From -8992535245583918943 X-Google-Thread: 7894ca11fe,a4bc43b401c1097f,start X-Google-Attributes: gid7894ca11fe,public,usenet X-Google-NewGroupId: yes X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news2.google.com!newsfeed.stanford.edu!news.kjsl.com!news.alt.net!frodo.cs.rpi.edu!not-for-mail From: SG Newsgroups: comp.std.c++ Subject: Is copy elision allowed for returning a function's parameter? Date: Tue, 30 Jun 2009 09:28:38 CST Organization: http://groups.google.com Lines: 44 Sender: cppmods@cs.rpi.edu Approved: james.dennett@gmail.com Message-ID: NNTP-Posting-Host: netlab.cs.rpi.edu Content-Type: text/plain; charset=ISO-8859-1 To: (Usenet) Return-Path: X-Original-Date: Mon, 29 Jun 2009 10:14:21 -0700 (PDT) X-Submission-Address: std-c++@netlab.cs.rpi.edu Xref: g2news2.google.com comp.std.c++:958 Hi! Is copy elision allowed for returning a function's parameter? In other words: string flip(string x) { reverse( x.begin(), x.end() ); return x; // <-- Do I need std::move() here? } In N2914.pdf we can read in section 12.8/15 [class.copy] that the elision of a copy is permitted under some circumstances including: - in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object with the same cv-qualified type as the function return type, the copy operation can be omitted by constructing the automatic object directly into the function's return value and later we can read that, if a copy elision is allowed but cannot be performed the compiler has to treat the source object as an rvalue when selecting a constructor for the return value. My question is whether a function's parameter which has been accepted by value can qualify as a "non-volatile automatic object". From what I know current C++ implementations don't support this kind of copy elision but that doesn't mean that it is not allowed. In case this elision is currently deemed illegal I have to ask why this is and whether I'm missing an obvious problem with letting the compiler treat such parameters as rvalues in a return expression. [N2914] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2914.pdf Cheers! SG -- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@netlab.cs.rpi.edu] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]