# std::ios_base

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< cpp‎ | io

Defined in header `<ios>` |  |
class ios_base; |  |

The class `ios_base` is a multipurpose class that serves as the base class for all I/O stream classes. It maintains several kinds of data:

1) state information: stream status flags
2) control information: flags that control formatting of both input and output sequences and the imbued locale
3) private storage: indexed extensible data structure that allows both long and void* members, which may be implemented as two arbitrary-length arrays or a single array of two-element structs or another container.
4) callbacks: arbitrary number of user-defined functions to be called from imbue(), copyfmt(), and ~ios_base()

Typical implementation holds member constants corresponding to all values of fmtflags, iostate, openmode, and seekdir shown below, member variables to maintain current precision, width, and formatting flags, the exception mask, the buffer error state, a resizeable container holding the callbacks, the currently imbued locale, the private storage, and a static integer variable for xalloc().

### Member functions

(constructor) | constructs the object (protected member function)
(destructor) [virtual] | destructs the object (virtual public member function)

##### Formatting

flags | manages format flags (public member function)
setf | sets specific format flag (public member function)
unsetf | clears specific format flag (public member function)
precision | manages decimal precision of floating point operations (public member function)
width | manages field width (public member function)

##### Locales

imbue | sets locale (public member function)
getloc | returns current locale (public member function)

##### Internal extensible array

xalloc [static] | returns a program-wide unique integer that is safe to use as index to pword() and iword() (public static member function)
iword | resizes the private storage if necessary and access to the long element at the given index (public member function)
pword | resizes the private storage if necessary and access to the void* element at the given index (public member function)

##### Miscellaneous

register_callback | registers event callback function (public member function)
sync_with_stdio [static] | sets whether C++ and C IO libraries are interoperable (public static member function)

### Member classes

failure | stream exception (public member class)
Init | initializes standard stream objects (public member class)

### Member types and constants

Type | Explanation

openmode

stream open mode type

The following constants are also defined:

Constant | Explanation
`app` | seek to the end of stream before each write
`binary` | open in binary mode
`in` | open for reading
`out` | open for writing
`trunc` | discard the contents of the stream when opening
`ate` | seek to the end of stream immediately after open

(typedef)

fmtflags

formatting flags type

The following constants are also defined:

Constant | Explanation
`dec` | use decimal base for integer I/O: see std::dec
`oct` | use octal base for integer I/O: see std::oct
`hex` | use hexadecimal base for integer I/O: see std::hex
`basefield` | dec|oct|hex|0. Useful for masking operations
`left` | left adjustment (adds fill characters to the right): see std::left
`right` | right adjustment (adds fill characters to the left): see std::right
`internal` | internal adjustment (adds fill characters to the internal designated point): see std::internal
`adjustfield` | left|right|internal. Useful for masking operations
`scientific` | generate floating point types using scientific notation, or hex notation if combined with fixed: see std::scientific
`fixed` | generate floating point types using fixed notation, or hex notation if combined with scientific: see std::fixed
`floatfield` | scientific|fixed|(scientific|fixed)|0. Useful for masking operations
`boolalpha` | insert and extract bool type in alphanumeric format: see std::boolalpha
`showbase` | generate a prefix indicating the numeric base for integer output, require the currency indicator in monetary I/O: see std::showbase
`showpoint` | generate a decimal-point character unconditionally for floating-point number output: see std::showpoint
`showpos` | generate a + character for non-negative numeric output: see std::showpos
`skipws` | skip leading whitespace before certain input operations: see std::skipws
`unitbuf` | flush the output after each output operation: see std::unitbuf
`uppercase` | replace certain lowercase letters with their uppercase equivalents in certain output output operations: see std::uppercase

(typedef)

iostate

state of the stream type

The following constants are also defined:

Constant | Explanation
`goodbit` | no error
`badbit` | irrecoverable stream error
`failbit` | input/output operation failed (formatting or extraction error)
`eofbit` | associated input sequence has reached end-of-file

(typedef)

seekdir

seeking direction type

The following constants are also defined:

Constant | Explanation
`beg` | the beginning of a stream
`end` | the ending of a stream
`cur` | the current position of stream position indicator

(typedef)

event | specifies event type (enum)
event_callback | callback function type (typedef)

##### Deprecated member types

Type | Explanation
`io_state`(deprecated) | integer type that may be used like `iostate`
`open_mode`(deprecated) | integer type that may be used like `openmode`
`seek_dir`(deprecated) | integer type that may be used like `seekdir`
`streamoff`(deprecated) | unspecified type that may be used like `off_type`, not necessarily std::streamoff
`streampos`(deprecated) | unspecified type that may be used like `pos_type`, not necessarily std::streampos

(until C++17)

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