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up vote 18 down vote favorite
2

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?)

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It might be eye-opening to add objects B b1,b2 and C c1,c2, c3. – MSalters 5 hours ago
up vote 23 down vote accepted

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.

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up vote 18 down vote

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)

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up vote 0 down vote

B and C both have static instances of A, these are seperate instances of A and would have different seperate instances of it's members as well. However, A::a is a static variable that is shared between all instances of A so:

&B::a1 != &C::a1 (the two a1 are seperate)

but

&B::a1.a == &C::a1.a (i.e. all A::a are the same, no matter the 'enclosing' instance of A)

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