Qt：QObject::deleteLater无法删除对象的问题
QObject::deleteLater并非每次都能删除掉对象。遇到的几种情况：
- QThread对象无法回收
先调用了QThread的quit函数结束线程，然后调用QThread->deleteLater()。此时QThread对象无法回收。原因是，deleteLater实际是发送一个异步的事件，然后到下一个事件处理函数中回收。而QThread线程结束后，消息循环也结束，自然就无法回收。
2. QWidget无法回收
这种情况未具体分析，所以不确定具体原因。大致是，先new一个父窗口，然后父窗口创建了子窗口，调用replaceWidget替换子窗口，再修改父窗口的父窗口，一系列操作之后，deleteLater也将无效。
两种情况都改为直接delete回收来代替。
void QObject::deleteLater()
Schedules this object for deletion.
The object will be deleted when control returns to the event loop. If the event loop is not running when this function is called (e.g. deleteLater() is called on an object before QCoreApplication::exec()), the object will be deleted once the event loop is started. If deleteLater() is called after the main event loop has stopped, the object will not be deleted. Since Qt 4.8, if deleteLater() is called on an object that lives in a thread with no running event loop, the object will be destroyed when the thread finishes.
Note that entering and leaving a new event loop (e.g., by opening a modal dialog) will not perform the deferred deletion; for the object to be deleted, the control must return to the event loop from which deleteLater() was called. This does not apply to objects deleted while a previous, nested event loop was still running: the Qt event loop will delete those objects as soon as the new nested event loop starts.
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