std::ios_base::event

From cppreference.com
< cpp‎ | io‎ | ios base
enum event;

Specifies the event type which is passed to functions registered by register_callback() on specific events. The following constants are defined:

Constant Explanation
erase_event issued on ~ios_base() or basic_ios::copyfmt() (before the copy of members takes place)
imbue_event issued on imbue()
copyfmt_event issued on basic_ios::copyfmt() (after the copy of members takes place, but before the exception settings are copied)

[edit] Example