220 36483 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 11:41:26 -0800 Lines: 154 Approved: news@gmane.org Message-ID: References: <4ff5fe2d-0945-491f-9888-479b8e866ace@isocpp.org> <3356554.N1GgDjyayG@tjmaciei-mobl1> <2794319.GCS1XuZPMK@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="94eb2c1b5110bc5f730561e46697" X-Trace: blaine.gmane.org 1515008379 13607 195.159.176.226 (3 Jan 2018 19:39:39 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 3 Jan 2018 19:39:39 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDLZJYWNDQIORY5U2ICRUBG5BTMGQ@isocpp.org Wed Jan 03 20:39:35 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f71.google.com ([74.125.82.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eWoso-0002mr-A1 for gclcip-std-proposals@m.gmane.org; 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[209.85.220.65]) by mx.google.com with SMTPS id v15sor1046646edl.35.2018.01.03.11.41.28 for (Google Transport Security); Wed, 03 Jan 2018 11:41:28 -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 10.80.194.74 with SMTP id t10mr4139867edf.116.1515008487610; Wed, 03 Jan 2018 11:41:27 -0800 (PST) Original-Received: by 10.80.176.162 with HTTP; Wed, 3 Jan 2018 11:41:26 -0800 (PST) In-Reply-To: <2794319.GCS1XuZPMK@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=s/yZQKqy; 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:36483 Archived-At: --94eb2c1b5110bc5f730561e46697 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, Jan 3, 2018 at 11:33 AM, Thiago Macieira wrote: > On Wednesday, 3 January 2018 17:05:10 -02 Arthur O'Dwyer wrote: > > 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 matte= r* > > 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. > > Indeed, but that was done because in those cases, any live references wer= e > about to become dangling anyway, so any well-defined program would never > use > them after this point. You snipped out the example of a well-defined program that used them after that point. Here it is again. https://wandbox.org/permlink/DIveFuEiL5JzSkOj Perfectly well-defined. No undefined behavior here. It was safe to move from this particular object. > I'd tend to agree with you there. However, please continue to remember that "safe" is a human concept, not a computerese concept. It's "safe" to move from `o` in this case because that's what the human programmer "expected." If the human programmer hadn't expected `o` to appear "moved from" in the second output =E2=80=94 such that that faulty expectation had caused the co= mputer to launch missiles or crash the plane or whatever =E2=80=94 then it wouldn'= t have been "safe" after all. > 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 > referenced > > by the `this` argument of its own destructor. Moves are *always* > > observable. Trying to "hide" moves or pretend that you can do them unde= r > > 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? > > All true. > =E2=80=93Arthur --=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/CADvuK0K1DGi-uv%2BFc-33wT%3D_VR0R6NDThAR-ZFK%2B%= 3DXcgtrwdrw%40mail.gmail.com. --94eb2c1b5110bc5f730561e46697 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Wed, Jan 3, 2018 at 11:33 AM, Thiago Macieira <thiag= o@macieira.org> wrote:
On Wedne= sday, 3 January 2018 17:05:10 -02 Arthur O'Dwyer wrote:
> 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 synta= ctic constructs simply
> turn into moves (not copies), *regardless* of the number of liv= e references
> to the operand.

Indeed, but that was done because in those cases, any live references were<= br> about to become dangling anyway, so any well-defined program would never us= e
them after this point.

You snipped out the = example of a well-defined program that used them after that point. Here it = is again.
Perfectly well-defined. No = undefined behavior here.


It was safe to move from this particular object.

I'd tend to agree with you= there. However, please continue to remember that "safe" is a hum= an concept, not a computerese concept. It's "safe" to move fr= om `o` in this case because that's what the human programmer "expe= cted." If the human programmer hadn't expected `o` to appear "= ;moved from" in the second output =E2=80=94 such that that faulty expe= ctation had caused the computer to launch missiles or crash the plane or wh= atever =E2=80=94 then it wouldn't have been "safe" after all.=


> Also, please remember that even in simple cases with no aliasing at al= l,
> 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 referenced
> by the `this` argument of its own destructor. Moves are *always= *
> observable. Trying to "hide" moves or prete= nd that you can do them under
> the as-if rule will never work. You *must* confront the human f= actor
> head-on: Which syntactic constructs do human programm= ers "expect" to
> produce moves, and which do they "expect" to produce copies?=

All true.

=E2=80=93Arthur=C2=A0<= /div>

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