220 11779 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 09:04:19 +0300 Lines: 58 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 1404972270 15833 80.91.229.3 (10 Jul 2014 06:04:30 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 10 Jul 2014 06:04:30 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRBY6Z7COQKGQECREIIUQ@isocpp.org Thu Jul 10 08:04:24 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qg0-f71.google.com ([209.85.192.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X57Sv-0005ta-AY for gclcip-std-proposals@m.gmane.org; Thu, 10 Jul 2014 08:04:21 +0200 Original-Received: by mail-qg0-f71.google.com with SMTP id a108sf1537895qge.10 for ; Wed, 09 Jul 2014 23:04:20 -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=kjBV8mrgaQ2bmk0fzCfkAnf0POdm4F4bGnNrfJwRy+o=; b=HX9I2vYkhBZCAtLK2XiyOfaBu1WtzTffLQie/hJdQlrQ2ItN5N+Km5CRdf3/Fhwh15 ZasCd3YrP6sHuPQh7dRBt/jr6+zI7jWIWmt4KJ1dDPjchxIF9eC6UPoZ6mahjlSUm2Ms KAkn1bh67icGZd9dvlnKNUDmCznUKRQwkj7GvzZ0RGcHcEoI8aiePK+0TtOgdQJsZo0F y2FvSYFVKOLl1JsftWLPihNXn1XNZbd1Qe8ba844mHGp32nWsRlGveaBI37hfndnK1L0 yF90w4x0WV9WzDdAHYsY+r/rNE/nrzLKe2/dU8RfClPY5Zm3kJdYKYRSmbTHQPRl83aG WS3A== X-Gm-Message-State: ALoCoQlOEM3XZSb12g2MAdAkHUwt54uTku1S8KBSx0G4E3ZyVliRtXDjbfq6ubX7cNA31orguLF1 X-Received: by 10.236.189.68 with SMTP id b44mr19880099yhn.4.1404972260128; Wed, 09 Jul 2014 23:04:20 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.26.241 with SMTP id 104ls2631570qgv.21.gmail; Wed, 09 Jul 2014 23:04:19 -0700 (PDT) X-Received: by 10.224.114.69 with SMTP id d5mr77173641qaq.96.1404972259332; Wed, 09 Jul 2014 23:04:19 -0700 (PDT) Original-Received: from mail-qc0-x22c.google.com (mail-qc0-x22c.google.com [2607:f8b0:400d:c01::22c]) by mx.google.com with ESMTPS id d2si25654845qge.13.2014.07.09.23.04.19 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 09 Jul 2014 23:04:19 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400d:c01::22c as permitted sender) client-ip=2607:f8b0:400d:c01::22c; Original-Received: by mail-qc0-f172.google.com with SMTP id l6so315550qcy.3 for ; Wed, 09 Jul 2014 23:04:19 -0700 (PDT) X-Received: by 10.224.87.130 with SMTP id w2mr76817236qal.5.1404972259220; Wed, 09 Jul 2014 23:04:19 -0700 (PDT) Original-Received: by 10.224.31.137 with HTTP; Wed, 9 Jul 2014 23:04:19 -0700 (PDT) In-Reply-To: <66AF41C1-CB37-4311-98EA-D47DBC6068DC@gmail.com> 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:c01::22c 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:11779 Archived-At: On 10 July 2014 08:48, David Krauss wrote: > > On 2014=E2=80=9307=E2=80=9310, at 1:11 PM, tomaszkam@gmail.com wrote: > 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 > rvalue. 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 > overloads. > > > I think you mean f().front(). As per this thread, or maybe it=E2=80=99s j= ust my > extrapolation, the solution is to add an rvalue-ref qualified overload > returning by value. The solution, as per this thread, is to not use auto&& for such cases. > 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". > > 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 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. --=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/. .