[Qt-interest] How do QApplication relates to QWidget

[wei]

Hi, all!

I am wondering how QApplication relates to QWidget in a qt application 
such as the following one:
   int main(int argc, char* argv[])
   {
      QApplication app(argc, argv);
      QMainWindow win;
      QPushButton* button = new QPushButton("exit", &win);
      button->setText("Exit");
      QObject::connect(button, SIGNAL(clicked()), &win, SLOT(close()));
      win.show();
      button->show();
      return app.exec();
   }

The QMainWindow object "win" is created after "app", and there's no 
parameter related to "app" in its constructor.
So how can "app" know it has a widget named "win"? If "app" don't know 
the fact, how can it process and dispatch messages related to win?

Thanks!