46 lines
1.3 KiB
C++
46 lines
1.3 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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#ifndef TEST_CONCEPT_CONSTANT_MINIMAL_HPP
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#define TEST_CONCEPT_CONSTANT_MINIMAL_HPP
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#include <boost/hana/concept/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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template <typename T>
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struct minimal_constant_tag {
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using value_type = T;
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};
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template <typename T, T v>
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struct minimal_constant {
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using hana_tag = minimal_constant_tag<T>;
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static constexpr T value_ = v;
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};
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namespace boost { namespace hana {
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template <typename T>
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struct value_impl<minimal_constant_tag<T>> {
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template <typename N>
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static constexpr T apply() { return N::value_; }
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};
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template <typename T, typename C>
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struct to_impl<minimal_constant_tag<T>, C, hana::when<
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hana::Constant<C>::value &&
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hana::is_convertible<typename C::value_type, T>::value
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>>
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: hana::embedding<hana::is_embedded<typename C::value_type, T>::value>
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{
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template <typename N>
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static constexpr auto apply(N const&)
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{ return minimal_constant<T, hana::value<N>()>{}; }
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};
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}} // end namespace boost::hana
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#endif // !TEST_CONCEPT_CONSTANT_MINIMAL_HPP
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