Is it possible to use C++ object composition where choice of object is determined at run-time?

Question

Say I have a class Face. I wish to use composition to build Face. Face has eyes, so I can create a class Eyes and use composition adding eyes to Face.

But what if I subclass eyes, e.g.

class Eyes { ... };
class BlueEyes : public Eyes { ... };
class BrownEyes : public Eyes { ... };

? (Assume interfaces are all identical.)

Is it possible to compose Face at run-time, say depending on some parameter provided to the constructor a Face would get either BlueEyes or BrownEyes?

Answer 1

You can do this by composing a pointer to the base class. E.g.,

enum EyeColor{Blue, Brown}; // Even better would be enum class

class Face
{
private:
    // Here is the composition - to a base-class ptr.
    std::unique_ptr<Eyes> m_eyes;

public:
    explicit Face(EyeColor c) 
    {
        // Use c to instantiate m_eyes below to the appropriate-class object.
    }
};

In fact, the C++ implementation of some design patterns, e.g., Strategy, often employ this.