Question

So I have a C++ function to which I pass a pointer to a base class, like:

void DoStuffAndAssignPtr(MyBase* ptr)
{
    MyBase* p;
    //do stuff
    if (stuff was awesome) p = new MyAwesome();
    else p = new MyBase();
          //vftable of p before the return is MyAwesome
    (*ptr) = (*p);
}

Now we need to invoke some virtual method that ‘MyAwesome’ and ‘MyBase’ implement differently. However, when checking the return value, as below:

void mainly()
{
    MyBase* passMe = new MyBase();
    DoStuffAndAssignPtr(passMe);
    //now passMe is always being returned with MyBase's vftable
}

We can see that ‘passMe’ is always a ‘MyBase’ (as far as the vftable shows while debugging in VS). Can anyone provide any guidance as to why this is happening, and how I can ensure that ‘passMe’ will invoke ‘MyAwesome’ implementations of virtual methods?

Answer 1

The problem is with the use of (*ptr)=(*p). You should use ptr=p.

Only pointer and reference have polymorphism. When you use dereference (the star *), you lose this property, and only the base part is copied from p to ptr. This is because ptr only have space for that part of information.