From kde-core-devel  Mon Apr 29 23:01:36 2013
From: Milian Wolff <mail () milianw ! de>
Date: Mon, 29 Apr 2013 23:01:36 +0000
To: kde-core-devel
Subject: Re: Initialization of QSharedDataPointer's for empty objects (i.e. Foo::Foo(void))
Message-Id: <15541471.LLbQ4kSKbW () minime>
X-MARC-Message: https://marc.info/?l=kde-core-devel&m=136727652121073
MIME-Version: 1
Content-Type: multipart/mixed; boundary="--nextPart1929980.9m2YljqxF4"


--nextPart1929980.9m2YljqxF4
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset="utf-8"

On Monday 29 April 2013 15:16:29 Thiago Macieira wrote:
> On segunda-feira, 29 de abril de 2013 21.52.37, Milian Wolff wrote:
> > I.e.: In the ctors that construct an "empty" object do not call "ne=
w
> > Private" but instead (re-)use a static shared empty object created =
on the
> > stack - see shared_empty and shared_null in qstring.{cpp,h}.
>=20
> You probably did not mean what you said: you do not want something al=
located
> on the stack. You want a global. Like QString :-)

Yes of course - thanks for the correction.

> To be *really* like QString, you want your private to be POD. That me=
ans you
> cannot derive from QSharedData, you cannot use QAtomicInt, QString or=

> QByteArray. That may be taking it a bit too far.
>=20
> If your private class is not POD, you should use a K/Q_GLOBAL_STATIC =
instead
> to hold your private.

No, I don't think making it a POD is required. {K,Q}_GLOBAL_STATIC with=
=20
Q_MOVABLE_TYPE should be sufficient in most cases I think.

> Examples:

<snip>

Interestingly I can't find a case  cases don't seem to be using=20
QSharedDataPointer, or am I missing something?

And is the following not OK due to random static initialization order i=
n=20
C++03? http://paste.kde.org/734738/

But in C++11 it should be fine, no? The alternative using {K,Q}_GLOBAL_=
STATIC=20
is imo much uglier, see: http://paste.kde.org/734750/

I could esp. not make it work with a private Private class as used in t=
he=20
previous code snippet. I.e. this: http://paste.kde.org/734756/ triggers=
 the=20
following compile error:

/home/milian/projects/foo/src/main.cpp: In function=20
=E2=80=98QSharedDataPointer<Foo::Private>* s_sharedEmpty()=E2=80=99:
/home/milian/projects/foo/src/main.cpp:21:56: error:=20
=E2=80=98QSharedDataPointer<Foo::Private>* s_sharedEmpty()=E2=80=99 was=
 declared =E2=80=98extern=E2=80=99 and=20
later =E2=80=98static=E2=80=99 [-fpermissive]
 Q_GLOBAL_STATIC(QSharedDataPointer<Foo::Private>, s_sharedEmpty)
                                                        ^
/home/milian/projects/foo/src/main.cpp:12:46: error: previous declarati=
on of=20
=E2=80=98QSharedDataPointer<Foo::Private>* s_sharedEmpty()=E2=80=99 [-f=
permissive]
     friend QSharedDataPointer<Foo::Private>* ::s_sharedEmpty();


Thanks for the useful input Thiago, much appreciated!
--=20
Milian Wolff
mail@milianw.de
http://milianw.de
--nextPart1929980.9m2YljqxF4
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: This is a digitally signed message part.
Content-Transfer-Encoding: 7Bit

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.19 (GNU/Linux)

iEYEABECAAYFAlF++9AACgkQDA6yEs0dE5M+BQCfYlpip0xvYCb0UNuYdiwoLAVL
1GMAoJPDuCv/n4fi9NHQnxzbEEp0BLkz
=7v6L
-----END PGP SIGNATURE-----

--nextPart1929980.9m2YljqxF4--