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.