From -7032494034170181546 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,a5e7fc89a8659411 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2003-07-22 11:46:46 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!newsfeed.icl.net!newsfeed.fjserv.net!kibo.news.demon.net!mutlu.news.demon.net!demon!mail2news.demon.co.uk!devnull From: algrant@myrealbox.com (Al Grant) Newsgroups: comp.std.c++ Subject: Re: Voided application of indirection operator to pointer-to-void Date: Tue, 22 Jul 2003 18:46:45 +0000 (UTC) Organization: http://groups.google.com/ Lines: 48 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: <5765b025.0307220144.1df0c5c1@posting.google.com> References: <5765b025.0307210232.6c571eff@posting.google.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Trace: mail2news.demon.co.uk 1058899605 24388 10.0.0.1 (22 Jul 2003 18:46:45 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Tue, 22 Jul 2003 18:46:45 +0000 (UTC) X-Received: from mulga.cs.mu.oz.au ([128.250.1.22]) by news.demon.co.uk with esmtp (Exim 4.12) id 19f29s-0006L3-00 for mail2news@news.news.demon.net; Tue, 22 Jul 2003 18:46:44 +0000 X-Received: from localhost (localhost [[UNIX: localhost]]) by mulga.cs.mu.OZ.AU id EAA03134; Wed, 23 Jul 2003 04:46:34 +1000 (EST) X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Path: comp-std-cpp-robomod!not-for-mail X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Delivered-To: std-c++@ucar.edu X-Newsgroups: comp.std.c++ X-NNTP-Posting-Date: 22 Jul 2003 09:44:01 GMT X-Spam-Status: No, hits=-5.8 required=5.0 tests=BAYES_01,QUOTED_EMAIL_TEXT,REFERENCES version=2.55 X-Spam-Checker-Version: SpamAssassin 2.55 (1.174.2.19-2003-05-19-exp) Xref: archiver1.google.com comp.std.c++:20266 do-not-spam-benh@bwsint.com (Ben Hutchings) wrote in message news:... > In article <5765b025.0307210232.6c571eff@posting.google.com>, Al Grant wrote: > > and 5.3.1 indicates that those generated by the indirection operator > > must have an object type. > > Or function type. Ok, I was forgetting that in C++ function designators are still lvalues. > > I can believe that the committee might have intended to allow creation > > of void lvalues, motivated either by orthogonality, or by compatibility > > with C. > > > I very much doubt it; I can't see that it would be meaningful to have a > void lvalue, and I don't believe C allows it either. But that's the wrong question. The question is whether it allows application of the indirection operator to a pointer-to-void, with a well defined result, whether or not that result is an lvalue. Reading 6.5.3.2 suggests this is in fact the case, and the result is not an lvalue: note the description of the operand of the unary & operator: "either a function designator, the result of a [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is not declared with the register storage-class specifier." Now this is curious wording if the result of unary * is always an lvalue/FD. So it presumably is not. Reading further in 6.5.3.2 and footnote 83 suggests that the result of *pv is simply a denotation of indirection - and that &*pv is well-defined and equal to pv, which implies that *pv is well-defined, though of limited usefulness. In particular, not being an lvalue, it is not subject to the usual void-context (in C) lvalue-to-rvalue conversion. There appears to be nothing to make it illegal. Note that the description of unary & is more extensive than in C89. In C89 the case of &*pv was considered in DR#12 (and ruled illegal) and the case of (void)*pv was considered in DR#106 (and ruled legal). DR 106 should be sufficient evidence that C++ introduced a compatibility issue for the original code example. --- [ 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 ]