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

This is an older version of Boost and was released in 2015. The current version is 1.92.0.

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 {};

extern const none_t none; // see below

} // namespace boost

Variable none has external linkage, however it is not required to link with any library to obtain its definition. Only by including this header file, the definition becomes available, by means of using template instantiation.