220 8287 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: Sun, 29 Dec 2013 20:07:21 -0800 Lines: 112 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: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1388376456 28100 80.91.229.3 (30 Dec 2013 04:07:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 30 Dec 2013 04:07:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDDM34EO6QDRBDPDQOLAKGQEJ5FGT4Q@isocpp.org Mon Dec 30 05:07:44 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f200.google.com ([209.85.214.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VxU8l-0004M5-PX for gclcip-std-proposals@m.gmane.org; Mon, 30 Dec 2013 05:07:44 +0100 Original-Received: by mail-ob0-f200.google.com with SMTP id wm4sf53054275obc.11 for ; Sun, 29 Dec 2013 20:07:42 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=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 :content-transfer-encoding; bh=+sMHIvpNTc4LLJ55LPt91T/k4P6Zr/AICOuhAj5KFzA=; b=E1uvqCHG4xyUYEWShV3LACFDOEt0JzUbDqpQK33HPinTu+NYHWYvFxGGi8kP8EYtiJ 6mJqYaAP4rzATT1LN456SEM5U2CTXn30czUvEHrSD7eHhmHWkB+Gve3J30ytw5t9//+v U/ZZkAdvx1bbU7HaLDZ1u2qb2/VoEqoEbYluT/BT7k7y7MPu8O6ywpHmy2umDJavVhti DQxac79nfRobFhMmwBop+i4P4A4HkvRv/IFjR2gxFROeBML3cpex3f11jI6dapCpVAmw rx9/xXBChJHAuMpdRa1DzEx0OXHFPCsqguPxOu1uVXqdOR0uKQErVrs0ncfLmHDvhcNM fOGw== 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:content-transfer-encoding; bh=+sMHIvpNTc4LLJ55LPt91T/k4P6Zr/AICOuhAj5KFzA=; b=IQV/UcPP9uYenPOHEiPrkAueUvpwdPFl7zbIlgBCV+4y0hKh/+UebSH1sil8Rh7W03 Qk/U0ZvhBq6f9sR2zofDfbNMRb/nL3H7LenS6JBCzgxZ6i7muDRqs7XKnPfKRMLhBn7t Ry5UN2PPzpXHFm1JmHyE8Urtuf/DutRp7T617QIEzdEOCsgzp4QJCultw+nooEPixNgO PA8sIm/74wOnRtcIu+z6jVgHF5awTFvn8TTczRF3T6uPHEzeriWrytgi8MAAqP7sKdnQ A37/om10eTCdXm+my/xzZ7USbtd5n78zeP3vZhj16aSxY6EWdiyi88IQuskrLOUszdpH ps4A== X-Gm-Message-State: ALoCoQmbq1j3ALjP5hqMnVtzdWbAkdswR8iOaCV17S2XW7nIJYkhv+DZtDJUl0ud2IeNcrq50+ka X-Received: by 10.50.12.68 with SMTP id w4mr26062391igb.0.1388376462313; Sun, 29 Dec 2013 20:07:42 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.85.162 with SMTP id i2ls3438595qez.39.gmail; Sun, 29 Dec 2013 20:07:41 -0800 (PST) X-Received: by 10.224.0.139 with SMTP id 11mr28677147qab.50.1388376461782; Sun, 29 Dec 2013 20:07:41 -0800 (PST) Original-Received: from mail-qe0-x232.google.com (mail-qe0-x232.google.com [2607:f8b0:400d:c02::232]) by mx.google.com with ESMTPS id w4si369921qaj.22.2013.12.29.20.07.41 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 29 Dec 2013 20:07:41 -0800 (PST) Received-SPF: pass (google.com: domain of jyasskin@google.com designates 2607:f8b0:400d:c02::232 as permitted sender) client-ip=2607:f8b0:400d:c02::232; Original-Received: by mail-qe0-f50.google.com with SMTP id 1so10927936qec.37 for ; Sun, 29 Dec 2013 20:07:41 -0800 (PST) X-Received: by 10.49.27.8 with SMTP id p8mr106681484qeg.64.1388376461565; Sun, 29 Dec 2013 20:07:41 -0800 (PST) Original-Received: by 10.229.208.138 with HTTP; Sun, 29 Dec 2013 20:07:21 -0800 (PST) In-Reply-To: X-Original-Sender: jyasskin@google.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of jyasskin@google.com designates 2607:f8b0:400d:c02::232 as permitted sender) smtp.mail=jyasskin@google.com; dkim=pass header.i=@google.com; dmarc=pass (p=REJECT dis=NONE) header.from=google.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:8287 Archived-At: On Sun, Dec 29, 2013 at 10:15 AM, Peter Bigot wrote: > On Sun, Dec 29, 2013 at 11:35 AM, Marshall Clow > wrote: >> >> >> 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 th= at 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, whi= ch >> 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 und= efined >> 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 wor= th > keeping in mind (libstdc++ missed that one). > >> >> >> Also, I can=92t find anywhere in the specification does it guarantee tha= t >> 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 t= o > 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. Yeah, because this all makes sv1 equivalent to string_view(foo, 0), and that guarantees that data()=3D=3Dfoo, the assert()s have to pass. Hopefully the new version at https://github.com/google/cxx-std-draft/blob/string-ref-paper/string_view.h= tml, which describes things in terms of exposition-only data_ and size_ members, makes this clearer. Jeffrey --=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/. .