ceph/src/boost/libs/hana/include/boost/hana/detail/canonical_constant.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::detail::CanonicalConstant`.
   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_DETAIL_CANONICAL_CONSTANT_HPP
  11 #define BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
  12
  13 #include <boost/hana/config.hpp>
  14
  15
  16 BOOST_HANA_NAMESPACE_BEGIN namespace detail {
  17     //! @ingroup group-details
  18     //! Tag representing a canonical `Constant`.
  19     //!
  20     //! This is an implementation detail used to provide many models for
  21     //! stuff like `Monoid`, `Group`, etc. To create a `CanonicalConstant`,
  22     //! simply create an object with a nested `hana_tag` equal to the proper
  23     //! specialization of `CanonicalConstant<T>`, and then also provide a
  24     //! `constexpr` static member `::%value` holding the value of the constant.
  25     template <typename T>
  26     struct CanonicalConstant {
  27         using value_type = T;
  28     };
  29 } BOOST_HANA_NAMESPACE_END
  30
  31
  32 #include <boost/hana/concept/constant.hpp>
  33 #include <boost/hana/concept/integral_constant.hpp>
  34 #include <boost/hana/core/to.hpp>
  35 #include <boost/hana/core/when.hpp>
  36
  37 #include <type_traits>
  38
  39
  40 BOOST_HANA_NAMESPACE_BEGIN
  41     //////////////////////////////////////////////////////////////////////////
  42     // Constant
  43     //////////////////////////////////////////////////////////////////////////
  44     template <typename T>
  45     struct value_impl<detail::CanonicalConstant<T>> {
  46         template <typename X>
  47         static constexpr decltype(auto) apply()
  48         { return X::value; }
  49     };
  50
  51     namespace detail {
  52         template <typename T, typename X>
  53         struct canonical_constant {
  54             static constexpr auto value = hana::to<T>(hana::value<X>());
  55             using hana_tag = detail::CanonicalConstant<T>;
  56         };
  57     }
  58
  59     template <typename T, typename C>
  60     struct to_impl<detail::CanonicalConstant<T>, C, when<
  61         hana::Constant<C>::value &&
  62         is_convertible<typename C::value_type, T>::value
  63     >>
  64         : embedding<is_embedded<typename C::value_type, T>::value>
  65     {
  66         template <typename X>
  67         static constexpr detail::canonical_constant<T, X> apply(X const&)
  68         { return {}; }
  69     };
  70
  71     //////////////////////////////////////////////////////////////////////////
  72     // IntegralConstant (when value_type is integral)
  73     //////////////////////////////////////////////////////////////////////////
  74     template <typename T>
  75     struct IntegralConstant<detail::CanonicalConstant<T>> {
  76         static constexpr bool value = std::is_integral<T>::value;
  77     };
  78 BOOST_HANA_NAMESPACE_END
  79
  80 #endif // !BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP