Behaves as an unformatted input function
, except that it does not count the number of characters extracted and
does not affect the value returned by subsequent calls to is.gcount().
constructing a sentry object extracts characters as long as the next available
is whitespace or until there are no more characters in the sequence.
Whitespace characters are distinguished with the same criterion as used by
stops extracting characters because there are no more available it sets