220 8647 <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> article Path: news.gmane.org!not-for-mail From: Marshall Clow Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 08:26:50 -0800 Lines: 109 Approved: news@gmane.org Message-ID: <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> References: <8045a4d2-721d-4725-8bb7-7a91b6f53ec8@isocpp.org> <186E0927-C962-4E34-AED0-E7509DD1F0A0@gmail.com> <7aad44d8-6b4e-438a-b309-bd898b994b34@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.1 \(1827\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_769C02A7-776F-4F23-B5F6-273D5042B65C" X-Trace: ger.gmane.org 1389976013 31932 80.91.229.3 (17 Jan 2014 16:26:53 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 16:26:53 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDHIXP53REBRBUNT4WLAKGQEDTVW5TY@isocpp.org Fri Jan 17 17:27:00 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qc0-f197.google.com ([209.85.216.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4CG2-00006P-EA for gclcip-std-proposals@m.gmane.org; Fri, 17 Jan 2014 17:26:58 +0100 Original-Received: by mail-qc0-f197.google.com with SMTP id e16sf6536530qcx.0 for ; Fri, 17 Jan 2014 08:26:57 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:message-id:mime-version:subject:date :references:to:in-reply-to: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; bh=lAc7lZ+w5neqkeXwqofmlr9Tz7Bjom+AyXaMTyJlzyk=; b=RqsjxXtBbQQgtJEsvC/oj1vHCE7z2gX8hg00H/VP7a35lXn0wtYl6UZa+w7fLyNNUx BjeWKk/zvLQCf/JiAEMxP/hqG42gZ/hQr4qT2D0rX2sIUwqI25xw5fj22MEWzCyMailE RTbf30G+lZaTzlMgO+vr5etbK6Qami9YSMqWFfhf+QugmiW8JWwd2pEHusiXJEI4t/47 JaNzC1j+jtqXSomCM7HCNarskNVPLiQXGMSkVdNlAhQ5IIFu9BViDUQ3c52alP45mdYt 0m/7J0xoo298T5Y/FNf5LhJVON1y5CC3uLwQna0LsGWFIJAwG80Acg6fRjGww5gpWXNB Blyw== X-Gm-Message-State: ALoCoQnVwv0BUyQqfVvjfGWDVHKJ+UMmnFPA0IFMagEeIihDvyTHVUOHM0Bt1k9K+11+aOg3SzvZ X-Received: by 10.236.94.113 with SMTP id m77mr969299yhf.33.1389976017622; Fri, 17 Jan 2014 08:26:57 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.95.134 with SMTP id i6ls488020qge.2.gmail; Fri, 17 Jan 2014 08:26:57 -0800 (PST) X-Received: by 10.236.131.19 with SMTP id l19mr2767484yhi.0.1389976017058; Fri, 17 Jan 2014 08:26:57 -0800 (PST) Original-Received: from mail-pb0-x236.google.com (mail-pb0-x236.google.com [2607:f8b0:400e:c01::236]) by mx.google.com with ESMTPS id q69si15893268yhd.220.2014.01.17.08.26.56 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Jan 2014 08:26:57 -0800 (PST) Received-SPF: pass (google.com: domain of mclow.lists@gmail.com designates 2607:f8b0:400e:c01::236 as permitted sender) client-ip=2607:f8b0:400e:c01::236; Original-Received: by mail-pb0-f54.google.com with SMTP id uo5so656043pbc.13 for ; Fri, 17 Jan 2014 08:26:55 -0800 (PST) X-Received: by 10.68.17.41 with SMTP id l9mr3106999pbd.76.1389976015894; Fri, 17 Jan 2014 08:26:55 -0800 (PST) Original-Received: from [192.168.16.50] (wsip-174-77-164-131.sd.sd.cox.net. [174.77.164.131]) by mx.google.com with ESMTPSA id nl7sm23985319pbc.6.2014.01.17.08.26.54 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Jan 2014 08:26:54 -0800 (PST) In-Reply-To: <7aad44d8-6b4e-438a-b309-bd898b994b34@isocpp.org> X-Mailer: Apple Mail (2.1827) X-Original-Sender: mclow.lists@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of mclow.lists@gmail.com designates 2607:f8b0:400e:c01::236 as permitted sender) smtp.mail=mclow.lists@gmail.com; dkim=pass header.i=@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:8647 Archived-At: --Apple-Mail=_769C02A7-776F-4F23-B5F6-273D5042B65C Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=windows-1252 On Jan 15, 2014, at 4:28 AM, Olaf van der Spek wrote= : > On Saturday, December 28, 2013 6:01:34 AM UTC+1, Marshall wrote: > if sv.size () =3D=3D 0 or sv.empty () [same thing], there is nothing that= you can do with sv.data() > (except compare it to null, and that doesn=92t tell you anything useful). >=20 > Are you sure? > IMO an empty string_view could still point into a string and I should be = able to use that pointer to for example construct a new string_view. A string view references a half-open contiguous range of characters; [ data= (), data()+size()) if size() =3D=3D 0, then the range of characters referenced by the string v= iew is empty, and accessing them (either via dereferencing begin() or data(= )) is (or should be) undefined behavior. (the same as any other container) If in your program, you =93know=94 that the pointer returned by data() poin= ts to something valid, well, that=92s on you to determine. Code example: char deref ( string_view sv ) { return *sv.data(); } string_view sv1; string_view sv2 { =93ABCDE=94 }; sv2.remove_prefix(5); assert ( sv1 =3D=3D sv2 ); deref ( sv1 ); // may or may not blow up in your face deref ( sv2 ); // this will probably =93work=94 ; but I don=92t know what = it will return. =97 Marshall --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --Apple-Mail=_769C02A7-776F-4F23-B5F6-273D5042B65C Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=windows-1252 On Jan 15, 2014, at 4:= 28 AM, Olaf van der Spek <olafvd= spek@gmail.com> wrote:

On Saturday, December 28, 201= 3 6:01:34 AM UTC+1, Marshall wrote:
if sv.size () =3D=3D 0 or sv= ..empty () [same thing], there is nothing that you can do with sv.data()
(except compare it to null, and that doesn=92t tell you anything use= ful).

Are you sure?
I= MO an empty string_view could still point into a string and I should be abl= e to use that pointer to for example construct a new string_view.

A string view references a half-op= en contiguous range of characters; [ data(), data()+size())

<= /div>
if size() =3D=3D 0, then the range of characters referenced by th= e string view is empty, and accessing them (either via dereferencing begin(= ) or data()) is (or should be) undefined behavior. (the same as any other c= ontainer)

If in your program, you =93know=94 that = the pointer returned by data() points to something valid, well, that=92s on= you to determine.

Code example:

ch= ar deref ( string_view sv ) { return *sv.data(); }

string_vi= ew sv1;
= string_view sv2 { =93ABCDE=94 };
sv2.remove_prefix(5);
assert ( sv1= =3D=3D sv2 );
deref ( sv1 ); // may or may not blow up in your face
deref ( sv2 ); // this will = probably =93work=94 ; but I don=92t know what it will return.
=97 Marshall

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