Question

There are 4 classes: A, B, C, and D. Class A “HAS A” B, and class C “HAS A” D. As it happens, C “IS A” A, and D “IS A” B. What’s the best way to model these relationships in C++?

The B that A has is the same D that C has.

Example: A building has an entrance, and a house has a door. A house is a building, and a door is an entrance.

Answer 1

I think there is a flaw in your design. If I understand correctly, you want to restrict the type of a base class member variable:

struct Entrance {};
struct Door : public Entrance {};

struct Building
{
    Entrance * entrance; // raw pointer just for the sake of clarity
};

struct House : public Building
{
    // Building::entrance must always be a door
};

You could achieve that using accessors (getter/setter) to entrance and checking the type, throwing an exception if the entrance is not a door, but that would break the Liskov substitution principle: you would not be able to manipulate Houses as if they were Buildings:

struct Drawbridge : public Entrance {};

House house;
Drawbridge bridge;

Building & building = house;
building.setEntrance(bridge); 
// Oups, I'm trying to install a drawbridge on my house!

Some libraries perform this kind of restrictions (for example, ReadOnlyCollection<T> throws an exception when one try to modify its content), but in my opinion, this is not a clean design. If collection’s interface states that I can add elements to the collection, then a read only collection is not a collection (since it does not support adding elements).

The same reasoning can be applied here: a House is not a Building since it cannot contain all kind of Entrances.