From -9070934885698482407 X-Google-Thread: f78e5,1130823ffefc6ebe X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII Path: g2news1.google.com!news1.google.com!news.glorb.com!border1.nntp.dca.giganews.com!nntp.giganews.com!peer01.cox.net!cox.net!peer-uk.news.demon.net!kibo.news.demon.net!mutlu.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: swelef@post.sk (Vladimir Marko) Newsgroups: comp.std.c++ Subject: Re: Unions up in arms! Date: Fri, 17 Sep 2004 06:14:59 GMT Organization: http://groups.google.com Lines: 66 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: <84523212.0409160407.d34ab40@posting.google.com> References: <9215d7ac.0409081832.4ab088d6@posting.google.com> <84523212.0409140907.46471b77@posting.google.com> <41476579$0$12975$626a14ce@news.free.fr> NNTP-Posting-Host: news.news.demon.net Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Trace: news.demon.co.uk 1095401703 14830 158.152.254.254 (17 Sep 2004 06:15:03 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Fri, 17 Sep 2004 06:15:03 +0000 (UTC) X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Spam-Checker-Version: SpamAssassin 2.64-mulga_r1 (2004-01-11) on mulga.cs.mu.OZ.AU X-Spam-Status: No, hits=-4.9 required=5.0 tests=BAYES_00 autolearn=ham version=2.64-mulga_r1 X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id i8H6ExKN006050; Fri, 17 Sep 2004 16:14:59 +1000 (EST) X-Path: comp-std-cpp-robomod!not-for-mail X-NNTP-Posting-Date: Thu, 16 Sep 2004 12:07:22 +0000 (UTC) X-Delivered-To: std-c++@ucar.edu X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Newsgroups: comp.std.c++ Xref: g2news1.google.com comp.std.c++:2684 tannhauser86549spam@free.fr (Falk Tannh�user) wrote in message news:<41476579$0$12975$626a14ce@news.free.fr>... > Vladimir Marko wrote: > > > kanze@gabi-soft.fr wrote in message news: posting.google.com>... > > > >> struct S { int i ; float f ; } ; > >> float ff ; > >> > >> S* ps ; > >> float* pf ; > [...] > >>For the rest, there are a lot of other rules which have to be taken > >>into account. Thus, for example, *(float*)ps is definitly not > >>a legal access, despite the text you quote. > > > > Oh, really? First, we have 9.2/17: > > A pointer to a POD-struct object, suitably converted using a > > reinterpret cast, points to its initial member (or if that member > > is a bit-field, then to the unit in which it resides) and vice versa. > > Thus, *(int*)ps is surely valid. > > OK. It's an lvalue referring to the same object as 'ps->i'. > > > Then, 9.5/1 says > > [snip] Each data member is allocated as if it were the sole member of > > a struct. [snip] > > But 'ps' in the above example is a pointer to struct, and 9.5/1 > concerns unions. I appologize. The thread is about unions and I misread the Kanze's example as if there was a union too. (And I also forgot to mention that 9.5 is about unions.) > If 'S' were a union, I agree that 'reinterpret cast(ps)' > would be well defined and equal to '&ps->f'. > > > It comes down to the question "How deep is that 'as if'?" Is it deep > > enough for the 9.2/17 to apply? > > Well, being "the sole member of a struct" implies being its initial > member, so 9.2/17 indirectly applies to each union member IMO. The wording is "allocated as if". This suggests that it only applies to the memory layout and 9.2/17 is about reinterpret_cast validity not about memory layout. It definitely can't apply to the paragraph 8.5.1/15 [dlc.init.aggr/unions]: When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer for the first member of the union. And what's the difference between "initial member" (in 9.2/17 and "first member" (in 8.5.1/15)? I'll just repeat myself by writing that I think that the standard says "undefined behaviour" while each reasonable implementation makes it well defined. Vladimir Marko --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ]