Type to indicate event.
  Type used as an argument to call functions registered by register_callback. This argument identifies the type of event occurred.

value	type of event
ios_base::copyfmt_event	copyfmt has been called.
ios_base::erase_event	destroying object.
ios_base::imbue_event	imbue has been called.

See also.
  register_callback
  ios_base class