From -3333914034063321903 X-Google-Thread: f78e5,1130823ffefc6ebe X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII Path: g2news1.google.com!news1.google.com!newshub.sdsu.edu!news-xfer.cox.net!peer01.cox.net!cox.net!peer-uk.news.demon.net!kibo.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: tannhauser86549spam@free.fr (=?ISO-8859-1?Q?Falk_Tannh=E4user?=) Newsgroups: comp.std.c++ Subject: Re: Unions up in arms! Date: Tue, 14 Sep 2004 22:59:42 GMT Organization: Guest of ProXad - France Lines: 46 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: <41476579$0$12975$626a14ce@news.free.fr> References: <9215d7ac.0409081832.4ab088d6@posting.google.com> <84523212.0409140907.46471b77@posting.google.com> NNTP-Posting-Host: news.news.demon.net Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: news.demon.co.uk 1095202788 20504 158.152.254.254 (14 Sep 2004 22:59:48 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Tue, 14 Sep 2004 22:59:48 +0000 (UTC) X-Received-SPF: none (mailbox2.ucsd.edu: domain of root@news.free.fr does not designate permitted sender hosts) X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.1) Gecko/20040707 X-Path: comp-std-cpp-robomod!not-for-mail X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id i8EMxgkb020653; Wed, 15 Sep 2004 08:59:42 +1000 (EST) X-NNTP-Posting-Date: 14 Sep 2004 23:41:13 MEST X-Delivered-To: std-c++@ucar.edu X-Spamscanner: mailbox2.ucsd.edu (v1.5 Aug 25 2004 09:28:35, 0.2/5.0 3.0.0-rc1) X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-MIME-Autoconverted: from 8bit to quoted-printable by mulga.cs.mu.OZ.AU id i8EMxhQs020663 X-Spam-Checker-Version: SpamAssassin 2.64-mulga_r1 (2004-01-11) on mulga.cs.mu.OZ.AU X-Accept-Language: de, fr, pl, en, en-us X-Spam-Status: No, hits=-4.9 required=5.0 tests=BAYES_00 autolearn=ham version=2.64-mulga_r1 X-Newsgroups: comp.std.c++ X-MailScanner: PASSED (v1.2.8 78657 i8ELfHYl088393 mailbox2.ucsd.edu) Xref: g2news1.google.com comp.std.c++:2628 Vladimir Marko wrote: > kanze@gabi-soft.fr wrote in message news:... >=20 >> 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. >=20 > 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 =A7 9.5/1 conce= rns 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 =A7 9.2/17 indirectly applies to each union member IMO. Falk Tannh=E4user --- [ 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 ]