mirror of https://gitee.com/bigwinds/arangodb
86 lines
2.3 KiB
C++
86 lines
2.3 KiB
C++
// Copyright Louis Dionne 2013-2016
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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#include <boost/hana/assert.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/core/make.hpp>
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#include <boost/hana/core/tag_of.hpp>
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#include <boost/hana/drop_front.hpp>
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#include <boost/hana/is_empty.hpp>
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#include <boost/hana/not.hpp>
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#include <boost/hana/tuple.hpp>
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#include <cstddef>
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namespace hana = boost::hana;
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struct RefTuple { };
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template <typename ...T>
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struct ref_tuple {
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hana::tuple<T&...> storage_;
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};
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namespace boost { namespace hana {
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template <typename ...T>
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struct tag_of<ref_tuple<T...>> {
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using type = RefTuple;
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};
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template <>
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struct make_impl<RefTuple> {
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template <typename ...T>
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static constexpr auto apply(T&& ...t) {
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return ref_tuple<T...>{{t...}};
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}
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};
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template <>
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struct at_impl<RefTuple> {
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template <typename Xs, typename N>
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static constexpr decltype(auto) apply(Xs&& xs, N const& n) {
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return hana::at(static_cast<Xs&&>(xs).storage_, n);
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}
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};
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template <>
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struct is_empty_impl<RefTuple> {
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template <typename Xs>
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static constexpr auto apply(Xs const& xs) {
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return hana::is_empty(xs.storage_);
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}
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};
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template <>
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struct drop_front_impl<RefTuple> {
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template <std::size_t n, typename T, typename ...U, std::size_t ...i>
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static constexpr auto helper(ref_tuple<T, U...> xs, std::index_sequence<i...>) {
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return hana::make<RefTuple>(hana::at_c<n + i>(xs.storage_)...);
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}
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template <typename ...T, typename N>
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static constexpr auto apply(ref_tuple<T...> xs, N const&) {
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return helper<N::value>(xs, std::make_index_sequence<(
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N::value < sizeof...(T) ? sizeof...(T) - N::value : 0
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)>{});
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}
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};
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}}
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int main() {
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int i = 0, j = 1, k = 2;
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auto refs = hana::make<RefTuple>(i, j, k);
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hana::at_c<0>(refs) = 3;
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BOOST_HANA_RUNTIME_CHECK(i == 3);
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BOOST_HANA_CONSTANT_CHECK(hana::not_(hana::is_empty(refs)));
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ref_tuple<int&, int&> tail = hana::drop_front(refs);
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hana::at_c<0>(tail) = 4;
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BOOST_HANA_RUNTIME_CHECK(j == 4);
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}
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