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 Hu Chunming
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/*!
@file
Defines `boost::hana::at_key`.
Copyright Louis Dionne 2013-2022
Copyright Jason Rice 2017
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_AT_KEY_HPP
#define BOOST_HANA_AT_KEY_HPP
#include <boost/hana/fwd/at_key.hpp>
#include <boost/hana/accessors.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/concept/searchable.hpp>
#include <boost/hana/concept/struct.hpp>
#include <boost/hana/config.hpp>
#include <boost/hana/core/dispatch.hpp>
#include <boost/hana/detail/decay.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/find.hpp>
#include <boost/hana/find_if.hpp>
#include <boost/hana/first.hpp>
#include <boost/hana/functional/on.hpp>
#include <boost/hana/index_if.hpp>
#include <boost/hana/length.hpp>
#include <boost/hana/optional.hpp>
#include <boost/hana/second.hpp>
#include <cstddef>
#include <utility>
namespace boost { namespace hana {
//! @cond
template <typename Xs, typename Key>
constexpr decltype(auto) at_key_t::operator()(Xs&& xs, Key const& key) const {
using S = typename hana::tag_of<Xs>::type;
using AtKey = BOOST_HANA_DISPATCH_IF(at_key_impl<S>,
hana::Searchable<S>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(hana::Searchable<S>::value,
"hana::at_key(xs, key) requires 'xs' to be Searchable");
#endif
return AtKey::apply(static_cast<Xs&&>(xs), key);
}
//! @endcond
template <typename S, bool condition>
struct at_key_impl<S, when<condition>> : default_ {
template <typename Xs, typename Key>
static constexpr auto apply(Xs&& xs, Key const& key) {
return hana::find(static_cast<Xs&&>(xs), key).value();
}
};
namespace at_key_detail {
template <typename T>
struct equal_to {
T const& t;
template <typename U>
constexpr auto operator()(U const& u) const {
return hana::equal(t, u);
}
};
}
template <typename S>
struct at_key_impl<S, when<hana::Sequence<S>::value>> {
template <typename Xs, typename Key>
static constexpr decltype(auto) apply(Xs&& xs, Key const& key) {
using Result = decltype(hana::index_if(
static_cast<Xs&&>(xs), at_key_detail::equal_to<Key>{key}));
return hana::at(static_cast<Xs&&>(xs), Result{}.value());
}
};
template <typename S>
struct at_key_impl<S, when<hana::Struct<S>::value>> {
template <typename X, typename Key>
static constexpr decltype(auto) apply(X&& x, Key const& key) {
auto accessor = hana::second(*hana::find_if(hana::accessors<S>(),
hana::equal.to(key) ^hana::on^ hana::first
));
return accessor(static_cast<X&&>(x));
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_AT_KEY_HPP
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