std::ios_base::event_callback (3) - Linux Man Pages
typedef void (*event_callback)(event type, ios_base& ios, int index);
The type of function callbacks that can be registered using register_callback() to be called on specific events.
type is a value of type ios_base::event which indicates the type of the event that will invoke this callback.
ios refers to the stream object for which the callback is invoked: *this is passed as the argument when callbacks are invoked by std::ios_base and std::basic_ios member functions.
index is the user-provided value passed to register_callback() when registering the function.
copyfmt (public member function of std::basic_ios<CharT,Traits>)
imbue (public member function)
destructor destructs the object
register_callback (public member function)