deps/boost_1_63_0/boost/hana/value.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::value`.
   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_VALUE_HPP
  11 #define BOOST_HANA_VALUE_HPP
  12
  13 #include <boost/hana/fwd/value.hpp>
  14
  15 #include <boost/hana/concept/constant.hpp>
  16 #include <boost/hana/concept/integral_constant.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19
  20 #include <type_traits>
  21
  22
  23 BOOST_HANA_NAMESPACE_BEGIN
  24     template <typename C, bool condition>
  25     struct value_impl<C, when<condition>> : default_ {
  26         template <typename ...Args>
  27         static constexpr auto apply(Args&& ...args) = delete;
  28     };
  29
  30     template <typename T>
  31     constexpr decltype(auto) value() {
  32         using RawT = typename std::remove_cv<
  33             typename std::remove_reference<T>::type
  34         >::type;
  35         using C = typename hana::tag_of<RawT>::type;
  36         using Value = BOOST_HANA_DISPATCH_IF(
  37             value_impl<C>, hana::Constant<C>::value
  38         );
  39
  40     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  41         static_assert(hana::Constant<C>::value,
  42         "hana::value<T>() requires 'T' to be a Constant");
  43     #endif
  44
  45         return Value::template apply<RawT>();
  46     }
  47
  48     template <typename I>
  49     struct value_impl<I, when<hana::IntegralConstant<I>::value>> {
  50         template <typename C>
  51         static constexpr auto apply()
  52         { return C::value; }
  53     };
  54 BOOST_HANA_NAMESPACE_END
  55
  56 #endif // !BOOST_HANA_VALUE_HPP