31 Input/output library [input.output]

31.7 Formatting and manipulators [iostream.format]

31.7.5 Input streams [input.streams]

31.7.5.5 Standard basic_istream manipulators [istream.manip]

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Each instantiation of the function template specified in this subclause is a designated addressable function ([namespace.std]).
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template<class charT, class traits> basic_istream<charT, traits>& ws(basic_istream<charT, traits>& is);
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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.
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Returns: is.