ceph/src/boost/libs/hana/example/misc/ref_tuple.cpp

   1 // Copyright Louis Dionne 2013-2017
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/assert.hpp>
   6 #include <boost/hana/at.hpp>
   7 #include <boost/hana/core/make.hpp>
   8 #include <boost/hana/core/tag_of.hpp>
   9 #include <boost/hana/drop_front.hpp>
  10 #include <boost/hana/is_empty.hpp>
  11 #include <boost/hana/not.hpp>
  12 #include <boost/hana/tuple.hpp>
  13
  14 #include <cstddef>
  15 #include <functional>
  16 namespace hana = boost::hana;
  17
  18
  19 // A tuple that holds reference_wrappers to its elements, instead of the
  20 // elements themselves.
  21
  22 struct RefTuple { };
  23
  24 template <typename ...T>
  25 struct ref_tuple;
  26
  27 template <typename ...T>
  28 struct ref_tuple<T&...> {
  29     hana::tuple<std::reference_wrapper<T>...> storage_;
  30 };
  31
  32
  33 namespace boost { namespace hana {
  34     template <typename ...T>
  35     struct tag_of<ref_tuple<T...>> {
  36         using type = RefTuple;
  37     };
  38
  39     template <>
  40     struct make_impl<RefTuple> {
  41         template <typename ...T>
  42         static constexpr auto apply(T& ...t) {
  43             return ref_tuple<T&...>{{std::ref(t)...}};
  44         }
  45     };
  46
  47     template <>
  48     struct at_impl<RefTuple> {
  49         template <typename Xs, typename N>
  50         static constexpr decltype(auto) apply(Xs&& xs, N const& n) {
  51             return hana::at(static_cast<Xs&&>(xs).storage_, n).get();
  52         }
  53     };
  54
  55     template <>
  56     struct is_empty_impl<RefTuple> {
  57         template <typename Xs>
  58         static constexpr auto apply(Xs const& xs) {
  59             return hana::is_empty(xs.storage_);
  60         }
  61     };
  62
  63     template <>
  64     struct drop_front_impl<RefTuple> {
  65         template <std::size_t n, typename T, typename ...U, std::size_t ...i>
  66         static constexpr auto helper(ref_tuple<T, U...> xs, std::index_sequence<i...>) {
  67             return hana::make<RefTuple>(hana::at_c<n + i>(xs.storage_).get()...);
  68         }
  69
  70         template <typename ...T, typename N>
  71         static constexpr auto apply(ref_tuple<T...> xs, N const&) {
  72             return helper<N::value>(xs, std::make_index_sequence<(
  73                 N::value < sizeof...(T) ? sizeof...(T) - N::value : 0
  74             )>{});
  75         }
  76     };
  77 }} // end namespace boost::hana
  78
  79
  80 int main() {
  81     int i = 0, j = 1, k = 2;
  82
  83     ref_tuple<int&, int&, int&> refs = hana::make<RefTuple>(i, j, k);
  84     hana::at_c<0>(refs) = 3;
  85     BOOST_HANA_RUNTIME_CHECK(i == 3);
  86
  87     BOOST_HANA_CONSTANT_CHECK(hana::not_(hana::is_empty(refs)));
  88
  89     ref_tuple<int&, int&> tail = hana::drop_front(refs);
  90     hana::at_c<0>(tail) = 4;
  91     BOOST_HANA_RUNTIME_CHECK(j == 4);
  92 }