Up-Casting and down-casting in c++

We are talking about casting only when converting the pointer or reference, calling a method on that pointer or reference has nothing to do with a cast. This statement is an assignment and an upcast. When you call a virtual method on the base class pointer then the override method of the derived class is executed through "dynamic dispatch", of course only if the actually created instance has a "more derived version" of the virtual function otherwise the base class virtual function gets executed. In case of nonvirtual method call there is nothing to explain...

The trick what happens in case of virtual function call: You cast the pointer to a base class pointer but objects having virtual methods also have a hidden internal "vtable" (virtual method table) that is initialized when you create the object. Since you created an instance of the DERIVED class, the vtable will point to the virtual method table of the DERIVED class even if you cast the pointer to a BASE pointer (and because of the vtable of the derived class the object will know from itself that its a DERIVED instance even if you access it through a base class pointer). By casting the pointer to a base class pointer you are accessing the very same virtual methods that you could access with a DERIVED pointer (if we overlook covariant return types that are rarely used and mostly unknown for programmers?), the upcast has effect only on the visibility of virtual/nonvirtual methods and member variables. With a base class pointer you simple don't see the member variables and nonvirtual methods defined in the derived class (this is quite obvious). It is also possible to increase the visibility of Base class members in derived classes (for example from protected to public, even in case of virtual methods) but this is rarely used in c++.