220 28076 <57C6F249.1030601@gmail.com> article Path: news.gmane.org!.POSTED!not-for-mail From: Matthew Woehlke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Specifier to cause the destructor to be called at last use instead of end of scope Date: Wed, 31 Aug 2016 11:05:45 -0400 Lines: 56 Approved: news@gmane.org Message-ID: <57C6F249.1030601@gmail.com> References: <4387866d-169e-44a8-a858-d9aa96d54680@isocpp.org> <85dfd0dc-bf1a-4fa8-81f5-2b36e8f6465c@isocpp.org> <57C48794.6020105@gmail.com> 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 1472655962 7252 195.159.176.226 (31 Aug 2016 15:06:02 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 31 Aug 2016 15:06:02 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC37LBFWUIFBBTHETO7AKGQEHCAV4GI@isocpp.org Wed Aug 31 17:05:53 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f197.google.com ([209.85.217.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bf75J-0000gx-BG for gclcip-std-proposals@m.gmane.org; Wed, 31 Aug 2016 17:05:49 +0200 Original-Received: by mail-ua0-f197.google.com with SMTP id m60sf108556325uam.3 for ; Wed, 31 Aug 2016 08:05:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:reply-to:message-id:date:user-agent :mime-version:in-reply-to:x-original-sender :x-original-authentication-results:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=RfgB1FHRTImXjWVc79dIA/xwEf+Xl7nkivwBQxlwIIU=; b=KhxYBRADeNoYUzpdvvPv2BDlCrbGWIL2Fhan0gMgolubOBWJ9Y7jZqqr2ewYh9RZSt f5SYYG0bKAK0kKFFs4MZwCKEky5Y9xvdjppsyD51upJ6nWn6e+HiyWRka2Qb07h+OYqN 2zY5AEA7BkVjDmxu0sP2IM2KcptqF3L0BpXCkcVNO0re6F3uztJqV4Urto9MAWCfjy3Y wca7H+uOKEbsU8ECyAKz/FBjAlhjVFNLRRVwkWWwPQPslGHTbYIKbNklUTU7UUltS6L/ oKlpGHQZosUlrS8h4Bsx+FtxsLlzRacFN4TY/9VRZa1Jl8JMveLBu5CfOWzOLifgyCEz a+Zw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:to:references:from:reply-to:message-id :date:user-agent:mime-version:in-reply-to:x-original-sender :x-original-authentication-results:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=RfgB1FHRTImXjWVc79dIA/xwEf+Xl7nkivwBQxlwIIU=; b=cKTxHfO8NZXjCBHJ+hsOIw8BFWB9tiXKK+Ed+82b+5237aoE/tqshKoQNGKcN/TvxR rYId4bpSeW5t/bQyrnFJkqggekQT6nickX2eCOCfC/0Y1E4LR0KHbTj3qLWkCNmqz7Ez vKxTiWRFFNq1eilNxPKb6bvdvCCYsmk3udKwcSReWTBUWdWHNOqJUNiXpN5UFumDZ5Xr wFhqp+vYJmwu2FFJm0q5YeCVrSH2cEh5fv1TpD8usUBcHSMli9aEW5w0EKnJDdnTXYnA h7LG86qr3fnLSrfw16k6VxCjsH3QnP1GIqaURLi5R+9fwUem35Q25wtDSWvbnc4xLOZ8 QCtA== X-Gm-Message-State: AE9vXwODcZikJgK2VEWLlx3e6Z0v48iz8hX6d+Yvf3C3r7t0oQ7ExZoh/oSatWuexP9A0A== X-Received: by 10.157.2.202 with SMTP id 68mr7447179otl.28.1472655949182; Wed, 31 Aug 2016 08:05:49 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.13.112 with SMTP id 103ls4091830oti.11.gmail; Wed, 31 Aug 2016 08:05:48 -0700 (PDT) X-Received: by 10.55.95.2 with SMTP id t2mr2376493qkb.144.1472655947967; Wed, 31 Aug 2016 08:05:47 -0700 (PDT) Original-Received: from mail-qk0-x235.google.com (mail-qk0-x235.google.com. 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[66.194.253.20]) by smtp.googlemail.com with ESMTPSA id w10sm169369qtc.28.2016.08.31.08.05.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 31 Aug 2016 08:05:46 -0700 (PDT) X-Enigmail-Draft-Status: N1110 In-Reply-To: X-Original-Sender: mwoehlke.floss@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of mwoehlke.floss@gmail.com designates 2607:f8b0:400d:c09::235 as permitted sender) smtp.mailfrom=mwoehlke.floss@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:28076 Archived-At: On 2016-08-29 16:38, Nicol Bolas wrote: > On Monday, August 29, 2016 at 3:05:59 PM UTC-4, Matthew Woehlke wrote: >> On 2016-08-29 14:46, Nicol Bolas wrote: >>> This problem is fundamentally identical to the temporary lifetime >>> extension problem. Namely, that this works: >>> >>> const auto &str = std::string(...); >>> //use str >>> >>> While this does not: >>> >>> const auto &str = std::string(...).c_str(); >>> //use str >> >> I think having explicit temporary types might help here: >> >> std::string::c_str() -> char const* register; // declaration >> >> // error: taking reference of register-qualified type. >> const auto &str = std::string(...).c_str(); >> >> Obviously, it is too late to change std::string this way, but it would >> help us write better API's going forward. > > How would that solve the problem? How does the compiler connect the return > value's lifetime to the lifetime of one of the function parameters?* Which* > parameter should it connect it to? All of them? ....yes? I'm not sure you're asking the right question; `register` here means that the value becomes invalid "almost immediately", i.e. as soon as the inputs that led to its "creation" change, the return value is invalidated. (So, yes, I want to say "all of them" is the correct answer.) > And why would you use `register` of all things for that? It grew out of the original idea of `register` being used to annotate something that is "more ephemeral than usual". (And also, because it's available.) Nothing says we couldn't use something else. > P0066 provided a more comprehensive solution to lifetime analysis. And it > was ultimately rejected > > . Ah, well, that's not reassuring. Oh, well. I'm not sure I agree that the problem is 'too niche', but... -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/57C6F249.1030601%40gmail.com. .