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 Hu Chunming
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/*!
@file
Defines `boost::hana::if_`.
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_IF_HPP
#define BOOST_HANA_IF_HPP
#include <boost/hana/fwd/if.hpp>
#include <boost/hana/concept/logical.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/eval_if.hpp>
namespace boost { namespace hana {
//! @cond
template <typename Cond, typename Then, typename Else>
constexpr decltype(auto) if_t::operator()(Cond&& cond, Then&& then_, Else&& else_) const {
using Bool = typename hana::tag_of<Cond>::type;
using If = BOOST_HANA_DISPATCH_IF(if_impl<Bool>,
hana::Logical<Bool>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Logical<Bool>::value,
"hana::if_(cond, then, else) requires 'cond' to be a Logical");
#endif
return If::apply(static_cast<Cond&&>(cond),
static_cast<Then&&>(then_),
static_cast<Else&&>(else_));
}
//! @endcond
namespace detail {
template <typename T>
struct hold {
T value;
constexpr T&& operator()() && { return static_cast<T&&>(value); }
};
}
template <typename L, bool condition>
struct if_impl<L, when<condition>> : default_ {
template <typename C, typename T, typename E>
static constexpr auto apply(C&& c, T&& t, E&& e) {
return hana::eval_if(static_cast<C&&>(c),
detail::hold<T&&>{static_cast<T&&>(t)},
detail::hold<E&&>{static_cast<E&&>(e)}
);
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_IF_HPP
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