220 8618 <79b41284-9085-46c7-8667-5e0149be1649@isocpp.org> article Path: news.gmane.org!not-for-mail From: abolz.lists@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: string_view::is_null() Date: Wed, 15 Jan 2014 12:27:05 -0800 (PST) Lines: 299 Approved: news@gmane.org Message-ID: <79b41284-9085-46c7-8667-5e0149be1649@isocpp.org> References: <8045a4d2-721d-4725-8bb7-7a91b6f53ec8@isocpp.org> <2875722.TjfBuHt6Pp@tjmaciei-mobl2> <2685690.E3vksNc10o@tjmaciei-mobl2> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_3526_10633220.1389817625417" X-Trace: ger.gmane.org 1389817619 31958 80.91.229.3 (15 Jan 2014 20:26:59 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 15 Jan 2014 20:26:59 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCAJPLFFWYPBBGW63OLAKGQEY7ZT4JI@isocpp.org Wed Jan 15 21:27:08 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f199.google.com ([209.85.214.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W3X3M-0006Jq-3J for gclcip-std-proposals@m.gmane.org; Wed, 15 Jan 2014 21:27:08 +0100 Original-Received: by mail-ob0-f199.google.com with SMTP id vb8sf5346760obc.6 for ; Wed, 15 Jan 2014 12:27:07 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; 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 :content-type; bh=9jhn3yRY86FdkADrFezF+eb78KL7IURPZ5TGKuNRwZ8=; b=lPTCiNfpfcKqKpdbEvz6VQ3TubGE9gni7lg/tajNQfGu/9VmzXQoVdF+1t5/XBhqZh 0mkj+hn+R9vePsZj4lFjyn/3Qbn0pNHjTjjUFhSwfRke/pqHcT3UUu0/E6i+R3PhmAH4 GXOC+/nKXSwkc6oPw+Olxlq09/EN4Mcls7QjeeRMl8wTd0BCRL4ZJBZJCNAy5npcocxE g6XxkHiGUHtyOBQ2xRF3fQNzMku8HtGUlU5sL41MpwQ39E5f+QbtW3QOZvrmuDssv1e3 eHovouYk57gAErFzRbiOuQfGdTKnSetqmerm48/o9n6XIM6VgBD1K6bs4FxcFFvUXLNB JMsQ== 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=9jhn3yRY86FdkADrFezF+eb78KL7IURPZ5TGKuNRwZ8=; b=S9OvHsRtPvMFceXYnqVy4PrtEH4pTewZHJ+4zTcynlSRUyyHyjn43KPeOQTVx8dKlA uFHUo5UMzaYvU4oIAXfvtKd+NxQUoHkBIBRC+j8rsQcpNF6pWOkWyZcNWQTU5x3Z8IZa /piwTc+vS2yyTolthyXYmy95DJpoPB9Y9jYuvkMyG5DUbX4aIiCptJ+mAy0p/RTLhHkT at9Gx0/CLP0hFju/SF7Eyl3tTCL8JRoVBHDF2doFLSoWs+qX+1lU1WOezonL5hAtVsVx pcOR0eXlYEHYt5TAQ4mOqIJeNx7HgSqJlWMPWW9DAGO8X5fQ7RQYL17TY7rY7O9dc4cr Z5tg== X-Gm-Message-State: ALoCoQncGeZsUDcYqLz/no30qNgBy3Lw0xX4QzY3XQu3Z+LrYG4EsJ5wk35QAaRwGABGz7YXDemH X-Received: by 10.182.87.2 with SMTP id t2mr1689332obz.2.1389817627110; Wed, 15 Jan 2014 12:27:07 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.92.163 with SMTP id b32ls31980qge.57.gmail; Wed, 15 Jan 2014 12:27:06 -0800 (PST) X-Received: by 10.140.94.11 with SMTP id f11mr29026qge.7.1389817626007; Wed, 15 Jan 2014 12:27:06 -0800 (PST) In-Reply-To: <2685690.E3vksNc10o@tjmaciei-mobl2> X-Original-Sender: abolzlists@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:8618 Archived-At: ------=_Part_3526_10633220.1389817625417 Content-Type: text/plain; charset=UTF-8 Am Mittwoch, 15. Januar 2014 20:45:46 UTC+1 schrieb Thiago Macieira: > > On quarta-feira, 15 de janeiro de 2014 10:26:14, Jeffrey Yasskin wrote: > > Can you look over some of the uses of isNull() in QT and see what > > fraction are ones where both empty and null are possible values, and > > the distinction is useful rather than just forcing the API to document > > which one is used? > > The far majority of the cases, people just need the empty case. That's > what we > recommend. And unless some API in specific documents the distinction, > using > isEmpty() is the right thing to do. > > The null option comes in handy when you need to distinguish a field that > might > be present but empty from a field that isn't present. QUrl makes use of > that > and the equivalent std::networking::uri proposal just uses > std::optional (String is a template). For example: > > QUrl url1("foo:/"), url2("foo://@/?#"); > QString query1 = url1.query(), query2 = url2.query(); > > Both query1 and query2 are empty, but only query1 is null, indicating that > the > query was not present. The same applies to userInfo(), host(), and > fragment(), > and would apply to authority() if the "@" weren't present. > > And you can do: > > url1.setQuery(""); > url2.setQuery(QString()); > > to invert the situation. > > The other common case for using nulls is in QVariant and that comes from > the > QtSql module: all entries are returned as QVariants and they need to > support > database tables that don't contain "NOT NULL" (that is, are nullable). So > QVariant can contain a null int that is different from a zero: > > QVariant v1, v2{0}; > v1.convert(QVariant::Int); > // v1.isNull() == true; v2.isNull() == false > > We don't recommend relying on the nullness of a string. We only condone on > the > above cases I described, but I wouldn't be surprised to find more uses. > And, > trust me, there's quite a lot of headache involved in keeping the nullness > of > certain types across transformations: > > QString().toUtf8().isNull() == true; > QString("").toUtf8().isNull() == false; > > And then there are weird questions like: > - does a null QString compare equal to an empty one? (yes) > - does a null QString startsWith() an empty one? Does the opposite? > - same for endsWith(), contains(), indexOf() > - is QString().left(1) null? How about QString().left(0)? > - and what about QString("hello").left(0)? > - if left, right and mid can return null, can leftRef, rightRef and midRef > > (which return QStringRef)? > In general I would say keep the nullness and otherwise treat null and empty strings the same. But sometimes it is convenient to distinguish between a null and an empty string. I'd be fine with the current proposal if a named constructor string_view::null() and a member function string_view::is_null() were added. > > I don't know the answer to most of those questions, which means I would > recommend no one rely on a specific behaviour. > > Most of it is unit-tested so we don't break it: > > http://code.woboq.org/qt5/qtbase/tests/auto/corelib/tools/qstring/tst_qstring.cpp.html#_ZN11tst_QString10startsWithEv > > http://code.woboq.org/qt5/qtbase/tests/auto/corelib/tools/qstring/tst_qstring.cpp.html#_ZN11tst_QString8endsWithEv > > http://code.woboq.org/qt5/qtbase/tests/auto/corelib/tools/qstring/tst_qstring.cpp.html#_ZN11tst_QString4leftEv > > http://code.woboq.org/qt5/qtbase/tests/auto/corelib/tools/qstring/tst_qstring.cpp.html#_ZN11tst_QString7leftRefEv > > and what isn't tested I'd feel free to change behaviour at any time. > > -- > Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org > Software Architect - Intel Open Source Technology Center > PGP/GPG: 0x6EF45358; fingerprint: > E067 918B B660 DBD1 105C 966C 33F5 F005 6EF4 5358 > > -- --- 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/. ------=_Part_3526_10633220.1389817625417 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Am Mittwoch, 15. Januar 2014 20:45:46 UTC+1 schrieb Thiago= Macieira:
On quarta-feira, 15 de janeiro de 20= 14 10:26:14, Jeffrey Yasskin wrote:
> Can you look over some of the uses of isNull() in QT and see what
> fraction are ones where both empty and null are possible values, a= nd
> the distinction is useful rather than just forcing the API to docu= ment
> which one is used?

The far majority of the cases, people just need the empty case. That's = what we=20
recommend. And unless some API in specific documents the distinction, u= sing=20
isEmpty() is the right thing to do.

The null option comes in handy when you need to distinguish a field tha= t might=20
be present but empty from a field that isn't present. QUrl makes use of= that=20
and the equivalent std::networking::uri proposal just uses=20
std::optional<String> (String is a template). For example:

        QUrl url1("foo:/"), url= 2("foo://@/?#");
        QString query1 =3D url1= ..query(), query2 =3D url2.query();

Both query1 and query2 are empty, but only query1 is null, indicating t= hat the=20
query was not present. The same applies to userInfo(), host(), and frag= ment(),=20
and would apply to authority() if the "@" weren't present.

And you can do:

        url1.setQuery("");
        url2.setQuery(QString(<= wbr>));

to invert the situation.

The other common case for using nulls is in QVariant and that comes fro= m the=20
QtSql module: all entries are returned as QVariants and they need to su= pport=20
database tables that don't contain "NOT NULL" (that is, are nullable). = So=20
QVariant can contain a null int that is different from a zero:

        QVariant v1, v2{0};
        v1.convert(QVariant::Int);
        // v1.isNull() =3D=3D t= rue; v2.isNull() =3D=3D false

We don't recommend relying on the nullness of a string. We only condone= on the=20
above cases I described, but I wouldn't be surprised to find more uses.= And,=20
trust me, there's quite a lot of headache involved in keeping the nulln= ess of=20
certain types across transformations:

        QString().toUtf8().isNull() =3D=3D true;
        QString("").toUtf8().isNull() =3D=3D false;

And then there are weird questions like:=20
- does a null QString compare equal to an empty one? (yes)
- does a null QString startsWith() an empty one? Does the opposite?
- same for endsWith(), contains(), indexOf()
- is QString().left(1) null? How about QString().left(0)?
- and what about QString("hello").left(0)?
- if left, right and mid can return null, can leftRef, rightRef and mid= Ref  =20
  (which return QStringRef)?

In general I would say keep the nullne= ss and otherwise treat null and empty strings the same.

But sometimes it is convenient to distinguish between a null and an e= mpty string.

I'd be fine with the current proposal= if a named constructor string_view::null() and a member
function= string_view::is_null() were added.
 

I don't know the answer to most of those questions, which means I would= =20
recommend no one rely on a specific behaviour.=20

Most of it is unit-tested so we don't break it:
http://code.woboq.org/qt5/qtbase/tests/= auto/corelib/tools/qstring/tst_qstring.cpp.html#_ZN11tst_QSt= ring10startsWithEv
http://code.woboq.org/qt5/qtbase/tests/auto/core= lib/tools/qstring/tst_qstring.cpp.html#_ZN11tst_QString8ends= WithEv
http://code.woboq.org/qt5/qtbase/tests/auto/corelib/t= ools/qstring/tst_qstring.cpp.html#_ZN11tst_QString4leftEv
http://code.woboq.org/qt5/qtbase/tests/auto/corelib= /tools/qstring/tst_qstring.cpp.html#_ZN11tst_QString7leftRef= Ev

and what isn't tested I'd feel free to change behaviour at any time.

--=20
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4= 5358

--
 
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