QT：在其他窗口中显示QMainWindow

//MyWindows.h
class Frame : public QFrame
{
public:
  Frame(QWidget *parent = nullptr);
};

//MyWindows.cpp

Frame::Frame(QWidget *parent) : QFrame(parent)
{
  QMainWindow *mainWindow = new QMainWindow(this);
  mainWindow->setWindowFlags(Qt::FramelessWindowHint);
    QLabel * label = new QLabel("Hello World!", mainWindow);
  QVBoxLayout *vLayout = new QVBoxLayout(this);
  vLayout->addWidget(mainWindow);  
}

//main.cpp
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Frame w;
    w.show();
    return a.exec();
}