Question

I have an object of a base class which is actually pointing to a derived class like this

Base *b = new Derived()

What I want to know, is it possible to pass the base class pointer by reference to some function which can cast the object back to Derived class. Something like this

Dummy_cast_fn(&b) // casts b to derived class

After calling Dummy_cast_fn, b should have a full copy of Derived class (no slicing).

Edit
I dont understand the part that there is no slicing since pointers are used. My problem is Derived class is returned from a function call to a shared library and I do not have access to .h file for the Derived. Only information I have is that Derived is based on Base class. I have access to Base.h so I can instantiate an object of Base but problem comes when I try to access the functions which are defined in Derived but not in Base. So I was wondering if I can typecast the Base to Derived type, then I will be able to access the function defined in Derived and not in Base.

Answer 1

As long as b is a pointer or reference to a Derived, you can always:

1. Down cast it to a Derived.
2. Treat it as a Base as long as you’re using b.

Namely, what determines what you can do with b is its static type, in this case Base. However, since it actually points on a Derived, you can always downcast it. Yet, to use it as a Derived you must have a variable whose type is Derived as well.

So, if the purpose of Dummy_cast_fn is just to fix something in b – it’s useless. If an object is sliced, nothing can fix it. But in your case, there’s no slicing, since you’re using pointers.

Edit according to the question’s edit:

First, you’re Derived object is not sliced. Let’s get that off the table. You have a pointer to a complete Derived (assuming that’s what you’ve been passed), but you only have access to its Base part when using the Base pointer. Now, you say you don’t have the definition of Derived. This means you won’t be able to downcast to that type, because the compiler doesn’t know how it’s defined. No casting will work here. There’s no legal C++ way you can call that sum function if you don’t have Derived‘s definition.

I do wonder why the author of Derived provided you its documentation without providing its definition. With this kind of polymorphism, the provider usually lets the user have some “interface”, leaving the actual type as an internal implementation detail. If you can’t use Derived because you don’t have its definition, there’s no point in letting you have its documentation.