boost/none_t.hpp

// Copyright (C) 2003, Fernando Luis Cacciola Carballal.
// Copyright (C) 2014, 2015 Andrzej Krzemienski.
//
// Use, modification, and distribution is subject to the Boost Software
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
//
// See http://www.boost.org/libs/optional for documentation.
//
// You are welcome to contact the author at:
//  fernando_cacciola@hotmail.com
//
#ifndef BOOST_NONE_T_17SEP2003_HPP
#define BOOST_NONE_T_17SEP2003_HPP

#include <boost/config.hpp>

namespace boost {

#ifdef BOOST_OPTIONAL_USE_OLD_DEFINITION_OF_NONE

namespace detail { struct none_helper{}; }
typedef int detail::none_helper::*none_t ;

#elif defined BOOST_OPTIONAL_USE_SINGLETON_DEFINITION_OF_NONE

class none_t {};

#else

struct none_t
{
  struct init_tag{};
  explicit BOOST_CONSTEXPR none_t(init_tag){} // to disable default constructor
};

#endif // old implementation workarounds

} // namespace boost

#endif // header guard

Boost C++ Libraries

boost/none_t.hpp