Accessing method of a class in an implementing class

Accessing method of a class in an implementing class

I have a class, suppose a.cpp. In the private property of this class, I
have a pointer to b.cpp and c.cpp where b.cpp is the implementation of a
virtual interface class, lets call d.cpp. Also, in the private property of
c.cpp, I have a pointer to the interface class, d.cpp.
I want to access one of the methods of class a.cpp in b.cpp class. How can
I do that?