Question

I have these C++ classes:

class Base
{
protected:
    static int method()
    {
        static int x = 0;
        return x++;
    }
};

class A : public Base
{

};

class B : public Base
{

};

Will the x static variable be shared among A and B, or will each one of them have it’s own independent x variable (which is what I want)?

Answer 1

There will only be one instance of x in the entire program. A nice work-around is to use the CRTP:

template <class Derived>
class Base
{
protected:
    static int method()
    {
        static int x = 0;
        return x++;
    }
};

class A : public Base<A> { };
class B : public Base<B> { };

This will create a different Base<T>, and therefore a distinct x, for each class that derives from it.

You may also need a “Baser” base to retain polymorphism, as Neil and Akanksh point out.