Mapping of model attributes with unknown values to a corresponding widget display

[Markus Elfring <Markus Elfring web de>]

Hello!

The class "Gtk::CheckButton" provides the feature to display an intermediate
status (by the interface "gtk_toggle_button_set_inconsistent"), doesn't it?
http://developer.gnome.org/gtk3/3.2/GtkToggleButton.html#gtk-toggle-button-set-inconsistent
http://developer.gnome.org/gtkmm/3.2/classGtk_1_1CheckButton.html#_details

I would like to know the recommend way to represent unknown values for model
attributes when it seems that a member function like
"Gtk::TreeView::append_column" does not support the use of three-valued logic
directly.
http://developer.gnome.org/gtkmm/3.2/classGtk_1_1TreeView.html#a0253a5477ce006bf84fae8eb205efc73
http://en.wikipedia.org/wiki/Three-valued_logic#Representation_of_values

How do you store a "NULL" (SQL) for a model attribute with a data type like
"bool" and update its visual appearance accordingly?

Regards,
Markus