Declarations are how names and/or entities are introduced (or re-introduced) into the C++ program. Not all declarations actually declare anything, and each kind of entity is declared differently. A
is a declaration that provides a complete definition for the introduced entity.
A declaration is one of the following:
(including
Partial template specialization
)
Explicit template instantiation
Explicit template specialization
Attribute declaration (
;)
(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
declarator- a function declarator This declaration must declare a constructor, destructor, or user-defined type
. It can only be used as part of a
,
, or explicit instantiation.block-declaration (a declaration that can appear inside a
), which, in turn, can be one of the following:
simple declaration
Note that the C++ standard distinguishes between declaration (a syntactic construct) and declaration (a C++ term). A declaration can be not only a declaration but also a
, an
, a
, and so on. For a name, the declaration that introduces it and the declaration of the entity it denotes are not always the same declaration:
template<classT>structA{};In this example, the declaration that introduces the name A is the class specifier, whereas the declaration that introduces the entity class template A is its enclosing
.
Simple declaration
A simple declaration is a statement that introduces, creates, and optionally initializes one or several identifiers, typically variables.
decl-specifier-seqinit-declarator-list(optional);(1) attrdecl-specifier-seqinit-declarator-list(optional);(2) (since C++11)decl-specifier-seq- sequence of
init-declarator-list- comma-separated list of init-declarator s (see below) attr- sequence of any number of
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:
declaratorinitializer(1) declaratorrequires-clause(optional)contract-specs(optional)(2) 1) A declarator with an initializer.
2) A declarator without an initializer.
declarator- a
initializer- an
requires-clause- (since C++20) a
contract-specs- (since C++26) a list of
requires-clause can only appear if declarator declares a
.
(since C++20)contract-specs can only appear if declarator declares a function or function template.
(since C++26)Specifiers
Declaration specifiers (decl-specifier-seq) is a sequence of the following whitespace-separated specifiers, in any order:
the
specifier. If present, the entire declaration is a
and each declarator introduces a new type name, not an object or a function.
function specifiers (
,
,
), only allowed in
.
the
specifier is also allowed on variable declarations.
(since C++17)the
specifier, allowed in class and function declarations.
the
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
specifier, only allowed in function and function template declarations.
the
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)
(
, (until C++17)
,
, (since C++11)
,
). 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
. Only the following specifiers are part of type-specifier-seq, in any order:
simple type specifier
,
, (since C++20)
,
, (since C++11)
,
,
,
,
,
,
,
,
,
previously declared class name (optionally
)
previously declared enum name (optionally
)
previously declared
or
(since C++11) (optionally
)
template name with template arguments (optionally
, optionally using
)
the keyword
,
, or
, followed by the identifier (optionally
), previously defined as the name of a class.
the keyword
,
, or
, followed by template name with template arguments (optionally
, optionally using
), previously defined as the name of a class template.
the keyword
followed by the identifier (optionally
), 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)
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 conststaticconst, or virtualinlinevirtual are errors, except that long is allowed to appear twice(since C++11).
Declarators
Each init-declarator in an init-declarator-listSD1,D2,D3; is processed as if it were a standalone declaration with the same specifiers: SD1;SD2;SD3;.
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 symbols 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-idattr(optional)(1) qualified-idattr(optional)(2) ...identifierattr(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
that is declared.
4)
: the declaration S*D; declares D as a pointer to the type determined by decl-specifier-seqS.
6)
: the declaration S&D; declares D as an lvalue reference to the type determined by decl-specifier-seqS.
7)
: the declaration S&&D; declares D as an rvalue reference to the type determined by decl-specifier-seqS.
8)
. noptr-declarator any valid declarator, but if it begins with *, &, or &&, it has to be surrounded by parentheses.
9)
. 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
. When appearing immediately after the identifier, it applies to the object being declared.
(since C++11)cv is a sequence of
qualifiers, where either qualifier may appear at most once in the sequence.
Due to
, when declarations of a type and another entity with the same name appear in the same scope, the type name is hidden. Note that the type can still be found via an
.
classFoo{};voidFoo();// Foo obj; // Error: ‘Foo’ is not a typeclassFooobj;// OKNotes
When a block-declaration appears
, and an identifier introduced by a declaration was previously declared in an outer block, the
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
,
, or by reaching its end), in order opposite to their order of initialization.
Example
Note: this example demonstrates how some complex declarations are parsed in terms of the language grammar. Other popular mnemonics are:
, reading
, and
. There is also an automated parser at
.
Run this code
#include<type_traits>structS{intmember;// 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 itinti=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 intint(*(*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]"// ...intmain(){static_assert(std::is_same_v<decltype(var1),decltype(var2)>);}Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 the declarators of redeclarations could not be qualified qualified declarators allowed
C++98 a single standalone semicolon was not a valid declaration it is an empty declaration,
which has no effect
C++98 repetition of a function specifier in a decl-specifier-seq was allowed repetition is forbidden See also