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 Hu Chunming
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/*!
@file
Defines `boost::hana::concat`.
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_CONCAT_HPP
#define BOOST_HANA_CONCAT_HPP
#include <boost/hana/fwd/concat.hpp>
#include <boost/hana/at.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>
#include <boost/hana/length.hpp>
#include <cstddef>
#include <type_traits>
#include <utility>
namespace boost { namespace hana {
//! @cond
template <typename Xs, typename Ys>
constexpr auto concat_t::operator()(Xs&& xs, Ys&& ys) const {
using M = typename hana::tag_of<Xs>::type;
using Concat = BOOST_HANA_DISPATCH_IF(concat_impl<M>,
hana::MonadPlus<M>::value &&
std::is_same<typename hana::tag_of<Ys>::type, M>::value
);
#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
static_assert(std::is_same<typename hana::tag_of<Ys>::type, M>::value,
"hana::concat(xs, ys) requires 'xs' and 'ys' to have the same tag");
static_assert(hana::MonadPlus<M>::value,
"hana::concat(xs, ys) requires 'xs' and 'ys' to be MonadPlus");
#endif
return Concat::apply(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys));
}
//! @endcond
template <typename M, bool condition>
struct concat_impl<M, when<condition>> : default_ {
template <typename ...Args>
static constexpr auto apply(Args&& ...) = delete;
};
template <typename S>
struct concat_impl<S, when<Sequence<S>::value>> {
template <typename Xs, typename Ys, std::size_t ...xi, std::size_t ...yi>
static constexpr auto
concat_helper(Xs&& xs, Ys&& ys, std::index_sequence<xi...>,
std::index_sequence<yi...>)
{
return hana::make<S>(
hana::at_c<xi>(static_cast<Xs&&>(xs))...,
hana::at_c<yi>(static_cast<Ys&&>(ys))...
);
}
template <typename Xs, typename Ys>
static constexpr auto apply(Xs&& xs, Ys&& ys) {
constexpr std::size_t xi = decltype(hana::length(xs))::value;
constexpr std::size_t yi = decltype(hana::length(ys))::value;
return concat_helper(static_cast<Xs&&>(xs), static_cast<Ys&&>(ys),
std::make_index_sequence<xi>{},
std::make_index_sequence<yi>{});
}
};
}} // end namespace boost::hana
#endif // !BOOST_HANA_CONCAT_HPP
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