From 6608914408462621906 X-Google-Thread: f78e5,bd52daa8ac2e51f1 X-Google-Thread: 109fba,bd52daa8ac2e51f1 X-Google-Attributes: gidf78e5,gid109fba,public,usenet X-Google-Language: ENGLISH,ASCII Path: g2news2.google.com!news2.google.com!news.glorb.com!news.alt.net!comp-std-cpp-robomod!not-for-mail From: "terminator(jam)" Newsgroups: comp.std.c++,comp.lang.c++ Subject: Re: pre return optimization Date: Tue, 16 Oct 2007 11:55:54 CST Organization: http://groups.google.com Lines: 146 Approved: Fergus Henderson , moderator of comp.std.c++ Message-ID: <1192533048.201465.58760@k35g2000prh.googlegroups.com> References: <1191919832.635392.256930@d55g2000hsg.googlegroups.com> <1192343829.469097.231500@i13g2000prf.googlegroups.com> <1192366892.063805.202140@k35g2000prh.googlegroups.com> <1192434165.863496.171650@q3g2000prf.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8bit X-Trace: posting.google.com 1192533048 22656 127.0.0.1 (16 Oct 2007 11:10:48 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Tue, 16 Oct 2007 11:10:48 +0000 (UTC) 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 User-Agent: G2/1.0 X-HTTP-UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Media Center PC 3.0; .NET CLR 1.0.3705; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2),gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: k35g2000prh.googlegroups.com; posting-host=87.247.164.107; posting-account=ps2QrAMAAAA6_jCuRt2JEIpn5Otqf_w0 X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au X-MIME-Autoconverted: from quoted-printable to 8bit by mulga.csse.unimelb.edu.au id l9GBB4gw014093 X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Xref: g2news2.google.com comp.std.c++:854 comp.lang.c++:22857 On Oct 15, 8:35 pm, Erik Wikstr�m wrote: > ===================================== MODERATOR'S COMMENT: > > Please trim unnecessary material when replying. > > ===================================== END OF MODERATOR'S COMMENT > On 2007-10-15 17:52, terminator(jam) wrote: > > > > > > > On Oct 14, 10:00 pm, Erik-wikst...@telia.com (Erik Wikstr�m) wrote: > >> On 2007-10-14 21:19, terminator wrote: > > >> > On Oct 14, 11:08 am, Greg Herlihy wrote: > >> >> 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. > > >> > I knew about RVO but not NRVO(that is what I am talking about).Is NRVO > >> > standard behavoir or unspecified? > >> > At least my compiler does not perform the NRVO. > > >> The standard allows this optimisation, but it does not require it (see > >> section 12.8 paragraph 15). I think that most newer compilers support > >> it, but you might have to turn up the optimisation level a bit. > > >> One important thing to remember is that when NRVO is in use the > >> destructor will not be called for the local object, nor will the copy- > >> constructor for the non-local object (m1 to m3 in the code above) be > >> called. Thus you should not depend on any side effects that the copy- > >> constructor or destructor might have. > > > prior to return and after return are two different case to me:I mean > > before return even move should be disabled ,while after the return > > either of move/copy must perform . > > Sorry, but you lost me there, what move? Of what to where? > Move ctor of course. regards, FM. --- [ 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 ]