220 11874 <8209A2E9-D947-4C00-A1F3-38F5676A4E52@gmail.com> 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: Sat, 12 Jul 2014 07:30:08 +0800 Lines: 93 Approved: news@gmane.org Message-ID: <8209A2E9-D947-4C00-A1F3-38F5676A4E52@gmail.com> 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> <6B925FF7-2940-44C7-BF1D-13209A703BA2@gmail.com> <944C9C01-865B-4215-8C91-95F01AEA256E@gmail.com> <8DC3E474-3957-40B2-B492-D4AF2FDDD451@gmail.com> <0ACAE525-4FB2-4932-BBD2-F0623C4EBBFB@gmail.com> <52187852-ff8e-4e59-a8cf-7a29fd0a405b@isocpp.org> 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; 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Fri, 11 Jul 2014 16:30:22 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c03::22a as permitted sender) client-ip=2607:f8b0:4001:c03::22a; Original-Received: by mail-ie0-f170.google.com with SMTP id rl12so146239iec.15 for ; Fri, 11 Jul 2014 16:30:22 -0700 (PDT) X-Received: by 10.42.198.136 with SMTP id eo8mr7756335icb.7.1405121422421; Fri, 11 Jul 2014 16:30:22 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.139]) by mx.google.com with ESMTPSA id jc2sm62294igb.19.2014.07.11.16.30.20 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 11 Jul 2014 16:30:22 -0700 (PDT) In-Reply-To: <52187852-ff8e-4e59-a8cf-7a29fd0a405b@isocpp.org> 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::22a 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:11874 Archived-At: --Apple-Mail=_1F73ABED-AE92-4FD5-999C-76FF7336FE37 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 2014-07-11, at 6:57 PM, tomaszkam@gmail.com wrote: > Do we found any other argumentation for T member()&& overload except maki= ng auto&& s =3D f; work for some cases?=20 >=20 > In the follwoing example: > template > delctype(auto) f(T&& t) { return std::addressof(*std::forward(t)); } > =20 > template > delctype(auto) g(T&& t) {=20 > auto&& ret =3D f(std::forward(t));=20 > //will create a dangling pointer, because the liftetime of result o= f f.value() is not longer same a liftme of the owner t Once again you've omitted the parens after f in the comment. Please proofre= ad before posting. f is a naked function and f() is a naked pointer. Neithe= r can have a member .value. You probably mean *f or f->value() under the as= sumption that f is type optional*. A function that converts a possibly-owning smart pointer to merely an obser= ving naked pointer is obviously not a candidate for move semantics, so the = example is not reasonable. I will provide a thorough analysis of value categories, member access expre= ssions, forwarding contexts, and function calls (or even better, someone el= se will), but this thread is not the right place. --=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=_1F73ABED-AE92-4FD5-999C-76FF7336FE37 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;07–11, at 6:57 PM, tomas= zkam@gmail.com wrote:

Do we found any other argumentation = for T member()&& overload except making auto&& s =3D f; wor= k for some cases?

In the follwoing example:
  template<t= ypename T>
  delctype(auto) f(T&& t) { return std::addre= ssof(*std::forward<T>(t)); }
  
  template<t= ypename T>
  delctype(auto) g(T&& t) {
  &= nbsp;   auto&& ret =3D f(std::forward<T>(t));
&= nbsp;     //will create a dangling pointer, because the= liftetime of result of f.value() is not longer same a liftme of the owner = t

Once again you’ve omitted= the parens after f in the comment. Please proofread before posting. f is a= naked function and f() is a naked pointer. Neither can have a member .value. You probably mean *= f or f->value() under the assumptio= n that f is type optional*.

A function that converts a possibly-owning smart pointer to merely = an observing naked pointer is obviously not a candidate for move semantics,= so the example is not reasonable.

I will provide = a thorough analysis of value categories, member access expressions, forward= ing contexts, and function calls (or even better, someone else will), but t= his thread is not the right place.

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