220 36478 article Path: news.gmane.org!.POSTED!not-for-mail From: "Arthur O'Dwyer" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Treating local variables as rvalues in return statements Date: Wed, 3 Jan 2018 11:05:10 -0800 Lines: 128 Approved: news@gmane.org Message-ID: References: <4ff5fe2d-0945-491f-9888-479b8e866ace@isocpp.org> <1823126.nxGvHgdJMA@tjmaciei-mobl1> <3356554.N1GgDjyayG@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="f403045c28b8fa3ae30561e3e423" X-Trace: blaine.gmane.org 1515006204 17119 195.159.176.226 (3 Jan 2018 19:03:24 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 3 Jan 2018 19:03:24 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDLZJYWNDQIOPUVU2ICRUBEGDPWJY@isocpp.org Wed Jan 03 20:03:19 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f200.google.com ([209.85.128.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eWoJh-0003XJ-K6 for gclcip-std-proposals@m.gmane.org; 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[209.85.220.65]) by mx.google.com with SMTPS id w46sor1015516eda.57.2018.01.03.11.05.11 for (Google Transport Security); Wed, 03 Jan 2018 11:05:11 -0800 (PST) Received-SPF: pass (google.com: domain of arthur.j.odwyer@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; X-Received: by 10.80.148.112 with SMTP id q45mr3893217eda.161.1515006310626; Wed, 03 Jan 2018 11:05:10 -0800 (PST) Original-Received: by 10.80.176.162 with HTTP; Wed, 3 Jan 2018 11:05:10 -0800 (PST) In-Reply-To: <3356554.N1GgDjyayG@tjmaciei-mobl1> X-Original-Sender: arthur.j.odwyer@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=fDMP3bO3; spf=pass (google.com: domain of arthur.j.odwyer@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=arthur.j.odwyer@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:36478 Archived-At: --f403045c28b8fa3ae30561e3e423 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, Jan 3, 2018 at 5:31 AM, Thiago Macieira wrote= : > On Wednesday, 3 January 2018 10:32:40 -02 Olaf van der Spek wrote: > > 2018-01-03 12:33 GMT+01:00 Thiago Macieira : > > > On Wednesday, 3 January 2018 08:51:05 -02 olafvdspek@gmail.com wrote: > > >> Wouldn't the idea be to do the analysis and only consider it a rvalu= e > if > > >> it's NOT accessed later in the statement / function? > > > > > > And its address has never been taken, which also means it must never > have > > > been passed by reference either. That limits the usefulness too much = to > > > make it a feature. > > > > Taking the address is fine as long as all references are dead by the > > time it's considered an rvalue. > > And how do you prove that they are? > Guys, you're going down an unnecessary rabbit hole here. The way C++11 solved the problem was just to say that *it doesn't matter* if there's a live reference or not. Certain syntactic constructs simply turn into moves (not copies), *regardless* of the number of live references to the operand. https://wandbox.org/permlink/DIveFuEiL5JzSkOj (The output "moved from" indicates that we're observing `Observable o` *after* it has been moved from, via the reference we stashed.) Also, please remember that even in simple cases with no aliasing at all, moving-from an object *does not end its lifetime*. Every object is accessed at least once after its "last" move-access =E2=80=94 namely, when it is ref= erenced by the `this` argument of its own destructor. Moves are *always* observable. Trying to "hide" moves or pretend that you can do them under the as-if rule will never work. You *must* confront the human factor head-on: Which syntactic constructs do human programmers "expect" to produce moves, and which do they "expect" to produce copies? HTH, Arthur --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/CADvuK0LhJMe01RiTW5M52A9WwsG4wzHrgcHiT5jXN59Gyoh= 6AQ%40mail.gmail.com. --f403045c28b8fa3ae30561e3e423 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Wed, Jan 3, 2018 at 5:31 AM, Thiago Macieira <thiag= o@macieira.org> wrote:
On Wedne= sday, 3 January 2018 10:32:40 -02 Olaf van der Spek wrote:
> 2018-01-03 12:33 GMT+01:00 Thiago Macieira <thiago@macieira.org>:
> > On Wednesday, 3 January 2018 08:51:05 -02 olafvdspek@gmail.com wrote:
> >> Wouldn't the idea be to do the analysis and only consider= it a rvalue if
> >> it's NOT accessed later in the statement / function?
> >
> > And its address has never been taken, which also means it must ne= ver have
> > been passed by reference either. That limits the usefulness too m= uch to
> > make it a feature.
>
> Taking the address is fine as long as all references are dead by the > time it's considered an rvalue.

And how do you prove that they are?

<= div>Guys, you're going down an unnecessary rabbit hole here.
= The way C++11 solved the problem was just to say that it doesn't = matter if there's a live reference or not. Certain syntactic co= nstructs simply turn into moves (not copies), regardless of t= he number of live references to the operand.

(The output "moved from&qu= ot; indicates that we're observing `Observable o` after it has b= een moved from, via the reference we stashed.)

Als= o, please remember that even in simple cases with no aliasing at all, movin= g-from an object does not end its lifetime. Every object is accessed= at least once after its "last" move-access =E2=80=94 namely, whe= n it is referenced by the `this` argument of its own destructor. Moves are = always observable. Trying to "hide" moves or pretend that = you can do them under the as-if rule will never work. You must confr= ont the human factor head-on: Which syntactic constructs do human programme= rs "expect" to produce moves, and which do they "expect"= ; to produce copies?

HTH,
Arthur

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