82 lines
2.5 KiB
C++
82 lines
2.5 KiB
C++
/*!
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@file
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Defines `boost::hana::zero`.
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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_ZERO_HPP
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#define BOOST_HANA_ZERO_HPP
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#include <boost/hana/fwd/zero.hpp>
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#include <boost/hana/concept/constant.hpp>
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#include <boost/hana/concept/monoid.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/to.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/detail/canonical_constant.hpp>
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#include <type_traits>
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BOOST_HANA_NAMESPACE_BEGIN
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template <typename M>
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struct zero_t {
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Monoid<M>::value,
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"hana::zero<M>() requires 'M' to be a Monoid");
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#endif
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constexpr decltype(auto) operator()() const {
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using Zero = BOOST_HANA_DISPATCH_IF(zero_impl<M>,
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hana::Monoid<M>::value
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);
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return Zero::apply();
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}
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};
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template <typename M, bool condition>
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struct zero_impl<M, when<condition>> : default_ {
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template <typename ...Args>
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static constexpr auto apply(Args&& ...) = delete;
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};
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//////////////////////////////////////////////////////////////////////////
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// Model for non-boolean arithmetic data types
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//////////////////////////////////////////////////////////////////////////
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template <typename T>
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struct zero_impl<T, when<
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std::is_arithmetic<T>::value &&
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!std::is_same<T, bool>::value
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>> {
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static constexpr T apply()
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{ return static_cast<T>(0); }
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};
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//////////////////////////////////////////////////////////////////////////
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// Model for Constants over a Monoid
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//////////////////////////////////////////////////////////////////////////
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namespace detail {
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template <typename C>
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struct constant_from_zero {
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static constexpr auto value = hana::zero<typename C::value_type>();
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using hana_tag = detail::CanonicalConstant<typename C::value_type>;
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};
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}
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template <typename C>
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struct zero_impl<C, when<
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hana::Constant<C>::value &&
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Monoid<typename C::value_type>::value
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>> {
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static constexpr decltype(auto) apply()
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{ return hana::to<C>(detail::constant_from_zero<C>{}); }
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_ZERO_HPP
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