From -662718750564581880 X-Google-Thread: f78e5,1130823ffefc6ebe X-Google-Thread: fc772,1130823ffefc6ebe X-Google-Attributes: gidf78e5,gidfc772,public X-Google-Language: ENGLISH,ASCII Path: g2news1.google.com!news1.google.com!news.glorb.com!border1.nntp.dca.giganews.com!nntp.giganews.com!newsfeed-east.nntpserver.com!nntpserver.com!newsfeed1.sea.pnap.net!newsfeed2.sea.pnap.net!newsfeed.pnap.net!brmea-news-1.sun.com!news1nwk.sfbay.sun.com!taumet!clamage From: kanze@gabi-soft.fr Newsgroups: comp.std.c++,comp.lang.c++.moderated Subject: Re: Unions up in arms! Date: Mon, 13 Sep 2004 22:47:09 +0000 (UTC) Organization: http://groups.google.com Lines: 133 Sender: cppmods@netlab.cs.rpi.edu Approved: stephen.clamage@sun.com (comp.std.c++) Message-ID: References: <9215d7ac.0409081832.4ab088d6@posting.google.com> NNTP-Posting-Host: taumet.sfbay.sun.com Mime-Version: 1.0 Content-Type: text/plain; charset=unknown-8bit Content-Transfer-Encoding: QUOTED-PRINTABLE X-Trace: news1nwk.SFbay.Sun.COM 1095115629 8388 129.146.79.208 (13 Sep 2004 22:47:09 GMT) X-Complaints-To: usenet@news1nwk.sfbay.sun.com NNTP-Posting-Date: Mon, 13 Sep 2004 22:47:09 +0000 (UTC) X-MIME-Autoconverted: from 8bit to quoted-printable by nwkea-mail-1.sun.com id i8DLqb4d003549 X-Original-NNTP-posting-date: Mon, 13 Sep 2004 20:41:40 +0000 (UTC) Delivered-to: std-c++@ucar.edu X-Original-Date: Mon, 13 Sep 2004 05:22:02 -0700 X-Submission-Address: c++-submit@netlab.cs.rpi.edu X-Auth: PGPMoose V1.1 PGP comp.lang.c++.moderated iQBVAwUAQUYGyEHMCo9UcraBAQEbRwIAoted+oXASqiQk+zc26Jq1LBEUE88nMsv GpF1x8lf0BWkZGVbtGXDZJPUnjeZlt1lJlXAH0aUtiAaPuSZ4fBQjQ== =ITUL X-Approved-For-Group: kuehl@inf.uni-konstanz.de comp.lang.c++.moderated X-NNTP-posting-host: netlab.cs.rpi.edu Original-recipient: rfc822;clamage@mpk-mail1.SFBay.Sun.COM Originator: clamage@taumet Xref: g2news1.google.com comp.std.c++:2601 comp.lang.c++.moderated:8137 glenlow@pixelglow.com (Glen Low) wrote in message news:<9215d7ac.0409081832.4ab088d6@posting.google.com>... > Color me stupid, but after months of staring I still can't grok thi= s > clause in the Standard 3.10/15: > ---- > If a program attempts to access the stored value of an object throu= gh > an lvalue of other than one of the following types the behavior is > undefined: > an aggregate or union type that includes one of the aforementioned > types among its members (including, recursively, a member of a > subaggregate or contained union), > ---- The wording is a bit difficult, to put it mildly. I think that the k= ey is in the (non-normative) footnote. These restrictions are basicly there to tell compiler writers what they can exclude (or rather what they cannot exclude) as possible aliases. 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 i= nto account. Thus, for example, *(float*)ps is definitly not a legal access, despite the text you quote. > 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 =3D 12; > int k =3D ((u&) j).i; This is a reinterpret_cast. According to =A75.2.10/7, "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. > B. > j =3D 12; > u v =3D (u&) j; Unspecified, see above. > C. > u v =3D {12}; > float g =3D v.f; The union was initialized with an int. You access the float field. Undefined behavior. On machines using IEEE, this can cause a trappin= g NaN. On machines where floats are larger than ints, who knows what m= ay happen. In this case, it doesn't even work in practice. > D. > j =3D 12; > float g =3D ((u&) j).f; You're using an unspecified pointer to access a field in a union that doesn't exist. What do you expect to happen. > And finally, what does "including, recursively, a member of a > subaggregate or contained union" mean? It means that if you have a float*, and a U*, for some union or aggregate, if that union or aggregate contains a float, directly, or indirectly, as a member of one of its members, then the compiler must consider that the the two pointers may possibly be aliases for the sa= me data, i.e. if there is a write through one of the pointers, the compi= ler must reread any value read through the other, since this write may ha= ve modified the value. -- James Kanze GABI Software http://www.gabi-soft.fr Conseils en informatique orient=E9e objet/ Beratung in objektorientierter Datenverarbeitung 9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 = 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ comp.std.c++ is moderated. To submit articles, try just posting wi= th ] [ your news-reader. 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