The Gtk::TextView is based on the MVC (Model, View, Controller) system.
What you need to do is obtain the Gtk::TextBuffer that is associated with your Gtl::TextView. Once you have got the Gtk::TextBuffer you can then get or set your text.
Glib::RefPtr <Gtk::TextBuffer> tb = text.get_buffer();
u_text = tb->get_text();
Here I obtained the text buffer using the member function Gtk::TextView::get_buffer(). This returns a reference pointer to the text buffer. The to get all the text I used the member function Gtk::TextBuffer::get_text().
The API for the Text View classes can be found at https://developer.gnome.org/gtkmm/unstable/group__TextView.html