Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!uunet!comp.vuw.ac.nz!actrix!Bruce.Hoult
From: Bruce.Hoult@bbs.actrix.gen.nz
Newsgroups: comp.lang.c++
Subject: Re: Comparison functions for qsort() and bsearch()
Keywords: Sun, C++
Message-ID: <1991Jan5.105957.23014@actrix.gen.nz>
Date: 5 Jan 91 10:59:57 GMT
References: <277640DD.4A70@tct.uucp> <1990Dec26.160636.15566@clear.com> <1991Jan2.191630.18581@Think.COM>
Sender: Bruce.Hoult@actrix.gen.nz (Bruce Hoult)
Organization: Actrix Information Exchange, Wellington, New Zealand
Lines: 35
Comment-To: barmar@think.com

Barry Margolin writes:
>Bob Martin writes:
>>		       Case in point is a class derived from multiple bases:
>>
>>		class AB : public A, public B;
>>
>>	Now take the following variable:
>>		AB *ab;
>>
>>	The following two casts work just fine:  
>>		(A *)ab;  (B *)ab;
>>
>>	But the following, though it may compile, is likely not to work properly
>>	because it miscalculates the address of at least one of the base classes.
>>		(A *)(void *)ab; or (B *)(void *)ab;
> 
>But the following do work:
>
>	(A *) (AB *) (void *) ab;
>	(B *) (AB *) (void *) ab;


That doesn't solve the problem in question, since the compare function will
most likely know that it is comparing A's or B's, but not that they are
embedded in AB's.  After all, if you're writing an A or B compare function
that *does* know this, then you might as well write a special one for AB's
anyway.

What *will* work is...

    (A*) (void*)(A*) ab;    or
    (B*) (void*)(B*) ab;
-- 
Bruce.Hoult@bbs.actrix.gen.nz   Twisted pair: +64 4 772 116
BIX: brucehoult                 Last Resort:  PO Box 4145 Wellington, NZ