31 Input/output library [input.output]

31.5 Iostreams base classes [iostreams.base]

31.5.2 Class ios_base [ios.base] Constructors and destructor [ios.base.cons]

Effects: Each ios_base member has an indeterminate value after construction.
The object's members shall be initialized by calling basic_ios​::​init before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.
Effects: Calls each registered callback pair (fn, idx) ([ios.base.callback]) as (*fn)(​erase_event, *this, idx) at such time that any ios_base member function called from within fn has well-defined results.
Then, any memory obtained is deallocated.