220 11778 <66AF41C1-CB37-4311-98EA-D47DBC6068DC@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: N4078: Rvalue reference overloads for value() method returns object by value Date: Thu, 10 Jul 2014 13:48:17 +0800 Lines: 109 Approved: news@gmane.org Message-ID: <66AF41C1-CB37-4311-98EA-D47DBC6068DC@gmail.com> References: <4DA018C3-2327-4F55-B7F8-7988EC7B789A@gmail.com> <1a858a77-310b-4591-aaf5-960c40d9f5fc@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_A330A76D-647A-412A-B8F6-CB5E58257283" X-Trace: ger.gmane.org 1404971316 6254 80.91.229.3 (10 Jul 2014 05:48:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 10 Jul 2014 05:48:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBLGS7COQKGQEYEX72DY@isocpp.org Thu Jul 10 07:48:30 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yh0-f70.google.com ([209.85.213.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X57DZ-000268-Ow for gclcip-std-proposals@m.gmane.org; Thu, 10 Jul 2014 07:48:30 +0200 Original-Received: by mail-yh0-f70.google.com with SMTP id a41sf33738791yho.9 for ; Wed, 09 Jul 2014 22:48:28 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:message-id:mime-version:subject:date :references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=MShBuRKueb3+Ckto5PzUNyu6EKKA6U9wNwHImKhiD8I=; b=E7awtGXcFklBkPGGudyDT9W6093AEUUxnX/0YOeOJvZtGCjrQCdafzenni7RbU+c4N 3X/ZN71Rxq8OsAY3pugpiZfo0WgTeKOMSFihS/AGSHP1gLXpg5dbopCrwEHhtv0UF435 6KyuWPBrT7VcYJwGnl2LNJvX2T9xnh4PpIQP0/cXcecrnBop7Y5es9vBgku9434li9oY i8j+PsC2woEz7qD+PRBFaZWHaTZBMFDw27zonK9g/fWWj5Yks1KlmgFkpvpWVrUPjrtK JSlRNIQzwiBvY3ga8DAH5yTGKv7q8MFNUI7/9Kol5i/xYGBzBp9HweDXsPUfMAj6NB+j dLUg== X-Gm-Message-State: ALoCoQlxCAsPkSx+TCPN+7TS4xFQWa+SWGLyC9z4+gxOmsSc9Zbw8Gdyu4CFx9gxy8XNanGnAaHB X-Received: by 10.58.19.227 with SMTP id i3mr21226108vee.2.1404971308827; Wed, 09 Jul 2014 22:48:28 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.79.135 with SMTP id j7ls1291217igx.20.gmail; Wed, 09 Jul 2014 22:48:28 -0700 (PDT) X-Received: by 10.42.11.74 with SMTP id t10mr50743971ict.27.1404971308158; Wed, 09 Jul 2014 22:48:28 -0700 (PDT) Original-Received: from mail-ig0-x232.google.com (mail-ig0-x232.google.com [2607:f8b0:4001:c05::232]) by mx.google.com with ESMTPS id is10si6559873igb.20.2014.07.09.22.48.28 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 22:48:28 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c05::232 as permitted sender) client-ip=2607:f8b0:4001:c05::232; Original-Received: by mail-ig0-f178.google.com with SMTP id hn18so2707756igb.17 for ; Wed, 09 Jul 2014 22:48:28 -0700 (PDT) X-Received: by 10.50.111.132 with SMTP id ii4mr19371159igb.18.1404971308000; Wed, 09 Jul 2014 22:48:28 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.63]) by mx.google.com with ESMTPSA id qa4sm22168495igb.10.2014.07.09.22.48.25 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 22:48:27 -0700 (PDT) In-Reply-To: <1a858a77-310b-4591-aaf5-960c40d9f5fc@isocpp.org> X-Mailer: Apple Mail (2.1878.6) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c05::232 as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:11778 Archived-At: --Apple-Mail=_A330A76D-647A-412A-B8F6-CB5E58257283 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 On 2014=E2=80=9307=E2=80=9310, at 1:11 PM, tomaszkam@gmail.com wrote: > W dniu czwartek, 10 lipca 2014 01:17:39 UTC+2 u=C5=BCytkownik David Kraus= s napisa=C5=82: >=20 > auto && has some strong proponents, and although nothing is a silver bull= et, it will always work except when a function returns an rvalue reference = =E2=80=94 which, as the title of this thread and most of the early messages= indicate, is a problem to be fixed. > No, the auto&& does not work with any function that return a any kind of = reference to subobject of it parameter (including this) is invoked on an rv= alue. In the examples above you my find: > std::vector f(); > auto&& s =3D f.front(); > //This creates a dangling lvalue reference, and front() has not r-value o= verloads. I think you mean f().front(). As per this thread, or maybe it=E2=80=99s jus= t my extrapolation, the solution is to add an rvalue-ref qualified overload= returning by value. As a very general policy in C++ library design, every accessor to a strictl= y owned resource should be lvalue-qualified and/or return by value. This be= st practice should be applied across the standard library. > The whole discussion is about that the changes was made to the optional that makes following work correctly in cost on introducing temporaries in= other places: 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, correctness is always worth the cost =E2=80=94 and copy elision= will probably eliminate it anyway. --=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/. --Apple-Mail=_A330A76D-647A-412A-B8F6-CB5E58257283 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 2014=E2=80= =9307=E2=80=9310, at 1:11 PM, tomasz= kam@gmail.com wrote:

W dniu czwartek, 10 lipca 2014 01:17:39 UTC+2 u=C5=BCytkownik David Kraus= s napisa=C5=82:

= auto && has some strong proponents, and although nothing is = a silver bullet, it will always work except when a function returns an rval= ue reference =E2=80=94 which, as the title of this thread and most of the e= arly messages indicate, is a problem to be fixed.

No, the auto&& does not work with any function that return a any k= ind of reference to subobject of it parameter (including this) is invoked o= n an rvalue. In the examples above you my find:
std::vector<std::stri= ng> f();
auto&& s =3D f.front();
//This creates a dangling= lvalue reference, and front() has not r-value overloads.

I think you mean f().= front(). As per this thread, or maybe it=E2=80=99s just my extrapola= tion, the solution is to add an rvalue-ref qualified overload returning by = value.

As a very general policy in C++ library des= ign, every accessor to a strictly owned resource should be lvalue-qualified= and/or return by value. This best practice should be applied across the st= andard library.

The whole discussion is about that the changes was made to the opt= ional<T> that makes following work correctly in cost on introducing t= emporaries in other places:

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 elision will probably eliminate it anyway.

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