When should Q_OBJECT be used?

Question

The documentation states that:

The Q_OBJECT macro must appear in the private section of a class definition that declares its own signals and sl

Answer 1

What @liaK said is correct (in short: you should always use the Q_OBJECT macro in any class that derives from QObject).

One thing that I haven't seen highlighted is that if you don't explicitly put the Q_OBJECT macro then using the sometimes very handy qobject_cast won't work!!!