set_modal() is used to prevent the other windows in an application from reacting to user activity until the modal window has had an appropriate user response. It is usually used where dialog or popup windows are acting as message boxes.

A similar effect can be achieved with most widgets by using the static function gtk::grab_add() ; however note that set_modal() is a slightly more complex method and is equipped to deal more fully with the scenarios likely to arise when using modal windows.