diff options
Diffstat (limited to 'boost/hana/fwd/none.hpp')
| -rw-r--r-- | boost/hana/fwd/none.hpp | 46 |
1 files changed, 46 insertions, 0 deletions
diff --git a/boost/hana/fwd/none.hpp b/boost/hana/fwd/none.hpp new file mode 100644 index 0000000000..158f4c93d7 --- /dev/null +++ b/boost/hana/fwd/none.hpp @@ -0,0 +1,46 @@ +/*! +@file +Forward declares `boost::hana::none`. + +@copyright Louis Dionne 2013-2016 +Distributed under the Boost Software License, Version 1.0. +(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) + */ + +#ifndef BOOST_HANA_FWD_NONE_HPP +#define BOOST_HANA_FWD_NONE_HPP + +#include <boost/hana/config.hpp> +#include <boost/hana/core/when.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + //! Returns whether all of the keys of the structure are false-valued. + //! @ingroup group-Searchable + //! + //! The keys of the structure must be `Logical`s. If the structure is not + //! finite, a true-valued key must appear at a finite "index" in order + //! for this method to finish. + //! + //! + //! Example + //! ------- + //! @include example/none.cpp +#ifdef BOOST_HANA_DOXYGEN_INVOKED + constexpr auto none = [](auto&& xs) -> decltype(auto) { + return tag-dispatched; + }; +#else + template <typename S, typename = void> + struct none_impl : none_impl<S, when<true>> { }; + + struct none_t { + template <typename Xs> + constexpr auto operator()(Xs&& xs) const; + }; + + constexpr none_t none{}; +#endif +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_FWD_NONE_HPP |