| 1 | // Copyright Louis Dionne 2013-2022 |
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| 2 | // Distributed under the Boost Software License, Version 1.0. |
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| 3 | // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt) |
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| 4 | |
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| 5 | #include <boost/hana/concept/constant.hpp> |
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| 6 | #include <boost/hana/concept/integral_constant.hpp> |
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| 7 | #include <boost/hana/core/when.hpp> |
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| 8 | #include <boost/hana/fwd/core/to.hpp> |
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| 9 | #include <boost/hana/value.hpp> |
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| 10 | namespace hana = boost::hana; |
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| 11 | |
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| 12 | |
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| 13 | // Define a simple model of IntegralConstant |
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| 14 | struct constant_tag { using value_type = int; }; |
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| 15 | template <int i> |
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| 16 | struct constant { |
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| 17 | static constexpr int value = i; |
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| 18 | using hana_tag = constant_tag; |
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| 19 | }; |
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| 20 | |
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| 21 | namespace boost { namespace hana { |
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| 22 | template <> |
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| 23 | struct IntegralConstant<constant_tag> { |
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| 24 | static constexpr bool value = true; |
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| 25 | }; |
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| 26 | |
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| 27 | template <typename From> |
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| 28 | struct to_impl<constant_tag, From, when<IntegralConstant<From>::value>> { |
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| 29 | template <typename N> |
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| 30 | static constexpr auto apply(N const&) |
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| 31 | { return constant<N::value>{}; } |
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| 32 | }; |
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| 33 | }} |
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| 34 | |
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| 35 | // Make sure we really satisfy IntegralConstant<>. |
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| 36 | static_assert(hana::IntegralConstant<constant<0>>::value, "" ); |
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| 37 | static_assert(hana::IntegralConstant<constant<1>>::value, "" ); |
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| 38 | static_assert(hana::IntegralConstant<constant<2>>::value, "" ); |
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| 39 | |
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| 40 | // Make sure we're also a model of Constant automatically. |
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| 41 | static_assert(hana::Constant<constant<0>>::value, "" ); |
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| 42 | static_assert(hana::Constant<constant<1>>::value, "" ); |
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| 43 | static_assert(hana::Constant<constant<2>>::value, "" ); |
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| 44 | |
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| 45 | // Make sure we have the hana::value<> function defined automatically. |
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| 46 | static_assert(hana::value<constant<0>>() == 0, "" ); |
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| 47 | static_assert(hana::value<constant<1>>() == 1, "" ); |
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| 48 | static_assert(hana::value<constant<2>>() == 2, "" ); |
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| 49 | static_assert(hana::value<constant<3>>() == 3, "" ); |
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| 50 | |
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| 51 | |
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| 52 | int main() { } |
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| 53 | |
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