How can I access the Gtk+ data from a gtkmm object?

Sometimes you may try to access a piece of Gtk+ functionality which is not supported through gtkmm (the gtkmm authors are busy people, as such gtkmm is quite complete but not perfect). In such a case you can call Gtk::Widget::gtkobj() which will return to you the plain C Gtk+ data structure your gtkmm object wraps. You can then operate directly on this C object as you would in any Gtk+ program.