Question

I have 2 class:

class A
{
    int aa;
};
class B
{
    int bb;
};
class C
{
    public:
        bool equal(A& av,B& bv)
        {
            return (av.aa==bv.bb);
        }
};

Of course, class C has compilation error cause of private members’ access.
Is there a way to implement that equal() member of class C ?

Answer 1

This is a great scenario for using friend functions:

// forwarding references to each of the classes
class A;
class B;
class C
{
public:
    bool equal(A& av,B& bv);
    // notice we cannot implement equal() here, 
    // because A and B have not been defined yet, 
    // even though they have been declared.
};

class A
{
private:
    int aa;

    // Simply register to be a friend of A with our 'C::equal' function,
    // so that we can access 'aa' 
    friend bool C::equal(A&, B&);
};
class B
{
private:
    int bb;

    // Once again, we register as a friend of C::equal,
    // this time to access 'bb'
    friend bool C::equal(A&, B&);
};

// finally, now that A and B have been fully defined,
// we can implement our equal method:
bool C::equal(A&av, B&bv)
{
    return (av.aa == bv.bb);
}

// Sample Usage
int main()
{
    A a = A();
    B b = B();

    C c = C();

    c.equal(a, b);
}