boost/hana/at_key.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::at_key`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Copyright Jason Rice 2017
0007 Distributed under the Boost Software License, Version 1.0.
0008 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0009  */
0010
0011 #ifndef BOOST_HANA_AT_KEY_HPP
0012 #define BOOST_HANA_AT_KEY_HPP
0013
0014 #include <boost/hana/fwd/at_key.hpp>
0015
0016 #include <boost/hana/accessors.hpp>
0017 #include <boost/hana/at.hpp>
0018 #include <boost/hana/concept/searchable.hpp>
0019 #include <boost/hana/concept/struct.hpp>
0020 #include <boost/hana/config.hpp>
0021 #include <boost/hana/core/dispatch.hpp>
0022 #include <boost/hana/detail/decay.hpp>
0023 #include <boost/hana/equal.hpp>
0024 #include <boost/hana/find.hpp>
0025 #include <boost/hana/find_if.hpp>
0026 #include <boost/hana/first.hpp>
0027 #include <boost/hana/functional/on.hpp>
0028 #include <boost/hana/index_if.hpp>
0029 #include <boost/hana/length.hpp>
0030 #include <boost/hana/optional.hpp>
0031 #include <boost/hana/second.hpp>
0032
0033 #include <cstddef>
0034 #include <utility>
0035
0036
0037 namespace boost { namespace hana {
0038     //! @cond
0039     template <typename Xs, typename Key>
0040     constexpr decltype(auto) at_key_t::operator()(Xs&& xs, Key const& key) const {
0041         using S = typename hana::tag_of<Xs>::type;
0042         using AtKey = BOOST_HANA_DISPATCH_IF(at_key_impl<S>,
0043             hana::Searchable<S>::value
0044         );
0045
0046     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0047         static_assert(hana::Searchable<S>::value,
0048         "hana::at_key(xs, key) requires 'xs' to be Searchable");
0049     #endif
0050
0051         return AtKey::apply(static_cast<Xs&&>(xs), key);
0052     }
0053     //! @endcond
0054
0055     template <typename S, bool condition>
0056     struct at_key_impl<S, when<condition>> : default_ {
0057         template <typename Xs, typename Key>
0058         static constexpr auto apply(Xs&& xs, Key const& key) {
0059             return hana::find(static_cast<Xs&&>(xs), key).value();
0060         }
0061     };
0062
0063     namespace at_key_detail {
0064         template <typename T>
0065         struct equal_to {
0066             T const& t;
0067             template <typename U>
0068             constexpr auto operator()(U const& u) const {
0069                 return hana::equal(t, u);
0070             }
0071         };
0072     }
0073
0074     template <typename S>
0075     struct at_key_impl<S, when<hana::Sequence<S>::value>> {
0076         template <typename Xs, typename Key>
0077         static constexpr decltype(auto) apply(Xs&& xs, Key const& key) {
0078             using Result = decltype(hana::index_if(
0079                 static_cast<Xs&&>(xs), at_key_detail::equal_to<Key>{key}));
0080
0081             return hana::at(static_cast<Xs&&>(xs), Result{}.value());
0082         }
0083     };
0084
0085     template <typename S>
0086     struct at_key_impl<S, when<hana::Struct<S>::value>> {
0087         template <typename X, typename Key>
0088         static constexpr decltype(auto) apply(X&& x, Key const& key) {
0089             auto accessor = hana::second(*hana::find_if(hana::accessors<S>(),
0090                 hana::equal.to(key) ^hana::on^ hana::first
0091             ));
0092             return accessor(static_cast<X&&>(x));
0093         }
0094     };
0095 }} // end namespace boost::hana
0096
0097 #endif // !BOOST_HANA_AT_KEY_HPP