I have derived QGraphicsItem and QGraphicsScene classes. I want the items to be able to call scene() and get a derviedGraphicsItem * instead of a QGraphicsItem *, so I reimplemented QGraphicsScene::itemAt to return a derived pointer.

DerivedItem* DerivedScene::itemAt( const QPointF &position, const QTransform &dt ) const
{
    return qobject_cast< DerivedItem * >(
            QGraphicsScene::itemAt(position, dt) );
}

I get the following error (Qt 4.6, GCC 4.4.3 on Ubuntut 10.4)

scene.cpp: In member function ‘DerivedItem* DerivedScene::itemAt(qreal, qreal, const QTransform&) const’:
scene.cpp:28: error: no matching function for call to ‘qobject_cast(QGraphicsItem*)’

I then noticed QGraphicsItem doesn't inherit QObject, so I made my derived QGraphicsItem class have multiple inheritance from QObject and QGraphicsItem, and after adding the Q_OBJECT macro and rebuilding the project I get the same error.

Am I going about this the wrong way? I know it's supposed to be bad design to try to cast a parent class as a child, but in this case it seems like what I want, since my derived item class has new functionality and its objects need a way to call that new functionality on items around themselves, and asking the items scene object with itemAt() seems like the best way - but I need itemAt() to return a pointer of the right type. I can get around this by having the derived items cast the QGraphicsItem * returned by QGraphicsScene::itemAt() using dynamic_cast, but I don't really understand why that works and not qobject_cast, or the benefits or disadvantages to using dynamic_cast vs. qobject_cast.