220 11859 <9838a60d-0527-4fb6-907a-acb41732ee19@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 03:25:22 -0700 (PDT) Lines: 115 Approved: news@gmane.org Message-ID: <9838a60d-0527-4fb6-907a-acb41732ee19@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> <7411f096-742f-4e9b-a265-1b05a1af5521@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_178_3937797.1405074322161" X-Trace: ger.gmane.org 1405074334 11900 80.91.229.3 (11 Jul 2014 10:25:34 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 11 Jul 2014 10:25:34 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDNPVXXG6IGBBE7X72OQKGQELNJXZKQ@isocpp.org Fri Jul 11 12:25:27 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f199.google.com ([209.85.214.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1X5Y16-0001wo-Se for gclcip-std-proposals@m.gmane.org; Fri, 11 Jul 2014 12:25:25 +0200 Original-Received: by mail-ob0-f199.google.com with SMTP id wn1sf3198183obc.2 for ; Fri, 11 Jul 2014 03:25:24 -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=UUvhKo1Y2YniP2R5Zx1zVi+Yqo7WhgUdhzfWnU23nSk=; b=HlGEHG28tJstDqurPsN6aVrdd34FsiBrIok4HfuArQY+6mtgvDRhoKswsom32Idirq Rl5sGPqdOl2592RKwOki4vN6OjHbUcIV0SWvAGr6mZMtk/lYmKpXbBl9Yqel3yR8Gjjg uudgclCVH6gRt2pxLZrWZfDBSwkXbB4nyNR8uXNTuH6XP2OCYsRnrIrVV+ZfjlVcqDNF kiX9mpkELenI/Pnb5+ITOlqiBKQ7cH3XFFdP7GzqLzeBREpgjohGR0eyhj8pBXLefIRq hrNA89fM3qYEqJt5ZKO6cOUP90Uvdgh2fFJvA/bxGnhFw1wemxy765YyiPEuSJ6JOnI6 YGoA== 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=UUvhKo1Y2YniP2R5Zx1zVi+Yqo7WhgUdhzfWnU23nSk=; b=j6ssUxhMvKaBZdQuk7zTYi8c3XIFAdfcpFEcK0BSUzZ1JHfcVsZydKmnVmQdiWYLmO pf156Kj6Y8su405j1GcvX2DCMpTu0zybGVgXzq5pd/IaQVDxuftv+W+aLyTL3gmlIvme ytASsPoW+n9QDSSrkf8NhopUT9W9bEOmhcvCVxPEax67HsBKNaB+E6PHzZNKpLMnkk3W 6wybqzAse6rssc31sc/EnwDc4xgEYQdRZCJRrDA/ky+b4gjkg/l7yP1bYWMw4P94+dJD UeNaCNDv+JdRcDpBDVwcoCxlkmTTJ8lKdtjPQ3TgQ1Wr8cnpfd9zQ1/NalYiqfL7V3H0 +Hbg== X-Gm-Message-State: ALoCoQk7f8VTetx+99Dreno8rcXeM5YilRCLdIF18khGFJrEBrLEzdBnE5cRoeXGVbGVIwrsHpaY X-Received: by 10.42.100.138 with SMTP id a10mr1538413ico.32.1405074323887; Fri, 11 Jul 2014 03:25:23 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.42.195 with SMTP id q3ls342261obl.97.gmail; Fri, 11 Jul 2014 03:25:23 -0700 (PDT) X-Received: by 10.182.153.200 with SMTP id vi8mr10922obb.23.1405074323021; Fri, 11 Jul 2014 03:25:23 -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:11859 Archived-At: ------=_Part_178_3937797.1405074322161 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable W dniu pi=C4=85tek, 11 lipca 2014 12:17:51 UTC+2 u=C5=BCytkownik David Krau= ss napisa=C5=82: > > > On 2014=E2=80=9307=E2=80=9311, at 5:56 PM, toma...@gmail.com wrote: > > And the really funny think. Your code will not work if container_handle i= s=20 > a lazy evaluator and * std::forward< container_handle >( ch ) acutally=20 > returns the container by value, ortogonalny to the fact if we have &&=20 > overloads and the solutions presented by me will actually fix the code.= =20 > Because the * std::forward< container_handle >( ch ) will be created and= =20 > used in one expression. The auto&& will create dangling reference. > > > operator[]() && is also assumed to return a prvalue. This is what gets=20 > bound to the reference, so it does not dangle. > But it current design is current state (without && qualifiers) in not work.= =20 And having && returning by value cause a lot of other problmes (see post=20 above) - especially copying in place wher is is not actually needed. The=20 auto&& f, will still not work if *ch[0] will use an expression template and= =20 return some operator_result by prvalue that has reference to ch. So if we= =20 cannot make the auto&& to work in generic code (the generic code should=20 handle expression template), there is no reason to introduce prvalue &&=20 overloads, especially when... > > Factoring into a second one-line function completely misses the point.=20 > Sometimes we want multiple lines in a function ;v) . > > ... we can use wiht(expr, [](auto&& ) { arbitrally code; }) to make this=20 work everwhere. And yes we can put more than one line in lambda. --=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_178_3937797.1405074322161 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


W dniu pi=C4=85tek, 11 lipca 2014 12:17:51 UTC+2 u= =C5=BCytkownik David Krauss napisa=C5=82:

On 2014=E2=80= =9307=E2=80=9311, at 5:56 PM, toma...@= gmail.com wrote:

An= d the really funny think. Your code will not work if container_handle is a = lazy evaluator and * std::forward< container_hand= le >( ch ) acutally returns the container by value, ortogonalny to the f= act if we have && overloads and the solutions presented by m= e will actually fix the code. Because the * std::for= ward< container_handle >( ch ) will be created and used in one= expression. The auto&& will create dangling reference.

operator[]() &&am= p; is also assumed to return a prvalue. This is what gets bound= to the reference, so it does not dangle.

But it current design is current state (without && qualifiers= ) in not work. And having && returning by value cause a lot of othe= r problmes (see post above) - especially copying in place wher is is not ac= tually needed. The auto&& f, will still not work if *ch[0] will use= an expression template and return some operator_result by prvalue that has= reference to ch. So if we cannot make the auto&& to work in generi= c code (the generic code should handle expression template), there is no re= ason to introduce prvalue && overloads, especially when...

Factoring into a second one-line function c= ompletely misses the point. Sometimes we want multiple lines in a function = ;v) .


.. we can use wiht(expr, [= ](auto&& ) { arbitrally code; }) to make this work everwhere. And y= es we can put more than one line in lambda.

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