220 30579 article Path: news.gmane.org!.POSTED!not-for-mail From: "'Scott Dolim' via ISO C++ Standard - Future Proposals" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Explicit temporary lifetime extension Date: Sat, 14 Jan 2017 15:02:52 -0800 (PST) Lines: 36 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3313_1952947547.1484434972365" X-Trace: blaine.gmane.org 1484434976 21156 195.159.176.226 (14 Jan 2017 23:02:56 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 14 Jan 2017 23:02:56 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBC27BZU67ADBBHO45LBQKGQEARVHPVY@isocpp.org Sun Jan 15 00:02:51 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f199.google.com ([209.85.192.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cSXLV-0004nJ-6U for gclcip-std-proposals@m.gmane.org; Sun, 15 Jan 2017 00:02:49 +0100 Original-Received: by mail-pf0-f199.google.com with SMTP id 201sf36555329pfw.5 for ; Sat, 14 Jan 2017 15:02:54 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=ntdxu7/bNPO/NCwmqKVakW98V+0JsZm3nkQHYFas3/4=; b=eH6iCIwKulucxKx2qMTe2ZxYug0UCeyBgfUdaYVBv2F+J+eYFcssFWwfjLjDRS/ddX gEoUu2ieKIzKHGkyPVxztnAtOkz4OHSUofRuLLenH4t2n4noN6WGcKS6lJ5ReOL3/H/B vEf+zxjEqjhCuR45+QDadwDHxOvJ9+ReFulbPnHE9NXsKqD5H2rE0hSMztr1W/wsCgBG 5RDHYq4tH4lVYphbZUiinAD/TRsj65L2/60HJzJuhawdMHSF2u/YG5hf5lB+y+65K3tO 8P4SRVYv8ecs7LDoEnrHCOEw5Oq/p6Xn1vjkLAXUEft9aCcMfDhHgGCV1jam04qd4aEb qrSQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=ntdxu7/bNPO/NCwmqKVakW98V+0JsZm3nkQHYFas3/4=; b=VvUKSFqE2lqbvdyO2dDP/t6o/3ypZQ2p3W6eq6Y8zDzyUtp5difvKuFxPSjoQ0Bnul hkBM2GynI8m/vunlGHcI1D5OlibsNx/XD3PIbiLDxfm40UwfUEKL+qsq+4jMkCf5NEJr h56JsjctCvVOsWzC0ZAiEUK5CmSFin50ChQ3hqj+17Ly8Xy9TEZlgbwzGcZU/bGVnIwL HbcDLtc7HzjgyA5+AqryWZy7ov/gi6bsDY6I3Mp9zmYO4jt8yNGX8zNhfgFtZGpIRRie p9pwe5VMpe0CNS1THe5ZNYd6jr7LBhnw1ZafDuVcAUQr2vMJh0R1xOFixae8+O+vPT6u tYeQ== X-Gm-Message-State: AIkVDXJSIYg8lVis0iGNnA7s/I+GHjZxbs/kxtF2pd7D5of8iuNS5VmUkw2vQ3IBP6E8HQ== X-Received: by 10.99.112.84 with SMTP id a20mr9238788pgn.168.1484434973569; Sat, 14 Jan 2017 15:02:53 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.8.69 with SMTP id 63ls12237021oty.30.gmail; Sat, 14 Jan 2017 15:02:52 -0800 (PST) X-Received: by 10.157.14.183 with SMTP id 52mr2330889otj.20.1484434972930; Sat, 14 Jan 2017 15:02:52 -0800 (PST) In-Reply-To: X-Original-Sender: sdolim@yahoo.com X-Original-From: Scott Dolim 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:30579 Archived-At: ------=_Part_3313_1952947547.1484434972365 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable I think it is worth putting a stake in the ground and *defining* the term "= lifetime extension" as the act of converting a temporary's value category t= o lvalue. Since temporary variables are introduced by the compiler to hold= transient intermediate values of a complex expression, those variables are= xvalues already (materialized from prvalue, by necessity). Normally the t= ransient value is meant to be immediately consumed by the operator of the t= erm it appears in. As I recall, lifetime extension was added to C++ in ord= er to handle cases the operator binds to the address of the temporary inste= ad, so a dangling reference would result if it actually did expire immediat= ely. The core idea being to delay the expiration past the point where any = other part of the overall expression could witness it (which would mean see= ing the temporary as an xvalue) so, as with the case of named rvalue refere= nce variables, this therefore categorizes them as lvalue, for the duration = the expression evaluation (or longer). =20 Basically, exactly what you said with the "auto&& unnamed" example, but fli= pping the logic so that the definition "lifetime extension =3D temporary is= now an lvalue" is the axiom which implies the example as an exposition of = how the mechanism is implemented. --=20 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 e= mail 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/d34038aa-e4d4-4795-997f-f9eef1b2cf1d%40isocpp.or= g. ------=_Part_3313_1952947547.1484434972365-- .