Question

Possible Duplicate:
Overriding static variables when subclassing

I have a set of classes that are all derived from a base class. Any of these derived classes declare the same static variable. It is however specific to each of the derived classes.

Consider the following code.

class Base {
    // TODO: somehow declare a "virtual" static variable here?
    bool foo(int y) { 
        return x > y; // error: ‘x’ was not declared in this scope
    }
};

class A : public Base {
    static int x;
};

class B : public Base {
    static int x;
};

class C : public Base {
    static int x;
};

int A::x = 1;
int B::x = 3;
int C::x = 5;

int main() {}

In my base class I wanted to implement some logic, that requires the knowledge of the derived-class-specific x. Any of the derived classes has this variable. Therefore I would like to be able to refer to this variable at base class scope.

This wouldn’t be a problem if it were a simple member variable. However, semantically, the variable is indeed not a property of the derived class’ instance, but rather of the derived class itself. Therefore it should be a static variable.

UPDATE I need the class hierarchy to preserve its polymorphic nature. That is, all my derived class’ instances need to be members of a common base class.

Then however, how can I get my hands on this variable from the base class method?

Answer 1

You can use the Curiously recurring template pattern.

// This is the real base class, preserving the polymorphic structure
class Base
{
};

// This is an intermediate base class to define the static variable
template<class Derived>
class BaseX : public Base
{
    // The example function in the original question
    bool foo(int y)
    { 
        return x > y;
    }

    inline static int x;
};

class Derived1 : public BaseX<Derived1>
{
};

class Derived2 : public BaseX<Derived2>
{
};

Now classes Derived1 and Derived2 will each have a static int x available via the intermediate base class! Also, Derived1 and Derived2 will both share common functionality via the absolute base class Base.
Note: The variable declaration uses C++17’s inline static member declaration to avoid the need for a separate template variable definition.