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Declarations

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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:

  • Attribute declaration (attr ;)
(since C++11)
  • Empty declaration (;)
  • 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:
(since C++11)
(since C++20)
(since C++11)
  • simple declaration

Contents

[edit] 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)

[edit] Specifiers

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

  • the inline specifier is also allowed on variable declarations.
(since C++17)
  • 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.
(since C++11)
  • 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.
(since C++20)
  • 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:
(since C++11)
(since C++26)
(since C++17)
  • 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.
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.

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).

[edit] Declarators

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

declarator initializer (optional) (1)
declarator requires-clause (2) (since C++20)
declarator - the declarator
initializer - optional initializer (except where required, such as when initializing references or const objects). See Initialization for details.
requires-clause - a requires-clause, which adds a constraint to a function declaration

Each init-declarator in an 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.
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. It may end with the optional trailing return type.(since C++11)

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

(since C++11)

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

[edit] 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.

[edit] Example

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)>);
}

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
CWG 482 C++98 the declarators of redeclarations could not be qualified qualified declarators allowed
CWG 569 C++98 a single standalone semicolon was not a valid declaration it is an empty declaration,
which has no effect
CWG 1830 C++98 repetition of a function specifier in a decl-specifier-seq was allowed repetition is forbidden

[edit] See also

C documentation for Declarations