Declarations

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:

• 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.

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

• 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

• 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, 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.

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

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]

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

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

Declarators

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

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:

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.

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

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.