understanding vptr in multiple inheritance?

Question

I am trying to make sense of the statement in book effective c++. Following is the inheritance diagram for multiple inheritance.

Answer 1

It all has to do with how compiler figures out the actual addresses of method functions. The compiler assumes that virtual table pointer is located at a known offset from the base of the object (typically at offset 0). The compiler also needs to know the structure of the virtual table for each class - in other words, how to lookup pointers to functions in the virtual table.

Class B and class C will have completely different structures of Virtual Tables since they have different methods. Virtual table for class D can look like a virtual table for class B followed by additional data for methods of class C.

When you generate an object of class D, you can cast it as a pointer to B or as a pointer to C or even as a pointer to class A. You may pass these pointers to modules that are not even aware of existence of class D, but can call methods of class B or C or A. These modules need to know how to locate the pointer to the virtual table of the class and they need to know how to locate pointers to methods of class B/C/A in the virtual table. That's why you need to have separate VPTRs for each class.

Class D is well aware of existence of class B and the structure of its virtual table and therefore can extend its structure and reuse the VPTR from object B.

When you cast a pointer to object D to a pointer to object B or C or A, it will actually update the pointer by some offset, so that it starts from vptr corresponding to that specific base class.