QT create a background mask, pop up the child window, and the background of the parent window turns black and dark

qt Realize the pop-up of a new interface , The original window is dimmed .
You can create a sub window , The background is black and has a certain transparent effect , Hide the parent window to realize .

    QWidget *m_widget = new QWidget(this);
    m_widget->resize (this->width(), this->height());
    m_widget->move (0,0);
    m_widget->setStyleSheet("background-color:rgba(0, 0, 0,80%);");
    m_widget->show();

effect

The background darkens after the prompt box pops up