598bfd3f
 Hu Chunming
提交_GLIBCXX_USE_CX...
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/*!
@file
Defines `boost::hana::zero`.
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_ZERO_HPP
#define BOOST_HANA_ZERO_HPP
#include <boost/hana/fwd/zero.hpp>
#include <boost/hana/concept/constant.hpp>
#include <boost/hana/concept/monoid.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/to.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/detail/canonical_constant.hpp>
#include <type_traits>
namespace boost { namespace hana {
//! @cond
template <typename M>
constexpr decltype(auto) zero_t<M>::operator()() const {
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Monoid<M>::value,
"hana::zero<M>() requires 'M' to be a Monoid");
#endif
using Zero = BOOST_HANA_DISPATCH_IF(zero_impl<M>,
hana::Monoid<M>::value
);
return Zero::apply();
}
//! @endcond
template <typename M, bool condition>
struct zero_impl<M, when<condition>> : default_ {
template <typename ...Args>
static constexpr auto apply(Args&& ...) = delete;
};
//////////////////////////////////////////////////////////////////////////
// Model for non-boolean arithmetic data types
//////////////////////////////////////////////////////////////////////////
template <typename T>
struct zero_impl<T, when<
std::is_arithmetic<T>::value &&
!std::is_same<T, bool>::value
>> {
static constexpr T apply()
{ return static_cast<T>(0); }
};
//////////////////////////////////////////////////////////////////////////
// Model for Constants over a Monoid
//////////////////////////////////////////////////////////////////////////
namespace detail {
template <typename C>
struct constant_from_zero {
static constexpr auto value = hana::zero<typename C::value_type>();
using hana_tag = detail::CanonicalConstant<typename C::value_type>;
};
}
template <typename C>
struct zero_impl<C, when<
hana::Constant<C>::value &&
Monoid<typename C::value_type>::value
>> {
static constexpr decltype(auto) apply()
{ return hana::to<C>(detail::constant_from_zero<C>{}); }
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_ZERO_HPP
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