A simple basic fundamental doubt !

[symsahoo]

Hi,

I was just trying to understand the concept behind qobject_cast and have landed in to some dilemma . I would be more than happy if some one could tell me the difference between the two representation of the same logic. I am here by trying to expand the q_object macro to see how it is using the meta object for dynamic identification of the object type.

Preconditions
----------------
I have a QObject subclass,

class QMyTestClass : public QObject
{
	Q_OBJECT
public:
	QMyTestClass(){}
	int Get(){return 23;}
};
 
QObject *n ;
QMyTestClass *o

class QMyTestClass : public QObject
{
	Q_OBJECT
public:
	QMyTestClass(){}
	int Get(){return 23;}
};

QObject *n ;
QMyTestClass *o

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First
-----

//QMyTestClass  
  QMyTestClass* l = reinterpret_cast<QMyTestClass*>(0);
  const QMetaObject m = l->staticMetaObject;
  n = m.cast(myptr);
  o = static_cast<QMyTestClass*> (n);

//QMyTestClass  
  QMyTestClass* l = reinterpret_cast<QMyTestClass*>(0);
  const QMetaObject m = l->staticMetaObject;
  n = m.cast(myptr);
  o = static_cast<QMyTestClass*> (n);

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Second
---------

n = reinterpret_cast<QMyTestClass*>(0)->staticMetaObject.cast(myptr); 
o = static_cast<QMyTestClass*>(n);

n = reinterpret_cast<QMyTestClass*>(0)->staticMetaObject.cast(myptr); 
o = static_cast<QMyTestClass*>(n);

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In the first scenario I am getting the cast as NULL (i.e. o = NULL) where as in the second case I am getting the same as the correct cast. Just wondering what's the difference.

Regards,
Subhransu