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// Copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
#include <boost/hana/assert.hpp>
#include <boost/hana/at.hpp>
#include <boost/hana/core/make.hpp>
#include <boost/hana/core/tag_of.hpp>
#include <boost/hana/drop_front.hpp>
#include <boost/hana/is_empty.hpp>
#include <boost/hana/not.hpp>
#include <boost/hana/tuple.hpp>
#include <cstddef>
#include <functional>
namespace hana = boost::hana;
// A tuple that holds reference_wrappers to its elements, instead of the
// elements themselves.
struct RefTuple { };
template <typename ...T>
struct ref_tuple;
template <typename ...T>
struct ref_tuple<T&...> {
hana::tuple<std::reference_wrapper<T>...> storage_;
};
namespace boost { namespace hana {
template <typename ...T>
struct tag_of<ref_tuple<T...>> {
using type = RefTuple;
};
template <>
struct make_impl<RefTuple> {
template <typename ...T>
static constexpr auto apply(T& ...t) {
return ref_tuple<T&...>{{std::ref(t)...}};
}
};
template <>
struct at_impl<RefTuple> {
template <typename Xs, typename N>
static constexpr decltype(auto) apply(Xs&& xs, N const& n) {
return hana::at(static_cast<Xs&&>(xs).storage_, n).get();
}
};
template <>
struct is_empty_impl<RefTuple> {
template <typename Xs>
static constexpr auto apply(Xs const& xs) {
return hana::is_empty(xs.storage_);
}
};
template <>
struct drop_front_impl<RefTuple> {
template <std::size_t n, typename T, typename ...U, std::size_t ...i>
static constexpr auto helper(ref_tuple<T, U...> xs, std::index_sequence<i...>) {
return hana::make<RefTuple>(hana::at_c<n + i>(xs.storage_).get()...);
}
template <typename ...T, typename N>
static constexpr auto apply(ref_tuple<T...> xs, N const&) {
return helper<N::value>(xs, std::make_index_sequence<(
N::value < sizeof...(T) ? sizeof...(T) - N::value : 0
)>{});
}
};
}} // end namespace boost::hana
int main() {
int i = 0, j = 1, k = 2;
ref_tuple<int&, int&, int&> refs = hana::make<RefTuple>(i, j, k);
hana::at_c<0>(refs) = 3;
BOOST_HANA_RUNTIME_CHECK(i == 3);
BOOST_HANA_CONSTANT_CHECK(hana::not_(hana::is_empty(refs)));
ref_tuple<int&, int&> tail = hana::drop_front(refs);
hana::at_c<0>(tail) = 4;
BOOST_HANA_RUNTIME_CHECK(j == 4);
}
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