[Qt-interest] Calling qRegisterMetaType on static initialization

[Adrien Guinet]

Le 21/01/2012 18:06, Karl Krach a écrit :
> Hello,
>
> "Static member variables are initialized before the 'real program'
> (main()) starts" - right? At least this is how I understand
> http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.14
>
> So I used this nice trick for calling qRegisterMetaType:

Why don't you simply use the Q_DECLARE_METATYPE macro that does this work for 
you ? (and works on gcc 4.6 AFAIK :)). Search for it in the Qt assistant for 
instance, it provides example for how to use it.

Actually, it is a bit different as what you try to do, as it declares a special 
instantiation of the QMetaTypeId template class. See the macro definition : 
(from Qt 4.7 sources, qmetatype.h)

#define Q_DECLARE_METATYPE(TYPE)                                        \
     QT_BEGIN_NAMESPACE                                                  \
     template <>                                                         \
     struct QMetaTypeId< TYPE >                                          \
     {                                                                   \
         enum { Defined = 1 };                                           \
         static int qt_metatype_id()                                     \
             {                                                           \
                 static QBasicAtomicInt metatype_id = 
Q_BASIC_ATOMIC_INITIALIZER(0); \
                 if (!metatype_id)                                       \
                     metatype_id = qRegisterMetaType< TYPE >(#TYPE,      \
                                reinterpret_cast< TYPE *>(quintptr(-1))); \
                 return metatype_id;                                     \
             }                                                           \
     };                                                                  \
     QT_END_NAMESPACE

I assume that qt_metatype_id is called by QVariant (and others) when needed for 
a custom type. From what I understand, qRegisterMetaType() will actually 
"register" the type only if it has never been registered, and thus return always 
the same id. See Qt 4.7 sources' qmetatype.cpp:464 for more informations !

-- 
Adrien.