[Help] c++ polymorphism and function overloading

I have some problems in the following examples:

// where code begins

class base
{
    public:
        virtual bool operator == (const base& b) const = 0;
}

class inheritanceA
{
    public:
        virtual bool operator == (const inheritanceA& b) const
        {cout << "operator == in inheritanceA"}
}

class inheritanceB
{
     public:
         virtual bool operator == (const inheritanceB& b) const
         {cout << "operator == in inheritanceB"}
}

int main()
{
    base* a = new inheritanceA;
    base* b = new inheritanceB;
    cout << (*a)==(*b) << endl;
    return 0;
}

problems coming:

but the codes above is wrong while compiling, if I change the method
function in class inheritanceA, virtual bool operator==(const base&
b), It will be Ok.

however, when I add a data base* c = new inheritanceB, and invokes
operator == like this: (*a)==(*c), It will invokes method operator ==
in class inheritanceA,
but what I want is the compiler says it to be wrong .

How can I do this in polymorphism?

thanks