31 Input/output library [input.output]

31.5 Iostreams base classes [iostreams.base]

31.5.2 Class ios_base [ios.base]

31.5.2.7 Callbacks [ios.base.callback]

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void register_callback(event_callback fn, int idx);
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Preconditions: The function fn does not throw exceptions.
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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.
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Remarks: Identical pairs are not merged.
A function registered twice will be called twice.