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From: thomasw@hpcupt1.HP.COM (Thomas Wang)
Newsgroups: comp.lang.c++
Subject: Multiple Inheritance flaw
Message-ID: <7050013@hpcupt1.HP.COM>
Date: 3 Jun 90 00:44:13 GMT
Organization: Hewlett Packard, Cupertino
Lines: 24


There is a serious flaw in C++'s multiple inheritance scheme.  The manual
reads "One can cast from a derived class to a virtual base class, but not
from a virtual base class to a derived class."

Normally, one would make a list class that operates on the base class.
With the run-time type information, it is safe to cast to a derived class.
This no longer works with multiple inheritance.  The base class is usually
declared 'public virtual'.  So even when it is obviously safe to type cast
upward, C++ still cannot do it.

Reading from manual, I understand this limitation is due to the use of one
way pointers.  However, I wonder how much effort would be required to
implement the backward pointer.

Maybe we can split the vtbl into two parts.  Part 1 is the function jump
table indexed by function.  Part 2 is the byte offset table indexed by
type.  This way, the base class can use the byte offset table to calculate the
backward pointer.

 -Thomas Wang
              (Everything is an object.)
                                                     wang@hpdmsjlm.cup.hp.com
                                                     thomasw@hpcupt1.cup.hp.com