It is (well?) known, that upon deletion of a parent all its children will be axed as well.

Unknown for me was, that QStateMachine::addState will put it in the same kind of relationship, so the added QState will be deleted as well. A short word on that in the documentation would be nice.

http://doc.qt.io/qt-5/qstatemachine.html#addState

In contrast: removeState states explicitly that the ownership is released.