220 8660 article Path: news.gmane.org!not-for-mail From: Jeffrey Yasskin Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 14:10:41 -0800 Lines: 109 Approved: news@gmane.org Message-ID: 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> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a11c16f5e4d0d9704f031d106 X-Trace: ger.gmane.org 1389996658 20608 80.91.229.3 (17 Jan 2014 22:10:58 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 22:10:58 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDDM34EO6QDRB5WU42LAKGQEE3Z6FCI@isocpp.org Fri Jan 17 23:11:06 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f70.google.com ([209.85.219.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4Hd2-0008Kq-Dw for gclcip-std-proposals@m.gmane.org; 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charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On Fri, Jan 17, 2014 at 2:04 PM, Daniel Kr=FCgler wrote: > 2014/1/17 Marshall Clow : > > > > 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. > > No, it is not, it undefined behaviour. > > > 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. > > Exactly this requirement makes above code undefined. There exists an > explicit rule in [res.on.arguments] p1 b2 for this kind of wording: > > "If a function argument is described as being an array, the pointer > actually passed to the function shall > have a value such that all address computations and accesses to > objects (that would be valid if the > pointer did point to the first element of such an array) are in fact > valid." > > Agreed. To mitigate this, if string_view::data() could return nullptr, I believe we could still create strings using std::string(null_sv.begin(), null_sv.end()). --=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/. --001a11c16f5e4d0d9704f031d106 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
On F= ri, Jan 17, 2014 at 2:04 PM, Daniel Kr=FCgler <da= niel.kruegler@gmail.com> wrote:
2014/1/17 Marshall Clow <mclow.lists@gmail.com>:
>
> 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.

No, it is not, it undefined behaviour.

> Section 21.4.2 says only:
> basic_string(const charT* s, size_type n,
> =A0 =A0 =A0 =A0 =A0 =A0 =A0const Allocator& a =3D Allocator()); > Requires: s points to an array of at least n elements of charT.

Exactly this requirement makes above code undefined. There exists an<= br> explicit rule in [res.on.arguments] p1 b2 for this kind of wording:

"If a function argument is described as being an array, the pointer actually passed to the function shall
have a value such that all address computations and accesses to
objects (that would be valid if the
pointer did point to the first element of such an array) are in fact valid.= "


Agreed. To mitigat= e this, if string_view::data() could return nullptr, I believe we could sti= ll create strings using std::string(null_sv.begin(), null_sv.end()).

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