220 35104 article Path: news.gmane.org!.POSTED!not-for-mail From: Nevin Liber Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Marking more std::string_view members noexcept Date: Wed, 25 Oct 2017 19:17:01 -0500 Lines: 109 Approved: news@gmane.org Message-ID: References: <3d03cd7d-2177-98fd-408d-f74b1f6a8552@gmail.com> <79ff7517-9d44-4770-8124-a2a036a2e6be@isocpp.org> <5949784.ta64GYsRuq@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a114159eec1cb92055c68194b" X-Trace: blaine.gmane.org 1508977072 15335 195.159.176.226 (26 Oct 2017 00:17:52 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 26 Oct 2017 00:17:52 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBCE35H5S6IDBBJ6TYTHQKGQEX6JPKKA@isocpp.org Thu Oct 26 02:17:46 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f71.google.com ([209.85.213.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e7Vrd-0002C6-D1 for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id f197sor1674881vkf.107.2017.10.25.17.17.42 for (Google Transport Security); Wed, 25 Oct 2017 17:17:42 -0700 (PDT) Received-SPF: pass (google.com: domain of nliber@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.31.167.151 with SMTP id q145mr2596363vke.49.1508977062036; Wed, 25 Oct 2017 17:17:42 -0700 (PDT) Original-Sender: nliber@gmail.com Original-Received: by 10.103.71.140 with HTTP; Wed, 25 Oct 2017 17:17:01 -0700 (PDT) In-Reply-To: <5949784.ta64GYsRuq@tjmaciei-mobl1> X-Original-Sender: nevin@eviloverlord.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=X9X8rFef; spf=pass (google.com: domain of nliber@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=nliber@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:35104 Archived-At: --001a114159eec1cb92055c68194b Content-Type: text/plain; charset="UTF-8" On Wed, Oct 25, 2017 at 6:09 PM, Thiago Macieira wrote: > > > template > > > constexpr basic_string_view(const charT(&str)[N]) noexcept; > > > > It hasn't been proposed, has it? > > > > My guess would be no, since there is a precondition that str[N-1] == 0, > > giving it a narrow contract. > > Why would it have such a precondition? > This constructor is unnecessary unless you want to differentiate between C-string literals and raw pointers (which is a breaking change BTW). The problem is, C-string literals are not represented in the type system. The closest, and by no means is it perfect, is a const char(&str)[N] where !str[N-1] holds. Take the following: string_view a("Hello"); const char s[] = { 'H', 'e', 'l', 'l', 'o' }; string_view b(s); string_view c("Hello\0World"); Right now: a.size() == 5; b.size() // UB c.size() == 5; With this change, what should a.size(), b.size() and c.size() be? Boost.Range has as_literal because you cannot in general deduce the intent from the type alone. -- Nevin ":-)" Liber +1-847-691-1404 -- 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 email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAGg_6%2BO6VhTD2vJoLVDONbbfw5EdSU5CW4D_KCdEvPT2V0DGrA%40mail.gmail.com. --001a114159eec1cb92055c68194b Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Wed, Oct 25, 2017 at 6:09 PM, Thiago Macieira <thiag= o@macieira.org> wrote:
> >= ; template<size_t N>
> > constexpr basic_string_view(const charT(&str)[N]) noexcept; >
> It hasn't been proposed, has it?
>
> My guess would be no, since there is a precondition that str[N-1] =3D= =3D 0,
> giving it a narrow contract.

Why would it have such a precondition?

This constructor is unnecessary unless you want to differentiate bet= ween C-string literals and raw pointers (which is a breaking change BTW).

The problem is, C-string literals are not represent= ed in the type system.

The closest, and by no mean= s is it perfect, is a const char(&str)[N] where !str[N-1] holds.
<= div>
Take the following:

string_view= a("Hello");
const char s[] =3D { 'H', 'e&#= 39;, 'l', 'l', 'o' };
string_view b(s);
string_view c("Hello\0World");

= Right now:

a.size() =3D=3D 5;
b.size() /= / UB
c.size() =3D=3D 5;

With this change= , what should a.size(), b.size() and c.size() be?

= Boost.Range has as_literal because you cannot in general deduce the intent = from the type alone.
--
= =C2=A0Nevin ":-)" Liber=C2=A0 <mailto:nevin@eviloverlord.com> =C2=A0+1= -847-691-1404

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