Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: Pointers and Multiple Inheritance
Message-ID: <54056@microsoft.UUCP>
Date: 12 Apr 90 17:44:29 GMT
References: <6170015@hpindda.HP.COM>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 35

In article <6170015@hpindda.HP.COM> daver@hpindda.HP.COM (Dave Richards) writes:
>This is probably a trivial question, but I've been hunting through my
>documentation and can't find it...

Neither could I, when I went looking for it....

>Given a pointer to an instance of a single-inherited subclass, it is
>legal to cast that pointer to any of the instance's superclasses.  

Yes, this is true.

>However given an instance of a multiply-inherited class is it legal to cast 
 that pointer to any of the inherited superclasses?  I would say not.  

Wrong guess, this is legal too, if I correctly understand your question.  
Given class FOOBAR multiply inheriting from FOO and BAR, a FOOBAR* can
be cast, either explicetly or implicetly, to either a FOO* or a BAR*

C++ compilers are smart enough to correctly do the pointer offsets.
Pointer coercions in C++ are not longer necessarily conceptual, but
can actually change the bit representations of the pointers involved.
But this was always [potentially, at least] true of C implementations too.

--If your derivations are private, you can still do all these pointer
coercions, but the pointer coercions must be explicet, not implicet.

>What then, can a pointer to a multiply-inherited class be cast to?

Implicetly, to any of its superclasses.  Explicetly, to almost anything.
The only restriction I can think of is in some instances of virtual
base classes.  These are the situations where the virtual base class is
handled via delegation.

Note:  Just 'cuz its "legal" to do these things doesn't necessarily
mean its "good" to so.