The C++ function ios_base::event_callback is for callback functions registered with member register_callback.
Following is the declaration for ios_base::event_callback.
typedef void (*event_callback) (event ev, ios_base& obj, int index);
Therefore it is a function returning no value and having taking three arguments as shown below −
ev − An object of enum member type event. When the callback function is called, this is set to one of the three possible values to indicate what type of event triggered the function call.
obj − When the callback function is called, this is a reference to the stream object on which the even is triggered (*this).
index − When the callback function is called, this is set to the same value used as index argument when the function was registered with member register_callback.