220 11849 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 17:29:40 +0800 Lines: 104 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=_5DB44DB9-2EAB-46AD-AF6E-4FB2EB298B76" X-Trace: ger.gmane.org 1405070999 2919 80.91.229.3 (11 Jul 2014 09:29:59 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 11 Jul 2014 09:29:59 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBDO572OQKGQELWYJEGY@isocpp.org Fri Jul 11 11:29:52 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f200.google.com ([209.85.214.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X5X9L-0005A0-9l for gclcip-std-proposals@m.gmane.org; Fri, 11 Jul 2014 11:29:51 +0200 Original-Received: by mail-ob0-f200.google.com with SMTP id nu7sf4159205obb.11 for ; Fri, 11 Jul 2014 02:29:50 -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=MehNiBvPD9Nugi5OvmztbEq7tvhPP2uPkKQMEPrCn9M=; b=MauC/pkIy4/NGm5NthykKz7DfOVlXZ/AXQKwfacg7L2cPWhnBNxe1Ie8CO2x8fUC4u HPgPTYO2KGjVwvmF12xDNI4mzzGLwFmCzJk1NDHD6MSsGT9vCF3WYuonagHsyd769o60 X5O3ayHsSxxbl6exRQ3z4zO59ZmVAAFRdomJIAlP3lrZo90hTFfbEYzi2I0AY17mQpJF Nwt0KAQPqwDfQL/Wk4ShS/XJA6+yCBfGIPf3UcISvCKTLIMbno5tTK8O0e6FGEwqLkk6 183tZHBCh0aUutZG8Y8v8zEpi/fnzRkfSXAFemLaGHSljwUD3Pgfhstkc8jzUCq7W/ld l6wg== X-Gm-Message-State: ALoCoQn2WFE2vateHzjFw4FNIkA2ez1GrW8Xnhg+nJUozOI+xOU6RJy4kxl4yHMcjxLeacvVf6qT X-Received: by 10.42.151.67 with SMTP id d3mr1495182icw.6.1405070990183; Fri, 11 Jul 2014 02:29:50 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.79.135 with SMTP id j7ls310585igx.6.canary; Fri, 11 Jul 2014 02:29:49 -0700 (PDT) X-Received: by 10.50.79.232 with SMTP id m8mr2737835igx.33.1405070989005; Fri, 11 Jul 2014 02:29:49 -0700 (PDT) Original-Received: from mail-ig0-x229.google.com (mail-ig0-x229.google.com [2607:f8b0:4001:c05::229]) by mx.google.com with ESMTPS id hs3si2359673igb.51.2014.07.11.02.29.48 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 11 Jul 2014 02:29:49 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c05::229 as permitted sender) client-ip=2607:f8b0:4001:c05::229; Original-Received: by mail-ig0-f169.google.com with SMTP id r10so351083igi.2 for ; Fri, 11 Jul 2014 02:29:48 -0700 (PDT) X-Received: by 10.43.140.193 with SMTP id jb1mr3205334icc.15.1405070988890; Fri, 11 Jul 2014 02:29:48 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.43]) by mx.google.com with ESMTPSA id hv1sm1428938igb.0.2014.07.11.02.29.45 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 11 Jul 2014 02:29:48 -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:c05::229 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:11849 Archived-At: --Apple-Mail=_5DB44DB9-2EAB-46AD-AF6E-4FB2EB298B76 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? template< typename container_handle > void operate_on_fifth_thing( container_handle && ch ) { // Save only the fifth element. We don't need the rest; OK to free thos= e resources. 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=_5DB44DB9-2EAB-46AD-AF6E-4FB2EB298B76 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;07–11, at 5:09 PM, Andrzej Krzemienski <akrzemi1@gmail.com> wrote:

2014-07-11= 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?

template< typename container_hand= le >
void operate_on_fifth_thing= ( container_handle && ch ) {
    // Save only the fifth element. We don’t need the res= t; OK to free those resources.
&nbs= p;   auto && fifth_thing =3D=3D ( * std::forward< container= _handle >( ch ) )[ 5 ];

    // This is illustrated a= s 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-overloaded operator* and operator[].

You generally might do the same things with a named = object as with a temporary. “Forwarding contexts” don’t a= ll end at the semicolon.

--

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