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- // Copyright Louis Dionne 2013-2017
- // Distributed under the Boost Software License, Version 1.0.
- // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
- #include <boost/hana/concept/constant.hpp>
- #include <boost/hana/concept/integral_constant.hpp>
- #include <boost/hana/core/when.hpp>
- #include <boost/hana/fwd/core/to.hpp>
- #include <boost/hana/value.hpp>
- namespace hana = boost::hana;
- // Define a simple model of IntegralConstant
- struct constant_tag { using value_type = int; };
- template <int i>
- struct constant {
- static constexpr int value = i;
- using hana_tag = constant_tag;
- };
- namespace boost { namespace hana {
- template <>
- struct IntegralConstant<constant_tag> {
- static constexpr bool value = true;
- };
- template <typename From>
- struct to_impl<constant_tag, From, when<IntegralConstant<From>::value>> {
- template <typename N>
- static constexpr auto apply(N const&)
- { return constant<N::value>{}; }
- };
- }}
- // Make sure we really satisfy IntegralConstant<>.
- static_assert(hana::IntegralConstant<constant<0>>::value, "");
- static_assert(hana::IntegralConstant<constant<1>>::value, "");
- static_assert(hana::IntegralConstant<constant<2>>::value, "");
- // Make sure we're also a model of Constant automatically.
- static_assert(hana::Constant<constant<0>>::value, "");
- static_assert(hana::Constant<constant<1>>::value, "");
- static_assert(hana::Constant<constant<2>>::value, "");
- // Make sure we have the hana::value<> function defined automatically.
- static_assert(hana::value<constant<0>>() == 0, "");
- static_assert(hana::value<constant<1>>() == 1, "");
- static_assert(hana::value<constant<2>>() == 2, "");
- static_assert(hana::value<constant<3>>() == 3, "");
- int main() { }
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