220 22535 article Path: news.gmane.org!not-for-mail From: leni536@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: std::vector (and other containers) to have a perfect forwarding constructor Date: Wed, 11 Nov 2015 23:35:16 -0800 (PST) Lines: 173 Approved: news@gmane.org Message-ID: References: <14fb89ea-443b-4696-93ec-29889f642bca@isocpp.org> <5cd9879e-a6e7-447f-8475-edc6b52d9716@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_9378_1156900636.1447313716235" X-Trace: ger.gmane.org 1447313724 31786 80.91.229.3 (12 Nov 2015 07:35:24 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 12 Nov 2015 07:35:24 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDXJR44QUQHBBNMCSGZAKGQEXRDU7SA@isocpp.org Thu Nov 12 08:35:23 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f70.google.com ([209.85.220.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZwmPf-00043A-Ko for gclcip-std-proposals@m.gmane.org; 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Wed, 11 Nov 2015 23:35:18 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.18.228 with SMTP id 97ls403276ios.75.gmail; Wed, 11 Nov 2015 23:35:17 -0800 (PST) X-Received: by 10.50.33.73 with SMTP id p9mr302820igi.4.1447313717370; Wed, 11 Nov 2015 23:35:17 -0800 (PST) In-Reply-To: X-Original-Sender: leni536@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:22535 Archived-At: ------=_Part_9378_1156900636.1447313716235 Content-Type: multipart/alternative; boundary="----=_Part_9379_710222595.1447313716236" ------=_Part_9379_710222595.1447313716236 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wednesday, November 11, 2015 at 6:42:29 PM UTC+1, Nevin ":-)" Liber=20 wrote: > > On 11 November 2015 at 04:53, > wrote: > >> Ok, this is my last try :). >> >> What if we use the first declaration: >> >> vector(size_type n,vector::forward_args_type,Args&&... args); >> >> But inside vector's constructor we convert every rvalue references in=20 >> args to const ref. >> > > But then it is no longer perfect forwarding, is it? > =20 I agree.=20 Worse, the signature looks like it *should* do perfect forwarding. > =20 I'm not familiar with the conventions of modern C++. Is it already settled= =20 that if you see a universal reference in a signature it should be moved or= =20 perfect forwarded (so eventually moved)? AFAIK rvalue references are only= =20 candidates for moving. Anyway this convention (if it exist) seems too=20 limiting. I only use a variadic universal reference because AFAIK it's the= =20 only way to catch mixed const and non-const lvalue references and keep=20 their "constness" in a variadic parameter. > For instance, if you have move-only parameters that get moved into the=20 > object being constructed, it looks like it ought to work even though it= =20 > won't compile. Take this example: > > unique_ptr p; > vector> v(1, forward_args_type(), std::move(p)); > =20 Well not compiling is not that bad. Maybe a static assert is possible with= =20 useful information.=20 =20 > >> I'm getting a little bit lost here though, maybe this suggestion adds=20 >> even more problems than what it solves. >> > > I think the idea was certainly worth exploring, but I'm leaning towards= =20 > the same conclusion. > =20 I worry more about more convoluted corner cases. I think this case is not= =20 that bad. - L=C3=A9n=C3=A1rd --=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_9379_710222595.1447313716236 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

On Wednesday, November 11, 2015 at 6:42:29 PM UTC+1, Nevin ":-= )" Liber wrote:



=C2=A0
Well not compiling is not that bad. Maybe a static asse= rt is possible with useful information.=C2=A0

=C2=A0
I'm getting a little bit lost here though, maybe this suggest= ion adds even more problems than what it solves.

I think the idea was certainly worth exploring, but I'= ;m leaning towards the same conclusion.
=C2=A0
I worry more about more convoluted corner cases. I t= hink this case is not that bad.

- L=C3=A9n=C3=A1rd=

--

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