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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.

Syntax

`static` data_member	(1)
`static` member_function	(2)

1) Declares a static data member.

2) Declares a static member function.

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).

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.

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.

A static data member may be declared inline. An inline static data member can be defined in the class definition and may specify a default member initializer. It does not need an out-of-class definition:

struct X
{
    inline static int n = 1;
};

(since C++17)

Constant static members

If a static data member of integral or enumeration type is declared const (and not volatile), it can be initialized with an 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;

If a static data member of LiteralType is declared constexpr, it must be initialized with an initializer in which every expression is a constant expression, right inside the class definition:

struct X {
    constexpr static int arr[] = { 1, 2, 3 };        // OK
    constexpr static std::complex<double> n = {1,2}; // OK
    constexpr static int k; // Error: constexpr static requires an initializer
};

(since C++11)

If a const non-inline (since C++17) static data member or a constexpr static data member (since C++11) is odr-used, a definition at namespace scope is still required, but it cannot have an initializer. This definition is deprecated for constexpr data members (since C++17).

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 (except for X::m in C++17)

If a static data member is declared constexpr, it is implicitly inline and does not need to be redeclared at namespace scope. This redeclaration without an initializer (formerly required as shown above) is still permitted, but is deprecated.