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

   1 /*!
   2 @file
   3 Defines `boost::hana::zero`.
   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_ZERO_HPP
  11 #define BOOST_HANA_ZERO_HPP
  12
  13 #include <boost/hana/fwd/zero.hpp>
  14
  15 #include <boost/hana/concept/constant.hpp>
  16 #include <boost/hana/concept/monoid.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/to.hpp>
  19 #include <boost/hana/core/dispatch.hpp>
  20 #include <boost/hana/detail/canonical_constant.hpp>
  21
  22 #include <type_traits>
  23
  24
  25 BOOST_HANA_NAMESPACE_BEGIN
  26     template <typename M>
  27     struct zero_t {
  28     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  29         static_assert(hana::Monoid<M>::value,
  30         "hana::zero<M>() requires 'M' to be a Monoid");
  31     #endif
  32
  33         constexpr decltype(auto) operator()() const {
  34             using Zero = BOOST_HANA_DISPATCH_IF(zero_impl<M>,
  35                 hana::Monoid<M>::value
  36             );
  37
  38             return Zero::apply();
  39         }
  40     };
  41
  42     template <typename M, bool condition>
  43     struct zero_impl<M, when<condition>> : default_ {
  44         template <typename ...Args>
  45         static constexpr auto apply(Args&& ...) = delete;
  46     };
  47
  48     //////////////////////////////////////////////////////////////////////////
  49     // Model for non-boolean arithmetic data types
  50     //////////////////////////////////////////////////////////////////////////
  51     template <typename T>
  52     struct zero_impl<T, when<
  53         std::is_arithmetic<T>::value &&
  54         !std::is_same<T, bool>::value
  55     >> {
  56         static constexpr T apply()
  57         { return static_cast<T>(0); }
  58     };
  59
  60     //////////////////////////////////////////////////////////////////////////
  61     // Model for Constants over a Monoid
  62     //////////////////////////////////////////////////////////////////////////
  63     namespace detail {
  64         template <typename C>
  65         struct constant_from_zero {
  66             static constexpr auto value = hana::zero<typename C::value_type>();
  67             using hana_tag = detail::CanonicalConstant<typename C::value_type>;
  68         };
  69     }
  70
  71     template <typename C>
  72     struct zero_impl<C, when<
  73         hana::Constant<C>::value &&
  74         Monoid<typename C::value_type>::value
  75     >> {
  76         static constexpr decltype(auto) apply()
  77         { return hana::to<C>(detail::constant_from_zero<C>{}); }
  78     };
  79 BOOST_HANA_NAMESPACE_END
  80
  81 #endif // !BOOST_HANA_ZERO_HPP