220 11781 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 14:12:50 +0800 Lines: 85 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 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_5D3D7484-C4AA-4437-8142-084D173768DC" X-Trace: ger.gmane.org 1404972784 21066 80.91.229.3 (10 Jul 2014 06:13:04 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 10 Jul 2014 06:13:04 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBZW57COQKGQE6ZS77XQ@isocpp.org Thu Jul 10 08:12:57 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 1X57bF-0004CK-1C for gclcip-std-proposals@m.gmane.org; Thu, 10 Jul 2014 08:12:57 +0200 Original-Received: by mail-pa0-f70.google.com with SMTP id lj1sf57151453pab.5 for ; Wed, 09 Jul 2014 23:12:55 -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=EE9cAXi+3kD24VjAWKnEklh5Rwm5zMHm1u3qMyck970=; b=URUI12G4TzUr+2fJ0zCfvruUympy2aHJpdBwmF96fZJuxGDdDnxxLDmFfqE46uB/OD y4ehYHVo3X/BPT77N19KfOjcrGhR/ITrFF9lW/CKocinZR7pHbcaT5/10PP15Wz3OHcI R3PUjpeufS5MzZoXjha7b7H9/RVxcBgdtXRyxkjwr/VQhnAlyeTViAHJF7X4/5h+EHi+ 3XjgCzIQATZtShRKtaabpGGn5JY+2TJ+XHD32TCr63O4P/r2jyHTlvazYAJKulzwsj6X 6F6UxO10OxuHSaLv0yBBIClehTBxOEXVVQIQagE7PEC4uPtKTA9Fr/6jpFBzJEBCP4mY /qLw== X-Gm-Message-State: ALoCoQmZ6H/qZONADsAv+Nh/+DUeJ9GtTjuAPfzIgwHVVnyEYgMMg7CjkDDse/AmBCKr2qZ0lHHg X-Received: by 10.66.121.2 with SMTP id lg2mr11786123pab.43.1404972775315; Wed, 09 Jul 2014 23:12:55 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.51.17.39 with SMTP id gb7ls822igd.17.gmail; Wed, 09 Jul 2014 23:12:54 -0700 (PDT) X-Received: by 10.42.240.20 with SMTP id ky20mr9995icb.97.1404972774750; Wed, 09 Jul 2014 23:12:54 -0700 (PDT) Original-Received: from mail-ie0-x22b.google.com (mail-ie0-x22b.google.com [2607:f8b0:4001:c03::22b]) by mx.google.com with ESMTPS id z7si60582669icx.45.2014.07.09.23.12.54 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 23:12:54 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c03::22b as permitted sender) client-ip=2607:f8b0:4001:c03::22b; Original-Received: by mail-ie0-f171.google.com with SMTP id at1so2518388iec.16 for ; Wed, 09 Jul 2014 23:12:54 -0700 (PDT) X-Received: by 10.50.70.99 with SMTP id l3mr18921714igu.1.1404972774552; Wed, 09 Jul 2014 23:12:54 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.63]) by mx.google.com with ESMTPSA id z6sm22355982igl.1.2014.07.09.23.12.53 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 23:12:54 -0700 (PDT) In-Reply-To: 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:c03::22b 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:11781 Archived-At: --Apple-Mail=_5D3D7484-C4AA-4437-8142-084D173768DC Content-Type: text/plain; charset=ISO-8859-1 On 2014-07-10, at 2:04 PM, Ville Voutilainen wrote: >> As a very general policy in C++ library design, every accessor to a strictly >> owned resource should be lvalue-qualified and/or return by value. This best >> practice should be applied across the standard library. > > I fail to see how that is "best practice". 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 incorrect to return a reference to it. Returning by rvalue reference may be tempting, 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 rvalue 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, correctness is always worth the cost -- and copy elision 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 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 semantic. -- --- 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 email 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-proposals/. --Apple-Mail=_5D3D7484-C4AA-4437-8142-084D173768DC Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;07–10, at 2:04 PM, Ville Voutilainen <ville.voutilainen@gmail.com> wrote:
As a very general pol= icy in C++ library design, every accessor to a strictly
owned resource s= hould be lvalue-qualified and/or return by value. This best
practice sho= uld be applied across the standard library.

I fail to s= ee how that is "best practice”.

T= he resource is strictly owned and the owner is represented by an xvalue. Th= erefore the resource will be going away very soon. It would be incorrect to= return a reference to it.

Returning by rvalue ref= erence may be tempting, but that is the semantic of move(). It does not tra= nsfer ownership, it only provides the caller with a handle to accept owners= hip. Returning an rvalue reference is something only special functions shou= ld do.

Whether= returning a direct member or something owned elsewhere, methods of
an x= value should always return by value. If a temporary might be introduced
= elsewhere, correctness is always worth the cost — and copy elision wi= ll
probably eliminate it anyway.

Except that copy el= ision will not eliminate it, and this correctness
is not worth
the co= st.

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

--

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