220 22449 article Path: news.gmane.org!not-for-mail From: Matthew Woehlke 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 09:34:28 -0500 Lines: 48 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: text/plain; charset=UTF-8 X-Trace: ger.gmane.org 1447252663 3778 80.91.229.3 (11 Nov 2015 14:37:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 11 Nov 2015 14:37:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC37LBFWUIFBB5NERWZAKGQENKOSJ4Q@isocpp.org Wed Nov 11 15:37:35 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f70.google.com ([209.85.215.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZwWW1-0007ZV-84 for gclcip-std-proposals@m.gmane.org; Wed, 11 Nov 2015 15:36:49 +0100 Original-Received: by lfdo63 with SMTP id o63sf11493967lfd.2 for ; Wed, 11 Nov 2015 06:36:38 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp_org.20150623.gappssmtp.com; s=20150623; h=to:from:subject:date:lines:message-id:references:mime-version :content-type:user-agent:in-reply-to: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=1nhzKvCWIIiLhTn65/jD/l/Ewx8/1XhadyxjrpC/Aj4=; b=Y+5ETDfqhJP6I1SpcgR2kINVGvFdFv7Fy+w3IqG8K7x7sxAueoY6FAiSrSGMfu3616 4snRmvA3Mk041AVJnZVWseQW+TnHOxB5gTUv099Pgucz3KVBvVyKV1DDFmVuUid/3Zh5 jI6zw6KJiiQwIe3qy/A5zd6Qa8pORcFyAbVz9oKmwYJO37JT16NC2B/WhSBgOGPHRwGm T9PwkJYfMGlG54ZsU0dZx9LBhCMf0CfQodg/M4sbIhuxTUWq/7qmQIC2hDooz9U66tPC t+HQxCQDaccqBcxUI12CbhCJm4lNzsjVWxL9no8ocdejzyzY9Lbm8l174PatMn/ZX4Zc nJ3 X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:to:from:subject:date:lines:message-id:references :mime-version:content-type:user-agent:in-reply-to: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=1nhzKvCWIIiLhTn65/jD/l/Ewx8/1XhadyxjrpC/Aj4=; b=bZJXbpMP+7n5LlFdCg2XJ4NI5SQ5UiN1RJJC80h/pdN08AoUGbUY/Mt/SIH1KsV41u e5I0iZh6uJYUf153LBEaqIq0Sb3fC9wkrZzdcsPIE3u5SGFjZNu8XaWZWQZsLdw+tFJm QevK0gs6vb8anQtbXpMDt1mof/PKOTgU7U0SZB1pkzMGjgKRMFUz0NtOTLmrDx7+HvYr YwVucbvDbblPdAEuBP5y8H6HQrrdf2BJf8ipuhPRj3LRz9eDpVCo6TetXPi1IMt6+IKN N3hPKgnP0mafNby7lWYro639WGesyEM5hcLrenGfe5Gov+O61a17F84TwIW+7lzq3g// X-Gm-Message-State: ALoCoQkLjx+uFodCG4qLpqN/i1+ewiMUXPlTVo26CLIvNYFsDSzBEgjLo56+OH336W8jLeFXJK+x X-Received: by 10.112.209.73 with SMTP id mk9mr1490074lbc.14.1447252598524; Wed, 11 Nov 2015 06:36:38 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.25.211.77 with SMTP id k74ls711958lfg.53.gmail; Wed, 11 Nov 2015 06:36:37 -0800 (PST) X-Received: by 10.25.162.131 with SMTP id l125mr4608320lfe.70.1447252597125; Wed, 11 Nov 2015 06:36:37 -0800 (PST) Original-Received: from plane.gmane.org (plane.gmane.org. [80.91.229.3]) by mx.google.com with ESMTPS id 73si6291729lfr.2.2015.11.11.06.36.37 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Wed, 11 Nov 2015 06:36:37 -0800 (PST) Received-SPF: pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) client-ip=80.91.229.3; Original-Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1ZwWUi-0006PW-FA for std-proposals@isocpp.org; Wed, 11 Nov 2015 15:35:28 +0100 Original-Received: from tripoint.kitware.com ([66.194.253.20]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Wed, 11 Nov 2015 15:35:28 +0100 Original-Received: from mwoehlke.floss by tripoint.kitware.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Wed, 11 Nov 2015 15:35:28 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 39 Original-X-Complaints-To: usenet@ger.gmane.org X-Gmane-NNTP-Posting-Host: tripoint.kitware.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 In-Reply-To: <14fb89ea-443b-4696-93ec-29889f642bca@isocpp.org> X-Original-Sender: mwoehlke.floss@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) smtp.mailfrom=gclcip-std-proposals@m.gmane.org; dmarc=fail (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-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:22449 Archived-At: On 2015-11-11 03:52, leni536@gmail.com wrote: > I didn't find a topic like this, so here it is. Something like this would > be nice: > > template > vector(size_type n,Args&&... args); > > 1) It could be faster than vector(n,value_type(args)) if value_type's copy > constructor is more expensive than its constructor with "args" parameters > (I don't know a good example though). Interesting idea. And since vector(n, value_type(...)) invokes value_type's copy ctor, it is in fact a subset of the proposed syntax, i.e. that form could be deprecated / removed in favor of the proposed "emplace" version. > 2) If value_type's different constructors are not deterministic then maybe > it's desirable to call the constructor multiple times instead of copying > one constructed element. One example if value_type has a constructor taking > a random generator as an argument. Hmm... for this case, you are thinking that one of the arguments is e.g. a functor? And that the value_type copy ctor copies existing data, while the ctor taking the functor uses the functor (which isn't stored) to generate a value? Interesting notion... > Workarounds: > 2) Construct an empty vector, use reserve then emplace_back newly > constructed elements one by one using value_type's constructor of choice. > I'm not sure if this would be equally efficient. Seems like it should be at least comparably efficient; I can't think offhand what vector's sized constructor would do differently that would make it more efficient. (Assuming that a default-constructed data has SOO or otherwise doesn't allocate storage that gets thrown out by the reserve().) -- Matthew -- --- 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/. .