Path: utzoo!attcan!uunet!aplcen!haven!ncifcrf!lhc!nih-csl!sparkler.dcrt.nih.gov!kgorlen
From: kgorlen@sparkler.dcrt.nih.gov (Keith Gorlen)
Newsgroups: comp.lang.c++
Subject: Re: Multiple Inheritance flaw
Message-ID: <148@nih-csl.nih.gov>
Date: 7 Jun 90 02:00:22 GMT
References: <7050013@hpcupt1.HP.COM> <58170024@hpclscu.HP.COM>
Sender: news@nih-csl.nih.gov
Organization: National Institutes of Health, Bethesda, MD
Lines: 53

In article <58170024@hpclscu.HP.COM> shankar@hpclscu.HP.COM (Shankar Unni) writes:
>> 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.
>
>I don't know about base classes being "usually" public virtual.
>
>Multiple inheritance is a very powerful and unwieldy tool. Virtual
>inheritance is the most tricky part of the whole setup to use - it should
>be used only in those cases where a base class defines some unique property
>for an object as a whole, and can be derived along *more than one path*.
>
>Remember the last part: you should not be using "virtual" (mainly for
>efficiency considerations) unless the last condition is true.

Or *might* be true!  This is what I encountered in making the NIH
Class Library, which follows a Smalltalk-80 -like tree, support MI.
Even though the library classes are organized in a tree and don't use
MI themselves, a user of the library *may* wish to multiply inherit
from two or more library classes.  To permit this, Object (the root
class of the tree), must be a virtual base class, for example.

From what I've done and heard about MI so far, it looks like virtual
base classes are frequently needed.  Jerry Schwarz, author of the AT&T
iostream library (an early application of C++ MI), even remarked a
while back that he thought virtual should be the *default* for base
class inheritance!  It's too bad it is so tricky to use.

>The reason why you cannot go back from a virtual base class to the derived
>class is mainly the above point: you could have got to the base class
>element along more than one path.  Even if, in your heirarchy, there is
>only one way to get from the base to the derived class, you still cannot do
>this safely.

Each class in the NIH Class Library implements the member function
castdown() to cast an Object* to a pointer to a derived class.  It
uses run-time type information to exhaustively search an object's
inheritance DAG to find the (unique) occurrence of the desired derived
class, and calls a member function of that class which returns its
"this" pointer as a void*.  It would be better (more efficient) if
downward casts from virtual bases were supported by C++.

--
	Keith Gorlen			phone: (301) 496-1111
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