Create a New Instance of a Derived Class Not Knowing it's Type

Question

If I have a class called "A" and I have two classes that derive from it lets say "B" and "C" how do I create a new instance of both B and C not knowing which is which. This probably sounds confusing so let me clarify and use similar code as mine.

Base class:

class A {
    stuff...
}

Derived classes:

class B : A{
    stuff...
}

class C : A {
    stuff...
}

Then in some other class I have:

class Z {
    void Derp() {
        foreach(A classA in listOfA's) {
            create new instance of derived class
        }
    }
}

I simply cannot just say B = new B() and C = new C() because maybe B isn't in the list at all or maybe there are three B's and one A in the list. Surly there is some way to do it but if not I could have a switch-case statement like:

class Z {
    void Derp() {
        foreach(A classA in listOfA's) {
            switch(classA) {
                case is B:
                    B = new B();
                    break;
                case is C:
                    C = new C();
                    break;
                }
            }
        }
    }
}

I would like to avoid doing it this way unless necessary because every time I create a new class that derives from class A I'll have to remember to add another case statement in there.

So how should I do this?

Thanks.

Answer 1

A common C++ solution to this is the so-called virtual constructor idiom: you define a virtual function called clone in the base class A, and then override it in B to return a new instance of B and in C to return a new instance of C.

If you need a blank instance instead of a clone, you could add a newInstance function that uses the default constructor.