Typically used for simple containers such as gtk-window, gtk-frame, or gtk-button; for more complicated layout containers such as gtk-box or gtk-grid, this function will pick default packing parameters that may not be correct. So consider functions such as gtk-box-pack-start and gtk-grid-attach as an alternative to gtk-container-add in those cases. A widget may be added to only one container at a time; you cannot place the same widget inside two different containers.