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Declarations - cppreference.com

Declarations are how names are introduced (or re-introduced) into the C++ program. Not all declarations actually declare anything, and 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 (including Partial template specialization)
• Explicit template instantiation
• Explicit template specialization
• Namespace definition
• Linkage specification

• Attribute declaration (attr ;)

• Empty declaration (;)
• A function declaration without a decl-specifier-seq :

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 declaration

• namespace alias definition
• using-declaration
• using directive

• simple declaration

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

init-declarator-list can only be omitted when declaring a named class or a named enumeration.

The syntax of init-declarator is defined as follows:

1) A declarator with an initializer.

2) A declarator without an initializer.

requires-clause can only appear if declarator declares a templated function.

contract-specs can only appear if declarator declares a function or function template.

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

• the 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 specifiers (inline, virtual, explicit), only allowed in function declarations.

• the inline specifier is also allowed on variable declarations.

• the friend specifier, allowed in class and function declarations.

• the constexpr specifier, only allowed in variable definitions, function and function template declarations, and the declaration of static data members of literal type.

• the consteval specifier, only allowed in function and function template declarations.
• the constinit specifier, only allowed in declaration of a variable with static or thread storage duration. At most one of the constexpr, consteval, and constinit specifiers is allowed to appear in a decl-specifier-seq.

• storage class specifier (register, (until C++17) 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(since C++11).
• 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, char8_t, (since C++20) char16_t, char32_t, (since C++11) wchar_t, bool, short, int, long, signed, unsigned, float, double, void

• 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)

• elaborated type specifier

• the keyword class, struct, or union, followed by the identifier (optionally qualified), previously defined as the name of a class.
• 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.

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

Repetitions of any specifier in a decl-specifier-seq, such as const static const, or virtual inline virtual are errors, except that long is allowed to appear twice(since C++11).

Each init-declarator in an init-declarator-list 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:

The

that is declared.

: the declaration

declares

as a pointer to the type determined by

.

: the declaration

declares

as an lvalue reference to the type determined by

.

: the declaration

declares

as an rvalue reference to the type determined by

.

.

any valid declarator, but if it begins with *, &, or &&, it has to be surrounded by parentheses.

.

any valid declarator, but if it begins with *, &, or &&, it has to be surrounded by parentheses.

10) Parenthesized declarator.

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.

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.

Note: this example demonstrates how some complex declarations are parsed in terms of the language grammar. Other popular mnemonics are: the spiral rule, reading inside-out, and declaration mirrors use. There is also an automated parser at https://cdecl.org.

#include <type_traits>
 
struct S
{
    int member;
    // decl-specifier-seq is "int"
    // declarator is "member"
} obj, *pObj(&obj);
// decl-specifier-seq is "struct S { int member; }"
// declarator "obj" declares an object of type S
// declarator "*pObj" declares a pointer to S,
//     and initializer "(&obj)" initializes it
 
int i = 1, *p = nullptr, f(), (*pf)(double);
// decl-specifier-seq is "int"
// declarator "i" declares a variable of type int,
//     and initializer "= 1" initializes it
// declarator "*p" declares a variable of type int*,
//     and initializer "= nullptr" initializes it
// declarator "f()" declares (but doesn't define)
//     a function taking no arguments and returning int
// declarator "(*pf)(double)" declares a pointer to function
//     taking double and returning int
 
int (*(*var1)(double))[3] = nullptr;
// decl-specifier-seq is "int"
// declarator is "(*(*var1)(double))[3]"
// initializer is "= nullptr"
 
// 1. declarator "(*(*var1)(double))[3]" is an array declarator:
//    Type declared is: "(*(*var1)(double))" array of 3 elements
// 2. declarator "(*(*var1)(double))" is a pointer declarator:
//    Type declared is: "(*var1)(double)" pointer to array of 3 elements
// 3. declarator "(*var1)(double)" is a function declarator:
//    Type declared is: "(*var1)" function taking "(double)",
//    returning pointer to array of 3 elements.
// 4. declarator "(*var1)" is a pointer declarator:
//    Type declared is: "var1" pointer to function taking "(double)",
//    returning pointer to array of 3 elements.
// 5. declarator "var1" is an identifier.
// This declaration declares the object var1 of type "pointer to function
// taking double and returning pointer to array of 3 elements of type int"
// The initializer "= nullptr" provides the initial value of this pointer.
 
// C++11 alternative syntax:
auto (*var2)(double) -> int (*)[3] = nullptr;
// decl-specifier-seq is "auto"
// declarator is "(*var2)(double) -> int (*)[3]"
// initializer is "= nullptr"
 
// 1. declarator "(*var2)(double) -> int (*)[3]" is a function declarator:
//    Type declared is: "(*var2)" function taking "(double)", returning "int (*)[3]"
// ...
 
int main()
{
    static_assert(std::is_same_v<decltype(var1), decltype(var2)>);
}

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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