Q_OBJECT and moc for inheritance

Question

I am working in a project where a Q_OBJECT macro is used in a class which I need to inherit.

The class where Q_OBJECT is already defined looks like this,

class cBaseObject : public QObject, public cinformation
{
    Q_OBJECT
    //...
    //...
}

I am creating a new class via public inheritance of cBaseObject. Should I need to write Q_OBJECT macro again? I tried with and without that macro and I am seeing no moc_XXX.cxx file is generated if I didn't include the QT_MACRO

class cEnhancedbaseObject : public cBaseObject
{
    Q_OBJECT   // if i didn't include this 
          //no moc__XXX.cxx file is  generated
} 

But when I inherit the class why the Q_OBJECT macro's functionality is also not inherited. Is it any problem if Q_OBJECT macro is defined twice, if it is inherited? How does the behavior of Q_OBJECT for multilevel inheritance. I have read that for multiple inheritance the QOBJECT class should be placed first. Is there anything similar for multilevel inheritance.

Answer 1

The presence of the Q_OBJECT macro marks the class for inclusion in Qt's meta-object system. If you want your class to have its own identity in this meta-object system, you must put the Q_OBJECT macro into it (and make sure it is directly or indirectly derived from QObject, naturally).

In your case of cBaseObject and cEnhancedbaseObject, if cEnhancedbaseObject does not include the Q_OBJECT macro, it will still work normally. However, as far Qt's meta-object system is concerned, objects of type cEnhancedbaseObject will be of meta-type cBaseObject. You can see that using such functions as myObject->metaObject()->className().