A simple basic fundamental doubt !

Hi,

I was just trying to understand the concept behind qobject_cast and have landed in to some dilemma :confused:. 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
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I have a QObject subclass,

Code:

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

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First
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Code:

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

---

Second
---------

Code:

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

---

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

The first one won't work. The point of this statement is to have a pointer to a class without actual object of this class. It's a terrible (or a very nice, depending on the point of view) hack to make the template mechanism cast the result to a proper class.

First, staticMetaObject ist static, so just use QMyTestClass::staticMetaObject.
Also, no need to reinterpret_cast a 0. If you just write QMyTestClass* l=0, it is just as well. (Better actually, because casts are bad, as is widely known...)

Code:

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n = QMyTestClass::staticMetaObject.cast(myptr); // better

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Ok, to your issue:
* what is myptr?
* the cast function is basically an internal of qobject_cast. It returns 0 if the pointer is not of the needed QObject subtype. Your static_cast does nothing but adjust (after the typecheck) to that subtype.
* The two examples should do the same.

Please give us your complete example. (Are you sure myptr is the same before executing the two versions of that code?)

Quote:

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Originally Posted by caduel

First, staticMetaObject ist static, so just use QMyTestClass::staticMetaObject.

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What if you don't know the class name? That's the point of the whole statement.

Quote:

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(Better actually, because casts are bad, as is widely known...)

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Hmm... why so? Bad casts are bad, good casts are good. qobject_cast<>() is among the latter.

Quote:

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Code:

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n = QMyTestClass::staticMetaObject.cast(myptr); // better

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Try making a template function that performs a cast based on the type passed to it. Or implement a series of functions for each and every QObject subclass (existing and future).

Quote:

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* The two examples should do the same.

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It's not that simple.

Quote:

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It's not that simple.

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Could you elaborate on that, pleaase?