220 21320 <31dedac5-7d51-402c-946e-0555c925f70d@isocpp.org> article Path: news.gmane.org!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Conversion from lvalue to rvalue Date: Mon, 5 Oct 2015 14:12:22 -0700 (PDT) Lines: 139 Approved: news@gmane.org Message-ID: <31dedac5-7d51-402c-946e-0555c925f70d@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_832_1673001065.1444079542262" X-Trace: ger.gmane.org 1444079545 15602 80.91.229.3 (5 Oct 2015 21:12:25 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 5 Oct 2015 21:12:25 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBN6PZOYAKGQE6Y7Z2JI@isocpp.org Mon Oct 05 23:12:25 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f197.google.com ([209.85.220.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZjD3Y-0001HL-LO for gclcip-std-proposals@m.gmane.org; Mon, 05 Oct 2015 23:12:24 +0200 Original-Received: by qkao72 with SMTP id o72sf238129605qka.2 for ; Mon, 05 Oct 2015 14:12:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :content-type:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=9w92VZCZoJWHlWy3XykhaF/9SbEnhiG0r34EE0RHH6E=; b=1FFGb2+Dg/vSlJxpEaN07MPuD+kuL4z+RWJsGAhOzvwJT4wgz2Ua0kxp3BXxB5TqJD Hu3yp9Ne+hwNOHL5Vho+mbBdEcx3TyslJNeOpX8kPnGLRlLThorEEM4Gwx4IfxZb5UK7 LscLwAjb61YPbEuo8PTuRh+aagoYY1QccjQR+VZoHNt2ZTUv+cyX9is2WkqqsEisDzJX ezuH7OVO6LuSQALyWKRh7gAa6eR7lsfn0058wsnsv/hGBxu7ctVmF3t5GdoFbqKcQwLW XyFQIM7A2cFb3xz9BZtgaAF2zQlb/cdZwpJla07XIhF0SGmvfTLa8s5YL8hamvMdhD6w lfQQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:content-type:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=9w92VZCZoJWHlWy3XykhaF/9SbEnhiG0r34EE0RHH6E=; b=I1bWwJENqXQMBA2fjUEuJz20cATItuC1XrmNMngxtWge1CP9FMOhcxVc8YGRpkfwKM 7bI9webys5A+P0a/rlTaAUkjzcxVvnGrUR7OXEBAgczJPni/UGUPplX33NlbfEsdEd4F LDJlKHVK7+jagd1QqX7rbNecXWLpFyABUkCgjfFXjDQf2VoDmqKsAAlEinD8EERvQ4lB SgFaXqcD1wF5/a8r3it7r/zqQCpPTy6Sj24uXDA3hnsBtqSXbvOMiBEx/idGHj0QGASL x3PljsVHQBKw6Dn6IYcESHrTxXFfnXWVHgm3MFEqiuY329boPmbhPzkKiywxJeTHxOcM jXVQ== X-Gm-Message-State: ALoCoQlFBwQr6tkYZ6hiXRnMdO1q76F01Ec7+8Veyw52BP/gWSIEgFx3AyDxGgOh9Sq9E8XkuZp6 X-Received: by 10.129.88.87 with SMTP id m84mr27147228ywb.3.1444079543851; Mon, 05 Oct 2015 14:12:23 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.5.139 with SMTP id 133ls1049671iof.107.gmail; Mon, 05 Oct 2015 14:12:23 -0700 (PDT) X-Received: by 10.50.112.4 with SMTP id im4mr134278igb.1.1444079543043; Mon, 05 Oct 2015 14:12:23 -0700 (PDT) In-Reply-To: X-Original-Sender: jmckesson@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:21320 Archived-At: ------=_Part_832_1673001065.1444079542262 Content-Type: multipart/alternative; boundary="----=_Part_833_1477843509.1444079542263" ------=_Part_833_1477843509.1444079542263 Content-Type: text/plain; charset=UTF-8 On Monday, October 5, 2015 at 4:24:39 PM UTC-4, Vlad from Moscow wrote: > > I am suggesting to use the following general syntax > > std::tie( a, b ) = std::minmax( auto( a ), auto( b ) ); > So you want `auto(a)` to result in a temporary that is copy/move constructed from `a`, with the type having been deduced from the type of `a` (minus references of course). You want a shorthand for: std::remove_reference_t(a) Your basic problem in this case is that fact that the destination function was taking and returning references to the parameters, and you were trying to store those values in the variables being referenced. Your solution is to create a pair of temporaries, pass them, and copy the temporaries back into your variables. Or... you could just create variables. You could even use `auto newA = a` to create it. And you can use scoping to make them temporary: { auto oldA = a, oldB = b; std::tie( a, b ) = std::minmax( oldA, oldB ); } Is this something we really need to modify the language to do? Is it common enough that we need specialized syntax to solve it? Do you have any other use cases for such functionality? -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On Monday, October 5, 2015 at 4:24:39 PM UTC-4, Vl= ad from Moscow wrote:
I am suggesting to use the following general= syntax

=C2=A0=C2=A0=C2=A0 std::tie( a, b ) =3D st= d::minmax(=C2=A0auto( a ),=C2=A0auto( b=C2=A0) );

So you want `auto(a)` to result in a temporary that is copy/mov= e constructed from `a`, with the type having been deduced from the type of = `a` (minus references of course). You want a shorthand for:

std::remove_reference_t<decltype(a)>(a)

Your basic problem in this case i= s that fact that the destination function was taking and returning referenc= es to the parameters, and you were trying to store those values in the vari= ables being referenced. Your solution is to create a pair of temporaries, p= ass them, and copy the temporaries back into your variables.

Or... y= ou could just create variables. You could even use `auto newA =3D a` to cre= ate it. And you can use scoping to make them temporary:

{
=C2=A0 auto oldA =3D a, oldB =3D b;
=C2=A0 st= d::= tie( a, b ) = =3D std::minmax( oldA, oldB );
}

Is this something we really need to modi= fy the language to do? Is it common enough that we need specialized syntax = to solve it?

Do you have any other use cases for such functionality?=

--

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