220 31906 article Path: news.gmane.org!.POSTED!not-for-mail From: Alberto Barbati Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: This variable should not be named: an identifier (not) to remember Date: Thu, 30 Mar 2017 19:31:39 +0200 Lines: 64 Approved: news@gmane.org Message-ID: References: <985b9b2a-c734-45eb-95f4-db4dc0d309a1@isocpp.org> <14211602.1P4LjlnQMh@tjmaciei-mobl1> <2235919.4GAIbDEcRC@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 (1.0) Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1490895108 12988 195.159.176.226 (30 Mar 2017 17:31:48 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 30 Mar 2017 17:31:48 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCPY5DV6RIMBB74B6XDAKGQEXOLELKI@isocpp.org Thu Mar 30 19:31:43 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f199.google.com ([209.85.128.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ctdv8-0002JW-H7 for gclcip-std-proposals@m.gmane.org; Thu, 30 Mar 2017 19:31:38 +0200 Original-Received: by mail-wr0-f199.google.com with SMTP id v44sf11608828wrc.9 for ; Thu, 30 Mar 2017 10:31:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=from:content-transfer-encoding:mime-version:date:subject:message-id :references:in-reply-to: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=7Ktkd0sLGA0WHdKxZP/lB5Yfrkpu3C1FvjqB8P5O/5Y=; b=d6XN9XRXgnKYuWXb/Jah7VMR3FRBMU0BCMYrfXW+pKMnEJseAcdWyLi9dUqEPG5aEp lGsDlEP43LxS1yXU5dOdP5ZuTSV3OupaO8H2vFY/gK/sHbv3pI73/guUIBnhvc9IFx4k U9w1ekvxok5LYOTmEYKS0ORI5YnJ9tykti74u5DgNY/aeBBRSMi/VT7MCE3aoUzq2vYi YLdFHMU3oV4T7NDm8Q9pARRzOlb20qLq7JobdlMPT90yXZis09Psoef8aLbgbnYxhGMA /Dwoq2CWcSDKWgllMjHM5GYARsZRmbHvrFSfKsv0ZcQCYjvQExCqALbWOLqg3+K9ulfQ CcNw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:content-transfer-encoding:mime-version:date :subject:message-id:references:in-reply-to: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=7Ktkd0sLGA0WHdKxZP/lB5Yfrkpu3C1FvjqB8P5O/5Y=; b=ntqm2/dwJNJiFp2Awi9OphHFO5RT2hyhOK6wDmFbUDV3LkaODGtxk/qnuLVEha3PYX G6RrRWBQUeX4YKUiUKJF1to1pjBPQELDaP9VD43LliBXQnrKc4+X7euGUPyvbhMPmXKK yaX96005LqlnIGPhqhXjWM4hc1EbYDpaIg1V2r9UMnKEIXiFB/9j750qsi8FrxQfHrWB MLZeIJXl7FWDgaR/2CoTPWEoW77XuOULQZDWogONwfRTRm8NlkUDQYA1jWSvewEzYv1r cxYL/kll9psl8np67scDzDLZDpNR8+XoPnwFUQbUc8wEx+TbEl9On/Btg+/QGJ0H9KfH YUk X-Gm-Message-State: AFeK/H1ueO9Iac6vuO5XuKHSPO/OHG++Kav7D/l/1kCdij+hvR9b5xZ7hp9MmLl7BQwlTA== X-Received: by 10.28.46.20 with SMTP id u20mr279238wmu.14.1490895104664; Thu, 30 Mar 2017 10:31:44 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.0.208 with SMTP id 199ls483178wma.0.gmail; Thu, 30 Mar 2017 10:31:43 -0700 (PDT) X-Received: by 10.28.11.130 with SMTP id 124mr1508089wml.3.1490895103390; Thu, 30 Mar 2017 10:31:43 -0700 (PDT) Original-Received: from mail-wr0-x230.google.com (mail-wr0-x230.google.com. [2a00:1450:400c:c0c::230]) by mx.google.com with ESMTPS id c58si4396299wrc.176.2017.03.30.10.31.43 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Mar 2017 10:31:43 -0700 (PDT) Received-SPF: pass (google.com: domain of albertobarbati@gmail.com designates 2a00:1450:400c:c0c::230 as permitted sender) client-ip=2a00:1450:400c:c0c::230; Original-Received: by mail-wr0-x230.google.com with SMTP id w11so69229301wrc.3 for ; Thu, 30 Mar 2017 10:31:43 -0700 (PDT) X-Received: by 10.28.132.130 with SMTP id g124mr4781160wmd.60.1490895102739; Thu, 30 Mar 2017 10:31:42 -0700 (PDT) Original-Received: from [172.20.10.6] ([91.252.253.48]) by smtp.gmail.com with ESMTPSA id i7sm3985210wmg.30.2017.03.30.10.31.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 30 Mar 2017 10:31:41 -0700 (PDT) In-Reply-To: X-Mailer: iPad Mail (14D27) X-Original-Sender: AlbertoBarbati@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of albertobarbati@gmail.com designates 2a00:1450:400c:c0c::230 as permitted sender) smtp.mailfrom=albertobarbati@gmail.com; dmarc=pass (p=NONE sp=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:31906 Archived-At: > Il giorno 30 mar 2017, alle ore 05:41, Vitali Lovich = ha scritto: >=20 > If a function returns a Shared_ptr of a string and string_view that refer= ences the string, if you do: >=20 > auto __ =3D getstr() > auto [__, view] =3D getstr() >=20 > The behavior is observably different between the two cases depending on w= hether the compiler elides the get(0). If you elide the get(0) in the seco= nd example isn't view going to point potentially freed memory?=20 Why would view point to potentially freed memory? Decomposition declaration= works like this: 1) initialize a hidden "main" object using the initializer 2) bind "element" names to subobjects of the main objects The elements are guaranteed not to outlive the main object. Point 2) can be= obtained differently according to the type of the main object. In the most= general case, get<>() may be invoked. The Standard does not mandate that g= et<>() function shall have no side effects, but, really, the intent should = be to get hold of a reference of a subobject of the main object, no signifi= cant task should be performed. > It seems like there needs to be a distinction for structured binding betw= een "don't care about name but keep alive for RAII" vs "don't care about va= lue at all". In the case of a decomposition declaration, all RAII logic should performed= by the main object itself, since it's guaranteed to be destroyed after the= elements. Having an element perform RAII logic is a misuse of the feature.= Remember that you are not required to always decompose the object with thi= s syntax. > Personally I think the "ignore" semantic is not as interesting - if perfo= rmance is a concern then you'll have to be slightly more explicit or if it'= s common enough a separate mechanism can be developed. I believe it is interesting. However, I understand what you mean. For regul= ar declarations we definitely want the "keep" semantic, I guess we agree on= that. For decomposition declarations we might get either a "keep" or "igno= re" semantic. In most cases (e.g.: aggregates, arrays and objects whose get= () functions have no side effect) the result is the same. In the remaining = corner case, I expect the "ignore" semantic to be more useful, but this is = a personal opinion. It's a fact, however that if we mandate the "ignore" se= mantic, the programmer might easily get the "keep" by just providing any id= entifier different from __. However if we mandate the "keep" semantic, ther= e's no way to get the other one. A. --=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/BD003722-5383-491F-8D6C-0B560A4D7B2F%40gmail.com= .. .