220 22475 article Path: news.gmane.org!not-for-mail From: Nevin Liber 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 11:41:47 -0600 Lines: 107 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/alternative; boundary=001a114c82c481a9da0524475941 X-Trace: ger.gmane.org 1447263750 8281 80.91.229.3 (11 Nov 2015 17:42:30 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 11 Nov 2015 17:42:30 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBCE35H5S6IDBBA74RWZAKGQEPGS47ZQ@isocpp.org Wed Nov 11 18:42:29 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f197.google.com ([209.85.213.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZwZPh-0000rn-C1 for gclcip-std-proposals@m.gmane.org; Wed, 11 Nov 2015 18:42:29 +0100 Original-Received: by igbhv6 with SMTP id hv6sf80908394igb.3 for ; Wed, 11 Nov 2015 09:42:28 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:from :date:message-id:subject:to:content-type:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=O89RQlF4s9RuzEHrcTwb2m8y2QhqhZJyLbtqR5vpXuc=; b=F6QlUo4+YTDtaxyFvtRypia//AnZIKivOmrGx80O08lineOSPhuhh1baNpgtAkWUya S4TCupWqVrDgValyqqY2jorcAEPAOnCYXOitGTiuYsBeA1B+3liGm25RYmhxR+AzV6Ij ukuOcNPy12Y+9ULueZ16vLVlsijDY1yfGOx0stY8WY4wwAo+cgL7xRq8FAAzwOwWZRA1 qnmo9f552IhuGSjFzhoN/nIKe5TGFW50D2tKjbU5LaprMNWjOxMK1hnVDXor6vEwsYnl Dh2to4AIdw/iBxDWahtqjtCNaS8FNyQ23iQLv4MvCekE1mqKavu7N81v2mQyeXtYnE47 plAA== X-Gm-Message-State: ALoCoQkzpSncw7PhpnNlPZ3dWBhEBePsLg/to9UV0llMzjp2bbd4rfI9jF3FdSoDhCWeVcRQx+4b X-Received: by 10.182.248.231 with SMTP id yp7mr8543378obc.17.1447263748554; Wed, 11 Nov 2015 09:42:28 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.104.209 with SMTP id a75ls632524qgf.45.gmail; Wed, 11 Nov 2015 09:42:27 -0800 (PST) X-Received: by 10.13.220.3 with SMTP id f3mr9506214ywe.330.1447263747191; Wed, 11 Nov 2015 09:42:27 -0800 (PST) Original-Received: from mail-yk0-x22c.google.com (mail-yk0-x22c.google.com. [2607:f8b0:4002:c07::22c]) by mx.google.com with ESMTPS id 204si6937290ywf.82.2015.11.11.09.42.27 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 11 Nov 2015 09:42:27 -0800 (PST) Received-SPF: pass (google.com: domain of nliber@gmail.com designates 2607:f8b0:4002:c07::22c as permitted sender) client-ip=2607:f8b0:4002:c07::22c; Original-Received: by ykfs79 with SMTP id s79so61001425ykf.1 for ; Wed, 11 Nov 2015 09:42:26 -0800 (PST) X-Received: by 10.129.91.214 with SMTP id p205mr11532686ywb.308.1447263746498; Wed, 11 Nov 2015 09:42:26 -0800 (PST) Original-Sender: nliber@gmail.com Original-Received: by 10.103.17.5 with HTTP; Wed, 11 Nov 2015 09:41:47 -0800 (PST) In-Reply-To: <5cd9879e-a6e7-447f-8475-edc6b52d9716@isocpp.org> X-Original-Sender: nevin@eviloverlord.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of nliber@gmail.com designates 2607:f8b0:4002:c07::22c as permitted sender) smtp.mailfrom=nliber@gmail.com; dkim=pass header.i=@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:22475 Archived-At: --001a114c82c481a9da0524475941 Content-Type: text/plain; charset=UTF-8 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 args > to const ref. > But then it is no longer perfect forwarding, is it? Worse, the signature looks like it *should* do perfect forwarding. For instance, if you have move-only parameters that get moved into the object being constructed, it looks like it ought to work even though it won't compile. Take this example: unique_ptr p; vector> v(1, forward_args_type(), std::move(p)); > I'm getting a little bit lost here though, maybe this suggestion adds even > more problems than what it solves. > I think the idea was certainly worth exploring, but I'm leaning towards the same conclusion. -- Nevin ":-)" Liber > (847) 691-1404 -- --- 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 email 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-proposals/. --001a114c82c481a9da0524475941 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On 11 November 2015 at 04:53, <leni536@gmail.com>= 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 a= rgs to const ref.

But then it i= s no longer perfect forwarding, is it?=C2=A0 Worse, the signature looks lik= e it should do perfect forwarding.

For inst= ance, if you have move-only parameters that get moved into the object being= constructed, it looks like it ought to work even though it won't compi= le.=C2=A0 Take this example:

unique_ptr<int>= p;
vector<unique_ptr<int>> v(1, forward_args_type(),= std::move(p));
=C2=A0
I'm getting a little bit lost here though, maybe t= his suggestion 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=A0Nevin ":-)" Liber=C2=A0 <mailto= :nevin@cpluscpl= usguy.com>=C2=A0 (847) 691-1404

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