void register_callback(event_callback fn, int idx);

Preconditions: The function fn does not throw exceptions.

Effects: Registers the pair (fn, idx) such that during calls to imbue() ([ios.base.locales]), copyfmt(), or ~ios_base() ([ios.base.cons]), the function fn is called with argument idx.

Functions registered are called when an event occurs, in opposite order of registration.

Functions registered while a callback function is active are not called until the next event.

Remarks: Identical pairs are not merged.

A function registered twice will be called twice.