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

I think D needs 2 or 3 vptrs.

Here A may or may not require a vptr. B needs one that should not be shared with A (because A is virtually inherited). C needs one that should not be shared with A (ditto). D can use B or C's vftable for its new virtual functions (if any), so it can share B's or C's.

My old paper "C++: Under the Hood" explains the Microsoft C++ implementation of virtual base classes. http://www.openrce.org/articles/files/jangrayhood.pdf

And (MS C++) you can compile with cl /d1reportAllClassLayout to get a text report of class memory layouts.

Happy hacking!