220 11886 <225BBAAC-B3FA-4014-9FB6-64C48DF4208E@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 15:50:54 +0800 Lines: 86 Approved: news@gmane.org Message-ID: <225BBAAC-B3FA-4014-9FB6-64C48DF4208E@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> <8209A2E9-D947-4C00-A1F3-38F5676A4E52@gmail.com> <4c42f7c9-0588-4ab9-9af5-b2e692513993@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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Sat, 12 Jul 2014 00:51:14 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4001:c03::234 as permitted sender) client-ip=2607:f8b0:4001:c03::234; Original-Received: by mail-ie0-f180.google.com with SMTP id at20so1741013iec.11 for ; Sat, 12 Jul 2014 00:51:14 -0700 (PDT) X-Received: by 10.42.43.5 with SMTP id v5mr9595955ice.62.1405151474284; Sat, 12 Jul 2014 00:51:14 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.57]) by mx.google.com with ESMTPSA id d4sm3458282igc.5.2014.07.12.00.51.12 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 12 Jul 2014 00:51:13 -0700 (PDT) In-Reply-To: <4c42f7c9-0588-4ab9-9af5-b2e692513993@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::234 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:11886 Archived-At: --Apple-Mail=_AB2BD471-1A28-40A3-B3E0-944E7AB57C56 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 2014-07-12, at 3:27 PM, tomaszkam@gmail.com wrote: > 3. The above code does not work if if the passed Nullable type define && = overload of operator* that returns by value. > Lifetime of object bound by auto&& value =3D *std::forward(= nullable); is not the same as lifetime of nullable. Yes, but in that case what does the std::forward express? Reading that func= tion, I would intuitively expect addressof(value) to become a dangling refe= rence. Any other interpretation requires thinking hard about what did *not*= happen to the result of forward. It wasn't passed to a function, but absen= ce of some common thing isn't a good discriminating factor. (And in fact, i= t was passed to a function, operator *.) Because function_taking_pointer(pointer) passes a non-owning observer, null= able must live at least through that call. The intent is best expressed by = simply value =3D * nullable; do not use forward(). This can be declared as = auto && or auto & equivalently. Am I missing something? --=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=_AB2BD471-1A28-40A3-B3E0-944E7AB57C56 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;07–12, at 3:27 PM, tomas= zkam@gmail.com wrote:

3. The above code does not work if i= f the passed Nullable type define && overload of operator* that ret= urns by value.
    Lifetime of object bound by auto&= amp;& value =3D *std::forward<Nullable>(nullable); is not the sam= e as lifetime of nullable.

Yes, but in that case what does the std::forward express? Reading that function, I would intuitively expect addressof(value) to become a dangling reference. Any ot= her interpretation requires thinking hard about what did *not* happe= n to the result of forward. It wasn&rsqu= o;t passed to a function, but absence of some common thing isn’t a go= od discriminating factor. (And in fact, it was passed to a function, operator *.)

Because function_taking_pointer(pointer) passes a non-= owning observer, nullable must live at least = through that call. The intent is best expressed by simply value =3D * nullable; do not use forward(). This can be declared as auto && or auto & equival= ently.

Am I missing something?

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