Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Reference

Reference

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>

Header <boost/none.hpp>

Synopsis

namespace boost {

class none_t
{
  friend constexpr bool operator==(none_t, none_t) = default;
  /* see below */
};

inline constexpr 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 class.

It is trivially copyable.

It is not default-constructible.

It models concept std::equality_comparable.

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