220 17431 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal: lambda move capture Date: Sun, 19 Apr 2015 17:26:41 +0300 Lines: 37 Approved: news@gmane.org Message-ID: 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 X-Trace: ger.gmane.org 1429453606 20745 80.91.229.3 (19 Apr 2015 14:26:46 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 19 Apr 2015 14:26:46 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRBIXWZ2UQKGQEXTL442Y@isocpp.org Sun Apr 19 16:26:45 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f200.google.com ([209.85.192.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YjqBI-0002n5-Mb for gclcip-std-proposals@m.gmane.org; Sun, 19 Apr 2015 16:26:44 +0200 Original-Received: by pdbqb6 with SMTP id qb6sf248631914pdb.3 for ; Sun, 19 Apr 2015 07:26:43 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from: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=vLPqBzsDclxi48WMj7auL6F20No/p61IZmXoKrnUaAg=; b=Nxw/WSFDaIeRBQxI5hTNeUDCJPzh9iZoQlKk93stztbSiXLBI/bvtavUM/Jb4bwTX9 GJHmbi+ppN+v0zGVSYKDoo1nogzNCzqz0RC2M3bZ6HQyqjS7vkElQHabKAOVnkv+l8uh T730wsULfKIXmHKGv0dtAUYwziNa3KRxkuv5H6xfmZTVW2QQOSCEKezUvk8Pz5t1dudq PBCc4d3Gp1fL2XXl4uPQtPhOmWim3Qd8VsmXumXmeYffTLYL0VQ50vk9H8XfzYWLrTom 5xvx0c1uGnIRQgvzuFQOF9R9Vf39zgKTc8mbpP65hIFVAJwh91AnVCGw1M1nH8CaiI7B SaMg== X-Gm-Message-State: ALoCoQlIkd5pzvDx7P99rg4/gbwykw+m+amNZEvloFxVKAxErXwozuHzojprjRx4uM8dDbiVb+Cs X-Received: by 10.66.182.166 with SMTP id ef6mr16865556pac.42.1429453603126; Sun, 19 Apr 2015 07:26:43 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.148.208 with SMTP id w199ls1829217iod.92.gmail; Sun, 19 Apr 2015 07:26:42 -0700 (PDT) X-Received: by 10.107.161.81 with SMTP id k78mr16633481ioe.41.1429453602392; Sun, 19 Apr 2015 07:26:42 -0700 (PDT) Original-Received: from mail-ie0-x233.google.com (mail-ie0-x233.google.com. [2607:f8b0:4001:c03::233]) by mx.google.com with ESMTPS id sb1si6026206igb.52.2015.04.19.07.26.42 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 19 Apr 2015 07:26:42 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4001:c03::233 as permitted sender) client-ip=2607:f8b0:4001:c03::233; Original-Received: by iebrs15 with SMTP id rs15so100662047ieb.3 for ; Sun, 19 Apr 2015 07:26:42 -0700 (PDT) X-Received: by 10.50.30.130 with SMTP id s2mr15413610igh.11.1429453601788; Sun, 19 Apr 2015 07:26:41 -0700 (PDT) Original-Received: by 10.36.213.70 with HTTP; Sun, 19 Apr 2015 07:26:41 -0700 (PDT) In-Reply-To: <0c39ed29-d389-4894-bc65-71a9bb79cbe5@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4001:c03::233 as permitted sender) smtp.mail=ville.voutilainen@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:17431 Archived-At: 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. -- --- 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/. .