From 1972246624704461825 X-Google-Thread: f78e5,86930e54cbf7375c X-Google-Attributes: gidf78e5,public,usenet X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news2.google.com!news.glorb.com!news.cs.univ-paris8.fr!proxad.net!feeder1-2.proxad.net!usenet-fr.net!grolier!newsfeed.vmunix.org!peer-uk.news.demon.net!kibo.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: AlbertoBarbati@libero.it (Alberto Ganesh Barbati) Newsgroups: comp.std.c++ Subject: Re: operator class&() { return *this; } considered harmful? Date: Fri, 2 Nov 2007 16:54:13 GMT Organization: [Infostrada] Lines: 73 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: <4726a73f$1@clarion.carno.net.au> <1193750417.817345.137430@v3g2000hsg.googlegroups.com> <4727abe3$1@clarion.carno.net.au> <4727ef35$1@clarion.carno.net.au> NNTP-Posting-Host: news.news.demon.net Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Trace: news.demon.co.uk 1194022463 12770 158.152.254.254 (2 Nov 2007 16:54:23 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Fri, 2 Nov 2007 16:54:23 +0000 (UTC) X-Original-To: std-c++@mailman.ucar.edu X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-User-Agent: Thunderbird 2.0.0.6 (Windows/20070728) X-IronPort-Anti-Spam-Result: Ao8CAAuVKkfBRsBm/2dsb2JhbACQNQ X-IronPort-Anti-Spam-Filtered: true X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au X-Path: comp-std-cpp-robomod!not-for-mail X-Received: (from fjh@localhost) by mulga.csse.unimelb.edu.au (8.13.8+Sun/8.13.8/Submit) id lA2GsDMv012530; Sat, 3 Nov 2007 03:54:13 +1100 (EST) X-NNTP-Posting-Date: Fri, 02 Nov 2007 11:14:11 MET X-Delivered-To: std-c++@mailman.ucar.edu X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-SMTP-Auth: no X-Newsgroups: comp.std.c++ Xref: g2news2.google.com comp.std.c++:952 [Note to mods: this is a repost. Although the previous post had been approved, it somehow didn't appear on the newsgroup.] Michael Norrish ha scritto: > > Alberto Ganesh Barbati wrote: >> >> Michael Norrish ha scritto: >>> >>> Clearly, the rvalue reference proposal solves all sorts of other >>> >>> interesting issues apart from the one in my example, but my question >>> >>> is really whether or not >>> >>> >>> >>> operator C&() { return *this; } >>> >>> >>> >>> will safely achieve what I want. >>> >>> >> >> >> >> I don't think so. According to my interpretation, 12.2/5 doesn't apply >> >> here, because in line: >> >> >> >> C &x = f(); >> >> >> >> the reference x is *not* bound to a temporary, but rather to the result >> >> of the call to operator C& which is a reference (such reference >> >> *happens* to be bound to a temporary but this fact is irrelevant to a >> >> strict interpretation of 12.2/5). >> >> >> >> I expect the temporary to be properly destroyed at the end of the >> >> full-expression, making x a dangling reference. > > > > If this is so, is there any point to the language in 8.5.3/5 allowing > > this? (This language is "if the initializer expression ... has a > > class type ... and can be implicitly converted to an lvalue....". The > > footnote emphasises that this situation can only arise when you have a > > conversion function returning a reference type.) Notice that in this case "the reference is bound to the lvalue result of the conversion" confirming my interpretation. > > That clause only applies when the initialisor is not an lvalue, so > > there will be a temporary here anyway. That temporary can only get > > turned into an l-value via a conversion function, but if you are > > right, then the conversion function can't usefully return a reference > > to the temporary it's called on because that temporary will disappear > > as soon as the declaration of the reference completes. (The > > conversion function could return a reference to something other than > > the temporary of course; some global perhaps). The fault in your reasoning is that it's not true that the initializer must be an temporary to trigger this case. Consider this: class B; class A { public: operator B& (); }; A a; B& b = a; In fact, this example shows a very good reason why this case is allowed. HTH, Ganesh --- [ 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.comeaucomputing.com/csc/faq.html ]