220 5202 article Path: news.gmane.org!not-for-mail From: Richard Smith Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Teachability problems with std::move Date: Mon, 24 Jun 2013 14:30:48 -0700 Lines: 116 Approved: news@gmane.org Message-ID: References: <8e156cc1-8136-48e6-96f2-8dca72fee7d4@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1372109452 32702 80.91.229.3 (24 Jun 2013 21:30:52 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 24 Jun 2013 21:30:52 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDVNBJG4YAIBBCPVUKHAKGQEIJK2UAY@isocpp.org Mon Jun 24 23:30:53 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vb0-f69.google.com ([209.85.212.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UrELa-0008Lm-Qm for gclcip-std-proposals@m.gmane.org; Mon, 24 Jun 2013 23:30:51 +0200 Original-Received: by mail-vb0-f69.google.com with SMTP id w16sf15650561vbb.4 for ; Mon, 24 Jun 2013 14:30:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-beenthere:mime-version:sender:in-reply-to:references:date :message-id:subject:from:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-google-group-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=s9t/agq1iseoKJVTiUqPeEFSmVhjo+ikYGkhGaZsRFc=; b=pL8YrpcfOFF9a7rZKlDG4P/nRrlADIP3RhmxytED0hGppeZvNkvYS6Tp+W21fcW+rm UnX5UjgXIzbK71nR9zwQkfO1eU+yGanQrDIldQ8x6cFAaYD88oMqMs8xGPr102qxnL95 DBLcC8DywxifyxCmMgzr585xjwo9s4py2HOAEmnNK+6411VrTOJRfXBUafE+rlBpM02f 1epNqMFF215ULBiGdGEl848SIxiyNIDLN//1Yb4O97lTe4q4oqxKQhXQ828oDenpQM2K wNIcMBqxtEZ0CNGIxr+M4+g5LglYebttxTc4/HcxgsSpVVdO7F8CTUW1613ZOUttFWiu vYJg== X-Received: by 10.236.45.4 with SMTP id o4mr14620376yhb.6.1372109449868; Mon, 24 Jun 2013 14:30:49 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.116.197 with SMTP id jy5ls2264215qeb.10.gmail; Mon, 24 Jun 2013 14:30:49 -0700 (PDT) X-Received: by 10.52.163.67 with SMTP id yg3mr10799222vdb.14.1372109449185; Mon, 24 Jun 2013 14:30:49 -0700 (PDT) Original-Received: from mail-ve0-x22f.google.com (mail-ve0-x22f.google.com [2607:f8b0:400c:c01::22f]) by mx.google.com with ESMTPS id f8si5412233vec.47.2013.06.24.14.30.49 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 24 Jun 2013 14:30:49 -0700 (PDT) Received-SPF: pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c01::22f as permitted sender) client-ip=2607:f8b0:400c:c01::22f; Original-Received: by mail-ve0-f175.google.com with SMTP id da11so9359553veb.34 for ; Mon, 24 Jun 2013 14:30:49 -0700 (PDT) X-Received: by 10.52.69.177 with SMTP id f17mr10898110vdu.48.1372109448970; Mon, 24 Jun 2013 14:30:48 -0700 (PDT) Original-Sender: metafoo@gmail.com Original-Received: by 10.58.125.73 with HTTP; Mon, 24 Jun 2013 14:30:48 -0700 (PDT) In-Reply-To: <8e156cc1-8136-48e6-96f2-8dca72fee7d4@isocpp.org> X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c01::22f as permitted sender) smtp.mail=metafoo@gmail.com; dkim=pass header.i=@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:5202 Archived-At: On Mon, Jun 24, 2013 at 1:08 PM, Xeo wrote: > I remember sending you and Mike a reminder about that some months ago, and > was wondering, did you actually talk about this at the Bristol meeting? No. For CWG, the Bristol meeting was almost entirely occupied by reviewing C++14 feature wording. I don't recall seeing this on the issues list, either, but perhaps I just missed it. > On Monday, June 24, 2013 9:14:17 PM UTC+2, Richard Smith wrote: >> >> On Mon, Jun 24, 2013 at 11:42 AM, Geoffrey Romer >> wrote: >> > It's surprisingly hard to come up with a good, teachable set of rules >> > for >> > when to use std::move. For example, it would be nice to be able to say >> > "you >> > can leave out std::move when returning a named local variable", but this >> > is >> > not always the case. Consider: >> > >> > std::unique_ptr FooFactory() { >> > std::unique_ptr result(new Foo); >> > // ... >> > return std::move(result); >> > } >> > >> > The "std::move" is mandatory here, because an lvalue can only be >> > implicitly >> > treated as an rvalue during copy operations, and in "return result;", >> > 'result' is not copied, but used as the input for an implicit >> > conversion. It >> > seems like this should be fixable, perhaps by expanding the >> > implicit-rvalue >> > rules, but I'm not sure what all the consequences would be (at a >> > minimum, >> > this could change the behavior of existing code that has separate rvalue >> > and >> > lvalue overloads for an implicit conversion operation). >> >> Yes, for NRVO, I think the "same cv-unqualified type as the function >> return type" restriction only needs to apply to copy-elision and not >> to implicit move. That is, I think we should treat 'result' as an >> xvalue here: >> >> T f() { >> U result; >> return result; >> } >> >> ... independent of the types of T and U, but we should only be >> permitted to perform copy-elision of they're the same cv-unqualified >> type. Most generalizations of the implicit-rvalue rules are fraught >> with peril, but this one seems relatively safe. >> >> > Conversely, it would be nice to be able to say "When in doubt, just use >> > std::move if you want move semantics, because it never hurts", but >> > std::move >> > can in fact be a pessimization: >> > >> > std::array MakeHugeArray() { >> > std::array result; >> > // ... >> > return std::move(result); >> > } >> >> I suggest you talk to your compiler vendor (Hi!) and ask for a warning >> for this case. >> >> > As written, the return statement incurs 10,000 invocations of Foo's move >> > constructor (or, worse, its copy constructor), but if std::move is >> > eliminated, the return statement becomes effectively free, because it's >> > an >> > elidable copy/move. In this case it's not at all clear to me how to fix >> > the >> > problem; it seems like a fix would either require special-casing >> > std::move >> > (which seems like a hack, and breaks the convention that the >> > core-language >> > standard tries not to refer to the library standard), or require the >> > implementation to 'see into' the implementation of a function called in >> > a >> > return statement (which seems impractical). >> > >> > I think it's worth trying to fix these issues, despite the difficulties, >> > because being able to provide reliable "rules of thumb" would >> > substantially >> > improve the learning curve for std::move. Can anyone suggest how these >> > issues could be fixed? >> >> Per the above, I think we can make "you can leave out std::move when >> returning a named local variable" be the right advice, with the added >> bonus of a compiler warning if you put in a pessimizing std::move. > > -- > > --- > 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/. > > -- --- 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. 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