Static Member Variable Wouldn't Initialize Anywhere

Class Definition:

class A {
    public:
        int a;
        static int b;
        A(int x) { a = x; }
        void changeA(int x) { a = x; }
        int getA() { return a; }
};

This wouldn't work:

A::b = 5; //This is outside the class definition and outside main

The error is:

error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

Do note that I'm not initializing it inside the class body.