220 22439 <5cd9879e-a6e7-447f-8475-edc6b52d9716@isocpp.org> 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 02:53:15 -0800 (PST) Lines: 156 Approved: news@gmane.org Message-ID: <5cd9879e-a6e7-447f-8475-edc6b52d9716@isocpp.org> 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_3216_194931952.1447239196036" X-Trace: ger.gmane.org 1447239200 6357 80.91.229.3 (11 Nov 2015 10:53:20 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 11 Nov 2015 10:53:20 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDXJR44QUQHBBHN4RSZAKGQEQMEODDY@isocpp.org Wed Nov 11 11:53:20 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qg0-f69.google.com ([209.85.192.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZwT1j-00019F-0C for gclcip-std-proposals@m.gmane.org; Wed, 11 Nov 2015 11:53:19 +0100 Original-Received: by qgdy74 with SMTP id y74sf29369636qgd.2 for ; Wed, 11 Nov 2015 02:53:18 -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=eC3R9zBCV/hqio/5hJp8lICWNH6l8ki1LN4ZSKFiSss=; b=wCEWHjtR8Y9/BwtWyVYBa/FXgUzkTMNuC1E8GeqayolTcRxX99hmPxDeImK3opLhzP /JW8O9RAdfyf+yGsa3Q5uzU0v7qLBmixZsZKMvcKYzw0C7x5MCDR2Z7xWn2NBYO7/oUO qwHh1Kk4i495xZZAs+IAnPMTEOkYuCDrVl+SpvJhJsEGTY82QpUw4KPLV3wJyMQh7Svt E4eeDpi0ayAgLjO/7mUF9qQGfWKjSAG8Nt3+U75ZtRNrDwjAA5865wq4vFGZxgOwySUi rJ4OU/cdtKDsSFzwV7dYyuMttvAKXSB2dtezLDTMoywUoSogF44sEDHVvXZF7TxAmk5v 45ZA== 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=eC3R9zBCV/hqio/5hJp8lICWNH6l8ki1LN4ZSKFiSss=; b=JeUTkRdeWceoKae/jF6rH462Wfsep/v1FxhnN8A/Er5iV3EfDwnCmDK7CI22AIoini Mt2rcRJStPABGhqaOU2BGDjqr96wlLZFdU16e6h4pfQiHBrZWNpG0XnSe+RZgpSExpRD hwWfNkLTUP+m2IDFae7qM6QrxizffYvXjZJLFScBmuw7QlurRmzvNMCqilkb2TxZdBMV zvYykrmneqP6dK3mIWeFtYdti2b09eMKMSydEN02AzFRt3jxX68u+6OtiHgo8NtJPd1C kCvaCPMS0fKbrb6qndm/jl8hyJH0vFUsXoCLz6HTzD84jUpE7T140kvGReILbhOwC7St O8hQ== 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=eC3R9zBCV/hqio/5hJp8lICWNH6l8ki1LN4ZSKFiSss=; b=Sx37s8e8uP4KDLjnI2oPGdLjc+jwqo2diVQ/kbUOSueUc/edQqeVXMkpNiP2l7CiAF AUpd7dCuJr/WTEkCnGLLnljGuiH6NBCP7A5VFiPTDAReNxEm6B/CtdO8S8sdxwo4D4cR qHu8h/FY2CX1GKERVPiRhU0KSI9GFmsT6uuaJIcrjhAlxT2QkUvjk8I+A/QIuJ34GZTz oETI2timcqaFIZWJKConrtWHXp1TXZgm6cKaExMeqbLGRXwmGJQkn67ohPgQUAGSGXSp 8YjK1qtvHCFLWDBQg6KP3FMOKntqQ9y+yXq7u1dR4yy+dWs8qrMFBwpBa/mu1b8W/EyK +Qcg== X-Gm-Message-State: ALoCoQkkw1Bu+m6uAsl9V+V6EsvIvtpNtO1Nb6cvDx6R0bMHsNy4D1mGNyVm0bX8xiSzh9XbaZp+ X-Received: by 10.13.192.134 with SMTP id b128mr7223609ywd.43.1447239198100; Wed, 11 Nov 2015 02:53:18 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.73.199 with SMTP id n7ls148749igv.9.gmail; Wed, 11 Nov 2015 02:53:16 -0800 (PST) X-Received: by 10.50.126.99 with SMTP id mx3mr211942igb.1.1447239196834; Wed, 11 Nov 2015 02:53:16 -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:22439 Archived-At: ------=_Part_3216_194931952.1447239196036 Content-Type: multipart/alternative; boundary="----=_Part_3217_812763143.1447239196036" ------=_Part_3217_812763143.1447239196036 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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= =20 to const ref. Of course this behavior should be well documented and I maybe= =20 this can be confusing. About converting rvalue references one can define the following type: template using forward_for_copy=3Dstd::conditional_t< std::is_rvalue_reference{}, const T&, T >; Can be used this way: template int f(Args&&... args) { h(static_cast>(args)...); return 0; } (Maybe "forward for copy" is not the best name though.) The main idea that= =20 vector's constructor would forward args this way to std::allocator's=20 "construct" member function. The constructor would accept literals, but it= =20 wouldn't move objects even if they are movable. I'm getting a little bit lost here though, maybe this suggestion adds even= =20 more problems than what it solves. - 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_3217_812763143.1447239196036 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Ok, this is my last try :).

What if we = use the first declaration:

vector(size_type n,vector::forward_args_type= ,= Args&&... args<= span style=3D"color: #660;" class=3D"styled-by-prettify">);

But inside vector's constructor we convert every rvalue = references in args to const ref. Of course this behavior should be well doc= umented and I maybe this can be confusing.

About c= onverting rvalue references one can define the following type:
<= code class=3D"prettyprint">
template<typename T>
using = forward_for_copy=3Dstd::conditional_t< std::is_rvalue_reference<T>= {},
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0const T&,
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0T >;

Can be used this way:
<= div>
template<class... Args>
int f(Args&&... args)
{
=C2=A0 =C2=A0 =C2=A0 =C2=A0 h(static_cast<= ;forward_for_copy<Args>>(args)...);
=C2=A0 =C2=A0 =C2=A0 =C2=A0 return 0;
}

(Maybe "forward for copy" is not the best nam= e though.) The main idea that vector's constructor would forward args t= his way to std::allocator's "construct" member function. The = constructor would accept literals, but it wouldn't move objects even if= they are movable.

I'm getting a little bit lo= st here though, maybe this suggestion adds even more problems than what it = solves.

- L=C3=A9n=C3=A1rd

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