220 11783 <8888e3af-c11a-4e44-970d-01c0ef795f97@isocpp.org> article Path: news.gmane.org!not-for-mail From: tomaszkam@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: N4078: Rvalue reference overloads for value() method returns object by value Date: Wed, 9 Jul 2014 23:18:42 -0700 (PDT) Lines: 139 Approved: news@gmane.org Message-ID: <8888e3af-c11a-4e44-970d-01c0ef795f97@isocpp.org> References: <4DA018C3-2327-4F55-B7F8-7988EC7B789A@gmail.com> <1a858a77-310b-4591-aaf5-960c40d9f5fc@isocpp.org> <66AF41C1-CB37-4311-98EA-D47DBC6068DC@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_31_922469.1404973122432" X-Trace: ger.gmane.org 1404973135 24836 80.91.229.3 (10 Jul 2014 06:18:55 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 10 Jul 2014 06:18:55 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDNPVXXG6IGBBQ7A7COQKGQEAZZVO6I@isocpp.org Thu Jul 10 08:18:49 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f70.google.com ([209.85.220.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X57gr-0000II-8F for gclcip-std-proposals@m.gmane.org; Thu, 10 Jul 2014 08:18:45 +0200 Original-Received: by mail-pa0-f70.google.com with SMTP id lj1sf57168065pab.1 for ; Wed, 09 Jul 2014 23:18:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=8WnHFw+ZwqQbOWBrHQSXB/CUWrvSviGLwqMvyvaOwwQ=; b=HdF2Wrym3vJFis1a9nUEaHEMlOiz+okrKexWjVWZI+Z79HXRfrAIbSGgSw1xbDWv5p 5N6pMUXIm3XTHcuHQj26kzOMYZh6TiMd8J+MY66g/WqUGTJC4gGazzLVlTRc2WV5pdLz ThbwuNrfkM/4Pq6A0GsdtYF+w4/S2Aiy55vBBst3Pyoh1lArpPD544eR/CLHpgDwe2iJ AfTHhLF/sCFja//QmgEBID36GaBoCG5KHr27InC32unids+Q9nHnXVOTbkp36vhsE4me xaDfv+qhcJ9R4dwQVZcsLltvqbnve4bXXiRMZBnzE+2eK2uS3RSE2vqFoS8rshcyu++V Bljw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=8WnHFw+ZwqQbOWBrHQSXB/CUWrvSviGLwqMvyvaOwwQ=; b=b2M5N7xvaIdvHXDswOKkGIWcgc7AxKmd5r0WJLuiczfCqg7LrbELOAx4T0Xx9lAwrC xhHt1grPEbCbh4JR2NMymWrjM0yPO2eO+RskwWzqgsDCiZ+bEGCwXntY4h5LQtWrXZj0 AvWAnDsB2bf71sgr5xIzx8z55UfdE7qA1Yagj4AivTeqQiDPjtTfAz5cMoRTGF+7OgIa OPwkQS/oexMFqEtHDn+na75NpgXtW4I3dlSVQ///KonOztEPeWrw6c5zDp4REJ+3QruS o5Y9YzSm5JQfn7FCDaT/SETCILsLNT0j+c8qxXPkW3Cs306eLF6burRH35hNe94VW9Yo XNaA== X-Gm-Message-State: ALoCoQk2dWCbbHGheiw0np5f/kq7GNhwBcY85tcmsPOMdH1xGTK7gg4w/WQilX31K/HTA8OUiRdA X-Received: by 10.66.182.130 with SMTP id ee2mr8096519pac.37.1404973124198; Wed, 09 Jul 2014 23:18:44 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.135.193 with SMTP id pu1ls1436030obb.19.gmail; Wed, 09 Jul 2014 23:18:42 -0700 (PDT) X-Received: by 10.182.196.34 with SMTP id ij2mr4937obc.24.1404973122957; Wed, 09 Jul 2014 23:18:42 -0700 (PDT) In-Reply-To: X-Original-Sender: tomaszkam@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:11783 Archived-At: ------=_Part_31_922469.1404973122432 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable W dniu czwartek, 10 lipca 2014 08:12:55 UTC+2 u=C5=BCytkownik David Krauss= =20 napisa=C5=82: > > > On 2014=E2=80=9307=E2=80=9310, at 2:04 PM, Ville Voutilainen > wrote: > > As a very general policy in C++ library design, every accessor to a=20 > strictly > owned resource should be lvalue-qualified and/or return by value. This be= st > practice should be applied across the standard library. > > > I fail to see how that is "best practice=E2=80=9D. > > > The resource is strictly owned and the owner is represented by an xvalue.= =20 > Therefore the resource will be going away very soon. It would be incorrec= t=20 > to return a reference to it. > > Returning by rvalue reference may be tempting, but that is the semantic o= f=20 > move(). It does not transfer ownership, it only provides the caller with = a=20 > handle to accept ownership. Returning an rvalue reference is something on= ly=20 > special functions should do. > > Whether returning a direct member or something owned elsewhere, methods o= f > an xvalue should always return by value. If a temporary might be introduc= ed > elsewhere, correctness is always worth the cost =E2=80=94 and copy elisio= n will > probably eliminate it anyway. > > > Except that copy elision will not eliminate it, and this correctness > is not worth > the cost. > > > Then perhaps we need more cases for copy elision. Sneaky rvalue reference= =20 > hacks will only provide surprises and make user code more brittle. If the= re=20 > must be an rvalue reference overload, it should be named move_value() to= =20 > indicate the different semantic. > > I dont see like introduing the rvalue reference overload that makes the=20 code works more coherent with current status qou, that is no reference=20 overlaod for getters provide more suprises that introducing temporaries in= =20 places that they were not present before in existing codebases. Please also= =20 remeber about forward.=20 --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. ------=_Part_31_922469.1404973122432 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


W dniu czwartek, 10 lipca 2014 08:12:55 UTC+2 u=C5= =BCytkownik David Krauss napisa=C5=82:

On 2014=E2=80=930= 7=E2=80=9310, at 2:04 PM, Ville Voutilainen <ville.vo...@gmail.com> wrote:

As a very general policy in C++ library de= sign, every accessor to a strictly
owned resource should be lvalue-quali= fied and/or return by value. This best
practice should be applied across= the standard library.

I fail to see how that is "best = practice=E2=80=9D.

The resource is stri= ctly owned and the owner is represented by an xvalue. Therefore the resourc= e will be going away very soon. It would be incorrect to return a reference= to it.

Returning by rvalue reference may be tempt= ing, but that is the semantic of move(). It does not transfer ownership, it= only provides the caller with a handle to accept ownership. Returning an r= value reference is something only special functions should do.

Whether returning a direct= member or something owned elsewhere, methods of
an xvalue should always= return by value. If a temporary might be introduced
elsewhere, correctn= ess is always worth the cost =E2=80=94 and copy elision will
probably el= iminate it anyway.

Except that copy elision will not el= iminate it, and this correctness
is not worth
the cost.

Then perhaps we need more cases for copy elision. Sneaky = rvalue reference hacks will only provide surprises and make user code more = brittle. If there must be an rvalue reference overload, it should be named = move_value() to indicate the different semant= ic.


I dont see like introdu= ing the rvalue reference overload that makes the code works more coherent w= ith current status qou, that is no reference overlaod for getters provide m= ore suprises that introducing temporaries in places that they were not pres= ent before in existing codebases. Please also remeber about forward.

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