Question

I was wondering if someone could help me.

Suppose I have some classes as follows:

class A { ... };

class B : public A { ... }

class C : public A { ... };

class D { ... };

class E : public D { ... };

class F : public D { ... };

Now I want class B to have an object of type E as a member and class C to have a object of type F as a member.

Is there a nice design pattern whereby I can have some sort of base pointer to class D in class A and then create a new E in class B and a new F in class C. I was wondering if abstract factory is similar?.

I hope this is not too confusing … it is hard to explain without UML diagrams!.

Answer 1

What’s wrong with just doing exactly as you say?

class A
{
    A(D* p) : ptr(p) {}
    D* ptr;
};

class B : public A
{
    B() : A(new E) {}
};

class C : public A
{
    C() : A(new F) {}
};

If you want to automate it a bit better, you can use an intermediate template class.

class A
{
    virtual D* getD();
};

template <class T>
class AImpl : public A
{
    AImpl() : m_d(new T) {}
    T* getD() { return m_d; }
    T* m_d;
};

class B : public AImpl<E> { };

class C : public AImpl<F> { };

Note that AImpl::getD() can return T* due to covariant return types.