From -543141276145967686 X-Google-Thread: f78e5,1130823ffefc6ebe X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news1.google.com!news.glorb.com!news.zanker.org!news.clara.net!wagner.news.clara.net!194.159.246.34.MISMATCH!peer-uk.news.demon.net!kibo.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: Tue, 14 Sep 2004 17:37:18 GMT Organization: http://groups.google.com Lines: 89 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: <84523212.0409140907.46471b77@posting.google.com> References: <9215d7ac.0409081832.4ab088d6@posting.google.com> 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 1095183443 20280 158.152.254.254 (14 Sep 2004 17:37:23 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Tue, 14 Sep 2004 17:37:23 +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 i8EHbItt000523; Wed, 15 Sep 2004 03:37:18 +1000 (EST) X-Path: comp-std-cpp-robomod!not-for-mail X-NNTP-Posting-Date: Tue, 14 Sep 2004 17:07:35 +0000 (UTC) X-Spam-Level: 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++:2618 kanze@gabi-soft.fr wrote in message news:... > glenlow@pixelglow.com (Glen Low) wrote in message > news:<9215d7ac.0409081832.4ab088d6@posting.google.com>... [snip] > Thus, if I have > > strut S { int i ; float f ; } ; > float ff ; > > S* ps ; > float* pf ; > > The compiler must assume that *pf is also accessible via ps. > > 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. Then, 9.5/1 says [snip] Each data member is allocated as if it were the sole member of a struct. [snip] It comes down to the question "How deep is that 'as if'?" Is it deep enough for the 9.2/17 to apply? Given the exact wording, I'd say no (i.e. you are right). On the other hand, making it behave wrong would require a special handling of this case in the implementation's reinterpret_cast. AFAICT the standard could be changed to allow it without any implications for the vendors. > > Suppose > > > int j; > > union u { int i; float f; }; > > > Then which of the following is illegal, if any, and why? (Putting > > aside, if possible, the legality of the popular type-punning practi > ce > > of putting x into a union and getting back y.) > > But you can't put it aside. The standard clearly says that the only > member of a union that can be accessed is the last one written. > > > A. > > > j = 12; > > int k = ((u&) j).i; > > This is a reinterpret cast. According to 5.2.10/7, "Except that > converting an rvalue of type 'pointer to T1' to the type 'pointer to > T2' (where T1 and T2 are object types and where the alignement > requirements of T2 are no stricter than those of T1) and back to its > original type yields the original pointer value, the result of such a > pointer conversion is unspecified." So there results here are > unspecified. > > Note that even if you converted back to the original type, if float > required stricter alignment (or if the compiler imposed stricter > alignment on unions than on its constituant members), the results wou > ld > be unspecified. But _if the union has no stricter alignment requirements_, the following is well defined: u* pu=(u*)&j; int k=*(int*)pu; And in the light of 9.2/17, the last line is pretty much equivalent to int k=pu->i; Casting references instead of pointers gives the same behaviour, thus the original example could be valid _if the union had no stricter alignment requirements_, couldn't it? Well, "pretty much equivalent" is not the same as "equivalent." 9.2/17 speaks about converting a pointer to POD struct and pu is _not_ a pointer to an POD struct, it is just a correctly typed pointer "somewhere." So, it's again something that should work with a reasonable implementation while beeing marked as undefined behaviour by the standard. This time, however, even if the standard could be changed to accept this, it should _not_ be changed. Vladimir Marko --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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