In gtkmm docs is written:
(http://www.gtkmm.org/gtkmm2/docs/reference/html/classGtk_1_1TreePath.html#a41)
Glib::ustring Gtk::TreePath::to_string (  ) const
Generates a string representation of the path.
This string is a ':' separated list of numbers. For example, "4:10:0:3"
would be an acceptable 
return value for this string.
Returns:
    A newly-allocated string. Must be freed with Glib::free(). "

Here "Must be freed with Glib::free()" is erroneous, these statement should
be removed.

Another refference in gtkmm docs:
(http://www.gtkmm.org/gtkmm2/docs/reference/html/classGtk_1_1TreeModel.html#a18)
"Glib::ustring Gtk::TreeModel::get_string (const iterator& iter) const
Generates a string representation of the iter.
This string is a ':' separated list of numbers. For example, "4:10:0:3"
would be an acceptable return value for this string.
Parameters:
    iter 	An Gtk::TreeIter.
Returns:
    A newly-allocated string. Must be freed with Glib::free()."
"Must be freed with Glib::free()" is erroneous, these statement should be
removed.