[solved] C++ derived class object as parameter of base class method?

Question

Is there any way for a method of base class to accept ONLY derived class object as its parameter?
My tries:
1. Directly write method to accept derived class const object by reference. Verdict: not working. Error: [derived class name] is not defined.
2. Play with references and full class definitions. Verdict: not working. Error: [either base or derived class name] is not fully defined, so cannot accept that type of parameters.
Thinking of:
1. Doing it with multiple files and checking if a file is included.
2. Accept derived class object as base class object (by const reference, too).

Answer 1

Not sure why you'd want that kind of behaviour, but if I understood your requirements correctly, it is possible. Here is a single file demonstration:
https://code.sololearn.com/c0h548DBS7C2/?ref=app

Answer 2

Just to make sure, but the const qualifier has no influence on it. What matters is the order of declaration, i.e. the base class only requires a forward declaration of the derived class for the method declaration, but the method definition needs a full definition of the derived class, hence the structure of the code.

Answer 3

Now I understood: method declaration can contain declared-only types. Thanks again!

Answer 4

Oh, all I have to do is to declare the *const* method and write its code after derived declaration? I did not mind to make the method const...
Anyway, thanks a lot!
If everything with the code will be fine, I will try to make it public, so you can see why I needed that.