How to implement access to Qt library classes?

Question

Qt

There is a class AMain declared in the main program (in Qt). There is a library (in Qt) where exactly the same class AMain and its subclass AMainJoin are declared. The main program knows nothing about AMainJoin other than that it inherits from AMain. Some functions are redefined in AMainJoin (what is not known).
Task: Call the virtual function AMainJoin and call the code AMainJoin

Answer 1

First, how are you going to work with the AMainJoin class if you don't know anything about it? To be honest, it is not clear how this is generally implemented. Secondly, what is the need to declare two classes with the same name? Thirdly,

Call the virtual function AMainJoin and call the AMainJoin code

there is some typo here.