220 31112 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 . 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[184.173.153.196]) by mx.google.com with ESMTPS id p3si1013331qtc.154.2017.02.24.09.46.06 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 24 Feb 2017 09:46:06 -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-f181.google.com (mail-wr0-f181.google.com [209.85.128.181]) by mxa.mailgun.org with ESMTP id 58b0715c.7fcc880d0d30-smtp-out-n03; Fri, 24 Feb 2017 17:46:04 -0000 (UTC) Original-Received: by mail-wr0-f181.google.com with SMTP id u47so17809944wrb.0 for ; Fri, 24 Feb 2017 09:46:04 -0800 (PST) X-Received: by 10.223.133.205 with SMTP id 13mr3986775wru.1.1487958363041; Fri, 24 Feb 2017 09:46:03 -0800 (PST) Original-Received: by 10.80.173.212 with HTTP; Fri, 24 Feb 2017 09:46:02 -0800 (PST) In-Reply-To: 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:31112 Archived-At: On Fri, Feb 24, 2017 at 8:48 AM, Nicol Bolas wrote: > auto &&r2 = X().n; > > For `r1`, if `f` returns an rvalue reference that refers to its parameter or > a subobject thereof, this will be a dangling reference. > > But `r2` is perfectly fine. The lifetime of the prvalue temporary will be > extended even though we are not actually referencing it directly. > [class.temporary]p6: > > [...] > > The idea of lifetime extension should not be that you provide lifetime > extension for all xvalues. But it also shouldn't be that you provide > lifetime extension for prvalues. It should be that you provide lifetime > extension for any expression for which lifetime extension is currently > valid. It should behave exactly as if you had created a reference variable > with that expression. I think we can find some compromise here, that is to extend lifetime for certain xvalues that we can, but not change any xvalue's value category, making it useful to interfaces which do not care about value categories, and still safe in general. Example: struct X { std::string s; }; string_view x = __extend_me X().s; // fine, extending s -- 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/CAGsORuD%3DUDBc7MPKcbyH7p4B-EpN8guiw9kOQHHOTmi%2B%3DgEqzQ%40mail.gmail.com. .