Question

I have the following test program (demonstrates what I am really trying to accomplish).

Does anyone know if the following is possible without a reinterpret_cast<>?

struct B;

struct A {
  A() { }
  A( const B &b) { }
  A( const B *b) { }
  A( B *b) { }

  A* operator=(const B *b) { }
  A* operator=(B *b) { }
};

struct B {
  B() { }
  B( const A &a) { }
  B( const B * b) { }
  B( B *b) { }

  B* operator=(const A *a) { }
  B* operator=(A *a) { }
};

int main(int argc, char *argv[])
{
  A *a = new A();
  B *b = new B();

  A *c = b;

  return 0;
}

I tried doing every conversion operator I could, but I just can’t seem to get

  A *c = b;

to not complain

error C2440: 'initializing' : cannot convert from 'B *' to 'A *'

Answer 1

No, it’s not possible, because a B is not an A. You either need to use inheritance to make B a subclass of A (so that a B is an A), or use composition and then refer to the appropriate subobject.