220 8658 <8E74B747-7450-41E0-8D1E-48C2D6F81535@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 13:55:41 -0800 Lines: 181 Approved: news@gmane.org Message-ID: <8E74B747-7450-41E0-8D1E-48C2D6F81535@gmail.com> References: <8045a4d2-721d-4725-8bb7-7a91b6f53ec8@isocpp.org> <186E0927-C962-4E34-AED0-E7509DD1F0A0@gmail.com> <7aad44d8-6b4e-438a-b309-bd898b994b34@isocpp.org> <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> 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=_6DD81798-CFCD-4E3C-8C18-83A4B141E511" X-Trace: ger.gmane.org 1389995743 11077 80.91.229.3 (17 Jan 2014 21:55:43 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 21:55:43 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDHIXP53REBRBYON42LAKGQEXUFOZKI@isocpp.org Fri Jan 17 22:55:49 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pb0-f70.google.com ([209.85.160.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4HOF-0001Z9-Hg for gclcip-std-proposals@m.gmane.org; Fri, 17 Jan 2014 22:55:47 +0100 Original-Received: by mail-pb0-f70.google.com with SMTP id rq2sf2346955pbb.1 for ; Fri, 17 Jan 2014 13:55:46 -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=hjiXRAZPhIGj3aWtPpXgWqvCf+v2zj40O3M6WdmmV5Y=; b=fM1YbWx98nzzBvz+RvldIu4kLw3+uFkG+/nRxBeghtsjA/FdRGFuwXVKBUEiX9fcZu nc355vaSsFlsM/nHNQjCLhNztfnzcYeEFVf96gu8c9ul5l4ClWt7sfvKSXSXkuq52mJK TpnDkuzdJ3DECa4fFyDZi45+PE5art+RrAXNXGRGHj/Xk2XJFsqcUdAEV1o4X9nbeQVB 7AzVsEoMa0JRCofP7jyDutqPPbbG9j2xS/vD8SXONNAzcah2sJudImbkKxe+TO9JC1mq ioqLzF86yjp/X2k0cw3IOkxFLnK1O4TkIINr7Ervp9NycFcd8S5A+3FoixT2jKZ4nqwa FN4Q== X-Gm-Message-State: ALoCoQmtTXoLjuOMqJ+YvPGxEfhXUL39lttVKzSB5W0uDx1C3/Z5Ibfo/MtzFJCAdXurinNTyehb X-Received: by 10.66.193.104 with SMTP id hn8mr1671145pac.46.1389995746359; Fri, 17 Jan 2014 13:55:46 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.96.69 with SMTP id dq5ls724566qeb.85.gmail; Fri, 17 Jan 2014 13:55:45 -0800 (PST) X-Received: by 10.236.147.107 with SMTP id s71mr3912908yhj.45.1389995745718; Fri, 17 Jan 2014 13:55:45 -0800 (PST) Original-Received: from mail-ob0-x22a.google.com (mail-ob0-x22a.google.com [2607:f8b0:4003:c01::22a]) by mx.google.com with ESMTPS id q66si4069978yhm.154.2014.01.17.13.55.45 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Jan 2014 13:55:45 -0800 (PST) Received-SPF: pass (google.com: domain of mclow.lists@gmail.com designates 2607:f8b0:4003:c01::22a as permitted sender) client-ip=2607:f8b0:4003:c01::22a; Original-Received: by mail-ob0-f170.google.com with SMTP id va2so1317947obc.1 for ; Fri, 17 Jan 2014 13:55:45 -0800 (PST) X-Received: by 10.182.230.135 with SMTP id sy7mr3699042obc.24.1389995745265; Fri, 17 Jan 2014 13:55:45 -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 qe2sm14225384obc.1.2014.01.17.13.55.43 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Jan 2014 13:55:44 -0800 (PST) In-Reply-To: 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:f8b0:4003:c01::22a 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:8658 Archived-At: --Apple-Mail=_6DD81798-CFCD-4E3C-8C18-83A4B141E511 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=windows-1252 On Jan 17, 2014, at 1:25 PM, Peter Bigot wrote: > On Fri, Jan 17, 2014 at 2:22 PM, Jeffrey Yasskin wr= ote: > On Fri, Jan 17, 2014 at 12:18 PM, Nevin Liber wr= ote: > > On 17 January 2014 14:11, Jeffrey Yasskin wrote: > >> > >> If it is, I intend to also > >> remove the precondition on the string_view(data, size) to let people > >> explicitly put nullptr in. > > > > > > I'd like to, mainly because it makes it easy to create a string_view fr= om a > > vector by using v.data() and v.size(). Having to special case a = check > > for nullptr (such as in string::string(const char*, size_t)) for what i= s > > otherwise a perfectly valid 0-sized range is just annoying. >=20 > Ah, vector.data() doesn't guarantee non-null results. :( That's a good ar= gument. >=20 > First, is that true? [data(), data()+size()) must be a valid range, but = I've been unable to find a definition of "valid range" in the standard so I= could understand whether an empty valid range permits its base to be a nul= l pointer. >=20 > Second, even if so std::string.data() does guarantee a non-null result, d= oesn't it? In that case it's also required that data()+i =3D=3D &operator[= ](i) for i in [0, size()] which led me to believe data() could not be null,= since the equality must hold for i=3D=3D0 and operator[] must return a ref= erence to an object (hence the underlying pointer cannot be null). I think you mean, [0,i), not [0,i] > I've been swayed by the argument that there's no such thing as a null std= ::string, so there should be no such thing as a null std::string_view. Thi= s is easily assured by disallowing std::string_view(s, n) for a null s just= as std::string(s, n) is disallowed for null s. The difference here is that string manages its own storage, while string_vi= ew does not. > I don't see it as unreasonable to require a check for that situation befo= re creating a std::string_view in situations where a char* pointer comes fr= om a source that might produce a null pointer. As Nevin points out, you ha= ve to do it for std::string too. Maybe I missed something earlier in the discussion, but I believe that std::string foo { nullptr, 0 }; is a perfectly legal way of creating an empty string. Section 21.4.2 says only: basic_string(const charT* s, size_type n, const Allocator& a =3D Allocator()); Requires: s points to an array of at least n elements of charT. > Personally, I'd find it more annoying to have to constantly check in ever= y utility function that operates on string_view instances whether an input = sv.data() is null in addition to whether sv.size() is zero.=20 I=92m afraid I don=92t see why you would do this. Can you give an example, = please? =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=_6DD81798-CFCD-4E3C-8C18-83A4B141E511 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=windows-1252
On Jan 1= 7, 2014, at 1:25 PM, Peter Bigot <bigo= tp@acm.org> wrote:

On Fri, Jan 17, 2014 at 2:22 PM, Jeffrey Yasskin <jyas= skin@google.com> wrote:
On Fri, Jan 17, 2014 at 12= :18 PM, Nevin Liber <nevin@evi= loverlord.com> wrote:
> On 17 January 2014 14:11, Jeffrey Yasskin <jyasskin@google.com> wrote:
>>
>> If it is, I intend to also
>> remove the precondition on the string_view(data, size) to let peop= le
>> explicitly put nullptr in.
>
>
> I'd like to, mainly because it makes it easy to create a string_view f= rom a
> vector<char> by using v.data() and v.size().  Having to spe= cial case a check
> for nullptr (such as in string::string(const char*, size_t)) for what = is
> otherwise a perfectly valid 0-sized range is just annoying.
<= /blockquote>

Ah, vector.data() doesn't guarantee non-null results. :( That's a goo= d argument.

First, is that true?  = [data(), data()+size()) must be a valid range, but I've been unable to find= a definition of "valid range" in the standard so I could understand whethe= r an empty valid range permits its base to be a null pointer.

Second, even if so std::st= ring.data() does guarantee a non-null result, doesn't it?  In that cas= e it's also required that data()+i =3D=3D &operator[](i) for i in [0, s= ize()] which led me to believe data() could not be null, since the equality= must hold for i=3D=3D0 and operator[] must return a reference to an object= (hence the underlying pointer cannot be null).

I think you mean, [0,i), not [0,i]

I've= been swayed by the argument that there's no such thing as a null std::stri= ng, so there should be no such thing as a null std::string_view.  This= is easily assured by disallowing std::string_view(s, n) for a null s just = as std::string(s, n) is disallowed for null s.
=

The difference here is that string manages its own storage, = while string_view does not.

I don't see it as unreasonable to requ= ire a check for that situation before creating a std::string_view in situat= ions where a char* pointer comes from a source that might produce a null po= inter.  As Nevin points out, you have to do it for std::string too.

Maybe I missed something earlier i= n the discussion, but I believe that
std::string foo { nullptr, 0 };
<= div>
is a perfectly legal way of creating an empty string.

Section 21.4.2 says only:
basic_string(const charT* = s, size_type n,
             const Allocat= or& a =3D Allocator());
Requires: s points to an array of at= least n elements of charT.

<= br>
Pe= rsonally, I'd find it more annoying to have to constantly check in every ut= ility function that operates on string_view instances whether an input sv.d= ata() is null in addition to whether sv.size() is zero. 
<= /blockquote>

I=92m afraid I don=92t see why you would do this= .. Can you give an example, please?

=97 Marshall

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