;

o  compound type:
      
       o  arrays of objects of a given type;
       o  functions, which have parameters of a given type returning
        void or objects;
       o  pointers to void, to objects, to functions, or to 
        non-static class members;
       o  references to objects or functions;
       o  class, union or enumeration types;

o  fundamental type:
       a built-in type;

o  integral type:
       all types whose values represent integral numbers, as well as bool
        and all built-in types representing (possibly unicode) characters;

o  literal type:
       a literal type is a scalar type; a trivial class type; or an array of
        literal type elements;

o  is_nothrow_... type trait:
       a type trait to determine whether its template argument supports the
        specified non-throwing member. Such type traits return false
        unless noexcept(true) is used at the function's declaration. E.g.,
           struct NoThrow
    {
        NoThrow &operator=(SomeType const &rhs) noexept(true);
    };

o  trivial type:
       trivial types are scalar types, trivial class types, arrays of such
        types and cv-qualified versions of these types;

o  trivial class type: 
       a class type having a trivial copy constructor, no non-trivial move
        constructor, a trivial destructor, a trivial default constructor or at
        least one constexpr constructor other than the copy or move
        constructor, and only has non-static data members and base classes of
        literal types; 

o  trivial member function: 
       trivial member functions are never declared (other than default) in
        their class interfaces and (for default constructors or assignment
        operators) only perform byte-by-byte actions. Here are two examples:
        struct Pod only has trivial members as it doesn't explicitly
        declare any member function and its data member is plain old
        data. struct Nonpod is not plain old data. Although it
        doesn't explictly declare any member function either, its data member
        is a std::string, which itself isn't plain old data as
        std::string has non-trivial constructors:
           struct Pod
    {
        int x;
    };

    struct Nonpod
    {
        std::string s;
    };

When type-condition applies to a type, it must be a complete type,
void, or an array of unknown size;

The following type traits are provided:
    
    o add_const<typename Type>::type
        to add const to Type;

o add_cv<typename Type>::type
        to add const volatile to Type;

o add_lvalue_reference<typename Type>::type
        to add an lvalue reference to Type;

o add_pointer<typename Type>::type
        to add a pointer to Type;

o add_rvalue_reference<typename Type>::type
        to add an rvalue reference to Type;

o add_volatile<typename Type>::type
        to add volatile to Type;

o conditional<bool cond, typename TrueType, typename FalseType>::type
        to conditionally use TrueType if cond is true,
        FalseType if not;

o template <typename Type> struct decay
        defines the typename type obtained from Type after
        removing all cv-qualifiers and references from the specified template
        type argument. Moreover, it converts lvalue types to rvalue types,
        and arrays and functions to pointers. It resembles what happens if an
        argument is passed to a value-type parameter.

o template <typename Type> decay_t
        is shorthand for typename decay<Type>::type.

o enable_if<bool cond, typename Type>::type
        to conditionally define Type if cond is true;

o is_abstract<typename Type>::value
        to determine whether Type is an abstract type
        (e.g., an abstract base class) (type-condition applies);

o is_arithmetic<typename Type>::value
        to determine whether Type is an arithmetic type;

o is_array<typename Type>::value
        to determine whether Type is an array type;

o is_assignable<typename To, typename From>::value
        to determine whether an object of type From can
        be assigned to an object of type To (type-condition applies);

o is_base_of<typename Base, typename Derived>::value
        to determine whether Base is a base class
        of type Derived;

o is_class<typename Type>::value
        to determine whether Type is a class type;

o is_compound<typename Type>::value
        to determine whether Type is a compound type;

o is_const<typename Type>::value
        to determine whether Type is a const type;

o is_constructible<typename Type, typename ...Args>::value
        to determine whether Type is constructible
        from arguments in the Args parameter pack (type-condition
        applies to all types in Args);

o is_convertible<typename From, typename To>::value
        to determine whether a type From may be
        converted to a type To using a static_cast;

o is_copy_assignable<typename Type>::value
        to determine whether Type supports copy
        assignment (type-condition applies);

o is_copy_constructible<typename Type>::value
        to determine whether Type supports copy
        construction (type-condition applies);

o is_default_constructible<typename Type>::value
        to determine whether Type supports a
        default constructor (type-condition applies);

o is_destructible<typename Type>::value
        to determine whether Type has a non-deleted
        destructor (type-condition applies);

o is_empty<typename Type>::value
        to determine whether Type is a class type (not a union
        type), without non-static data members, virtual members, virtual or
        non-empty base classes (type-condition applies);

o is_enum<typename Type>::value
        to determine whether Type is an enum type;

o is_floating_point<typename Type>::value
        to determine whether Type is a floating point
        type;

o is_function<typename Type>::value
        to determine whether Type is a function type;

o is_fundamental<typename Type>::value
        to determine whether Type is a fundamental type;

o is_integral<typename Type>::value
        to determine whether Type is an integral type;

o is_literal_type<typename Type>::value
        to determine whether Type is a literal type
        (type-condition applies);

o is_lvalue_reference<typename Type>::value
        to determine whether Type is an lvalue
        reference;

o is_member_function_pointer<typename Type>::value
        to determine whether Type is a
        pointer to a non-static member function;

o is_member_object_pointer<typename Type>::value
        to determine whether Type is a pointer
        to a non-static data member;

o is_member_pointer<typename Type>::value
        to determine whether Type is a pointer to a
        member function;

o is_move_assignable<typename Type>::value
        to determine whether Type supports move
        assignment (type-condition applies);

o is_move_constructible<typename Type>::value
        to determine whether Type supports move
        construction (type-condition applies);

o is_nothrow_assignable<typename To, typename From>::value
        to determine whether Type supports an
        assignment operator not throwing exceptions (type-condition
        applies).

o is_nothrow_constructible<typename Type, typename ...Args>::value
        to determine whether a Type object can
        be constructed from arguments of types mentioned in the parameter pack
        not throwing exceptions (type-condition applies);

o is_nothrow_copy_assignable<typename Type>::value
        to determine whether Type supports a
        copy-assignment operator not throwing exceptions (type-condition
        applies);

o is_nothrow_copy_constructible<typename Type>::value
        to determine whether Type
        supports copy construction not throwing exceptions (type-condition
        applies);

o is_nothrow_default_constructible<typename Type>::value
        to determine whether Type
        supports a default constructor not throwing exceptions
        (type-condition applies);

o is_nothrow_destructible<typename Type>::value
        to determine whether Type supports a
        destructor not throwing exceptions (type-condition applies).

o is_nothrow_move_assignable<typename Type>::value
        to determine whether Type supports
        move assignment not throwing exceptions (type-condition applies);

o is_nothrow_move_constructible<typename Type>::value
        to determine whether Type
        supports a move constructor not throwing exceptions
        (type-condition applies);

o is_object<typename Type>::value
        to determine whether Type is an object (in contrast
        to scalar) type;

o is_pod<typename Type>::value
        to determine whether Type is an aggregate 
            (plain old data,
        type-condition applies);

o is_pointer<typename Type>::value
        to determine whether Type is a pointer type;

o is_polymorphic<typename Type>::value
        to determine whether Type is a polymorphic type
        (type-condition applies);

o is_reference<typename Type>::value
        to determine whether Type is an (lvalue or rvalue)
        reference;

o is_rvalue_reference<typename Type>::value
        to determine whether Type is an rvalue
        reference;

o is_same<typename First, typename Second>::value
        to determine whether types First and Second are
        identical;

o is_scalar<typename Type>::value
        to determine whether Type is a scalar type (in
        contrast to an object type);

o is_signed<typename Type>::value
        to determine whether Type is a signed type;

o is_standard_layout<typename Type>::value
        to determine whether Type offers the
        standard layout (type-condition applies);

o is_trivial<typename Type>::value
        to determine whether Type is a trivial type
        (type-condition applies);

o is_trivially_assignable<typename Dest, typename Src>::value
        to determine whether an object or value of
        type Src can trivially be assigned to an object of type Dest
        (type-condition applies);

o is_trivially_constructible<typename Type, typename ...Args>::value
        to determine whether Type is
        trivially constructible from arguments in the Args parameter pack
        (type-condition applies to all types in Args);

o is_trivially_copy_assignable<typename Type>::value
        to determine whether Type supports
        a trivial assignment operator (type-condition applies);

o is_trivially_copy_constructible<typename Type>::value
        to determine whether Type is
        trivially copy-constructible (type-condition applies);

o is_trivially_copyable<typename Type>::value
        to determine whether Type is trivially
        copyable (type-condition applies);

o is_trivially_default_constructible<typename Type>::value
        to determine whether Type
        supports a trivial default constructor (type-condition applies);

o is_trivially_default_destructible<typename Type>::value
        to determine whether Type
        supports a trivial default destructor (type-condition applies);

o is_trivially_move_assignable<typename Type>::value
        to determine whether Type supports
        a trivial assignment operator (type-condition applies);

o is_trivially_move_constructible<typename Type>::value
        to determine whether Type is
        trivially move-constructible (type-condition applies);

o is_union<typename Type>::value
        to determine whether Type is a union type;

o is_unsigned<typename Type>::value
        to determine whether Type is an unsigned type;

o is_void<typename Type>::value
        to determine whether Type is void;

o is_volatile<typename Type>::value
        to determine whether Type is a volatile
        qualified type;

o make_signed<typename Type>::type
        to construct a signed type;

o make_unsigned<typename Type>::type
        to construct an unsigned type;

o remove_all_extents<typename Type>::type
        if Type is a (possibly multidimensional)
        array of ElementType values or objects then typedef type
        equals ElementType;

o remove_const<typename Type>::type
        to remove const from Type;

o remove_cv<typename Type>::type
        to remove const and/or volatile from Type;

o remove_extent<typename Type>::type
        if Type is an array of ElementType values or
        objects then typedef type equals ElementType. With
        multi-dimensional arrays ElementType is the type of the array from
        which its first array dimension has been removed;

o remove_pointer<typename Type>::type
        to remove a pointer from Type;

o remove_reference<typename Type>::type
        to remove a reference from Type;

o remove_volatile<typename Type>::type
        to remove volatile from Type;


23.7: Defining `ErrorCodeEnum' and 'ErrorConditionEnum'
                          enumerations
 
    In section 4.3.1 the class std::error_code was introduced. One of
its constructors accepts ErrorCodeEnum values, where ErrorCodeEnum is
a template type name for enumerations that we may define ourselves containing
symbols that are used as error code values. Another constructor expects an
int-valued error code and a specification of an error category that uses
those error codes.

Several error code enumerations and error categories are predefined by C++
but it is also possible to define new ErrorCodeEnums and error
categories. In this section constructing new ErrorCodeEnums is covered, in
the next section designing new error categories is covered.

Defining new error code enumerations is an option when using error_code
objects is attractive, but standard error code values (like the values defined
by enum class errc) aren't appropriate. For example, when designing an
interactive calculator, several errors may be encountered that are related to
the way expressions are entered by the user. For those situations you might
want to develop your own error code enumeration.

In this and the next section a bare bones approach to defining error code
enumerations and error categories is adopted. No concrete, real-life like
class is developed. I think the advantage of this is that this way it's easier
to apply the principles to new real-life situations than if you first have to
abstract the content of a real-life example yourself. Here we go:
   
   o  Our first step consists of defining our own enumeration. The
    enumeration contains symbols listing causes of errors. Newly defined
    error code enumerations must not associate value 0 with any of its
    symbols, as by convention value 0 indicates `no error'. 

The enum class CatErr lists causes of errors that are associated
    with our (as yet to be designed) error category: 
            enum class CatErr
    {
        Err1 = 1,           // causes of errors
        Err2,
        Err3
    };

o  By itself, defining an enum class does not allow us to pass its
    values to error_code constructors. Before we can do that the enum
    must be `promoted' to an error_code_enum. This `promotion' is
    realized by specializing the trait class
    std::is_error_code_enum after which the
    error_code(ErrorCodeEnum) member template and the make_error_code
    function accept CatErr enumeration values. Interestingly, this
    requires us to add code to the std namespace. Normally this is not
    allowed, but in this case it is. The C++ standard states:

20.5.4.2.1 Namespace std

The behavior of a C++ program is undefined if it adds declarations or
        definitions to namespace std or to a namespace within namespace std
        unless otherwise specified.

A program may add a template specialization for any standard library
        template to namespace std only if the declaration depends on a
        user-defined type and the specialization meets the standard library
        requirements for the original template and is not explicitly
        prohibited.  

Here is how the is_error_code_enum trait class is specialized:
            namespace std
    {
        template <>
        struct is_error_code_enum<CatErr>: public true_type
        {};
    }

This completes the definition of our own error code enumeration, whose symbols
are now accepted by error_code's constructor.

Before we're able to design our own error category we must also have a look at
`higher order' causes of errors as represented by objects of the class
std::error_condition (cf. section 10.9.2). Error conditions
represent platform independent errors like syntax errors or non-existing
requests.

In our bare bones implementation of an error category these higher order
causes of errors are enumerated in the enum class Cond enumeration. It's
defined similarly to CatErr.
   
   o  Our first step consists of defining the enumeration. As with error
    code enums its symbols should not be given the value 0. Here is the
    enumeration Cond whose symbols presumably represent platform
    independent causes of errors: 
       enum class Cond
    {
        NoCond = -1,
        Cond1 = 1,
        Cond2,
        Cond3,
    };

o  By itself, defining an enum class does not allow us to pass its
    values to error_condition constructors. Before we can do that the
    enum must be `promoted' to an error_condition_enum. Also similar
    to CatErr this `promotion' is realized by specializing the trait class
    std::is_error_condition_enum

Here is how the is_error_condition_enum trait class is specialized:
            namespace std
    {
        template <>
        struct is_error_condition_enum<Cond>: public true_type
        {};
    }

We're now ready for designing our own error_category class.


23.7.1: Deriving classes from std::error_category
 
    In the previous section the error code enumerations CatErr and Cond
were developed.  The values of these enumerations specify, respectively, the
direct and the platform independent causes of errors that may be encountered
in the context of the new error category developed in this section.

Classes derived from std::error_category are designed as singleton classes
and implement their own name, message and an equivalent members. Our
class Category also declares a static member instance returning a
reference to the class's singleton object, which is compile-time initialized
and is available by the time instance is called. Alternatively a dedicated
function (like Category_category), analogously to the function
generic_category, returning a reference to the Category object could
be defined.

CatErr values, Cond values and textual descriptions of CatErr's
values are combined in a std::unordered_map using CatErr as key, and a
struct POD as value type. This map allows us to retrieve the platform
independent error types and the descriptions that are associated with
CatErr values.

Here is the interface of the class Category:
        class Category: public std::error_category
    {
        static Category s_instance;
        struct POD
        {
            Cond        cond;
            char const *msg;
        };
    
        static std::unordered_map<CatErr, POD> s_map;
    
        public:
            Category(Category const &other) = delete;
    
            static Category &instance();
    
            bool equivalent(std::error_code const &ec, int condNr)
                                                const noexcept override;
            bool equivalent(int ev, std::error_condition const &condition)
                                                const noexcept override;
            std::error_condition default_error_condition(int ev)
                                                const noexcept override;
    
            std::string message(int ce) const override;
            char const *name() const noexcept override;
    
        private:
            Category() = default;
    
            template <typename Enum>
            static constexpr Enum as(int err);
    };

Its unordered_map s_map provides the Cond values and verbal
descriptions of the CatErr values given those CatErr values:
        unordered_map<CatErr, Category::POD> Category::s_map =
    {
        { CatErr::Err1, { Cond::Cond1, "Err1" } },
        { CatErr::Err2, { Cond::Cond2, "Err2" } },
        { CatErr::Err3, { Cond::Cond1, "Err3" } },
    };

The functions make_error_code and make_error_condition return,
respectively, error_code and error_condition objects from,
respectively, CatErr values and Cond values.

Their declarations can be provided below the Category class interface and
their implementations pass the Category object to their constructors:
        std::error_code make_error_code(CatErr ce)
    {
        return { static_cast<int>(ce), Category::instance() };
    }

    std::error_condition make_error_condition(Cond ec)
    {
        return { static_cast<int>(ec), Category::instance() };
    }

The member name must be defined by classes derived from
error_category. It simply returns a short string naming the category
(e.g., "Category" for our Category class). Likewise, the member
message must be redefined. Its implementation usually is slightly more
complex than name's implementation: it expects a (cast to an int)
CatErr value and uses that value to find the corresponding textual
description in s_map. If found the description is returned; if not found
then a short fallback message is returned:
        std::string Category::message(int ce) const
    {
        auto iter = s_map.find(static_cast<CatErr>(ce));
        return iter != s_map.end() ? iter->second.msg : "No CatErr value";
    }

The member default_error_condition receives a (cast to int) CatErr
value. That value is used to find the associated Cond value. If the
int received by the function does not represent a valid CatErr value
then the fallback value Cond::NoCond is used.  The function returns an
error_condition object created by make_error_condition which receives
the determined Cond value as its argument:
        std::error_condition Category::default_error_condition(int ev)
                                                            const noexcept
    {
        auto iter = s_map.find(as<CatErr>(ev));
    
        return make_error_condition(
                    iter == s_map.end() ? Cond::NoCond : iter->second.cond
                );
    }

What's left is implementing the two equivalent members. The first
equivalent member (receiving a reference to an error_code object and a
(cast to int) Cond value) determines the equivalence of
    the Cond value that is associated with the error_code object
and 
    the Cond value that is specified as the function's second argument.
 If these values are equal and the error_code object's category is equal
to Category then the equivalence has been established and true is
returned.  Here is its implementation:
        bool Category::equivalent(std::error_code const &ec, int condNr)
                                                            const noexcept
    {
        if (*this != ec.category())
            return false;
    
        if (ec.value() == 0)                            // no error in ec?
            return condNr == 0;                         // then condNr must
                                                        // also be 0
    
        auto iter = s_map.find(as<CatErr>(ec.value())); // info associated
                                                        // with ec's CatErr
    
        return iter == s_map.end() ?
                            false                       // not found or
                        :                               // compare Cond values
                            iter->second.cond == as<Cond>(condNr);
    }

The second equivalent member (receiving (as an int) CatErr value 
and an error_condition object) determines the equivalence of an
    error_condition object that is constructed from the Cond value
    that is associated with the CatErr value that was passed (as int)
    to the function
and 
    the error_condition object that was passed to the function as its
    second argument.

Here a prerequisite for concluding equivalence is that the error condition's
category is Category. If that's the case then the function returns
true if its int argument equals zero and the condition object also
indicates no error. Alternatively, if the condition argument is equal to
the error_condition object made from the Cond value associated with
the CatErr value passed to the function as its first argument the
equivalence has also been established and true is returned.  Here is its
implementation:
        bool Category::equivalent(int ev, error_condition const &condition)
                                                            const noexcept
    {
        if (ev == 0)                                    // no error? then
            return condition.category() == *this and    // categories must
                   not static_cast<bool>(condition);    // be equal and
                                                        // condition must
                                                        // indicate no error
    
        auto iter = s_map.find(as<CatErr>(ev));     // find ev's Cond
    
        return iter == s_map.end()?
                    false                           // no such CatErr
                :                                   // or compare conditions
                    condition == make_error_condition(iter->second.cond);
    }

So, in order to define your own category:
    
    o  define an enumeration which is `promoted' to an error_code_enum;
    o  define an enumeration which is promoted to an
        error_condition_enum; 
    o  define a new error_category class by deriving a class from
        std::error_category, define it as a singleton class and override
        its default_error_condition, equivalent, message, and name
        members. 


23.8: Using `noexcept' when offering the `strong guarantee'

When throwing exceptions while trying to achieve the strong guarantee a
function's actions are usually split in two parts
    
    o  First, usually on a temporary object, all operations that may throw
are performed (which doesn't affect the target object)
    o  Then, the target object is modified using operations that offer the
nothrow guarantee.

The actions in the first step might be made move aware by using
std::move (e.g., to assign the source's values to a (possibly temporary)
destination). However, using std::move can easily affect the source (e.g.,
when extending the source's memory, moving the existing data to its new
locations), which breaks the first step's assumption, as the target object is
now modified. 

In this case (and generally) the move operation should not be allowed to throw
exceptions. This, in turn, implies that it is difficult to write code which
must offer a non-throwing moving constructor, if it uses (external) 
data types over which the moving constructor has no control. E.g.,
            template <typename Type>
    class MyClass
    {
        Type d_f;
        public:
            MyClass() = default;
            MyClass(MyClass &&tmp)
            :
                d_f(move(tmp.d_f))
            {}
    };

Here, MyClass's author has no control over the design of Type. If
MyClass is instantiated with the type Foreign, merely having a
(possibly throwing) copy constructor, then the following code breaks the
no-throw assumption underlying move constructors:
            MyClass<Foreign> s2{ move(MyClass<Foreign>()) };

If templates are able to detect whether Type has non-throwing move
constructors then their implementations may be optimized by
calling these move constructors (already modifying their targets in the first
part of code offering the strong guarantee) in situations where otherwise the
non-modifying, but more expensive copy constructor has to be used.

The noexcept keyword was introduced to allow such templates to perform
such optimizations. As with throw lists, checking for noexcept is
a run-time check, but the consequence of violating a noexept declaration
are more serious than violating a throw list: violating noexcept
results in calling std::terminate, terminating the program, possibly
without unwinding the stack. In the context of the previous example, the
following code is flawlessly accepted by the compiler, demonstrating that
there is no compile-time checking of noexcept:
            class Foreign
    {
        public:
            Foreign() = default;
            Foreign(Foreign const &other) noexcept
            {
                throw 1;
            }
    };

However, when this class's copy constructor is called, execution aborts
with the following message:
            terminate called after throwing an instance of 'int'
    Abort

Keep in mind that the current purpose of noexcept is to allow templates to
optimize their code by using move operations where the code must also be able
to offer the strong exception guarantee. Since noexcept also offers the
conditional noexcept(condition) syntax (with noexcept(true) and
noexcept having identical semantics), noexcept can be made
conditional to the `noexcepting' nature of template types. Note that this is
not possible with throw lists. 

The following  rules of thumb may be used to decide whether
or not to use noexcept in your code:
    
    o  General rule: don't use noexcept (this is identical to the advise
        given for throw lists);
    o  Default implementations of constructors, copy- and move-assignment
        operators and destructors are provided with noexcept(true) if the
        compiler can deduce that composing types also offer
        noexcept(true), allowing template optimizations using move
        operations where possible.
    o  Functions provided with noexcept declarations may still throw
        exceptions (see the example given above). In the end noexcept
        merely means that if such a function throws an exception
        std::terminate rather than std::unexpected is called. 
    o  Functions previously provided with an empty throw list (throw())
        should be provided with noexcept.
    o  noexcept specifications are required when using the following std
        traits (declared in the <type_traits> header file):
        
        o is_nothrow_constructible
        o is_nothrow_default_constructible
        o is_nothrow_move_constructible
        o is_nothrow_copy_constructible
        o is_nothrow_assignable
        o is_nothrow_move_assignable
        o is_nothrow_copy_assignable
        o is_nothrow_destructible

These type traits provide the member constant value which is
        true if the class (and possibly its argument type list) matches
        the characteristic after which the trait was named. E.g., if
        MyClass(string const &) noexcept is a constructor, then
            std::is_nothrow_constructible<MyClass, string>::value

equals true. For the named members (like
        is_nothrow_move_constructible) parameter types do not have to be
        specified, as they are implied. E.g.,
            std::is_nothrow_move_constructible<MyClass>::value

returns true if the move constructor has the noexcept
        modifier. 


23.9: More conversions to class types



23.9.1: Types to types

    Although class templates may be partially specialized, function
templates may not. At times this is annoying. Assume a function template is
available implementing a certain unary operator that could be used with the
transform generic algorithm (cf. section 19.1.56):
            template <typename Return, typename Argument>
    Return chop(Argument const &arg)
    {
        return Return{ arg };
    }

Furthermore assume that if Return is std::string then the above
implementation should not be used. Instead, with std::string a second
argument 1 should always be provided. If Argument is a C++ string,
this would allow us to, e.g., return a copy of arg from which its first
character has been chopped off.

Since chop is a function, it is not possible to define a partial
specialization like this:
            template <typename Argument>        // This won't compile!
    std::string chop<std::string, Argument>(Argument const &arg)
    {
        return std::string{ arg, 1 };
    }

Although a function template cannot be partially specialized it is
possible to use overloading, defining a second,  dummy, string parameter:
            template <typename Argument>
    std::string chop(Argument const &arg, std::string)
    {
        return std::string{ arg, 1 };
    }

Instead of providing a string dummy argument the functions could
use the IntType template (cf. section 23.2.1.1) to select the proper
overloaded version. E.g., IntType<0> could be defined as the type of the
second argument of the first overloaded chop function, and IntType<1>
could be used for the second overloaded function. From the point of view of
program efficiency this is an attractive option, as the provided IntType
objects are extremely lightweight. IntType objects contain no data at
all. But there's also an obvious disadvantage as there is no intuitively clear
association between the int value used and the intended type.

Instead of defining arbitrary IntType types it is more attractive to
use another lightweight solution, using an automatic type-to-type
association. The struct TypeType is a lightweight type wrapper, much like
IntType. Here is its definition:
            template <typename T>
    struct TypeType
    {
        using Type = T;
    };

TypeType is also a lightweight type as it doesn't have any data fields
either. TypeType allows us to use a natural type association for
chop's second argument. E.g, the overloaded functions can now be defined
as follows:
            template <typename Return, typename Argument>
    Return chop(Argument const &arg, TypeType<Argument> )
    {
        return Return{ arg };
    }

    template <typename Argument>
    std::string chop(Argument const &arg, TypeType<std::string> )
    {
        return std::string{ arg, 1 };
    }

Using the above implementations any type can be specified for
Result. If it happens to be a std::string the appropriate overloaded
version is automatically selected. The following additional overload of the
function chop capitalizes on this:
            template <typename Result>
    Result chop(char const *txt)    // char const * could also be a 2nd
    {                               // template type parameter
        return chop(std::string{ txt }, TypeType<Result>{});
    }

Using the third chop function, the following statement produces the
text `ello world':
            cout << chop<string>{ "hello world" } << '\n';

Template functions do not support partial specializations. But they can be
overloaded. By providing overloads with dummy type-arguments that depend
on other parameters and calling these overloads from a overloaded function
that does not require the dummy type argument a situation similar to partial
specializations with class templates can often be realized.


23.9.2: An empty type

    At times (cf. section 23.10) an empty
        
    struct is a useful tool. It can be used as a type acting
analogously to the final 0-byte in NTBSs.
It can simply be defined as:
            struct NullType
    {};


23.9.3: Type convertibility

    In what situations can a type T be used as a `stand in' for another type
U? Since C++ is a strongly typed language the answer is surprisingly
simple: Ts can be used instead of Us if a T is accepted as
argument in cases where Us are requested.

This reasoning is behind the following class which can be used to determine
whether a type T can be used where a type U is expected. The
interesting part is that no code is actually generated or executed.  All
decisions are made by the compiler.

In the second part of this section we'll show how the code developed in
the first part can be used to detect whether a class B is a base class of
another class D (the is_base_of template (cf. section 23.6.2)
also provides an answer to this question). The code developed here closely
follows the example provided by Alexandrescu  (2001,
p. 35).

First, a function test is designed accepting a type U.
The function test returns a value
of the as yet unknown type Convertible:
            Convertible test(U const &);

The function test is never implemented. It is only declared.  If a type
T can be used instead of a type U then T can also be passed as
argument to the above test function.

On the other hand, if the alternate type T cannot be used where a U is
expected, then the compiler won't be able to use the above test
function. Instead, it uses an alternative function that has a lower
selection priority but that can always be used with any T type.

C (and C++) offer a very general parameter list, a parameter list that
is always considered acceptable. This parameter list is the familiar
  ellipsis which represents the worst case the compiler may
encounter.  If everything else fails, then the function defining an ellipsis
as its parameter list is selected.

Usually that's not a productive alternative, but in the current situation it
is exactly what is needed. When confronted with two candidate functions,
one of which defines an ellipsis parameter, the compiler selects the
function defining the ellipsis parameter only if the alternative(s) can't be
used.

Following the above reasoning an alternative function test(...) is
declared as well. This alternate function does not return a Convertible
value but a NotConvertible value:
            NotConvertible test(...);

If test's argument is of type T and if T can be converted to U
then test's return type is Convertible.  Otherwise NotConvertible
is returned.

This situation clearly shows similarities with the situation encountered in
section 23.6.1 where the value isClass had to be determined
compile time. Here two related problems must be solved:
    
    o  how do we obtain a T argument? This is more difficult than might
be expected at first sight as it might not be possible to define a T. If
type T does not define any constructor then no T object can be
defined.
    o  how can Convertible be distinguished from NotConvertible?

The first problem is solved by realizing that no T needs to be
defined. After all, the intent is to decide compile-time whether a type is
convertible and not to define a T value or object. Defining objects is not
a compile-time but a run-time matter.

By simply declaring a function returning a T we can
tell the compiler where it should assume a T:
            T makeT();

This mysterious function has the magical power of enticing the compiler
into thinking that a T object comes out of it. However, this function
needs a small modification before it will actually suit our needs. If, for
whatever reason, T happens to be an array then the compiler will choke on
T makeT() as functions cannot return arrays. This, however, is easily
solved, as functions can return references to arrays. So the above
declaration is changed into:
            T const &makeT();

Next we pass a T const & to test:
following code:
            test(makeT())

Now that the compiler sees test being called with a T const &
argument it decides that its return value is Convertible if a
conversion is in fact possible. Otherwise it decides that its return value
is NotConvertible (as the compiler, in that case, selected test(...)).

The second problem, distinguishing Convertible from NotConvertible
is solved exactly the way isClass could be determined in section
23.6.1, viz. by making their sizes different. Having done so the
following expression determines whether T is convertible from U or
not:
            isConvertible = sizeof(test(makeT())) == sizeof(Convertible);

By using char for Convertible and Char2 (cf. section
23.6.1) for NotConvertible the distinction can be made.

The above can be summarized in a class template LconvertibleToR,
having two template type parameters:
            template <typename T, typename U>
    class LconvertibleToR
    {
        struct Char2
        {
            char array[2];
        };
        static T const &makeT();
        static char test(U const &);
        static Char2 test(...);

        public:
            LconvertibleToR(LconvertibleToR const &other) = delete;
            enum { yes = sizeof(test(makeT())) == sizeof(char) };
            enum { sameType = 0 };
    };

    template <typename T>
    class LconvertibleToR<T, T>
    {
        public:
            LconvertibleToR(LconvertibleToR const &other) = delete;
            enum { yes = 1 };
            enum { sameType = 1 };
    };

    As the class template deletes its copy constructor no object can be
created. Only its enum values can be interrogated. The next example writes
1 0 1 0 when run from a main function:
            cout <<
        LconvertibleToR<ofstream, ostream>::yes << " " <<
        LconvertibleToR<ostream, ofstream>::yes << " " <<
        LconvertibleToR<int, double>::yes << " " <<
        LconvertibleToR<int, string>::yes <<
        "\n";



23.9.3.1: Determining inheritance

        Now that it is possible to determine type convertibility, it's easy to
determine whether a type Base is a (public) base class of a type
Derived.

Inheritance is determined by inspecting convertibility of (const)
pointers. Derived const * can be converted to Base const * if
    
    o  both types are identical;
    o  Base is a public and unambiguous base class of Derived;
    o  and (usually not intended) if Base is void.

Assuming the last conversion isn't used inheritance can be determined
using the following trait class LBaseRDerived. LBaseRDerived provides
an enum yes which is 1 if the left type is a base class of the right type
and both types are different:
        template <typename Base, typename Derived>
    struct LBaseRDerived
    {
        LBaseRDerived(LBaseRDerived const &) = delete;
        enum {
            yes =
                LconvertibleToR<Derived const *, Base const *>::yes &&
                not LconvertibleToR<Base const *, void const *>::sameType
        };
    };


If code should not consider a class to be its own base class, then the
trait class LBaseRtrulyDerived can be used to perform a  strict test. This
trait class adds a test for type-equality:
        template <typename Base, typename Derived>
    struct LBaseRtrulyDerived
    {
        LBaseRtrulyDerived(LBaseRtrulyDerived const &) = delete;
        enum {
            yes =
                LBaseRDerived<Base, Derived>::yes &&
                not LconvertibleToR<Base const *, Derived const *>::sameType
        };
    };

    Example: the next statement displays 1: 0, 2: 1, 3: 0, 4: 1, 5: 0
when executed from a main function:
        cout << "\n" <<
        "1: " << LBaseRDerived<ofstream, ostream>::yes << ",  " <<
        "2: " << LBaseRDerived<ostream, ofstream>::yes << ",  " <<
        "3: " << LBaseRDerived<void, ofstream>::yes << ",  " <<
        "4: " << LBaseRDerived<ostream, ostream>::yes << ",  " <<
        "5: " << LBaseRtrulyDerived<ostream, ostream>::yes <<
        "\n";



23.10: Template TypeList processing

    This section serves two purposes. It illustrates capabilities of the various
template meta-programming techniques, which can be used as a source of
inspiration when developing your own templates; and it offers a concrete
example, illustrating some of the power offered by these techniques.

This section itself was inspired by Andrei Alexandrescu's (2001) book
        
    Modern C++ design. It diverts from Alexandrescu's book in its use of
variadic templates which were not yet available when he wrote his book. Even
so, the algorithms used by Alexandrescu are still useful when using variadic
templates.

C++ offers the tuple to store and retrieve
values of multiple types. Here the focus is merely on processing types.  A
simple struct TypeList is going to be used as our working horse for the
upcoming subsections. Here is its definition:
            template <typename ...Types>
    struct TypeList
    {
        TypeList(TypeList const &) = delete;
        enum { size = sizeof ...(Types) };
    };

    A typelist allows us to store any number of types. Here is an example
storing the three types char, short, int in a TypeList:
            TypeList<char, short, int>


23.10.1: The length of a TypeList

    As the number of types in a parameter pack may be obtained using the sizeof
operator (cf. section 22.5) it is easy to obtain the number of types
that were specified with a certain TypeList. For example, the following
statement displays the value 3:
            std::cout << TypeList<int, char, bool>::size << '\n';

However, it's illustrative to see how the number of types specified with a
TypeList could be determined if sizeof hadn't been available.

To obtain the number of types that were specified with a TypeList
the following algorithm is used:
    
    o  If the TypeList contains no types, its size equals zero;
    o  If the TypeList contains types, its size equals 1 plus the number
        of types that follow its  first type.

The algorithm uses recursion to define the length of a TypeList. In
executable C++ recursion could also be used in comparable situations. For
example recursion can be used to determine the length of an NTBS:
            size_t c_length(char const *cp)
    {
        return *cp == 0 ? 0 : 1 + c_length(cp + 1);
    }

While C++ functions usually use iteration rather than recursion,
iteration is not available to template meta programming algorithms. In
template meta programming repetition must be implemented using
recursion. Furthermore, while C++ run-time code may use conditions to
decide whether or not to start the next recursion template meta programming
cannot do so. Template meta programming algorithms must resort to (partial)
specializations. The specializations are used to select alternatives.

The number of types that are specified in a TypeList can be computed
using the following alternate implementation of TypeList, using a generic
struct declaration and two specialization for the empty and non-empty
TypeList (cf. the above description of the algorithm):
            template <typename ...Types>
    struct TypeList;

    template <typename Head, typename ...Tail>
    struct TypeList<Head, Tail...>
    {
        enum { size = 1 + TypeList<Tail...>::size };
    };
    template <>
    struct TypeList<>
    {
        enum { size = 0 };
    };


23.10.2: Searching a TypeList

    To determine whether a particular type (called SearchType below) is
present in a given TypeList, an algorithm is used that either defines
`index' as -1 (if SearchType is not an element of the TypeList ) or it
defines `index' as the index of the first occurrence of SearchType in
the TypeList. The following algorithm is used:
    
    o  If the TypeList is empty, `index' is -1;
    o  If the TypeList's first element equals   SearchType, `index' is 0;
    o  Otherwise  `index' is:
        
        o  -1 if searching for SearchType in TypeList's tail results in
            `index' == -1;
        o  Otherwise (SearchType was found in TypeList's tail) index is
            set to 1 + the index obtained when searching for SearchType in the
            TypeList's tail.

The algorithm is implemented using a variadic template struct
ListSearch expecting a parameter pack:
            template <typename ...Types>
    struct ListSearch
    {
        ListSearch(ListSearch const &) = delete;
    };

    Specializations handle the alternatives mentioned with the algorithm:
    
    o  If TypeList is empty, `index' is -1:
            template <typename SearchType>
    struct ListSearch<SearchType, TypeList<>>
    {
        ListSearch(ListSearch const &) = delete;
        enum { index = -1 };
    };


o  If TypeList's head  equals SearchType, `index'
        is 0. Note that SearchType is explicitly mentioned as the
            TypeList's first element:
            template <typename SearchType, typename ...Tail>
    struct ListSearch<SearchType, TypeList<SearchType, Tail...>>
    {
        ListSearch(ListSearch const &) = delete;
        enum { index = 0 };
    };


o  Otherwise a search is performed on the TypeList's tail. The index
            value returned by this search is stored in a tmp enum value,
            which is then used to determine index's value.
            template <typename SearchType, typename Head, typename ...Tail>
    struct ListSearch<SearchType, TypeList<Head, Tail...> >
    {
        ListSearch(ListSearch const &) = delete;
        enum {tmp = ListSearch<SearchType, TypeList<Tail...>>::index};
        enum {index = tmp == -1 ? -1 : 1 + tmp};
    };


Here is an example showing  how ListSearch can be used:
            std::cout <<
        ListSearch<char, TypeList<int, char, bool>>::index << "\n" <<
        ListSearch<float, TypeList<int, char, bool>>::index << "\n";



23.10.3: Selecting from a TypeList

    The inverse operation of determining the index of a certain type in a
TypeList is retrieving the type given its index. This inverse operation is
the topic of this section.

The algorithm is implemented using a struct TypeAt. TypeAt specifies a
using-declaration to define the type matching a given index. But the index
might be out of bounds. In that case we have several options:
    
    o  Use a static_assert to stop the compilation. This
is an appropriate action if the index should simply not be out of bounds;
    o  Define a local type (e.g., Null) that should not be used as a
type in the TypeList. This type is going to be returned when the index is
out of bounds. Using this local type as one of the types in a TypeList is
considered an error as its would conflict with the special meaning of Null
as the type returned at an invalid index.

    To prevent Null from being returned by TypeAt a static_assert
is used to catch the Null type when it is encountered while evaluating
TypeAt;
    o  The struct TypeAt may define an enum value validIndex set to
true if the index was valid and set to false if not.

The first alternative is implemented below. The other alternatives are not
difficult to implement and are left as exercises for the reader. Here's how
TypeAt works:
    
    o  The foundation consists of a variadic template struct TypeAt,
        expecting an index and a TypeList:
                template <size_t index, typename Typelist>
    struct TypeAt;


o  If the typelist is empty a static_assert ends the compilation
                template <size_t index>
    struct TypeAt<index, TypeList<>>
    {
        static_assert(index < 0, "TypeAt index out of bounds");
        using Type = TypeAt;
    };


o  If the search index equals 0, define Type as the first type in
        the TypeList:
                template <typename Head, typename ...Tail>
    struct TypeAt<0, TypeList<Head, Tail...>>
    {
        using Type = Head;
    };


o  Otherwise, Type is defined as Type defined by TypeAt<index
        - 1> operating on the TypeList's tail:
                template <size_t index, typename Head, typename ...Tail>
    struct TypeAt<index, TypeList<Head, Tail...>>
    {
        using Type = typename TypeAt<index - 1, TypeList<Tail...>>::Type;
    };


Here is how typeAt can be used. Uncommenting the first variable
definition causes a TypeAt index out of bounds compilation error:
            using list3 = TypeList<int, char, bool>;

//    TypeAt<3, list3>::Type invalid;
    TypeAt<0, list3>::Type intVariable = 13;
    TypeAt<2, list3>::Type boolVariable = true;

    cout << "The size of the first type is " <<
                sizeof(TypeAt<0, list3>::Type) << ", "
            "the size of the third type is " <<
                sizeof(TypeAt<2, list3>::Type) << "\n";

    if (typeid(TypeAt<1, list3>::Type) == typeid(char))
        cout << "The typelist's 2nd type is char\n";

    if (typeid(TypeAt<2, list3>::Type) != typeid(char))
        cout << "The typelist's 3nd type is not char\n";



23.10.4: Prefixing/Appending to a TypeList

    Prepending or appending a type to a TypeList is easy and doesn't require
recursive template meta programs. Two variadic template structs Append and
Prefix and two specializations are all it takes.

Here are the declarations of the two variadic template structs:
                template <typename ...Types>
    struct Append;

    template <typename ...Types>
    struct Prefix;


To append or prefix a new type to a typelist, specializations expect a
typelist and a type to add. Then, they simply define a new TypeList also
including the new type. The Append specialization shows that a template
pack does not have to be used as the first argument when defining another
variadic template type:
            template <typename NewType, typename ...Types>
    struct Append<TypeList<Types...>, NewType>
    {
        using List = TypeList<Types..., NewType>;
    };

    template <typename NewType, typename ...Types>
    struct Prefix<NewType, TypeList<Types...>>
    {
        using List = TypeList<NewType, Types...>;
    };



23.10.5: Erasing from a TypeList

    It is also possible to erase types from a TypeList. Again, there are
several possibilities, each resulting in a different algorithm.
    
    o  The type to erase is specified, and the first
occurrence of that type is removed from the TypeList;
    o  The index of the type to erase is specified, and the type at that
index position  is erased from the TypeList;
    o  The type to erase is specified, and all
occurrences of that type are removed from the TypeList.
    o  As a variant of erasure: we may want to erase all multiply
occurring types in a TypeList, keeping each type only once.

Doubtlessly there are other ways of erasing types from a
TypeList. Which ones are eventually  implemented depends of course on
the circumstances. As template meta programming is very powerful most if not
all algorithms can probably be implemented. As an illustration of how to erase
types from a TypeList the above-mentioned algorithms are now developed in
the upcoming subsections.


23.10.5.1: Erasing the first occurrence

        To erase the first occurrence of a specified EraseType from a TypeList
a recursive algorithm is used once again. The template meta program uses a
generic Erase struct and several specializations. The specializations
define a type List containing the resulting TypeList after the
erasure. Here is the algorithm:
    
    o  The foundation of the algorithm consists of a struct template
Erase expecting the type to erase and a TypeList:
                template <typename EraseType, typename TypeList>
    struct Erase;

    o  If the typelist is empty, there's nothing to erase, and an empty
        TypeList results:
                template <typename EraseType>
    struct Erase<EraseType, TypeList<>>
    {
        using List = TypeList<>;
    };

    o  If the TypeList's head matches the type to erase, then List
        becomes a TypeList containing the original TypeList's tail
        types:
                template <typename EraseType, typename ...Tail>
    struct Erase<EraseType, TypeList<EraseType, Tail...>>
    {
        using List = TypeList<Tail...>;
    };

    o  In all other cases the erase operation is applied to the
        TypeList's tail. This results in a TypeList to which the
        orginal TypeList's head must be prefixed. The TypeList
        returned by the prefix operation is then returned as Erase::List:
                template <typename EraseType, typename Head, typename ...Tail>
    struct Erase<EraseType, TypeList<Head, Tail...>>
    {
        using List =  typename
            Prefix<Head,
                typename Erase<EraseType, TypeList<Tail...>>::List
            >::List;
    };


Here is a statement showing how Erase can be used:
                cout <<
            Erase<int, TypeList<char, double, int>>::List::size << '\n' <<
            Erase<char, TypeList<int>>::List::size << '\n' <<
            Erase<int, TypeList<int>>::List::size << '\n' <<
            Erase<int, TypeList<>>::List::size << "\n";




23.10.5.2: Erasing a type by its index

        To erase a type from a TypeList by its index we again use a
recursive template meta program. EraseIdx expects a size_t index value
and a TypeList from which its idx^th (0-based) type must be
erased. EraseIdx defines the type List containing the resulting
TypeList. Here is the algorithm:
    
    o  The foundation of the algorithm consists of a struct template
EraseIdx expecting the index of the type to erase and a TypeList:
                template <size_t idx, typename TypeList>
    struct EraseIdx;

    o  If the typelist is empty, there's nothing to erase, and an empty
        TypeList results:
                template <size_t idx>
    struct EraseIdx<idx, TypeList<>>
    {
        using List = TypeList<>;
    };

    o  The recursion otherwise ends once idx becomes 0. At that point
        the TypeList's first type is ignored and List is initialized
        to a TypeList containing the types in the orginal TypeList's
        tail:
                template <typename EraseType, typename ...Tail>
    struct EraseIdx<0, TypeList<EraseType, Tail...>>
    {
        using List = TypeList<Tail...>;
    };

    o  In all other cases EraseIdx is applied to the TypeList's
        tail, providing it with a decremented value of idx. To the
        resulting TypeList the orginal TypeList's head is
        prefixed. The TypeList returned by the prefix operation is then
        returned as EraseIdx::List:
                template <size_t idx, typename Head, typename ...Tail>
    struct EraseIdx<idx, TypeList<Head, Tail...>>
    {
        using List = typename Prefix<
                    Head,
                    typename EraseIdx<idx - 1, TypeList<Tail...>>::List
                >::List;
    };


Here is a statement showing how EraseIdx can be used:
                if
    (
        typeid(TypeAt<2,
                EraseIdx<1,
                   TypeList<int, char, size_t, double, int>>::List
                >::Type
        )
        == typeid(double)
    )
        cout << "the third type is now a double\n";



23.10.5.3: Erasing all occurrences of a type

        Erasing all types EraseType from a TypeList can easily be accomplished
by applying the erasure procedure not only to the head of the TypeList but
also to the  TypeList's tail.

Here is the algorithm, described in a slightly different order than
Erase's algorithm:
    
    o  If the TypeList is empty, there's nothing to erase, and an empty
        TypeList results. This is exactly what we do with Erase, so we
        can use inheritance to prevent us from having to duplicate elements of
        a template meta program:
                template <size_t idx>
    struct EraseIdx<idx, TypeList<>>
    {
        using List = TypeList<>;
    };

    o  The foundation of the algorithm is therefore a struct template
        EraseAll expecting the type to erase and a TypeList that is
        derived from Erase, thus already offering the empty TypeList
        handling specialization:
                template <typename EraseType, typename TypeList>
    struct EraseAll: public Erase<EraseType, TypeList>
    {};

    o  If TypeList's head matches EraseType EraseAll is also
        applied to the TypeList's tail, thus removing all occurrences of
        EraseType from TypeList:
                template <typename EraseType, typename ...Tail>
    struct EraseAll<EraseType, TypeList<EraseType, Tail...>>
    {
        using List = typename EraseAll<EraseType, TypeList<Tail...>>::List;
    };

    o  In all other cases (i.e., TypeList's head does not match
        EraseType) EraseAll is applied to the TypeList's tail.
        The returned TypeList consists of the original TypeList's
        initial type and the types of the TypeList returned by the
        recursive EraseAll call:
                template <typename EraseType, typename Head, typename ...Tail>
    struct EraseAll<EraseType, TypeList<Head, Tail...>>
    {
        using List = typename Prefix<
            Head,
            typename EraseAll<EraseType, TypeList<Tail...>>::List
        >::List;
    };


Here is a statement showing how EraseAll can be used:
           cout <<
        "After erasing size_t from "
            "TypeList<char, int, size_t, double, size_t>\n"
            "it contains " <<
                EraseAll<size_t,
                         TypeList<char, int, size_t, double, size_t>
                >::List::size << " types\n";



23.10.5.4: Erasing duplicates

        To remove all duplicates from a TypeList all the TypeList's first
elements must be erased from the TypeList's tail, applying the procedure
recursively to the TypeList's tail. The algorithm, outlined below, merely
expects a TypeList:
    
    o  First, the generic EraseDup struct template is
declared. EraseDup structures define a type List representing the
TypeList that they generate. EraseDup calls expect a TypeList as
their template type parameters:
            template <typename TypeList>
    struct EraseDup;

    o  If the TypeList is empty it can be returned empty and we're done:
            template <>
    struct EraseDup<TypeList<>>
    {
        using List = TypeList<>;
    };

    o  In all other cases
        
        o  EraseDup is first applied to the original TypeList's
tail. By definition this results in a TypeList from which all
duplicates have been removed;
        o  The TypeList returned by the previous step might contain the
original TypeList's initial type. If so, it is removed by applying
Erase on the returned TypeList, specifying the original TypeList's
initial type as the type to remove;
        o  The returned TypeList consists of the original TypeList's
initial type to which the types of the TypeList produced by the previous
step are appended.

This specialization is implemented like this:
            template <typename Head, typename ...Tail>
    struct EraseDup<TypeList<Head, Tail...>>
    {
        using UniqueTail = typename EraseDup<TypeList<Tail...>>::List;
        using NewTail = typename Erase<Head, UniqueTail>::List;

        using List = typename Prefix<Head, NewTail>::List;
    };


Here is an example showing how EraseDup can be used:
            cout <<
        "After erasing duplicates from "
             "TypeList<double, char, int, size_t, int, double, size_t>\n"
        "it contains " <<
        EraseDup<
            TypeList<double, char, int, size_t, int, double, size_t>
        >::List::size << " types\n";



23.11: Using a TypeList

In the previous sections the definition and some of the features of typelists
were discussed. Most C++ programmers consider typelists both
exciting and an intellectual challenge, honing their skills in the area of
recursive programming.

But there's more to typelist than a mere intellectual challenge. In the final
sections of this chapter the following topics are covered:
    
    o  Creating classes from a typelist.

       Here the aim is to construct a new class consisting of instantiations
        of an existing basic template for each of the types mentioned in a
        provided typelist;
    o  Accessing data members from the thus constructed conglomerate class
        by index, rather than name.

Again, much of the material covered by these sections was inspired by
        
    Alexandrescu's (2001) book.


23.11.1: The Wrap and Multi class templates

    To illustrate template meta programming concepts the template class Multi
is now developed. The class template Multi creates a new class from a
template template parameter Policy defining the data storage policy and a
series of types from which Multi is eventually derived. It does so by
passing its template parameters to its base class MultiBase that in turn
creates a final class inheritance tree. Since we don't know how many types are
going to be used Multi is defined as a variadic class template using a
template pack ...Types.

In fact, the types that are specified with Multi aren't that
interesting. They primarily serve to `seed' the class Policy. Therefore,
rather than forwarding Multi's types to MultiBase they are passed to
Policy and the sequence of Policy<Type> types is then forwarded to
MultiBase. Multi's constructor expects initialization values for its
various Policy<Type>s which are perfectly forwarded to MultiBase.

The class Multi (implementing its constructor in-class to save some
space) shows how a
    
    
 template pack can be wrapped into a policy. Here is Multi's definition:
        template <template <typename> class Policy, typename ...Types>
    struct Multi: public MultiBase<0, Policy<Types>...>
    {
        using PlainTypes = TypeList<Types...>;
        using Base = MultiBase<0, Policy<Types>...>;

        enum { size = PlainTypes::size };

        Multi(Policy<Types> &&...types)
        :
            MultiBase<0, Policy<Types>...>(
                            std::forward<Policy<Types>>(types)...)
        {}
    };


Unfortunately, the design as described contains some flaws.
    
    o  As the Policy template template parameter is defined as
template <typename> class Policy it can only accept policies expecting one
type argument. Contrary to this, std::vector is a template expecting two
template arguments, the second one defining the allocation scheme used by
std::vector. This allocation scheme is hardly ever changed, and most
applications merely define objects of types like vector<int>,
vector<string> etc.. Template template parameters must, however, be specified
with the correct number and types of required template parameters so
vector can't be specified as a policy for Multi. This can be solved by
    
wrapping a more complex template in a simpler wrapper template, like so:
            template <class Type>
    struct Vector: public std::vector<Type>
    {
        Vector(std::initializer_list<Type> iniValues)
        :
            std::vector<Type>(iniValues)
        {}
    };

    Now Vector provides std::vector's second parameter using its
default template argument. Alternatively, a template using declaration
could be used.
    o  If the TypeList contains two types like int and double
and the policy class is Vector, then the MultiBase class
eventually inherits from vector<int> and vector<double>. But if the
TypeList contains identical types, like two int type specifications
MultiBase would inherit from two vector<int> classes. Classes
cannot be derived from identical base classes as that would make it impossible
to distinguish among their members. Regarding this, Alexandrescu (2001)
     writes (p.67):
    
   There is one major source of annoyance...: you cannot use it when you
    have duplicate types in your TypeList.

   .... There is no easy way to solve the ambiguity, [as the eventually
    derived class/FBB] ends up inheriting [the same base class/FBB] twice.

There is a way around the problem of duplicate base class types. If
instead of inheriting directly from base classes these base classes are first
wrapped in unique type defining classes, then these unique classes can be used
to access the base classes using principles of inheritance. As these unique
type-defining wrapper classes are merely classes that are derived from the
`real' base classes they inherit (and thus: offer) the functionality of their
base classes. A unique type defining wrapper class can be designed after the
class IntType, defined earlier.  The wrapper class we're
looking combines class derivation with the uniqueness offered by
IntType. The class template UWrap has two template parameters: one
non-type parameter idx and one type parameter. By ensuring that each
UWrap definition uses a unique idx value unique class types are
created. These unique class types are then used as base classes of the derived
class MultiBase:
        template <size_t nr, typename Type>
    struct UWrap: public Type
    {
        UWrap(Type const &type)
        :
            Type(type)
        {}
    };

    Using UWrap it's easy to distinguish, e.g., two vector<int>
classes: UWrap<0, vector<int>> could refer to the first vector<int>,
UWrap<1, vector<int>> to the second vector.

Uniqueness of the various UWrap types is assured by the class
template MultiBase as discussed in the next section.

It must also be possible to initialize a  Multi class object. Its
constructor therefore expects the initialization values for all its Policy
values. So if a Multi is defined for Vector, int, string then its
constructor can receive the matching initialization values. E.g.,
            Multi<Vector, int, string> mvis({1, 2, 3}, {"one", "two", "three"});


23.11.2: The MultiBase class template

    The class template MultiBase is Multi's base class. It
defines a class that, eventually, is derived from the list of Policy
types that, in turn, were created by Multi using any additional types that
were  passed to it.

MultiBase itself has no concept of a Policy. To MultiBase the
world appears to consist of a simple template pack whose types are used to
define a class from. In addition to the PolicyTypes template pack,
MultiBase also defines a size_t nr non-type parameter that is used to
create unique UWrap types. Here is MultiBase's generic class
declaration:
        template <size_t nr, typename ...PolicyTypes>
    struct MultiBase;


Two specializations handle all possible MultiBase invocations. One
specialization is a recursive template. This template handles the first type
of MultiBase's template parameter pack and recursively uses itself to
handle the remaining types. The second specialization is invoked once the
template parameter pack is exhausted and does nothing. Here is the definition
of the latter specialization:
        template <size_t nr>
    struct MultiBase<nr>
    {};


The recursively defined specialization is the interesting one. It performs
the following tasks:
    
    o  It is derived from a unique UWrap type. The uniqueness is
guaranteed by using MultiBase's nr parameter when defining
UWrap. In addition to nr the UWrap class receives the first type
of the template parameter pack made available to MultiBase;
    o  It is also recursively derived from itself. The recursive
MultiBase type is defined using as its first template argument an
incremented nr value (thus ensuring the uniqueness of the UWrap types
defined by recursive MultiWrap types). Its second template argument is the
tail of the template parameter pack made available to MultiBase

An illustration showing the layout of the MultiBase class
hierarchy is provided in figure 30.

------------------------------------------------------------------Insert Figure 30(Layout of a MultiBase class hierarchy)about here (file: advancedtemplates/multibase)------------------------------------------------------------------

MultiBase's constructor simply receives the initialization values that
were (originally) passed to the Multi object. Perfect forwarding is used
to accomplish this. MultiBase's constructor passes its first parameter
value to its UWrap base class, also using perfect forwarding.
MultiBase's recursive definition is:
            template <size_t nr, typename PolicyT1, typename ...PolicyTypes>
    struct MultiBase<nr, PolicyT1, PolicyTypes...> :
                                public UWrap<nr, PolicyT1>,
                                public MultiBase<nr + 1, PolicyTypes...>
    {
        using Type = PolicyT1;
        using Base = MultiBase<nr + 1, PolicyTypes...>;

        MultiBase(PolicyT1 && policyt1, PolicyTypes &&...policytypes)
        :
            UWrap<nr, PolicyT1>(std::forward<PolicyT1>(policyt1)),
            MultiBase<nr + 1, PolicyTypes...>(
                              std::forward<PolicyTypes>(policytypes)...)
        {}
    };



23.11.3: Support templates

    The Multi class template defines PlainTypes as the TypeList
holding all the types of its parameter pack. Each MultiBase derived from a
UWrap type also defines a type Type representing the policy type that
was used to define the UWrap type and a type Base representing the
type of its nested MultiBase class.

These three type definitions allow us to access the types from which the
Multi object was created as well as the values of those types.

The class template typeAt, is a pure template meta program class template
(it has no run-time executable code). It expects a size_t idx template
argument specifying the index of the policy type in a Multi type object as
well as a Multi class type. It defines the type Type as the Type
defined by Multi's MultiBase<idx, ...> base class. Example:
            typeAt<0, Multi<Vector, int, double>>::Type // Type is vector<int>

The class template typeAt defines (and uses) a nested class template
PolType doing all the work. PolType's generic definition specifies two
template parameters: an index used to specify the index of the requested type
and a typename initialized by a MultiBase type argument. PolType's
recursive definition recursively reduces its index non-type parameter,
passing the next base class in MultiBase's inheritance tree to the
recursive call. As PolType eventually defines the type Type to be the
requested policy type the recursive definition defines its Type as the
type defined by the recursive call. The final (non-recursive) specialization
defines the initial policy type of the MultiBase type as Type. Here is
typeAt's definition:
            template <size_t index, typename Multi>
    class typeAt
    {
        template <size_t idx, typename MultiBase>
        struct PolType;

        template <size_t idx,
                  size_t nr, typename PolicyT1, typename ...PolicyTypes>
        struct PolType<idx, MultiBase<nr, PolicyT1, PolicyTypes...>>
        {
            using Type = typename PolType< idx - 1, MultiBase<nr + 1,
                                           PolicyTypes...> >::Type;
        };

        template <size_t nr, typename PolicyT1, typename ...PolicyTypes>
        struct PolType<0, MultiBase<nr, PolicyT1, PolicyTypes...>>
        {
            using Type = PolicyT1;
        };
    public:
        typeAt(typeAt const &) = delete;
        using Type = typename PolType<index, typename Multi::Base>::Type;
    };


The types specified by Multi's parameter pack can also be retrieved using
a second helper class template: plainTypeAt. Example:
            plainTypeAt<0, Multi<Vector, int, double>>::Type // Type is int

The class template plainTypeAt uses a comparable (but simpler)
implementation than typeAt. It is also a pure template meta program class
template defining a nested class template At. At is implemented like
typeAt but it visits the types of the original template pack that was
passed to Multi, and made available by Multi as its PlainTypes
type. Here is plainTypeAt's definition:
            template <size_t index, typename Multi>
    class plainTypeAt
    {
        template <size_t idx, typename List>
        struct At;

        template <size_t idx, typename Head, typename ...Tail>
        struct At<idx, TypeList<Head, Tail...>>
        {
            using Type = typename At<idx - 1, TypeList<Tail...>>::Type;
        };

        template <typename Head, typename ...Tail>
        struct At<0, TypeList<Head, Tail...>>
        {
            using Type = Head;
        };

    public:
        plainTypeAt(plainTypeAt const &) = delete;
        using Type = typename At<index, typename Multi::PlainTypes>::Type;
    };


Arguably the neatest support template is get. This is a function template
defining size_t idx as its first template parameter and typename Multi
as its second template parameter. The function template get defines one
function parameter: a reference to a Multi, so it can deduce Multi's
type by itself. Knowing that it's a Multi, we reason that it is also a
UWrap<nr, PolicyType> and therefore also a PolicyType, as the latter
class is defined as a base class of UWrap.

Since class type objects can initialize references to their base classes the
PolicyType & can be initialized by an appropriate UWrap reference,
which in turn can be initialized by a Multi object. Since we can
determine PolicyType using TypeAt (note that evaluating typename
typeAt<idx, Multi>::Type is a purely compile-time matter), the get
function can very well be implemented inline by a single return
statement:
            template <size_t idx, typename Multi>
    inline typename typeAt<idx, Multi>::Type &get(Multi &multi)
    {
        return static_cast<
                UWrap<idx, typename typeAt<idx, Multi>::Type> &>(multi);
    }

    The intermediate UWrap cast is required to disambiguate between
identical policy types (like two vector<int> types). As UWrap is
uniquely determined by its nr template argument and this is the number
argument that is passed to get ambiguities can easily be prevented.


23.11.4: Using Multi

    Now that Multi and its support templates have been developed, how can a
Multi be used?

A word of warning is in place. To reduce the size of the developed classes
they were designed in a minimalist way. For example, the get function
template cannot be used with Multi const objects and there is no default,
or move constructor available for Multi types. Multi was designed to
illustrate some of the possibilities of template meta programming and
hopefully Multi's implementation served that purpose well. But can it be
used? If so, how?

This section provides some annotated examples. They may be concatenated to
define a series of statements that could be placed in a main function's
body, which would result in a working program.


    o  A simple Policy could be defined:
            template <typename Type>
    struct Policy
    {
        Type d_type;
        Policy(Type &&type)
        :
            d_type(std::forward<Type>(type))
        {}
    };

    Policy defines a data member and it can be used to define Multi
objects:
            Multi<Policy, string> ms{ Policy<string>{ "hello" } };
    Multi<Policy, string, string> ms2s{ Policy<string>{ "hello" },
                                       Policy<string>{ "world" } };


     using MPSI = Multi<Policy, string, int>;
     MPSI mpsi{ string{ "hello" }, 4 };


o  To obtain the number of types defined by a Multi class or object
    either use the ::size enum value (using the Multi class) or the
.size member (using the Multi object):
            cout << "There are " << MPSI::size << " types in MPSI\n"
            "There are " << mpsi.size << " types in mpsi\n";


o  Variables of constituting types can be defined using plainTypeAt:
            plainTypeAt<0, MPSI>::Type sx = "String type";
    plainTypeAt<1, MPSI>::Type ix = 12;


o  Raw static casts can be used to obtain the constituent type:
            cout << static_cast<Policy<string> &>(mpsi).d_type << '\n' <<
            static_cast<Policy<int> &>(mpsi).d_type << '\n';


o  However, this won't work when the template parameter pack contains
identical types, as a cast can't distinguish between identical
Policy<Type> types. In that case get still works fine:
            using MPII = Multi<Policy, int, int>;
    MPII mpii{ 4, 18 };

    cout << get<0>(mpii).d_type << ' ' << get<1>(mpii).d_type << '\n';


o  Here is an example wrapping a std::vector in a Vector:
            using MVID = Multi<Vector, int, double>;
    MVID mi{ {1, 2, 3}, {1.2, 3.4, 5.6, 7.8} };


o  Such a vector can be defined by its Multi type:
            typeAt<0, Multi<Vector, int>>::Type vi = {1, 2, 3};


o  Knowing that a Vector is a std::vector, the reference
returned by get support index operators that can be used as left hand side
or right hand side operands:
            cout << get<0>(mi)[2] << '\n';
    get<1>(mi)[3] = get<0>(mi)[0];
    cout << get<1>(mi)[3] << '\n';



23.12: Expression Templates



Assume we are processing std::vector objects. Vectors may be assigned to
each other, but that's about it. We've seen (cf. section 12.3.2) that its
member functions tend to operate on the current vector, but arithmetic
operations like addition, subtraction, multiplication and the like cannot
be applied to pairs of vectors.

Implementing the, e.g., addition operator for vectors is not difficult. If
VecType is our vector type, then implementing free functions like
    VecType &&operator+(VecType const &lhs, VecType const &rhs) and 
    VecType &&operator+(VecType &&lhs, VecType const &rhs) 
        performing the additions is a simple exercise (cf. chapter
11).

Now consider an expression like one + two + three + four. It takes four
steps to compute this sum: first, tmp = one is computed, creating the
eventual return value. The vector tmp becomes the eventual return
value. Once it is available tmp += two is computed, followed by tmp +=
three, and finally by tmp += four (of course
we shouldn't implement std::vector::operator+= as the std namespace is
off-limits to us, and we shouldn't derive a class from std::vector
offering operator+= according to Liskov's Substitution Principle
(cf. section 14.7), but we could get around that.
     
    Here we simply assume operator+= is available).

Here's how we might implement operator+= for VecType:
             VecType &VecType::operator+=(VecType const &rhs)
    {
        for (size_t idx = 0, end = size(); idx != end; ++idx)
            (*this)[idx] += rhs[idx];
        return *this;
    }

Consider this implementation: once we add VecType objects and such
objects have N elements then we have to perform 2 * N index
evaluations. When adding k VecType objects this adds up to 2 * N * k
index expression evaluations (as eventually we also have to assign the
elements of the resulting temporary object to the destination object): lots
of index expression evaluations.

If instead we could manage to perform the evaluations `row by row', we
would only have to access each vector element only once (which in particular
applies to the temporary object). In that case, when adding k objects,
assigning the sums of their respective elements to a destination vector we
have to compute N * (k + 1) index expressions (`k' for each of the
vectors, `1' for the destination vector).

For k == 1 the two methods are equally efficient in terms of index
computations. But that's not addition, that is assignment. So when adding any
number of vectors, assigning their sum to a destination vector using expression
templates 
     
  is more efficient than the ordinary implementation of the addition
operator. We'll have a look at the design and implementation of expression
templates in the coming sections.


23.12.1: Designing an Expression Template

    As we've seen, when using a standard implementation of an expression like
one + two + three + four, where the objects are vectors having n
elements, then if we have k vectors we have to perform a total of k * 2
* n index evaluations.

Expression templates allow us to avoid many of these evaluations. When using
expression templates these templates may access the vectors, but their
elements are not accessed during addition operations. 

Assuming our expression template is named ET, and we want to add one + two
+ three, then the first + operator merely creates ET(one, two). Note
that no addition is actually performed, ET merely stores (constant)
references to one (becoming ET's lhs data member) and two
(becoming ET's rhs data member). In general, ET stores references to
the two arguments that are passed to its constructor.

At the next addition operator another ET is created. Its constructor
arguments are, respectively, the ET object that has just been constructed
for one and two, and the vector three. Again, no addition is
performed by the ET objects.

This algorithm easily generalizes to any number of vectors. Parentheses can
also be used. E.g., (one + two) + (three + four) results in
            ET(ET(one, two), ET(three, four))

Presumably, at some point we want to obtain the sum of the vectors. For this
the expression template is provided with a conversion operator, converting the
ET object to a vector, or maybe an assignment operator doing the same.

The conversion operator looks like this:
            operator ET::VecType() const
    {                                                        
        VecType retVal;
        retVal.reserve(size());

        for (size_t ix = 0, end = size(); ix != end; ++ix)
            new(&retVal[ix]) value_type((*this)[ix]);
                                                     
        return retVal;
    }

Placement new is used for efficiency reasons: there's no need to
initialize retVal with default values first. The really interesting part,
however, is hidden behind the (*this)[idx] expression: at this point the
real addition takes place.

ET's index operator simply adds the values returned by the
corresponding index expressions of its lhs and rhs data members. If a
data member refers to a vector then the corresponding vector element is used,
adding it to the other data member's value. If a data member itself refers to
an ET object, then that nested ET object's index operator performs the
same addition on its own data members, returning their sum. So, an expression
like (*this)[0] returns first[0] + second[0] + third[0], and the
computed sum is then stored in retVal[0] using placement new.

In this case the required number of index expression evaluations are n
* k (for the n elements of the k vectors) plus n (for the n elements of
retVal, adding up to (k + 1) * n). 

Since (k + 1) * n < 2 * k * n for k > 1 expression templates evaluate
the requested addition more efficiently than the traditional implementation of
operator+. An additional benefit of using expression templates is that
they do not create additional temporary vector objects when parenthesized
expressions are used.


23.12.2: Implementing an Expression Template

    In this section we specify using IntVect = std::vector<int> to illustrate
the construction of an expression template. 

Starting point is a simple main function, in which several IntVect
objects are added. E.g., 
            int main()
    {
        IntVect one;
        IntVect two;
        IntVect three;
        IntVect four;
        
        // ... assume the IntVects somehow receive values

        four = one + two + three + four;
    }

At this point the code does not suggest that expression templates are
going to be used. However, operator+'s implementation is special: it's a
template merely returning an object constructed by operator+:
            template<typename LHS, typename RHS>
    BinExpr<LHS, RHS, plus> operator+(LHS const &lhs, RHS const &rhs)
    {
        return BinExpr<LHS, RHS, plus>{ lhs, rhs };
    }

Our expression template is called BinExpr. It has three template type
parameters: two object types and a template template parameter performing the
requested operation. Its declaration looks like this:
            template<typename LHS, typename RHS, template<typename> class Operation>
    struct BinExpr;

Since LHS and RHS can either be the data type that is processed by
the expression template, or a BinExpr two different typenames are
required. Operation is the operation that is performed by the expression
template. By using a template template parameter we can use BinExpr to
perform any operation we want, not just addition. Predefined function
templates like std::plus can be used for the standard arithmetic
operators; for other operators we can define our own function templates.

BinExpr's constructor initializes constant references to lhs and
rhs. Its in-class implementation is
            BinExpr(LHS const &lhs, RHS const &rhs)
    :
        d_lhs(lhs),
        d_rhs(rhs)
    {}

To retrieve the resulting IntVect a conversion operator is defined. We
already encountered its implementation (in the previous section). Here is it,
as an in-class implemented BinExpr member:
            operator ObjType() const
    {                                                        
        ObjType retVal;
        retVal.reserve(size());

        for (size_t idx = 0, end = size(); idx != end; ++idx)
            new(&retVal[idx]) value_type((*this)[idx]);
                                                     
        return retVal;
    }

We return to the type ObjType below. At this point it can be
considered an IntVect. The member size() simply returns
d_lhs.size(): in any sequence of IntVect additions LHS eventually
is an IntVect, and so every BinExpr defines a valid size() like
so:
            size_t size() const
    {
        return d_lhs.size();
    }

The only remaining member to implement is operator[]. Since it
receives an index, it only needs to perform the requested operation on the
corresponding index elements of its d_lhs and d_rhs data members. The
beauty of expression templates is that if either one itself is a BinExpr
that expression template in turn calls its operator[], eventually
performing the requested operation on all corresponding elements of all
IntVect objects. Here is its implementation:
            value_type operator[](size_t ix) const
    {
        static Operation<value_type> operation;

        return operation(d_lhs[ix], d_rhs[ix]);
    }

This implementation uses another type: value_type which is the 
type of the elements of the vector type that is processed by the expression
template. Like ObjType before, its definition is covered below. The static
data member operation simply is an instantiation of the Operation type
that is specified when constructing an ExprType object. 

In the next section we take a closer look at ObjType and value_type.


23.12.3: The BasicType trait class and ordering classes

    The BinExpr expression template needs to be aware of two types before it
can instantiate objects. First, ObjType must be known, as this is the 
type of object that is handled by the expression template. ObjType objects
contain values, and we require that the type of these values can be determined
as ObjType::value_type. E.g., for our IntVect data type value_type
is int. 

In expressions like one + two + three, the BinExpr expression template
receives two IntVect objects. This is always true: the BinExpr that is
first constructed receives two IntVect objects. In this case ObjType
is simply LHS, and ObjType::value_type is also available: either
value_type is already defined by LHS or BinExpr requires that
it defines type value_type. 

Since arguments to BinExpr objects are not always of the basic
ObjType type (BinExpr objects at the next nesting level receive at
least one BinExpr argument) we need a way to determine ObjType from a
BinExpr. For this we use a trait class. The 
trait class BasicType receives a typename template argument, and equates
its type ObjType to the received template type argument:
            template<typename Type>
    struct BasicType
    {
        using ObjType = Type ;
    };

A specialization handles the case where Type in fact is a BinExpr:
    template<typename LHS, typename RHS, template<typename> class Operation>
            template<typename LHS, typename RHS, template<typename> class Operation>
    struct BasicType<BinExpr<LHS, RHS, Operation>>
    {
        using ObjType = BinExpr<LHS, RHS, Operation>::ObjType ;
    };

Since BinExpr requires that ObjType::value_type is a defined
type, value_type has automatically been taken care of. 

As BinExpr refers to BasicType and BasicType refers to
BinExpr somewhere we must provide a forward declaration. As BinExpr's
declaration has already been provided, we start with that declaration,
resulting in:
            BinExpr's declaration

    BasicType's definition

    BasicType's specialization (for BinExpr)

    template<typename LHS, typename RHS, template<typename> class Operation>
    class BinExpr 
    {
        LHS const &d_lhs;
        RHS const &d_rhs;

        public:
            using DataType = BasicType<RHS>::DataType ;
            using type = DataType::value_type value_;

            // all BinExpr member functions
    };


23.13: Concepts



C++ is a strongly typed language: a function add(int lhs, int rhs)
doesn't accept std::string arguments, even though the actual operations
(lhs + rhs) are identical for ints and strings. 

Templates were introduced so we could design recipes for the compiler,
allowing it to construct type-safe overloaded versions of functions and
classes while keeping their type-safety. 

A basic addition function template adding two values looks like this:
            template <typename Type>
    Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }
        

When this function template is called with arguments of types that do not
support operator+ then the compiler notices this, and it will generate an
error. E.g., when calling
            add(std::cerr, std::cout);
    the g++ compiler produces some 140 lines of error messages. It notices
that there's no operator+ for std::ostream objects, and then tells us
what else we might have done (like adding two ints), and where the
construction of the add function that should accept std::ostream
arguments went wrong. In fact, 140 lines of error messages is rather benign.
Getting several hundreds of lines is quite common, and sometimes the
location of the error isn't mentioned at the top but somewhere near the end of
the error message output. 

The C++26 standard introduced  concepts allowing us to specify
requirements for template types. When applying an appropriate concept to
the definition of the add function template the compiler immediately
pinpoints the error, telling us where and why the error occurred in some 15
instead of 140 lines of error messages.

The reduction of the number of lines of error messages by itself is a
boon. But the fact that concepts allow us to consciously develop our
templates, realizing what the precise requirements are for their use, is at
least as important: it improves the template's documentation, and thus our
understanding of templates.

Concepts may be considered the template's answer to the philosphy that lies
behind a strongly typed language. By applying concepts to templates we can
specify type-requirements rather than using the traditional `shotgun
empiricism' approach where templates are bluntly used, knowing that the
compiler will complain if things are incorrect. In that sense concepts provide
type definitions of types. Concepts have names and can (among other) be used
in template headers where the concept names replace the traditional
typename keywords. 

As an opening illustration, assume that a concept Addable exists
specifying that operator+ must have been defined for the template's
type. The above function template add can now be formulated as:
            template<Addable Type>
    Type add(Type const &lhs, Type const &rhs)
    {
        return lhs + rhs;
    }

From now on every type that is actually passed to add must be 
satisfy the Addable requirements. Here are two expressions
using add:
            add("first"s, "second"s);               // (1)
    add(map<int, int>{}, map<int, int>{});  // (2)

Expression (1) flawlessly compiles as string objects can be added;
expression (2) fails with the compiler reporting something like
        error: use of function `Type add(const Type&, const Type&)
    [with Type = std::unordered_map<int, int>]' with unsatisfied constraints
    add(unordered_map<int, int>{}, unordered_map<int, int>{});
    note: constraints not satisfied

    Type add(const Type&, const Type&) 
    ...
    note: the required expression `(lh + rh)' is invalid

The error message's final `note' clearly states the cause of the problem:
you can't add maps. 

The difference between the compiler's report using concepts and not using
concepts again is impressive. When using the traditional typename Type
specification in the template header the compiler produces some 17 kB of error
messages, spread out over more than 200 lines.

In the following sections we cover how concepts are defined, what kind of
requirements can be formulated, and how they can be used in practice.


23.13.1: Defining concepts

    As a prelude before actually looking at how concepts are defined it is noted
that concept names, like class names, type names, function names, and variable
names should suggest their purposes. Don't name a concept `Constraint' or
`Concept', but use names like `Addable' and `HasValueType'.

Concepts are templates. They start with a template header (the
template headers shown in the examples define a single template type
parameter, but multiple template parameters are also used). 

In the previous section we used the concept Addable. Here is how it can
be defined:
        template <typename Type>
    concept Addable =
        requires(Type lh, Type rh)
        {
            lh + rh;
        };

The concept's template header is followed by the keyword concept, the
concept's name, and the assignment operator. Following the assignment operator
requirement specifications are provided.

Semicolons end concept definitions. This concept uses a
simple requirement (cf. section 23.13.2.1) indicating that
operator+ must have been defined for Addable templates' types.

Requirements come in many forms. A very simple form consists of just a
bool value, which is sometimes useful when developing a concept. Such a
concept looks like this:
        template <typename Type>
    concept IsTrue = 
        true;
        
    But in most situations requires specifications are used. They resemble
function definitions having parameter lists optionally defining variables of
the types that specified in the concept's template header and compound
statements specifying requirements.

Concepts are never instantiated. They are used compile-time to verify that
template types satisfy the imposed requirements.  Thus there's no need
to use refererences in parameter lists of requires specifications. The
concept Addable simply uses
            requires(Type lh, Type rh)

and there's no need to specify
            requires(Type const &lh, Type const &rh)
    (That is, usually there is no need for this. In section 23.13.2.4 we
encounter a situation where a more specific parameter definition might be
appropriate.)

Here are two examples of templates using concept Addable. The first
example uses Addable instead of typename when specifying the template
header, the second example appends the concept specification to the template
header itself:
        template<Addable Type>
    Type add2(Type const &x, Type const &y)
    {
        return x + y;
    }
    
    template<typename Type>
        requires Addable<Type>
    Type add(Type const &x, Type const &y)
    {
        return x + y;
    }

Template declarations using concepts are specified accordingly. Simply replace
the function template's body by a semicolon.

Concepts may also be defined by extending or combining existing concepts. 
Nesting concepts is covered in section 23.13.2.4.

Although concepts are templates, they cannot be specialized. If a concept
should recognize specializations then these specializations must be handled by
the concepts' definitions. See section 23.13.2.3 for an illustration.


23.13.2: Requirements

    The bodies of requires declarations contain define constraints to apply to
template parameters. There are four types of requirements:
    
    o  Simple requirements: constraints on facilities (like
        requiring the availability of operator+);
    o  Type requirements: requiring the availability of (sub)types (like 
        the sub-type value_type) which must be available when using
        standard push_back functions;
    o  Compound requirements: requiring types resulting from applying
        operators or returned by called (member) functions;
    o  Nested requirements: defining concepts in terms of already
        existing concepts. 

Constraints must be compile-time verifiable.

When multiple constraints are specified, they must all be compile-time
verifiable, and an actual type is only accepted by the compiler if all
requirements could be satisfied. 


23.13.2.1: Simple requirements

        We've already encountered various examples of simple requirements: they
specify the operations that must be supported by the variable(s) declared in
the parameter lists of requires specifications. When the requirements
refer to single variables single Type parameters suffice; when 
requirements involve different types then the concept's template head declares
those different types and the requires parameter list will usually define
variables of those different types. The concept BasicMath specifies two
types, and uses four simple requirements to specify the four basic arithmetic
operations:
        template <typename LhsType, typename RhsType>
    concept BasicMath =
        requires(LhsType lhs, RhsType rhs)
        {
            lhs + rhs;      // addition must be available
            lhs - rhs;      // subtraction must be available
            lhs * rhs;      // multiplication must be available
            lhs / rhs;      // division must be available
        };

Specifying constraints does not necessarily mean that the constraints as
specified literally apply to run-time situations. To require the existence of
the index operator the following simple requirement can be used:
        template <typename Type>
    concept HasIndex =
        requires(Type tp)
        {
            tp[0];
        };
    
    template <HasIndex Type>
    auto idx(Type const &obj, size_t idx)
    {
        return obj[idx];
    }

Here the stand-in argument 0 is used to specify the index operator's
argument. The argument value used in the simple requirement really is a
stand-in. The following code fragment compiles, as string supports the
index operator. Although argument 0 is used in the simple requirement
specification the argument 5 is in fact being used:
            string str;
    idx(str, 5);

Other than int index types can be specified analogously. Here is an
example showing how to define and use a concept HasStrIndex requiring the
availability of std::string arguments of index operators:
        template <typename Type>
    concept HasStrIndex =
        requires(Type tp)
        {
            tp[std::string{}];
        };
    
    template <HasStrIndex Type>
    auto value(Type &obj, std::string const &key)
    {
        return obj[key];
    }
    
    int main()
    {
        std::map<std::string, double> msd;
        value(msd, "hi");
    }


23.13.2.2: Type requirements

        Sometimes templates must use sub-types of template types. Examples are
templates assuming the existence of iterator (like
vector<int>::iterator) or value_type which is used when calling
std::push_back.

To specify that a sub-type must be available the concept's requires
specification needs no parameters, but can directly refer to the subtype. It's
also possible to combine type requirements with requirements that do
define a non-empty parameter list, and so the requires's parameter list
does not have to be empty. Here is a concept that specifies a plain type
requirement:
            template <typename Type> 
    concept HasValueType =
        requires()
        {
            typename Type::value_type;
        };
    and here's a concept that combines a simple requirement with a type
requirement: 
            template <typename Type>
    concept Iteratable =
        requires(Type tp)
        {
            typename Type::iterator;
            tp.begin();
        };
    
    template <Iteratable Type>
    auto iter(Type const &obj)
    {
        return obj.begin();
    }

Calling iter with a string argument succeeds, calling it with a
queue argument results in two error notes: no Type::iterator and no
tp.begin().

Types specified in type requirements don't necessarily have to refer to
types. They may also specify be the names of nested classes or
enumerations defined by Type. When specifying enumerations they do not
have to be strongly typed.


23.13.2.3: Compound requirements

        When return types of operations must satisfy certain requirements then
compound requirements should be used. Compound requirements define type
constraints on expressions embedded in compound statements.  The C++26 standard
defines several concepts that can be used to specify such requirements (see
also section 23.13.3 below). Here is an example:
            template <typename Type, typename ReturnType> 
    concept Return =
        requires(Type par)
        {
                          // par[..] must return a `ReturnType'
            { par[0] } -> std::same_as<ReturnType>; 
        };

This concept can now be used to specify requirements of template type
parameters. E.g.,
            template <typename Type, typename RetType> 
        requires Return<Type, RetType>
    RetType fun(Type tp)
    {
        return tp[0];
    }

Here arguments passed to fun must satify two requirements:
    
    o  they must provide an index operator accepting integral argument
        values; 
    o  their index operators must return std::string values.

You may have noticed that the std::same_as concept receives only one
template type argument, which (as if by magic) compares it with the type
returned by the par[0] expression. When peeking at the available concepts
in section 23.13.3 you will see that several of those concepts in
fact define two template type parameters. When these concepts are used in
compound requirements then the compiler passes the deduced type of the
expression in the concept's compound statement (so that's the type of
par[0] in the above example) to the concept's first type, and passes the
explicitly specified type to the concept's second type.

Knowing this we can define our own concepts to use in compound
expressions. We may define our own same_as concept as follows, using a
separate class template SameTypes. SameTypes defines a
bool value `value' which is used to decide about the concept's
requirement. The class template SameTypes uses a specialization to handle
the situation where both types are equal. Note that concepts 
     themselves cannot be specialized:
            template <typename Lhs, typename Rhs>
    struct SameTypes            // generic: any two types
    {
        static bool const value = false;
    };
    template <typename Lhs>
    struct SameTypes<Lhs, Lhs>  //specialization: equal types
    {
        static bool const value = true;
    };

    template<typename Compound, typename Specified>
    concept Same = SameTypes::value;

Now the concept Same can be used instead of std::same_as by merely
specifying the required type:
            template <typename Type, typename ReturnType> 
    concept Return =
        requires(Type par)
        {
                          // par[..] must return a `ReturnType'
            { par[0] } -> Same<ReturnType>; 
        };
    Although in this case it isn't important which actual type is used as
argument for which concept type parameter, the compiler specifies the compound
expression's type as template argument for Same's Compound parameter
whereas ReturnType is used as template argument for Same's Specified
parameter.

Multiple type requirements can be specified by providing multiple compound
requirements as in the following example:
            template <typename Type> 
    concept MultiArgs =
        requires(Type lhs, Type rhs)
        {
            { lhs + rhs   } -> std::same_as<Type>;  
            { lhs += rhs  } -> std::same_as<Type &>;
            { lhs.c_str() } -> std::same_as<char const *>;
        };

If it is required that the compound operation doesn't throw exceptions then
noexcept can be written immediately following the compound requirement's
late return type arrow (->). The noexcept specification itself may
then optionally be followed by a type constraint.

Finally, the late return type specifications itself is optional, in which case
the compound requirement acts like a simple requirement: it requires the
existence of the expression that's specified in the compound statement. In
this case: don't forget to add the semicolon following the closing parenthesis
of the compound requirement:
            template <typename Type> 
    concept Increment =
        requires(Type par)
        {
            { ++par };  
            // same as:
            ++par;
        };


23.13.2.4: Nested requirements

        Concepts can be nested. Being able to nest concepts is very useful as it
allows us to hierarchically order concepts and to define concepts in terms of
existing concepts.

In chapter 18 iterators were introduced (section
18.2). Commonly five conceptually different iterator types are
distinguished:
    
    o  Input iterators are incrementable, and they support dereferencing to
        const values;
    o  Output iterators are like Input iterators, but they refer to
        non-const values;
    o  Forward iterators combine Input and Output iterators;
    o  Bidirectional iterators are like Forward iterators, but 
        they also support decrement operators;
    o  RandomAccess iterators are like Bidirectional iterators, but these
        iterators also support addition and subtraction by any stepsize.

------------------------------------------------------------------Insert Figure 31(Concept Hierarchy)about here (file: advancedtemplates/concepts/nested)------------------------------------------------------------------

All iterator types support (in)equality checks and increment operators. Thus,
at the basis of all iterators we find the requirements that iterators must be
comparable and incrementable. Concepts covering those requirements are easily
constructed (see also figure 31):
            template <typename Type>
    concept Comparable =
        requires (Type lhs, Type rhs)
        {
            lhs == rhs;
            lhs != rhs;
        };
    
    template <typename Type>
    concept Incrementable =
        requires (Type type)
        {
            ++type;
            type++;
        };

Note that no type is specified following the lhs == rhs and lhs !=
rhs requirements, as those types are implied by their operators.

Two more concepts are defined: one allowing dereferencing pointers
returning constant references and one returning modifiable references. To
allow the compiler to verify those requirements we also implicitly require the
(commonly encountered) existence of typename Type::value_type:
            template <typename Type>
    concept Dereferenceable =
        requires(Type type)
        {
            { *type } -> std::same_as<typename Type::value_type &>;
        };

    template <typename Type>
    concept ConstDereferenceable =
        requires(Type type)
        {
            { *type } -> std::same_as<typename Type::value_type const &>;
        };

Not much of a hierarchy so far, but that changes now that we're about to
define concepts for iterators. 

An input iterator is an iterator that is comparable, incrementable and
const-dereferenceable.  For each of these requirements concepts were defined
which can be combined using boolean operators when defining the concept
InIterator. Note that template type parameters of concepts must use
the typename keyword. Concepts'
     template parameters cannot be constrained by
specifying them in terms of existing concepts (which is possible when
defining function and class templates). 

Here is the definition of the concept InIterator. The function template
inFun (below the concept InIterator) illustrates how a
constrained template parameter type can be specified in template headers:
            template <typename Type>
    concept InIterator =
        Comparable<Type> and Incrementable<Type> and
        ConstDereferenceable<Type>;
    
    template <InIterator Type>
    void inFun(Type tp)
    {}

The concept for output iterators (and its use, as in the function
template outFun) is defined analogously. This time requiring
dereferenceable types rather than const-dereferenceable types:
            template <typename Type>
    concept OutIterator =
        Comparable<Type> and Incrementable<Type> and
        Dereferenceable<Type>;
    
    template <OutIterator Type>
    void outFun(Type tp)
    {}
    

For forward iterators the concept FwdIterator is defined. A forward
iterator combines the characteristics of input and output iterators, and we
may want to define a forward iterator by requiring the requirements of the
InIterator and OutIterator concepts.

However, there's a slight problem. The following class (struct) defines const
and non-const dereference operators and may be therefore be passed to
functions expecting input or output iterators:
            struct Iterable
    {
        using value_type = int;
    
        Iterable &operator++();
        Iterable operator++(int);
    
        int const &operator*() const;
        int &operator*();
    };
    
    bool operator==(Iterable const &lh, Iterable const &rh);
    bool operator!=(Iterable const &lh, Iterable const &rh);
    int  operator-(Iterable const &lh, Iterable const &rh);

But when a function template requires ConstDerefenceable arguments then
the compiler notices that the overloaded member int &operator*() doesn't
return an int const &. Even though int const &operator*() const is
available compilation fails. This problem can be solved in two ways: noting
that an int & can be converted to an int const & the predefined
concept std::convertible_to instead of std::same_as can be used in
ConstDereferenceable; alternatively its requires clause can specify
Type const &type instead of just Type type. Here is a definition of
ConstDereferenceable that, when defining the concept FwdIter, can be
used in combination with Dereferenceable:
            template <typename Type>
    concept ConstDereferenceable =
        requires(Type const &type)
        {
            { *type } -> std::same_as<typename Type::value_type const &>;
        };

The final two iterator types pose no problems: the concept BiIterator
requires the constraints of the concept FwdIterator as well as decrement
operators, and finally the concept RndIterator requires the constraints of
BiIterator and in addition iterator increments decrements for any step
size as well as the possibility to subtract iterators:
            template <typename Type>
    concept BiIterator =
        FwdIterator<Type> and
        requires(Type type)
        {
            --type;
            type--;
        };
    
    template <typename Type>
    concept RndIterator =
        BiIterator<Type>
        and
        requires(Type lhs, Type rhs)
        {
            lhs += 0;
            lhs -= 0;
            lhs + 0;
            lhs - 0;
            { lhs - rhs } -> std::same_as<int>;
        };


23.13.3: Predefined concepts

    In the previous section we covered defining concept requirements. When
specifying some of the requirements already available concepts were used, like
std::same_as. The C++26 standard provides some 30 predefined concepts which
may be used to specify type requirements, to specify conversion requirements,
and to specify more advanced requirements, sometimes accepting variadic
template parameters. The currently predefined concepts are covered in the
following subsections.


23.13.3.1: Concepts specifying one template type parameter

        The following concepts specify just one template type parameter. Their generic
form is 
        template <typename Type>
    concept Name = 
        ... requirements ...
        ;
    When used in compound requirements only their names have to be
specified. For example (using the concept std::boolean (see below)), to
require that a function fun receiving an argument of some type Type
returns a boolean value, the following concept could be defined:
        template<typename Type>
    concept BoolFun = 
        requires(Type param)
        {
            { fun(param) } -> std::boolean;
        };


o boolean:
requires that its type can be used in boolean expressions

o copy_constructible:
requires that objects of its type supports copy- and
    move-construction;

o copyable:
requires that objects of its type support copy- and move-
    construction and assignment, and that two objects of its type can be
    swapped; 

o default_initializable:
requires that objects of its type support default
    constructors; 

o destructible:
requires that the destructor of its type is defined as
    noexcept(true);

o equality_comparable:
requires that operator== is available to compare
    two objects of its type;

o floating_point:
requires that its type is a floating-point type;

o integral:
requires that its type is an integral  type;

o movable:
requires that its type supports moving and swapping. Note that
    full support for moving requires that its type supports move construction
    and move assignment;

o move_constructible:
requires that its type supports move construction;

o regular:
requires that its type satisfies the requirements of the
    semiregular and equality_comparable concepts;

o semiregular:
requires that its type supports default construction,
    copying, moving, and swapping;

o signed_integral:
requires that its type is a signed integral type;

o swappable:
requires that two objects of the same type can be swapped. The
    generic variant (see below) is named swappable_with;

o unsigned_integral:
requires that its type is an unsigned integral type;

o totally_ordered:
requires that two objects of identical
    types can be ordered using the operators ==, !=, <, <=, >, and
    >=. The requirements for ordering is strict:
    for any two objects one and two either one < two, one == two,
    or one > two is true. The generic variant is named
    totally_ordered_with.


23.13.3.2: Concepts specifying two template type parameters

        The following concepts define two template type parameters. Their generic
form is 
        template <typename LHS, typename RHS>
    concept Name = 
        ... requirements ...
        ;
    When used in compound requirements the compiler deduces the type of the
compound expression, and then uses that type as LHS. In the type requirement
following the compound statement only the RHS type is specified. For example
(using the concept std::same_as (see below)), to require that a function
fun, receiving an argument of some type Type, returns a
std::string the following concept can be defined:
        template<typename Type>
    concept StringFun = 
        requires(Type param)
        {
            { fun(param) } -> std::same_as<std::string>;
        };


o assignable_from:
requires that RHS-typed expressions
    can be assigned to LHS-typed expressions;

o common_reference_with:
requires that both types can be converted to an
    identical (reference) type. The concept is satisfied for two identical
    types, but also for two types where one of them is derived from the
    other. E.g., 
   template <typename LHS, typename RHS>
concept CommonRef = std::common_reference_with<LHS, RHS>;

template <typename T1, typename T2>
    requires CommonRef<T1, T2>
void fun(T1 &&t1, T2 &&t2)
{}

struct B
{};

struct D1: public B
{
};

int main()
{
    fun(4, 'a');
    fun(4.5, 'a');

    D1 d1;
    B b;

    fun(b, d1);     // objects, rvalue refs:
    fun(D1{}, B{}); // all OK
}
 

o common_with:
works like the previous concept;

o convertible_to:
requires that the LHS type can automatically be converted
    to the RHS type:
   template <typename LHS, typename RHS>
concept Convertible =
    requires(LHS lhs)
    {
        { lhs } -> std::convertible_to<RHS>;
    };

template <typename RHS, typename LHS>
    requires Convertible<LHS, RHS>
void fun(LHS lhs)
{}

int main()
{                           // note: LHS is the <...>'s type
    fun<double>(12);        // from int to double
    fun<int>(12.5);         // from double to int
    fun<std::string>("a");  // from NTBS to string
//  fun<std::string>(12);   // constraints not satisfied
}

o derived_from:
requires that the LHS type is derived from the RHS type;

o equality_comparable_with:
requires that operators operator== and
    operator!= are available to compare LHS- and RHS-type variables (in
    any order);

o same_as:
requires that the LHS type is the same as the RHS type. Note
    that this concept acts rather strictly. For example, std::same_as<long
    int, int> does not satisfy the requirement. If such a strict equality
    isn't really required then convertible_to might offer a workable
    alternative;

o swappable_with:
requires that two objects of possibly different types can
    be swapped. The more restrictive variant requiring objects of identical
    types is named swappable;

o totally_ordered_with:
requires that two objects of possibly different
    types can be ordered using the operators ==, !=, <, <=, >, and
    >=. The requirements for ordering is strict:
    for any two objects one and two either one < two, one == two,
    or one > two is true. The more restrictive variant requiring objects
    of identical types is named totally_ordered.


23.13.3.3: Concepts specifying multiple template type parameters
 
        Most predefined concepts expecting more than two parameters are variadic
(cf. section 23.13.4). 


o constructible_from:
requires a LHS type and a variadic template parameter
    representing the types from which LHS can be constructed (which may be
    empty if a default constructor is suppported). Here is an example:
   template <typename LHS, typename ...Args>
concept Constructible = std::constructible_from<LHS, Args...>;

template <typename T1, typename ...Args>
    requires Constructible<T1, Args...>
T1 fun(Args &&...t2)
{
    return T1( std::forward<Args>(t2)... );
}

int main()
{
    std::string s{ fun<std::string>(5, 'a') };
    std::string s2{ fun<std::string>() };   // slightly weird...
}

o equivalence_relation:
this concept defines three template type
    parameters. It is a synonym of the concept std::relation (see below);

o invocable:
requires a LHS type and a variadic template parameter
    representing the types which are forwared as arguments to the function or
    functor specified as invocable's first argument. Here is an example:
   template <typename Function, typename ...Params>
void fun(Function &&fun, Params &&...params)
    requires std::invocable<Function, Params ...>
{
    fun(std::forward<Params>(params)...);
}

void hello(int value, char const *txt, std::string const &str)
{
    std::cout << value << ' ' << txt << ' ' << str << '\n';
}

int main()
{
    fun(hello, 1, "text", "string");    // promotions are still OK
    // fun(hello, 1);                   // WC: not invocable
}

o predicate:
requires a LHS type which is a predicate (functor or function
    returning a bool value), expecting arguments of the variadic template
    parameter RHS. Here is an example:
   template <typename Container, typename Predicate>
concept Find_ifConcept =
    std::predicate<Predicate, typename Container::value_type>
    and
    requires(Container container)
    {
        { container.begin() } -> Iter;
        { container.end() }   -> Iter;
        { container.size() }  -> std::same_as<size_t>;
    };

template <typename Container, typename Predicate>
size_t findIdx_if(Container const &cont, size_t from, Predicate const &pred)
    requires Find_ifConcept<Container, Predicate>
{
    auto iter = std::find_if(cont.begin() + from, cont.end(), pred);
    return iter == cont.end() ? cont.size() : iter - cont.begin();
}

int main()
{
    std::cout << "Index at " <<
        findIdx_if(std::string{ "hello world" }, 0,
            [&](int ch)
            {
                return ch == ' ';
            }
        ) << '\n';

    int target = 4;
    fun(
        [&](int value)
        {
            return value == target;
        },
        4
    );
}


o regular_invocable:
synonym of invocable;

o relation:
this concept defines three template type parameters. The first
    parameter is a predicate whose function call operator expects
    two arguments. The arguments are of the types of the second and/or third
    template type parameters (any combination, any order). 
    For example, the requirements of the std::relation concept are
    satisfied if the predicate's first template argument is std::less, the
    second argument is int and the third argument is double. 

In the following example struct Less is a functor comparing the
    two arguments of its function call operator. Next, the function cmp
    accepts such a struct type as predicate, verifying that the predicate and
    the arguments that are passed to the function satisfy the requirements of
    the std::relation concept. If so the functor's return value is
    returned.

Finally, in main cmp is first called with int and double
    arguments (which succeeds) and then with an int and a NTBS arguments:
    for the second call the constraints are not satisfied as ints and
    NTBSs can't be compared, and consequently compilation fails:
   template <typename LHS, typename RHS>
struct Less
{
    bool operator()(LHS lhs, RHS rhs) const
    {
        return lhs < rhs;
    }
};

template <template<typename LHS, typename RHS> typename Pred,
          typename LHS, typename RHS>
bool cmp(LHS lhs, RHS rhs)
    requires std::relation<Pred<LHS, RHS>, LHS, RHS>
{
    return Pred<LHS, RHS>{}(lhs, rhs);
}

int main()
{
    std::cout << cmp <Less>(5, 4.9) << '\n';
    std::cout << cmp <Less>(5, "hello world") << '\n';
}


o strict_weak_order:
this concept defines three template type
    parameters. The first parameter is the type of a predicate whose function
    call operator expects two arguments. The arguments are of the types of the
    second and/or third template type parameters (any combination, any
    order). The concept is satisfied if the predicate can verify that a strict
    weak order applies to its argument types. A relation is a strict weak
    ordering if 
    
    o  the predicate returns false if an object is compared to itself;
    o  the relation is transitive: if pred(one, two) and pred(two,
        three) are both true, then pred(one, three) is also true;


23.13.4: Applying concepts to template parameter packs

    As we have seen (cf. section 23.13.3.3) concepts may process 
template parameter packs. Such concepts are called 
     variadic concepts.  When defining
concept-protected variadic function or class templates variadic concepts
aren't always required. Consider the following function:
        template <HasSize ...Types>
    void fun(Types &&...obj)
    {
        sum(std::forward<Types &&>(obj)...);
    }

Here we see a variadic template, but it defines all its parameters as
constrained types by simply mentioning the concept HasSize instead of just 
typename. The HasSize concept is very basic: it merely requires that
type.size() exists, returning a size_t:
        template <typename Types>
    concept HasSize =
        requires (Types type)
        {
            { type.size() } -> std::same_as<size_t>;
        };

Once fun has verified that all its argument types satisfy the HasSize
requirements no additional checks are necessary. The fun function
template merely forwards its arguments to sum, a variadic template, that
simply adds the return values of the size() members of its arguments:
        size_t sum()
    {
        return 0;
    }
    
    template <typename First, typename  ...Types>
    size_t sum(First &&first, Types &&...types)
    {
        return first.size() + sum(std::forward<Types>(types)...);
    }

The wrapper function fun isn't really required. The variadic template
function summing the various size() values itself can also be defined so
that its types themselves must satisfy the HasSize concept. Here is the
definition of the variadic function template sum2 requiring precisely
that:
        size_t sum2()
    {
        return 0;
    }
    
    template <HasSize First, HasSize  ...Types>
    size_t sum2(First &&first, Types &&...types)
    {
        return first.size() + sum2(std::forward<Types>(types)...);
    }

And here is a main function calling fun and sum2:
        int main()
    {
        fun(queue<int>{}, vector<int>{}, string{});
        cout << sum2(queue<int>{}, vector<int>{}, string{}) << '\n';
    }

On the other hand, the predefined concept std::constructible_from is a
variadic concept, as it accepts a LHS template parameter and a RHS parameter
pack. This concept is satisfied if the LHS parameter can be constructed
from the types specified in its RHS parameter pack. After including
type_trait defining and using such a concept is not very hard:
        template <typename Class, typename ...Params>
    concept Constructible = std::is_constructible<Class, Params ...>::value;
    
    template <typename Class, typename ...Params>
        requires Constructible<Class, Params ...>
    void fun(Class &&type, Params &&...params)
    {}

The recipe for writing variadic concepts is not very complex:
    
    o  start the concept's definition with a template header specifying a
        parameter pack; 
    o  pass the parameters to a type trait handling the pack;

To use the variadic concept in a function or class template its template
parameters are simply forwarded to the concept (as shown in the above
example). 

When no predefined variadic type trait is available the variadic concept must
use other means to determine whether its constraints are satisfied or not. In
those cases define your own variadic type traits. For illustration let's
assume we are looking for a variadic concept that can be used to verify that
the types of all the arguments that are passed to a variadic function template
are integral types. In this case there is no predefined type trait we can use,
so we have to define it ourselves. We define the concept IntegralOnly as a
variadic concept using our self-defined type trait allIntegralTypes, and
thereupon use it when defining a function requiring that all of its arguments
are integral values:
        template <typename ...Types>
    concept IntegralOnly = allIntegralTypes<Types ...>::value;
    
    template <IntegralOnly ...Types>
    void fun(Types ...types)
    {}

The generic type trait allIntegralTypes merely specifies that it accepts
any number of type parameters and uses specializations to handle specific
cases. One specific case is the case were no types are specified which simply
defines a true static bool const value:
        template <typename ...Types>
    struct allIntegralTypes;
    
    template <>
    struct allIntegralTypes<>
    {
        static bool const value = true;
    };

The type trait's partial specialization does the hard work: it determines
whether the first type is integral and combines that (using and) with the
value made available by the struct allIntegralType receiving the
remaining types:
        template <typename First, typename ...Types>
    struct allIntegralTypes<First, Types ...>
    {
        static bool const value = std::is_integral<First>::value and
                                  allIntegralTypes<Types ...>::value;
    };

The function fun can now be called with any number of arguments. As long
as the arguments are integral types the compilation succeeds and fun can
safely do its job.


23.13.5: Applying concepts to free functions

        
Concepts are most often used in combination with classes. But concepts can
also be used with mere function templates, restricting their argument types.
Once a concept is applied to a function that function automatically becomes a
function template. Usually that's clear as a template header is used, but
a function can also define constrained parameter types without having to
provide its definition with a template header.

To illustrate the various ways concepts can be used when defining function
templates the concept Addable (cf. section 23.13.1) is used in the
following examples.
    
    o  The requirement can be specified immediately following the template
        header: 
        template <typename Type>
        requires Addable<Type>
    auto add(Type const &lhs, Type const &rhs) 
    {
        return lhs + rhs;
    }

o  The requirement can also be specified immediately following the
        function header: 
        template <typename Type>
    auto add(Type const &lhs, Type const &rhs) requires Addable<Type>
    {
        return lhs + rhs;
    }

These variants allow us to specify the requirements in the most flexible
way. E.g., if the parameters should also be integral values, then the
Addable requirement is not enough, by we also need the std::integral
requirement, resulting in a function definition like
        template <typename Type>
        requires Addable<Type> and std::integral<Type>
    auto add(Type const &lhs, Type const &rhs) 
    {
        return lhs + rhs;
    }
    (which can also be used with the trailing requires specification).

If the Addable concept completely covers the arguments' requirements, then
the following abbreviated definitions can be used:
    
    o  The template header uses the concept name instead of typename:
        template <Addable Type>
    auto add(Type const &lhs, Type const &rhs) 
    {
        return lhs + rhs;
    }

o   The concept name itself is used
        as the parameter type. Note that in this form the template header
        isn't used, and the keyword auto follows the concept's name:
        auto informs the compiler that Addable is not the name of a
        plain type but the name of a concept:
        auto add(Addable auto const &lhs, Addable auto const &rhs) 
    {
        return lhs + rhs;
    }


23.13.6: Implementing constrained class members

    When defining members of class templates outside of their class interfaces the
members' template headers must match the class templates' template
headers. This is no different when using concepts.

In the following example the concept Addable is used when defining the
class template Data. The class Data declares a 
member process, implemented below the class interface. Like the class of
which it is a member its header must also specify Addable (cf. section
23.13.1): 
            template <Addable Type>
    class Data
    {
        void process();
    };

    template <Addable Tp>       // The concept must be specified,
    void Data<Tp>::process()    // but the formal type name 
    {                           // doesn't have to be `Type'
        ...
    }

Comparably, if a class template member function can only be used when a
constraint has been satisfied (but no additional constraints apply to other
class members), the class template's header can use typename and the
(additional) constraint can be tailored to members where applicable:
            template <typename Type>    // generic template type parameter
    class Data
    {
        void process() requires Addable<Type>;  // additional requirement
    };

    template <typename X>
    void Data<X>::process() requires Addable<X>
    ...

Types of member templates themselves may also be constrained. Here too the
rule applies that the template headers of member implementations must match
those of their declarations:
            template <typename Type>
    class Data
    {
        template <Addable Tp>       // constraint applied to
        void process(Tp par);       // a member template
    };

    template <typename Type>
    template <Addable Tp>
    void Data<Type>::process(Tp par)
    {
        ...
    }


23.13.7: Constrained partial specializations

    Class templates can be (partially) specialized. Specializations are commonly
used to fine-tune implementations for specific types. Concepts can also be
used when specializations are defined. Consider a struct Handler having
the following generic implementation:
        template <typename Tp>
    struct Handler
    {
        Handler()
        {
            std::cout << "Generic Handler\n";
        }
    };

In addition to possibly type-related specializations (like a struct
Handler<Tp *> ...) a specialization requiring the availability of the
addition operator on Tp can be defined by requiring the concept
Addable:
        template <Addable Tp>       // constrain Tp to addable types
    struct Handler<Tp>
    {
        Handler()
        {
            std::cout << "Handler for types supporting operator+\n";
        }
    };

When used in the following program (assuming all required headers were
included), the first line of the output shows Generic Handler, while
the second line shows Handler for types supporting operator+:
        int main()
    {
        Handler<std::vector<int>>{};    // generic
        Handler<int>{};                 // specialized
    }

The compiler, compiling main's first statement, first looks for
a specialized version of Handler. Although it finds one, that
specialization requires the availability of operator+. As that operator is
not available for std::vector the compiler does not use that
specialization. Had this been the only available implementation, then the
compiler would have reported a constraints not satisfied error. However,
there's still the generic definition which can be used for
std::vector. Therefore the compiler uses the generic definition (which is
at the same time provides a nice illustration of the SFINAE (cf. section
21.15) principle). 

When instantiating the second Handler object the addition operator is
available, and so in that case the compiler selects the specialized version:
where available, specializations are used; if not, then generic template
definitions are used.


23.13.7.1: Function- and class template declarations

        Constrained function- or class-templates can be declared as usual:
instead of the implementations semicolons are used. When declaring a function-
or class-template without constraint specifications then the function or class
template is unconstrained and won't match existing constrained overloaded
versions of such function or class templates. On the other hand,
     concepts cannot be declared. So if a concept
definition must be used in multiple source or header files then the concept
definition normally is provided in a header file of its own which is then
included by files using the concept.

Here are some simple examples illustrating how constrained 
function templates are declared:
        template <typename Type>    // advice: define concepts in
    concept Addable =           // separate headers.
        requires(Type lh, Type rh)
        {
            lh + rh;
        };
    
    template <typename Type>    // declares an unconstrained
    void fun();                 // function template
    
    template <Addable Type>     // declares a constrained overloaded
    void fun();                 // function template
    
    template <typename Type>    // same, requirement follows fun
    void fun() requires Addable<Type>;
    
    template <typename Type>    // same, requirement precedes fun
    requires Addable<Type> void fun();

When declaring class templates their requires-clauses must precede the class
names. Also, when unconstrained class templates are available the constrained
class templates are in fact specializations and must be declared accordingly:
        template <typename Type>    // unconstrained
    struct Data;                // declaration
    
    template <Addable Type>     // constrained declaration
    struct Data<Type>;          // (i.e., a specialization)
    
    template <typename Type>    // same specialization
    requires Addable<Type> struct Data<Type>;

Multiple constraints can also be declared:
        template <typename Type>        // used concepts
    concept C1 = true;
    template <typename Type>
    concept C2 = true;
    
    template <C1 Type>              // multiply constrained
    requires C2<Type> void fun();   //    function template
    
    template <typename Type>        // same, using 'and'
    requires C1<Type> and C2<Type> void fun();
    
    template <typename Type>        // same, trailing 'requires'
    void fun() requires C1<Type> and C2<Type>;
    
    template <typename Type>
    struct Multi;
    
    template <C1 Type>              // multiply constrained
    requires C2<Type> struct Multi<Type>; // class template

Although specializations may define different constraints (e.g., there may
also be a concept Subtractable), a Data specialization for
subtractable types might also be defined:
            template <Subtractable Type>
    struct Data<Type>
    {};

But this is probably not what you want: when defining Data<vector<int>>{},
where template<typename Type> Data is merely declared, the compiler
complains about an incomplete type `struct Data<std::vector<int>>' as it
cannot use the specialization for either Addable or Subtractable. So
it falls back on the generic template, but for that one no implementation is
available, and hence it's incomplete.

Defining a template requiring two types, the first being Addable and the
second template argument being unrestricted, while a specialization is defined
requiring a Subtractable type and an int, then that also
does'n work as intended. In that case, the templates might be:
        template <typename t1, typename t2> requires Addable<t1>
    struct Data
    {};
    
    template <Subtractable Type>
    struct Data<Type, int>
    {};

Here, if the first template argument isn't a subtractable type (like a
vector<int>), and the second argument is an int then the compiler
simply won't use it because the 1st argument isn't a subtractable type. 

Therefore it falls back to the first (generic) template definition. However,
that one doesn't work either, because the first argument also isn't
addable, and you receive complaints about (lh + rh) being ill-formed. 

Now, as you specified int as the template's second argument chances are
that you expected a complaint about (lh - rh) being ill formed, but that
doesn't happen.  In other words: using concepts still requires you to
understand what's going on. Concepts help the compiler to pinpoint reasons
for compilation failures, but in the end it's you who has to understand what
you're doing in order to grasp what the compiler is trying to tell you.


23.13.7.2: Bound free-operators

        Earlier, in section 22.10.2.1 the construction of free operators of
nested classes of template classes was covered. There the nested classes
defined typenames which were thereupon used to select the appropriate free
operators by defining template parameters of the typenames that were defined
by the nested classes.

Concepts provide yet another way to define free operators, bound to the types of
the nested classes' template types. When using concepts the class
templates and their nested classes can be defined in their most basic form, as
in: 
        template <typename Data>
    struct String
    {
        struct iterator
        {
            using value_type        = Data;
    
            std::string::iterator d_iter;
    
                // Note the <>: operator== is a function template
                // specialization as 'iterator' is a class template
            friend bool operator==<>(iterator const &lhs, iterator const &rhs);
        };
        iterator begin()
        {
            return iterator{};
        }
    };


Once the class interface (struct String) has been specified the concept
can be formulated. It simply requires that the arguments of the free operators
are String<Data>::iterator objects:
        template<typename Type>
    concept StringIterator =
        std::same_as<Type,
                     typename String<typename Type::value_type>::iterator>;


The free operator(s) can now be defined as a function template using the
abbreviated StringIterator auto type specification:
        inline bool operator==(StringIterator auto const &lhs,
                           StringIterator auto const &rhs)
    {
        return lhs.d_iter == rhs.d_iter;
    }


By using concepts when defining free operators of nested classes of class
templates we achieve that those operators are bound to the template types of
those class templates, and that the free operators perfectly match those
(nested) classes. Furthermore, when designing the class templates the software
engineer can concentrate on the class's essential characteristics without
having to consider special type-definitions, which are required when using the
sfinae approach covered in section 22.10.2.1.



Chapter 24: Coroutines

Consider the following assignment: design a program that offers a function
next returning the next fibonacci number at subsequent
calls.\ (Fibonacci numbers start with 0 and 1. The next fibonacci number
is the sum of the last two fibonacci numbers. The sequence, therefore, starts
with 0, 1, 1, 2, 3, 5, etc. In this and the following examples fibonacci
sequences are frequently used for illustration, and not because they are
inherently related to coroutines.)

Here is an example of how such a program could be designed: it defines a class
Fibo and in main Fibo::next is called to compute the next fibonacci
number (for brevity the program uses a single source file):
            #include <iostream>
    #include <string>

    using namespace std;

    class Fibo
    {
        size_t d_return = 0;
        size_t d_next = 1;

        public:
            size_t next();
    };

    size_t Fibo::next()
    {
        size_t ret = d_return;      // the next fibonacci number

        d_return = d_next;          // at the next call: return d_next;
        d_next += ret;              // prepare d_next as the sum of the
                                    // original d_return and d_next
        return ret;
    }

    int main(int argc, char **argv)
    {
        Fibo fibo;                  // create a Fibo object

        size_t sum = 0;

                                    // use its 'next' member to obtain
        for (                       // the sequence of fibonacci numbers
            size_t begin = 0, end = argc == 1 ? 10 : stoul(argv[1]);
                begin != end;
                    ++begin
        )
            sum += fibo.next();

        cout << sum << '\n';
    }

Clearly the next member isn't all that complicated. But when it's called
several actions are performed which themselves are unrelated to computing
fibonacci numbers:
    
    o  Calling a function boils down to (ignoring
            the complications when copying and destroying value type argument
            objects and value type local objects):
        
        o  pushing its arguments on the stack (note that next is a
            member function, so it does have an argument: the address of
            the object for which next is called); 
        o  pushing the number of bytes required for those arguments on the
            stack; 
        o  pushing the location of the next instruction to execute after
            completing the next function call on the stack;
        o  pushing the location of the current function's stack frame on the
            stack;
        o  copying the current stack pointer's value to a register (commonly
            called the base pointer), which register is then used to
            access next's arguments and local variables;
        o  continuing execution at next's first instruction;
        o  which first makes room on the stack for all of next's
            local variables.

o  once next's return statement has been executed:
        
        o  the stack pointer is reset to the value stored in the base
            pointer, thus removing local variables from the stack;
        o  popping the base pointer, thus restoring access to the caller's
            stack frame;
        o  popping the value of the caller's next instruction to execute
            from the stack;
        o  reducing the size of the stack by the value currently at the top
            of the stack, holding the size of the function's arguments.

These steps, although they look like a lot, in practice don't take that much
time, because most of them are performed by very fast register operations, and
the computer' architecture is usually highly optimized for these steps. 

Nonetheless, in situations where the functions themselves are short and simple
(like Fibo::next) these steps, requiring stack- and register
manipulations, might be considered unwelcome, raising the question whether
they may be avoided.

C++ coroutines allow us to avoid executing the steps that are required
when calling ordinary functions. The upcoming sections cover coroutines in
depth, but here is, for starters, the coroutine's equivalent of
Fibo::next:
        
             1: #include "main.ih"
     2: 
     3: Fibo fiboCoro()
     4: {
     5:     size_t returnFibo = 0;
     6:     size_t next = 1;
     7: 
     8:     while (true)
     9:     {
    10:         size_t ret = returnFibo;
    11: 
    12:         returnFibo = next;
    13:         next += ret;
    14: 
    15:         co_yield ret;
    16:     }
    17: }

Already now we can observe some characteristics of the fiboCoro coroutine:
    
    o  although coroutines can be defined as class members, this
        particular coroutine is not. It's a free function, which therefore
        doesn't have a (hidden) pointer pointing to the object calling the
        coroutine;
    o  more importantly: it defines local variables (especially those in
        lines 5, 6), which keep their values between successive
        calls. Such local variables are like static variables, but they're
        not. Each coroutine-call receives its own instances of its local
        variables;
    o  Lines 8 thru 16 define a continuous loop where each cycle computes
        the next fibonacci number;
    o  Once a fibonacci number is available the coroutine is suspended
        (at line 15) without loosing the just computed values, passing ret
        to the coroutine's caller (the keyword co_yield is one of three
        new keywords which can be used by coroutines; the other two being
        co_await and co_return).

While fiboCoro lays the foundation for obtaining a sequence of
fibonacci numbers, resuming a coroutine doesn't mean calling a function the
way the above member Fibo::next is called: there are no arguments; there
is no preparation of local variables; and there is no stack
handling. Instead there's merely a direct jump to the instruction just beyond
the coroutine's suspension point. When the coroutine's code encounters the
next suspension point (which occurs in fiboCoro at it's next cycle,
when it again reaches its co_yield statement) then the program's execution
simply jumps back to the instruction following the instruction that resumed
the coroutine.

The main function using the coroutine looks very similar to the main
function using the class Fibo:
             1: #include "main.ih"
     2: 
     3: int main(int argc, char **argv)
     4: {
     5:     Fibo fibo = fiboCoro();
     6: 
     7:     size_t sum = 0;
     8: 
     9:     for (                       // the sequence of fibonacci numbers
    10:         size_t begin = 0, end = argc == 1 ? 10 : stoul(argv[1]);
    11:             begin != end;
    12:                 ++begin
    13:     )
    14:         sum += fibo.next();
    15: 
    16:     cout << sum << '\n';
    17: }

At line 5 fiboCoro is called, returning an (as yet not covered)
fibo thing. This fibo thing is called a 
coroutine handler, and when (in line 14) fibo.next() is called, then
that call resumes fiboCoro, which is then again suspended at its
co_yield statement, returning the next available value through the
handler's next function.

The core feature of coroutines is thus that they may suspend their execution,
while keeping their current state (values of variables, location of the next
instruction to be executed, etc.). Normally, once suspended, the coroutine's
caller resumes its work beyond the instruction that returned the coroutine's
next value, so those two functions (the coroutine and its caller) closely
cooperate to complete the implemented algorithm. Co-routines are therefore
also known as
    cooperating routines, which should not be confused with
concurrent (multi-threaded) routines.

How this whole process works, and what its characteristics are, is covered in
the upcoming sections. 


24.1: Defining a coroutine



When defining coroutines the <coroutine> header file must be included.

A function is a coroutine once it uses the keywords co_yield,
co_await, or co_return. A coroutines cannot use the return
keyword, cannot define variadic parameters, and its return type must be an
existing type, which defines the  coroutine's
handler.

Although coroutines appear to return objects (as suggested by the Fibo
return type of the Fibo fiboCoro() coroutine defined in the
previous section), in fact they do not. Instead coroutines return
so-called `handlers'. Such a handler is fibo, defined and used in the
previous section's main function:
            int main(int argc, char **argv)
    {
        auto fibo = fiboCoro();
        ...
        sum += fibo.next();
        ...
    }

The class Fibo itself defines the characteristics allowing the compiler to
generate code storing the coroutine's arguments, its local variables, the
location of the next instruction to execute when the coroutine returns or is
suspended, and its so-called promise_type object on the heap. This only
happens once, so when the coroutine is activated (as in sum +=
fibo.next()) the steps which are normally taken when a function is called are
avoided, and instead the coroutine is immediately executed, using its already
available local variables and arguments.
     
    Coroutine's handler classes are sometimes called Future, and their
nested state classes must be known as the handler's promise_type. The
names future and promise_type are completely unrelated to the
std::future (cf. section 20.8) and std::promise (cd. section
20.12) types which are used in the context of multi threading. In fact,
coroutines themselves are unrelated to multi threading, but are known as
cooperating routines. Because the coroutines' handler and state classes
are unrelated to the future and promise classes used in the context of
multi threading in this chapter the terms Handler and State are
generally used.

It's one thing to define a coroutine, but when using a coroutine its
handler-class (the Fibo class in the current example) must also be
defined. In addition,  such a handler-class must define a
nested class whose name must be publicly available as the handler's
promise_type. The name promise_type doesn't very well cover its
purpose, and using a more descriptive class name might be preferred. In that
case a simple using declaration in the handler class's public section can be
used, as shown in the following basic design of the Fibo handler-class:
            #include <coroutine>

    class Fibo
    {
        class State     // keeps track of the coroutine's state
        {
            ...
        };

        std::coroutine_handle<State> d_handle;

        public:
            using promise_type = State;

            explicit Fibo(std::coroutine_handle<State> handle);
            ~Fibo();

            size_t next();
    };

The coroutine's handler class has the following characteristics:
    
    o  it has a nested class (here: State) keeping track of the
        couroutine's state;
    o  it commonly defines a private data member of the type
        std::coroutine_handle<State>, e.g.,
        std::coroutine_handle<State> d_handle, whose members are covered
        below;
    o  Unless the handling class's nested class is called promise_type a
        using declaraction must be specified to make the nested class name
        also known as promise_type;
    o  Other members are optional, although usually there is at least a
        member returning a value that's available in the coroutine's state,
        like the member next which was used in the example's main
        function: 
       
    sum += fibo.next();
       
       The next member's current implementation resumes the coroutine.
        This doesn't mean that when next is called for the first time,
        that it's the very first activation of the coroutine: the call auto
        fibo = fiboCoro() comes first, and constructing and returning the
        coroutine's handler (thereby suspending the coroutine at its very
        first statement) is done automatically. At that point the caller
        receives the coroutine's handler object, and constructing and
        returning the handler object isn't visible in the coroutine's code
        (more about that in the next section). Once suspended at co_yield
        the value that's available in the coroutine's State is returned
        and made available to the coroutine's caller:
           size_t Fibo::next()
    {
        d_handle.resume();
        return d_handle.promise().value();
    }

The following members can be called via the Handler's d_handle data
        member:
    
    o void *address()  returning the address of the handler's
        State object;
    o void destroy(),  returning the State object's memory
        to the operating system. It ends the State object's
        existence. Usually the handler class's destructor calls
        d_handle.destroy();
    o bool done(),  returning true when the coroutine has
        returned, and false if it's currently suspended;
    o coroutine_handle from_address(void *address)  returns
        a coroutine_handle corresponding to the address of a handler's
        State object, which address is passed to the function as its
        argument. A nullptr can also be passed to from_address;
    o explicit operator bool(),  returning
        true if d_handle is not a null-pointer. It's commonly used in
        the handler's destructor's if (d_handle) phrase. It returns
        false after assigning 0 (or nullptr) to d_handle. This
        operator and static_cast<bool>(d_handle.address()) act identically
        (note that d_handle.address() is still valid after assigning 0 to
        d_handle, in which case it returns 0);
    o State &promise(),  returning a reference to
        the Handler's State class;
    o void resume()  
        (or void operator()()) resumes the execution of a suspended
        coroutine. Resuming a coroutine is only defined if the coroutine is
        actually suspended.

The Handler's State class keeps track of the coroutine's state. Its basic
elements are covered in the next section.


24.1.1: The coroutine's State class (promise_type)

    The class Handler::State keeps track of the coroutine's state. It must
publicly be known as the class Handler::promise_type, which can be
realized using a public using-declaration associating a more appropriately
named class with `promise_type'.

In the current example the class name State is used, having the following
interface:
                class State
        {
            size_t d_value;

            public:
                Fibo get_return_object();

                std::suspend_always yield_value(size_t value);

                static std::suspend_always initial_suspend();
                static std::suspend_always final_suspend() noexcept;
                static void unhandled_exception();

                static void return_void();

                size_t value() const;
        };

This State class doesn't declare a constructor, so its default
constructor is used. It's also possible to declare and define the default
constructor. Alternatively, by declaring and defining a constructor that has
the same parameters as its coroutine (or parameters that can be initialized by
the coroutine's parameters) that constructor is called when the coroutine
returns its handling object. E.g., if a coroutine's signature is
            Handler coro(int value, string const &str);
    and the State class has a constructor
            Handler::State::State(int value, string const &str);
    then that constructor is called. State's default constructor is called
if such a constructor is not available. In addition to calling State's
constructor a coroutine can also use an awaiter to pass arguments to the
handler's State class. This method is covered in section 24.5.

The data member d_value and member function value() are specifically
used by the class Fibo, and other coroutine state classes might declare
other members. The remaining members are required, but the members returning
 std::suspend_always could also be declared
as members returning  std::suspend_never. 

By returning the (empty) structs suspend_always the coroutine's actions
are suspended until resumed. In practice suspend_always is used, and so
the ..._suspend members can be declared static, using these basic
implementations:
            inline std::suspend_always Fibo::State::initial_suspend() 
    { 
        return {}; 
    }
    inline std::suspend_always Fibo::State::final_suspend() noexcept
    { 
        return {}; 
    } 
    Likewise, the unhandled_exception member can be declared static when
it simply retrows exceptions that may be thrown by the coroutine:
            inline void Fibo::State::unhandled_exception()
    {               // don't forget: #include <future>
        std::rethrow_exception(std::current_exception());
    }
    The (required) member Fibo::State::get_return_object returns an
object of the coroutine's handling class (so: Fibo). The recipe is:
    
    o  pass the current object (which is the coroutine's state
        object) to the member from_promise of an anonymous object of the
        class std::coroutine_handle<State>;
    o  that anonymous object is a handle which is passed as argument to
        the constructor of the coroutine's handler class (i.e., the class
        Fibo);
    o  this Fibo object is then returned by
        State::get_return_object.

Here is Fibo::State::get_return_object's implementation:
            inline Fibo Fibo::State::get_return_object()
    {
        return Fibo{ std::coroutine_handle<State>::from_promise(*this) };
    }
  The member Fibo:State::yield_value can be overloaded for different
argument types. In our Fibo::State there's only one yield_value
member, storing its parameter value in the State::d_value data member. It
also suspends the coroutine's execution as it returns std::suspend_always:
       inline std::suspend_always Fibo::State::yield_value(size_t value) 
    {
        d_value = value;
        return {};
    }
  Now that the coroutine's handling class and its State subclass have been
covered, let's have a closer look at what happens when the main function
from the introductory section is executed. Here's main once again:
             1: int main(int argc, char **argv)
     2: {
     3:     auto fibo = fiboCoro();
     4: 
     5:     size_t sum = 0;
     6: 
     7:     for (                       // the sequence of fibonacci numbers
     8:         size_t begin = 0, end = argc == 1 ? 10 : stoul(argv[1]);
     9:             begin != end;
    10:                 ++begin
    11:     )
    12:         sum += fibo.next();
    13: 
    14:     cout << sum << '\n';
    15: }

When called with argument `2' the following happens:
    
    o  at line 2 the program starts;
    o  at line 3 it looks as though fiboCoroutine is called (see the
        introduction section [FIBOCORO] for its definition),
        but before that: 
       
        o State::get_return_object is called, returning a Fibo
            object. Note here that fiboCoroutine itself nowhere returns a
            Fibo object, even though its definition suggests that it does.
            The get_return_object member does call Fibo's
            constructor though, and this Fibo object is returned at line
            3.
        o  immediately after constructing the Fibo object the
            coroutine's execution is suspended (as 
            Fibo::State::initial_suspend is automatically called).

Next, still at line 3, the returned Fibo object is assigned to
        fibo. The current implementation uses auto fibo = ..., but
        Fibo fibo = ... may also be used. Using auto might be
        attractive if the name of the coroutine handling class's type is
        rather convoluted;
    o main's execution continues at line 12 where fibo.next() is
        called;
    o fibo.next() resumes the coroutine by calling d_handle.resume().
        As this is the very first time that the coroutine is explicitly called
        it now starts its execution at the first statement written by the
        coroutine's author;
    o  the coroutine continues its execution until it reaches a
        co_yield, co_await, or co_return keyword. In this case it's
        co_yield, returning the current fibonacci value (size_t ret);
    o  since a State::yield_value member whose parameter type matches
        ret's type exists, that member is called, receiving ret's
        value as its argument. As yield_value returns
        std::suspend_always the coroutine is again suspended;
    o  once suspended control is returned to Fibo::next's next
        statement, where o Fibo's d_handle uses its member promise() to
        reach the handler's State object, allowing next to return 
        State::value() (which is the most recently computed fibonacci
        value, stored in the State 
        object when State::yield_value was called). 
        Fibo::next() returns that value;
    o  after completing line 12 the for-statement continues at line 7,
        once again reaching line 12.
    o  at subsequent iterations the coroutine continues beyond the statement
        where it was suspended (at its co_yield statement). So it doesn't
        continue at the coroutine's first statement but continues its
        execution by performing the statements of its while (true)
        statement until again reaching its co_yield statement, as
        described above; 
    o  after two iterations the for-statement ends, and just before the
        program ends (at line 15) Fibo's destructor is automatically
        called, returning the memory allocated for the coroutine's data to the
        common pool.


24.1.1.1: What if `suspend_never' is used?

        What if, instead of returning std::suspend_always State's members return
std::suspend_never? In that case the coroutine, once it has started, is is
never suspended. If the program computing fibonacci numbers is then called
with argument 2, the following happens:
    
    o  at line 2 the program starts;
    o  at line 3 it looks as though fiboCoroutine is called,
        but before that: 
       
        o State::get_return_object is called, returning a Fibo object
            at line 3.
        o  immediately after constructing the Fibo object the
            coroutine's execution continues, as this time
            Fibo::State::initial_suspend doesn't suspend. The fibo
            object, however, has been constructed since auto fibo = ...
            isn't an assignment but an initialization of the fibo object.

o  Since the coroutine's execution isn't suspended, it starts to
        iterate, and so it calls co_yield. But although
        Fibo::State::yield_value is called at co_yield calls the
        coroutine isn't suspended, as yield_value now returns
        suspend_never.
    o  So the coroutine continues its loop, assigning the next fibonacci
        number to State::d_value at each subsequent iteration.  Since the
        loop isn't suspended and since there's no other exit from the
        loop, the program continues until it's terminated by some signal (like
        ctrl-C).


24.1.2: Simplifying the state class

    Since the coroutine handler's state classes can often use the shown minimal
implementations for its members, it might be attractive to define those
members in a separate base-class, thus simplifying the state class's interface
and implementation.

Looking at the Fibo::State class, its members initial_suspend,
final_suspend and unhandled_exception are good candidates for such a base
class. By defining the base class as a class template, receiving the coroutine
handler's class name and the handler's state class name as its template type
parameters then it can also provide the handler's get_return_object
member. 

Here is how this base class can be defined. It is used by the coroutine
handler's state classes developed in this chapter:
        #include <cstddef>
    #include <future>
    #include <coroutine>
    
    template <typename Handler, typename State>
    struct PromiseBase
    {
        Handler get_return_object();
    
        static std::suspend_always initial_suspend();
        static std::suspend_always final_suspend() noexcept;
        static void unhandled_exception();
        static void return_void();
    };
    
    
    template <typename Handler, typename State>
    inline void PromiseBase<Handler, State>::return_void()
    {}
    
    template <typename Handler, typename State>
    inline std::suspend_always PromiseBase<Handler, State>::initial_suspend()
    {
        return {};
    }
    
    template <typename Handler, typename State>
    inline std::suspend_always PromiseBase<Handler, State>::final_suspend() noexcept
    {
        return {};
    }
    
    template <typename Handler, typename State>
    inline void PromiseBase<Handler, State>::unhandled_exception()
    {
        std::rethrow_exception(std::current_exception());
    }
    
    template <typename Handler, typename State>
    inline Handler PromiseBase<Handler, State>::get_return_object()
    {
        return Handler{ std::coroutine_handle<State>::from_promise(
                                                    static_cast<State &>(*this) )
                      };
    }


24.2: Embedding coroutines in classes

Coroutines do not have to be free functions (i.e., outside of classes). They
can also very well be defined as class members, in which case they have full
access to the members of their class.

In this section we develop a class Floats that can either write or read 
binary float values to or from files. An object of this
class is used in main, calling its member run to either write or read
a binary file\ (the full program is available in the distribution's
coroutines/demo/readbinary directory):
        int main(int argc, char **argv)
    {
        Floats floats(argc, argv);
        floats.run();
    }

The program is called with two arguments: r for reading, or w for
writing, and the name of the binary file as its second argument.

The member Floats:run uses pointers to members to call either read or
write:
        class Reader;
    class Writer;
    
    class Floats
    {
        enum Action
        {
            READ,
            WRITE,
            ERROR,
        };
    
        Action d_action;
        std::string d_filename;                     // name of the binary file
    
        static void (Floats::*s_action[])() const;  // pointers to read and write
    
        public:
            Floats(int argc, char **argv);
            void run() const;
    
        private:
            void read() const;
            Reader coRead() const;
    
            void write() const;
            static Writer coWrite();
    
    };
    
    inline void Floats::run() const
    {
        (this->*s_action[d_action])();
    }

The member read reads the binary file, using the coroutine
coRead. When coRead is called the usual actions are performed:
implicitly the coroutine Reader's State member get_return_object is
called to obtain the coroutine's handler, and the coroutine is suspended. Next
the handler returned by get_return_object is made available as the
read function's reader object:
        void Floats::read() const
    {
        Reader reader = coRead();           // coRead: the coroutine
                                            // reader: the coroutine's handler
        while (auto opt = reader.next())    // retrieve the next value
            cout << opt.value() << ' ';
    
        cout << '\n';
    }

Once the reader object is available the member read enters a while
loop repeatedly calling reader.next(). At this point the following
happens:
    
    o  the coroutine is resumed, reading the next value from the binary file
        (if it's available);
    o  the coroutine suspends itself again, returning the just obtained
        value (or it indicates that all values have been processed);
    o  once next has returned, the read function again continues,
        either ending its while-statement or showing the retrieved
        float. 

When resumed for the first time (so when reader.next() is called for the
first time) the coRead coroutine opens the file, and then, in a
while-statement, determines the next available value. If that succeeds the
coroutine is again suspended, using co_yield to pass the just read value
on to read, or (if no value could be obtained) the coroutine ends by
calling co_return. Here is the Floats::coRead coroutine:
        Reader Floats::coRead() const
    {
        ifstream in{ d_filename };
    
        while (true)
        {
            float value;
            in.read(reinterpret_cast<char *>(&value), sizeof(float));
    
            if (not in)
                co_return;          // if not: end the coroutine
    
            co_yield value;
        }
    }

Likewise, the member write (re)writes the binary file, using the coroutine
coWrite, following the same procedure as used by read to obtain the
writer coroutine handler:
        void Floats::write() const
    {
        ofstream out{ d_filename };
    
        Writer writer = coWrite();          // coWrite: the coroutine,
                                            // writer: the coroutine's handler
        cout << "Enter values (one per prompt), enter 'q' to quit\n";
    
        while (true)
        {
            cout << "? ";
            auto opt = writer.next();       // retrieve the next value
            if (not opt)                    // stop if no more values
                break;
            out.write(&opt.value()[0], sizeof(float));
        }
    }

The member Floats::coWrite behaves like Floats::coRead, but writes
instead of reads values to the binary file. Here is coWrite, defined as a
normal (non-static) member, as it uses Floats::d_filename:
        Writer Floats::coWrite()
    {
        while (true)
        {
            float value;
            if (not (cin >> value))       // get the next value
                co_return;          // if not: end the coroutine
    
    
            Writer::ValueType ret;  // value to return
    
            ret = Writer::ValueType{ string{
                        reinterpret_cast<char const *>(&value),
                        reinterpret_cast<char const *>(&value + 1) }
                    };
    
            co_yield ret;
        }
    }

The Reader and Writer handler classes are covered next.


24.2.1: The `Reader' coroutine handler

    The essence of the Reader class is that its State subclass receives a
value from the coroutine (coRead) at the coroutine's co_yield
statement. Reader::State receives the value that's passed to co_yield
as argument of its yield_value member, which stores the received float
value in its std::optional<float> d_value data member.

The Reader class itself must define a constructor receiving a handle to
its State class , and should define a destructor. Its next member
simply returns the value that's stored in its State class to next's
caller. Here is Reader's complete header file:
            #include <iostream>
    #include <optional>
    
    #include "../../promisebase/promisebase.h"
    
    struct Reader
    {
        using ValueType = std::optional<float>;
    
        private:
            class State: public PromiseBase<Reader, State>
            {
                ValueType d_value;
    
                public:
                    std::suspend_always yield_value(float value);
                    void return_void();
                    ValueType const &value() const;
            };
    
            std::coroutine_handle<State> d_handle;
    
        public:
            using promise_type = State;
    
            explicit Reader(std::coroutine_handle<State> handle);
            ~Reader();
    
            ValueType const &next();
    };

Reader's and Reader::State's members have (except for Reader::next)
very short implementations which can very well be defined inline:
            inline std::suspend_always Reader::State::yield_value(float value)
    {
        d_value = value;
        return {};
    }
    
    inline void Reader::State::return_void()
    {
        d_value = ValueType{};
    }
    
    inline Reader::ValueType const &Reader::State::value() const
    {
        return d_value;
    }
    
    inline Reader::Reader(std::coroutine_handle<State> handle)
    :
        d_handle(handle)
    {}
    
    inline Reader::~Reader()
    {
        if (d_handle)
            d_handle.destroy();
    }

Reader::next performs two tasks: it resumes the coroutine, and then, once
the coroutine is again suspended (at its co_yield statement), it returns
the value stored in the Reader::State object. 
     It can access its State class object via
d_handle.promise(), returning the value stored in that object:
            Reader::ValueType const &Reader::next()
    {
        d_handle.resume();
        return d_handle.promise().value();
    }


24.2.2: The `Writer' coroutine handler

    The Writer class closely resembles the Reader class. It uses a
different value type, as it must write float values to the output stream
using their binary representations, but other than that there aren't that many
difference with the Reader class. Here is its interface and the
implementations of its yield_value member that differs from that of the
Reader class:
            struct Writer
    {
        using ValueType = std::optional<std::string>;
    
        private:
            class State: public PromiseBase<Writer, State>
            {
                ValueType d_value;
    
                public:
                    std::suspend_always yield_value(ValueType &value);
                    void return_void();
                    ValueType const &value() const;
            };
    
            std::coroutine_handle<State> d_handle;
    
        public:
            using promise_type = State;
    
            explicit Writer(std::coroutine_handle<State> handle);
            ~Writer();
    
            ValueType const &next();
    };
    
    inline std::suspend_always Writer::State::yield_value(ValueType &value)
    {
        d_value = std::move(value);
        return {};
    }


24.3: `Awaitables', `Awaiters' and `co_await'

So far we've encountered co_yield and co_return. What about
co_await? The verb to await is more formal than to wait, but the
two verbs mean the same. The added level of formality of to await is
illustrated by a second description offered by the Merrian Webster
dictionary: to remain in abeyance until, and abeyance's meaning takes
us home again: a state of temporary inactivity or suspension. So when
co_await is used the coroutine enters a state of temporary inactivity,
i.e., it is suspended. In that sense co_yield is no different, as it also
suspends the coroutine, but different from co_yield co_await expects a
so-called awaitable expression. I.e., an expression resulting in an
Awaitable, or that is convertible to an Awaitable
object (see also figure 32).
    ------------------------------------------------------------------Insert Figure 32(co_await)about here (file: coroutines/coawait)------------------------------------------------------------------
    Figure 32 shows that the expression that's passed to
co_await may be an Awaitable object, or if the coroutine handle's
State class has a member await_transform accepting an argument of some
expr's type, the value returned by await_transform is the 
Awaitable (cf. figure 33). These await_transform members
may be overloaded, so in any concrete situation several Awaitable types
could be used.
    ------------------------------------------------------------------Insert Figure 33(awaitable)about here (file: coroutines/awaitable)------------------------------------------------------------------

The Awaiter type that's eventually used is either an object of
co_await's expr's type, or it is the return type of the (implicitly called
when defined) coroutine handler's State::await_transform(expr) member.

Thus, the Awaitable object is a middle-man, that's only used to obtain
an Awaiter object. The Awaiter is the real work-horse in the context
of co_await.

 Awaitable classes may define a member
Awaiter Awaitable::operator co_await(), which may also be provided as a
free function (Awaiter operator co_await(Awaitable &&)). If such a
co_await conversion operator is available then it is used to obtain the
Awaiter object from the Awaitable object. If such a conversion
operator is not available then the Awaitable object is the
Awaiter object.

As an aside: the types Awaitable and Awaiter are used here as formal
class names, and in actual programs the software engineer is free to use other
(maybe more descriptive) names.


24.4: The class `Awaiter'

     Once the Awaiter object is available its member bool
await_ready() is called. If it returns true then the coroutine is not
suspended, but continues beyond its co_await statement (in which case
the Awaitable object and Awaiter::await_ready were apparently able to
avoid suspending the coroutine).

 If await_ready returns false
Awaiter::await_suspend(handle) is called. Its handle argument is the
handle (e.g., d_handle) of the current coroutine's handler object. Note
that at this point the coroutine has already been suspended, and the
coroutine's handle could even be transferred to another thread (in which case
the current thread must of course not be allowed to resume the current
coroutine). The member await_suspend may return void, bool, or some
coroutine's handle (optionally its own handle). As illustrated in figure
34, when returning void or true the coroutine is
suspended and the coroutine's caller continues its execution beyond the
statement that activated the coroutine. If false is returned the coroutine
is not suspended, and resumes beyond the co_await statement. If a
coroutine's handle is returned (not a reference return type, but value return
type) then the coroutine whose handle is returned is resumed (assuming that
another coroutine's handle is returned than the current coroutine is
suspended, and the other coroutine (which was suspended up to now) is resumed;
in the next section this process is used to implement a finite state automaton
using coroutines)

------------------------------------------------------------------Insert Figure 34(awaiter)about here (file: coroutines/awaiter)------------------------------------------------------------------

 If, following await_suspend, the current coroutine is
again resumed, then just before that the Awaiter object calls
Awaiter::await_resume(), and await_resume's return value is returned
by the co_await expression (await_resume) frequently defines a
void return type, as in
            static void Awaiter::await_resume() const
    {}
        

In the next section a finite state automaton is implemented using
coroutines. Their handler classes are also Awaiter types, with
await_ready returning false and await_resume doing nothing. Thus
their definitions can be provided by a class Awaiter acting as base class
of the coroutines' handler classes. Awaiter only needs a simple header
file: 
            struct Awaiter
    {
        static bool await_ready();
        static void await_resume();
    };
    
    inline bool Awaiter::await_ready()
    {
        return false;
    }
    
    inline void Awaiter::await_resume()
    {}


24.5: Accessing State from inside coroutines

    As we've seen, when a coroutine starts it constructs and returns an object of
its handler class. The handler class contains a subclass whose object keeps
track of the coroutine's state. In this chapter that subclass is named
State, and a using declaration is used to make it known as
promise_type which is required by the standard facilities made available
for coroutines.

When coroutines are suspended at co_yield statements, the yielded values
are passed to State class's yield_value members whose parameter
types match the types of the yielded values.

In this section we reverse our point of view, and discuss a method allowing
the coroutine to reach facilities of the State class. We've already
encountered one way to pass information from the coroutine to the State
class: if the State class's constructor defines the same parameters as the
coroutine itself then that constructor is used, receiving the coroutine's
parameters as arguments.

But let's assume that the coroutine performs a continuous loop containing
several, maybe conditional, co_yield statements, and we want to inform the
State class what the current iteration cycle is. In that case a parameter
is less suitable, as tracking the cycle number is in fact a job for one of the
local variables of the coroutine, which would look something like this:
            Handler coroutine()
    {
        size_t cycleNr = 0;
        // make cycleNr available to tt(Handler's State) class
        while (true)
        {
            ++cycleNr;      // now also known to tt(Handler's State)
            ...             // the coroutine at work, using various co_yield
                            // statements
        }
    }
    Awaiters can also be used in these kinds of situations, setting up
communication lines between coroutines and the State classes of their
Handler class objects. As an illustration, the original 
 fibocoro coroutine [FIBOCORO] was slightly modified:
          1: Fibo fiboCoroutine()
     2: {
     3:     size_t returnFibo = 0;
     4:     size_t next = 1;
     5:     size_t cycle = 0;
     6: 
     7:     co_await Awaiter{ cycle };
     8:     cerr << "Loop starts\n";
     9: 
    10:     while (true)
    11:     {
    12:         ++cycle;
    13: 
    14:         size_t ret = returnFibo;
    15: 
    16:         returnFibo = next;
    17:         next += ret;
    18: 
    19:         co_yield ret;
    20:     }
    21: }


    o  At line 5 size_t cycle is defined, keeping track of the
        coroutine's iteration cycle;
    o  Line 7 contains a co_await statemenent, passing co_await an
        object that receives the cycle, which is the variable the
        Handler's State should know about;
    o  At line 12 cycle is incremented, so it contains the current
        iteration cycle.

The Awaiter object, since there's no State::await_transform member,
is an awaitable. Neither does Awaiter have a Type operator
co_await(), so the anonymous Awaiter object is indeed an Awaiter.

Being the Awaiter, it defines three members: await_ready, merely returning
false, as the coroutine's execution must be suspended at the co_await
statement; await_suspend(handle), receiving a handle to the coroutine's
Handler's State object; and await_resume, which doesn't have to do
anything at all:
         class Awaiter
    {
        size_t const &d_cycle;
    
        public:
            Awaiter(size_t const &cycle);
    
            bool await_suspend(Fibo::Handle handle) const;
    
            static bool await_ready();
            static void await_resume();
    };
    
    inline Awaiter::Awaiter(size_t const &cycle)
    :
        d_cycle(cycle)
    {}
    
    inline bool Awaiter::await_ready()
    {
        return false;
    }
    
    inline void Awaiter::await_resume()
    {}

The member await_suspend uses the received handle to access the State
object, passing cycle to State::setCycle:
       bool Awaiter::await_suspend(Fibo::Handle handle) const
    {
        handle.promise().setCycle(d_cycle);
        return false;
    }

In the next section (24.6) we use await_suspend to switch from
one coroutine to another, but that's not required here. So the member returns
false, and thus continues its execution once it has passed cycle to
State::setCycle.  This way coroutines can pass information to the
Handler's State object, which could define a data member size_t const
*d_cycle and a member setCycle, using d_cycle in, e.g.,
yield_value:
       inline void Fibo::State::setCycle(size_t const &cycle)
    {
        d_cycle = &cycle;
    }

    std::suspend_always Fibo::State::yield_value(size_t value)
    {
        std::cerr << "Got " << value << " at cycle " << *d_cycle << '\n';
        d_value = value;
        return {};
    }


24.6: Finite State Automatons via coroutines

Finite state automatons (FSAs) are usually implemented via state x input
matrices. For example, when using Flexc++ to recognize letters, digits or
other characters it defines three input categories, and 4 states (the first
state being the INITIAL state determining the next state based on the
character read from the scanner's input stream, the other three being the
states that handle characters from their specific categories).

FSAs can also be implemented using coroutines. When using coroutines each
coroutine handles a specific input category, and determines the category to
use next, given the current input category. Figure 35 shows a simple
FSA: at Start a digit takes us to Digit, a letter to Letter, at
any other character we remain in Start, and at end of file (EOF) we end
the FSA at state Done. Digit and Letter act analogously.

------------------------------------------------------------------Insert Figure 35(Finite State Automaton)about here (file: coroutines/fsa)------------------------------------------------------------------

This FSA uses four coroutines: coStart, coDigit, coLetter, and coDone,
each returning their own handlers (like a Start handler returned by
coStart, a Digit handler by coDigit, etc.).  Here is 
coStart:
             1: Start coStart()
     2: {
     3:     char ch;
     4:     while (cin.get(ch))
     5:     {
     6:         if (isalpha(ch))
     7:         {
     8:             cout << "at `" << ch << "' from start to letter\n";
     9:             co_await g_letter;
    10:         }
    11:         else if (isdigit(ch))
    12:         {
    13:             cout << "at `" << ch << "' from start to digit\n";
    14:             co_await g_digit;
    15:         }
    16:         else
    17:             cout << "at char #" << static_cast<int>(ch) <<
    18:                     ": remain in start\n";
    19:     }
    20:     co_await g_done;
    21: }

The flow of this coroutine is probably self-explanatory, but note the
co_await statements at lines 9, 14, and 20: at these lines the
co_awaits realize the switch from the current coroutine to another. How
that's realized is soon described.

The coroutines coDigit and coLetter perform similar actions, but
coDone, called at EOF, simply returns, thus ending the coroutine-based
processing of the input stream. Here's coDone, simply using co_return
to end its lifetime:
            Done coDone()
    {
        cout << "at EOF: done\n";
        co_return;
    }

Now take a look at this short input file to be processed by the program:
            a
    1
    a1
    1a

when processing this input the program shows its state changes:
            at `a' from start to letter
    at char #10: from letter to start
    at `1' from start to digit
    at char #10: from digit to start
    at `a' from start to letter
    at `1' from letter to digit
    at char #10: from digit to start
    at `1' from start to digit
    at `a' from digit to letter
    at char #10: from letter to start
    at EOF: done

Since coroutines are normally suspended once activated, the Start handler
privides a member go starting the FSA by resuming its coroutine:
            void  Start::go()
    {
        d_handle.resume();      // maybe protect using 'if (d_handle)'
    }

The main function merely activates the Start coroutine, but the
coroutines might of course also be embedded in something like a class FSA,
and main might offer an option to process a file argument instead of using
redirection. Here's main:
            int main()
    {
        g_start.go();
    }


24.6.1: The `Start' handler class

    As illustrated, there are various ways to obtain an Awaiter from a co_await
expr statement. The shortest route goes like this:
    
    o  According to figure 32 expr is an awaitable if the
        current coroutine's State class does not have an
        await_transform member. In that case expr is the awaitable;
    o  According to figure 33 if expr's type does not
        have an operator co_await member then expr's type is the
        Awaiter. 

So, the nested Start::State class only has to provide the standard
members of the coroutine handler's State class. As those are all provided
by the generic PromiseBase class (section 24.1.2) State needs
no additional members:
                        // nested under the Start handler class:
    struct State: public PromiseBase<Start, State> 
    {};
    Similar considerations apply to the other three handler classes: their
State classes are also derived from PromiseBase<Handler,
State>. However, as the coDone coroutine also uses co_return, the
Done::State state class must have its own a return_void member:
                        // nested under the Done handler class:
    struct State: public PromiseBase<Done, State> 
    {
        void return_void() const;
    };
                // implementation in done.h:
    inline void Done::State::return_void() const
    {}
    As our FSA allows transitions from Digit and Letter back to
Start the Start handler class itself is an Awaiter (as are Digit,
Letter, and Done). Section 24.4 described the requirements and
basic definition of Awaiter classes.

From the point of view of FSAs the most interesting part is how to switch from
one coroutine to another. As illustrated in figure 34 this
requires a member await_suspend which receives the handle of the
coroutine using the co_await statement, and returns some coroutine's
handle. So:
    
    o co_await expr is used, where expr is a coroutine's handler, which
        is also an Awaiter;
    o  the current coroutine passes its own handle to expr's
        await_suspend member;
    o  the handle returned by expr's await_suspend member determines
        which coroutine is resumed, suspending the coroutine using
        co_await;
    o expr returns its own handle, thereby realizing the FSA's state switch
        from the current coroutine to expr's coroutine.

Here is the interface of coStart's handler class as well as the definition
of its await_suspend member. Since the coStart coroutine may be
resumed by several other coroutines it is unknown which coroutine's handle was
passed to Start::await_suspend, and so await_suspend is a member
template, which simply returns Start's handle.
            class Start: public Awaiter
    {
        struct State: public PromiseBase<Start, State>
        {};
    
        std::coroutine_handle<State> d_handle;
    
        public:
            using promise_type = State;
            using Handle = std::coroutine_handle<State>;
    
            explicit Start(Handle handle);
            ~Start();
    
            void go();
    
                    // this and the  members in Awaiter are required for Awaiters
            template <typename HandleType>
            std::coroutine_handle<State> await_suspend(HandleType &handle);
    };
    
    template <typename HandleType>
    inline std::coroutine_handle<Start::State>
                                      Start::await_suspend(HandleType &handle)
    {
        return d_handle;
    }

As the member Start's wait_suspend returns Start's d_handle, the
coroutine containing the co_await g_start statement is suspended, and the
co_start coroutine is resumed (see also figure 34).

The implementations of the Start handler's constructor and destructor are
straightforward: the constructor stores the coroutine's handle in its
d_handle data member, the destructor uses the (language provided) member
destroy to properly end the Start::State's coroutine handle. Here are
their implementations:
            Start::Start(Handle handle)
    :
        d_handle(handle)
    {}

    Start::~Start()
    {
        if (d_handle)
            d_handle.destroy();
    }


24.6.2: Completing the Finite State Automaton

    The Digit and Letter coroutines handler classes are implemented
like Start. Like coStart, which continues its execution when a
non-digit and non-letter character is received, coDigit continues for as
long as digit characters are received, and coLetter continues for as long
as letter characters are received.

As we've seen in section 24.6 the implementation of coDone is a
bit different: it doesn't have to do anything, and the coDone coroutine
simply ends at its co_return statement. Coroutine execution (as well as
the FSA program) then ends following main's g_start.go() call.

The complete implementation of the coroutine-based FSA program is available in
the Annotation's distribution under the directory
yo/coroutines/demo/fsa. 


24.7: Recursive coroutines

Like ordinary functions coroutines can recursively be called. An
     essential characteristic of
coroutines is that when they're used they look no different than ordinary
functions. It's merely in the implementation that coroutines differ from
ordinary functions.

For starters, consider a very simple interactive program that produces a
series of numbers until the user ends the program or enters q:
         1: int main()
     2: {
     3:     Recursive rec = recursiveCoro(true);
     4: 
     5:     while (true)
     6:     {
     7:         cout << rec.next() << "\n"
     8:                 "? ";
     9: 
    10:         string line;
    11:         if (not getline(cin, line) or line == "q")
    12:             break;
    13:     }
    14: }

At line 3 the recursiveCoro coroutine is called, returning its handler
rec. In line 7 its member next is called, returning the next value
produced by recursiveCoro. The function recursiveCoro could
have been any function returning an object of a class that has a next
member. For now ignoring recursion, recursiveCoro could look like this:
         1: namespace
     2: {
     3:     size_t s_value = 0;
     4: }
     5: 
     6: Recursive recursiveCoro(bool recurse)
     7: {
     8:     while (true)
     9:     {
    10:         for (size_t idx = 0; idx != 2; ++idx)
    11:             co_yield ++s_value;
    12: 
    13:         // here recursiveCoro will recursively be called 
    14: 
    15:         for (size_t idx = 0; idx != 2; ++idx)
    16:             co_yield ++s_value;
    17:     }
    18: }

The coroutine merely produces the sequence of non-negative integral
numbers, starting at 0. Its two for-loops (lines 10 and 15) are there merely
for illustrative purposes, and the recursive call will be placed between those
for-loops. The variable s_value is defined outside the coroutine (instead
of using static s_value = 0 inside), as recursively called coroutines must
all access the same s_value variable. There's no magic here: just two
for-statements in a continuously iterating while-statement.

The interface of the returned Recursive object isn't complex either:
         1: class Recursive
     2: {
     3:     class State: public PromiseBase<Recursive, State>
     4:     {
     5:         size_t d_value;
     6: 
     7:         public:
     8:             std::suspend_always yield_value(size_t value);
     9:             size_t value() const;
    10:     };
    11: 
    12:     private:
    13:         using Handle = std::coroutine_handle<State>;
    14:         Handle d_handle;
    15: 
    16:     public:
    17:         using promise_type = State;
    18: 
    19:         explicit Recursive(Handle handle);
    20:         ~Recursive();
    21: 
    22:         size_t next();
    23:         bool done() const;
    24: };

The required members of its State class are available in
PromiseBase (cf. section 24.1.2) and do not have to be
modified. As recursiveCoro co_yields values, the State::yield_value 
member stores those values in its d_value data member:
        std::suspend_always Recursive::State::yield_value(size_t value)
    {
        d_value = value;
        return {};
    }

Its member value is an accessor, returning d_value. When recursion
is used the recursive calls end at some point. When recursiveCoro
functions end State::return_void is called. It doesn't have to do
anything, so PromiseBase's empty implementation perfectly does the job.

The Recursive handling class's own interface starts at line 12. Its
d_handle data member (line 14) is initialized by its constructor (line
19), which is all the constructor has to do. The handler's destructor only has
to call d_handle.destroy() to return the memory used by its State
object.

The remaining members are next and done. These, too, are implemented
straight-forwardly. The member done will shortly be used in the recursive
implementation of recursiveCoro, and it just returns the value returned by
d_handle.done().

When the member next is called the coroutine is in its suspended state
(which is what happens when it's initially called (cf. line 3 in the above
main function) and thereafter when it uses co_yield (lines 11 and 16
in the above implementation of recursiveCoro)). So it resumes the
coroutine, and when the coroutine is again suspended, it returns the (next
available) value stored in the handler's State object:
        size_t Recursive::next()
    {
        d_handle.resume();
        return d_handle.promise().value();
    }


24.7.1: Recursively calling recursiveCoro

    Now we change the non-recursive recursiveCoro coroutine into a recursively
called coroutine. To activate recursion recursiveCoro is modified by
adding some extra statements below line 8:
         1: Recursive recursiveCoro(bool recurse)
     2: {
     3:     while (true)
     4:     {
     5:         for (size_t idx = 0; idx != 2; ++idx)
     6:             co_yield ++s_value;
     7: 
     8:         // here recursiveCoro will recursively be called
     9: 
    10:         if (not recurse)
    11:             break;
    12: 
    13:         Recursive rec = recursiveCoro(false);
    14: 
    15:         while (true)
    16:         {
    17:             size_t value = rec.next();
    18:             if (rec.done())
    19:                 break;
    20: 
    21:             co_yield value;
    22:         }
    23: 
    24:         for (size_t idx = 0; idx != 2; ++idx)
    25:             co_yield ++s_value;
    26:     }
    27: }

Recursion is activated when the parameter recurse is true, which is
passed to recursiveCoro when initially called by main. It is then
recursively called in line 13, now using false as its argument.  Consider
what happens when it's recursively called: the while-loop is entered and the
for-statement at line 5 is executed, `co_yielding' two values. Next, in line
10, the loop ends, terminating the recursion. This implicitly calls
co_return. It's also possible to do that explicitly, using
            if (not recurse)
        co_return;
    Going back to the initial call: once rec (line 13) is available, a
nested while-loop is entered (line 15), receiving the next value obtained by
the recursive call (line 17). That next call resumes the nested coroutine,
which, as just described, returns two values when executing line 5's
for-statement. But then, when it's resumed for the third time, it doesn't
actually co_yield a newly computed value, but calls co_return (because
of lines 10 and 11), thus ending the recursive call. At that point 
the coroutine's State class's member done returns true, which
value is available through ret.done() (line 18). Once that happens the
while loop at line 15 ends, and the non-recursive coroutine continues at line
24. If the recursively called coroutine does compute a value,
rec.done() returns false, and value produced by rec is
`co_yielded' by the non-recursively called coroutine, making it available to
main. So in that latter case the value co_yielded by the recursively
called coroutine is co_yielded by the initially called coroutine, where it is
retrieved by main: there's a sequence of co_yield statements from the
most deeply nested coroutine to the coroutine that's called by main,
at which point the value is finally collected in main.

The next..done implementation used here resembles the way streams are
read: first try to extract information from a stream. If that succeeds, use
the value; if not, do something else:
            while (true)
    {
        cin >> value;
        if (not cin)
            break;
        process(value);
    }
    Functions like getline and overloaded extraction operators may combine
the extraction and the test. That's of course also possible when using
coroutines. Defining next as
            bool Recursive::next(size_t *value) 
    {
        d_handle.resume();
        if (d_handle.done())        // no more values
            return false;
        *value = d_handle.promise().value();
        return true;
    }
  allows us to change the while loop at line 15 into:
            size_t value;
    while (rec.next(&value))
        co_yield value;


24.7.2: Beyond a single recursive call

    In the introductory section the fiboCoro coroutine was presented. In 
this section the fiboCoro coroutine is going to be used by the
recursiveCoro coroutine, using multiple levels of recursion.

To concentrate on the recursion process the fiboCoroutine's handler is
defined in main as a global object, so it can directly be used by every
recursive call of recursiveCoro. Here is the main function, using
bool Recursive::next(size_t *value), and it also defines the global object
g_fibo:
        Fibo g_fibo = fiboCoro();
    
    int main()
    {
        Recursive rec = recursiveCoro(0);
    
        size_t value;
        while (rec.next(&value))
        {
            cout << value << "\n"
                    "? ";
    
            string line;
            if (not getline(cin, line) or line == "q")
                break;
        }
    }

The Recursive class interface is identical to the one developed in the
previous section, except for the Recursive::done member (which is not used
anymore and was therefore removed from the interface), and changing next
member's signature as shown. It's implementation was altered accordingly:
        bool Recursive::next(size_t *value)
    {
        d_handle.resume();
    
        if (d_handle.done())
            return false;
    
        *value = d_handle.promise().value();
        return true;
    }

In fact, the only thing that has to be modified to process deeper recursion
levels is the recursiveCoro coroutine itself. Here is its modified
version:
         1: Recursive recursiveCoro(size_t level)
     2: {
     3:     while (true)
     4:     {
     5:         for (size_t idx = 0; idx != 2; ++idx)
     6:             co_yield g_fibo.next();
     7: 
     8:         if (level < 5)
     9:         {
    10:             Recursive rec = recursiveCoro(level + 1);
    11:             size_t value;
    12:             while (rec.next(&value))
    13:                 co_yield value;
    14:         }
    15: 
    16:         for (size_t idx = 0; idx != 2; ++idx)
    17:             co_yield g_fibo.next();
    18: 
    19:         if (level > 0)
    20:             break;
    21:     }
    22: }

This implementation strongly resembles the 1-level recursive coroutine. Now
multiple levels of recursion are allowed, and the maximum recursion level is
set at 5. The coroutine knows its own recursion level via its size_t level
parameter, and it recurses as long as level is less than 5 (line 8). At
each level two series of two fibonacci values are computed (in the
for-statements at lines 5 and 16). After the second for-statement the
coroutine ends unless it's the coroutine that's called from main, in which
case level is 0. The decision to end (recursively called) coroutines is
made in line 19.

In this implementation the maximum recursion level is set to a fixed
value. It's of course also possible that the coroutine itself decides that
further recursion is pointless. Consider the situation where 
directory entries are examined, and where subdirectories are handled
recursively. The recursive directory visiting coroutine might then have an
implementation like this:
         1: Recursive recursiveCoro(string const &directory)
     2: {
     3:     chdir(directory.c_str());               // change to the directory
     4: 
     5:     while ((entry = nextEntry()))           // visit all its entries
     6:     {
     7:         string const &name = entry.name();
     8:         co_yield name;                      // yield the entry's name
     9: 
    10:         if (entry.type() == DIRECTORY)      // a directory?
    11:         {                                   // get the full path
    12:             string path = pathName(directory, name);
    13:             co_yield path;                  // yield the full path
    14: 
    15:             auto rec = recursiveCoro(path); // visit the entries of 
    16:             string next;                    // the subdir (and of its
    17:             while (rec.next(&next))         // subdirs)
    18:                 co_yield next;              // and yield them
    19:         }
    20:     }
    21: }

In this variant the (not implemented here) function nextEntry (line 5)
produces all directory entries in sequence, and if an entry represents a
directory (line 10), the same process is performed recursively (line 15),
yielding its entries to the current coroutine's caller (line 18).


24.8: Coroutine iterators

The previous examples predominantly used while-statements to obtain the values
returned by coroutines, many generic algorithms (as well as range-based
for-loops) depend on the availability of begin and end members
returning iterators. 

Coroutines (or actually, their handling classes) may also define begin and
end members returning iterators. In practice those iterators are input
iterators (cf. section 18.2), providing access to the values
co_yielded by their coroutines. Section 22.14 specifies their
requirements. For plain types (like size_t which is co_yielded by
Fibo::next) iterators should provide the following members:
    
    o  a prefix increment operator (Iterator &operator++());
    o  a dereference operator (Type &operator*());
    o  comparison operators (bool operator==(Iterator const &other)) 
        (and maybe operator!= returning its complement).

The Iterator class is a value class. However, except for copy- and
move-constructions, Iterator objects can only be constructed by
Recursive's begin and end members. It has a private constructor
and declares Recursive as its friend:
        class Iterator
    {
        friend bool operator==(Iterator const &lhs, Iterator const &rhs);
        friend class Recursive;

        Handle d_handle;

        public:
            Iterator &operator++();
            size_t operator*() const;

        private:
            Iterator(Handle handle);
    };

Iterator's constructor receives Recursive::d_handle, so it can use 
its own d_handle to control recursiveCoro's behavior:
        Recursive::Iterator::Iterator(Handle handle)
    :
        d_handle(handle)
    {}

The member Recursive::begin ensures that Iterator::operator* can
immediately provide the next available value by resuming the coroutine. If
that succeeds it passes d_handle to Iterator's constructor. If
there are no values it returns 0, which is the Iterator that's also
returned by Recursive::end:
        Recursive::Iterator Recursive::begin()
    {
        if (d_handle.promise().level() == 0)
            g_fibo.reset();
    
        d_handle.resume();
        return Iterator{ d_handle.done() ? 0 : d_handle };
    }
    
    Recursive::Iterator Recursive::end()
    {
        return Iterator{ 0 };
    }

The dereference operator simply calls and returns the value returned by
State::value() and the prefix increment operator resumes the coroutine. If
no value was produced it assigns 0 to its d_handle, resulting in true
when compared to the iterator returned by Recursive::end:
        size_t Recursive::Iterator::operator*() const
    {
        return d_handle.promise().value();
    }
    
    Recursive::Iterator &Recursive::Iterator::operator++()
    {
        d_handle.resume();
    
        if (d_handle.done())
            d_handle = 0;
    
        return *this;
    }


24.9: Visiting directories using coroutines

Because coroutines are usually suspended once they have produced some
intermediate but useful result they offer an alternative to stack-based
approaches in which recursion is often used.

This section covers a coroutine that visits all elements of (nested)
directories, listing all their path-names relative to the original starting
directory. First a more traditional approach is covered, using a class having
a member that recursively visits directory elements. Thereafter a
coroutine is described performing the same job. Finally, some statistics about
execution times of both approaches are discussed.


24.9.1: The `Dir' class showing directory entries

    Here a class Dir is developed (recursively) showing all entries
in and below a specified directory. The program defines a class Dir,
used by main:
        int main(int argc, char **argv)
    {
        Dir dir{ argc == 1 ? "." : argv[1] };
    
        while (char const *entryPath = dir.entry())
            cout << entryPath << '\n';
    }

The class Dir, like the coroutine based implementation in the next
section, uses the `dirent' C struct. As we prefer typenames starting
with capitals, Dir specifies a simple using DirEntry = dirent
so C's typename doesn't have to be used.

Dir defines just a few data members: 
    d_dirPtr stores the pointer returned by C's function opendir;
    d_recursive points to a Dir entry that's used to handle a
        sub-directory of the current directory;
    d_entry is the name of the directory
        returned by Dir::entry member, which is refreshed at each call;
    d_path stores the name of the directory visited by a Dir object;
    and d_entryPath is d_entry's path name, starting at the initial
        directory name.
    Here is Dir's class interface:
        class Dir
    {
        using DirEntry = dirent;
    
        DIR *d_dirPtr = 0;
        Dir *d_recursive = 0;
    
        char const *d_entry;        // returned by entry()
        std::string d_path;         // Dir's directory name, ending in '/'
        std::string d_entryPath;
    
        public:
            Dir(char const *dir);   // dir: the name of the directory to visit
            ~Dir();
    
            char const *entry();
    };

Dir's constructor prepares its object for inspection of the entries of the
directory whose name is received as its argument: it calls
opendir for that directory, and prepares its d_path data member:
            Dir::Dir(char const *dir)
    :
        d_dirPtr(opendir(dir)),             // prepare the entries
        d_path( dir )
    {
        if (d_path.back() != '/')           // ensure that dirs end in '/'
            d_path += '/';
    }

Once a Dir object's lifetime ends its destructor simply calls
closedir to return the memory allocated by opendir:
        inline Dir::~Dir()
    {
        closedir(d_dirPtr);
    }

The member entry performs two tasks: first, if a recursion is active then
if a recursive entry is available, that entry is returned. Otherwise, if
no recursive entry is available d_recursive's memory is deleted, and
d_recursive is set to 0:
            // first part
    if (d_recursive)                                    // recursion active
    {
        if (char const *entry = d_recursive->entry())   // next entry
            return entry;                               // return it

        delete d_recursive;                             // delete the object
        d_recursive = 0;                                // end the recursion
    }

The second part is executed if there's no recursion or once all the recursive
entries have been obtained. In that case all entries of the current directory
are retrieved, skipping the two mere-dot entries. If the thus obtained entry
is the name of a directory then d_recursive stores the address of a newly
allocated Dir object (which is then handled at Dir::entry's next call)
and the just received entry name is returned:
            // second part
    while (DirEntry const *dirEntry = readdir(d_dirPtr))// visit all entries
    {
        char const *name = dirEntry->d_name;            // get the name

        if (name == "."s or name == ".."s)              // ignore dot-names
            continue;

        name = (d_entryPath = d_path + name).c_str();   // entry-name
                                                        // (as path)

        if (dirEntry->d_type == DT_DIR)                 // a subdir?
            d_recursive = new Dir{ name };              // handle it next

        return name;                                    // return the entry
    }

The member Dir::entry itself consists of these two parts, returning zero
(no more entries) once the second part's while-loop ends:
        char const *Dir::entry()
    {
        // first part here
    
        // second part here
    
        return 0;
    }

Thus, the class Dir essentially requires one single member function, using
recursion to visit all directory entries that exist in or below the specified
starting directory. All sources of this program are available in the
distribution's yo/coroutines/demo/dir directory.


24.9.2: Visiting directories using coroutines

    In this section a coroutine-based implementation of a program recursively
showing all directory entries is discussed. The program was based on
facilities offered by Lewis Baker's 
    cppcoro library.

The source files of this program are available  in the distribution's
yo/coroutines/demo/corodir directory. It uses the same DirEntry
type definition as used in the previous section, and specifies
    using Pair = std::pair<DirEntry, char const *> to access a
DirEntry and its path name.

The program's main function strongly resembles the main function using
the class Dir, but this time main uses the visitAllEntries
coroutine:
        int main(int argc, char **argv)
    {
        char const *path = argc == 1 ? "." : argv[1];
    
        for (auto [entry, entryPath ]: visitAllEntries(path))
            cout << entryPath << '\n';
    }

The main function uses a range-based for-loop to show the entries produced
by the visitAllEntries coroutine, which are the files and directories that
are (recursively) found in a specified starting directory. 

Three coroutines are used to process directories. The visitAllEntries
coroutine returns a RecursiveGenerator<Pair> as its handler.  Like
main, the visitAllEntries coroutine also uses a range-based for-loop
(line 3) to retrieve directory entries. The coroutine yields Pair objects
(line 5) or the results from nested directories (line 9). Its handler (a
RecursiveGenerator) is a class template, defined in Lewis Baker's
cppcoro library:
             1: RecursiveGenerator<Pair> visitAllEntries(char const *path)
     2: {
     3:     for (auto &entry_pair: dirPathEntries(path))
     4:     {
     5:         co_yield entry_pair;
     6: 
     7:         auto [entry, entry_path] = entry_pair;
     8:         if (entry.d_type == DT_DIR)
     9:             co_yield visitAllEntries(entry_path);
    10:     }
    11: }

Directory entries are made available by a second coroutine,
dirPathEntries. At each entry visitAllEntries is suspended (line 5),
allowing main to show its full path. At lines 7 and 8 the types of the
entries are inspected. If the received entry refers to a sub-directory then
visitAllEntries yields, recursively calling itself, and thus yielding the
sub-directory's entries. Once all entries have been processed the range-based
for-loop ends, and the coroutine ends by automatically calling co_return.

The coroutine yielding directory entries is dirPathEntries, whose handler
is an object of another cppcoro class, Generator<Pair>:
             1: Generator<Pair> dirPathEntries(char const *path)
     2: {
     3:     for (auto const &entry: dirEntries(path))
     4:         co_yield make_pair(entry,
     5:                     (string{path} + '/' + entry.d_name).c_str());
     6: }

The dirPathEntries coroutine performs a cosmetic task: it receives the
path name of a directory, and calls a third coroutine (dirEntries) to
retrieve the successive elements of that directory (line 3). As long as there
are entries the coroutine is suspended, yielding Pair objects consisting
of the values returned by dirEntry and the full path names of those
entries (lines 4 and 5). Eventually, as with visitAllEntries, co_return
ends the coroutine.

The third coroutine is dirEntries, returning a Generator<DirEntry
handler:
             1: Generator<DirEntry> dirEntries(char const *path)
     2: {
     3:     DIR *dirPtr = opendir(path);
     4: 
     5:     while (auto entry = readdir(dirPtr))
     6:     {
     7:         if (accept(*entry))
     8:             co_yield *entry;
     9:     }
    10:     closedir(dirPtr);
    11: }

This coroutine, like the Dir class from the previous section, uses
C's opendir, readdir, and closedir triplet of functions. As
coroutines resume their actions beyond their suspension points these functions
can now all be used in a single coroutine. When dirEntries starts, it
calls opendir (line 3). Then, as long as there are entries (line 5) and
those entries are neither the current nor the parent directory (line 7,
checked by accept, not listed here), the coroutine is suspended, yielding
the obtained entry (line 8). Its while-loop ends once all entries have been
retrieved. At that point closedir is called (line 10), and the coroutine
ends.


24.9.3: Functions vs. coroutines

    Coroutines might be considered severe competitors of ordinary functions. After
all, there's no repeated stack handling required between successive
activations: co_yields simply suspend coroutines, leaving all their data,
ready to be (re)used, in the heap.

In many situations coroutines are considered more attractive at an intuitive
level: already in the introductory section of this chapter they were
characterized as cooperating routines, where the coroutine cooperates with
its caller, producing the caller's requested information as if the coroutine
is part of the caller's code. But although it formally isn't it can
conceptually be considered part of the caller's code, as it doesn't have to be
called again and again during the caller's lifetime: once activated the
coroutine remains available in memory and doesn't have to be called again when
it's resumed. Coroutines are cooperating in that sense: they can in fact
be considered part of the caller's code. In this way they really differ from
independent functions which, when called repeatedly, are called again and
again `from scratch', implying intensive stack operations.

On the other hand, using coroutines is way more complex than using
`traditional' functions. The source files of the discussed Dir class
required some 100 lines of source code, whereas the coroutine based
implementation needed about 700 lines of code. But maybe that's not a fair
comparison. Maybe the cppcoro library's sources shouldn't be considered,
like when we're using --say-- strings, streams and vectors, in which case we
also ignore the sizes of their sources when they're used in our programs. If
we do ignore the sizes of cppcoro's sources, then the coroutine based
implementation in fact requires fewer lines of code than the Dir
class, as the Generator and RecursiveGenerator handler classes are
provided by the cppcoro library.

Eventually, implementing parts of algorithms with coroutines, instead of using
the (functions based) structured programming approach, might simply be a
matter of taste. But maybe, as coroutines allow us to split up algorithms in
separate parts which are not using stack-based activations, the efficiency of
coroutine-based implementations exceeds the efficiency of implementations
using separate supoprt functions. To get some information about the efficiency
of programs using coroutines vs. programs that use separate support functions
the class Dir based program and the coroDir based program was each run
five times, processing a large multi-directory, multi-file structure,
containing over 400.000 entries. The execution times of each of the five runs
are highly comparable. The following table shows the average clock-times, the
average user-times, and the average system-times for the class Dir based
program and the coroDir based program:



    

                ------------------------------------------------------------

                                                 time        

        
                                         real     user        system    

        
                class Dir           82      22          59    

                coroDir                  88      25          62    

        
    

  

    Although the class Dir implementation uses slightly less time than the
coroDir implementatin, the differences are small, and should not be
interpreted as an indication that (maybe different from what was expected)
coroutine based implementations are inherently slower than function based
implementations. Furthermore, coroutines themselves often call ordinary
functions (like readdir called by coroDir's  coroutine
dirEntries), which still require stack-handling.

The conclusion at the end of this chapter is therefore that, yes, coroutines
are available in C++, but they require lots of effort before they can be
used.  Some libraries (like cppcoro) are available, but they're not (yet?)
part of the software that comes standard with your C++ compiler.  However,
the underlying philosophy (being able to use cooperating routines) certainly
is attractive, although it doesn't necessarily result in more efficient
programs than programs which are developed using the traditional structured
programming approach. And so, in the end, whether or not to use coroutines
might simply boil down to a matter of taste.



Chapter 25: Modules



Since the introduction of header files in the C programming language
header files have been the main tool for declaring elements that are not
defined but are used in source files. E.g., to use printf in main
the preprocessor directive #include <stdio.h> had to be specified.

Header files are still extensively used in C++, but gradually some
drawbacks emerged. One minor drawback is that, in C++, header files
frequently not merely contain function and variable declarations but often
also type definitions (like class interfaces and enum definitions). When
designing header files the software engineer therefore commonly distinguishes
headers which are merely used inside a local (class) context (like the
internal-header approach advocated in the C++ Annotations) and header files
which are used externally. Those latter header files need include guards to
prevent them from being processed repeatedly by sources (indirectly) including
them. Another, important, drawback is that a header file is processed again
for every source file including it. Such a task is not a trivial one. E.g., if
a header file includes iostream and string then that forces a compiler
like g++ 14.2.0 to process over 900,000 bytes of code for every source
file including that header.

To speed up compilations precompiled headers  were
introduced. Although the binary format of precompiled headers does indeed
allow the compiler to parse the content of header files much faster than
their standard text format, they are also very large. A precompiled header
merely including iostream and string exceeds 25 MB: a bit more
than its uncompiled text-file equivalent....

Modules were introduced to avoid those complications. Although modules can
still include header files, it's a good design principle to avoid including
header files when designing modules. In general: once a module has been
designed its use doesn't require processing header files anymore, and
consequenly programs that merely use modules are compiled much faster than
corresponding programs that use header files.

There is another, conceptual, feature of modules. The initial high-level
programming languages (like Fortran and Algol) (but also assembly languages)
provided functions (a.k.a. subroutines and procedures) to distinguish
conceptually different task levels.  Functions implement specific tasks. A
program reading data, then processing the data, and finally showing the
results can easily be designed using functions, and is much easier to
understand than replacing the function calls by their actual implementations:
            int main()
    {
        readData();
        processData();
        showResults();
    }
        
    Often such functions use their own support functions, etc, etc, until
trivial decomposition levels are reached where simple flow control and
expression statements are used.

This decomposition methodology works very good. It still does. But at the
global level a problem does exist: there's little integrity
protection. Function parameters may help to maintain the program's data
integrity, but it's difficult to ensure the integrity of global data.

In this respect classes do a much better job. Their private sections
offer means for class-designers to guarantee the integrity of the classes'
data.

Modules allow us to take the next step up the (separation or integrity)
ladder. Conceptually modules offer program sections which are completely
separated from the rest of the program. Modules define what the outside world
can use and reach, whether they are variables, functions, or types (like
classes). Modules resemble factories: visitors can go to showrooms and meeting
halls, but the locations where the actual products are being designed and
constructed are not open to the public.

In this chapter we cover the syntax, design, and implementation of modules as
offered by the C++ programming language. To use modules with the current
edition of the Gnu g++ compiler (version 14.2.0) --std=c++26 (or more
recent) should be specified as well as the  module compilation
flag -fmodules-ts. E.g.,
            g++ -c --std=c++26 -fmodules-ts -Wall modsource.cc

Unfortunately, currently it's not all sunshine and roses. One (current?)
consequence of using modules is that the standard that was specified when
compiling those modules is also required when compiling sources using those
modules. If the specified standards differ (e.g., the modules were compiled
with option --std=c++26, but for a source file using those modules
--std=c++23 was specified) then the compilation fails with an error like
        error: language dialect differs 'C++26/coroutines', expected 'C++23/coroutines'
    
    A similar error is reported when the modules were compiled with
--std=c++23 and the module using source file is compiled specifying
--std=c++26. Therefore, once a new standard becomes available, and a
module defining source files is recompiled using the new standard then module
source files using that module must also be recompiled using that standard.

At https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103524 an overview is
published of the (many) currently reported compiler bugs when modules are
compiled by Gnu's g++ compiler. Many of those bugs refer to internal
compiler errors, sometimes very basic code that correctly compiles when
modules are not used but that won't compile when using modules. Sometimes
reported errors are completely incomprehensible. Another complexity is
introduced by the fact that, e.g., a class which is defined inside a module is
no longer declared in an interface providing header file. Instead, it is
defined in a module defining source file. Consequently, those module-interface
defining source files must be compiled before the member functions of such a
class can be compiled. But it's not the module's interface's object file
that's important at that point. When the module-interface defining source file
is compiled the compiler defines a `module-name'.gcm file in a
sub-directory gcm.cache (cf. section 25.3). Whenever a source
file that uses the module is compiled the compiler must be able to read that
gcm.cache/module-name.gcm file. As a software engineer you cannot simply
compile such module using source files, but you must ensure that the
compiler has access to the proper module-name.gcm files.


25.1: Syntax

Modules introduce new syntax to C++. In this section the new syntax is
summarized. Note that each of the following syntax specifications end in 
semicolons.

Modules may define sub-components (partitions) defining facilities which
may be completely inaccessible outside of the modules
themselves. Partitions are covered in section 25.6.


o export module Name;

    defines a module interface unit. It must be specified on the source
    file's first line. Name is the module's name, its module-compiled
    interface unit becomes available in ./gcm.cache/Name.gcm;

o module Name;

   source files implementing a module (partition) component must start with
    the line module Name;, where Name is the module's name.

o export import Name;

   inside module (partition) interface units or module (partition) source
    files existing modules are made available using export import Name;
    specifications. Using export is optional. In module interface units
    specifying export means that the exported module is also available
    when sources import the module.  E.g.,
     // module interface unit:
    export module Group;
    export import <iostream>;
    ...

    // source file:
    import Group;
    int main()
    {           // cout available via Group
        std::cout << "hello world\n";
    }        


o export module Name:Partition;
 
   defines a module partition interface unit. It must be specified on the
    source file's first line. Name is the module's name, Partition is
    a partition of module Name. Its compiled interface unit becomes
    available in ./gcm.cache/Name-Partition.gcm;

o export import :Partition;

   inside module (partition) interface units or module (partition) source files
    existing module partitions are made available using export import
    :Partition;. In module interface units using export is required,
    otherwise using export is optional.


25.2: Modules

Modules (and partitions, cf. section 25.6) are designed using
source files instead of header files. A module always defines a 
    module interface unit. The module's interface unit is the module's
equivalent of a header file. Consider (as used in the C++ Annotations) defining
the module's interface in a file named `modname.cc' (where name is the
(possibly lowercase) module's name), located in a sub-directory having the
(possibly lowercase) module's name. Using plain (internal) header files should
be avoided when defining and/or using modules.

Here's an example of a module's interface. The module's name is Square and
it declares a function, a class, and a variable:
            export module Square;                           // source: modsquare.cc

    export
    {
        double square(double value);

        class Square
        {
            double d_amount;

            public:
                Square(double amount = 0);          // initialize
                void amount(double value);          // change d_amount
                double amount() const;              // return d_amount

                double lastSquared() const;         // returns g_squared
                double square() const;              // returns sqr(d_amount)
        };
    }

    extern double g_squared;
  
    This module interface merely serves as an illustration. In practice module
interfaces don't contain many different items, but usually just a single
class or, alternatively, a series of utility functions. For now, however, the
slightly overpopulated module Square is used as an initial illustration.

The interface's top-line exports and defines the module name. This must be the
first line of the module's interface unit. Next, the function square and
class Square are declared inside an 
     export compound. It is possible to export
componentsindividually, but using an `export compound' is convenient. Exported
components can be used outside of the module. Non-exported components (like
g_squared) are only available to the module's components, and are
therefore like global components, albeit with a restricted (module) scope.

Also note that the variable g_squared is listed in the interface as
extern double g_squared: it is therefore a declaration, not a
definition. To define variables in a module omit extern (as in 
double g_squared), but to avoid overpopulating modules it's advised to
merely put declarations in module interface units. 

The modsquare.cc file can now be compiled, requiring the -fmodules-ts
compiler option:
           g++ -c --std=c++26 -fmodules-ts -Wall modsquare.cc

Compiling modsquare.cc not only produces the file modsquare.o, but
also a sub-directory gcm.cache, containing the `module compiled interface
file' Square.gcm, which is somewhat comparable to a traditional
pre-compiled header file.  This .gcm file must be available when compiling
source files implementing components of module Square, and it must also be
available to other source files that import (i.e., use) module
Square. Consequently, the directories containing such files must therefore
also have gcm.cache sub-directories containing the Square.gcm
file. This requirement is complex, and in practice a so-called modmapper
(cf. section 25.7) is used to handle the complexity. A simple way to
make module compiled interface files available to all sections of a project is
by defining a top-level directory gcm.cache and using soft-links (like
square/gcm.cache) to the top-level's gcm.cache directory. The
top-level gcm.cache directory and the soft-links in the project's
sub-directories can be prepared before compiling square/modsquare.cc
resulting in the following directory structure:
        
    .
    +-- gcm.cache
    |   +-- Square.gcm
    |
    +-- square
        +-- gcm.cache -> ../gcm.cache
        

All components of the module Square must specify that they're part of
that module. Traditionally this is realized by using (internal) header
files. But projects using modules should no longer need header files. Instead
of the (traditionally used) square.h and square.ih files a module
     frame file, tailored to the modules' requirements
can be used when defining source files belonging to modules. In this example
the requirement for the remaining source files of the Square module is
simple: just specify that the source file belongs to the Square
module. Here's a basic frame file, tailored to the members of the class
Square:
            module Square;

    Square::()
    {
    }
        

The function square isn't part of the class Square, so when it's
defined the Square:: scope is omitted:
            module Square;

    double square(double value)
    {
        return g_squared = value * value;
    }
  

But the members of the class Square can be defined as usual after copying
the frame file to the source filew defining those members. Here is
Square's constructor:
            module Square;

    Square::Square(double amount)
    :
        d_amount(amount)
    {}
  
and the other members are defined analogously:
            module Square;

    void Square::amount(double value)
    {
        d_amount = value;
    }
  
            module Square;

    double Square::amount() const
    {
        return d_amount;
    }
  
            module Square;

    double Square::lastSquared() const
    {
        return g_squared;
    }
  
            module Square;

    double square(double value)
    {
        return g_squared = value * value;
    }
  

As an aside: the members of this class Square are all very simple, and
instead of defining them in separate source files they could also be
defined inline in the module.cc file itself. Since the class
Square's interface is exported its members are too, and inline
implementations of its members don't have to be provided in the export 
compound.

The module can now be used by, e.g., the program's main function. Source
files importing a module must import that module, and if multiple source
files are defined in the top-level directory (like main.cc, usage.cc,
etc.), that directory can define its own frame file. In this initial
example there's only a single main.cc source file, which merely has to
import module Square. But since it also uses std::cout it must import
the
    module compiled system header iostream (cf. section 25.3.1):
        import Square;
    import <iostream>;

    int main(int argc, char **argv)
    {
        std::cout << "the square of " << argc << " is " << square(argc) << '\n';
        Square obj{12};
        std::cout << "the square of 12 is " << obj.square() << "\n"
                    "the last computed square is " << obj.lastSquared() << '\n';

    }


The gcm.cache directories are only required during compilation time.  The
linker doesn't use them and once the source files have been compiled a
program can be constructed as before, by linking the object files, resulting
in a binary program.

This example illustrates several characteristics of modules:
    
    o  module interfaces start with export module followed by the
        module's name. Module names are identifiers, possibly followed by
        `. identifier' sequences (like My.Module). 

o  module names containing a colon (like :name or name1:name2)
        are also encountered. Such names refer to module partitions,
        covered in section 25.6.

o  module interface units must begin with an export module
        'module-name' line. Therefore module interface files cannot define
        multiple modules;

o  components which are accessible to sources outside of the module
        itself must be specified in an export compound or must be
        specified using their own initial export keyword. If no export
        is used the component is only accessible to the components of the
        module;

o  When using modules project header files are not used anymore.  Module
        compiled system headers (like in import <iostream>;) are 
        used, but preprocessor directives like #include <iostream> aren't.

o  Since module interface units are source files include guards aren't
        required anymore.

o  Since header files are not used anymore the compiler doesn't have to
        read thousands of lines of header files anymore, significantly
        reducing compilation times.


25.3: The gcm.cache directory

When using modules the module interface units must be compiled before other
sources using the modules or implementing their components can be
compiled. This also holds true for partition interface units (cf. section
25.6). When a module interface unit is compiled the compiler stores
the module's .gcm file (the equivalent of a compiled header file) in the
gcm.cache sub-directory of the module interface unit. If the module's name
is Square, defined in the modsquare.cc source file then the compiler
produces gcm.cache/Sqaure.gcm as well as the modsquare.o object
file. The .gcm file must be available to any source file importing the
module or implementing one of the module's components.

Usually projects define several sub-directories, and files in those
sub-directories (as well as files in the top-level directory) may import
modules defined in other sub-directories. To ensure that module-compiled
interface units are available to all of the program's source files the
following setup can be adopted:
    
    o  The top-level directory defines a gcm.cache sub-directory
    o  Each of the project's sub-directories defines a soft-link
        gcm.cache -> ../gcm.cache

This setup works fine as long as all the project's modules have different
names, but that by itself is a good design principle.


25.3.1: Module-compiled system header files

    Traditionally system header files were included to declare classes like
std::string and std:ostream. But when using modules including headers
is deprecated and instead their module-compiled equivalents should be imported
using import statements.

 
To avoid recompiling system header files for different projects consider
storing module-compiled headers in /usr/include/c++/14 (here, '14' is
g++'s main version number: update it to your actually installed
version). Using the procedure described in this section project source files
can import module-compiled system headers (assuming that projects have defined
./gcm.cache directories as described in the previous section).
    
    o  In the project's gcm.cache directory define the soft-link 
        usr -> /usr; 
    o  To construct the module-compiled version of a system header file
        (e.g., iostream -> iostream.gcm) then as execute root in
        /usr/include/c++/14 execute the command
     g++ --std=c++26 -fmodules-ts -x c++-system-header iostream
       (if the C++ standard isn't c++26 then adapt it to the 
        C++ standard that should be used)
    o  Then move the file iostream.gcm to the current directory:
     mv gcm.cache/usr/include/c++/14/iostream.gcm .

The same procedure can also be used to module-comile header files of installed
libraries. E.g., the bobcat(7) library stores its headers in
/usr/include/bobcat. To module-compile its arg header file:
    
    o cd /usr/include
    o g++ --std=c++26 -fmodules-ts -x c++-system-header bobcat/arg
    o mv gcm.cache/usr/include/bobcat/arg.gcm bobcat/

Note: currently (using g++ version 14.2.0) compilation sequence issues may
be encountered when module-compiling system headers. For some system headers
module-compilation fails if some other module-compiled system headers are
already available. Those issues can usually be `solved' by first moving all
existing .gcm files to, e.g., a ./tmp sub-directory, followed by the
module-compilation of the intended system header file, and then moving the
tmp/*.gcm files back to the current directory.


25.3.2: Module-compiled headers of locally developed libraries

    In addition to header files used for system libraries locally developed
libaries exist. Such libraries may very well have traditionally been
developed, using standard header files. But once project start using modules
it might be attractive to convert the traditional header files to
module-compiled header files. 

To module-compile a standard header file (e.g., an existing support.h
header file in a support sub-directory) to a module-compiled header file
issue the command
     g++ --std=c++26 -Wall -fmodules-ts -x c++-header support.h
    The compiler then writes the module-compiled header file support.h.gcm
in the ./gcm.cache/,/ sub-directory (note: in gcm.cache's `comma'
sub-directory).  

When a library is constructed there are usually multiple header files, between
which some hierarchy may exist. Consider a library consisting of three
classes: Top, depending on Middle, which in turn depends on Base
Each class is defined in its own sub-directory. E.g., base/base.h:
        #ifndef INCLUDED_BASE_
    #define INCLUDED_BASE_

    #include <fstream>

    class Base
    {
        std::ifstream d_in;
        public:
            Base();
    };

    inline Base::Base()
    :
        d_in("demo.in")
    {}

    #endif

middle/middle.h:
        #ifndef INCLUDED_MIDDLE_
    #define INCLUDED_MIDDLE_

    #include <string>

    #include "../base/base.h"

    class Middle
    {
        std::string d_text;
        Base d_base;
    };

    #endif

and top/top.h:
        #ifndef INCLUDED_TOP_
    #define INCLUDED_TOP_

    #include <vector>
    #include "../middle/middle.h"

    class Top
    {
        std::vector<std::string> d_vect;
        Middle d_middle;
    };

    #endif

To module-compile the library's headers the gcm.cache organization as
described in section 25.2 is used (a gcm.cache directory is
defined at the project's top-level directory, while sub-directories define
gcm.cache soft-links to the top-level gcm.cache directory). Next each
header file is module-compiled whereafter the gcm.cache subdirectory
contains: 
 
    ,/base.h.gcm
    ,/middle.h.gcm
    ,/top.h.gcm
    usr -> /usr
 

This, however, is not the organization which is expected by programs that need
to use the module-compiled headers. To compare: the module-compiled system
header files are, together with the system header files themselves, located in
/usr/include/c++/14 (like iostream and iostream.gcm). When 
module-compiled headers should be used by programs a similar organization is
expected. But that's easily realized by defining a sub-directory (e.g.,
hdrs) containing soft-links to each of the library's header files and
to each of the library's module-compiled header files. If hdrs is a
sub-directory of the libraary's top-level directory then it contains
 
    base.h       -> ../base/base.h
    base.h.gcm   -> ../gcm.cache/,/base.h.gcm
    middle.h     -> ../middle/middle.h
    middle.h.gcm -> ../gcm.cache/,/middle.h.gcm
    top.h        -> ../top/top.h
    top.h.gcm    -> ../gcm.cache/,/top.h.gcm
 

As elaborated in the following two sub-sections there are two ways programs
using the local library can import the library's module-compiled headers: by
using a relative path to the hdrs sub-directory or by using an absolute
path to the hdrs subdirectory.


25.3.2.1: Using a relative location

        Once the local library's headers and module-compiled headers are available via
the local library's hdr sub-directory they can be imported by source
files which do not belong to the library. 

Suppose the library's header files are imported by main.cc, defined in the
directory ~/project:
        import "base.h";
    import "middle.h";
    import "top.h";

    int main()
    {
        Base base;
        Middle middle;
        Top top;
    }


Furthermore, assume the local library's top-level directory is
~/support/locallib (and so ~/support/locallib/hdrs contains the
soft-links to its header files and module-compiled header files).

To make the library's headers available to main.cc two soft-links to
relative destinations can be defined:
    
    o  in ~/project define a soft-link to the library's hdrs
        directory: 
     ln -s ../support/locallib/hdrs .
    o  in the ~/project/gcm.cache/,/ (note: comma!) sub-directory define
        a soft-link to the just defined hdrs soft-link:
     ln -s ../../hdrs .

Once these links are available main.cc can be compiled using
     g++ -c --std=c++26 -fmodules-ts -isystem hdrs main.cc


    o  In practice the program using the library's headers also needs linker
        options: these are used as always, and need no further modifications.
    o  The soft-links must be relative links: if the local library's hdr
        is located in a completely different location then the soft-link
        ~/project/hdrs must point to that directory using a relative path
        specification. 
    o  The names of the two soft-links must be identical, but the name of
        the project's soft-link to the library's hdrs directory does not
        have to be hdrs.


25.3.2.2: Using an absolute location

        When importing a local library's header using two soft-links to relative
destinations it's also possible to define a single soft-link to the absolute
location of the directory containing the library's headers. There's no
compelling reason for using relative or absolute destinations. Maybe if the
relative path specification becomes complex using the absolute destination is
slightly more attractive. 

Using the ~/project/main.cc from the previous section define a soft-link
to the absolute location of the library's hdrs sub-directory in the
project's gcm.cache sub-directory (note: here no comma is used):
     ln -s ~/support/locallib/hdrs gcm.cache
    Next compile main.cc using
 g++ -c -fmodules-ts -isystem ~/support/locallib/hdrs main.cc


o  In gcm.cache the full path to hdrs is not required, but at least
    first sub-directory name must be be specified (since ~/... refers to
    the user's home directory, which commonly begins at /home/... the link
    could be defined as ln -s /home gcm.cache/).
o  The soft-link name itself can freely be chosen and doesn't have to be
    equal to the final destination. E.g., after defining the links
     ln -s ~/support/locallib/hdrs /tmp/locallib
    ln -s /tmp gcm.cache
    the compilation command is:
     g++ -c -fmodules-ts -isystem /tmp/locallib main.cc


25.3.3: Local header files

    Although local header files can also be module-compiled doing so is deprecated 
as using (local) headers is probably a left-over from the traditional program
development era.

But, for the sake of completeness, local header files can be
module-compiled. But note that once a header file is compiled using
namespace declarations specified in header files are lost when the header
file is either included or imported. Compiled local header files are
stored in the gcm.cache/,/ (note: a comma) sub-directory.  They're
automatically used when either their header files are included (e.g.,
#include "header.h") or imported (using import "header.h";).


25.4: Namespaces

Modules and namespaces are mutually independent. Modules (and/or partitions,
cf. section 25.6)
can define (contain) namespaces and/or they can define components in
namespaces. Source files importing such modules must either explicitly specify
the namespace of such components or they can specify a using namespace
declarations avoiding the explicit namespace specification when referring to
components living in those namespaces.

Here is a simple example of a module exporting a variable which is defined in
a namespace:
        export module NS;

    export import <cstddef>;

    export namespace FBB
    {
        extern size_t g_count;
    }

  It's a very basic example, illustrating the essence of defining a namespace
completely covering the module's export compound. Variations are possible: a
namespace could completely surround the export compund, a namespace could be
defined inside the export compound, only some of the module's components could
belong to a namespace, non-exported components could belong to a namespace,
multiple namespaces could be used, etc., etc..

To use the variable g_count, its module is imported and its namespace is
specified as usual. E.g.,
        import NS;

    import <iostream>;

    int main()
    {
        ++FBB::g_count;

        std::cout << FBB::g_count << '\n';
    }


Alternatively the source file could specify some using namespace
declarations so the repeated namespace specifications can be avoided:
        import NS;
    import <iostream>;

    using namespace std;
    using namespace FBB;

    int main()
    {
        ++g_count;
        cout << g_count << '\n';
    }


Some notes:
    
    o  Since module interfaces must start with export module modules
        cannot be defined inside namespaces.  Constructions like
     namespace Area
    {
        export module Nested;
        ... declarations of components of Nested
    }
 
        won't compile. But as shown, defining a namespace section inside a
        module is fine.
    o  Modules cannot be imported inside a namespace area as import
        declarations cannot be nested (i.e., they must be at global scope) So
        constructions like the following also won't compile:
     namespace FBB
    {
        import NS;
    }
 


25.5: Templates

Templates are recipes used by the compiler to construct code tailored to
specific types. Since they're recipes they cannot be compiled to running
code. Traditionally templates are implemented in header files, and when used
the compiler instantiates templates with the correct types using the
templates' recipes.

Since modules are defined in source files there are no header files anymore
when using modules. Modules specify their components in source files,
replacing the traditional header files: by defining templates in a module
interface unit the template's recipe character is kept, and they are
instantiated when needed.

Here's an initial example. A module Adder exports a function template
add adding two values and returning their sum:
        export module Adder;

    export
    {
        template <typename Type>
        Type add(Type const &t1, Type const &t2)
        {
            return t1 + t2;
        }
    }


Source files importing Adder can now add values of types supporting the
+ operator:
        import Adder;

    import <iostream>;
    import <string>;

    using namespace std;

    int main()
    {
        cout << add(1, 2) << '\n' <<
                add(1.1, 2.2) << '\n' <<
                add("hello "s, "world"s) << '\n';
    }


producing the following output:
            3
    3.3
    hello world
        


25.5.1: Class templates

    In the previous section a function template was defined in a module interface
unit. This section covers the definition of a class template. The class
template SortMap offers the facilities of an unordered_map, but also
has two sort members. Both return a vector with pointers to the elements
of the unordered map. One member returns pointers to the elements sorted by
their key values, the other member receives a functor, returning a vector of
pointers to the elements which are sorted according to the functor's
decisions.

The module interface unit defines the SortMap module. It declares the
class template SortMap. The module, like any module, can be imported in
other source files, like a source file main.cc. The main function
inserts its arguments into SortMap and then shows the inserted values:
first ordered by the map's keys, then ordered by the map's values:
        import SortMap;

    import <iostream>;
    import <string>;

    using namespace std;

    int main(int argc, char **argv)
    {
        SortMap<string, size_t> sortMap;

        for (; argc--; )                                    // fill sortMap
            sortMap.emplace(argv[argc], argc);

        for (auto const *ptr: sortMap.sort())               // sort by key
            cout << ptr->first << ' ' << ptr->second << "; ";
        cout.put('\n');

        for (auto const *ptr: sortMap.sort(                 // sort by value
                                    [&](auto const &lhs, auto const &rhs)
                                    {
                                        return lhs->second < rhs->second;
                                    }
                                )
        )
            cout << ptr->first << ' ' << ptr->second << "; ";
        cout.put('\n');
    }

When calling a.out one two three four five six it outputs
     a.out 0; five 5; four 4; one 1; six 6; three 3; two 2
    a.out 0; one 1; two 2; three 3; four 4; five 5; six 6
 

All members of class templates are templates, and as with header files
containing class templates there's only one source file defining the module
interface unit: it contains the class template's interface and also the
implementations of its members. There is, however, an escape route: it's
covered in section 25.6. But in this section the standard way to 
implement class templates is covered. 

When using a single file there's maybe one drawback: the module interface
unit, as it must contain the full implementation of the class template, may
quickly grow to a very large file, which is hard to maintain. To simplify
maintenance it is advised to adopt here the one function, one file design
principle also: the module interface unit itself exports the class interface,
and then uses an #include preprocessor directive to add the class members'
implementations:
        export module SortMap;

    export import <unordered_map>;
    export import <vector>;
    import <algorithm>;

    export
    {
        template <typename Key, typename Value>
        class SortMap: public std::unordered_map<Key, Value>
        {
            using UMap = std::unordered_map<Key, Value>;
            using ValueType = typename UMap::value_type;
            using Vect = std::vector<ValueType const *>;

            private:
                Vect d_sortVect;

            public:
                Vect const &sort();             // sort the keys        // 1.f

                template <typename Functor>     // use a functor        // 2.f
                Vect const &sort(Functor const &functor);
        };
    }

    #include "sortmap.f"

The design of the class SortMap itself is standard. At the top of the
module interface unit the SortMap module name is defined followed by
importing some module compiled system header files. Since algorithm is
only used internally by the member functions it's not exported. But imported
components must be specified at the top of module source files, and so
algorithm cannot be imported by, e.g., sortmap.f. 

The file sortmap.f itself merely includes the files implementing the two
sort members, keeping the module interface unit as clean as possible. Here
is sortmap.f:
        #include "sort1.f"
    #include "sort2.f"

Each of its included files contain the definition of one member
function. A comment in the class's interface indicates which function is
defined in which file. Here's the first sort member (sort1.f):
        template <typename Key, typename Value>
    SortMap<Key, Value>::Vect const &SortMap<Key, Value>::sort()
    {
        d_sortVect.clear();

        for (auto const &el: *this)
            d_sortVect.push_back(&el);

        std::sort(d_sortVect.begin(), d_sortVect.end(), 
            [&](auto const *lhs, auto const *rhs)
            {
                return lhs->first < rhs->first;
            }
        );

        return d_sortVect;
    }

And here's the second sort member, accepting a functor (sort2.f)
        template <typename Key, typename Value>
    template <typename Functor>
    SortMap<Key, Value>::Vect const &SortMap<Key, Value>::sort(
                                            Functor const &functor)
    {
        d_sortVect.clear();

        for (auto const &el: *this)
            d_sortVect.push_back(&el);

        std::sort(d_sortVect.begin(), d_sortVect.end(), functor);

        return d_sortVect;
    }


25.6: Partitions



In the introduction section of this chapter it was stated that modules offer
new, conceptual features: modules offer program sections which are completely
separated from the rest of the program, but modules themselves can define
subsections (called partitions) which can define components
(classes, types, variables, etc.) which are only accessible to the module
and its partitions.

Within the restriction that partitions can be defined so that they're only
accessible to their module and their sibling-partitions the access rights of
classes defined in partitions are identical to those of classes in
general: public members are accessible to their users, protected members are
available to classes derived from those classes, while private members cannot
be accessed from outside of those classes.

------------------------------------------------------------------Insert Figure 36(Partitions of the `Math' module)about here (file: modules/partition)------------------------------------------------------------------

Figure 36 illustrates a simple module (Math) having two
partitions. It shows the relationships between the module and its partitions
in the same way as commonly used for classes: the most basic (least dependent)
partition is on top, the partition depending on the topmost partition is in
the middle, and the module itself, depending on both partitions, is at the
bottom. The Math module defines a class Math, having a member
returning the sum of two size_t values and a member count returning
the number of performed additions. It's a very basic design merely
illustrating the way partitions are designed. 

The object of class Add performs an addition, and the object of class
Utility keeps track of the number of additions that were performed.
Both Add and Utility are defined as classes in their own partitions:
Math:Utility and Math:Add.

 Note that partitions themselves are not classes. When
defining a partition interface unit only a single colon is used instead
of two, as used then defining cass member functions (so Math:Utility and
not Mat::Utility).

The Math interface unit exports class Math, allowing code which does
not belong to the module Math itself to define Math objects after
importing the Math module:
        import Math;
    void fun()
    {
        Math math;
        ...
    }


Since the class Math has data members Utility d_util and Add d_add
these classes must be known inside the Math module interface unit
(modmath.cc), but also by software defining objects of class Math
(comparable to the requirement to, e.g., include <string> when a class
defines a std::string data member). Therefore the Math module
interface unit specifies export import for the Utility and Add
partitions. As we'll shortly see these export specifications do not imply
that software merely using the facilities of the Math module can also
define, e.g., Math:Utility objects: the partitions can still remain
`private' to the Math module. Here is the Math module interface unit,
defined in math/modmath.cc:
        export module Math;

    export import :Utility;
    export import :Add;

    export
    {
        class Math
        {
            Utility d_util;
            Add d_add;

            public:
                Math();
                size_t count() const;
                size_t add(size_t lhs, size_t rhs);
        };
    }

The design of the module Math defining partitions introduces a completely
new level of separation: the components of partitions are like nested classes
but don't result in `class overpopulation': partitions are not defined
inside modules but are completely separately defined from their modules,
while making their facilities only available to their modules and sibling
partitions. And although probably not illustrating good design: by defining
export compounds in partition interface units the elements in those
compounds become directly available to using software essentially turning the
partition into a module.


25.6.1: The Math:Utility partition

    The Math:Utility partition is the most basic of the two Math
partitions. It does not depend on features of either the Math module or
the Math:Add partition. Since it does use the size_t type, it
specifies export import <cstddef>. Here, because of its export
specification, the definitions in cstddef are also available when
Utility is imported, and since the Math module itself exports
:Utility also to software importing Math.

As Utility is a Math partition its partition interface unit starts by
specifing this:
        export module Math:Utility;
    The colon indicates that this is not a plain module but a partition (note
that the first line must start by specifying export). It contains
only non-exported components, but as it's a partition of a module, all the
partition's components are fully available to its module and its sibling
partitions. Its class Utility has two simple members, both very simple,
never changing one-liners. Because of that they're implemented in-line. Here's
the Utility partition interface unit:
        export module Math:Utility;

    export import <cstddef>;

    class Utility
    {
        size_t d_count;

        public:
            Utility();

            void inc();
            size_t count() const;
    };

    inline void Utility::inc()
    {
        ++d_count;
    }

    inline size_t Utility::count() const
    {
        return d_count;
    }

Defining members inline is not required, but is an option. For example, its
constructor could very well also have been defined inline, but (for
illustration purposes) is defined in a separate source file. As Utility's
constructor is a partition source file it starts by specifying its module
(Math), followed by importing the partition component (:Utility) which
is defined in this source file:
        module Math;
    import :Utility;

    Utility::Utility()
    :
        d_count(0)
    {}


25.6.2: The Math:Add partition

    Different from the Math:Utility partition the Math:Add partition
does depend on another partition: it depends on Math:Utility. Like
Utility's modutility.cc it starts its modadd.cc file by exporting its
partition name. But then it imports :Utility, since that partition is used
by Math:Add, making available all components of that partition. 

Since :Utility already imports <sstddef> :Add can also use
size_t. For :Add the :Utility partition wouldn't have to specify
export in front of import <stddef> since all components of imported
partitions are available to both the module and its partitions. But by using
export import <cstddef> users of the Math module automatically receive
<cstddef>'s definitions (among which size_t). Here's the Math::Add
partition interface unit (again: notice that the interface doesn't contain an
export compound):
        export module Math:Add;

    import :Utility;

    class Add
    {
        Utility &d_utility;

        public:
            Add(Utility &utility);
            size_t sum(size_t lhs, size_t rhs);
    };

For this partition no members were defined inline (although that would also
have been possible). Instead all members were defined in separate source
files. Here is class Add's constructor:
        module Math;
    import :Add;

    // export import :Utility;  OK, but superfluous

    Add::Add(Utility &utility)
    :
        d_utility(utility)
    {}

and its sum member:
        module Math;
    import :Add;

    size_t Add::sum(size_t lhs, size_t rhs)
    {
        d_utility.inc();
        return lhs + rhs;
    }


25.6.3: The Math module's members

    Now that the interfaces of the Math module and its partitions have been
defined (cf. figure 36) the Math module's members can be 
defined. The constructor of class Add in the Math:Add partition
needs a reference to a Utility object which it receives from Math
constructor:
        #include "math.i"
    //import Math;

    Math::Math()
    :
        d_add(d_util)
    {}

Since class Utility's count member is public, it can be called by the
Math::count member:
        import Math;

    size_t Math::count() const
    {
        return d_util.count();
    }

and finally, Math's add member calls Add's sum member, defined in the
Math:Add partition to obtain the sum of two positive integral values:
        import Math;

    size_t Math::add(size_t lhs, size_t rhs)
    {
        return d_add.sum(lhs, rhs);
    }


25.6.4: Using the Math module

    Now that the Math module has been developed it can be used by a
program. The exported facilities offered by a module are available after
importing the module. Note that although a module may export import its
partitions only partition components defined in their export compound are
available to source files not belonging to the module itself. As
Math:Utility exports <cstddef> source files importing Math
automatically also can use cstddef's features.

Here's a main function importing Math and using its facilities to add
two positive integral numbers. Header files are not used anymore, and the
implementation of the main function is equal to the implementation when
using header files. However, compared to the latter implementation the
compilation of a program using modules requires significantly less time, and
the design of modules using partitions may offer better isolation of
components that are mere building blocks of the those modules. 

Here is the implementation of the main function using the Math module:
        import Math;
    import <iostream>;

    using namespace std;

    int main()
    {
        Math math;

        cout << "Initial number of additions: " << math.count() << "\n"
                "Enter two pos. values to add: ";

        size_t lhs;
        size_t rhs;
        cin >> lhs >> rhs;
        cout << "their sum is " << math.add(lhs, rhs) << "\n"
                "total number of performed additions: " << math.count() << '\n';
    }


25.6.5: Partitioned class templates

    In section 25.5.1 the a class template SortMap was defined in a
module interface unit. Like class templates in header files class templates in
a module interface files may grow large because the file not only contains the
class template's interface but also the implementations of its
members. Consequently a full recompilation is required when only a single
member function is modified. Recompilations cannot completely be avoided, but
by using module partitions complete recompilation can often be avoided.

Some maintenance complications, however, remain: once a class template's
member is modified the modification is not visible in members that use that
member. But this dependency issue always plays a role when using templates:
once a template has been modified all code using the template needs to be
recompiled.

In the current section class templates are implemented using module
partitions, each containing a member of the class template. The previously
developed SortMap module consists of three components: the class interface
and two sort members functions. Now when using partitions each
component will be defined in its own partition, and the module itself contains
export import statements for each partition which must be accessible by
code using the module. Consequently the modsortmap.cc module interface
unit is remarkably simple:
        export module SortMap;

    export import :Interface;
    export import :Sort1;
    export import :Sort2;

Note that even the class interface is defined in a partition: it cannot be
defined in the module interface unit itself, since partitions can depend
on other partitions, but not on the module itself as the partitions must (as
they are imported by the module) be available before the module itself can be
compiled. Partitions, however, can depend on other partitions, so a
partition implementing a sort function can import a partition providing
the class's interface.

As the :Interface partition merely specifies what's offered by the
class SortMap it doesn't depend on the other partitions. So it merely
declares but does not contain the implementations of the sort members, and
thus it does not itself have to import the alorithm module-compiled
header:
        export module SortMap:Interface;

    export import <unordered_map>;
    export import <vector>;

    export
    {
        template <typename Key, typename Value>
        class SortMap: public std::unordered_map<Key, Value>
        {
            using UMap = std::unordered_map<Key, Value>;
            using ValueType = typename UMap::value_type;
            using Vect = std::vector<ValueType const *>;

            private:
                Vect d_sortVect;

            public:
                Vect const &sort();             // sort the keys        // 1.f

                template <typename Functor>     // use a functor        // 2.f
                Vect const &sort(Functor const &functor);
        };
    }

On the other hand, the implementations of the two sort members do
depend on the interface, since they implement their declarations. They
also depend on the facilities provided by the algorithm module-compiled
header. Both are therefore imported. Here's the implementation of the first
sort member in the :Sort1 partition:
        export module SortMap:Sort1;

    import :Interface;
    import <algorithm>;

    export
    {
        template <typename Key, typename Value>
        SortMap<Key, Value>::Vect const &SortMap<Key, Value>::sort()
        {
            d_sortVect.clear();

            for (auto const &el: *this)
                d_sortVect.push_back(&el);

            std::sort(d_sortVect.begin(), d_sortVect.end(),
                [&](auto const *lhs, auto const *rhs)
                {
                    return lhs->first < rhs->first;
                }
            );

            return d_sortVect;
        }
    }

The second sort member is implemented analogously:
        export module SortMap:Sort2;

    import :Interface;
    import <algorithm>;

    export
    {
        template <typename Key, typename Value>
        template <typename Functor>
        SortMap<Key, Value>::Vect const &SortMap<Key, Value>::sort(
                                                Functor const &functor)
        {
            d_sortVect.clear();

            for (auto const &el: *this)
                d_sortVect.push_back(&el);

            std::sort(d_sortVect.begin(), d_sortVect.end(), functor);

            return d_sortVect;
        }
    }

The main function can remain as provided in section 25.5.1. When
constructing the program the partition dependencies must be taken into
account:
    
    o  first the :Interface partition is compiled;
    o  then, in any order, the :Sort1 and :Sort2 partitions are
        compiled; 
    o  finaly main.cc is compiled and the compiled object files are
        linked resulting in the final program.


25.7: Module mapping

Modules cannot mutually import each other. Consequently there's always a
linear, non-circular module hierarchy, which should be acknowledged when
compiling files defining and using modules. In this section the requirements
for projects using modules are covered, as well as a procedure that can be
adopted when constructing such projects.

With modules traditional headers (.h files) should be avoided.  System
header files (cf. section 25.3.1) still exist, but for those import
statements should be used or made available. For locally developed libraries
module-compiled header files can also be used (cf. section 25.3.2).

It can be difficult to determine which module sections can
immediately be compiled, and which ones depend on already compiled
sections. Moreover, source files belonging to modules but not defining their
interface units may import modules which aren't yet available when
compiling the module's interface unit. For example, module
Basic's interface unit is independent of other modules:
            export module Basic;

    export
    {
        class Basic
        {
            public:
                void hello() const;
         };
    }

But a module Second imports Basic and therefore the compilation of
its module interface file depends on the availability of
gcm.cache/Basic.gcm:
            export module Second;

    export import Basic;
    export import <iosfwd>;

    export
    {
        class Second
        {
            public:
                static std::string hello();
        };
    }

Consequently basic/modbasic.cc must be compiled before compiling
second/modsecond.cc. However, module Second may export a class
Second, and an object of class Second could be defined by a member
of the class Basic, exported by the Basic module:
            module Basic;

    import Second;
    import <iostream>;
    import <string>;

    using namespace std;

    void Basic::hello() const
    {
        cout << Second::hello() << '\n';
    }

In such situations the module interface units must first, and in the right
order, be compiled. Then, once they are available the remaining source files
of the modules can be compiled.

This two-step process (first compile the module interface units in the right
order and then compile the remaining source files) quickly becomes a
challenge, which is commonly handled by using a module mapper. Module
mappers inspect the modules of a project, building their dependency tree, and
maybe compiling the module interface units in the right order. The module
mapper icmodmap(1), a utility program of the icmake(1) project, is
such a module mapper.

By default (but configurable via options) icmodmap(1) expect projects
using modules to be organized as follows:
    
    o  The project's top-level directory contains a file CLASSES, where
        each line (ignoring empty lines and C++ comment) specifies the
        name of a sub-directory implementing one of the project's
        components. Partitions can be defined in their own sub-directories or
        in sub-directories of the module to which they belong. Sub-directories
        do not have to define modules, but all of the project's source files
        may import modules;
    o  The top-level directory itself may also define a module;
    o  If the project defines a program, the project's top-level directory
        contains source files (like main.cc defining the main
        function);
    o  To simplify locating source files defining module or partition
        interface units it is advises to start their names with mod,
        followed by the (optionally lower-case) name of the module or
        partition they define. Following this convention a module Support
        is defined in the file modsupport.cc;
    o  It's considered good practice to export only a single class from a
        module or parition unit, but (like traditional header files) this is
        not a formal requirement.

Icmodmap(1) inspects each of the source files in the project's top-level
directory and in each sub-directory specified in the CLASSES file. If a
source file's first line starts with export module it's a module or
partition interface unit. If so, its module or partition name is stored in a
data-base. Since module and partition interface units may themselves import
modules and/or partitions the current interface depends on those imported
components. Those imported components are declared in its data-base
registering that the current interface unit depends on those imported
components.

Likewise, if another source file imports modules or implements a component of
a module or partition then those modules (and/or paritions) are also declared
in its data-base.

Once all source files were inspected icmodmap(1) determines the module
dependencies: its data-base must indicate that each imported module or
partition also has a module/partition interface unit, and that there are no
circular dependencies among the module/partition interface units. If these
requirement are satified then icmodmap by default calls the compiler to
compile the interface units in their proper order. By default the object files
are stored in the standard icmake(1) fashion: each compiled interface unit
is stored in the project's tmp/o sub-directory and their names begin with
a number which is equal to the line number in the CLASSES file specifying
the inspected sub-directory (like 1modfirst.o, 2modsecond.o,) using prefix
0 for module(s) defined in the project's top-level directory.

When the inspection and compilation successfully completes then the interface
units' object files are stoed in tmp/o, and the project's top-level
directory contains a sub-directory gcm.cache containing the .gcm files
of all modules and partitions. In addition each inspected sub-directory has
received a soft-link gcm.cache to its parent's gcm.cache
sub-directory, so each of the project's source files can import each module.

Now that the module and parition interface files are available the remaining
source files can be compiled as usual. They can import and use the components
of the the defined modules (and where applicable the defined paritions).


25.7.1: Modifying interface units

    When module/partition interface units are modified components are
added, removed, or altered. E.g., an interface unit may declare a class which
is modified: the ordering of its data members may change, new data members may
be added, existing data members may be removed.

When a module interface unit is modified and it's recompiled then the modified
definition replaces the old one. Consider this interface unit:
     export module Demo;
    export
    {
        class Demo
        {
            int d_value = 100;
            public:
                int value() const;
        };
    }
 
    The member function Demo::value (returning d_value) is implemented
in its own source file, and is called by main:
        import Demo;
    import <iostream>;

    using namespace std;

    int main()
    {
        Demo demo;
        cout << demo.value() << '\n';
    }

When the program is run it outputs 100. Now the module interface unit is
modified: d_first is added to the class as its first data member:
        export module Demo;

    export
    {
        class Demo
        {
            int d_first = 13;
            int d_value = 100;
            public:
                int value() const;
        };
    }

Next moddemo.cc and main.cc, are recompiled and the three object files
are linked constructing the binary program. The new binary outputs 13: 
value's object file returns the first bytes of the Demo object as an
int, but since the class Demo was modified those bytes no longer
contain the value 100.

Such complications are familiar: when using a traditional header declaring a
class and the class changes the organization of its data members then in
practice all source files using the class must be recompiled (and recursively:
if another class is derived from, or contains a data member of the modified
class then all source files using the other class must also be
recompiled). When using modules such recompilations are also required. The
icmodmap(1) support program has an option to recompile module (partition)
interface units depending on modified interface units and either to modify the
last write times of source files using those modules or to write their names
to a file so a build utility can recompile those module using source files.


25.7.2: External modules

    Modules can import module-compiled system header files (cf. section
25.3.1) and/or module-compiled header files of locally developed libraties
(cf. section 25.3.2).

What if a library's design merely uses modules? In that case the library must
make available its module-compiled interface files (i.e., .gcm files) to
its users. Copying the library's .gcm files to the gcm.cache
sub-directory of a using project is not necessary (and probably a bad design
anyway, since modifications of the library's .gcm files will not be
noted), but soft-links should be used. However, often modules import other
modules or partitions, and consequently such library-modules in turn may
depend on other modules (or partitions). When a project must import a
library's module not only the module's .gcm file must be available but
also (recursively) the module/partition .gcm files that module depends
on. 

As an illustration consider the situation where a project defines three
modules: Module1, Module2, and Module3. Module1 imports External,
which is a module offered by some library; Module2 imports
Module1, and Module3 imports Module2 (cf. figure 37). 

------------------------------------------------------------------Insert Figure 37(Three modules and an external module)about here (file: modules/threemods)------------------------------------------------------------------

The External module doesn't belong to the current project, and so the
External.gcm file lives elsewhere. When constructing the project its
gcm.cache directory must make External.gcm available. This is realized
by defining a soft-link to the actual location of External.gcm. But in
addition soft-links to the modules/partitions imported by External.gcm
must be made available in the project's gcm.cache sub-directory. 

The icmodmap(1) support program can be used to determine and to satisfy
the requirements of externally defined modules.
     
    To determine the modules' dependencies icmodmap is called in the
project's top-level directory specifying its --dependencies (or -d)
option. It shows the dependencies amount the modules and reports an error as
the module External isn't found:
     [Error 1] 1 UNKNOWN module
    Dependencies:
       LOCAL module Module1
           imports UNKNOWN module External
       UNKNOWN module External
       LOCAL module Module2
           imports LOCAL module Module1
       LOCAL module Module3
           imports LOCAL module Module2
 
    And indeed, there's an error: the External module wasn't developed in
the context of the current project. It's defined in an external library,
offering its module/partition compiled interface units to its users. By
informing icmodmap where the external .gcm files are located the error
is solved. The relative or absolute path to the directory containing the
library's .gcm files is either specified as a command-line option or it's
specified in a file. Either of these can be passed to icmodmap using its
--extern (or -e) option. E.g., if the library's .gcm files are in
/tmp/library/gcm.cache then by calling
 
    icmodmap -d -e /tmp/library/gcm.cache
    the error disappears and icmodmap reports:
     Dependencies:
       LOCAL module Module1
           imports EXTERN module External
       EXTERN module External
       LOCAL module Module2
           imports LOCAL module Module1
       LOCAL module Module3
           imports LOCAL module Module2
 
    specifying --extern also defines the soft-links to the external
modules: the project's gcm.cache sub-directory now contains the soft-link
     External.gcm -> /tmp/library/gcm.cache/External.gcm
    Now that the requirements of the project's module interface files are all
satisfied they can be compiled (they are compiled by icmodmap if the
-d option isn't specified), followed by the compilation of the project's 
remaining source files. Finally, all object files can be linked to the used
object files of the external library in the usual way (specifying, e.g., the
linker's -L and -l options).


25.8: Design

When designing projects using modules the same approach as used with
traditional projects (using headers) can be followed. Some additional
conventions are suggested that specifically apply to modules. Filenames are just
filenames. In the C++ Annotations following conventions are adopted:
    
    o  the names of files containing module- or partition-interface units
       begin with mod, followed by the lower-case module- or
       partition-names, using .cc extensions (e.g. modgroup.cc);
    o  module's source files are in sub-directories having
        their lower-case module names (e.g., ./group) 
    o  partitions are defined in sub-directories of their modules'
       directories. E.g., if the module Group is defined in ./group,
       than its partiton Group:Support is defined in ./group/support;
    o  the project's main directory has a sub-directory gcm.cache,
        initially containing a soft-link usr/ to the system's /usr
        sub-directory allowing source files to import module-compiled
        system header files (e.g., import <iostream>;);
    o  module or partition sub-directories have a soft-link gcm.cache
        to ../gcm.cache, so all compiled module- and partition-interface
        units are available in the (top-level) gcm.cache sub-directory.

The compiler writes compiled interface units in their gcm.cache
sub-directories. The .gcm filenames of modules are equal to the names of the
modules (e.g., for Group it is Group.gcm). The filename of
a module-compiled partition starts with its module name, followed by a dash
(-), followed by the partitions's name (e.g., for Group:Support it is 
Group-Support.gcm).

To avoid overfilling interface units they should (like class headers) not
define components but merely declare them.  Their definitions are
provided in separate source files, defined in their interface unit's
sub-directory, using the familiar the one component, one source file
design principle.


25.8.1: 'header' and 'frame' files

    Modules and partitions usually their components in separate source files. The
initial lines of those source files are identical: they specify the module or
the partition to which they belong. Since these source files do not include
(internal) header files, they may also declare namespaces. E.g., the member
implementations of the Math:Add partition (cf. section 25.6)
all start with
     module Math;
    import :Add;
 
    Since the Math:Add partition interface unit declares a class Add
most or all of its source files implement a member of class Add. In these
cases it's convenient to have a predefined frame file containing the
essential elements of its source files. E.g., for Math::Add sources this
could be
     module Math;
    import :Add;

    using namespace std;

    void Add::()
    {
    }
 
    When implementing a member copy the frame to the intended source file,
specify the member's name, define its body and maybe change its return type.



Chapter 26: Concrete Examples

In this chapter concrete examples of C++ programs, classes and templates
are presented.  Topics covered by the C++ Annotations such as virtual
functions, static members, etc. are illustrated in this chapter. The
examples roughly follow the organization of earlier chapters.

As an additional topic, not just providing examples of C++ the subjects of
scanner and parser generators are covered. We show how these tools may
be used in C++ programs. These additional examples assume a certain
familiarity with the concepts underlying these tools, like grammars,
parse-trees and
        
        
    parse-tree decoration. Once the input for a program exceeds a certain
level of complexity, it's attractive to use scanner- and parser-generators to
create the code doing the actual input processing. One of the
examples in this chapter describes the usage of these
tools in a C++ environment.


26.1: Using file descriptors with `streambuf' classes



26.1.1: Classes for output operations

        Reading and writing from and to file descriptors
are not part of the C++ standard. But on most operating systems file
descriptors are available and can be considered a device. It seems
natural to use the class std::streambuf as the starting point
for constructing classes interfacing such file descriptor devices.

Below we'll construct classes that can be used to write to a device given
its file descriptor. The devices may be files, but they could also be pipes
or sockets. Section 26.1.2 covers reading from such devices; section
26.2.3 reconsiders redirection, discussed earlier in section
6.6.2.

Using the streambuf class as a base class it is relatively easy to
design classes for output operations. The only member function that must
be overridden is the (virtual) member
 
    int streambuf::overflow(int c). This member's responsibility is to
write characters to the device. If fd is an output file descriptor and if
output should not be buffered then the member overflow() can simply be
implemented as:
            class UnbufferedFD: public std::streambuf
    {
        public:
            int overflow(int c) override;
            ...
    };

    int UnbufferedFD::overflow(int c)
    {
        if (c != EOF)
        {
            if (write(d_fd, &c, 1) != 1)
                return EOF;
        }
        return c;
    }

The argument received by overflow is either written to the file
descriptor (and returned from overflow), or EOF is returned.

This simple function does not use output buffering. For various reasons,
using a buffer is usually a good idea (see also the next section).

When output buffering is used, the overflow member is a bit more
complex as it is only called when the buffer is full. Once the buffer is full,
we first have to flush the buffer. Flushing the buffer is the
responsibility of the (virtual) function streambuf::sync.  Since
sync is a virtual function, classes derived from streambuf may
redefine sync to flush a buffer streambuf itself doesn't know about.

Overriding sync and using it in overflow is not all that has to be
done. When the object of the class defining the buffer reaches the end of its
lifetime the buffer may be only partially full. In that situation the buffer
must also be flushed. This is easily done by simply calling sync from the
class's destructor.

Now that we've considered the consequences of using an output buffer,
we're almost ready to design our derived class. Several more features
are added as well, though:
    
    o  First, we should allow the user of the class to specify the size of
the output buffer.
    o  Second, it should be possible to construct an object of our class
before the file descriptor is actually known. Later, in section 26.2
we'll encounter a situation where this feature is actually used.

To save some space in the C++ Annotations, the successful completion of the
functions designed here is not checked in the example code. In `real life'
implementations these checks should of course not be omitted.  Our class
OFdnStreambuf has the following characteristics:
    
    o  Its member functions use low-level functions operating on file
descriptors. So apart from streambuf the <unistd.h> header file must
have been read by the compiler before its member functions can be compiled.
    o  The class is derived from std::streambuf.
    o  It defines three data members. These data members keep track of,
respectively, the size of the buffer, the file descriptor, and the buffer
itself. Here is the full class interface
            class OFdnStreambuf: public std::streambuf
    {
        int     d_fd = -1;
        size_t  d_bufsize = 0;
        char   *d_buffer = 0;

        public:
            OFdnStreambuf() = default;
            OFdnStreambuf(int fd, size_t bufsize = 1);
            ~OFdnStreambuf() override;
            void open(int fd, size_t bufsize = 1);
        private:
            int sync() override;
            int overflow(int c) override;
    };

    o  Its default constructor merely initializes the buffer to 0. Slightly
more interesting is its constructor expecting a file descriptor and a buffer
size. This constructor passes its arguments on to the class's open
member (see below). Here are the constructors:
            inline OFdnStreambuf::OFdnStreambuf(int fd, size_t bufsize)
    {
        open(fd, bufsize);
    }

    o  The destructor calls sync, flushing any characters stored in the
output buffer to the device. In implementations not using a buffer the
destructor can be given a default implementation:
            inline OFdnStreambuf::~OFdnStreambuf()
    {
        if (d_buffer)
        {
            sync();
            delete[] d_buffer;
        }
    }

    This implementation does not close the device. It is left as an exercise
to the reader to change this class in such a way that the device is optionally
closed (or optionally remains open). This approach was adopted by, e.g., the
        Bobcat library.
         See also section
26.1.2.2.
    o  The open member initializes the buffer.  Using
streambuf::setp, the begin and end points of the buffer are
defined. This is used by the streambuf base class to initialize
streambuf::pbase, streambuf::pptr, and
streambuf::epptr:
            inline void OFdnStreambuf::open(int fd, size_t bufsize)
    {
        d_fd = fd;
        d_bufsize = bufsize == 0 ? 1 : bufsize;
        delete[] d_buffer;
        d_buffer = new char[d_bufsize];
        setp(d_buffer, d_buffer + d_bufsize);
    }

    o  The member sync flushes the as yet unflushed content of the
buffer to the device. After the flush the buffer is reinitialized using
setp. After successfully flushing the buffer sync returns 0:
            inline int OFdnStreambuf::sync()
    {
        if (pptr() > pbase())
        {
            write(d_fd, d_buffer, pptr() - pbase());
            setp(d_buffer, d_buffer + d_bufsize);
        }
        return 0;
    }

    o  The member streambuf::overflow is also
overridden. Since this member is called from the streambuf base class when
the buffer is full it should first call sync to flush the buffer to the
device. Next it should write the character c to the (now empty)
buffer. The character c should be written using pptr and
streambuf::pbump.  Entering a character into the buffer should be
implemented using available streambuf member functions, rather than `by
hand' as doing so might invalidate streambuf's internal bookkeeping. Here
is overflow's implementation:
            inline int OFdnStreambuf::overflow(int c)
    {
        sync();
        if (c != EOF)
        {
            *pptr() = c;
            pbump(1);
        }
        return c;
    }


The next program uses the OFfdStreambuf class to copy its standard
input to file descriptor STDOUT_FILENO, which is the symbolic name of the
file descriptor used for the standard output:
            #include <string>
    #include <iostream>
    #include <istream>
    #include "fdout.h"
    using namespace std;

    int main(int argc, char **argv)
    {
        OFdnStreambuf   fds(STDOUT_FILENO, 500);
        ostream         os(&fds);

        switch (argc)
        {
            case 1:
                for (string  s; getline(cin, s); )
                    os << s << '\n';
                os << "COPIED cin LINE BY LINE\n";
            break;

            case 2:
                cin >> os.rdbuf();      // Alternatively, use:  cin >> &fds;
                os << "COPIED cin BY EXTRACTING TO os.rdbuf()\n";
            break;

            case 3:
                os << cin.rdbuf();
                os << "COPIED cin BY INSERTING cin.rdbuf() into os\n";
            break;
        }
    }



26.1.2: Classes for input operations

        When classes for input operation are derived from
std::streambuf, they should be provided with an input buffer
of at least one character. The one-character input buffer allows for the use
of the member functions istream::putback or istream::ungetc.  Strictly
speaking it is not necessary to implement a buffer in classes derived from
streambuf. But using buffers in these classes is strongly advised. Their
implementation is very simple and straightforward and the applicability of
such classes is greatly improved. Therefore, all our classes derived from the
class streambuf define a buffer of at least one character.


26.1.2.1: Using a one-character buffer

            When deriving a class (e.g., IFdStreambuf) from streambuf using a
buffer of one character, at least its member
 
    streambuf::underflow should be overridden, as this member eventually
receives all requests for input. The member
 
    streambuf::setg is used to inform the streambuf base class of the
size and location of the input buffer, so that it is able to set up its input
buffer pointers accordingly. This ensures that 
streambuf::eback,  streambuf::gptr, and 
streambuf::egptr return correct values.

The class IFdStreambuf is designed like this:
    
    o  Its member functions use low-level functions operating on file
descriptors. Therefore, in addition to streambuf, the <unistd.h>
header file must have been read by the compiler before its member functions
can be compiled.
    o  Like most classes designed for input operations, this class is derived
from std::streambuf as well.
    o  The class defines two data members, one of them a fixed-sized one
character buffer. The data members are defined as protected data members
so that derived classes (e.g., see section 26.1.2.3) can access them. Here
is the full class interface:
            class IFdStreambuf: public std::streambuf
    {
        protected:
            int     d_fd;
            char    d_buffer[1];
        public:
            IFdStreambuf(int fd);
        private:
            int underflow() override;
    };

    o  The constructor initializes the buffer. However, the initialization
sets gptr's return value equal to egptr's return value. This
implies that the buffer is empty so underflow is immediately called
to fill the buffer:
            inline IFdStreambuf::IFdStreambuf(int fd)
    :
        d_fd(fd)
    {
        setg(d_buffer, d_buffer + 1, d_buffer + 1);
    }

    o  Finally underflow is overridden. The buffer is refilled by
reading from the file descriptor. If this fails (for whatever reason),
EOF is returned. More sophisticated implementations could act more
intelligently here, of course. If the buffer could be refilled, setg is
called to set up streambuf's buffer pointers correctly:
            inline int IFdStreambuf::underflow()
    {
        if (read(d_fd, d_buffer, 1) <= 0)
            return EOF;

        setg(d_buffer, d_buffer, d_buffer + 1);
        return static_cast<unsigned char>(*gptr());
    }


The following main function shows how IFdStreambuf can be used:
            int main()
    {
        IFdStreambuf fds(STDIN_FILENO);
        istream      is(&fds);

        cout << is.rdbuf();
    }



26.1.2.2: Using an n-character buffer

            How complex would things get if we decided to use a buffer of
substantial size? Not that complex. The following class allows us to specify
the size of a buffer, but apart from that it is basically the same class as
IFdStreambuf developed in the previous section. To make things a bit more
interesting, in the class IFdNStreambuf developed here, the member
streambuf::xsgetn is also overridden, to optimize reading a
series of characters. Also a default constructor is provided that can be used
in combination with the open member to construct an istream object
before the file descriptor becomes available. In that case, once the
descriptor becomes available, the open member can be used to initiate
the object's buffer. Later, in section 26.2, we'll encounter such a
situation.

To save some space, the success of various calls was not checked. In `real
life' implementations, these checks should of course not be omitted. The
class IFdNStreambuf has the following characteristics:
    
    o  Its member functions use low-level functions operating on file
descriptors. So apart from streambuf the <unistd.h> header file must
have been read by the compiler before its member functions can be compiled.
    o  As usual, it is derived from std::streambuf.
    o  Like the class IFdStreambuf (section 26.1.2.1), its data
members are protected. Since the buffer's size is configurable, this size is
kept in a dedicated data member, d_bufsize:
                class IFdNStreambuf: public std::streambuf
    {
        protected:
            int         d_fd = -1;
            size_t      d_bufsize = 0;
            char*       d_buffer = 0;
        public:
            IFdNStreambuf() = default;
            IFdNStreambuf(int fd, size_t bufsize = 1);
            ~IFdNStreambuf() override;
            void open(int fd, size_t bufsize = 1);
        private:
            int underflow() override;
            std::streamsize xsgetn(char *dest, std::streamsize n) override;
    };

    o  The default constructor does not allocate a buffer. It can be used
to construct an object before the file descriptor becomes known. A second
constructor simply passes its arguments to open. Open will then
initialize the object so that it can actually be used:
                inline IFdNStreambuf::IFdNStreambuf(int fd, size_t bufsize)
    {
        open(fd, bufsize);
    }

    o  Once the object has been initialized by open, its destructor will
both delete the object's buffer and use the file descriptor to close the
device:
                IFdNStreambuf::~IFdNStreambuf()
    {
        if (d_bufsize)
        {
            close(d_fd);
            delete[] d_buffer;
        }
    }

    Even though the device is closed in the above implementation this may not
always be desirable. In cases where the open file descriptor is already
available the intention may be to use that descriptor repeatedly, each time
using a newly constructed IFdNStreambuf object. It is left as an exercise
to the reader to change this class in such a way that the device may
optionally be closed. This approach was followed in, e.g., the
        Bobcat library.
        
    o  The open member simply allocates the object's buffer. It is
assumed that the calling program has already opened the device. Once the
buffer has been allocated, the base class member setg is used to ensure
that  streambuf::eback  streambuf::gptr and 
streambuf::egptr return correct values:
                void IFdNStreambuf::open(int fd, size_t bufsize)
    {
        d_fd = fd;
        d_bufsize =  bufsize == 0 ? 1 : bufsize;
        delete[] d_buffer;
        d_buffer = new char[d_bufsize];
        setg(d_buffer, d_buffer + d_bufsize, d_buffer + d_bufsize);
    }

    o  The overridden member underflow is implemented almost
identically to IFdStreambuf's (section 26.1.2.1) member. The only
difference is that the current class supports buffers of larger
sizes. Therefore, more characters (up to d_bufsize) may be read from the
device at once:
                int IFdNStreambuf::underflow()
    {
        if (gptr() < egptr())
            return *gptr();

        int nread = read(d_fd, d_buffer, d_bufsize);

        if (nread <= 0)
            return EOF;

        setg(d_buffer, d_buffer, d_buffer + nread);
        return static_cast<unsigned char>(*gptr());
    }

    o  Finally xsgetn is overridden. In a loop, n is reduced until
0, at which point the function terminates. Alternatively, the member returns
if underflow fails to obtain more characters. This member optimizes the
reading of series of characters. Instead of calling 
streambuf::sbumpc n times, a block of avail characters is copied
to the destination, using streambuf::gbump to consume avail
characters from the buffer using one function call:
                std::streamsize IFdNStreambuf::xsgetn(char *dest, std::streamsize n)
    {
        int nread = 0;

        while (n)
        {
            if (!in_avail())
            {
                if (underflow() == EOF)
                    break;
            }

            int avail = in_avail();

            if (avail > n)
                avail = n;

            memcpy(dest + nread, gptr(), avail);
            gbump(avail);

            nread += avail;
            n -= avail;
        }

        return nread;
    }


The member function xsgetn is called by streambuf::sgetn,
which is a streambuf member. Here is an example illustrating the use of
this member function with an IFdNStreambuf object:
            #include <unistd.h>
    #include <iostream>
    #include <istream>
    #include "ifdnbuf.h"
    using namespace std;

    int main()
    {
                                    // internally: 30 char buffer
        IFdNStreambuf fds(STDIN_FILENO, 30);

        char buf[80];               // main() reads blocks of 80
                                    // chars
        while (true)
        {
            size_t n = fds.sgetn(buf, 80);
            if (n == 0)
                break;
            cout.write(buf, n);
        }
    }



26.1.2.3: Seeking positions in `streambuf' objects

            When devices support seek operations, classes derived from
 std::streambuf should override the members 
streambuf::seekoff and streambuf::seekpos. The class
IFdSeek, developed in this section, can be used to read information from
devices supporting seek operations. The class IFdSeek was derived from
IFdStreambuf, so it uses a character buffer of just one character. The
facilities to perform seek operations, which are added to our new class
IFdSeek, ensure that the input buffer is reset when a seek operation is
requested. The class could also be derived from the class
IFdNStreambuf. In that case the arguments to reset the input buffer
must be adapted so that its second and third parameters point beyond the
available input buffer. Let's have a look at the characteristics of
IFdSeek:
    
    o  As mentioned, IFdSeek is derived from IFdStreambuf. Like the
latter class, IFdSeek's member functions use facilities declared in
unistd.h. So, the header file <unistd.h> must have been read by the
compiler before it can compile the class's members functions.  To reduce the
amount of typing when specifying types and constants from streambuf and
std::ios, several using-declarations are defined by the class.
These using-declarations refer to types that are defined in the header file
<ios>, which must therefore also be included before the compiler can
compile IFdSeek's class interface:
            class IFdSeek: public IFdStreambuf
    {
        using pos_type = std::streambuf::pos_type;
        using off_type = std::streambuf::off_type;
        using seekdir = std::ios::seekdir;
        using openmode = std::ios::openmode;

        public:
            IFdSeek(int fd);
        private:
            pos_type seekoff(off_type offset, seekdir dir, openmode);
            pos_type seekpos(pos_type offset, openmode mode);
    };

    o  The class has a very basic interface. Its (only)
constructor expects the device's file descriptor. It has no special tasks
to perform and just calls its base class constructor:
            inline IFdSeek::IFdSeek(int fd)
    :
        IFdStreambuf(fd)
    {}

    o  The member seek_off is responsible for performing the actual
seek operations. It calls lseek to seek a new position in a device whose
file descriptor is known. If seeking succeeds, setg is called to define
an already empty buffer, so that the base class's underflow member
refills the buffer at the next input request.
            IFdSeek::pos_type IFdSeek::seekoff(off_type off, seekdir dir, openmode)
    {
        pos_type pos =
            lseek
            (
                d_fd, off,
                (dir ==  std::ios::beg) ? SEEK_SET :
                (dir ==  std::ios::cur) ? SEEK_CUR :
                                          SEEK_END
            );

        if (pos < 0)
            return -1;

        setg(d_buffer, d_buffer + 1, d_buffer + 1);
        return pos;
    }

    o  Finally,  the companion function seekpos is overridden as well:
it is actually defined as a call to seekoff:
            inline IFdSeek::pos_type IFdSeek::seekpos(pos_type off, openmode mode)
    {
        return seekoff(off, std::ios::beg, mode);
    }


Here is an example of a program using the class IFdSeek. If
this program is given its own source file using input redirection then
seeking is supported (and with the exception of the first line, every other
line is shown twice):
            #include "fdinseek.h"
    #include <string>
    #include <iostream>
    #include <istream>
    #include <iomanip>
    using namespace std;

    int main()
    {
        IFdSeek fds(0);
        istream is(&fds);
        string  s;

        while (true)
        {
            if (!getline(is, s))
                break;

            streampos pos = is.tellg();

            cout << setw(5) << pos << ": `" << s << "'\n";

            if (!getline(is, s))
                break;

            streampos pos2 = is.tellg();

            cout << setw(5) << pos2 << ": `" << s << "'\n";

            if (!is.seekg(pos))
            {
                cout << "Seek failed\n";
                break;
            }
        }
    }



26.1.2.4: Multiple `unget' calls in `streambuf' objects

        Streambuf classes and classes derived from streambuf should support
at least ungetting the last read character. Special care must be taken
when series of unget calls must be supported. In this section the
construction of a class supporting a configurable number of istream::unget
or istream::putback calls is discussed.

Support for multiple (say `n') unget calls is implemented by
reserving an initial section of the input buffer, which is gradually filled up
to contain the last n characters read. The class is implemented as
follows:
    
    o  Once again, the class is derived from std::streambuf. It
defines several data members, allowing the class to perform the bookkeeping
required to maintain an unget-buffer of a configurable size:
            class FdUnget: public std::streambuf
    {
        int     d_fd;
        size_t  d_bufsize;
        size_t  d_reserved;
        char   *d_buffer;
        char   *d_base;
        public:
            FdUnget(int fd, size_t bufsz, size_t unget);
            ~FdUnget() override;
        private:
            int underflow() override;
    };

    o  The class's constructor expects a file descriptor, a buffer size
and the number of characters that can be ungot or pushed back as its
arguments. This number  determines the size of a reserved area,
defined as the first d_reserved bytes of the class's input buffer.
        
        o  The input buffer will always be at least one byte larger than
d_reserved. So, a certain number of bytes may be read. Once d_reserved
bytes have been read at most d_reserved bytes can be ungot.
        o  Next, the starting point for reading operations is configured. It
is called d_base, pointing to a location d_reserved bytes beyond the
location represented by d_buffer. This is always the location where buffer
refills start.
        o  Now that the buffer has been constructed, we're ready to define
streambuf's buffer pointers using setg. As no characters have been
read yet, all pointers are set to point to d_base. If unget is
called at this point, no characters are available, and unget
(correctly) fails.
        o  Eventually, the refill buffer's size is determined as the
number of allocated bytes minus the size of the reserved area.

Here is the class's constructor:
                FdUnget::FdUnget(int fd, size_t bufsz, size_t unget)
    :
        d_fd(fd),
        d_reserved(unget)
    {
        size_t allocate =
                bufsz > d_reserved ?
                    bufsz
                :
                    d_reserved + 1;

        d_buffer = new char[allocate];

        d_base = d_buffer + d_reserved;
        setg(d_base, d_base, d_base);

        d_bufsize = allocate - d_reserved;
    }

    o  The class's destructor simply returns the memory allocated for the
buffer to the common pool:
                inline FdUnget::~FdUnget()
    {
        delete[] d_buffer;
    }

    o  Finally,  underflow is overridden as follows:
        
        o  First underflow determines the number of characters that
could potentially be ungot. If that number of characters are ungot, the input
buffer is exhausted. So this value may be any value between 0 (the initial
state) or the input buffer's size (when the reserved area has been filled up
completely, and all current characters in the remaining section of the buffer
have also been read);
        o  Next the number of bytes to move into the reserved area is
computed. This number is at most d_reserved, but it is set equal to the
actual number of characters that can be ungot if this value is smaller;
        o  Now that the number of characters to move into the reserved area
is known, this number of characters is moved from the input buffer's end to
the area immediately before d_base;
        o  Then the buffer is refilled. This all is standard, but notice
that reading starts from d_base and not from d_buffer;
        o  Finally, streambuf's read buffer pointers are set up.
                
            Eback is set to move locations before d_base, thus
defining the guaranteed unget-area,
            gptr is set to d_base, since that's the location of the
first read character after a refill, and
            egptr is set just beyond the location of the last character
read into the buffer.

Here is underflow's implementation:
                int FdUnget::underflow()
    {
        size_t ungetsize = gptr() - eback();
        size_t move = std::min(ungetsize, d_reserved);

        memcpy(d_base - move, egptr() - move, move);

        int nread = read(d_fd, d_base, d_bufsize);
        if (nread <= 0)       // none read -> return EOF
            return EOF;

        setg(d_base - move, d_base, d_base + nread);

        return static_cast<unsigned char>(*gptr());
    }


An example using FdUnget

The next example program illustrates the use of the class FdUnget. It
reads at most 10 characters from the standard input, stopping at
EOF. A guaranteed unget-buffer of 2 characters is defined in a buffer
holding 3 characters. Just before reading a character, the program tries to
unget at most 6 characters. This is, of course, not possible; but the program
nicely ungets as many characters as possible, considering the actual
number of characters read:
            #include "fdunget.h"
    #include <string>
    #include <iostream>
    #include <istream>
    using namespace std;

    int main()
    {
        FdUnget fds(0, 3, 2);
        istream is(&fds);
        char    c;

        for (int idx = 0; idx < 10; ++idx)
        {
            cout << "after reading " << idx << " characters:\n";
            for (int ug = 0; ug <= 6; ++ug)
            {
                if (!is.unget())
                {
                    cout
                    << "\tunget failed at attempt " << (ug + 1) << "\n"
                    << "\trereading: '";

                    is.clear();
                    while (ug--)
                    {
                        is.get(c);
                        cout << c;
                    }
                    cout << "'\n";
                    break;
                }
            }

            if (!is.get(c))
            {
                cout << " reached\n";
                break;
            }
            cout << "Next character: " << c << '\n';
        }
    }
    /*
        Generated output after 'echo abcde | program':

        after reading 0 characters:
                unget failed at attempt 1
                rereading: ''
        Next character: a
        after reading 1 characters:
                unget failed at attempt 2
                rereading: 'a'
        Next character: b
        after reading 2 characters:
                unget failed at attempt 3
                rereading: 'ab'
        Next character: c
        after reading 3 characters:
                unget failed at attempt 4
                rereading: 'abc'
        Next character: d
        after reading 4 characters:
                unget failed at attempt 4
                rereading: 'bcd'
        Next character: e
        after reading 5 characters:
                unget failed at attempt 4
                rereading: 'cde'
        Next character:

        after reading 6 characters:
                unget failed at attempt 4
                rereading: 'de
        '
         reached
    */



26.1.3: Fixed-sized field extraction from istream objects

    Usually when extracting information from istream objects operator>>, the
standard extraction operator is perfectly suited for the task as in most
cases the extracted fields are white-space (or otherwise clearly) separated
from each other. But this does not hold true in all situations. For example,
when a web-form is posted to some processing script or program, the receiving
program may receive the form field's values as url-encoded
characters: letters and digits are sent unaltered, blanks are sent as +
characters, and all other characters start with % followed by the
character's
    ascii-value represented by its two digit hexadecimal value.

When decoding url-encoded information, simple hexadecimal extraction won't
work, as  that extracts as many hexadecimal characters as available,
instead of just two. Since the letters a-f` and 0-9 are legal
hexadecimal characters, a text like My name is `Ed', url-encoded as
            My+name+is+%60Ed%27

results in the extraction of the hexadecimal values 60ed and 27,
instead of 60 and 27. The name Ed disappears from view, which is
clearly not what we want.

In this case, having seen the %, we could extract 2 characters, put
them in an istringstream object, and extract the hexadecimal value from
the istringstream object. A bit cumbersome, but doable. Other approaches
are possible as well.

The class Fistream for fixed-sized field istream defines
an istream class supporting both fixed-sized field extractions and
blank-delimited extractions (as well as unformatted read calls). The
class may be initialized as a wrapper around an existing istream, or
it can be initialized using the name of an existing file. The class is derived
from istream, allowing all extractions and operations supported by
istreams in general. Fistream defines the following data members:
    
    o d_filebuf: a filebuffer used when Fistream reads its information
        from a named (existing) file. Since the filebuffer is only needed in
        that case, and since it must be allocated dynamically, it is defined
        as a unique_ptr<filebuf> object.
    o d_streambuf: a pointer to Fistream's streambuf. It points
        to d_filebuf when Fistream opens a file by name. When an
        existing istream is used to construct an Fistream, it
        points to the existing istream's streambuf.
    o d_iss: an istringstream object used for the fixed field
        extractions.
    o d_width: a size_t indicating the width of the field to
        extract. If 0 no fixed field extractions is used, but
        information is extracted from the istream base class object
        using standard extractions.

Here is the initial section of Fistream's class interface:
            class Fistream: public std::istream
    {
        std::unique_ptr<std::filebuf> d_filebuf;
        std::streambuf *d_streambuf;
        std::istringstream d_iss;
        size_t d_width;

    As stated, Fistream objects can be constructed from either a
filename or an existing istream object. The class interface therefore
declares two constructors:
                    Fistream(std::istream &stream);
            Fistream(char const *name,
                std::ios::openmode mode = std::ios::in);

    When an Fistream object is constructed using an existing istream
object, the Fistream's istream part simply uses the stream's
streambuf object:
        Fistream::Fistream(istream &stream)
:
    istream(stream.rdbuf()),
    d_streambuf(rdbuf()),
    d_width(0)
{}

    When an fstream object is constructed using a filename, the
istream base initializer is given a new filebuf object to be used as
its streambuf. Since the class's data members are not initialized before
the class's base class has been constructed, d_filebuf can only be
initialized thereafter. By then, the filebuf is only available as
rdbuf, returning a streambuf. However, as it is actually a
filebuf, a static_cast is used to cast the streambuf pointer
returned by rdbuf to a filebuf *, so d_filebuf can be initialized:
        Fistream::Fistream(char const *name, ios::openmode mode)
:
    istream(new filebuf()),
    d_filebuf(static_cast<filebuf *>(rdbuf())),
    d_streambuf(d_filebuf.get()),
    d_width(0)
{
    d_filebuf->open(name, mode);
}



26.1.3.1: Member functions and example

            There is only one additional public member: setField(field const
&). This member defines the size of the next field to extract. Its
parameter is a reference to a field class, a manipulator class
defining the width of the next field.

Since a field & is mentioned in Fistream's interface, field
must be declared before Fistream's interface starts. The class field
itself is simple and declares Fistream as its friend. It has two data
members: d_width specifies the width of the next field, and d_newWidth
which is set to true if d_width's value should actually be used. If
d_newWidth is false, Fistream returns to its standard extraction
mode. The class field has two constructors: a default
constructor, setting d_newWidth to false, and a second constructor
expecting the width of the next field to extract as its value. Here is the
class field:
            class field
    {
        friend class Fistream;
        size_t d_width;
        bool     d_newWidth;

        public:
            field(size_t width);
            field();
    };

    inline field::field(size_t width)
    :
        d_width(width),
        d_newWidth(true)
    {}

    inline field::field()
    :
        d_newWidth(false)
    {}

    Since field declares Fistream as its friend, setField may
inspect field's members directly.

Time to return to setField. This function expects a reference to a
field object, initialized in one of three different ways:
    
    o field(): When setField's argument is a field object
        constructed by its default constructor the next extraction will use
        the same field width as the previous extraction.
    o field(0): When this field object is used as setField's
        argument, fixed-sized field extraction stops, and the Fistream
        acts like any standard istream object again.
    o field(x): When the field object itself is initialized by a
        non-zero size_t value x, then the next field width is x
        characters wide. The preparation of such a field is left to
        setBuffer, Fistream's only private member.

Here is setField's implementation:
        std::istream &Fistream::setField(field const &params)
{
    if (params.d_newWidth)                  // new field size requested
        d_width = params.d_width;           // set new width

    if (!d_width)                           // no width?
        rdbuf(d_streambuf);                 // return to the old buffer
    else
        setBuffer();                        // define the extraction buffer

    return *this;
}


The private member setBuffer defines a buffer of d_width + 1
characters and uses read to fill the buffer with d_width
characters. The buffer is an NTBS. This buffer is used to initialize the
d_iss member. Fistream's rdbuf member is used to extract the
d_str's data via the Fistream object itself:
        void Fistream::setBuffer()
{
    char *buffer = new char[d_width + 1];

    rdbuf(d_streambuf);                         // use istream's buffer to
    buffer[read(buffer, d_width).gcount()] = 0; // read d_width chars,
                                                // terminated by a 0-byte
    d_iss.str(buffer);
    delete[] buffer;

    rdbuf(d_iss.rdbuf());                       // switch buffers
}


Although setField could be used to configure Fistream to use or
not to use fixed-sized field extraction, using manipulators is probably
preferable. To allow field objects to be used as manipulators an
overloaded extraction operator was defined. This extraction operator accepts
istream & and a field const & objects. Using this extraction
operator, statements like
        fis >> field(2) >> x >> field(0);

are possible (assuming fis is a Fistream object). Here is the
overloaded operator>>, as well as its declaration:
        istream &std::operator>>(istream &str, field const &params)
{
    return static_cast<Fistream *>(&str)->setField(params);
}

    Declaration:
        namespace std
{
    istream &operator>>(istream &str, FBB::field const &params);
}


Finally, an example. The following program uses a Fistream object to
url-decode url-encoded information appearing at its standard input:
            int main()
    {
        Fistream fis(cin);

        fis >> hex;
        while (true)
        {
            size_t x;
            switch (x = fis.get())
            {
                case '\n':
                    cout << '\n';
                break;
                case '+':
                    cout << ' ';
                break;
                case '%':
                    fis >> field(2) >> x >> field(0);
                // FALLING THROUGH
                default:
                    cout << static_cast<char>(x);
                break;
                case EOF:
                return 0;
            }
        }
    }
    /*
        Generated output after:
            echo My+name+is+%60Ed%27 | a.out

        My name is `Ed'
    */



26.2: The `fork' system call

From the C programming language the fork system call is well
known. When a program needs to start a new process, system can be used.
The function system requires the program to wait for the
        child process
    to terminate. The more general way to spawn subprocesses is to use
fork.

In this section we investigate how C++ can be used to wrap classes around
a complex system call like fork. Much of what follows in this section
directly applies to the Unix operating system, and the discussion therefore
focuses on that operating system. Other systems usually provide
comparable facilities. What follows is closely related to the
 Template Design Pattern (cf. Gamma et al. (1995)
     Design Patterns, Addison-Wesley)

When fork is called, the current program is duplicated in memory, thus
creating a new process. Following this duplication both processes continue
their execution just below the fork system call. The two processes may
inspect fork's return value: the return value in the
original process (called the parent process) differs from the return
value in the newly created process (called the child process):
    
    o  In the parent process fork returns the process ID of the
(child) process that was created by the fork system call. This is a
positive integer value.
    o  In the child process fork returns 0.
    o  If fork fails, -1 is returned.


26.2.1: A basic Fork class

    
    A basic Fork class should hide all bookkeeping details of a system
call like fork from its users. The class Fork developed here does
just that. The class itself only ensures the proper execution of the fork
system call. Normally, fork is called to start a child process, usually
boiling down to the execution of a separate process. This child process may
expect input at its standard input stream and/or may generate output to its
standard output and/or standard error streams. Fork does not know all
this, and does not have to know what the child process will do. Fork
objects should be able to start their child processes.

Fork's constructor cannot know what actions its child
process should perform. Similarly, it cannot know what actions the parent
process should perform. For these kind of  situations, the
        template method design pattern
        
    was developed. According to Gamma c.s., the template method design
pattern
    
    ``Define(s) the skeleton of an algorithm in an operation, deferring some
    steps to subclasses. [The] Template Method (design pattern) lets
    subclasses redefine certain steps of an algorithm, without changing
    the algorithm's structure.''

This design pattern allows us to define an abstract base class
        
    already providing the essential steps related to the fork system call,
deferring the implementation of other parts of the fork system call to
subclasses.

The Fork abstract base class has the following characteristics:
    
    o  It defines a data member d_pid. In the parent process this data
member contains the child's process id and in the child process it has
the value 0. Its public interface declares only two members:
    
    o  a fork member function, responsible for the actual forking (i.e.,
        it creates the (new) child process);
    o  a virtual destructor ~Fork (having an empty body).

Here is Fork's interface:
                class Fork
    {
        int d_pid;

        public:
            virtual ~Fork();
            void fork();

        protected:
            int pid() const;
            int waitForChild();                 // returns the status

        private:
            virtual void childRedirections();
            virtual void parentRedirections();

            virtual void childProcess() = 0;    // pure virtual members
            virtual void parentProcess() = 0;
    };

    o  All other non-virtual member functions are declared in the class's
protected section and can thus only be used by derived classes. They
are:
        
        o pid(): The member function pid allows derived classes to
            access the system fork's return value:
                    inline int Fork::pid() const
    {
        return d_pid;
    }

        o waitForChild(): The member int waitForChild can be called by
            parent processes to wait for the completion of their child
            processes (as discussed below). This member is declared in the
            class interface. Its implementation is:
                    #include "fork.ih"

    int Fork::waitForChild()
    {
        int status;

        waitpid(d_pid, &status, 0);

        return WEXITSTATUS(status);
    }

            This simple implementation returns the child's exit status to
            the parent. The called system function waitpid blocks
            until the child terminates.

o  When fork system calls are used,
            parent processes 
        and
            child processes 
        must always be distinguished. The main distinction between these
        processes is that d_pid becomes the child's process-id in the
        parent process, while d_pid becomes 0 in the child process
        itself. Since these two processes must always be distinguished (and
        present), their implementation by classes derived from Fork is
        enforced by Fork's interface: the members childProcess,
        defining the child process' actions and parentProcess, defining
        the parent process' actions were defined as pure virtual functions.

o  communication between parent- and child processes
        may use standard streams or other facilities, like pipes
        (cf. section 26.2.5). To facilitate this inter-process
        communication, derived classes may implement:
        
        o childRedirections(): this member should be overridden by derived
            classes if any standard stream (cin, cout,) or cerr must
            be redirected in the child process (cf. section
            26.2.3). By default it has an empty implementation;
        o parentRedirections(): this member should be overridden by derived
            classes if any standard stream (cin, cout,) or cerr must
            be redirected in the parent process. By default it has an
            empty implementation.

Redirection of the standard streams is necessary if parent and
child processes must communicate with each other via the standard streams.
Here are their default definitions. Since these functions are virtual
functions they should not be implemented inline, but in their own source file:
                void Fork::childRedirections()
    {}
    void Fork::parentRedirections()
    {}



26.2.2: Parents and Children

        The member function fork calls the system function fork
(Caution: since the system function fork is called by a member
function having the same name, the :: scope resolution operator must be
used to prevent a recursive call of the member function itself).
The function ::fork's  return value determines whether parentProcess
or  childProcess is called. Maybe redirection is
necessary. Fork::fork's implementation calls childRedirections
just before calling childProcess, and parentRedirections just
before calling parentProcess:
            #include "fork.ih"

    void Fork::fork()
    {
        if ((d_pid = ::fork()) < 0)
            throw "Fork::fork() failed";

        if (d_pid == 0)                 // childprocess has pid == 0
        {
            childRedirections();
            childProcess();
            exit(1);                    // we shouldn't come here:
        }                               // childProcess() should exit

        parentRedirections();
        parentProcess();
    }

    In fork.cc the class's internal header file
fork.ih is included. This header file takes care of the inclusion of the
necessary system header files, as well as the inclusion of fork.h
itself. Its implementation is:
            #include "fork.h"
    #include <cstdlib>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/wait.h>


Child processes should not return: once they have completed their tasks,
they should terminate. This happens automatically when the child process
performs a call to a member of the exec... family, but if the child
itself remains active, then it must make sure that it terminates properly. A
child process normally uses exit to terminate itself, but note that
exit prevents the activation of destructors of objects
        
        
    defined at the same or more superficial nesting levels than the level at
which exit is called. Destructors of globally defined objects are
activated when exit is used. When using exit to terminate
childProcess, it should either itself call a support member function
defining all nested objects it needs, or it should define all its objects in a
compound statement (e.g., using a throw block) calling exit beyond
the compound statement.

Parent processes should normally wait for their children to complete.
Terminating child processes inform their parents that they are about to
terminate by sending a signal that should be caught by their parents.  If
child processes terminate and their parent processes do not catch those
signals then such child processes remain visible as so-called zombie
processes.

If parent processes must wait for their children to complete, they may
call the member waitForChild. This member returns the exit status of a
child process to its parent.

There exists a situation where the child process continues to
live, but the parent dies. This is a fairly natural event: parents tend to
die before their children do. In our context (i.e. C++), this is called a
daemon program. In a daemon the parent process dies and the child program
continues to run as a child of the basic init process. Again, when the
child eventually dies a signal is sent to its `step-parent' init. This
does not create a zombie as init catches the termination signals of all
its (step-) children.  The construction of a daemon process is very simple,
given the availability of the class Fork (cf. section 26.2.4).


26.2.3: Redirection revisited

        Earlier, in section 6.6.2 streams were  redirected
using the ios::rdbuf member function. By assigning the
streambuf of a stream to another stream, both stream objects access the
same streambuf, thus implementing redirection at the level of the
programming language itself.

This may be fine within the context of a C++ program, but once we
leave that context the redirection terminates.  The operating system does not
know about streambuf objects. This situation is encountered, e.g., when a
program uses a system call to start a subprogram. The example program at
the end of this section uses C++ redirection to redirect the information
inserted into cout to a file, and then calls
            system("echo hello world")

to echo a well-known line of text. Since echo writes its information
to the standard output, this would be the program's redirected file if the
operating system would recognize C++'s redirection.

But redirection doesn't happen. Instead, hello world still appears at
the program's standard output and the redirected file is left untouched.  To
write hello world to the redirected file redirection must be realized at
the operating system level.  Some operating systems (e.g., Unix and
friends) provide system calls like dup and dup2 to accomplish
this. Examples of the use of these system calls are given in section
26.2.5.

Here is the example of the failing redirection at the system level
following  C++ redirection using streambuf redirection:
            #include <iostream>
    #include <fstream>
    #include <cstdlib>
    using namespace std;

    int main()
    {
        ofstream of("outfile");

        streambuf *buf = cout.rdbuf(of.rdbuf());
        cout << "To the of stream\n";
        system("echo hello world");
        cout << "To the of stream\n";
        cout.rdbuf(buf);
    }
    /*
        Generated output: on the file `outfile'

        To the of stream
        To the of stream

        On standard output:

        hello world
    */



26.2.4: The `Daemon' program

        Applications exist in which the only purpose of fork is to start a
child process. The parent process terminates immediately after spawning the
child process. If this happens, the child process continues to run as a child
process of init, the always running first process on Unix systems. Such
a process is often called a daemon, running as a background process.

Although the next example can easily be constructed as a plain C
program, it was included in the C++ Annotations because it is so closely
related to the current discussion of the Fork class. I thought about
adding a daemon member to that class, but eventually decided against it
because the construction of a daemon program is very simple and requires no
features other than those currently offered by the class Fork.  Here is an
example illustrating the construction of such a daemon program. Its child
process doesn't do exit but throw 0 which is caught by the catch
clause of the child's main function. Doing this ensures that any objects
defined by the child process are properly destroyed:
            #include <iostream>
    #include <unistd.h>
    #include "fork.h"

    class Daemon: public Fork
    {
        void parentProcess() override       // the parent does nothing.
        {}
        void childProcess() override        // actions by the child
        {
            sleep(3);
                                            // just a message...
            std::cout << "Hello from the child process\n";
            throw 0;                        // The child process ends
        }
    };

    int main()
    try
    {
        Daemon{}.fork();
    }
    catch (...)
    {}

    /*
        Generated output:
    The next command prompt, then after 3 seconds:
    Hello from the child process
    */



26.2.5: The class `Pipe'

        Redirection at the system level requires the use of 
        file descriptor, created by the pipe system
call. When two processes want to communicate using such file descriptors, the
following happens:
    
    o  The process constructs two associated file descriptors using the
pipe system call. One of the file descriptors is used for writing, the
other file descriptor is used for reading.
    o  Forking takes place (i.e., the system fork function is called),
duplicating the file descriptors. Now we have four file descriptors as
the child process and the parent process both have their own copies of the two
file descriptors created by pipe.
    o  One process (say, the parent process) uses the file descriptors
for reading. It should close its file descriptor intended for writing.
    o  The other process (say, the child process) uses the file descriptors
for writing. It should therefore close its file descriptor intended for
reading.
    o  All information written by the child process to the file
descriptor intended for writing, can now be read by the parent process from
the corresponding file descriptor intended for reading, thus establishing a
communication channel between the child and the parent process.

Though basically simple, errors may easily creep in. Functions of file
descriptors available to the two processes (child or parent) may easily get
mixed up. To prevent bookkeeping errors, the bookkeeping may be properly set
up once, to be hidden thereafter inside a class like the Pipe class
developed here. Let's have a look at its characteristics (before using
functions like pipe and dup the compiler must have read the
<unistd.h> header file):
    
    o  The pipe system call expects a pointer to two int values,
representing, respectively, the file descriptor used for reading and the file
descriptor used for writing. To avoid confusion, the class Pipe defines an
enum having values associating the indices of the array of 2-ints with
symbolic constants. The two file descriptors themselves are stored in a data
member d_fd. Here is the initial section of the class's interface:
            class Pipe
    {
        enum    RW { READ, WRITE };
        int     d_fd[2];

    o  The class only needs a default constructor. This constructor
calls pipe to create  a set of associated file descriptors used for
accessing both ends of a pipe:
            Pipe::Pipe()
    {
        if (pipe(d_fd))
            throw "Pipe::Pipe(): pipe() failed";
    }

    o  The members readOnly and readFrom are used to configure the
pipe's reading end. The latter function is used when using redirection. It is
provided with an alternate file descriptor to be used for  reading from the
pipe. Usually this alternate file descriptor is STDIN_FILENO, allowing
cin to extract information from the pipe.  The former function is merely
used to configure the reading end of the pipe. It closes the matching writing
end and returns a file descriptor that can be used to read from the pipe:
            int Pipe::readOnly()
    {
        close(d_fd[WRITE]);
        return d_fd[READ];
    }
    void Pipe::readFrom(int fd)
    {
        readOnly();

        redirect(d_fd[READ], fd);
        close(d_fd[READ]);
    }

    o  writeOnly and two writtenBy members are available to
configure the writing end of a pipe. The former function is only used to
configure the writing end of the pipe. It closes the reading end, and
returns a file descriptor that can be used for writing to the pipe:
            int Pipe::writeOnly()
    {
        close(d_fd[READ]);
        return d_fd[WRITE];
    }
    void Pipe::writtenBy(int fd)
    {
        writtenBy(&fd, 1);
    }
    void Pipe::writtenBy(int const *fd, size_t n)
    {
        writeOnly();

        for (size_t idx = 0; idx < n; idx++)
            redirect(d_fd[WRITE], fd[idx]);

        close(d_fd[WRITE]);
    }

    For the latter member two overloaded versions are available:
        
        o writtenBy(int fd) is used to configure single
redirection, so that a specific file descriptor (usually STDOUT_FILENO
or STDERR_FILENO) can be used to write to the pipe;
        o (writtenBy(int const *fd, size_t n)) may be used
to configure multiple redirection, providing an array argument containing
file descriptors. Information written to any of these file descriptors is
actually written to the pipe.

o  The class has one private data member, redirect, used to set up
redirection through the dup2 system call. This function expects two file
descriptors. The first file descriptor represents a file descriptor that can
be used to access the device's information; the second file descriptor is an
alternate file descriptor that may also be used to access the device's
information. Here is redirect's implementation:
            void Pipe::redirect(int d_fd, int alternateFd)
    {
        if (dup2(d_fd, alternateFd) < 0)
            throw "Pipe: redirection failed";
    }


Now that redirection can be configured easily using one or more Pipe
objects, we'll use Fork and Pipe in various example programs.


26.2.6: The class `ParentSlurp'

        The class ParentSlurp, derived from Fork, starts a child process
executing a stand-alone program (like /bin/ls). The (standard) output of
the executed program is not shown on the screen but is read by the parent
process.

For demonstration purposes the parent process writes the lines it
receives to its standard output stream, prepending linenumbers to the
lines. It is attractive to redirect the parent's standard input stream to
allow the parent to read the output from the child process using its
std::cin input stream. Therefore, the only pipe in the program is used
as an input pipe for the parent, and an output pipe for the child.

The class ParentSlurp has the following characteristics:
    
    o  It is derived from Fork. Before starting ParentSlurp's class
interface, the compiler must have read fork.h and pipe.h. The class
only uses one data member, a Pipe object d_pipe.
    o  As Pipe's constructor already defines a pipe, and as d_pipe
is automatically initialized by ParentSlurp's default constructor, which
is implicitly provided, all additional members only exist for
ParentSlurp's own benefit so they can be defined in the class's (implicit)
private section. Here is the class's interface:
            class ParentSlurp: public Fork
    {
        Pipe    d_pipe;

        void childRedirections() override;
        void parentRedirections() override;
        void childProcess() override;
        void parentProcess() override;
    };

    o  The childRedirections member configures the writing end of the
pipe. So, all information written to the child's standard output stream ends
up in the pipe. The big advantage of this is that no additional streams are
needed to write to a file descriptor:
            inline void ParentSlurp::childRedirections()
    {
        d_pipe.writtenBy(STDOUT_FILENO);
    }

    o  The parentRedirections member, configures the reading end of
the pipe. It does so by connecting the reading end of the pipe to the parent's
standard input file descriptor (STDIN_FILENO). This allows the parent to
perform extractions from cin, not requiring any additional streams for
reading.
            inline void ParentSlurp::parentRedirections()
    {
        d_pipe.readFrom(STDIN_FILENO);
    }

    o  The childProcess member only needs to concentrate on its own
actions. As it only needs to execute a program (writing information to its
standard output), the member can consist of  one single statement:
            inline void ParentSlurp::childProcess()
    {
        execl("/bin/ls", "/bin/ls", 0);
    }

    o  The parentProcess member simply `slurps' the information
appearing at its standard input. Doing so, it actually reads the child's
output. It copies the received lines to its standard output stream prefixing
line numbers to them:
            void ParentSlurp::parentProcess()
    {
        std::string     line;
        size_t    nr = 1;

        while (getline(std::cin, line))
            std::cout << nr++ << ": " << line << '\n';

        waitForChild();
    }


The following program simply constructs a ParentSlurp object, and
calls its fork() member. Its output consists of a numbered list of files
in the directory where the program is started.  Note that the program also
needs the fork.o, pipe.o and waitforchild.o object files (see
earlier sources):
            int main()
    {
        ParentSlurp{}.fork();
    }
    /*
        Generated Output (example only, actually obtained output may differ):

        1: a.out
        2: bitand.h
        3: bitfunctional
        4: bitnot.h
        5: daemon.cc
        6: fdinseek.cc
        7: fdinseek.h
        ...
    */



26.2.7: Communicating with multiple children

        The next step up the ladder is the construction of a child-process
monitor. Here, the parent process is responsible for all its child processes,
but it also must read their standard output. The user enters information at
the standard input of the parent process. A simple command language is
used for this:
    
    o start: this starts a new child process. The parent returns the
child's ID (a number) to the user. The ID is thereupon be used to identify a
particular child process;
    o <nr> text sends ``text'' to the child process having ID
<nr>;
    o stop <nr> terminates the child process having ID <nr>;
    o exit terminates the parent as well as all its child processes.

If a child process hasn't received text for some time it will complain by
sending a message to the parent-process. Those messages are simply transmitted
to the user by copying them to the standard output stream.

A problem with programs like our monitor is that they allow
        asynchronous input from multiple sources. Input may appear at the
standard input as well as at the input-sides of pipes. Also, multiple output
channels are used. To handle situations like these, the select system
call was developed.


26.2.7.1: The class `Selector': interface

            The select system call was developed to handle asynchronous
        I/O multiplexing.
    The select system call is used to handle, e.g., input appearing
simultaneously at a set of file descriptors.

The select function is rather complex, and its full discussion is
beyond the C++ Annotations' scope.  By encapsulating select in a class
Selector, hiding its details and offering an intuitively attractive
interface, its use is simplified. The Selector class has these
features:
    
    o  Efficiency. As most of Select's members are very small,
most members can be implemented  inline. The class requires quite a few data
members. Most of these data members belong to types that require some system
headers to be included first:
            #include <limits.h>
    #include <unistd.h>
    #include <sys/time.h>
    #include <sys/types.h>

    o  The class interface can now be defined. The data type fd_set is a
type designed to be used by select and variables of this type contain the
set of file descriptors on which select may sense some
activity. Furthermore, select allows us to fire an
        asynchronous alarm. To set the alarm time, the class Selector
defines a timeval data member. Other members are used for internal
bookkeeping purposes. Here is the class Selector's interface:
            class Selector
    {
        fd_set          d_read;
        fd_set          d_write;
        fd_set          d_except;
        fd_set          d_ret_read;
        fd_set          d_ret_write;
        fd_set          d_ret_except;
        timeval         d_alarm;
        int             d_max;
        int             d_ret;
        int             d_readidx;
        int             d_writeidx;
        int             d_exceptidx;

        public:
            Selector();

            int exceptFd();
            int nReady();
            int readFd();
            int wait();
            int writeFd();
            void addExceptFd(int fd);
            void addReadFd(int fd);
            void addWriteFd(int fd);
            void noAlarm();
            void rmExceptFd(int fd);
            void rmReadFd(int fd);
            void rmWriteFd(int fd);
            void setAlarm(int sec, int usec = 0);

        private:
            int checkSet(int *index, fd_set &set);
            void addFd(fd_set *set, int fd);
    };



26.2.7.2: The class `Selector': implementation

            Selector's member functions serve the following tasks:
    
    o Selector(): the (default) constructor. It
clears the read, write, and execute fd_set variables, and switches off the
alarm. Except for d_max, the remaining data members do not require
specific initializations:
            Selector::Selector()
    {
        FD_ZERO(&d_read);
        FD_ZERO(&d_write);
        FD_ZERO(&d_except);
        noAlarm();
        d_max = 0;
    }

    o int wait(): this member blocks until the alarm times
out or until activity is sensed at any of the file descriptors monitored by
the Selector object. It throws an exception when the select system
call itself fails:
            int Selector::wait()
    {
        timeval t = d_alarm;

        d_ret_read = d_read;
        d_ret_write = d_write;
        d_ret_except = d_except;

        d_readidx = 0;
        d_writeidx = 0;
        d_exceptidx = 0;

        d_ret = select(d_max, &d_ret_read, &d_ret_write, &d_ret_except,
                       t.tv_sec == -1 && t.tv_usec == -1 ? 0 : &t);

        if (d_ret < 0)
            throw "Selector::wait()/select() failed";

        return d_ret;
    }

    o int nReady: this member function's return value is only
defined when wait has returned. In that case it returns 0 for an
alarm-timeout, -1 if select failed, and otherwise the number of file
descriptors on which activity was sensed:
            inline int Selector::nReady()
    {
        return d_ret;
    }

    o int readFd(): this member function's return
value is also only defined after wait has returned. Its return value is
-1 if no (more) input file descriptors are available. Otherwise the next file
descriptor available for reading is returned:
            inline int Selector::readFd()
    {
        return checkSet(&d_readidx, d_ret_read);
    }

    o int writeFd(): operating analogously to readFd, it
returns the next file descriptor to which output is written.  It uses
d_writeidx and d_ret_read and is implemented analogously to
readFd;
    o int exceptFd(): operating analogously to readFd, it
returns the next exception file descriptor on which activity was sensed.  It
uses d_except_idx and d_ret_except and is implemented analogously to
readFd;
    o void setAlarm(int sec, int usec = 0): this member
activates Select's alarm facility. At least the number of seconds to wait
for the alarm to go off must be specified. It simply assigns values to
d_alarm's fields. At the next Select::wait call, the alarm fires
(i.e., wait returns with return value 0) once the configured
alarm-interval has passed:
            inline void Selector::setAlarm(int sec, int usec)
    {
        d_alarm.tv_sec  = sec;
        d_alarm.tv_usec = usec;
    }

    o void noAlarm(): this member switches off the alarm, by
simply setting the alarm interval to a very long period:
            inline void Selector::noAlarm()
    {
        setAlarm(-1, -1);
    }

    o void addReadFd(int fd): this member adds a
file descriptor to the set of input file descriptors monitored by the
Selector object. The member function wait returns once input is
available at the indicated file descriptor:
            inline void Selector::addReadFd(int fd)
    {
        addFd(&d_read, fd);
    }

    o void addWriteFd(int fd): this member adds a file
descriptor to the set of output file descriptors monitored by the Selector
object. The member function wait returns once output is available at
the indicated file descriptor. Using d_write, it is implemented
analogously to addReadFd;
    o void addExceptFd(int fd): this member adds a file
descriptor to the set of exception file descriptors to be monitored by the
Selector object.  The member function wait returns once activity
is sensed at the indicated file descriptor. Using d_except, it is
implemented analogously to addReadFd;
    o void rmReadFd(int fd): this member removes a file
descriptor from the set of input file descriptors monitored by the
Selector object:
            inline void Selector::rmReadFd(int fd)
    {
        FD_CLR(fd, &d_read);
    }

    o void rmWriteFd(int fd): this member removes a file
descriptor from the set of output file descriptors monitored by the
Selector object.  Using d_write, it is implemented analogously to
rmReadFd;
    o void rmExceptFd(int fd): this member removes a file
descriptor from the set of exception file descriptors to be monitored by the
Selector object.  Using d_except, it is implemented analogously to
rmReadFd;

The class's remaining (two) members are support members, and should not be
used by non-member functions. Therefore, they are declared in the class's
private section:
    
    o  The member addFd adds a file descriptor to a fd_set:
            void Selector::addFd(fd_set *set, int fd)
    {
        FD_SET(fd, set);
        if (fd >= d_max)
            d_max = fd + 1;
    }

    o  The member checkSet tests whether a file descriptor (*index)
is found in a fd_set:
            int Selector::checkSet(int *index, fd_set &set)
    {
        int &idx = *index;

        while (idx < d_max && !FD_ISSET(idx, &set))
            ++idx;

        return idx == d_max ? -1 : idx++;
    }



26.2.7.3: The class `Monitor': interface

            The monitor program uses a Monitor object doing most of the
work. The class Monitor's public interface only offers a default
constructor and one member, run, to perform its tasks.  All other member
functions are located in the class's private section.

Monitor defines the private enum Commands, symbolically
listing the various commands its input language supports, as well as several
data members.  Among the data members are a Selector object and a map
using child order numbers as its keys and pointer to Child objects (see
section 26.2.7.7) as its values. Furthermore, Monitor has a static array
member s_handler[], storing pointers to member functions handling user
commands.

A destructor should be implemented as well, but its implementation is left
as an exercise to the reader. Here is Monitor's interface, including the
interface of the nested class Find that is used to create a function
object:
            class Monitor
    {
        enum Commands
        {
            UNKNOWN,
            START,
            EXIT,
            STOP,
            TEXT,
            sizeofCommands
        };

        using MapIntChild = std::map<int, std::shared_ptr<Child>>;

        friend class Find;
        class Find
        {
            int     d_nr;
            public:
                Find(int nr);
                bool operator()(MapIntChild::value_type &vt) const;
        };

        Selector    d_selector;
        int         d_nr;
        MapIntChild d_child;

        static void (Monitor::*s_handler[])(int, std::string const &);
        static int s_initialize;

        public:
            enum Done
            {};

            Monitor();
            void run();

        private:
            static void killChild(MapIntChild::value_type it);
            static int initialize();

            Commands    next(int *value, std::string *line);
            void    processInput();
            void    processChild(int fd);

            void    createNewChild(int, std::string const &);
            void    exiting(int = 0, std::string const &msg = std::string{});
            void    sendChild(int value, std::string const &line);
            void    stopChild(int value, std::string const &);
            void    unknown(int, std::string const &);
    };


Since there's only one non-class type data member, the class's constructor
is a very simple function which could be implemented inline:
            inline Monitor::Monitor()
    :
        d_nr(0)
    {}



26.2.7.4: The class `Monitor': s_handler

        The array
s_handler, storing pointers to functions needs to be initialized as
well. This can be accomplished in several ways:
    
    o  Since the Commands enumeration only specifies a fairly limited
set of commands, compile-time initialization could be considered:
            void (Monitor::*Monitor::s_handler[])(int, string const &)  =
    {
        &Monitor::unknown,              // order follows enum Command's
        &Monitor::createNewChild,       // elements
        &Monitor::exiting,
        &Monitor::stopChild,
        &Monitor::sendChild,
    };

    The advantage of this is that it's simple, not requiring any run-time
effort. The disadvantage is of course relatively complex maintenance. If for
some reason Commands is modified, s_handler must be modified as
well. In cases like these, compile-time initialization often is
asking for trouble. There is a simple alternative though.
    o  Looking at Monitor's interface we see a static data member
s_initialize and a static member function initialize. The static
member function handles the initialization of the s_handler array. It
explicitly assigns the array's elements and any modification in ordering of
enum Commands' values is automatically accounted for by recompiling
initialize:
            void (Monitor::*Monitor::s_handler[sizeofCommands])(int, string const &);

    int Monitor::initialize()
    {
        s_handler[UNKNOWN] =    &Monitor::unknown;
        s_handler[START] =      &Monitor::createNewChild;
        s_handler[EXIT] =       &Monitor::exiting;
        s_handler[STOP] =       &Monitor::stopChild;
        s_handler[TEXT] =       &Monitor::sendChild;
        return 0;
    }

    The member initialize is a static member and so it can be
called to initialize s_initialize, a static int variable. The
initialization is enforced by placing the initialization statement in the
source file of a function that is known to be executed. It could be main,
but if we're Monitor's maintainers and only have control over the library
containing Monitor's code then that's not an option. In those cases the
source file containing the destructor is a very good candidate. If a class
has only one constructor and it's not defined inline then the
constructor's source file is a good candidate as well. In Monitor's
current implementation the initialization statement is put in run's source
file, reasoning that s_handler is only needed when run is used.


26.2.7.5: The class `Monitor': the member `run'

            Monitor's core activities are performed by run. It
performs the following tasks:
    
    o  Initially, the Monitor object only monitors its standard
input. The set of input file descriptors to which d_selector listens
is initialized to STDIN_FILENO.
    o  Then, in a loop  d_selector's wait function is called.
If input on cin is available, it is processed by processInput.
Otherwise, the input has arrived from a child process. Information sent by
children is processed by processChild.
    o  To prevent zombies, the child processes must catch their
children's termination signals. This is discussed below.

As noted by Ben Simons (ben at mrxfx dot com) Monitor must not catch
the termination signals.  Instead, the process spawning child processes has
that responsibility (the underlying principle being that a parent process is
responsible for its child processes; a child process, in turn, is responsible
for its own child processes).
    o  As stated, run's source file also defines and initializes
s_initialize to ensure the proper initialization of the s_handler
array.

Here is run's implementation and s_initialize's definition:
            #include "monitor.ih"

    int Monitor::s_initialize = Monitor::initialize();

    void Monitor::run()
    {
        d_selector.addReadFd(STDIN_FILENO);

        while (true)
        {
            cout << "? " << flush;
            try
            {
                d_selector.wait();

                int fd;
                while ((fd = d_selector.readFd()) != -1)
                {
                    if (fd == STDIN_FILENO)
                        processInput();
                    else
                        processChild(fd);
                }
                cout << "NEXT ...\n";
            }
            catch (char const *msg)
            {
                exiting(1, msg);
            }
        }
    }


The member function processInput reads the commands entered by the
user using the program's standard input stream. The member itself is rather
simple. It calls next to obtain the next command entered by the user, and
then calls the corresponding function using the matching element of the
s_handler[] array. Here are the members processInput and next:
            void Monitor::processInput()
    {
        string line;
        int    value;
        Commands cmd = next(&value, &line);
        (this->*s_handler[cmd])(value, line);
    }

            Monitor::Commands Monitor::next(int *value, string *line)
    {
        if (!getline(cin, *line))
            exiting(1, "Monitor::next(): reading cin failed");

        if (*line == "start")
            return START;

        if (*line == "exit" || *line == "quit")
        {
            *value = 0;
            return EXIT;
        }

        if (line->find("stop") == 0)
        {
            istringstream istr(line->substr(4));
            istr >> *value;
            return !istr ? UNKNOWN : STOP;
        }

        istringstream istr(line->c_str());
        istr >> *value;
        if (istr)
        {
            getline(istr, *line);
            return TEXT;
        }

        return UNKNOWN;
    }


All other input sensed by d_select is created by child
processes. Because d_select's readFd member returns the corresponding
input file descriptor, this descriptor can be passed to
processChild. Using a IFdStreambuf (see section 26.1.2.1), its
information is read from an input stream. The communication protocol used here
is rather basic. For every line of input sent to a child, the child replies by
sending back exactly one line of text. This line is then read by
processChild:
            void Monitor::processChild(int fd)
    {
        IFdStreambuf ifdbuf(fd);
        istream istr(&ifdbuf);
        string line;

        getline(istr, line);
        cout << d_child[fd]->pid() << ": " << line << '\n';
    }

    The construction d_child[fd]->pid() used in the above source deserves
some special attention. Monitor defines the data member map<int,
shared_ptr<Child>> d_child.  This map contains the child's order number
as its key, and a (shared) pointer to the Child object as its value. A
shared pointer is used here, rather than a Child object, since we want to
use the facilities offered by the map, but don't want to copy a Child
object time and again.


26.2.7.6: The class `Monitor': example

            Now that run's implementation has been covered, we'll concentrate on
the various commands users might enter:
    
    o  When the start command is issued, a new child process is started.
A new element is added to d_child by the member createNewChild.  Next,
the Child object should start its activities, but the Monitor object
can not wait for the child process to complete its activities, as there is no
well-defined endpoint in the near future, and the user probably wants to be
able to enter more commands. Therefore, the Child process must run as a
daemon. So the forked process terminates immediately, but its own child
process continues to run (in the background). Consequently,
createNewChild calls the child's fork member. Although it is the
child's fork function that is called, it is still the monitor program
wherein that fork function is called. So, the monitor program is
duplicated by fork. Execution then continues:
    
    o  At the Child's parentProcess in its parent process;
    o  At the Child's childProcess in its child process

As the Child's parentProcess is an empty function, returning
immediately, the Child's parent process effectively continues immediately
below createNewChild's cp->fork() statement.  As the child process
never returns (see section 26.2.7.7), the code below cp->fork() is never
executed by the Child's child process. This is exactly as it should be.

In the parent process, createNewChild's remaining code simply
adds the file descriptor that's available for reading information from the
child to the set of input file descriptors monitored by d_select, and
uses d_child to  establish the association between that
file descriptor and the  Child object's address:
            void Monitor::createNewChild(int, string const &)
    {
        Child *cp = new Child{ ++d_nr };

        cp->fork();

        int fd = cp->readFd();

        d_selector.addReadFd(fd);
        d_child[fd].reset(cp);

        cerr << "Child " << d_nr << " started\n";
    }

    o  Direct communication with the child is required for the stop <nr>
and <nr> text commands. The former command terminates child process
<nr>, by calling stopChild. This function locates the child process
having the order number using an anonymous object of the class Find,
nested inside Monitor. The class Find simply compares the
provided nr with the children's order number returned by their nr
members:
            inline Monitor::Find::Find(int nr)
    :
        d_nr(nr)
    {}
    inline bool Monitor::Find::operator()(MapIntChild::value_type &vt) const
    {
        return d_nr == vt.second->nr();
    }

    If the child process having order number nr was found, its file
descriptor is removed from d_selector's set of input file
descriptors. Then the child process itself is terminated by the static member
killChild. The member killChild is declared as a static member
function, as it is used as function argument of the for_each generic
algorithm by exiting (see below). Here is killChild's
implementation:
            void Monitor::killChild(MapIntChild::value_type it)
    {
        if (kill(it.second->pid(), SIGTERM))
            cerr << "Couldn't kill process " << it.second->pid() << '\n';

        // reap defunct child process
        int status = 0;
        while( waitpid( it.second->pid(), &status, WNOHANG) > -1)
            ;
    }

    Having terminated the specified child process, the corresponding Child
object is destroyed and its pointer is removed from d_child:
            void Monitor::stopChild(int nr, string const &)
    {
        auto it = find_if(d_child.begin(), d_child.end(), Find{ nr });

        if (it == d_child.end())
            cerr << "No child number " << nr << '\n';
        else
        {
            d_selector.rmReadFd(it->second->readFd());
            d_child.erase(it);
        }
    }

    o  The command <nr> text sends text to child process nr
using the member function sendChild. This function also uses a Find
object to locate the child-process having order number nr, and simply
inserts the text into the writing end of a pipe connected to that child
process:
            void Monitor::sendChild(int nr, string const &line)
    {
        auto it = find_if(d_child.begin(), d_child.end(), Find(nr));

        if (it == d_child.end())
            cerr << "No child number " << nr << '\n';
        else
        {
            OFdnStreambuf ofdn{ it->second->writeFd() };
            ostream out(&ofdn);

            out << line << '\n';
        }
    }

    o  When users enter exit or quit the member exiting is
called.  It terminates all child processes using the for_each generic
algorithm (see section 19.1.18) to visit all elements of
d_child. Then the program itself ends:
            void Monitor::exiting(int value, string const &msg)
    {
        for_each(d_child.begin(), d_child.end(), killChild);
        if (msg.length())
            cerr << msg << '\n';
        throw value;
    }


The program's main function is simple and needs no further comment:
            int main()
    try
    {
        Monitor{}.run();
    }
    catch (int exitValue)
    {
        return exitValue;
    }



26.2.7.7: The class `Child'

            When the Monitor object starts a child process, it creates an object
of the class Child. The Child class is derived from the class
Fork, allowing it to operate as a daemon (as discussed in the
previous section). Since Child is a daemon class, we know that its parent
process must be defined as an empty function.  Its childProcess member
has a non-empty implementation. Here are the characteristics of the class
Child:
    
    o  The Child class has two Pipe data members, to handle
communications between its own child- and parent processes. As these pipes are
used by the Child's child process, their names refer to the child
process. The child process reads from d_in, and writes to d_out. Here
is the interface of the class Child:
            class Child: public Fork
    {
        Pipe                d_in;
        Pipe                d_out;

        int         d_parentReadFd;
        int         d_parentWriteFd;
        int         d_nr;

        public:
            Child(int nr);
            ~Child() override;
            int readFd() const;
            int writeFd() const;
            int pid() const;
            int nr() const;
        private:
            void childRedirections() override;
            void parentRedirections() override;
            void childProcess() override;
            void parentProcess() override;
    };

    o  The Child's constructor simply stores its argument, a
child-process order number, in its own d_nr data member:
            inline Child::Child(int nr)
    :
        d_nr(nr)
    {}

    o  The Child's child process obtains commands from its standard
input stream and writes its output to its standard output stream. Since the
actual communication channels are pipes, redirections must be used. The
childRedirections member looks like this:
            void Child::childRedirections()
    {
        d_in.readFrom(STDIN_FILENO);
        d_out.writtenBy(STDOUT_FILENO);
    }

    o  Although the parent process performs no actions, it must configure
some redirections. Realizing that the names of the pipes indicate their
functions in the child process. So the parent writes to d_in and
reads from d_out. Here is parentRedirections:
            void Child::parentRedirections()
    {
        d_parentReadFd = d_out.readOnly();
        d_parentWriteFd = d_in.writeOnly();
    }

    o  The Child object exists until it is destroyed by the
Monitor's stopChild member. By allowing its creator, the Monitor
object, to access the parent-side ends of the pipes, the Monitor object
can communicate with the Child's child process via those pipe-ends. The
members readFd and writeFd allow the Monitor object to access
these pipe-ends:
            inline int Child::readFd() const
    {
        return d_parentReadFd;
    }
    inline int Child::writeFd() const
    {
        return d_parentWriteFd;
    }

    o  The Child object's child process performs two tasks:
        
        o  It must reply to information appearing at its standard input
stream;
        o  If no information has appeared within a certain time frame (the
implementations uses an interval of five seconds), then a message is written
to its standard output stream.

To implement this behavior, childProcess defines a local
Selector object, adding STDIN_FILENO to its set of monitored input
file descriptors.

Then, in an endless loop, childProcess waits for selector.wait()
to return. When the alarm goes off it sends a message to its standard output
(hence, into the writing pipe). Otherwise, it echoes the messages appearing
at its standard input to its standard output. Here is the childProcess
member:
            void Child::childProcess()
    {
        Selector    selector;
        size_t    message = 0;

        selector.addReadFd(STDIN_FILENO);
        selector.setAlarm(5);

        while (true)
        {
            try
            {
                if (!selector.wait())       // timeout
                    cout << "Child " << d_nr << ": standing by\n";
                else
                {
                    string  line;
                    getline(cin, line);
                    cout << "Child " << d_nr << ":" << ++message << ": " <<
                                                        line << '\n';
                }
            }
            catch (...)
            {
                    cout << "Child " << d_nr << ":" << ++message << ": " <<
                                "select() failed" << '\n';
            }
        }
        exit(0);
    }

    o  Two accessors are defined allowing the Monitor object to obtain
the Child's process ID and its order number:
            inline int Child::pid() const
    {
        return Fork::pid();
    }
    inline int Child::nr() const
    {
        return d_nr;
    }

    o  A Child process terminates when the user enters a stop
command. When an existing child process number was entered, the corresponding
Child object is removed from Monitor's d_child map. As a result,
its destructor is called. Child's destructor calls kill to terminate
its child, and then waits for the child to terminate. Once its child has
terminated, the destructor has completed its work and returns, thus completing
the erasure from d_child. The current implementation fails if the child
process doesn't react to the SIGTERM signal. In this demonstration program
this does not happen. In `real life' more elaborate killing-procedures may be
required (e.g., using SIGKILL in addition to SIGTERM). As discussed in
section 10.12 it is important to ensure the proper
destruction. Here is the Child's destructor:
            Child::~Child()
    {
        if (pid())
        {
            cout << "Killing process " << pid() << "\n";
            kill(pid(), SIGTERM);
            int status;
            wait(&status);
        }
    }




26.3: Adding binary operators to classes

As we've seen in section 11.7 binary operators expecting
const & arguments can be implemented using a member implementing the
operation, only offering the basic exception guarantee.
This latter function can in turn be implemented using the binary
assignment member. The following examples illustrated this approach for a
fictitious class Binary:
            class Binary
    {
        public:
            Binary();
            Binary(int value);
                // copy and move constructors are available by default, or
                // they can be explicitly declared and implemented.

            Binary &operator+=(Binary const &other) &;    // see the text
            Binary &&operator+=(Binary const &other) &&;

        private:
            void add(Binary const &rhs);

        friend Binary operator+(Binary const &lhs, Binary const &rhs);
        friend Binary operator+(Binary &&lhs, Binary const &rhs);
    };


Eventually, the implementation of binary operators depends on the availability
of the member implementing the basic binary operation, modifying the object
calling that member (i.e., void Binary::add(Binary const &) in the
example).

Since template functions are not instantiated before they are actually used we
can call non-existing functions from template functions that are never
instantiated. If such a template function is never instantiated, nothing
happens; if it is (accidentally) instantiated, then the compiler generates an
error message, complaining about the missing function.

This allows us to implement all binary operators, movable and non-movable, as
templates. In the following subsections we develop the class template
Binops, prividing binary operators.  A complete implementation of a class
Derived illustrating how addition and insertion operators can be added to
a class is provided in the file
annotations/yo/concrete/examples/binopclasses.cc in the C++ Annotations'
source archive.


26.3.1: Merely using operators

    In section 11.7 addition operators are implemented in terms of
a support member add. This is less attractive when developing function
templates, as add is a private member, requiring us to provide friend
declarations for all function templates so they may access the private add
member.

At the end of section 11.7 we saw that add's implementation
can be provided by operator+=(Class const &rhs) &&. This operator may
thereupon be used when implementing the remaining addition operators:
            inline Binary &operator+=(Binary const &rhs) &
    {
        return *this = Binary{*this} += rhs;        
    }

    Binary operator+(Binary &&lhs, Binary const &rhs)
    {
        return std::move(lhs) += rhs;
    }

    Binary operator+(Binary const &lhs, Binary const &rhs)
    {
        return Binary{lhs} += rhs;
    }

In this implementation add is no longer required. The plain binary
operators are free functions, which supposedly can easily be converted to
function templates. E.g.,
            template <typename Binary>
    Binary operator+(Binary const &lhs, Binary const &rhs)
    {
        return Binary{lhs} += rhs;
    }


26.3.1.1: To namespace or not to namespace?

        When using the function template Binary operator+(Binary const &lhs, Binary
const &rhs), however, we may encounter a subtle and unexpected
complication. Consider the following program. When run, it displays the value
12, rather than 1:
            enum Values
    {
        ZERO,
        ONE
    };
    
    template <typename Tp>
    Tp operator+(Tp const &lhs, Tp const &rhs)
    {
        return static_cast<Tp>(12);
    };
    
    int main()
    {
        cout << (ZERO + ONE);       // shows 12
    }

This complication can be avoided by defining the operators in their own
namespace, but then all classes using the binary operator also have to be
defined in that namespace, which is not a very attractive
restriction. Fortunately, there is a better alternative: using the CRTP
(cf. section 22.12).


26.3.2: The CRTP and defining operator function templates

    When deriving classes from a class template Binops, using the CRTP the
operators are defined for arguments of the class Binops<Derived>: a base
class receiving the derived class as its template argument. 

Thus the class Binops as well as the additional operators are defined,
expecting Binops<Derived> type of arguments:
            template <class Derived>
    struct Binops
    {
        Derived &operator+=(Derived const &rhs) &;
    };

    template <typename Derived>
    Derived operator+(Binops<Derived> const &lhs, Derived const &rhs)
    {
        return Derived{static_cast<Derived const &>(lhs) } += rhs;
    }
    // analogous implementation for Binops<Derived> &&lhs

This way, a class that derives from Binops, and that provides an
operator+= member which is bound to an rvalue reference object, suddenly
also provides all other binary addition operators:
            class Derived: public Binops<Derived>
    {
        ...
        public:
            ...
            Derived &&operator+=(Derived const &rhs) &&
    };

All, but one.... 

The operator that's not available is the compound addition operator,
bound to an lvalue reference. As its function name is identical to the one in
the class Derived, it is not automatically visible at the user level.

Although this problem can simply be solved by providing the class Derived
with a using Binops<Derived>::operator+= declaration, it is not a very
attractive solution, as separate using declarations have to be provided for
each binary operator that is implemented in the class Derived.

But a much more attractive solution exists. A beautiful out-of-the-box
solution, completely avoiding the hidden base class operator, was proposed
by Wiebe-Marten Wijnja. Wiebe-Marten conjectured that operator+=, bound
to an lvalue reference could also very well be defined as a free
function. In that case no inheritance is used and therefore no function hiding
occurs. Consequently, the using directive can be avoided. 

The implementation of this free operator+= function looks like this:
            template <class Derived>
    Derived &operator+=(Binops<Derived> &lhs, Derived const &rhs) 
    {
        Derived tmp{ Derived{ static_cast<Derived &>(lhs) } += rhs };
        tmp.swap(static_cast<Derived &>(lhs));
        return static_cast<Derived &>(lhs);
    }

The flexibility of this design can be further augmented once we realize that
the right-hand side operand doesn't have to be a Derived class
object. Consider operator<<: oftentimes shifts are bit-shifts, using a
size_t to specify the number of bits to shift. In fact, the type of the
right-hand side operand can completely be generalized by defining a second
template type parameter, which is used to specify the right-hand side's
operand type. It's up to the Derived class to specify the argument type of
its operator+= (or any other binary compound operator), whereafter the
compiler will deduce the types of the right-hand side operands for the
remaining binary operators. Here is the final implementation of the free
operator+= function:
            template <class Derived, typename Rhs>
    Derived &operator+=(Binops<Derived> &lhs, Rhs const &rhs) 
    {
        Derived tmp{ Derived{ static_cast<Derived &>(lhs) } += rhs };
        tmp.swap(static_cast<Derived &>(lhs));
        return static_cast<Derived &>(lhs);
    }


26.3.3: Insertion and extraction

    Classes also frequently define overloaded insertion and extraction
operators. Since there are no `compound insertion operators' the design shown
so far cannot be used when overloading these operators. Instead using
standardized member function signatures is advocated: void
insert(std::ostream &out) const to insert an object into an ostream and
void extract(std::istream &in) const to extract an object from an
istream. As these functions are only used by, respectively, the insertion
and extraction operators, they can be declared in the Derived class's
private interface. Instead of declaring the insertion and extraction operators
friends of the class Derived a single friend Binops<Derived> is
specified. This allows Binops<Derived> to define private, inline iWrap
and eWrap members, merely calling, respectively, Derived's insert and
extract members:
            template <typename Derived>
    inline void Binops<Derived>::iWrap(std::ostream &out) const
    {
        static_cast<Derived const &>(*this).insert(out);
    }

Binops<Derived> then declares the insertion and extraction operators as
its friends, allowing these operators to call, respectively, iWrap and
eWrap. Note that the software engineer designing the class Derived
only has to provide a friend Binops<Derived> declaration. Here is the
implementation of the overloaded insertion operator:
            template <typename Derived>
    std::ostream &operator<<(std::ostream &out, Binops<Derived> const &obj)
    {
        obj.iWrap(out);
        return out;
    }

This completes the coverage of the essentials of a class template Binops
potentially offering binary operators and insertion/extraction operators for
any class derived from Binops. Finally, as noted at the beginning of this
section, a complete implementation of a class offering addition and insertion
operators is provided in the file
annotations/yo/concrete/examples/binopclasses.cc in the C++ Annotations'
source archive.


26.4: Distinguishing lvalues from rvalues with operator[]()

    A problem with operator[] is that it can't distinguish between its
    
 use as an lvalue and as an rvalue. It is a familiar misconception to
think that
            Type const &operator[](size_t index) const

is used as rvalue (as the object isn't modified), and
    that
            Type &operator[](size_t index)

is used as lvalue (as the returned value can be modified).

The compiler, however, distinguishes between the two operators only by the
const-status of the object for which operator[] is called. With
const objects the former operator is called, with non-const objects
the latter is always used. It is always used, irrespective of it being used as
lvalue or rvalue.

Being able to distinguish between lvalues and rvalues can be very
useful. Consider the situation where a class supporting operator[] stores
data of a type that is very hard to copy. With data like that reference
counting (e.g., using shared_ptrs) is probably used to prevent needless
copying.

As long as operator[] is used as rvalue there's no need to copy the data,
but the information must be copied if it is used as lvalue.

The Proxy Design Pattern (cf. Gamma et al. (1995)) can
be used to distinguish between lvalues and rvalues. With the Proxy Design
Pattern an object of another class (the Proxy class) is used to act as a
stand in for the `real thing'. The proxy class offers functionality that
cannot be offered by the data themselves, like distinguishing between its use
as lvalue or rvalue. A proxy class can be used in many situations where access
to the real data cannot or should not be directly provided. In this regard
iterator types are examples of proxy classes as they create a layer
between the real data and the software using the data. Proxy classes could
also dereference pointers in a class storing its data by pointers.

In this section we concentrate on the distinction between using operator[]
as lvalue and rvalue. Let's assume we have a class Lines storing lines
from a file. Its constructor expects the name of a stream from which the
lines are read and it offers a non-const operator[] that can be used as
lvalue or rvalue (the const version of operator[] is omitted as it
causes no confusion because it is always used as rvalue):
            class Lines
    {
        std::vector<std::string> d_line;

        public:
            Lines(std::istream &in);
            std::string &operator[](size_t idx);
    };


To distinguish between lvalues and rvalues we must find distinguishing
characteristics of lvalues and rvalues that we can exploit. Such
distinguishing characteristics are operator= (which is always used as
lvalue) and the conversion operator (which is always used as rvalue). Rather
than having operator[] return a string & we can let it return a
Proxy object that is able to distinguish between its use as lvalue
and rvalue.

The class Proxy thus needs operator=(string const &other) (acting as
lvalue) and operator std::string const &() const (acting as rvalue). Do we
need more operators? The std::string class also offers operator+=, so
we should probably implement that operator as well. Plain characters can also
be assigned to string objects (even using their numeric values). As
string objects cannot be constructed from plain characters
promotion cannot be used with operator=(string const &other) if the
right-hand side argument is a character. Implementing operator=(char
value) could therefore also be considered. These additional operators are
left out of the current implementation but `real life' proxy classes should
consider implementing these additional operators as well. Another subtlety is
  
that Proxy's operator std::string const &() const is not used
when using ostream's insertion operator or istream's extraction
operator as these operators are implemented as templates not recognizing our
Proxy class type. So when stream insertion and extraction is required (it
probably is) then Proxy must be given its own overloaded insertion and
extraction operator. Here is an implementation of the overloaded insertion
operator inserting the object for which Proxy is a stand-in:
        inline std::ostream &operator<<(std::ostream &out, Lines::Proxy const &proxy)
{
    return out << static_cast<std::string const &>(proxy);
}


There's no need for any code (except Lines) to create or copy Proxy
objects. Proxy's constructor should therefore be made private, and
Proxy can declare Lines to be its friend. In fact, Proxy is
intimately related to Lines and can be defined as a nested class. In the
revised Lines class operator[] no longer returns a string but
instead a Proxy is returned. Here is the revised Lines class,
including its nested Proxy class:
            class Lines
    {
        std::vector<std::string> d_line;

        public:
            class Proxy;
            Proxy operator[](size_t idx);
            class Proxy
            {
                friend Proxy Lines::operator[](size_t idx);
                std::string &d_str;
                Proxy(std::string &str);
                public:
                    std::string &operator=(std::string const &rhs);
                    operator std::string const &() const;
            };
            Lines(std::istream &in);
    };


Proxy's members are very lightweight and can usually be implemented
inline:
            inline Lines::Proxy::Proxy(std::string &str)
    :
        d_str(str)
    {}
    inline std::string &Lines::Proxy::operator=(std::string const &rhs)
    {
        return d_str = rhs;
    }
    inline Lines::Proxy::operator std::string const &() const
    {
        return d_str;
    }


The member Lines::operator[] can also be implemented inline: it merely
returns a Proxy object initialized with the string associated with
index idx.

Now that the class Proxy has been developed it can be used in a
program. Here is an example using the Proxy object as lvalue or rvalue. On
the surface Lines objects won't behave differently from Lines objects
using the original implementation, but by adding an identifying cout
statement to Proxy's members it can be shown that operator[] behaves
differently when used as lvalue or as rvalue:
            int main()
    {
        ifstream in("lines.cc");
        Lines lines(in);

        string s = lines[0];        // rvalue use
        lines[0] = s;               // lvalue use
        cout << lines[0] << '\n';   // rvalue use
        lines[0] = "hello world";   // lvalue use
        cout << lines[0] << '\n';   // rvalue use
    }



26.5: Implementing a `reverse_iterator'

In section 22.14.1 the construction of iterators and
reverse iterators was discussed. In that section the iterator was constructed
as an inner class in a class derived from a vector of pointers to strings.

An object of this nested iterator class handles the dereferencing of the
pointers stored in the vector. This allowed us to sort the strings
pointed to by the vector's elements rather than the pointers.

A drawback of this is that the class implementing the iterator is closely
tied to the derived class as the iterator class was implemented as a nested
class. What if we would like to provide any class derived from a container
class storing pointers with an iterator handling pointer-dereferencing?

In this section a variant of the earlier (nested class) approach is
discussed. Here the iterator class is defined as a class template,
    
    not only parameterizing the data type to which the container's elements
point but also the container's iterator type itself. Once again, we
concentrate on developing a RandomIterator as it is the most complex
iterator type.

Our class is named RandomPtrIterator, indicating that it is a random
iterator operating on pointer values. The class template defines three
template type parameters:
    
    o  The first parameter specifies the derived class type
(Class). Like before, RandomPtrIterator's
constructor is private. Therefore friend declarations are needed to
allow client classes to construct RandomPtrIterators.  However, a
friend class Class cannot be used as template parameter types cannot be
    
used in friend class ... declarations. But this is a minor problem as not
every member of the client class needs to construct iterators. In fact, only
Class's begin and end members must construct
iterators. Using the template's first parameter, friend declarations can be
specified for the client's begin and end members.

o  The second template parameter parameterizes the container's iterator
type (BaseIterator);

o  The third template parameter indicates the data type to which the
pointers point (Type).

RandomPtrIterator has one private data member, a
BaseIterator. Here is the class interface and the constructor's
implementation:
            #include <iterator>
    #include <compare>

    template <typename Class, typename BaseIterator, typename Type>
    struct RandomPtrIterator;

    #define PtrIterator RandomPtrIterator<Class, BaseIterator, Type>
    #define PtrIteratorValue RandomPtrIterator<Class, BaseIterator, value_type>

    template <typename Class, typename BaseIterator, typename Type>
    bool operator==(PtrIterator const &lhs, PtrIterator const &rhs);

    template <typename Class, typename BaseIterator, typename Type>
    auto operator<=>(PtrIterator const &lhs, PtrIterator const &rhs);

    template <typename Class, typename BaseIterator, typename Type>
    int operator-(PtrIterator const &lhs, PtrIterator const &rhs);

    template <typename Class, typename BaseIterator, typename Type>
    struct RandomPtrIterator
    {
        using iterator_category = std::random_access_iterator_tag;
        using difference_type   = std::ptrdiff_t;
        using value_type        = Type;
        using pointer           = value_type *;
        using reference         = value_type &;


        friend PtrIterator Class::begin();
        friend PtrIterator Class::end();

        friend bool operator==<>(RandomPtrIterator const &lhs,
                                 RandomPtrIterator const &rhs);
        friend auto operator<=><>(RandomPtrIterator const &lhs,
                                  RandomPtrIterator const &rhs);
        friend int operator-<>(RandomPtrIterator const &lhs,
                               RandomPtrIterator const &rhs);
        private:
            BaseIterator d_current;

        public:
            int operator-(RandomPtrIterator const &rhs) const;
            RandomPtrIterator operator+(int step) const;
            value_type &operator*() const;
            RandomPtrIterator &operator--();
            RandomPtrIterator operator--(int);
            RandomPtrIterator &operator++();
            RandomPtrIterator operator++(int);
            RandomPtrIterator operator-(int step) const;
            RandomPtrIterator &operator-=(int step);
            RandomPtrIterator &operator+=(int step);
            value_type *operator->() const;

        private:
            RandomPtrIterator(BaseIterator const &current);
    };

    template <typename Class, typename BaseIterator, typename value_type>
    PtrIteratorValue::RandomPtrIterator(BaseIterator const &current)
    :
        d_current(current)
    {}

    Looking at its friend declarations, we see that the members begin
and end of a class Class, returning a RandomPtrIterator object for
the types Class, BaseIterator and Type are granted access to
RandomPtrIterator's private constructor.  That is exactly what we
want. The Class's begin and end members are declared as bound
friends.

All RandomPtrIterator's remaining members are public. Since
RandomPtrIterator is just a generalization of the nested class
iterator developed in section 22.14.1, re-implementing the required
member functions is easy and only requires us to change iterator into
RandomPtrIterator and to change std::string into Type. For
example, operator<, defined in the class iterator as
        
    is now implemented as:
            template <typename Class, typename BaseIterator, typename Type>
    inline auto operator<=>(PtrIterator const &lhs, PtrIterator const &rhs)
    {
        return **lhs.d_current <=> **rhs.d_current;
    }


    Some additional examples: operator*, defined in the class
iterator as
        inline std::string &StringPtr::iterator::operator*() const
{
    return **d_current;
}

    is now implemented as:
            template <typename Class, typename BaseIterator, typename value_type>
    value_type &PtrIteratorValue::operator*() const
    {
        return **d_current;
    }

    The pre- and postfix increment operators are now implemented as:
            template <typename Class, typename BaseIterator, typename value_type>
    PtrIteratorValue &PtrIteratorValue::operator++()
    {
        ++d_current;
        return *this;
    }
    template <typename Class, typename BaseIterator, typename value_type>
    PtrIteratorValue PtrIteratorValue::operator++(int)
    {
        return RandomPtrIterator(d_current++);
    }

    Remaining members can be implemented accordingly, their actual
implementations are left as exercises to the reader (or can be obtained from
the cplusplus.yo.zip archive, of course).

Re-implementing the class StringPtr developed in section 22.14.1
is not difficult either. Apart from including the header file defining the
class template RandomPtrIterator, it only requires a single modification.
Its iterator using-declaration must now be associated with a
RandomPtrIterator. Here is the full class interface and the class's inline
member definitions:
            #ifndef INCLUDED_STRINGPTR_H_
    #define INCLUDED_STRINGPTR_H_

    #include <vector>
    #include <string>
    #include "iterator.h"

    class StringPtr: public std::vector<std::string *>
    {
        public:
            using iterator =
                    RandomPtrIterator
                    <
                        StringPtr,
                        std::vector<std::string *>::iterator,
                        std::string
                    >;

            using reverse_iterator = std::reverse_iterator<iterator>;

            iterator begin();
            iterator end();
            reverse_iterator rbegin();
            reverse_iterator rend();
    };

    inline StringPtr::iterator StringPtr::begin()
    {
        return iterator(this->std::vector<std::string *>::begin() );
    }
    inline StringPtr::iterator StringPtr::end()
    {
        return iterator(this->std::vector<std::string *>::end());
    }
    inline StringPtr::reverse_iterator StringPtr::rbegin()
    {
        return reverse_iterator(end());
    }
    inline StringPtr::reverse_iterator StringPtr::rend()
    {
        return reverse_iterator(begin());
    }
    #endif


Including StringPtr's modified header file into the program given in
section 22.14.2 results in a program behaving identically to its
earlier version. In this case StringPtr::begin and StringPtr::end
return iterator objects constructed from a template definition.


26.6: Using `bisonc++' and `flexc++'

    The example discussed below digs into the peculiarities of using
parser- and scanner generators generating C++ sources. Once the
input for a program exceeds a certain level of complexity, it becomes
attractive to use scanner- and parser-generators generating the code which
does the actual input recognition.

The examples in this and subsequent sections assume that the reader knows how
to use the
    scanner generator flex and the parser generator bison. Both
bison and flex are well documented elsewhere. The original
predecessors of bison and flex, called yacc and lex are
described in several books, e.g. in
    
 O'Reilly's book `lex & yacc'.

Scanner- and parser generators are also available as free software. Both
bison and flex are usually part of software distributions or they can
be obtained from
    
    ftp://prep.ai.mit.edu/pub/non-gnu. Flex creates a C++ class
when %option c++ is specified.

For parser generators the program bison is available. In the early 90's
Alain Coetmeur (coetmeur@icdc.fr) created a
C++ variant (bison++) creating a parser class. Although the
bison++ program produces code that can be used in C++ programs it also
shows many characteristics that are more suggestive of a C context than a
C++ context. In January 2005 I rewrote parts of Alain's bison++
program, resulting in the original version of the program
        
    bisonc++. Then, in May 2005 a complete rewrite of the bisonc++
parser generator was completed (version number 0.98). Current versions of
bisonc++ can be downloaded from
    https://fbb-git.gitlab.io/bisoncpp/. Binary versions for various
architectures are available as, e.g., Debian
package (including bisonc++'s documentation).

Bisonc++ creates a cleaner parser class than bison++. In particular,
it derives the parser class from a base-class, containing the parser's token-
and type-definitions as well as all member functions which should not be
(re)defined by the programmer. As a result of this approach, the generated
parser class is very small, declaring only members that are actually defined
by the programmer (as well as some other members, generated by bisonc++
itself, implementing the parser's parse() member). One member that is
not implemented by default is lex, producing the next lexical
token. When the directive %scanner (see section 26.6.2.1) is used,
bisonc++ produces a standard implementation for this member; otherwise it
must be implemented by the programmer.

In early 2012 the program
    
    flexc++ http://flexcpp.org/ reached its initial release. Like
bisonc++ it is part of the Debian linux
distribution.

Jean-Paul van Oosten (jp@jpvanoosten.nl <jp@jpvanoosten.nl>) and Richard Berendsen
(richardberendsen@xs4all.nl <richardberendsen@xs4all.nl>) started the flexc++ project in 2008
and the final program was completed by Jean-Paul and me between 2010 and 2012.

These sections of the C++ Annotations focus on bisonc++ as our
 parser generator and flexc++ as our lexical scanner
generator. Previous releases of the C++ Annotations were using flex as the
scanner generator.

Using flexc++ and bisonc++ class-based scanners and parsers are
generated. The advantage of this approach is that the interface to the scanner
and the parser tends to become cleaner than without using class
interfaces. Furthermore, classes allow us to get rid of most if not all global
variables, making it easy to use multiple parsers in one program.

Below two example programs are developed. The first example only uses
flexc++. The generated scanner monitors the production of a file from
several parts. That example focuses on the lexical scanner and on switching
files while churning through the information. The second example uses both
flexc++ and bisonc++ to generate a scanner and a parser transforming
standard arithmetic expressions to their postfix notations, commonly used in
code generated by compilers and in HP-calculators.  In the second example
the emphasis is mainly on bisonc++ and on composing a scanner object
inside a generated parser.


26.6.1: Using `flexc++' to create a scanner

    The lexical scanner developed in this section is used to monitor the
production of a file from several subfiles. The setup is as follows: the
input-language defines #include directives, followed by a text
string specifying the file (path) which should be included at the location of
the #include.

In order to avoid complexities irrelevant to the current example, the format
of the #include statement is restricted to the form #include
<filepath>.  The file specified between the angle brackets should be
available at the location indicated by filepath. If the file is not
available, the program terminates after issuing an error message.

The program is started with one or two filename arguments. If the program is
started with just one filename argument, the output is written to the
standard output stream cout. Otherwise, the output is written to
the stream whose name is given as the program's second argument.

The program defines a maximum nesting depth. Once this maximum is exceeded,
the program terminates after issuing an error message. In that case, the
filename stack indicating where which file was included is printed.

An additional feature of the program is that (standard C++) comment-lines
are ignored. Include-directives in comment-lines are also ignored.

The program is created in five major steps:
    
    o  First, the file lexer is constructed, containing the
input-language specifications.
    o  From the specifications in lexer the requirements for the
class Scanner evolve. The Scanner class derives from the base class
ScannerBase generated by flexc++.
    o  Next, main is constructed. A Scanner object is created
inspecting the command-line arguments. If successful, the scanner's member
lex is called to produce the program's output.
    o  Now that  the global setup of the program has been specified,
the member functions of the various classes are implemented.
    o  Finally, the program is compiled and linked.


26.6.1.1: The derived class `Scanner'

            The function matching the regular expression rules (lex) is a
member of the class Scanner. Since Scanner is derived from
ScannerBase, it has access to all of ScannerBase's protected members
that execute the lexical scanner's regular expression matching algorithm.

Looking at the regular expressions themselves, notice that we need rules
to recognize comment, #include directives, and all remaining characters.
This all is fairly standard practice. When an #include directive is
sensed, the directive is parsed by the scanner. This too is common
practice. Our lexical scanner performs the following tasks:
    
    o  As usual, preprocessor directives are not
analyzed by a parser, but by the lexical scanner;
    o  The scanner uses a mini scanner to extract the filename from the
directive, throwing a exception if this fails;
    o  If the filename could be extracted, processing switches to the next
stream, controlling for a maximum nesting depth.
    o  Once the end of the current file has been reached processing
automatically returns to the previous file, restoring the previous file name
and line number. The scanner returns 0 if all files have been processed.


26.6.1.2: The lexical scanner specification file

            The lexical scanner specification file is organized comparably to the
one used for flex in C contexts. However, in C++ contexts,
flexc++ creates a class Scanner, rather than just a scanner function.

Flexc++'s specification file consists of two sections:
    
    o  The specification file's first section is flexc++'s
        symbol area, used to define symbols, like a mini scanner, or
options. The following options are suggested:
        
        o  %debug: includes debugging
            
        code into the code generated by flexc++. Calling the member
function
            
    setDebug(true) activates this debugging code at run-time. When
activated, information about the matching process is written to the
standard output stream. The execution of debug code is suppressed after
calling the member function setDebug(false).
        o  %filenames: defines the base-name of the class header files
generated by flexc++. By default the class name (itself using the default
Scanner) is used.

Here is the specification files' symbol area:
            %filenames scanner
%debug
%max-depth 3

%x      comment
%x      include

    o  The specification file's second section is a rules section in
which the regular expressions and their associated actions are defined.
    In the example developed here,  the lexer should
copy information from the standard input stream (std::cin) to the
standard output stream (std::cout). For this  the predefined macro
ECHO can be used.  Here are the rules:
            %%
    // The comment-rules: comment is ignored.
"//".*                  // ignore eoln comment
"/*"                    begin(StartCondition__::comment);
<comment>{
    .|\n                // ignore all characters in std C comment
    "*/"                begin(StartCondition__::INITIAL);
}

    // File switching: #include <filepath>
#include[ \t]+"<"      begin(StartCondition__::include);
<include>{
    [^ \t>]+            d_nextSource = matched();
    ">"[ \t]*\n         switchSource();
    .|\n                throw runtime_error("Invalid include statement");
}

    // The default rule: echo anything else to std::cout
.|\n                    echo();



26.6.1.3: Implementing `Scanner'

            The class Scanner is generated once by flexc++. This class has
access to several members defined by its base class ScannerBase. Some of
these members have public access rights and can be used by code external to
the class Scanner. These members are extensively documented in the
flexc++(1) man-page, and the reader is referred to this man-page for
further information.

Our scanner performs the following tasks:
    
    o  it matches regular expressions, ignoring comment, and writing the
matched text to the standard output stream;
    o  it switches to other files, and returns to the previous file once a
file has completely been processed, ending the lexical scan once the end of
the first input file has been reached.

The #include statements in the input allow the scanner to distill the
name of the file where the scanning process must continue. This file name is
stored in a local variable d_nextSource and a member stackSource
handles the switch to the next source. Nothing else is required. Pushing and
popping input files is handled by the scanner's members pushStream and
popStream, provided by flexc++. Scanner's interface, therefore,
only needs one additional function declaration: switchSource.

Switching streams is handled as follows: once the scanner has extracted a
filename from an #include directive, a switch to another file is realized
by switchSource.  This member calls pushStream, defined by
flexc++, to stack the current input stream and to switch to the stream
whose name is stored in d_nextSource. This also ends the include
mini-scanner, so to return the scanner to its default scanning mode
begin(StartCondition__::INITIAL) is called. Here is its source:
        #include "scanner.ih"

void Scanner::switchSource()
{
    pushStream(d_nextSource);
    begin(StartCondition__::INITIAL);
}

    The member pushStream, defined by flexc++, handles all necessary
checks, throwing an exception if the file could not be opened or if too many
files are stacked.

The member performing the lexical scan is defined by flexc++ in
Scanner::lex, and this member can be called by code to process the tokens
returned by the scanner.


26.6.1.4: Using a `Scanner' object

            The program using our Scanner is very simple. It expects a filename
indicating where to start the scanning process.

The program first checks the number of arguments. If at least one argument was
given, then that argument is passed to Scanner's constructor, together
with a second argument "-", indicating that the output should go to the
standard output stream.

If the program receives more than one argument debug output, extensively
documenting the lexical scanner's actions, is written to the standard output
stream as well.

Next the Scanner's lex member is called. If anything fails, a
std::exception is thrown, which is caught by main's try-block's catch
clause. Here is the program's source:
        #include "lexer.ih"

int main(int argc, char **argv)
try
{
    if (argc == 1)
    {
        cerr << "Filename argument required\n";
        return 1;
    }

    Scanner scanner(argv[1], "-");

    scanner.setDebug(argc > 2);

    return scanner.lex();
}
catch (exception const &exc)
{
    cerr << exc.what() << '\n';
    return 1;
}



26.6.1.5: Building the program

            The final program is constructed in two steps. These steps are given for a
Unix system, on which flexc++ and the GNU C++ compiler g++
have been installed:
    
    o  First, the lexical scanner's source is created using flexc++. For
this the following command can be given:
            flexc++ lexer

o  Next, all sources are compiled and linked:
            g++ -Wall *.cc

Flexc++ can be downloaded from
    
    https://fbb-git.gitlab.io/flexcpp/, 
        and requires the bobcat library, which can be downloaded from
    http://fbb-git.gitlab.io/bobcat/.


26.6.2: Using `bisonc++' and `flexc++'

    Once an input language exceeds a certain level of complexity, a parser
is often used to control the complexity of the language. In this case, a
parser generator can be used to generate the code verifying the input's
grammatical correctness.  The lexical scanner (preferably composed into the
parser) provides chunks of the input, called tokens. The parser
then processes the series of tokens generated by the lexical scanner.

Starting point when developing programs that use both parsers and scanners is
the grammar. The grammar defines a set of tokens that can be returned
by the lexical scanner (called the scanner below).

Finally, auxiliary code is provided to `fill in the blanks': the actions
performed by the parser and by the scanner are not normally specified
literally in the grammar rules or lexical regular expressions, but
should be implemented in member functions, called from  the parser's
rules or which are associated with the scanner's regular expressions.

In the previous section we've seen an example of a C++ class generated by
flexc++. In the current section we concentrate on the parser. The parser
can be generated from a grammar specification file, processed by the program
bisonc++. The grammar specification file required by bisonc++ is
similar to the file processed by bison (or bison++, bisonc++'s
predecessor, written in the early nineties by
     Alain Coetmeur).

In this section a program is developed converting infix expressions,
where binary operators are written between their operands, to postfix
expressions, where operators are written behind their operands. Also, the
unary operator - is converted from its prefix notation to a postfix form.
The unary + operator is ignored as it requires no further actions. In
essence our little calculator is a micro compiler, transforming numeric
expressions into assembly-like instructions.

Our calculator recognizes a rather basic set of operators:
multiplication, addition, parentheses, and the unary minus. We'll
distinguish real numbers from integers, to illustrate a subtlety in
bison-like grammar specifications. That's all. The purpose of this section is,
after all, to illustrate the construction of a C++ program that uses both
a parser and a lexical scanner, rather than to construct a full-fledged
calculator.

In the coming sections we'll develop the grammar specification for
bisonc++. Then, the regular expressions for the scanner are
specified. Following that, the final program is constructed.


26.6.2.1: The `bisonc++' specification file

        The grammar specification file required by bisonc++ is comparable to
the specification file required by bison. Differences are related to the
class nature of the resulting parser. Our calculator distinguishes real
numbers from integers, and supports a basic set of arithmetic operators.

Bisonc++ should be used as follows:
    
    o  As usual, a grammar is  defined. With bisonc++ this is no
        different, and bisonc++ grammar definitions are for all practical
        purposes identical to bison's grammar definitions.
    o  Having specified the grammar and (usually) some declarations
        bisonc++ can generate files defining the parser class and
        the implementation of the member function parse.
    o  All class members (except those that are required for the proper
        functioning of the member parse) must be
        separately implemented.  Of course, they should also be
        declared in the parser class's header.  At the very least the member
        lex must be implemented. This member is called by parse to
        obtain the next available token.  However, bisonc++ offers a
        facility providing a standard implementation of the function
        lex. The member function
            error(char const *msg)
       is given a simple default implementation that may be modified by the
        programmer. The member function error is called when parse
        detects (syntactic) errors.
    o  The parser can now be used in a program. A very simple example would
        be:
    int main()
{
    Parser parser;
    return parser.parse();
}

The bisonc++  specification file has two
sections:
    
    o  The declaration section. In this section bison's tokens, and the
        priority rules for the operators are declared. However, bisonc++
        also supports several new declarations.  These new declarations are
        important and are discussed below.
    o  The rules section. The grammatical rules define the
        grammar. This section is identical to the one required by bison,
        albeit that some members that were available in bison and
        bison++ are obsolete in bisonc++, while other members can be
        used in a wider context.  For example, ACCEPT and ABORT can be
        called from any member called from the parser's action blocks to
        terminate the parsing process.

Readers familiar with bison may note that there is no
    header section anymore. Header sections are used by bison to provide
for the necessary declarations allowing the compiler to compile the C
function generated by bison. In C++ declarations are part of or
already used by class definitions. Therefore, a parser generator generating a
C++ class and some of its member functions does not require a header
section anymore.


The declaration section

            The declaration section contains several sets of declarations, among
which definitions of all the tokens used in the grammar and the priorities and
associativities of the mathematical operators. Moreover, several new and
important specifications can be used here as well. Those relevant to our
current example and only available in bisonc++ are discussed here. The
reader is referred to bisonc++'s man-page for a full description.
    
    o  %baseclass-preinclude header

          Use header as the pathname to the file pre-included in the
           parser's base-class header.  This declaration is useful in
           situations where the base class header file refers to types which
           might not yet be known. E.g., with %union a std::string *
           field might be used. Since the class std::string might not yet
           be known to the compiler once it processes the base class header
           file we need a way to inform the compiler about these classes and
           types. The suggested procedure is to use a pre-include header file
           declaring the required types. By default header is
           surrounded by double quotes (using, e.g., #include "header").
           When the argument is surrounded by angle brackets #include
           <header> is included. In the latter case, quotes might be
           required to escape interpretation by the shell (e.g., using -H
           '<header>').
    o  %filenames header 

           Defines the generic name of all generated files, unless overridden
            by specific names. By default the generated files use the
           class-name as the generic file name.
    o  %scanner header

           Use header as the pathname to the file pre-included in the
            parser's class header. This file should define a class
            Scanner, offering a member int lex() producing the next
            token from the input stream to be analyzed by the parser generated
            by bisonc++. When this option is used the parser's member
            int lex() is predefined as (assuming the default parser class
            name Parser is used):
            inline int Parser::lex()
{
    return d_scanner.lex();
}

and an object Scanner d_scanner is composed into the
            parser. The d_scanner object is constructed by its
            default constructor. If another constructor is required, the
            parser class may be provided with an appropriate (overloaded)
            parser constructor after having constructed the default parser
            class header file using bisonc++.  By default header is
            surrounded by double quotes (using, e.g., #include
            "header").  When the argument is surrounded by angle brackets
            #include <header> is included.
    o  %stype typename 

           The type of the semantic value of tokens.  The specification
            typename should be the name of an unstructured type (e.g.,
            size_t). By default it is int. See YYSTYPE in
            bison.  It should not be used if a %union specification is
            used.  Within the parser class, this type may be used as
            STYPE.
    o  %union union-definition 

           Acts identically to the bison declaration.  As with bison
            this generates a union for the parser's semantic type.  The union
            type is named STYPE. If no %union is declared, a simple
            stack-type may be defined using the %stype declaration. If no
            %stype declaration is used, the default stacktype (int) is
            used.

An example of a %union declaration is:
            %union
    {
        int     i;
        double  d;
    };

In pre-C++11 code a union cannot contain objects as its fields, as
constructors cannot be called when a union is created.  This means that
     a string cannot be a member of the
union. A string *, however, is a possible union member.  It might also
be possible to use unrestricted unions (cf. section 9.9), having
class type objects as fields.

As an aside: the scanner does not have to know about such a union. It
can simply pass its scanned text to the parser through its matched member
function. For example using a statement like
            $$.i = A2x(d_scanner.matched());

matched text is converted to a value of an appropriate type.

Tokens and non-terminals can be associated with union fields. This is
strongly advised, as it prevents type mismatches, since the compiler may then
check for type correctness. At the same time, the bison specific
variables $$, $1, $2, etc. may be used, rather than the full field
specification (like $$.i).  A non-terminal or a token may be associated
with a union field using the
    <fieldname> specification. E.g.,
            %token <i> INT          // token association (deprecated, see below)
           <d> DOUBLE
    %type  <i> intExpr      // non-terminal association

In the example developed here, both the tokens and the non-terminals can
be associated with a union field. However, as noted before, the scanner does
not have to know about all this. In our opinion, it is cleaner to let the
scanner do just one thing: scan texts. The parser, knowing what the input
is all about, may then convert strings like "123" to an integer
value. Consequently, the association of a union field and a token is
discouraged. Below, while describing the grammar's rules, this is further
illustrated.

In the %union discussion the %token and %type specifications
should be noted. They are used to specify the tokens (terminal symbols) that
can be returned by the scanner, and to specify the return types of
non-terminals.  Apart from %token the token declarators %left,
%right, and %nonassoc can be used to specify the associativity of
operators. The tokens mentioned at these indicators are interpreted as tokens
indicating operators, associating in the indicated direction. The precedence
of operators is defined by their order: the first specification has the lowest
priority. To overrule a certain precedence in a certain context %prec can
be used. As all this is standard bisonc++ practice, it isn't further
elaborated here. The documentation provided with bisonc++'s distribution
should be consulted for further reference.

Here is the specification of the calculator's declaration section:
        %filenames parser
%scanner ../scanner/scanner.h

%union {
    int i;
    double d;
};

%token  INT DOUBLE

%type   <i> intExpr
%type   <d> doubleExpr

%left   '+'
%left   '*'
%right  UnaryMinus

    In the declaration section %type specifiers are used, associating the
intExpr rule's value (see the next section) to the i-field of the
semantic-value union, and associating doubleExpr's value to the
d-field. This approach, admittedly, is rather complex, as expression rules
must be included for each of the supported union types. Alternatives are
definitely possible, and involve the use of polymorphic semantic
values, covered in detail in the 
    Bisonc++ user guide.


The grammar rules

            The rules and actions of the grammar are specified as usual. The grammar
for our little calculator is given below. There are quite a few rules, but
they illustrate various features offered by bisonc++. In particular, note
that no action block requires more than a single line of code. This keeps the
grammar simple, and therefore enhances its readability and
understandability. Even the rule defining the parser's proper termination (the
empty line in the line rule) uses a single member function called
done. The implementation of that function is simple, but it is worth while
noting that it calls Parser::ACCEPT, showing that ACCEPT can be called
indirectly from a production rule's action block. Here are the grammar's
production rules:
            lines:
        lines
        line
    |
        line
    ;

    line:
        intExpr
        '\n'
        {
            display($1);
        }
    |
        doubleExpr
        '\n'
        {
            display($1);
        }
    |
        '\n'
        {
            done();
        }
    |
        error
        '\n'
        {
            reset();
        }
    ;

    intExpr:
        intExpr '*' intExpr
        {
            $$ = exec('*', $1, $3);
        }
    |
        intExpr '+' intExpr
        {
            $$ = exec('+', $1, $3);
        }
    |
        '(' intExpr ')'
        {

            $$ = $2;
        }
    |
        '-' intExpr         %prec UnaryMinus
        {
            $$ = neg($2);
        }
    |
        INT
        {
            $$ = convert<int>();
        }
    ;

    doubleExpr:
        doubleExpr '*' doubleExpr
        {
            $$ = exec('*', $1, $3);
        }
    |
        doubleExpr '*' intExpr
        {
            $$ = exec('*', $1, d($3));
        }
    |
        intExpr '*' doubleExpr
        {
            $$ = exec('*', d($1), $3);
        }
    |
        doubleExpr '+' doubleExpr
        {
            $$ = exec('+', $1, $3);
        }
    |
        doubleExpr '+' intExpr
        {
            $$ = exec('+', $1, d($3));
        }
    |
        intExpr '+' doubleExpr
        {
            $$ = exec('+', d($1), $3);
        }
    |
        '(' doubleExpr ')'
        {
            $$ = $2;
        }
    |
        '-' doubleExpr         %prec UnaryMinus
        {

            $$ = neg($2);
        }
    |
        DOUBLE
        {
            $$ = convert<double>();
        }
    ;

    This grammar is used to implement a simple calculator in which integer and
real values can be negated, added, and multiplied and in which standard
priority rules can be overruled by parentheses. The grammar shows the use of
typed nonterminal symbols: doubleExpr is linked to real (double) values,
intExpr is linked to integer values. Precedence and type association is
defined in the parser's definition section.


The Parser's header file

        Several class members called from the grammar are defined as member
templates. Bisonc++ generates multiple files, among which the file
defining the parser's class. Functions called from the production rule's
action blocks are usually member functions of the parser. These member
functions must be declared and defined. Once bisonc++ has generated the
header file defining the parser's class, that header file isn't automatically
rewritten, allowing the programmer to add new members to the parser class
whenever required. Here is `parser.h' as used in our little calculator:
        #ifndef Parser_h_included
#define Parser_h_included

#include <iostream>
#include <sstream>
#include <bobcat/a2x>

#include "parserbase.h"
#include "../scanner/scanner.h"

#undef Parser
class Parser: public ParserBase
{
    std::ostringstream d_rpn;
    // $insert scannerobject
    Scanner d_scanner;

    public:
        int parse();

    private:
        template <typename Type>
        Type exec(char c, Type left, Type right);

        template <typename Type>
        Type neg(Type op);

        template <typename Type>
        Type convert();

        void display(int x);
        void display(double x);
        void done() const;
        void reset();
        void error(char const *msg);
        int lex();
        void print();

        static double d(int i);

    // support functions for parse():

        void executeAction(int d_ruleNr);
        void errorRecovery();
        int lookup(bool recovery);
        void nextToken();
        void print__();
};

inline double Parser::d(int i)
{
    return i;
}

template <typename Type>
Type Parser::exec(char c, Type left, Type right)
{
    d_rpn << " " << c << " ";
    return c == '*' ? left * right : left + right;
}

template <typename Type>
Type Parser::neg(Type op)
{
    d_rpn << " n ";
    return -op;
}

template <typename Type>
Type Parser::convert()
{
    Type ret = FBB::A2x(d_scanner.matched());
    d_rpn << " " << ret << " ";
    return ret;
}

inline void Parser::error(char const *msg)
{
    std::cerr << msg << '\n';
}

inline int Parser::lex()
{
    return d_scanner.lex();
}

inline void Parser::print()
{}

#endif



26.6.2.2: The `flexc++' specification file

            The flex-specification file used by the calculator is simple: blanks are
ignored, single characters are returned, and numeric values are returned as
either Parser::INT or Parser::DOUBLE tokens.

The flexc++ directive %interactive is provided since the
calculator is a program actively interacting with its human user.

Here is the complete flexc++ specification file:
        %interactive
%filenames scanner

%%

[ \t]                       // ignored

[0-9]+                      return Parser::INT;

"."[0-9]+                   |
[0-9]+"."[0-9]*             return Parser::DOUBLE;

.|\n                        return matched()[0];



26.6.2.3: Building the program

        The calculator is built using bisonc++ and flexc++.  Here is the
implementation of the calculator's main function:
        #include "parser/parser.h"

using namespace std;

int main()
{
    Parser parser;

    cout << "Enter (nested) expressions containing ints, doubles, *, + and "
            "unary -\n"
            "operators. Enter an empty line to stop.\n";

    return parser.parse();
}


The parser's files parse.cc and parserbase.h are generated by the
command:
            bisonc++ grammar

The file parser.h is created only once, to allow the developer to add
members to the Parser class occe the need for them arises.

The program flexc++ is used to create a lexical scanner:
            flexc++ lexer

On Unix systems a command like
            g++ -Wall -o calc *.cc -lbobcat -s

can be used to compile and link the source of the main program and the
sources produced by the scanner and parser generators. The example uses the
A2x class, which is part of the bobcat library (cf. section
26.6.1.5) (the bobcat library is available on systems offering either
bisonc++ or flexc++). Bisonc++ can be downloaded from


    http://fbb-git.gitlab.io/bisoncpp/.


