If multiple classes have a static variable in common, are they shared (within the same scope?)

Question

I have the following example code:

class A {     public:         static int a; }; int A::a = 0;  class B {     public:         static A a1; }; A B::a1;  class C {     public:         static A a1; }; A C::a1;   int main(int argc, const char * argv[]) {     C::a1.a++;     B::a1.a++;     std::cout << B::a1.a << " " << C::a1.a << std::endl;     return 0; } 

Class B and C have class A as a static member variable.

I expected the program to print "1 1", however it prints "2 2".

If multiple classes have a static variable in common, are they shared (within the same scope?)

Answer 1

The static members belong to class, it has nothing to do with objects.

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.

For your code, there's only one A::a, which is independent of B::a1 and C::a1 (which are objects of class A). So both B::a1.a and C::a1.a refer to A::a.

Answer 2

You're not looking at multiple classes here. Both B::a1 and C::a1 are of type A. And A has a static variable a, that you accessed twice. If you also wrote A::a++, your program would have printed 3 3

To modify your example slightly:

struct A {     static int a;     int b; }; int A::a;  struct B {     static A a1; }; A B::a1{0};  struct C {     static A a2; }; A C::a2{0}; 

and the user code:

B::a1.a = 1; // A's static variable changed B::a1.b = 2; // B's A's b changed to 2 cout << B::a1.a << ",  " << B::a1.b << endl; cout << C::a2.a << ",  " << C::a2.b << endl; 

It will print:

1, 2 1, 0 

That's because all As share a, but all As have their own b. And both C and B have their own A (that they respectively share between objects of their type)