220 8651 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 12:11:33 -0800 Lines: 87 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> 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 1389989509 1899 80.91.229.3 (17 Jan 2014 20:11:49 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 20:11:49 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDDM34EO6QDRBCU542LAKGQELEVMEUI@isocpp.org Fri Jan 17 21:11:57 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vb0-f71.google.com ([209.85.212.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4Flj-0006ND-Q8 for gclcip-std-proposals@m.gmane.org; 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Fri, 17 Jan 2014 12:11:54 -0800 (PST) Received-SPF: pass (google.com: domain of jyasskin@google.com designates 2607:f8b0:400d:c00::231 as permitted sender) client-ip=2607:f8b0:400d:c00::231; Original-Received: by mail-qa0-f49.google.com with SMTP id w8so3648519qac.22 for ; Fri, 17 Jan 2014 12:11:53 -0800 (PST) X-Received: by 10.224.30.133 with SMTP id u5mr6575655qac.47.1389989513818; Fri, 17 Jan 2014 12:11:53 -0800 (PST) Original-Received: by 10.229.208.138 with HTTP; Fri, 17 Jan 2014 12:11:33 -0800 (PST) In-Reply-To: <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> 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:c00::231 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:8651 Archived-At: On Fri, Jan 17, 2014 at 8:26 AM, Marshall Clow wrot= e: > On Jan 15, 2014, at 4:28 AM, Olaf van der Spek wro= te: > > On Saturday, December 28, 2013 6:01:34 AM UTC+1, Marshall wrote: >> >> 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)= .. > > > Are you sure? > IMO an empty string_view could still point into a string and I should be > able to use that pointer to for example construct a new string_view. > > > A string view references a half-open contiguous range of characters; [ > data(), data()+size()) > > if size() =3D=3D 0, then the range of characters referenced by the string= view > is empty, and accessing them (either via dereferencing begin() or data())= is > (or should be) undefined behavior. (the same as any other container) > > If in your program, you =93know=94 that the pointer returned by data() po= ints to > something valid, well, that=92s on you to determine. > > Code example: > > char deref ( string_view sv ) { return *sv.data(); } > > string_view sv1; > string_view sv2 { =93ABCDE=94 }; > sv2.remove_prefix(5); > assert ( sv1 =3D=3D sv2 ); > deref ( sv1 ); // may or may not blow up in your face > deref ( sv2 ); // this will probably =93work=94 ; but I don=92t know what= it will > return. The string_view spec guarantees the value data() returns after substr() and remove_prefix() operations, so the 'sv2' case is actually guaranteed to return the '\0' at the end of "ABCDE". However, *sv2.begin() has undefined behavior, because it goes through the iterator type. The use I intended for this was the ability to write things like: const char str[] =3D "ABCDE"; string_view sv{str}; consume_letters(sv); if (sv.data() =3D=3D std::end(str)) finished(); but I don't see a way to enable that without letting people also dereference data() when the string_view is empty but they know something about the value. (This assumes that knowing ptr1=3D=3Dptr2 means that *ptr1 and *ptr2 are equally valid, which isn't the case for things like realloc(), but I don't want to put any such corner cases in the string_view spec if I can help it.) I do see that this is a somewhat awkward middle ground, so I'm definitely going to ask for a straw-poll in Issaquah about whether the LWG wants data()=3D=3Dnullptr to be allowed. If it is, I intend to also remove the precondition on the string_view(data, size) to let people explicitly put nullptr in. I'm not sure if string_view() should be guaranteed to produce data()=3D=3Dnullptr if that change happens. It'll also be possible to change this after the first version of the TS ships with string_view inside, if field experience indicates that it's the wrong choice. --=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/. .