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.