From -782287096994093926 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,f5d433881cc29cff X-Google-Attributes: gidf78e5,public From: bparker@gil.com.au (Brian Parker) Subject: Re: Named return value optimisation- reprise. Date: 1997/06/23 Message-ID: <33ab49d6.3482939@news.ipswich.gil.com.au>#1/1 X-Deja-AN: 252085406 References: <33a345bf.7425526@news.ipswich.gil.com.au> <33A478C1.329@acm.org> <33a5f9a1.5157214@news.ipswich.gil.com.au> <33a87d7b.3835586@news.ipswich.gil.com.au> X-Original-Date: Sat, 21 Jun 1997 04:41:40 GMT Organization: Global Info-Links News Server X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBVAwUBM67f40y4NqrwXLNJAQFflQIAvY/Ta6rfsYQuXymqJcypTHQ95JzW0Xaq X0K4J4UXhOr/f20gVu/m80DAp9WoqyPNKQn9HwuMvIVkG00E6KrStQ== =p8ay Newsgroups: comp.std.c++ Originator: austern@isolde.mti.sgi.com On 20 Jun 1997 10:19:52 PDT, Jason Merrill wrote: >... >15Whenever a class object is copied and the original object and the copy > have the same type, and the lifetime of the original object ends > immediately* after the copy is created, an implementation is permitted to > treat the original and the copy as two different ways of referring to the > same object and not perform a copy at all. In that case, the object is > destroyed when the copy would have been destroyed without the > optimization. > >* For the purposes of this rule, implicit destructor calls are not > considered in the lifetime of an object. On further consideration it has occurred to me that the elision of a temporary created as a result of a conversion operator on a function call would not be allowed by this wording of 12.8/15, as after the temporary is copied there is an entire function call before the temp is destructed at the end of the full-expression. In every case except for the NRVO, the copy being elided is a copy of a newly-constructed class temporary (rvalue). So perhaps rather than attempting to have one simple rule that includes the NRVO, it may be better to treat it as a special case i.e. " 12.8/15 In a copy-initialisation (8.5/12), if the source type is identical to the destination type and is a class rvalue, or in a function return (6.63) is an lvalue of automatic storage duration, then the copy can be elided and the source object constructed directly in the destination's storage. (Note: *this in a member function of a class temporary is an lvalue expression and hence this paragraph does not apply when copying it.)" I put the note about *this to make it clear that copies of *this in a member function are (correctly) not elided even if "this" is an rvalue. Any comments on this wording? ,Brian Parker (bparker@gil.com.au) --- [ comp.std.c++ is moderated. To submit articles: Try just posting with your newsreader. If that fails, use mailto:std-c++@ncar.ucar.edu comp.std.c++ FAQ: http://reality.sgi.com/austern/std-c++/faq.html Moderation policy: http://reality.sgi.com/austern/std-c++/policy.html Comments? mailto:std-c++-request@ncar.ucar.edu ]