220 8697 <20140119192238.GA19497@faust.lysator.liu.se> article Path: news.gmane.org!not-for-mail From: Magnus Fromreide Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Sun, 19 Jan 2014 20:22:40 +0100 Lines: 52 Approved: news@gmane.org Message-ID: <20140119192238.GA19497@faust.lysator.liu.se> References: <104254A6-C69A-4571-8455-89D7E466D493@gmail.com> <6a95bf9c-55c9-4ae2-966f-d1b1f4df6a37@isocpp.org> <52D9E336.1060801@knejp.de> <1B534FD7-B385-4A7E-A718-3C73F0AB61DC@gmail.com> <11f13f8f-1270-427c-8942-94027a181ff4@isocpp.org> <20140119174431.GA16875@faust.lysator.liu.se> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1390159356 26675 80.91.229.3 (19 Jan 2014 19:22:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 19 Jan 2014 19:22:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDELLREETMBRBAWM6CLAKGQEVOHKPIY@isocpp.org Sun Jan 19 20:22:43 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-we0-f198.google.com ([74.125.82.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4xxD-0001G1-5t for gclcip-std-proposals@m.gmane.org; Sun, 19 Jan 2014 20:22:43 +0100 Original-Received: by mail-we0-f198.google.com with SMTP id p61sf10599903wes.9 for ; Sun, 19 Jan 2014 11:22:42 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:subject:message-id:mail-followup-to :references:mime-version:in-reply-to:user-agent:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type:content-disposition; bh=E6Mv2FZP2nKhP8pyUMA94WbAzD2RJhfJBYyi0Ry7Rqw=; b=lRCJZ00m7KNCUH4ESzLqoYdk5x+TmFBxbyVT2gc0AzmAbHV1OXqYMoNCTPgsJ4H5Oz K+qvoRO7nFT87iIRIGwoJxnBrMZmgkKuY6x3Qr1NywtCrjedwYk2amHKeK9yrTt3DKZC jUHjixu2x58+MuP7uEy6msGfoc9tJGClQq1cMSaZCC/Lk5I2gGz3x0kQh+sulYGKw2KW icoLeLeFAO8/gyz2B7WPYOsdLAa8Lj98qVLxrik1dmTDjatNYI6Gv5VzhLtueuvYGmeg uCsBXijLViepD/FDpxBtLRkA9XLdmkS+OnI6IWWwamQTQRca/Uv9/jpzQWKHPbCxXkMG X-Gm-Message-State: ALoCoQkJm7mmOUdWny9V1D7oqDn3ucC2bKZsr9XfojTI9McvtEPoiIv84k+9mc2QyEPmodGRQ94z X-Received: by 10.205.34.71 with SMTP id sr7mr3223256bkb.4.1390159362646; Sun, 19 Jan 2014 11:22:42 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.82.227 with SMTP id l3ls382500wiy.11.canary; Sun, 19 Jan 2014 11:22:41 -0800 (PST) X-Received: by 10.204.235.9 with SMTP id ke9mr3658170bkb.9.1390159361861; Sun, 19 Jan 2014 11:22:41 -0800 (PST) Original-Received: from mail.lysator.liu.se (mail.lysator.liu.se. [2001:6b0:17:f0a0::3]) by mx.google.com with ESMTPS id kb4si13182651bkb.43.2014.01.19.11.22.41 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Sun, 19 Jan 2014 11:22:41 -0800 (PST) Received-SPF: pass (google.com: domain of magfr@lysator.liu.se designates 2001:6b0:17:f0a0::3 as permitted sender) client-ip=2001:6b0:17:f0a0::3; Original-Received: from mail.lysator.liu.se (localhost [127.0.0.1]) by mail.lysator.liu.se (Postfix) with ESMTP id 3004940026 for ; Sun, 19 Jan 2014 20:22:41 +0100 (CET) Original-Received: from faust.lysator.liu.se (faust.lysator.liu.se [IPv6:2001:6b0:17:f0a0::d3]) by mail.lysator.liu.se (Postfix) with SMTP id 2476F4001F for ; Sun, 19 Jan 2014 20:22:40 +0100 (CET) Original-Received: by faust.lysator.liu.se (sSMTP sendmail emulation); Sun, 19 Jan 2014 20:22:40 +0100 Mail-Followup-To: std-proposals@isocpp.org In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) X-Virus-Scanned: ClamAV using ClamSMTP X-Original-Sender: magfr@lysator.liu.se X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of magfr@lysator.liu.se designates 2001:6b0:17:f0a0::3 as permitted sender) smtp.mail=magfr@lysator.liu.se 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: , Content-Disposition: inline Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:8697 Archived-At: On Sun, Jan 19, 2014 at 12:31:19PM -0600, Peter Bigot wrote: > On Sun, Jan 19, 2014 at 11:44 AM, Magnus Fromreide wrote: > > > On Sun, Jan 19, 2014 at 07:55:32AM -0800, Peter Bigot wrote: > > > On Friday, January 17, 2014 10:19:07 PM UTC-6, Marshall wrote: > > > > > > > > I have thought in a similar direction, but the problem is if you have two > > string_view's, s1 and s2. Assume that s1 i empty, does the statement > > s2 = s1; imply that s2.data() == s1.data()? > > > > Then what happens if s1 is deallocated and it's memory is returned to the > > system, won't s2.m_ptr then hold an illegal pointer value, one of those > > where even loading it could trigger a hardware trap on some architectures. > > > > In my approach I detect in the assignment operator and copy constructor > whether the RHS is default-constructed, and if so invoke clear() on the > LHS/new instance. So in all cases either the reference is to a sequence > outside the instance, or it's an empty default-constructed (cleared) > instance where data() is a non-dereferenceable pointer to an internal charT > object: no cross-object pointers. > > Now that you point it out, this does result in behavior non-conformant with > the current draft specification for the copy constructor and operator= > which are specified as =default. > > On further reflection it'd better to store a null pointer in m_ptr to > represent a default-constructed value but to return &m_nul from data() when > m_ptr is null. Then the default implementations are retained. This still > means that data() will be always be non-null, but sv1.data() will not > compare equal to sv2.data() if one or both of the instances have been > cleared/default-constructed. (sv1 == sv2 will still hold, of course.) > > Does that address the objection? The check at copy time seems to solve the problem. The use of null as a flag value seems to interfere with the "allow null string_views" and "have an optimized optional" proposals as both allow null's for other purposes. /MF -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. .