C++ unexpected calling of function of inherited classes

Question

I made following 3 classes:

struct Parent1
{
    virtual void f()
    {
        cout << "\nParent1::f";
    }
};
struct Parent2
{
    virtual void g()
    {
        cout << "\nParent2::g";
    }
    virtual void z()
    {
        cout << "\nParent2::z";
    }
};
struct Child : public Parent1, public Parent2
{
    virtual void h()
    {
        cout << "\nChild::h";
    }
};

In main, when I call function z of Parent2, it instead calls function h of the child class. Why is it happening so? Following is the main function:

int main()
{
    Child obj;
    Parent2 * p2 = (Parent2*)(Parent1*)&obj;
    p2->z();
    return 0;
}

Answer 1

The first explicit conversion from &obj i.e. Child* to Parent1* is an upcast. The result will point to the base class sub-object. The next explicit conversion is from Parent1* to Parent2*. Since these classes are not directly related, this is a reinterpretation cast. But the types are not pointer-interconvertible, so when you call the function through the reinterpreted pointer, the behaviour of the program is undefined.

You should avoid using C-style casts to prevent mistakes like this. In this case, no explicit cast is needed at all. This works correctly:

Parent2 * p2 = &obj;

And never reinterpret pointers unless you know what it means and that it is OK to do so.