220 22437 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 01:20:01 -0800 (PST) Lines: 118 Approved: news@gmane.org Message-ID: References: <14fb89ea-443b-4696-93ec-29889f642bca@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_761_1031011390.1447233601836" X-Trace: ger.gmane.org 1447233608 11772 80.91.229.3 (11 Nov 2015 09:20:08 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 11 Nov 2015 09:20:08 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDXJR44QUQHBBQUQRSZAKGQE45DQCTQ@isocpp.org Wed Nov 11 10:20:07 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f69.google.com ([209.85.220.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZwRZU-0002on-Ec for gclcip-std-proposals@m.gmane.org; Wed, 11 Nov 2015 10:20:04 +0100 Original-Received: by paak3 with SMTP id k3sf3726844paa.3 for ; Wed, 11 Nov 2015 01:20:03 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp_org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :content-type:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=buJz/xXDiJ+WgvHlj1+9+xnFw9xt0X32QvMi7kjKXBs=; b=OTxMm1xqGn04dlCF20TgFoX9hIa8iSxtSgV3YJcHHxyV4FvM93daPAF1oOig53GAJg /FRVfv5oM/jPZxOj1+W5tflFCOghMcb5jNyTqvAJQD8kgfMd997C2oRGKPMbA8qAvm8y c70+MS3Pw40mLCI+MwRMI+zbW1gY31b0ZdeJYxFyEa8zBQHsIy9wm4eihD3dfrRYkN/e LMA51G9Mvt5QNzLhlmAmfwXysEQTdvAGJNJVoO2Fw1C4PAeqp84cx1gQwZQkGX9r8YlH hzv2sfNvWsH5mPW6/RcC36Oi4DXdKAV6XEqR2dIAfmfGLu8gPT7NuRSyy59EA1F1nCDE /zDA== 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 :content-type:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=buJz/xXDiJ+WgvHlj1+9+xnFw9xt0X32QvMi7kjKXBs=; b=LTqnQSBB0mShArNUaxMiC+6l1X7iGM7wgaIK2HMv8fZ9N3ktj6b2zfjJX7uipY0Xx9 Hou22jAQueObtvTKTwLFf62pFsOEZtNnhhdqAQsZe0VcvnjZhOQEzlEQeqjaVw0FqCDB OOyay4njYHf/TlUzUaOfUo8Wx3G+bZOgw80M1B52XI2iMkLBnTLSGtMKHSwcJLnwKZaZ 4MVCaRIbOfbOa2FfjQXRwtcM8WBw7ZyZGI6JKrYTM+Y4xBJkZkB7+Es8z2Z4TGIA6YVy gJPtl1xc7zYJRCRDL3Fzr46GoSe/tRKGtL41wY4afuRx9xrkEW+bHGumjk61n5bi+v2m ECHA== 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:content-type:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=buJz/xXDiJ+WgvHlj1+9+xnFw9xt0X32QvMi7kjKXBs=; b=c3whFZjQLOP5ol0wH4dOEWtCynx6W3ZaKnZkdsEk0f6fNeroLbv5DX/7tIC19dLJVI sqYbfzyLga70fTL3kX86zSLSGapijz9xE5OFgPw/dEsYdWJ2xJNcKfCvHpzkmbcpDJtU 1vVoaLQIizPvIZM+LVvEoa/j84CCtjQGFrcM4Lxa6IuZLjUYcliHwRajq/R5VHm7UJiM lFqlQEeK1Ct+Z2IPndVbt5rcdBlX7inGkV4H5hNaLL843bO5LyPG1xKJ0gK8Nc9MHaGk sDMjuzpEWKl/fLrVgivs8ej1T1uXhTUYazsSNxNU0+jHTTiXuzWi5dsFCduKcp8zLmci fZzw== X-Gm-Message-State: ALoCoQmI8P2m1+TCrZgmTURWCFUF0vrPBE09giNkWiDbZXALsXzRN9zngKS//vIvadgMPnZJbyDn X-Received: by 10.68.196.169 with SMTP id in9mr841931pbc.2.1447233603291; Wed, 11 Nov 2015 01:20:03 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.11.139 with SMTP id 11ls138793iol.94.gmail; Wed, 11 Nov 2015 01:20:02 -0800 (PST) X-Received: by 10.50.142.66 with SMTP id ru2mr748431igb.3.1447233602485; Wed, 11 Nov 2015 01:20:02 -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:22437 Archived-At: ------=_Part_761_1031011390.1447233601836 Content-Type: multipart/alternative; boundary="----=_Part_762_1085229922.1447233601837" ------=_Part_762_1085229922.1447233601837 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wednesday, November 11, 2015 at 10:01:56 AM UTC+1, Daniel Kr=C3=BCgler w= rote: > > I consider the combination of "perfect forwarding" and the additional=20 > size information as a potential dangerous combination. If a=20 > constructor with size information would get any rvalues via args, the=20 > first vector element construction might move the arguments into the=20 > target and the second, third, etc. vector element constructed from the=20 > same argument pack would try to become constructed from already moved=20 > arguments. For exactly this reason any existing container member=20 > function that provides a size information and a "prototype" value=20 > expects that value as const value_type& and not as a potentially=20 > movable argument.=20 > > - Daniel=20 > Yeah, that was truly a mistake by me, perfect forwarding really doesn't=20 make sense in this context. Shouldn't this work though? vector(size_type n,vector::forward_args_type,Args&... args); =20 This way even if something in args is an rvalue reference its type would be= =20 deduced to be an lvalue reference. - 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_762_1085229922.1447233601837 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

On Wednesday, November 11, 2015 at 10:01:56 AM UTC+1, = Daniel Kr=C3=BCgler wrote:
I co= nsider the combination of "perfect forwarding" and the additional
size information as a potential dangerous combination. If a
constructor with size information would get any rvalues via args, the
first vector element construction might move the arguments into the
target and the second, third, etc. vector element constructed from the
same argument pack would try to become constructed from already moved
arguments. For exactly this reason any existing container member
function that provides a size information and a "prototype" v= alue
expects that value as const value_type& and not as a potentially
movable argument.

- Daniel

Yeah, that was truly a mistake by me, = perfect forwarding really doesn't make sense in this context. Shouldn&#= 39;t this work though?

vector(size_type n,vector::forward_args_ty= pe,Args&... args);
=C2=A0
=
This way even if something in args is an rvalue reference its ty= pe would be deduced to be an lvalue reference.

- L= =C3=A9n=C3=A1rd

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