为什么qobject_cast和dynamic_cast在连接到QWidget的slot中失败::销毁?

I have a class Subclass, subclassed from QObject. I want to know when the item is deleted, so I connected this slot to the QWidget::destroyed() signal inherited by Subclass. But when I try to cast the argument to with qobject_cast, I get a zero result. The same result obtains from C++'s dynamic_cast. Why?I have a class Subclass, subclassed from QObjec