220 17432 <5533C099.2050307@cloudius-systems.com> article Path: news.gmane.org!not-for-mail From: Avi Kivity Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal: lambda move capture Date: Sun, 19 Apr 2015 17:50:01 +0300 Lines: 45 Approved: news@gmane.org Message-ID: <5533C099.2050307@cloudius-systems.com> References: <0c39ed29-d389-4894-bc65-71a9bb79cbe5@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; format=flowed X-Trace: ger.gmane.org 1429455018 8708 80.91.229.3 (19 Apr 2015 14:50:18 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 19 Apr 2015 14:50:18 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDHJFOWD7YCRBHEBZ6UQKGQEB3QDLNY@isocpp.org Sun Apr 19 16:50:09 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-la0-f70.google.com ([209.85.215.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YjqXt-0007xv-VS for gclcip-std-proposals@m.gmane.org; Sun, 19 Apr 2015 16:50:06 +0200 Original-Received: by labgx2 with SMTP id gx2sf33993505lab.1 for ; Sun, 19 Apr 2015 07:50:05 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:message-id:date:from:user-agent:mime-version:to :subject:references:in-reply-to:content-type:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=+9C28LE67/SHFDwz4MjoFYWyBkLcu/V2hReCVnF5zmw=; b=SoHLo54zI4p9nlJJnsx/6S3Yi4SHl+NLA/rCOE3KYIZ3B55I1wrvbxIoGvmBKxsBXa ap1oDMrBjtGb3Npz65ARIqgCwDyQ14jfYK4pG3Avsw5Nq8T42P7vu3eSwcp0CAiAqxMo coEXGV1sJtrIK/MNXoUXRycyDmbU5qtwk6cPWBddJO9ZriMvikUJhhw8QOKrb6ySb2KR 2Nvijb9ODWgIcfiSh5HGzQITJ6o+OEO6M9Bguui94ouzAn3d007PY4CFfcDIl/72UbME B+Z2UBuXZ7WWNyb9u8bMPROtF6dwIS5af0/Q8rva9bA72CfT+Pk6wvmbvoZwo7YL6fG/ e26w== X-Gm-Message-State: ALoCoQmxDUxdGxWzdHC6fKZBqZuTmeJRtoWrTKVUEspt9dHmv5H+E5VYiBEtRKgEGp7nf+dmXUUT X-Received: by 10.112.203.168 with SMTP id kr8mr5437722lbc.10.1429455005304; Sun, 19 Apr 2015 07:50:05 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.216.105 with SMTP id op9ls369675wic.7.canary; Sun, 19 Apr 2015 07:50:04 -0700 (PDT) X-Received: by 10.194.179.38 with SMTP id dd6mr23196285wjc.149.1429455004237; Sun, 19 Apr 2015 07:50:04 -0700 (PDT) Original-Received: from mail-wg0-f53.google.com (mail-wg0-f53.google.com. [74.125.82.53]) by mx.google.com with ESMTPS id f18si27821898wjz.182.2015.04.19.07.50.03 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 19 Apr 2015 07:50:03 -0700 (PDT) Received-SPF: pass (google.com: domain of avi@cloudius-systems.com designates 74.125.82.53 as permitted sender) client-ip=74.125.82.53; Original-Received: by wgso17 with SMTP id o17so155444238wgs.1 for ; Sun, 19 Apr 2015 07:50:03 -0700 (PDT) X-Received: by 10.180.104.4 with SMTP id ga4mr17817494wib.86.1429455003836; Sun, 19 Apr 2015 07:50:03 -0700 (PDT) Original-Received: from avi.cloudius ([212.143.139.214]) by mx.google.com with ESMTPSA id gi17sm13109935wjc.8.2015.04.19.07.50.02 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 19 Apr 2015 07:50:03 -0700 (PDT) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 In-Reply-To: X-Original-Sender: avi@cloudius-systems.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of avi@cloudius-systems.com designates 74.125.82.53 as permitted sender) smtp.mail=avi@cloudius-systems.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:17432 Archived-At: On 04/19/2015 05:26 PM, Ville Voutilainen wrote: > On 19 April 2015 at 17:06, Avi Kivity wrote: >> Lambdas currently support three styles of capture: >> >> 1. Init capture (fully generic) [x = expression] >> 2. Copy capture: [x], equivalent to [x = x] >> 3. Reference capture: [&x], roughly equivalent to [x = std::ref(x)] >> >> As I find myself typing [x = std::move(x)] more and more, I propose to >> support a fourth style: >> >> 4. Move capture: [&&x], equivalent to [x = std::move(x)] >> >> This would be very useful for move-only types such as unique_ptr<>. > > I don't see that as a good idea. It looks like a > capture-by-rvalue-reference, but > you don't want to capture an rvalue reference, you want to capture a > value initialized > from an rvalue. Consistently, when you want to move into a function or > into a constructor, > you need to use std::move. For a lambda, you're initializing a > capture, which is rather > similar to calling a constructor, so using std::move there makes sense. > > This syntax has been proposed in the committee too - we found it undesirable > due to the points above, and went for init-captures since they can do much more > than initialize from an rvalue. Well, it's a pity. In a not-so-large code base I have about 100 instances of this pattern, some with multiple variables move-captured in the same lambda. This syntax would greatly reduce verbosity for a very repetitive pattern. But I appreciate the subtle different between &x and &&x (clobbering of the source). -- --- 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. 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