C++ : Initializing base class constant static variable with different value in derived class ?

Question

I have a base class A with a constant static variable a. I need that instances of class B have a different value for the static variable a. How could this be achieved, preferably with static initialization ?

class A {
public:
    static const int a;
};
const int A::a = 1;

class B : public A {
    // ???
    // How to set *a* to a value specific to instances of class B ?
};

Answer 1

You can't. There is one instance of the static variable that is shared by all derived classes.

Answer 2

Static members are unique in the application. There is a single A::a constant in your system. What you can do is create a B::a static constant in B that will hide the A::a static (if you don't use the fully qualified name:

class A {
public:
   static const int a = 10;
};
static const int A::a;
class B : public A {
public:
   static const int a = 20;
   static void test();
};
static const int B::a;
void B::test() {
   std::cout << a << std::endl;    // 20: B::a hides A::a
   std::cout << A::a << std::endl; // 10: fully qualified
}

Answer 3

You can do this with Curiously recurring template pattern (you'll have to lose the const though).

template <typename T>
class A {
public:
    static int a;
};

template <typename T>
int A<T>::a = 0;

class B : public A<B> {
    struct helper { // change the value for A<B>::a
        helper() { A<B>::a = 42; }
    };
    static helper h;
};
B::helper B::h;