Can't get Gdk::Window from Gtk::Window

[Adam Nielsen <a nielsen shikadi net>]

Hi all,

I'm just starting out with gtkmm (and GTK in general) and I'm trying to 
make one of these fancy semi-transparent windows.  I have some example C 
code, but I'm trying to port it to C++ with gtkmm.

I'm stuck trying to figure out how to get a Gdk::Window from a 
Gtk::Window.  The docs at 
http://www.gtkmm.org/docs/gtkmm-2.4/docs/reference/html/classGtk_1_1Widget.html 
reflect the C code and say I should do this:

gtk_widget_realize (window);
gdk_window_set_back_pixmap (window->window, <tt>0</tt>, <tt>false</tt>);
gtk_widget_show (window);

I'm guessing the C code (and HTML!) were included in the C++ docs by 
accident, but the best I can come up with as a C++ translation is this: 
(which is in a class descended from Gtk::Window)

  this->realize();
  gdk_window_set_back_pixmap(GTK_WIDGET(this->gobj())->window, 0,
    false);
  this->show();

Which looks horrible, and I don't even know if it works.  I thought 
maybe I could use Gtk::Window::get_frame() to get a Gdk::Window instead, 
but that function always returns NULL, apparently because of the 
underlying windowing system.

At any rate, even if I could get a valid Gdk::Window object, the 
documentation at 
http://www.gtkmm.org/docs/gtkmm-2.4/docs/reference/html/classGdk_1_1Window.html 
says I should pass 0 as the first parameter to 
Gdk::Window::set_back_pixmap(), but this gives me a compile-time error, 
because the first parameter is not an integer (it's a const 
Glib::RefPtr<Pixmap>&)

Am I looking at the latest version of the documentation?  Does anyone 
know how to make this work?  There seems to be very little use of this 
in C++, all the example code I can find is in C.

Many thanks,
Adam.