Path: utzoo!attcan!uunet!cs.utexas.edu!rutgers!rochester!cornell!peierls
From: peierls@svax.cs.cornell.edu (Tim Peierls)
Newsgroups: comp.lang.c++
Subject: Re: Pointer conversion macro brainteaser
Message-ID: <43906@cornell.UUCP>
Date: 30 Jul 90 18:53:48 GMT
References: <43645@cornell.UUCP> <56042@microsoft.UUCP>
Sender: nobody@cornell.UUCP
Reply-To: peierls@svax.cs.cornell.edu (Tim Peierls)
Distribution: comp
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 25

>In article <43645@cornell.UUCP> peierls@cs.cornell.edu (Tim Peierls) writes:
>|The question is, for all you spec hackers, under what conditions is
>|this macro guaranteed by E&S to work? ...
>|
>|#define bp2dp(B,D,p)  ((p)?((D*)(((char*)(p))+1-((size_t)(B*)(D*)(void*)1))):0)
>|It works for cfront 2.0 and probably everywhere else.  
>

In article <56042@microsoft.UUCP> jimad@microsoft.UUCP (Jim ADCOCK) writes:
>I disagree with both the first part and the second part of this statement.
>
>When I tried this macro on a cfront 2.0 derivative with B a virtual base
>class of D ...
>

I did say in my original posting that D had to be derived non-virtually
from B.

I was looking for a characterization of implementations where this macro
fails rather than an enumeration of implementation dependent behavior,
but perhaps what you are saying is that no useful characterization exists
beyond such an enumeration.  I'll buy that.
Name    : Tim Peierls
Mail	: peierls@svax.cs.cornell.edu
Scotch  : Laphroaig