220 11782 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen 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 09:17:16 +0300 Lines: 55 Approved: news@gmane.org Message-ID: 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: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1404973044 23672 80.91.229.3 (10 Jul 2014 06:17:24 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 10 Jul 2014 06:17:24 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRB3G77COQKGQEWS2XWHQ@isocpp.org Thu Jul 10 08:17:18 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qg0-f70.google.com ([209.85.192.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X57fS-0007ZJ-14 for gclcip-std-proposals@m.gmane.org; Thu, 10 Jul 2014 08:17:18 +0200 Original-Received: by mail-qg0-f70.google.com with SMTP id z60sf19583952qgd.5 for ; Wed, 09 Jul 2014 23:17:17 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from: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:content-transfer-encoding; bh=p+1Sji5ILxDUiiqELHoeUUii6/yt1dw5bGTzY5HsL+Q=; b=jTPNICC96+AnWaBgkTLuFIa8djgtxec+wZ5D1G5w+9WgIATL6hgAtpsNE9cCcbEiyU ZIeQuLeXQ2qetEa6RNqZaT/pKcrYryfmFfQEpI6rgPGSdvBCODTpQc1ZIGgF0hDME6DK Ccpk0ZmUM7MGgLa08ASZKqtacE/y5wZ343MVt/t3eGUA3aHT0WKcaPnzJy0K/aUacmY3 ciBWHprySHBgwetnC/iCfj+PBm0cH45KTDbLiERGkczeDh+76okk0SGX+R5PX+Hb8+ij +G/vh4HYPUGMRSXaclcB8sIgecBlDxoKPUQkOsOV4Y9bkK4yrh2kfRP1OJOvX6GFGhJx De6Q== X-Gm-Message-State: ALoCoQnnk/GVKcgQPu14w6gNXR2cyk6MuWww0VepnRcM/hE/JlslYN4AsiORBn2pgScMN9FvqgD2 X-Received: by 10.236.151.74 with SMTP id a50mr19732740yhk.32.1404973037262; Wed, 09 Jul 2014 23:17:17 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.36.229 with SMTP id p92ls2695362qgp.92.gmail; Wed, 09 Jul 2014 23:17:16 -0700 (PDT) X-Received: by 10.140.97.53 with SMTP id l50mr71297417qge.29.1404973036753; Wed, 09 Jul 2014 23:17:16 -0700 (PDT) Original-Received: from mail-qg0-x229.google.com (mail-qg0-x229.google.com [2607:f8b0:400d:c04::229]) by mx.google.com with ESMTPS id h11si42154731qgf.82.2014.07.09.23.17.16 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 23:17:16 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400d:c04::229 as permitted sender) client-ip=2607:f8b0:400d:c04::229; Original-Received: by mail-qg0-f41.google.com with SMTP id i50so7257799qgf.14 for ; Wed, 09 Jul 2014 23:17:16 -0700 (PDT) X-Received: by 10.224.163.146 with SMTP id a18mr76946721qay.31.1404973036640; Wed, 09 Jul 2014 23:17:16 -0700 (PDT) Original-Received: by 10.224.31.137 with HTTP; Wed, 9 Jul 2014 23:17:16 -0700 (PDT) In-Reply-To: X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400d:c04::229 as permitted sender) smtp.mail=ville.voutilainen@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:11782 Archived-At: On 10 July 2014 09:12, David Krauss wrote: > > 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 stric= tly > 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. > Therefore the resource will be going away very soon. It would be incorrec= t > to return a reference to it. There's two examples in this thread which explain why it's quite correct in order to avoid extraneous temporaries and to allow forwarding to work without metaprogramming tricks. > Returning by rvalue reference may be tempting, but that is the semantic o= f > move(). It does not transfer ownership, it only provides the caller with = a > handle to accept ownership. Returning an rvalue reference is something on= ly > special functions should do. I have no idea where this guideline popped out of. > Then perhaps we need more cases for copy elision. Sneaky rvalue reference > hacks will only provide surprises and make user code more brittle. If the= re > must be an rvalue reference overload, it should be named move_value() to > indicate the different semantic. That would be equally detrimental to users who want to forward. --=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/. .