Initialization of QSharedDataPointer's for empty objects (i.e. Foo::Foo(void))

[Milian Wolff]

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 "new
> > Private" but instead (re-)use a static shared empty object created on the
> > stack - see shared_empty and shared_null in qstring.{cpp,h}.
> 
> You probably did not mean what you said: you do not want something allocated
> 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 means you
> cannot derive from QSharedData, you cannot use QAtomicInt, QString or
> QByteArray. That may be taking it a bit too far.
> 
> 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 
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 
QSharedDataPointer, or am I missing something?

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

But in C++11 it should be fine, no? The alternative using {K,Q}_GLOBAL_STATIC 
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 the 
previous code snippet. I.e. this: http://paste.kde.org/734756/ triggers the 
following compile error:

/home/milian/projects/foo/src/main.cpp: In function 
‘QSharedDataPointer<Foo::Private>* s_sharedEmpty()’:
/home/milian/projects/foo/src/main.cpp:21:56: error: 
‘QSharedDataPointer<Foo::Private>* s_sharedEmpty()’ was declared ‘extern’ and 
later ‘static’ [-fpermissive]
 Q_GLOBAL_STATIC(QSharedDataPointer<Foo::Private>, s_sharedEmpty)
                                                        ^
/home/milian/projects/foo/src/main.cpp:12:46: error: previous declaration of 
‘QSharedDataPointer<Foo::Private>* s_sharedEmpty()’ [-fpermissive]
     friend QSharedDataPointer<Foo::Private>* ::s_sharedEmpty();


Thanks for the useful input Thiago, much appreciated!
-- 
Milian Wolff
mail at milianw.de
http://milianw.de
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