std::ios_base::event
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   enum event {erase_event, imbue_event, copyfmt_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) | 
Example
| This section is incomplete Reason: no example  |