81 lines
2.7 KiB
C++
81 lines
2.7 KiB
C++
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/*!
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@file
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Defines `boost::hana::detail::CanonicalConstant`.
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@copyright Louis Dionne 2013-2016
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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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*/
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#ifndef BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
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#define BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
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#include <boost/hana/config.hpp>
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BOOST_HANA_NAMESPACE_BEGIN namespace detail {
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//! @ingroup group-details
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//! Tag representing a canonical `Constant`.
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//!
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//! This is an implementation detail used to provide many models for
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//! stuff like `Monoid`, `Group`, etc. To create a `CanonicalConstant`,
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//! simply create an object with a nested `hana_tag` equal to the proper
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//! specialization of `CanonicalConstant<T>`, and then also provide a
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//! `constexpr` static member `::%value` holding the value of the constant.
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template <typename T>
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struct CanonicalConstant {
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using value_type = T;
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};
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} BOOST_HANA_NAMESPACE_END
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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/to.hpp>
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#include <boost/hana/core/when.hpp>
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#include <type_traits>
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BOOST_HANA_NAMESPACE_BEGIN
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//////////////////////////////////////////////////////////////////////////
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// Constant
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//////////////////////////////////////////////////////////////////////////
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template <typename T>
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struct value_impl<detail::CanonicalConstant<T>> {
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template <typename X>
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static constexpr decltype(auto) apply()
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{ return X::value; }
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};
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namespace detail {
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template <typename T, typename X>
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struct canonical_constant {
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static constexpr auto value = hana::to<T>(hana::value<X>());
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using hana_tag = detail::CanonicalConstant<T>;
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};
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}
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template <typename T, typename C>
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struct to_impl<detail::CanonicalConstant<T>, C, when<
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hana::Constant<C>::value &&
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is_convertible<typename C::value_type, T>::value
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>>
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: embedding<is_embedded<typename C::value_type, T>::value>
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{
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template <typename X>
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static constexpr detail::canonical_constant<T, X> apply(X const&)
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{ return {}; }
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};
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//////////////////////////////////////////////////////////////////////////
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// IntegralConstant (when value_type is integral)
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//////////////////////////////////////////////////////////////////////////
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template <typename T>
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struct IntegralConstant<detail::CanonicalConstant<T>> {
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static constexpr bool value = std::is_integral<T>::value;
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
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