598bfd3f
 Hu Chunming
提交_GLIBCXX_USE_CX...
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/*!
@file
Defines `boost::hana::empty`.
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_EMPTY_HPP
#define BOOST_HANA_EMPTY_HPP
#include <boost/hana/fwd/empty.hpp>
#include <boost/hana/concept/monad_plus.hpp>
#include <boost/hana/concept/sequence.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/core/make.hpp>
namespace boost { namespace hana {
//! @cond
template <typename M>
constexpr auto empty_t<M>::operator()() const {
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::MonadPlus<M>::value,
"hana::empty<M>() requires 'M' to be a MonadPlus");
#endif
using Empty = BOOST_HANA_DISPATCH_IF(empty_impl<M>,
hana::MonadPlus<M>::value
);
return Empty::apply();
}
//! @endcond
template <typename M, bool condition>
struct empty_impl<M, when<condition>> : default_ {
template <typename ...Args>
static constexpr auto apply(Args&& ...) = delete;
};
template <typename S>
struct empty_impl<S, when<Sequence<S>::value>> {
static constexpr auto apply() {
return hana::make<S>();
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_EMPTY_HPP
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