30 Input/output library [input.output]

Effects: Behaves as an unformatted input function ([istream.unformatted]), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.

gcount().

After constructing a sentry object extracts characters as long as the next available character c is whitespace or until there are no more characters in the sequence.

Whitespace characters are distinguished with the same criterion as used by sentry​::​sentry ([istream::sentry]).

If ws stops extracting characters because there are no more available it sets eofbit, but not failbit.

Returns: is.