30 Input/output library [input.output]

30.7 Formatting and manipulators [iostream.format]

30.7.4 Input streams [input.streams]

30.7.4.4 Standard basic_­istream manipulators [istream.manip]

Each instantiation of the function template specified in this subclause is a designated addressable function ([namespace.std]).
template<class charT, class traits> basic_istream<charT, traits>& ws(basic_istream<charT, traits>& is);
Effects: 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().
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.
If ws stops extracting characters because there are no more available it sets eofbit, but not failbit.
Returns: is.