From cppreference.com
enumevent{erase_event,imbue_event,copyfmt_event};Specifies the event type which is passed to functions registered by
on specific events. The following constants are defined:
Enumerator Meaning erase_eventissued on ~ios_base() or
(before the copy of members takes place) imbue_eventissued on
copyfmt_eventissued on
(after the copy of members takes place, but before the exception settings are copied) Example