Class hash_ptr

tests/ordered_map_tests.cpp:1517–1530

 */
BOOST_AUTO_TEST_CASE(test_heterogeneous_lookups) {
  struct hash_ptr {
    std::size_t operator()(const std::unique_ptr<int>& p) const {
      return std::hash<std::uintptr_t>()(
          reinterpret_cast<std::uintptr_t>(p.get()));
    }

    std::size_t operator()(std::uintptr_t p) const {
      return std::hash<std::uintptr_t>()(p);
    }

    std::size_t operator()(const int* const& p) const {
      return std::hash<std::uintptr_t>()(reinterpret_cast<std::uintptr_t>(p));
    }
  };

  struct equal_to_ptr {
    using is_transparent = std::true_type;