220 21337 <2C19913E-B6BC-423D-8570-939C3A573CDF@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Conversion from lvalue to rvalue Date: Tue, 6 Oct 2015 15:34:54 +0800 Lines: 134 Approved: news@gmane.org Message-ID: <2C19913E-B6BC-423D-8570-939C3A573CDF@gmail.com> References: <31dedac5-7d51-402c-946e-0555c925f70d@isocpp.org> <116a66a2-90ea-4269-ac85-3432ff7094b0@isocpp.org> <42447957-4276-4f59-89b6-ab4be37c47c6@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2104\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_D01DCD90-2953-4100-9334-C1E4B27ABB33" X-Trace: ger.gmane.org 1444116910 14909 80.91.229.3 (6 Oct 2015 07:35:10 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 6 Oct 2015 07:35:10 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBJPTZWYAKGQEJBFHO2Y@isocpp.org Tue Oct 06 09:35:05 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f197.google.com ([209.85.192.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZjMm8-0006Hb-Ib for gclcip-std-proposals@m.gmane.org; Tue, 06 Oct 2015 09:35:05 +0200 Original-Received: by pdnr2 with SMTP id r2sf365159807pdn.0 for ; Tue, 06 Oct 2015 00:35:02 -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:content-type: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:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=aDeA9S5hZfQknTJuxd00p8GN2kfO/gNEqXut0+SCN2M=; b=fs0cp5kYDc8c5oEq/Ksid5iG7txi4ZdAOcVY8+gwg88LM6MPvnu+9fnnSW/8Z9Sjn1 WYJ7yumMQkHZkn3Blm3bi20xk08qrADtrSYu0sdxyYL7ojcIeiN1PRTz/wbrFZDh/056 N2BlDUNPZIID02HbBrPZQ+HBY4XWsxmIc5+fUMRWUprjFKzeHXnbNIH9E6RHKVl0r3yG 2oskpXUfZ/g2Oxbw8v9T9zBMqeQX0ESKHPaCSqSf45HNANRw/8HAgjDbpqYD2KXfQvv7 bo0pHyL20VJLL/IZNC/yJxNLlq4N60KvPg4p9WuEEB6Wof/mEhxASBktB4ZKEUKPBTf2 ulaQ== X-Gm-Message-State: ALoCoQn81PECAjmjahvnHs2egEJ3K5hXpll+lsHrv7lbpaFdtiZHvEDXL/piZHM/K05uAynl0r0r X-Received: by 10.68.248.137 with SMTP id ym9mr29678428pbc.14.1444116902371; Tue, 06 Oct 2015 00:35:02 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.12.194 with SMTP id 63ls1873027iom.103.gmail; Tue, 06 Oct 2015 00:35:01 -0700 (PDT) X-Received: by 10.66.121.110 with SMTP id lj14mr44956488pab.61.1444116901352; Tue, 06 Oct 2015 00:35:01 -0700 (PDT) Original-Received: from mail-pa0-x234.google.com (mail-pa0-x234.google.com. [2607:f8b0:400e:c03::234]) by mx.google.com with ESMTPS id i16si42560585pbq.81.2015.10.06.00.35.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 06 Oct 2015 00:35:01 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::234 as permitted sender) client-ip=2607:f8b0:400e:c03::234; Original-Received: by pacfv12 with SMTP id fv12so206390324pac.2 for ; Tue, 06 Oct 2015 00:35:01 -0700 (PDT) X-Received: by 10.68.57.175 with SMTP id j15mr45490526pbq.34.1444116901153; Tue, 06 Oct 2015 00:35:01 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.89]) by smtp.gmail.com with ESMTPSA id qn5sm31786449pac.41.2015.10.06.00.34.58 for (version=TLSv1/SSLv3 cipher=OTHER); Tue, 06 Oct 2015 00:35:00 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.2104) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::234 as permitted sender) smtp.mailfrom=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-Spam-Checked-In-Group: std-proposals@isocpp.org 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:21337 Archived-At: --Apple-Mail=_D01DCD90-2953-4100-9334-C1E4B27ABB33 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 2015=E2=80=9310=E2=80=9306, at 9:37 AM, Agust=C3=ADn K-ballo Berg=C3= =A9 wrote: >=20 > This would do wasteful copies from rvalues. If you look at 30.2.6 [thread= ..decaycopy] you'll see it's defined as: >=20 > template decay_t decay_copy(T&& v) > { return std::forward(v); } >=20 > I guess `auto` would do too. decay_copy would always perform a copy, from an lvalue, xvalue, or prvalue. auto{} would allow copy elision for prvalue operands. It would copy lvalue,= xvalue, and derived-type prvalues. The ideal solution would return only lvalues by value, and rvalues always b= y reference. An overloaded function can do this, but the references would t= end to dangle without lifetime extension of function operands a la P0066 . However, that still doesn=E2=80=99t (yet) cover ass= ignment operations like to std::tie in the OP. --=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=_D01DCD90-2953-4100-9334-C1E4B27ABB33 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 2015=E2=80=9310= =E2=80=9306, at 9:37 AM, Agust=C3=ADn K-ballo Berg=C3=A9 <kaballo86@hotmail.com> wrote:<= /div>
This would d= o wasteful copies from rvalues. If you look at 30.2.6 [thread.decaycopy] yo= u'll see it's defined as:

   template <class T> decay= _t<T> decay_copy(T&& v)
     { return std::forward<T>(v); }

I= guess `auto` would do too.
<= /blockquote>

decay_copy would always perform a copy= , from an lvalue, xvalue, or prvalue.

auto{} would allow copy elision for prvalue ope= rands. It would copy lvalue, xvalue, and derived-type prvalues.

The ideal solution would ret= urn only lvalues by value, and rvalues always by reference. An overloaded f= unction can do this, but the references would tend to dangle without lifeti= me extension of function operands a la P0066. However, that still doesn=E2=80=99t (yet) cover a= ssignment operations like to std::ti= e in the OP.

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