From -6845388283303152565 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,f5d433881cc29cff X-Google-Attributes: gidf78e5,public From: Mike Ferrel Subject: Re: Named return value optimisation- reprise. Date: 1997/06/23 Message-ID: <33AACCA6.6C30@sr.hp.com>#1/1 X-Deja-AN: 252083341 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> <33a9da21.2670962@news.ipswich.gil.com.au> X-Original-Date: Fri, 20 Jun 1997 11:32:06 -0700 Organization: Hewlett Packard Sonoma County X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBVAwUBM67fw0y4NqrwXLNJAQELFgIAny5X6GcVsU3JSupiqMNGWTTbhiyQ8oFi ZT+4dFyQuL+LQSath5k06WphcIhvS6ceQVzk7ZSWePxLN+910VXDzg== =ofgz Newsgroups: comp.std.c++ Originator: austern@isolde.mti.sgi.com Brian Parker wrote: > > I wonder if a better rewording of 12.8/15 would be to simply add a > sentence preventing its use on pass-by-value. > > i.e. change 12.8/15 to - > > "Whenever a class object [intro.object] is copied via a *copy > constructor* and the original object and the copy have the same type, > *except on pass-by-value (5.2.2 Function call)* if the implementation > can prove ... (rest of wording the same)." > > And remove the current examples, replacing them with a NRVO example. > > This would still allow copies to be elided on NRVO, RVO and > initialisation of variables. > I have been following this thread with some bemusement, and can't understand how a compiler could possibly replace a call-by-value with a call by reference. The function body would have to contain entirely different code. For this to be possible, the compiler would have to generate different function bodies containing code to accept value parameters as value parameters, and as reference parameters. For functions with more than one value parameter, the exponential increase in numbers of distinct function bodies would cause impossible code bloat. The only way I see the compiler could possibly reasonably alter call semantics is if the function were declared static, and its definition occured after all calls, and the compiler had determined that the optimisation could be used in all calls. It seems like that's an awful lot of compiler housekeeping for an exceedingly rare occurence. I can't imagine any implementer going to the effort. Mike Ferrel --- [ 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 ]