ceph/src/boost/libs/hana/include/boost/hana/empty.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::empty`.
   4
   5 @copyright Louis Dionne 2013-2016
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_EMPTY_HPP
  11 #define BOOST_HANA_EMPTY_HPP
  12
  13 #include <boost/hana/fwd/empty.hpp>
  14
  15 #include <boost/hana/concept/monad_plus.hpp>
  16 #include <boost/hana/concept/sequence.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/core/make.hpp>
  20
  21
  22 BOOST_HANA_NAMESPACE_BEGIN
  23     template <typename M>
  24     struct empty_t {
  25     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  26         static_assert(hana::MonadPlus<M>::value,
  27         "hana::empty<M>() requires 'M' to be a MonadPlus");
  28     #endif
  29
  30         constexpr auto operator()() const {
  31             using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>,
  32                 hana::MonadPlus<M>::value
  33             );
  34
  35             return Empty::apply();
  36         }
  37     };
  38
  39     template <typename M, bool condition>
  40     struct empty_impl<M, when<condition>> : default_ {
  41         template <typename ...Args>
  42         static constexpr auto apply(Args&& ...) = delete;
  43     };
  44
  45     template <typename S>
  46     struct empty_impl<S, when<Sequence<S>::value>> {
  47         static constexpr auto apply() {
  48             return hana::make<S>();
  49         }
  50     };
  51 BOOST_HANA_NAMESPACE_END
  52
  53 #endif // !BOOST_HANA_EMPTY_HPP