如何在Qt中启用关闭按钮

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow *ui;
};

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    setWindowFlags(windowFlags() | Qt::WindowCloseButtonHint); // 启用关闭按钮
    // setWindowFlags(windowFlags() & ~Qt::WindowCloseButtonHint); // 关闭关闭按钮
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{
    close(); // 点击按钮关闭窗口
}