Invoke the copy construction of a derived class from a pointer to the base class?

Question

I have

1- class A {int m;};
2- class B: public A
{float n;};
3- class C: public A
{string n;};

I store all instances of class A in

vector <class A*> myInstansesofA;

It is stored as a pointer because I dynamically create them according to some if statement

if (some condition)
{
      A* newA = new B();
      myInstancesofA.push_back(newA );
}
else
{
      A* newA = new C();
      myInstancesofA.push_back(newA );
}

Now I need to create an exact copy of the vector. With new set of pointers, but exact copy of the data.

Problem is in copying the data for each instance of A (which are actually B and C) to the new vector.

I tried the following but it doesn't work.

for (vector<A*>::const_iterator it = myInstancesofA.begin(); it != myInstancesofA.end(); ++it)
    {
        A* newA = new A(*(*it));
        myInstancesofA.push_back(newA );
    }

I suspect this error occurs because of "new A" invokes only the copy constructor for type A and does not invoke of B or C (I dubugged and found this was happening).

How can I solve this ?

Answer 1

I believe you cannot invoke the constructor like that, and there is no straight-forward way in C++ to do something similar (it is pretty straight-forward for language with Reflection, like Java and C#).

One way is to make use of prototype pattern

class A {
public:
    virtual A* clone() {
        return new A {...};
    }
};

class B : public A {
public:
    A* clone() override {
        return new B {.........};
    }
};

class C : public A {
public:
    A* clone() override {
        return new C {.........};
    }
};

The object itself is responsible to create a clone of itself. Of course more thing to improve on the design but at least it gives you a taste of it.