void set_child_secondary(GtkWidget child, bool is_secondary);

This group appears after the other children if the style is Gtk::BUTTONBOX_START, Gtk::BUTTONBOX_SPREAD or GTK::BUTTONBOX_EDGE, and before the other children if the style is GTK::BUTTONBOX_END. For horizontal button boxes, the definition of before/after depends on direction of the widget (see set_direction() ). If the style is Gtk::BUTTONBOX_START or Gtk::BUTTONBOX_END, then the secondary children are aligned at the other end of the button box from the main children. For the other styles, they appear immediately next to the main children.

See also: get_child_secondary()