问QDockWidgets合并不正确

EN

// docker.hpp
class Docker : public QDockWidget
{
    Q_OBJECT
public:
    Docker(QString title, QWidget* parent = 0);
}

// docker.cpp
Docker::Docker(QString title, QWidget* parent): QDockWidget(title, parent)
{
    QWidget* widget = new QWidget(this);
    widget.setMinimumSize(200, 200);
    setWidget(widget);
    widget->setStyleSheet("border:5px solid gray;");

    setAllowedAreas(Qt::AllDockWidgetAreas);
}

// mainwindow.hpp
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget* parent);
private slots:
    void createDockers();
};

// mainwindow.cpp
MainWindow::MainWindow(QWidget* parent): QMainWindow(parent)
{
    setDockOptions(dockOptions() |
               QMainWindow::AllowTabbedDocks |
               QMainWindow::GroupedDragging);

    // The following line of code does not change the situation.
    // setTabPosition(Qt::RightDockWidgetArea, QTabWidget::East);

    // There are some other codes which connect a button to the void createDockers() method
}

void createDockers()
{
    Docker* dock = new Docker("Docker", this);
    dock->setFloating(true);
    dock->show();
}