220 8672 <6a95bf9c-55c9-4ae2-966f-d1b1f4df6a37@isocpp.org> article Path: news.gmane.org!not-for-mail From: Bengt Gustafsson Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 17:00:12 -0800 (PST) Lines: 156 Approved: news@gmane.org Message-ID: <6a95bf9c-55c9-4ae2-966f-d1b1f4df6a37@isocpp.org> 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> <8E74B747-7450-41E0-8D1E-48C2D6F81535@gmail.com> <104254A6-C69A-4571-8455-89D7E466D493@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_2_12412770.1390006812394" X-Trace: ger.gmane.org 1390006813 28529 80.91.229.3 (18 Jan 2014 01:00:13 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 18 Jan 2014 01:00:13 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCRIRSPDTQIRBHNE46LAKGQE23MMFOA@isocpp.org Sat Jan 18 02:00:20 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f197.google.com ([209.85.223.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4KGl-0006rP-9F for gclcip-std-proposals@m.gmane.org; Sat, 18 Jan 2014 02:00:15 +0100 Original-Received: by mail-ie0-f197.google.com with SMTP id e14sf18471488iej.4 for ; Fri, 17 Jan 2014 17:00:14 -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:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=zf4/EBQ+iypp0kSNGsfwrSV5PqL7dotn3L91javRxpc=; b=BrqEQ1z0ED4UlakCseumc5sEwxHBIrXp3lTJj2226ImeH5pZ2iRx1gzXgMRpHXMl9R SF6C42Dms1bzvNGwqyKKynLg0FPst/9ivP7qkOTeJWHwBcBf7bY0xdFuErPiF6eldsul 6JrQXKhk5hGu0GEnpDzhSGc5cuVsNjA2Nk4bRIV24Bs+2QG7lFU78z1NPwlhOiEtmiMb eJala36wc+Q2LBgswZAbueu0Rj2Wbwuhi3baDNPDBD9MV16HUyMT4GftyLYtnNBXFL5M auxDU6kYfRpgr9qS0OOVDBrNaWcYA5afVMfiWOFTiErLr0iytDSWrsOj1odKtybVn5JN CgqA== X-Gm-Message-State: ALoCoQmnrzptdKxckZkqUYLhzHSAigYGhy2XRV8t9CbTWuTRl4W4Z8In8EXx75Afd5XMflRaIX3n X-Received: by 10.182.219.226 with SMTP id pr2mr1921443obc.44.1390006814347; Fri, 17 Jan 2014 17:00:14 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.176.202 with SMTP id ck10ls738813igc.1.canary; Fri, 17 Jan 2014 17:00:13 -0800 (PST) X-Received: by 10.50.107.67 with SMTP id ha3mr26997igb.7.1390006813497; Fri, 17 Jan 2014 17:00:13 -0800 (PST) In-Reply-To: X-Original-Sender: bengt.gustafsson@beamways.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:8672 Archived-At: ------=_Part_2_12412770.1390006812394 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable I have not followed this entire thread, instead I hacked up a little=20 implementation which I used in implementing something for another thread on= =20 an improved "printf" functionality. What I noticed was: - I had use for a C array of string_view to keep the parts of the original= =20 string that fell between each data inserted. This means that I must be able= =20 to default construct string_view as there is no other way to construct an= =20 array element. - It is logical to do this default construction by setting m_begin and=20 m_end to nullptr, or m_begin to nullptr and m_size to 0. - It must be allowed to take a default constructed string_view as the range= =20 of a range based for statement __or__ to check if the range is in default= =20 constucted state using something like os_null(). - both of these methods are impossible if the presumptions stated above=20 "begin() should never return nullptr" and "we don't need a special=20 is_null()". Well, not entirely. Of course you could create some bogus=20 object and use its address instead of nullptr. Someone stated above that the \0 at the end of the string should be=20 accessible even if the string_view is empty. This is clearly impossible as= =20 a string_view may be constructed as a part of a string which continues on.= =20 Thus there is no \0 at *end() anytime. This said I think that the very simple solution is to just set the=20 pointer(s) to nullptr in the default ctor, don't implement any special=20 is_null but return "null" iterators from begin and end in default=20 constructed string_view, which of course results in zero length if they are= =20 subtracted. I don't really see a problem here, isn't this how all=20 containers do it. For instance a newly default constructed vector? Is the= =20 issue whether you can rely on this to know wheter the string_view (or=20 vector) has been non-empty before? If so, follow the example of vector and= =20 give the same promises on the standard level! Den l=C3=B6rdagen den 18:e januari 2014 kl. 01:06:18 UTC+1 skrev Jeffrey Ya= sskin: > > On Fri, Jan 17, 2014 at 3:51 PM, Marshall Clow >=20 > wrote:=20 > > On Jan 17, 2014, at 2:10 PM, Jeffrey Yasskin >=20 > wrote:=20 > >=20 > > Agreed. To mitigate this, if string_view::data() could return nullptr, = I=20 > > believe we could still create strings using std::string(null_sv.begin()= ,=20 > > null_sv.end()).=20 > >=20 > >=20 > > I thought that=E2=80=99s why we had to_string().=20 > > We have to_string mostly for convenience, I think, although it is=20 > another way to get from a null string_view to a std::string.=20 > --=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/. ------=_Part_2_12412770.1390006812394 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I have not followed this entire thread, instead I hacked u= p a little implementation which I used in implementing something for anothe= r thread on an improved "printf" functionality. What I noticed was:
- I had use for a C array of string_view to keep the parts of t= he original string that fell between each data inserted. This means that I = must be able to default construct string_view as there is no other way to c= onstruct an array element.
- It is logical to do this default con= struction by setting m_begin and m_end to nullptr, or m_begin to nullptr an= d m_size to 0.
- It must be allowed to take a default constructed= string_view as the range of a range based for statement __or__ to check if= the range is in default constucted state using something like os_null().

- both of these methods are impossible if the presu= mptions stated above "begin() should never return nullptr" and "we don't ne= ed a special is_null()". Well, not entirely. Of course you could create som= e bogus object and use its address instead of nullptr.

=
Someone stated above that the \0 at the end of the string should be ac= cessible even if the string_view is empty. This is clearly impossible as a = string_view may be constructed as a part of a string which continues on. Th= us there is no \0 at *end() anytime.

This said I t= hink that the very simple solution is to just set the pointer(s) to nullptr= in the default ctor, don't implement any special is_null but return "null"= iterators from begin and end in default constructed string_view, which of = course results in zero length if they are subtracted. I don't really see a = problem here, isn't this how all containers do it. For instance a newly def= ault constructed vector? Is the issue whether you can rely on this to know = wheter the string_view (or vector) has been non-empty before? If so, follow= the example of vector and give the same promises on the standard level!


Den l=C3=B6rdagen den 18:e januari 2014 kl. 01:0= 6:18 UTC+1 skrev Jeffrey Yasskin:
On Fri, Jan 17, 2014 at 3:51 PM, Marshall Clow <mclow...@gmail.com> wrote:
> On Jan 17, 2014, at 2:10 PM, Jeffrey Yasskin <jyas...@google.com> wrote:
>
> Agreed. To mitigate this, if string_view::data() could return null= ptr, I
> believe we could still create strings using std::string(null_sv.be= gin(),
> null_sv.end()).
>
>
> I thought that=E2=80=99s why we had to_string().

We have to_string mostly for convenience, I think, although it is
another way to get from a null string_view to a std::string.

--
 
---
You received this message because you are subscribed to the Google Groups &= quot;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-proposals/<= /a>.
------=_Part_2_12412770.1390006812394-- .