220 8649 <3379bbae-dd54-461c-bf90-228e476ca7db@isocpp.org> article Path: news.gmane.org!not-for-mail From: Peter Bigot Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Fri, 17 Jan 2014 10:34:37 -0800 (PST) Lines: 217 Approved: news@gmane.org Message-ID: <3379bbae-dd54-461c-bf90-228e476ca7db@isocpp.org> 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: multipart/alternative; boundary="----=_Part_6_3650826.1389983677618" X-Trace: ger.gmane.org 1389983675 28616 80.91.229.3 (17 Jan 2014 18:34:35 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Jan 2014 18:34:35 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDA3LUEAQACBBPXP4WLAKGQEXG3PAJI@isocpp.org Fri Jan 17 19:34:43 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f197.google.com ([209.85.214.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W4EFc-0001Ci-0X for gclcip-std-proposals@m.gmane.org; Fri, 17 Jan 2014 19:34:40 +0100 Original-Received: by mail-ob0-f197.google.com with SMTP id gq1sf8882183obb.4 for ; Fri, 17 Jan 2014 10:34:39 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=C7VPy1V56dj1r1ljNlYQHq2C4b/gQ4gfRw13veP0VUc=; b=ZgHQ1xT/QH1mXSxeiQ6Q61i5kN5FVQ71kPwRwoH8EEfhqjWyEzN96z6rSYd02aAnoq WR6ncQ9oXMu/mNwpbEgQWprASf/H7HfYWNIh9peQoFh4Uy7G2DMQqAzlVLai2kjTvMeU +kV/iWssdRF/k0aIrixu5S3T0gcePUc/bciGJUVmCmxQhent3+HBiRu8zT2oGiN6Va1e QYp5VDPQ/BDE+wojMAZC7kp2E+J0sCX4wClxWMrSSN3LLCJZcdsHudgdJV2/UzQrYjfa TuEU9g9JFOVeMfFWHRi5P/hoQsm1RxxF+L7xLvEbTWPQqiw+Meh1nuxjzOx7VWQ3LxSb A4Jg== X-Gm-Message-State: ALoCoQnnUqdQ320+W2zTCv8PX326KqUHiRbgd9dUfDLAZJI1soMRb/aW1NqirD2kS2JPP6yvyzdU X-Received: by 10.182.186.73 with SMTP id fi9mr1305132obc.48.1389983678995; Fri, 17 Jan 2014 10:34:38 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.47.4 with SMTP id z4ls655455obm.93.gmail; Fri, 17 Jan 2014 10:34:38 -0800 (PST) X-Received: by 10.182.154.38 with SMTP id vl6mr18454obb.40.1389983678103; Fri, 17 Jan 2014 10:34:38 -0800 (PST) In-Reply-To: <9C2C439F-7983-4224-B272-53CAD0E2E456@gmail.com> X-Original-Sender: bigotp@acm.org 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:8649 Archived-At: ------=_Part_6_3650826.1389983677618 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Friday, January 17, 2014 10:26:50 AM UTC-6, Marshall wrote: > > On Jan 15, 2014, at 4:28 AM, Olaf van der Spek >=20 > wrote: > > 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=20 >> can do with sv.data() >> (except compare it to null, and that doesn=E2=80=99t tell you anything u= seful). >> > > Are you sure? > IMO an empty string_view could still point into a string and I should be= =20 > able to use that pointer to for example construct a new string_view. > > > A string view references a half-open contiguous range of characters; [=20 > data(), data()+size()) > > if size() =3D=3D 0, then the range of characters referenced by the string= view=20 > is empty, and accessing them (either via dereferencing begin() or data())= =20 > is (or should be) undefined behavior. (the same as any other container) > > If in your program, you =E2=80=9Cknow=E2=80=9D that the pointer returned = by data() points=20 > to something valid, well, that=E2=80=99s on you to determine. > > Code example: > > char deref ( string_view sv ) { return *sv.data(); } > > string_view sv1; > string_view sv2 { =E2=80=9CABCDE=E2=80=9D }; > 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 =E2=80=9Cwork=E2=80=9D ; but I don= =E2=80=99t know what it=20 > will return. > > =E2=80=94 Marshall > > While the half-open range argument is true in isolation, it's missing the= =20 point that the value of data() is well-known based on how the string_view= =20 was created and what operations have been performed on it. Adjusting the=20 string_view to the point where size() is zero does not invalidate data(). Based on how sv2 is constructed in that example, I believe it's guaranteed= =20 that deref(sv2) will return the '\0' that terminates the string literal=20 used to initialize sv2, and that doing so does not invoke undefined=20 behavior. This follows from how sv2 is created and the definition of=20 remove_prefix. Here's a more relevant example. There is no need for scare quotes when=20 stating that the programmer knows delim.data() always points to something= =20 valid in the test code. Peter string_view comma_delimiter (string_view sv) { string_view::size_type p{sv.find(',')}; if (string_view::npos =3D=3D p) { return sv.substr(sv.size()); } return sv.substr(p, 1); } string_view case_change_delimiter (string_view sv) { for (string_view::size_type i =3D 1; i < sv.size(); ++i) { string_view::value_type c1 =3D sv[i-1]; string_view::value_type c2 =3D sv[i]; if (isalpha(c1) && isalpha(c2) && (islower(c1) !=3D islower(c2))) { return sv.substr(i, 0); } } return sv.substr(sv.size()); } TEST(Delimiter, Example) { const std::string base("A,Bcde"); string_view sv(base); ASSERT_EQ(base.data(), sv.data()); /* Locate the comma as a single-character delimiter */ string_view delim(comma_delimiter(sv)); ASSERT_EQ(1, delim.size()); ASSERT_EQ(1, (delim.data() - sv.data())); ASSERT_EQ(",", delim); /* Locate the case change as zero-width delimiter */ delim =3D case_change_delimiter(sv); ASSERT_EQ(0, delim.size()); ASSERT_EQ(3, (delim.data() - sv.data())); ASSERT_EQ("cde", sv.substr(delim.data()-sv.data())); } --=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/. ------=_Part_6_3650826.1389983677618 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Friday, January 17, 2014 10:26:50 AM UTC-6, Mar= shall wrote:
On Jan 15, 2014, at 4:28 AM, Olaf van der Spek <olafv...@gmail.com> wrote:
On Saturday, December 28, 2013= 6:01:34 AM UTC+1, Marshall wrote:
if sv.size () =3D=3D 0 or sv.empt= y () [same thing], there is nothing that you can do with sv.data()
(except compare it to null, and that doesn=E2=80=99t tell you anything us= eful).

Are you sure?
= IMO an empty string_view could still point into a string and I should be ab= le to use that pointer to for example construct a new string_view.

A string view references a half-o= pen contiguous range of characters; [ data(), data()+size())

=
if size() =3D=3D 0, then the range of characters referenced by t= he 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 =E2=80=9Cknow= =E2=80=9D that the pointer returned by data() points to something valid, we= ll, that=E2=80=99s on you to determine.

Code examp= le:

char d= eref ( string_view sv ) { return *sv.data(); }

string_view sv1;
string_view sv2 { =E2=80=9CABCDE=E2=80=9D };
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 =E2=80=9Cwork=E2=80=9D ; but I = don=E2=80=99t know what it will return.

=E2=80=94 = Marshall


While the half-ope= n range argument is true in isolation, it's missing the point that the valu= e of data() is well-known based on how the string_view was created and what= operations have been performed on it. Adjusting the string_view to the poi= nt where size() is zero does not invalidate data().

Based on how sv2= is constructed in that example, I believe it's guaranteed that deref(sv2) = will return the '\0' that terminates the string literal used to initialize = sv2, and that doing so does not invoke undefined behavior.  This follo= ws from how sv2 is created and the definition of remove_prefix.

Here= 's a more relevant example.  There is no need for scare quotes when st= ating that the programmer knows delim.data() always points to something val= id in the test code.

Peter

string_view
comma_delimiter (st= ring_view sv)
{
  string_view::size_type p{sv.find(',')};
&nb= sp; if (string_view::npos =3D=3D p) {
    return sv.subst= r(sv.size());
  }
  return sv.substr(p, 1);
}

str= ing_view
case_change_delimiter (string_view sv)
{
  for (stri= ng_view::size_type i =3D 1; i < sv.size(); ++i) {
    = string_view::value_type c1 =3D sv[i-1];
    string_view::= value_type c2 =3D sv[i];
    if (isalpha(c1) && i= salpha(c2) && (islower(c1) !=3D islower(c2))) {
  &nbs= p;   return sv.substr(i, 0);
    }
  }<= br>  return sv.substr(sv.size());
}

TEST(Delimiter, Example)=
{
  const std::string base("A,Bcde");
  string_view sv(= base);
  ASSERT_EQ(base.data(), sv.data());

  /* Locate= the comma as a single-character delimiter */
  string_view delim(c= omma_delimiter(sv));
  ASSERT_EQ(1, delim.size());
  ASSERT= _EQ(1, (delim.data() - sv.data()));
  ASSERT_EQ(",", delim);
  /* Locate the case change as zero-width delimiter */
  deli= m =3D case_change_delimiter(sv);
  ASSERT_EQ(0, delim.size());
&= nbsp; ASSERT_EQ(3, (delim.data() - sv.data()));
  ASSERT_EQ("cde", = sv.substr(delim.data()-sv.data()));
}

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