How can I assign a child class to a base class?

Question

I know there are solved questions related to this issue, but I still can't figure out how to resolve my problem.

I have something like this:

class Base
{
   static Base* createBase()
   {
      Base *b = new Base();
      ... //does a lot of weird things
      return b;
   }
}

class Child : public Base
{
   static Child* createChild()
   {
      Child *c = createBase(); // error
      return c;          
   }
}

I know why it doesn't work, but I have to find a way to do it. The createBase function does a lot of things so I don't want to recode it.

Any suggestions?

Answer 1

Why do you expect that to work? You can't treat a Base object as if it were a Child object, because the Child class might have all sorts of additional data that Base does not.

In order to get the effect you're looking for, there are two ways to do it:

The first way, and probably the best idea, is to move the logic from createBase into the Base constructor. The Base constructor will run whether you're creating a Base or something derived from it. It looks like you're trying to do the work of initializing the base object, and that's exactly what constructors are for!

If for some reason this will not work in your case, the other option is to create a protected initialize method in Base which accepts a Base* and does all the work that you are currently doing in createBase, e.g.

class Base
{
  public:

   static Base* createBase()
   {
       Base* b = new Base();
       initialize(b);
       return b;
   }

  protected:

   static void initialize(Base* b)
   {
      ... //does a lot of weird things
   }
}

class Child : public Base
{
  public:
   static Child* createChild()
   {
      Child *c = new Child(); 
      initialize(c):
      return c;          
   }
}

Note that this works since, while you can't treat a Base* as if it were a Child*, you can go the other way and treat a Child* as if it were a Base*, because the Child class is guaranteed to have at least everything that the Base class does, due to the nature of inheritance.

Edit: I saw you post in a comment to another answer that you cannot modify the definition of Base. In that case, you are completely out of luck and you will have to accept the need to copy-and-paste, given the restrictions in play. You are not going to be able to call createBase and get back a pointer to an object of any type other than Base if you cannot modify its code.