220 35112 <545c118e-8d6d-456b-a640-0e3c0784f583@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: pavel.kryukov@phystech.edu Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Marking more std::string_view members noexcept Date: Thu, 26 Oct 2017 02:30:45 -0700 (PDT) Lines: 239 Approved: news@gmane.org Message-ID: <545c118e-8d6d-456b-a640-0e3c0784f583@isocpp.org> References: <3d03cd7d-2177-98fd-408d-f74b1f6a8552@gmail.com> <5005582.pCRZNEiGFO@tjmaciei-mobl1> <3446274.dRsjijLxpX@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_235_2019475208.1509010245388" X-Trace: blaine.gmane.org 1509010266 28610 195.159.176.226 (26 Oct 2017 09:31:06 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 26 Oct 2017 09:31:06 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBC3LNQ5G3UMRBRWWY3HQKGQEQWCEDGY@isocpp.org Thu Oct 26 11:31:02 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f72.google.com ([209.85.213.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e7eUq-0004oL-4e for gclcip-std-proposals@m.gmane.org; Thu, 26 Oct 2017 11:30:40 +0200 Original-Received: by mail-vk0-f72.google.com with SMTP id v3sf1244334vkb.20 for ; Thu, 26 Oct 2017 02:30:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=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; bh=RDXiLxGx7CcMobdeHsFwOsm5T4RfL6FgCVFWqA5vA20=; b=BtoXhr2WWvC7nM9L8w4khfYgaXsFStx5daBkGbhwu6YnKp1PUxCZvJjnKKwfukwdOB isFAm6DjVzArD/3NQIoX/NrnAWMKchW9Z0QteZKAEWCuw9MShobOfBZ4TxlKQsZkCXQ+ vTq/pKc2E8FS7j5G7+/O8bT6t+9hW++hr9YocTECd4JVXuhF/k2I+yEwq4eY3LZL9L6n O3coyeGkZ+sXHCNmsUuKY63LrJjFL0s9+wuj1oTFLUcIFDyW590CWLi7Hy2pgc+rmtYx QS0FpcmHuMppVQeV2kKPakgpHdvV+muKnnX1LcJH8x1h+OI0+qP0zutaWCQ+0ZPzNhPR 6FRg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; 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:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=RDXiLxGx7CcMobdeHsFwOsm5T4RfL6FgCVFWqA5vA20=; b=GvkAizTbnpJiHPuJaAW3JoUxQjuSfLdgIrzP4BiCFOLO8yb83AKFnCEoTR2kQTTBtU xXcqHyzqpcVymewNiA7f4tpcocuTtUpvtk6usqGiqzzX9DcnrAhWWupUalCxZTq3Rslk yuUit01wjUY4MWLcRMnFpX/b6FWoogLiFhzimW3ajU6oGvnSqv45re7VnMghfPcFGPv+ gRI5pY9k6YVHAP3ovsoTLjt6aZEIZ4TLspWUFv5if3bN6YebaU9t3xihBqXwmS80IlpC CZSmna8eM6xTHlyQI+nrx9js+hUi0oIdu2mDhmYujsUXFj6AFbbez7ZHpxHdWM/GRaRH 47jg== X-Gm-Message-State: AMCzsaUhEkTQHE+3GGxbN+mXri197yeUsoh36hY82Kh0uwjOybqdNr3k RM5I2gFNwH6OvOLwTwXXJ6CM/g== X-Google-Smtp-Source: ABhQp+Ro2ZnLPvIJixe56Splvwxykm43mW0zs8oKX5Mj6ASJNT5K5pxK2gEGiwKObchOaWmBCMAFMw== X-Received: by 10.176.68.194 with SMTP id n60mr2545418uan.10.1509010247367; Thu, 26 Oct 2017 02:30:47 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.95.212 with SMTP id t203ls1348779vkb.0.gmail; Thu, 26 Oct 2017 02:30:45 -0700 (PDT) X-Received: by 10.31.50.18 with SMTP id y18mr424592vky.6.1509010245813; Thu, 26 Oct 2017 02:30:45 -0700 (PDT) In-Reply-To: X-Original-Sender: pavel.kryukov@phystech.edu 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:35112 Archived-At: ------=_Part_235_2019475208.1509010245388 Content-Type: multipart/alternative; boundary="----=_Part_236_1993802447.1509010245388" ------=_Part_236_1993802447.1509010245388 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable > This one still has the precondition that [Char, N) is a valid range. It= =20 > might not be:=20 > char bug[512];=20 > auto s =3D string_view(bug);=20 Should std::string_view have such precondition, isn't it just a wrapper=20 around pointer and size which may dangle? You may construct a nullptr string_view using argument-less ctor (it is=20 even noexcept) or get a valid string_view from a valid string, but then=20 break it. auto tmp =3D new std::string("Hello World"); std::string_view sv(tmp.c_str()); delete tmp; Thanks, -- Pavel =D1=87=D0=B5=D1=82=D0=B2=D0=B5=D1=80=D0=B3, 26 =D0=BE=D0=BA=D1=82=D1=8F=D0= =B1=D1=80=D1=8F 2017 =D0=B3., 11:55:14 UTC+3 =D0=BF=D0=BE=D0=BB=D1=8C=D0=B7= =D0=BE=D0=B2=D0=B0=D1=82=D0=B5=D0=BB=D1=8C Marc Mutz =D0=BD=D0=B0=D0=BF=D0= =B8=D1=81=D0=B0=D0=BB: > > On 2017-10-26 05:04, Thiago Macieira wrote:=20 > > On Wednesday, 25 October 2017 19:30:21 PDT Nevin Liber wrote:=20 > >> > QStringView has both a pointer and an array constructor. That means:= =20 > >> > const char16_t str[] =3D u"Hello\0World";=20 > >> >=20 > >> > QStringView(str).size() =3D=3D 11 // constexpr=20 > >> > QStringView(+str).size() =3D=3D 5 // not constexpr=20 > >>=20 > >> I'm not sure what constexpr has to do with it. Isn't it array vs.=20 > >> pointer=20 > >> that matters?=20 > >=20 > > Just a dig at the inability to overload on constexprness.=20 > > In QStringView, it's just an optimisation, because...=20 > > > Calculating the length of char16_t(&)[N] is easy: N - 1.=20 > > ... this is C++11-constexpr (which we still need to care for, in Qt)=20 > while ...=20 > > > Calculating the length of char16_t* can't be done in a constexpr way.= =20 > > ... is only C++14-constexpr (well, if you want to support reasonably=20 > long strings, otherwise you could come up with a recursive=20 > implementation).=20 > > We should probably document that the QStringView array ctor has the=20 > precondition that it contains no embedded NULs, but it seems qdoc is too= =20 > clever for it's own good:=20 > https://doc-snapshots.qt.io/qt5-5.10/qstringview.html doesn't have the=20 > docs for the array ctor.=20 > > >> It also means=20 > >>=20 > >> QStringView(b ? u"Hello\0World" : u"Howdy\0World") has subtly=20 > >> different=20 > >> behavior than QStringView(b ? u"Hello\0World" : u"Goodbye\0World"),=20 > >> doesn't=20 > >> it?=20 > >=20 > > Uh... I don't know. godbolting to the rescue...=20 > >=20 > > Why does this call the pointer version?=20 > > template void f(const char (&)[N]);=20 > > void f(const char *);=20 > >=20 > > void g(bool b)=20 > > {=20 > > const char str[] =3D "World";=20 > > f(str);=20 > > }=20 > > Because ordinary functions are preferred over templates. You need=20 > constrained templates to overload on pointer vs. array:=20 > https://www.kdab.com/qstringview-diaries-masters-overloads/ (can't seem= =20 > to link to a section, C-F for 'third').=20 > > Thanks,=20 > Marc=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/545c118e-8d6d-456b-a640-0e3c0784f583%40isocpp.or= g. ------=_Part_236_1993802447.1509010245388 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
> This one still has the precondition that [Char, N) is= a valid range. It=C2=A0
> might not be:=C2=A0

>=C2=A0 =C2= =A0 char bug[512];=C2=A0
>=C2=A0 =C2=A0 auto s =3D string_view(bug);= =C2=A0

Should std::string_view have such precondition, i= sn't it just a wrapper around pointer and size which may dangle?
<= div>You may construct a nullptr string_view using argument-less ctor (it is= even noexcept) or get a valid string_view from a valid string, but then br= eak it.

auto tmp =3D new std::string("Hello W= orld");
std::string_view sv(tmp.c_str());
delete t= mp;

Thanks,
--
Pavel

=D1=87=D0=B5=D1=82=D0=B2=D0=B5=D1=80=D0=B3, 26 =D0=BE=D0=BA=D1=82=D1= =8F=D0=B1=D1=80=D1=8F 2017 =D0=B3., 11:55:14 UTC+3 =D0=BF=D0=BE=D0=BB=D1=8C= =D0=B7=D0=BE=D0=B2=D0=B0=D1=82=D0=B5=D0=BB=D1=8C Marc Mutz =D0=BD=D0=B0=D0= =BF=D0=B8=D1=81=D0=B0=D0=BB:
On= 2017-10-26 05:04, Thiago Macieira wrote:
> On Wednesday, 25 October 2017 19:30:21 PDT Nevin Liber wrote:
>> > QStringView has both a pointer and an array constructor. = That means:
>> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 const char16_t str[] =3D u&qu= ot;Hello\0World";
>> >
>> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 QStringView(str).size() =3D= =3D 11 =C2=A0 // constexpr
>> > =C2=A0 =C2=A0 =C2=A0 =C2=A0 QStringView(+str).size() =3D= =3D 5 =C2=A0 // not constexpr
>>=20
>> I'm not sure what constexpr has to do with it. =C2=A0Isn&#= 39;t it array vs.=20
>> pointer
>> that matters?
>=20
> Just a dig at the inability to overload on constexprness.

In QStringView, it's just an optimisation, because...

> Calculating the length of char16_t(&)[N] is easy: N - 1.

... this is C++11-constexpr (which we still need to care for, in Qt)=20
while ...

> Calculating the length of char16_t* can't be done in a constex= pr way.

... is only C++14-constexpr (well, if you want to support reasonably=20
long strings, otherwise you could come up with a recursive=20
implementation).

We should probably document that the QStringView array ctor has the=20
precondition that it contains no embedded NULs, but it seems qdoc is to= o=20
clever for it's own good:=20
https://doc-snapshots.qt.io/qt5-5.10/qstringview.html does= n't have the=20
docs for the array ctor.

>> It also means
>>=20
>> QStringView(b ? u"Hello\0World" : u"Howdy\0Worl= d") has subtly=20
>> different
>> behavior than QStringView(b ? u"Hello\0World" : u&qu= ot;Goodbye\0World"),=20
>> doesn't
>> it?
>=20
> Uh... I don't know. godbolting to the rescue...
>=20
> Why does this call the pointer version?
> template <unsigned N> void f(const char (&)[N]);
> void f(const char *);
>=20
> void g(bool b)
> {
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0const char str[] = =3D "World";
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0f(str);
> }

Because ordinary functions are preferred over templates. You need=20
constrained templates to overload on pointer vs. array:=20
https://www.kdab.com/qstringview-diaries-mas= ters-overloads/ (can't seem=20
to link to a section, C-F for 'third').

Thanks,
Marc

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/545c118e-8d6d-456b-a640-0e3c0784f583= %40isocpp.org.
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