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