Gtk::Window::fullscreen_on_monitor()

Can someone give an example how to use Gtk::Window::fullscreen_on_monitor()
method?

The docs only tell me:
screen	A Gdk::Screen to draw to.
monitor	Which monitor to go fullscreen on.

I have no idea how to fill the parameter "screen", respectively from
what instance/class/window I get the value. The documentation
https://developer.gnome.org/gtkmm/stable/classGdk_1_1Screen.html#details
is quite unclear how to get an instance of that type.

If I have multiple Xservers running or can use a remote X server, how to
get the screen from that?


Thanks
  Klaus