220 8661 article Path: news.gmane.org!not-for-mail From: "Billy O'Neal" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 14:17:53 -0800 Lines: 162 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=001a11c309463040c104f031ec77 X-Trace: ger.gmane.org 1389997108 26363 80.91.229.3 (17 Jan 2014 22:18:28 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 22:18:28 +0000 (UTC) To: std-proposals Original-X-From: std-proposals+bncBDKLBLVE6ADBBOWY42LAKGQEHD2QKLY@isocpp.org Fri Jan 17 23:18:36 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f72.google.com ([209.85.219.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4HkJ-0007X3-Gh for gclcip-std-proposals@m.gmane.org; Fri, 17 Jan 2014 23:18:35 +0100 Original-Received: by mail-oa0-f72.google.com with SMTP id o6sf17331866oag.7 for ; Fri, 17 Jan 2014 14:18:34 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject: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=6r8Yf7S21zWclHbeYVNMwEFE/R+tm0X/lj/IGfqpIyQ=; b=kqpUsdetLpW5fej35zlSSmduU/9FtsQ/tV32Y8C8eK3ef9SRYmtKQuYoq+ovymNiyv D7vbpq2N6CjnGq35678KCQ4vOcYaKp+Y64gRmOmFZy7mqqdQR+JpuV5ksftyLU8jqFq7 GzF3Zo+ZM9Fzug5vfe/NcSgANLrB/SdVJmvxHFIQYd9V3Js6YsW6hTFGpXnNLt1HhFXa hw1nE/iBnipBwJodFsu7dRtw+1JvuTX1I+ncSlybO0R+4V9VlXY0mQ8mIBfJpGWIshjD Qa9SOd3gpvuR+vLH4QlABvRlh4z5aGvu77hEeAjUZ1QFdotNKkAMlX/Fq9JiC9/4qUDI Gfng== X-Gm-Message-State: ALoCoQnR8RhUipH1YYJBdznrDRgC6As8QrqA2WHIVNwXcm5ycQrgnDPF+QemQv3ALJWECO4ORxMJ X-Received: by 10.42.64.74 with SMTP id f10mr1523510ici.12.1389997114551; Fri, 17 Jan 2014 14:18:34 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.23.80 with SMTP id 74ls534453qgo.61.gmail; Fri, 17 Jan 2014 14:18:33 -0800 (PST) X-Received: by 10.236.209.168 with SMTP id s28mr4054868yho.82.1389997113970; Fri, 17 Jan 2014 14:18:33 -0800 (PST) Original-Received: from mail-oa0-x232.google.com (mail-oa0-x232.google.com [2607:f8b0:4003:c02::232]) by mx.google.com with ESMTPS id v3si15508816yhd.38.2014.01.17.14.18.33 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Jan 2014 14:18:33 -0800 (PST) Received-SPF: pass (google.com: domain of billy.oneal@gmail.com designates 2607:f8b0:4003:c02::232 as permitted sender) client-ip=2607:f8b0:4003:c02::232; Original-Received: by mail-oa0-f50.google.com with SMTP id n16so2255416oag.37 for ; Fri, 17 Jan 2014 14:18:33 -0800 (PST) X-Received: by 10.60.50.202 with SMTP id e10mr3624145oeo.39.1389997113489; Fri, 17 Jan 2014 14:18:33 -0800 (PST) Original-Received: by 10.182.87.37 with HTTP; Fri, 17 Jan 2014 14:17:53 -0800 (PST) In-Reply-To: X-Original-Sender: billy.oneal@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of billy.oneal@gmail.com designates 2607:f8b0:4003:c02::232 as permitted sender) smtp.mail=billy.oneal@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:8661 Archived-At: --001a11c309463040c104f031ec77 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable >"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.= " So to clarify, if it were an array of size 0, you would be allowed to increment the pointer by 1. (You would not be allowed to dereference the pointer) You are never allowed to increment a null pointer, so data() returning nullptr does not meet this requirement. Billy O'Neal https://github.com/BillyONeal/ http://stackoverflow.com/users/82320/billy-oneal On Fri, Jan 17, 2014 at 2:04 PM, Daniel Kr=C3=BCgler 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." > > - Daniel > > -- > > --- > 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. > Visit this group at > http://groups.google.com/a/isocpp.org/group/std-proposals/. > --=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/. --001a11c309463040c104f031ec77 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
>"If a function argument is described as bein= g an array, the pointer=C2=A0actually passed to the function shall have a v= alue such that all address computations and accesses to=C2=A0objects (that = would be valid if the pointer did point to the first element of such an arr= ay) are in fact valid."

So to clarify, if it were an array of size 0, you would= be allowed to increment the pointer by 1. (You would not be allowed to der= eference the pointer) You are never allowed to increment a null pointer, so= data() returning nullptr does not meet this requirement.



On Fri, Jan 17, 2014 at 2:04 PM, Daniel = Kr=C3=BCgler <daniel.kruegler@gmail.com> wrote:
<= blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px= #ccc solid;padding-left:1ex"> 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,
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=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.= "

- Daniel

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