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/*!
@file
Defines `boost::hana::detail::CanonicalConstant`.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
#define BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
#include <boost/hana/config.hpp>
namespace boost { namespace hana { namespace detail {
//! @ingroup group-details
//! Tag representing a canonical `Constant`.
//!
//! This is an implementation detail used to provide many models for
//! stuff like `Monoid`, `Group`, etc. To create a `CanonicalConstant`,
//! simply create an object with a nested `hana_tag` equal to the proper
//! specialization of `CanonicalConstant<T>`, and then also provide a
//! `constexpr` static member `::%value` holding the value of the constant.
template <typename T>
struct CanonicalConstant {
using value_type = T;
};
} }} // end namespace boost::hana
#include <boost/hana/concept/constant.hpp>
#include <boost/hana/concept/integral_constant.hpp>
#include <boost/hana/core/to.hpp>
#include <boost/hana/core/when.hpp>
#include <type_traits>
namespace boost { namespace hana {
//////////////////////////////////////////////////////////////////////////
// Constant
//////////////////////////////////////////////////////////////////////////
template <typename T>
struct value_impl<detail::CanonicalConstant<T>> {
template <typename X>
static constexpr decltype(auto) apply()
{ return X::value; }
};
namespace detail {
template <typename T, typename X>
struct canonical_constant {
static constexpr auto value = hana::to<T>(hana::value<X>());
using hana_tag = detail::CanonicalConstant<T>;
};
}
template <typename T, typename C>
struct to_impl<detail::CanonicalConstant<T>, C, when<
hana::Constant<C>::value &&
is_convertible<typename C::value_type, T>::value
>>
: embedding<is_embedded<typename C::value_type, T>::value>
{
template <typename X>
static constexpr detail::canonical_constant<T, X> apply(X const&)
{ return {}; }
};
//////////////////////////////////////////////////////////////////////////
// IntegralConstant (when value_type is integral)
//////////////////////////////////////////////////////////////////////////
template <typename T>
struct IntegralConstant<detail::CanonicalConstant<T>> {
static constexpr bool value = std::is_integral<T>::value;
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_DETAIL_CANONICAL_CONSTANT_HPP
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