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Declarations

From cppreference.com
< cpp‎ | language

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:

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:

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 decomposition 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)
  • 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:
(since C++11)
(since C++17)
  • 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.
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)

Decomposition 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 - 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 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 decomposition declaration introduces all identifiers named in the identifier-list 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)) 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>::value is a well-formed integer constant (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 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 identifier introduced by a decomposition declaration.

(since C++17)

[edit] 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.

[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] 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.