Question

Suppose I have a pair of base classes:

class A
{};

class B
{
    virtual void foo(A* ref);
    virtual void foo2(A* ref);
};

And from them, a few derived classes:

class C : virtual public A
{
   int data;
};

class D : public B
{
    virtual void foo(A* ref)
    {
        ((C*) (ref)).data = 5;
    }
};

class E : virtual public A
{
    int otherData;
};

class F : public B
{
    virtual void foo2(A* ref)
    {
        ((E*) (ref)).otherData = 6;
    }
};

And finally, we have a class that follows suit, as such:

class G : public E, public C
{
};

with a main function that goes as follows:

int main()
{
   B* myD = new D();
   B* myF = new F();

   A* myA = new G();

   myD->foo(myA);
   myF->foo2(myA);

   return 0;
}

Ok, IGNORING the obvious fact that this is a ‘dreaded diamond’ (which, the “multiple As” problem has been averted due to virtual), the general idea is that I want to have all my objects be Gs, Ds and Fs, but stored as references to As and Bs. I want to be able to use B’s (which is actually always either D or F) virtual functions to modify the values of G… However, A and B do not know of any of their child classes, and thus cannot be declared using them. This means that the virtual function arguments must always be a pointer to an A. However, D wants to have it’s input always be a C, and F wants it’s input to be an E. While this might not have been an issue if G was inheriting from only one parent, our current G is inheriting from 2 parents; I do not know how the this pointers actually work in the underlying context of situations like this… however, I do not feel like this is going to be a possible situation.

Would anyone be willing to shed some light on the mechanisms of casting pointers to parent/child classes, and if there would be any way this could be pulled off?

Answer 1

Assuming you have RTTI switched on, which you most likely do, you can use dynamic_cast to attempt to up-cast a base class pointer to a derived (or sibling) class pointer. The referenced object is tested at runtime to see if it is a compatible class, if this is not a valid upcast than it will return 0, in which case you can test for that and branch.

http://en.wikipedia.org/wiki/Dynamic_cast