# Declarations

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Declarations introduce (or re-introduce) names into the C++ program. Each kind of entity is declared differently. Definitions are declarations that are sufficient to use the entity identified by the name.

A declaration is one of the following:

- Function definition
- Template declaration
- Explicit template instantiation
- Explicit template specialization
- Namespace definition
- Linkage specification
- Attribute declaration (attr `;`) (since C++11)
- Empty declaration (`;`) (since C++11)
- A function declaration without a decl-specifier-seq:

attr(optional) declarator `;` |  |

attr (since C++11) | - | sequence of any number of attributes
declarator | - | A function declarator.

  This declaration must declare a constructor, destructor, or user-defined type conversion function. It can only be used as part of a template declaration, explicit specialization, or explicit instantiation.

- Block declaration (a declaration that can appear inside a block), which, in turn, can be one of the following:

  - asm definition
  - type alias declaration (since C++11)
  - namespace alias definition
  - using declaration
  - using directive
  - static_assert declaration (since C++11)
  - opaque enum declaration (since C++11)
  - simple declaration

### Simple declaration

A simple declaration is a statement that introduces, creates, and optionally initializes one or several identifiers, typically variables.

decl-specifier-seq init-declarator-list(optional) `;` | (1) |
attr decl-specifier-seq init-declarator-list`;` | (2) |

attr (since C++11) | - | sequence of any number of attributes
decl-specifier-seq | - | sequence of specifiers (see below).
init-declarator-list | - | comma-separated list of declarators with optional initializers. init-declarator-list is optional when declaring a named class/struct/union or a named enumeration

A structured binding declaration is also a simple declaration. (since C++17)

### Specifiers

Declaration specifiers (decl-specifier-seq) is a sequence of the following whitespace-separated specifiers, in any order:

- typedef specifier. If present, the entire declaration is a typedef declaration and each declarator introduces a new type name, not an object or a function.
- function specifier (inline, virtual, explicit), only allowed in function declarations

- the inline specifier is also allowed on variable declarations.

(since C++17)

- friend specifier, allowed in class and function declarations.
- constexpr specifier, only allowed in variable definitions, function and function template declarations, and the declaration of static data members of literal type.
- storage class specifier (register, static, thread_local (since C++11), extern, mutable). Only one storage class specifier is allowed, except that `thread_local` may appear together with `extern` or `static`.
- Type specifiers (type-specifier-seq), a sequence of specifiers that names a type. The type of every entity introduced by the declaration is this type, optionally modified by the declarator (see below). This sequence of specifiers is also used by type-id. Only the following specifiers are part of type-specifier-seq, in any order:

  - class specifier
  - enum specifier
  - simple type specifier
    - char, char16_t, char32_t (since C++11), wchar_t, bool, short, int, long, signed, unsigned, float, double, void

    - auto
    - decltype specifier

(since C++11)

    - previously declared class name (optionally qualified)
    - previously declared enum name (optionally qualified)
    - previously declared typedef-name or type alias (since C++11) (optionally qualified)
    - template name with template arguments (optionally qualified, optionally using template disambiguator)

    - template name without template arguments (optionally qualified): see class template deduction

(since C++17)

  - elaborated type specifier
    - the keyword class, struct, or union, followed by the identifier (optionally qualified), previously defined as the name of a class, struct, or union.
    - the keyword class, struct, or union, followed by template name with template arguments (optionally qualified, optionally using template disambiguator), previously defined as the name of a class template.
    - the keyword enum followed by the identifier (optionally qualified), previously declared as the name of an enumeration.
  - typename specifier
  - cv qualifier
  only one type specifier is allowed in a decl-specifier-seq, with the following exceptions:
  - `const` can be combined with any type specifier except itself.
  - `volatile` can be combined with any type specifier except itself.
  - `signed` or `unsigned` can be combined with `char`, `long`, `short`, or `int`.
  - `short` or `long` can be combined with `int`.
  - `long` can be combined with `double`.

  - `long` can be combined with `long`.

(since C++11)

Attributes may appear in decl-specifier-seq, in which case they apply to the type determined by the preceding specifiers.

The only specifier that is allowed to appear twice in a decl-specifier-seq is `long` (which can appear twice in a row). All other repeats, such as `const static const`, or `virtual inline virtual` are errors. | (since C++17)

### Structured binding declaration

attr(optional) cv-auto ref-operator(optional) `[` identifier-list `]` `=` expression `;` | (1) | (since C++17)
attr(optional) cv-auto ref-operator(optional) `[` identifier-list `]` `{` expression `}` `;` | (2) | (since C++17)
attr(optional) cv-auto ref-operator(optional) `[` identifier-list `]` `(` expression `)` `;` | (3) | (since C++17)

attr | - | sequence of any number of attributes
cv-auto | - | possibly cv-qualified type specifier `auto`
ref-operator | - | either `&` or `&&`
identifier-list | - | list of comma-separated identifiers (structured bindings) introduced by this declaration
expression | - | an expression that does not have the comma operator at the top level, and has either array or non-union class type

A structured binding declaration introduces all identifiers in the identifier-list as names (called structured bindings) and binds them to subobjects or elements of the object returned by expression, as follows:

- First, a uniquely-named variable `e` is introduced as follows:

  - If expression has array type `A` and no ref-operator is present, then `e` has type cv `A`, where cv is the cv-qualifiers in the cv-auto sequence, and each element of `e` is copy- (for (1)) or direct- (for (2,3)) initialized from the corresponding element of expression.
  - Otherwise `e` is defined as if by using its name instead of `[` identifier-list `]` in the declaration.
    Let `E` denote type of the expression e (note that `E` is never a reference type).

- If `E` is an array type whose element type is `T`, then each identifier in identifier-list becomes the name of an lvalue that refers to the corresponding element of the array. The referenced type for each identifier is `T` (note that if `E` is cv-qualified, so is `T`). The number of identifiers must equal the number of array elements:

    int a[2] = {1,2};
    auto f() -> int(&)[2] { return a; }

    auto [x,y] = f(); // creates e[2], copies a into e, then x refers to e[0], y refers to e[1]
    auto& [xr, yr] = f(); // xr refers to a[0], yr refers to a[1]

- Otherwise (if expression has non-union class type),

  - If std::tuple_size<E> names a complete type (e.g. `E` is a specialization of std::pair, std::tuple, or std::array), then first, the identifier `get` is looked up in the scope of `E` by class member access lookup. If that finds at least one declaration, each identifier in identifier-list is initialized by the expression e.get<i>(). Otherwise, each identifier in identifier-list is initialized by the expression get<i>(e), where get is looked up by ADL only, ignoring non-ADL lookup. In these expressions, e is an lvalue if the type of the entity `e` is an lvalue reference and an xvalue otherwise, and <i> is always interpreted as a template parameter list. Each identifier becomes a variable whose type is reference to std::tuple_element<i,E>::type: lvalue reference if its corresponding initializer is an lvalue, rvalue reference otherwise. The referenced type for the i-th identifier is std::tuple_element<i, E>::type. The expression std::tuple_size<E>::value must be a well-formed integer constant expression, and the number of identifiers must equal std::tuple_size<E>::value.

    std::set<string> myset;
    if (auto [iter, success] = myset.insert("Hello"); success)
        do_something_with(iter);

  - Otherwise (if `E` is a type for which std::tuple_size is not defined), every non-static data member of `E` must be a public direct member of `E` or the same unambiguous public base of `E`, and `E` may not have any anonymous union member. Each identifier in identifier-list becomes an lvalue that refers to the next member of `e` in declaration order (bit fields are supported); the type of the lvalue is `cv T_i`, where `cv` is the cv-qualifiers of `E` and `T_i` is the declared type of the i-th member. The referenced type of the i-th identifier is `cv T_i`. The number of identifiers must equal the number of non-static data members.

    struct S {
        int x1 : 2;
        volatile double y1;
    };
    S f();

    const auto [x, y] = f(); // x is a const int lvalue identifying the 2-bit bit field
                             // y is a const volatile double lvalue

The referenced type defined above is the type returned by `decltype` when applied to an unparenthesized structured binding.

(since C++17)

### Declarators

init-declarator-seq is a comma-separated sequence of one or more init-declarators, which have the following syntax:

declarator initializer(optional) | (1) |

declarator | - | the declarator
initializer | - | optional initializer (except where required, such as when initializing references or const objects). See Initialization for details.

Each init-declaractor in a init-declarator sequence S D1, D2, D3; is processed as if it were a standalone declaration with the same specifiers: S D1; S D2; S D3;.

Each declarator introduces exactly one object, reference, function, or (for typedef declarations) type alias, whose type is provided by decl-specifier-seq and optionally modified by operators such as & (reference to) or [] (array of) or () (function returning) in the declarator. These operators can be applied recursively, as shown below.

A declarator is one of the following:

unqualified-id attr(optional) | (1) |
qualified-id attr(optional) | (2) |
`...` identifier attr(optional) | (3) | (since C++11)
`*` attr(optional) cv(optional) declarator | (4) |
nested-name-specifier `*` attr(optional) cv(optional) declarator | (5) |
`&` attr(optional) declarator | (6) |
`&&` attr(optional) declarator | (7) | (since C++11)
noptr-declarator `[` constexpr(optional) `]` attr(optional) | (8) |
noptr-declarator `(` parameter-list `)` cv(optional) ref(optional) except(optional) attr(optional) | (9) |

1) The name that is declared.
2) A declarator that uses a qualified identifier (qualified-id) defines or redeclares a previously declared namespace member or class member.
3) Parameter pack, only appears in function parameter lists for variadic function templates.
4) Pointer declarator: the declaration S * D; declares `D` as a pointer to the type determined by decl-specifier-seq `S`.
5) Pointer to member declaration: the declaration S C::* D; declares `D` as a pointer to member of `C` of type determined by decl-specifier-seq `S`. nested-name-specifier is a sequence of names and scope resolution operators `::`
6) Lvalue reference declarator: the declaration S & D; declares `D` as an lvalue reference to the type determined by decl-specifier-seq `S`.
7) Rvalue reference declarator: the declaration S && D; declares `D` as an rvalue reference to the type determined by decl-specifier-seq `S`.
8) Array declarator. noptr-declarator any valid declarator, but if it begins with *, &, or &&, it has to be surrounded by parentheses.
9) Function declarator. noptr-declarator any valid declarator, but if it begins with *, &, or &&, it has to be surrounded by parentheses. Note that the outermost function declarator may end with the optional trailing return type.

In all cases, attr is an optional sequence of attributes. When appearing immediately after the identifier, it applies to the object being declared.

cv is a sequence of const and volatile qualifiers, where either qualifier may appear at most once in the sequence.

| This section is incomplete Reason: explain declaration name hiding rules; how a variable/function declaration hides a class (but not a typedef) with the same name

### Notes

When a block declaration appears inside a block, and an identifier introduced by a declaration was previously declared in an outer block, the outer declaration is hidden for the remainder of the block.

If a declaration introduces a variable with automatic storage duration, it is initialized when its declaration statement is executed. All automatic variables declared in a block are destroyed on exit from the block (regardless how the block is exited: via exception, goto, or by reaching its end), in order opposite to their order of initialization.

### Examples

Run this code

    class C {
        std::string member; // decl-specifier-seq is "std::string"
                            // declarator is "member"
    } obj, *pObj(&obj);
    // decl-specifier-seq is "class C { std::string member; }"
    // declarator "obj" defines an object of type C
    // declarator "*pObj(&obj)" declares and initializes a pointer to C

    int a = 1, *p = NULL, f(), (*pf)(double);
    // decl-specifier-seq is int
    // declarator a=1 defines and initializes a variable of type int
    // declarator *p=NULL defines and initializes a variable of type int*
    // declarator (f)() declares (but doesn't define)
    //                  a function taking no arguments and returning int
    // declarator (*pf)(double) defines a pointer to function
    //                  taking double and returning int

    int (*(*foo)(double))[3] = NULL;
    // decl-specifier-seq is int
    // 1. declarator "(*(*foo)(double))[3]" is an array declarator:
    //    the type declared is "/nested declarator/ array of 3 int"
    // 2. the nested declarator is "(*(*foo)(double))", which is a pointer declarator
    //    the type declared is "/nested declarator/ pointer to array of 3 int"
    // 3. the nested declarator is "(*foo)(double)", which is a function declarator
    //    the type declared is "/nested declarator/ function taking double and returning
    //        pointer to array of 3 int"
    // 4. the nested declarator is "(*foo)" which is a (parenthesized, as required by
    //        function declarator syntax) pointer declarator.
    //    the type declared is "/nested declarator/ pointer to function taking double
    //        and returning pointer to array of 3 int"
    // 5. the nested declarator is "foo", which is an identifier.
    // The declaration declares the object foo of type "pointer to function taking double
    //     and returning pointer to array of 3 int"
    // The initializer "= NULL" provides the initial value of this pointer.

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