From -4976750708421112924 X-Google-Thread: f78e5,bd52daa8ac2e51f1 X-Google-Thread: 109fba,bd52daa8ac2e51f1 X-Google-Attributes: gidf78e5,gid109fba,public,usenet X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news2.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local01.nntp.dca.giganews.com!news.giganews.com.POSTED!not-for-mail NNTP-Posting-Date: Sun, 14 Oct 2007 02:10:24 -0500 Return-Path: X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Original-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@mailman.ucar.edu X-SMTP-Auth: no From: Greg Herlihy Newsgroups: comp.std.c++,comp.lang.c++ Subject: Re: pre return optimization Organization: http://groups.google.com Message-ID: <1192343829.469097.231500@i13g2000prf.googlegroups.com> References: <1191919832.635392.256930@d55g2000hsg.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Complaints-To: groups-abuse@google.com User-Agent: G2/1.0 X-HTTP-UserAgent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-us) AppleWebKit/523.1 (KHTML, like Gecko) Version/3.0.3 Safari/523.1,gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: i13g2000prf.googlegroups.com; posting-host=209.204.165.36; posting-account=ps2QrAMAAAA6_jCuRt2JEIpn5Otqf_w0 X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Approved: Fergus Henderson , moderator of comp.std.c++ X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Date: Sun, 14 Oct 2007 02:08:09 CST Lines: 128 X-Usenet-Provider: http://www.giganews.com X-Trace: sv3-wLjmP7Q0yv8aZ67ABZnPG2e1df0kpO4uTwScXpNUoZdf6s4pioR3eDQt94y/hsn1L4U2BzPffsfMPp6!9oW0cu5T/HKn/7dkDp0yNvaJK7e2Sjpf06HyY3Y74dxmrZclSJf+i7uoCjh+lPsGNGhORfuevBzC!YtMCLEnZ0ys= X-Complaints-To: abuse@giganews.com X-DMCA-Notifications: http://www.giganews.com/info/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.36 Xref: g2news2.google.com comp.std.c++:833 comp.lang.c++:22539 On Oct 9, 8:56 am, "terminator(jam)" wrote: > consider: > > struct memory_pig{//a really large type: > > memory_pig(){ > std::cout<<"mem pig default\n"; > //etc... > }; > > memory_pig(memory_pig const&){ > std::cout<<"mem pig copy\n"; > //etc... > }; > > ~memory_pig(){ > std::cout<<"mem pig finish\n"; > //etc... > }; > > //etc... > > };///struct memory_pig > > memory_pig foo(){ > memory_pig result; > result=something; > //etc... > result=something_else; > return result; > > }; > > any time 'foo' is called the output will contain the following > sequence: > > mem pig default > mem pig copy > mem pig finish > > the last line of output may repeat based on how the result is > stored(rvo) or not. > So,two objects of a large type will be constructed and at least one is > destructed on every call to 'foo' The C++ Standard already allows the "(Named) Return Value Optimization" (NRVO or RVO for short). The optimization allows (under certain conditions) the compiler to construct the result of a function call "in place" - that is, directly in the object initialized with the function call result. So, to take advantage of this optimization, a program should use the result of a function call to initialize an object, instead of assigning the function result to an existing object. For example: #include using std::cout; struct memory_pig { // a really large type: memory_pig() { cout << "mem pig default\n"; } memory_pig(memory_pig const&) { cout << "mem pig copy\n"; } ~memory_pig() { cout << "mem pig finish\n"; } }; memory_pig foo() { memory_pig result; // ... return result; } int main() { memory_pig m1 = foo(); memory_pig m2 = foo(); memory_pig m3 = foo(); } I compiled the above program twice, once with and once without NRVO. The output of both programs is shown in the two columns below. As this comparison shows, NRVO can be a particular effective optimization - even for a small C++ program like the one used in this example. Program Output Without NRVO: With NRVO: mem pig default mem pig default mem pig copy mem pig default mem pig finish mem pig default mem pig copy mem pig finish mem pig finish mem pig finish mem pig default mem pig finish mem pig copy mem pig finish mem pig copy mem pig finish mem pig default mem pig copy mem pig finish mem pig copy mem pig finish mem pig finish mem pig finish mem pig finish 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 ]