Questions about "QPaintEngineEx"

Hey guys, I'm reading Qt GUI resources,,, i met some basic problems..that is
QRasterPaintEngine inherited by QPaintEngineEx, not QPaintEngine directly...
I'm so confused by this class "QPaintEngineEx, what exactly it implements?
Thank you so much guys...

Without looking at the code it is most likely a helper.
I.e. it implements things that are common to many paint engine implementations but not all (otherwise it would be implemented in QPaintEngine already).

Cheers,
_