static members
Inside a class definition, the keyword static declares members that are not bound to class instances.
Outside a class definition, it has a different meaning: see storage duration.
Contents |
[edit] Syntax
| static data_member | (1) | ||||||||
| static member_function | (2) | ||||||||
[edit] Explanation
Static members of a class are not associated with the objects of the class: they are independent objects with static storage duration or regular functions defined in namespace scope, only once in the program.
The static keyword is only used with the declaration of a static member, inside the class definition, but not with the definition of that static member:
class X { static int n; }; // declaration (uses 'static') int X::n = 1; // definition (does not use 'static')
The declaration inside the class body is not a definition and may declare the member to be of incomplete type (other than void), including the type in which the member is declared:
struct Foo; struct S { static int a[]; // declaration, incomplete type static Foo x; // declaration, incomplete type static S s; // declaration, incomplete type (inside its own definition) }; int S::a[10]; // definition, complete type struct Foo {}; Foo S::x; // definition, complete type S S::s; // definition, complete type
To refer to a static member m of class T, two forms may be used: qualified name T::m or member access expression E.m or E->m, where E is an expression that evaluates to T or T* respectively. When in the same class scope, the qualification is unnecessary:
struct X { static void f(); // declaration static int n; // declaration }; X g() { return X(); } // some function returning X void f() { X::f(); // X::f is a qualified name of static member function g().f(); // g().f is member access expression referring to a static member function } int X::n = 7; // definition void X::f() // definition { n = 1; // X::n is accessible as just n in this scope }
Static members obey the class member access rules (private, protected, public).
[edit] Static member functions
Static member functions are not associated with any object. When called, they have no this pointer.
Static member functions cannot be virtual, const, or volatile.
The address of a static member function may be stored in a regular pointer to function, but not in a pointer to member function.
[edit] Static data members
Static data members are not associated with any object. They exist even if no objects of the class have been defined. If the static member is declared thread_local(since C++11), there is one such object per thread. Otherwise, there is only one instance of the static data member in the entire program, with static storage duration.
Static data members cannot be mutable.
Static data members of a class in namespace scope have external linkage if the class itself has external linkage (i.e. is not a member of unnamed namespace). Local classes (classes defined inside functions) and unnamed classes, including member classes of unnamed classes, cannot have static data members.
[edit] Constant static members
If a static data member of integral or enumeration type is declared const (and not volatile), it can be initialized with a initializer in which every expression is a constant expression, right inside the class definition:
struct X { const static int n = {1}; const static int m{2}; // since C++11 const static int k; }; const int X::k = 3;
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If a static data member of struct X { constexpr static std::complex<double> n = {1,2}; constexpr static int k; // Error: constexpr static requires an initializer }; |
(since C++11) |
If such a member is odr-used, a definition at namespace scope is still required, but it cannot have an initializer:
struct X { static const int n = 1; static constexpr int m = 4; }; const int *p = &X::n, *q = &X::m; // X::n and X::m are odr-used const int X::n, X::m; // … so a definition is necessary
[edit] References
- C++11 standard (ISO/IEC 14882:2011):
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- 9.4 Static members [class.static]
- C++98 standard (ISO/IEC 14882:1998):
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- 9.4 Static members [class.static]