std::ios_base::event_callback
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                    |   typedef void (*event_callback)(event type, ios_base& ios, int index);  | 
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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.
[edit] See also
|    copies formatting information  (public member function of std::basic_ios) 
 | |
|    sets locale   (public member function)  | |
|    [virtual]  | 
  destructs the object  (virtual public member function)  | 
|    registers event callback function   (public member function)  | |