C++ static


Example

The static storage class specifier has three different meanings.

  1. Gives internal linkage to a variable or function declared at namespace scope.

    // internal function; can't be linked to
    static double semiperimeter(double a, double b, double c) {
        return (a + b + c)/2.0;
    }
    // exported to client
    double area(double a, double b, double c) {
        const double s = semiperimeter(a, b, c);
        return sqrt(s*(s-a)*(s-b)*(s-c));
    }
    
  2. Declares a variable to have static storage duration (unless it is thread_local). Namespace-scope variables are implicitly static. A static local variable is initialized only once, the first time control passes through its definition, and is not destroyed every time its scope is exited.

    void f() {
        static int count = 0;
        std::cout << "f has been called " << ++count << " times so far\n";
    }
    
  3. When applied to the declaration of a class member, declares that member to be a static member.

    struct S {
        static S* create() {
            return new S;
        }
    };
    int main() {
        S* s = S::create();
    }
    

Note that in the case of a static data member of a class, both 2 and 3 apply simultaneously: the static keyword both makes the member into a static data member and makes it into a variable with static storage duration.