Listing 3

struct app_frame : ... {
    app_frame();
    int show_label;
};
app_frame::app_frame() {
  set_cmd_chooser(&app_frame::show_label,this)
    .for_id(ID_label_left, 1)
    .for_id(ID_label_center, 2)
    .for_id(ID_label_right, 3);
  // initially set it to "Left" state
  show_label = 1;
}