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Diffstat (limited to 'boost/hana/empty.hpp')
-rw-r--r-- | boost/hana/empty.hpp | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/boost/hana/empty.hpp b/boost/hana/empty.hpp new file mode 100644 index 0000000000..f20a5e5fff --- /dev/null +++ b/boost/hana/empty.hpp @@ -0,0 +1,53 @@ +/*! +@file +Defines `boost::hana::empty`. + +@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_EMPTY_HPP +#define BOOST_HANA_EMPTY_HPP + +#include <boost/hana/fwd/empty.hpp> + +#include <boost/hana/concept/monad_plus.hpp> +#include <boost/hana/concept/sequence.hpp> +#include <boost/hana/config.hpp> +#include <boost/hana/core/dispatch.hpp> +#include <boost/hana/core/make.hpp> + + +BOOST_HANA_NAMESPACE_BEGIN + template <typename M> + struct empty_t { + #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS + static_assert(hana::MonadPlus<M>::value, + "hana::empty<M>() requires 'M' to be a MonadPlus"); + #endif + + constexpr auto operator()() const { + using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>, + hana::MonadPlus<M>::value + ); + + return Empty::apply(); + } + }; + + template <typename M, bool condition> + struct empty_impl<M, when<condition>> : default_ { + template <typename ...Args> + static constexpr auto apply(Args&& ...) = delete; + }; + + template <typename S> + struct empty_impl<S, when<Sequence<S>::value>> { + static constexpr auto apply() { + return hana::make<S>(); + } + }; +BOOST_HANA_NAMESPACE_END + +#endif // !BOOST_HANA_EMPTY_HPP |