4.14 Static

Static qualifiers allocate the memory address of a variable in a higher enclosing scope.

For a function body, the variable is allocated in the block where the function is defined; so in the code

struct s {
  int count() {
    static int c=0;
    ++c;
    return c;
  }
}

there is one instance of the variable c for each object s (as opposed for each call of count).

Similarly, in

int factorial(int n) {
  int helper(int k) {
    static int x=1;
    x *= k;
    return k == 1 ? x : helper(k-1);
  }
  return helper(n);
}

there is one instance of x for every call to factorial (and not for every call to helper), so this is a correct, but ugly, implementation of factorial.

Similarly, a static variable declared within a structure is allocated in the block where the structure is defined. Thus,

struct A {
  struct B {
    static pair z;
  }
}

creates one object z for each object of type A created.

In this example,

int pow(int n, int k) {
  struct A {
    static int x=1;
    void helper() {
      x *= n;
    }
  }
  A operator init() {return new A;}
  for (int i=0; i < k; ++i) {
    A a;
    a.helper();
  }
  return A.x;
}

there is one instance of x for each call to pow, so this is an ugly implementation of exponentiation.