It helps if you know the underlying mechanisms. C++ formalizes some coding techniques used by C programmers, "classes" replaced using "overlays" - structs with common header sections would be used to handle objects of different types but with some common data or operations. Normally the base struct of the overlay (the common part) has a pointer to a function table which points to a different set of routines for each object type. C++ does the same thing but hides the mechanisms i.e. the C++ ptr->func(...)ptr->func(...) where func is virtual as C would be (*ptr->func_table[func_num])(ptr,...)(*ptr->func_table[func_num])(ptr,...), where what changes between derived classes is the func_table contents. [A non-virtual method ptr->func() just translates to mangled_func(ptr,..).]
The upshot of that is that you only need to understand the base class in order to call the methods of a derived class, i.e. if a routine understands class A, you can pass it a derived class B pointer then the virtual methods called will be those of B rather than A since you go through the function table B points at.
