220 8284 article Path: news.gmane.org!not-for-mail From: Peter Bigot Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Sun, 29 Dec 2013 12:15:50 -0600 Lines: 212 Approved: news@gmane.org Message-ID: References: <8045a4d2-721d-4725-8bb7-7a91b6f53ec8@isocpp.org> <186E0927-C962-4E34-AED0-E7509DD1F0A0@gmail.com> <4E250ADA-5977-4F29-94B4-24D9F3E63EC2@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=e89a8ff1cbda307d6304eeb051f6 X-Trace: ger.gmane.org 1388340947 22188 80.91.229.3 (29 Dec 2013 18:15:47 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 29 Dec 2013 18:15:47 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDA3LUEAQACBBV6NQGLAKGQE5COAVUA@isocpp.org Sun Dec 29 19:15:54 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f198.google.com ([209.85.214.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VxKu0-0007nj-SY for gclcip-std-proposals@m.gmane.org; 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Sun, 29 Dec 2013 10:15:50 -0800 (PST) In-Reply-To: <4E250ADA-5977-4F29-94B4-24D9F3E63EC2@gmail.com> X-Original-Sender: bigotp@acm.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of pabigot@gmail.com designates 2607:f8b0:4003:c02::235 as permitted sender) smtp.mail=pabigot@gmail.com; dkim=pass header.i=@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:8284 Archived-At: --e89a8ff1cbda307d6304eeb051f6 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable On Sun, Dec 29, 2013 at 11:35 AM, Marshall Clow wrot= e: > > On Dec 28, 2013, at 11:21 AM, Peter Bigot wrote: > > > On Friday, December 27, 2013 11:01:34 PM UTC-6, Marshall wrote: >> >> On Dec 27, 2013, at 4:52 AM, abolz...@gmail.com wrote: >> > This should not affect data() which returns a non-null pointer. >> >> I disagree here with the proposal as currently written. >> I believe that the requirement that data() always return a non-null >> pointer is misguided. >> >> if sv.size () =3D=3D 0 or sv.empty () [same thing], there is nothing tha= t you >> can do with sv.data() >> (except compare it to null, and that doesn=92t tell you anything useful)= .. >> > > My understanding (which I'd like confirmed) is that sv.data() is still > useful as it provides a valid position within the referenced object, whic= h > might be referenced by other string_view instances or in its native form. > > > a string view refers to a half open range (just like an iterator pair). > sv.data() a pointer to the beginning of that range, just like sv.begin() > is an iterator to the beginning of that range. > Agreed. > > If sv.size() =3D=3D 0, then dereferencing sv.data() or sv.begin() is unde= fined > behavior. > Agreed. Also, per [string.view.access] the requirements disallow expression sv[0] in this case (unlike str[0] if str is a std::string, where the expression is permitted but the referenced object may not be modified). A subtlety worth keeping in mind (libstdc++ missed that one). > > Also, I can=92t find anywhere in the specification does it guarantee that > the following code works: > > const char *foo =3D =93abc=94; > string_view sv1 ( foo, 3 ); // size() =3D=3D 3 > The definition of the constructor in section x.1 [string.view.cons] guarantees sv1.data() =3D=3D foo. > sv1. remove_suffix (3); // size() =3D=3D 0 > Per [string.view.modifiers] this is equivalent to sv1 =3D sv1.substr(0, sv1.size() - 3). Per [string.view.ops] the definition of substr makes the RHS equivalent to string_view(sv1.data() + 0, sv1.size() - 3). Same constructor as above, so if that temporary object is tv, tv.data() =3D= =3D sv1.data()+0 =3D=3D foo+0 =3D=3D foo. > assert ( sv1.size() =3D=3D 0); > assert ( sv1.data() =3D=3D foo ); // will probably work - but might not. > AFAICT this must be true. If my reasoning is wrong, string_view becomes significantly less valuable to me, so do please point out my error. FWIW my first reading of N3593 was probably correct and zero-length string_view values returned from a delimiter find() may point into the middle of an input range; they only indicate "not found" when the data() member references the end position of the input. Peter --=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/. --e89a8ff1cbda307d6304eeb051f6 Content-Type: text/html; charset=windows-1252 Content-Transfer-Encoding: quoted-printable
On S= un, Dec 29, 2013 at 11:35 AM, Marshall Clow <mclow.lists@gmail.com> wrote:



On Friday, December 27, = 2013 11:01:34 PM UTC-6, Marshall wrote:
On Dec 27, 2013, at 4:52 AM, abolz..= ..@gmail.com wrote:

This should not affect data() which returns a non-nu= ll pointer.

I disagree here with the propo= sal as currently written.
I believe that the requirement that data() always return a non-nu= ll pointer is misguided.

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 usefu= l).

My understanding (which I'd like c= onfirmed) is that sv.data() is still useful as it provides a valid position= within the referenced object, which might be referenced by other string_vi= ew instances or in its native form.

a string view refers to= a half open range (just like an iterator pair).
sv.data() a pointer to= the beginning of that range, just like sv.begin() is an iterator to the be= ginning of that range.

Agreed.
=A0

= If sv.size() =3D=3D 0, then dereferencing sv.data() or sv.begin() is undefi= ned behavior.

Agreed.

Also, per [string.vie= w.access] the requirements disallow expression sv[0] in this case (unlike s= tr[0] if str is a std::string, where the expression is permitted but the re= ferenced object may not be modified).=A0 A subtlety worth keeping in mind (= libstdc++ missed that one).
=A0

Also, I can=92t find anywhere in the specification= does it guarantee that the following code works:

const char= *foo =3D =93abc=94;
string_view sv1 ( foo, 3 ); //= size() =3D=3D 3

The definition of the constructor in= section x.1 [string.view.cons] guarantees sv1.data() =3D=3D foo.
=A0
sv1.=A0remove_suffix (3); <= /span>// size() =3D=3D 0

Per [s= tring.view.modifiers] this is equivalent to sv1 =3D sv1.substr(0, sv1.size(= ) - 3).

Per [string.view.ops] the definition of substr makes the RHS equivalent= to string_view(sv1.data() + 0, sv1.size() - 3).

Same constructor as= above, so if that temporary object is tv, tv.data() =3D=3D sv1.data()+0 = =3D=3D foo+0 =3D=3D foo.

=A0
assert ( sv1.size() =3D=3D 0);
assert ( sv1.data() =3D= =3D foo ); // will probably wor= k - but might not.

AFAICT this = must be true.

If my reasoning is wrong, string_view becomes significantly = less valuable to me, so do please point out my error.

FWIW my first reading of N3593 was probably correct and zero-length= string_view values returned from a delimiter find()=A0 may point into the = middle of an input range; they only indicate "not found" when the= data() member references the end position of the input.
=A0
Peter

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