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Header <boost/none.hpp>
Header <boost/optional/bad_optional_access.hpp>
Header <boost/optional/optional_io.hpp>
Header <boost/optional/optional_fwd.hpp>
Header <boost/optional/optional.hpp>
Header <boost/optional.hpp>

namespace boost {

class none_t {/* see below */};

const none_t none (/* see below */);

} // namespace boost

Class none_t is meant to serve as a tag for selecting appropriate overloads of from optional's interface. It is an empty, trivially copyable class with disabled default constructor.

Constant none is used to indicate an optional object that does not contain a value in initialization, assignment and relational operations of optional.