220 11851 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: Fri, 11 Jul 2014 17:36:03 +0800 Lines: 114 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> <66c38b61-7a95-4ace-b34f-2346bb334fb9@isocpp.org> <6F4B3C72-9962-48DC-8527-0C89EA1B9710@gmail.com> <35873CE5-ED8C-4C03-A6A1-E4132DECD91F@gmail.com> <335948E6-8140-4673-B958-930356A52070@gmail.com> <76ACA175-5A44-4BE8-B49F-BAC6DA65E6B1@gmail.com> <4D3852AD-1AEB-44BA-941C-D645BAD19405@gmail.com> <6a4b6425-1167-40c1-acd8-3945c8cd4c74@isocpp.org> <13DD0D7A-A146-4667-9F30-6FB3739CDBE0@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=_D9B675E4-E67D-49EA-A350-7A0FA65E0481" X-Trace: ger.gmane.org 1405071382 7031 80.91.229.3 (11 Jul 2014 09:36:22 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 11 Jul 2014 09:36:22 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBDHA72OQKGQEYY4I52Y@isocpp.org Fri Jul 11 11:36:14 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yk0-f199.google.com ([209.85.160.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X5XFV-0003NR-DT for gclcip-std-proposals@m.gmane.org; Fri, 11 Jul 2014 11:36:13 +0200 Original-Received: by mail-yk0-f199.google.com with SMTP id 200sf2081774ykr.10 for ; Fri, 11 Jul 2014 02:36:12 -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=ih+kZC0fFEfGap98EdFvQcatAXiEgs3fQ+BzK8UI8Ls=; b=jC4RXTzeja3xKEwSqwxWWCk3W53/3D0ROi05FP5xpXge5mR4wqEYZvYWIhwjG85qqC zHzZUi3adjL2G7Bd6hJkWE5rjCoW7wG2zT/EkMPpP0V/cZeEpowgeCc0rAtzMb/tauGW OGJ1s8vvFJcyg2CTJbGqKMGQFdN1LP1C8o3BoZWW5z0fTxFlH5GnjC/aik/WRasjKhWt 03C10hCr5MuOhqP8lSagsVOx6Ts5SFGQKK1wzEI6Nlja7QFg6XNuHKRGAQLazsr04zZT CrJkpcUMmLXj7nJXFretqqh7gws9pJyuPHZT8+tPPziaU36E8XweuXSZb2xRgGpIcAVO FcEA== X-Gm-Message-State: ALoCoQnz0wllS/DHIsXl6MRZ69N80nXjtUO5nSxjscHRnnPj+VBdQSYlQ92fHWQazDNFCn+zGDV2 X-Received: by 10.236.14.99 with SMTP id c63mr22759283yhc.40.1405071372574; Fri, 11 Jul 2014 02:36:12 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.136.168 with SMTP id qb8ls265934igb.43.gmail; Fri, 11 Jul 2014 02:36:12 -0700 (PDT) X-Received: by 10.50.25.130 with SMTP id c2mr2905646igg.7.1405071371994; Fri, 11 Jul 2014 02:36:11 -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 ii2si2382087igb.62.2014.07.11.02.36.11 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 11 Jul 2014 02:36:11 -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 at1so682985iec.2 for ; Fri, 11 Jul 2014 02:36:11 -0700 (PDT) X-Received: by 10.42.239.196 with SMTP id kx4mr3041825icb.71.1405071371860; Fri, 11 Jul 2014 02:36:11 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.43]) by mx.google.com with ESMTPSA id qa4sm4456543igb.10.2014.07.11.02.36.09 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 11 Jul 2014 02:36:11 -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:11851 Archived-At: --Apple-Mail=_D9B675E4-E67D-49EA-A350-7A0FA65E0481 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 2014-07-11, at 5:09 PM, Andrzej Krzemienski wrote: > 2014-07-11 11:02 GMT+02:00 David Krauss : > As was discussed earlier, return-by-xvalue does not work with local unive= rsal references. Universal references are not just a cute thing that trendy= programmers like to sprinkle around, they provide a way for generic code t= o bind a name to something which may or may not be a reference. >=20 > David, would it be possible to illustrate with an example what you mean h= ere? I've adjusted the comments slightly, just for exposition of what forwarding= in this case actually does. template< typename container_handle > void operate_on_fifth_thing( container_handle && ch ) { // Get only the fifth element. We don't need the rest; OK to free those= resources. // However, if not assuming ownership, there's no need to make a copy. auto && fifth_thing =3D=3D ( * std::forward< container_handle >( ch ) )= [ 5 ]; // This is illustrated as a function call, but there might as well be a= loop here. process_for_a_long_time( std::forward< decltype( thing ) >( fifth_thing= ) ); } I don't think this is far-fetched at all, aside from presupposing rvalue-ov= erloaded operator* and operator[]. You generally might do the same things with a named object as with a tempor= ary. "Forwarding contexts" don't all end at the semicolon. --=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=_D9B675E4-E67D-49EA-A350-7A0FA65E0481 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2= 014–07–11, at 5:09 PM, Andrzej Krzemienski <akrzemi1@gmail.com> wrote:

2014-0= 7-11 11:02 GMT+02:00 David Krauss <potswa@gmail.com>:
As was discussed earlier, return-by-xvalue does not work with local univ= ersal references. Universal references are not just a cute thing that trend= y programmers like to sprinkle around, they provide a way for generic code = to bind a name to something which may or may not be a reference.

David, would it be possible to illustrate = with an example what you mean here?

I’ve adjusted the comments slightly, just = for exposition of what forwarding in this case actually does.

template< typename container_handle ><= /div>
void operate_on_fifth_thing( container_han= dle && ch ) {
   = // Get only the fifth element. We don’t need the rest; OK to free th= ose resources.
    // How= ever, if not assuming ownership, there’s no need to make a copy.=
    auto && fift= h_thing =3D=3D ( * std::forward< container_handle >( ch ) )[ 5 ];

    // This is illustrated as a function call, but there m= ight as well be a loop here.
 =   process_for_a_long_time( std::forward< decltype( thing ) >( f= ifth_thing ) );
}
=
I don’t think this is far-fetched at all, aside from p= resupposing rvalue-overloaded operator* and <= font face=3D"Courier">operator[].

You gener= ally might do the same things with a named object as with a temporary. &ldq= uo;Forwarding contexts” don’t all end at the semicolon.

--

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