Effects: Registers the pair (fn,index) such that during calls to imbue() ([ios.base.locales]), copyfmt(), or ~ios_base() ([ios.base.cons]), the function fn is called with argument index. 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.
Requires: The function fn shall not throw exceptions.
Remarks: Identical pairs are not merged. A function registered twice will be called twice.