220 11848 <6ff861ad-4711-48f9-981f-dcd41babf567@isocpp.org> article Path: news.gmane.org!not-for-mail From: tomaszkam@gmail.com 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 02:26:24 -0700 (PDT) Lines: 125 Approved: news@gmane.org Message-ID: <6ff861ad-4711-48f9-981f-dcd41babf567@isocpp.org> 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 Content-Type: multipart/alternative; boundary="----=_Part_117_13150077.1405070784540" X-Trace: ger.gmane.org 1405070796 637 80.91.229.3 (11 Jul 2014 09:26:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 11 Jul 2014 09:26:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDNPVXXG6IGBBQW372OQKGQEBGP6QYQ@isocpp.org Fri Jul 11 11:26:29 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f200.google.com ([209.85.223.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X5X63-0001hz-UC for gclcip-std-proposals@m.gmane.org; Fri, 11 Jul 2014 11:26:28 +0200 Original-Received: by mail-ie0-f200.google.com with SMTP id at20sf4271579iec.3 for ; Fri, 11 Jul 2014 02:26:27 -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 :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=6ap7dmTmxbAZ09ncSp77SvWBWew1yf0SV6H7hIHL0Hc=; b=e293Ye5sFt2M1JVz+SyANGmGD8dlJLqGq0TfU7ZPLJaom9guc9cf9GcoOu+70plS5q GAt0EvnMEaZHcfkTt4UIXc9ps7Kr1hvWA/BHaPLUKBnCC10zjjGVe6ftVBDnPe2bazEW H2SVDGcVynpRdl/p/Gpcbo+2IDdNV5t+uO7H2dFdg2/m0N3CiwisHnLNva5J4jrTuYD/ X5FpnZw2C3XOUFd1S1pazWk8xv6gLIPlc12YeTpH4dPQGaZYTqA8Vu0d7BHLaALQ5LZo rBb8LVPdd7rmjLKRVH6KnI8Yy5DGQtiv2cGfpNIRIL55Kr8ihh1qbz3lajSIKurk/BmA 3GcQ== 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:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=6ap7dmTmxbAZ09ncSp77SvWBWew1yf0SV6H7hIHL0Hc=; b=SewMFeh53l3/lpQKy720cZhKHuvNuvWzCOwx+VwaptcoY/T4aPzNpIaddPL45x1W1s wCvLwfM8CzLgJjf0DqVO6v6cLQCya4JW8XDmQ+s53Wnps3il+lnCO3NZEWF4DmWMh2HM JGJBmj/AfgsfJLwtsl2iFNGjzXAToZ1IuVA2d/UGLA6XKDBAHu4BuMGkTLtgTNl03Lgi CoDXAWhkvhxRWA06T7KH2M7IzIX62ufCurQH7+CSFOiwXKiFnSA0h1babD/4XwUXseA5 aiYNptZ7eFWDy11h36gHb99uQHnN4dXEEy6IWTMxCfmisttEjjXbNzw/itHItq4+k2mN n0JQ== X-Gm-Message-State: ALoCoQkOGIWpj1T8RtCpc4XZQuhC5ErisXLJjetPmdDlmlw1xssl+d3sN5rd+RlfrDvoxweMAvLo X-Received: by 10.182.125.4 with SMTP id mm4mr18486711obb.49.1405070786959; Fri, 11 Jul 2014 02:26:26 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.49.194 with SMTP id w2ls289530obn.56.gmail; Fri, 11 Jul 2014 02:26:25 -0700 (PDT) X-Received: by 10.182.98.136 with SMTP id ei8mr383obb.39.1405070785909; Fri, 11 Jul 2014 02:26:25 -0700 (PDT) In-Reply-To: X-Original-Sender: tomaszkam@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:11848 Archived-At: ------=_Part_117_13150077.1405070784540 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable But this still does not work with current overloads, so it wont be broken.= =20 In addition the auto&& will never work with expression templates even if=20 operator auto/using auto is introduced. If we have a matrix class that uses= =20 expression templates for multuplication the following expression m1*m2 will return object by value (mult_result) that stores=20 references so following: auto s =3D m1*m2; auto&& s =3D m1*m2; Will not work, becuase the object referenced by mult_result will be=20 destroyed by end of expression. The operator auto fixes only the first case= .. But actually doing: with(expr, [](auto&& cs) { /*code that uses it*/ });=20 Will fix every dangling reference. Or: auto f =3D []() -> delctype(auto) { return /*some long expr*/l } somelongexprtakingbeforeresultbrokeninparts(f()); Becuaxse all temporary objects and code that uses it will be run in single= =20 expression. W dniu pi=C4=85tek, 11 lipca 2014 11:02:08 UTC+2 u=C5=BCytkownik David Krau= ss napisa=C5=82: > > > On 2014=E2=80=9307=E2=80=9311, at 4:58 PM, Tomasz Kami=C5=84ski > wrote:=20 > > > Could you please provide me actuall code example that is actually broke= n=20 > by introduction of T&& operator*() && to existing set of overloads? Id di= d=20 > not found any in your post, only The generic statment tha will broek the= =20 > library/code.=20 > > As was discussed earlier, return-by-xvalue does not work with local=20 > universal references. Universal references are not just a cute thing that= =20 > trendy programmers like to sprinkle around, they provide a way for generi= c=20 > code to bind a name to something which may or may not be a reference.=20 > > --=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/. ------=_Part_117_13150077.1405070784540 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
But this still d= oes not work=20 with current overloads, so it wont be broken.

In addition the auto&= amp;& will never work with expression templates even if operator auto/u= sing auto is introduced. If we have a matrix class that uses expression tem= plates for multuplication the following
expression m1*m2 will return obj= ect by value (mult_result) that stores references so following:
  a= uto s =3D m1*m2;
  auto&& s =3D m1*m2;
Will not work, be= cuase the object referenced by mult_result will be destroyed by end of expr= ession. The operator auto fixes only the first case.

But actua= lly doing:
with(expr, [](auto&& cs) { /*code that uses it*/ });
Will fix every dangling reference.
Or:
auto f =3D []() -> = delctype(auto) { return /*some long expr*/l }
somelongexprtakingbe= foreresultbrokeninparts(f());
Becuaxse all temporary objects and code that uses it will be run in s= ingle expression.


W dniu pi=C4=85tek, 11 lipca 2014 11:02:08 UTC= +2 u=C5=BCytkownik David Krauss napisa=C5=82:

On 2014=E2=80=9307=E2=80=9311, at 4:58 PM, Tomasz Kami=C5=84ski <toma...@gmail.com> wrote:

> Could you please provide me actuall code example that is actually = broken by introduction of T&& operator*() && to existing se= t of overloads? Id did not found any in your post, only The generic statmen= t tha will broek the library/code.

As was discussed earlier, return-by-xvalue does not work with local uni= versal references. Universal references are not just a cute thing that tren= dy 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.

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