220 31124 article Path: news.gmane.org!.POSTED!not-for-mail From: Zhihao Yuan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: P0577: No lifetime extension for xvalue . Date: Fri, 24 Feb 2017 17:00:34 -0600 Lines: 66 Approved: news@gmane.org Message-ID: References: <0f4944b0-11a6-4220-9d48-38086babc97a@isocpp.org> <2bde698e-c5be-4e20-bca0-d9fa0f90af12@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: blaine.gmane.org 1487977242 25805 195.159.176.226 (24 Feb 2017 23:00:42 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 24 Feb 2017 23:00:42 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBCSKRWMD4EHBBFPWYLCQKGQEMFRDD4I@isocpp.org Sat Feb 25 00:00:37 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f71.google.com ([209.85.218.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1chOqm-0005yV-5c for gclcip-std-proposals@m.gmane.org; Sat, 25 Feb 2017 00:00:32 +0100 Original-Received: by mail-oi0-f71.google.com with SMTP id 62sf12582118oih.5 for ; Fri, 24 Feb 2017 15:00:38 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:mime-version:in-reply-to:references:from :date:message-id:subject: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=QmYW0oIQREPe8gZ5oryy8k/iRdVrIG8sZnz6bO2HE18=; b=SDhx+4n1ejTIakmiXP6a3MM3aNI3+/mbr12NQGuEhHCpZv/FB+6KPLjQd5E1YFyF/N u93X0EdHYz1H0+IxkX2NjfI/15y/1bdk732f4T+FnMriVILDslSbyLL2Gr3G3uepugJd vI9wA9C9KK0lGGympmKac/4SRMG4JUJUCxJ2KjqdxNKsAmYmsFQPimW9RlAAyGhuee39 YGmffgnbaogycwBuwh50i3B++5e7DF38NMJIhM4uAhgPy3MzCTHCbMHUxShE7cCQSAX3 r/Zw1pXcPP90YoV3Kiw8YcK9MpKV7HaN7W1cnIne9qBBoDPILUFDVcoaD01qGCgELiBj gzmg== X-Gm-Message-State: AMke39lmKiFGHG8xpvLb/20/rVoUf8w46YkrDbcqx8ASFsfmqozj8wYZv+swmJ1/Za0F9Q== X-Received: by 10.157.52.218 with SMTP id t26mr1840832otd.63.1487977237790; Fri, 24 Feb 2017 15:00:37 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.52.248 with SMTP id t53ls8484071otd.42.gmail; Fri, 24 Feb 2017 15:00:37 -0800 (PST) X-Received: by 10.55.190.129 with SMTP id o123mr5699672qkf.110.1487977237068; Fri, 24 Feb 2017 15:00:37 -0800 (PST) Original-Received: from mail-s68.mailgun.info (mail-s68.mailgun.info. [184.173.153.196]) by mx.google.com with ESMTPS id k66si6619442qkb.260.2017.02.24.15.00.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 24 Feb 2017 15:00:36 -0800 (PST) Received-SPF: pass (google.com: domain of bounce+3f9131.69110-std-proposals=isocpp.org@miator.net designates 184.173.153.196 as permitted sender) client-ip=184.173.153.196; Original-Sender: zy@miator.net X-Mailgun-Sending-Ip: 184.173.153.196 X-Mailgun-Sid: WyI3MTBkYiIsICJzdGQtcHJvcG9zYWxzQGlzb2NwcC5vcmciLCAiNjkxMTAiXQ== Original-Received: from mail-wr0-f174.google.com (mail-wr0-f174.google.com [209.85.128.174]) by mxa.mailgun.org with ESMTP id 58b0bb14.7fb30425f4b0-smtp-out-n01; Fri, 24 Feb 2017 23:00:36 -0000 (UTC) Original-Received: by mail-wr0-f174.google.com with SMTP id g10so20171492wrg.2 for ; Fri, 24 Feb 2017 15:00:36 -0800 (PST) X-Received: by 10.223.130.114 with SMTP id 105mr4510107wrb.41.1487977235242; Fri, 24 Feb 2017 15:00:35 -0800 (PST) Original-Received: by 10.80.173.212 with HTTP; Fri, 24 Feb 2017 15:00:34 -0800 (PST) In-Reply-To: <2bde698e-c5be-4e20-bca0-d9fa0f90af12@isocpp.org> X-Gmail-Original-Message-ID: X-Original-Sender: zy@miator.net X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@miator.net; spf=pass (google.com: domain of bounce+3f9131.69110-std-proposals=isocpp.org@miator.net designates 184.173.153.196 as permitted sender) smtp.mailfrom=bounce+3f9131.69110-std-proposals=isocpp.org@miator.net 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:31124 Archived-At: On Fri, Feb 24, 2017 at 4:04 PM, Nicol Bolas wrote: > Can you give an example where such code actually makes sense? Where it is > actually valid and reasonable code, in both the prvalue and xvalue case? I don't have such code, because my paper hasn't landed :) Whether it's worth to address, it's question worth to ask EWG -- maybe next meeting, since I already received some other comments and I need to address those -- before presenting it. > > auto &&x1 = X().s; //This works. > auto &&x2 = X(); //This works. > > auto &&x3 = register X().s; //This works. > auto &&x4 = register X(); //This doesn't work. auto&& binds to everything so of course x4 works. > > You could use this same reasoning about lots of things we don't allow, like > casting away `const`-ness with `static_cast`, requiring people to use > `std::move` in order to move from lvalues, and so forth. Compilers could > have warned about those things, but we explicitly made them errors. But you may also noticed that std::move never moves const and we didn't make that into an error. Move over, it copies. Good or bad, sometimes concerns with generic programming out weight getting benefits in other ways -- especially when we are not making the situation any worse. > > It seems to me that having either mechanism alone is overall less effective > than having both. And your paper doesn't really address this. Exactly, and that is why. Neither solely fully works, so we propose the one which always works, and is forward compatible with any others. > > Also, the idea I suggested is not particularly "smart". It makes no attempt > to follow variables through to return values and so forth. It only extends > parameters. When me and Tim were designing `register` for quite a while we had a design can do that if I understand what you are describing correctly -- extending lifetime by calling a function, right? And we had a nice explanation given existing mechanism in the language -- but we dropped that later. We believe that we need to do one thing and do it well. If we want explicit control, we don't try to expand the scope. -- Zhihao Yuan, ID lichray The best way to predict the future is to invent it. ___________________________________________________ 4BSD -- http://blog.miator.net/ -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAGsORuD5otmjmzVgY7h35uF2PZg1wHRsMzUoHUEddtNuD-p6AA%40mail.gmail.com. .