我要初始化子控件在主窗口的相对位置，所以必须要创建了这个类之后，显示出来以后才能初始化，不知道Qt有没有类似MFC的InitialDlg这样的函数啊？
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木有，可以写在resizeEvent里。
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可以重载showEvent()
当QWidget被显式显示时,触发这个
void QWidget::showEvent ( QShowEvent * event ) [virtual protected]
This event handler can be reimplemented in a subclass to receive widget show events which are passed in the event parameter.
Non-spontaneous show events are sent to widgets immediately before they are shown. The spontaneous show events of windows are delivered afterwards.
Note: A widget receives spontaneous show and hide events when its mapping status is changed by the window system, e.g. a spontaneous hide event when the user minimizes the window, and a spontaneous show event when the window is restored again. After receiving a spontaneous hide event, a widget is still considered visible in the sense of isVisible().
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在构造函数中应该也可以吧