From 5348192069391090820 X-Google-Thread: f78e5,86930e54cbf7375c X-Google-Attributes: gidf78e5,public,usenet X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news1.google.com!news.glorb.com!peer1.news.newnet.co.uk!194.159.246.34.MISMATCH!peer-uk.news.demon.net!kibo.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: greghe@mac.com (Greg Herlihy) Newsgroups: comp.std.c++ Subject: Re: operator class&() { return *this; } considered harmful? Date: Fri, 2 Nov 2007 05:33:54 GMT Organization: Sonic.Net Lines: 54 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="US-ASCII" Content-Transfer-Encoding: 7bit X-Trace: news.demon.co.uk 1193981656 29414 158.152.254.254 (2 Nov 2007 05:34:16 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Fri, 2 Nov 2007 05:34:16 +0000 (UTC) X-Thread-Topic: operator class&() { return *this; } considered harmful? X-Original-To: std-c++@mailman.ucar.edu X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-User-Agent: Microsoft-Entourage/11.3.6.070618 X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au X-Thread-Index: Acgc4Nh3FxqMPojUEdyyeAAWy5Ss6w== 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 lA25XsWM017694; Fri, 2 Nov 2007 16:33:54 +1100 (EST) X-NNTP-Posting-Date: 01 Nov 2007 23:42:24 GMT 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++:948 On 10/30/07 11:18 PM, in article 4727ef35$1@clarion.carno.net.au, "Michael Norrish" wrote: > 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.) There is little point in defining a conversion to a reference of the same class type - because an object can always be used to initialize a reference of its own type. In other words, adding the C&() operator has no real effect on the program (except to make it more difficult for the compiler to warn that the function f() is returning a reference to a de-allocated object.) Moreover, having f() return a C object by value would not only be safe, but would also be slightly more efficient than the current implementation of f() (owing to the Named Return Value Optimization). Greg --- [ 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 ]