Equality Test for Derived Classes in C++ [duplicate]

Question

Possible Duplicate:
What’s the right way to overload operator== for a class hierarchy?

In C++, how can derived classes override the base class equality test in a meaningful way?

For example, say I have a base class A. Classes B and C derive from A. Now given two pointers to two A objects, can I test if they are equal (including any subclass data)?

class A {
    public: int data;
};

class B : public A {
    public: float more_data; bool something_else;
};

class C : public A {
    public: double more_data;
};


    A* one = new B;
    A* two = new B;
    A* three = new C;

    //How can I test if one, two, or three are equal
    //including any derived class data?

Is there a clean way of doing it? What's my best bet?

Thanks!