void QWidget::setLayout(QLayout *layout)
Sets the layout manager for this widget to layout.
If there already is a layout manager installed on this widget, QWidget won't let you install another. You must first delete the existing layout manager (returned by layout()) before you can call setLayout() with the new layout.
If layout is the layout manager on a different widget, setLayout() will reparent the layout and make it the layout manager for this widget.
Example:
QVBoxLayout *layout = new QVBoxLayout;
layout->addWidget(formWidget);
setLayout(layout);
如果这个小部件上已经安装了布局管理器，那么QWidget将不允许您安装其他布局管理器。您必须首先删除现有的布局管理器（由layout（）返回），然后才能使用新布局调用setLayout（）。
void QMainWindow::setCentralWidget(QWidget *widget)
Sets the given widget to be the main window's central widget.
Note: QMainWindow takes ownership of the widget pointer and deletes it at the appropriate time.
See also centralWidget().
将给定的小部件设置为主窗口的中心小部件。注意：QMainWindow获取小部件指针的所有权，并在适当的时候删除它。
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