/// Primary template for future.
  template<typename _Res>
    class future : public __basic_future<_Res>
    {
      friend class promise<_Res>;
      template<typename> friend class packaged_task;
      template<typename _Fn, typename... _Args>
        friend future<typename result_of<_Fn(_Args...)>::type>
        async(launch, _Fn&&, _Args&&...);

      typedef __basic_future<_Res> _Base_type;
      typedef typename _Base_type::__state_type __state_type;

      explicit
      future(const __state_type& __state) : _Base_type(__state) { }

    public:
      constexpr future() noexcept : _Base_type() { }

      /// Move constructor
      future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }

      // Disable copying
      future(const future&) = delete;
      future& operator=(const future&) = delete;

      future& operator=(future&& __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      _Res
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        return std::move(this->_M_get_result()._M_value());
      }

      shared_future<_Res> share();
    };

  /// Partial specialization for future<R&>
  template<typename _Res>
    class future<_Res&> : public __basic_future<_Res&>
    {
      friend class promise<_Res&>;
      template<typename> friend class packaged_task;
      template<typename _Fn, typename... _Args>
        friend future<typename result_of<_Fn(_Args...)>::type>
        async(launch, _Fn&&, _Args&&...);

      typedef __basic_future<_Res&> _Base_type;
      typedef typename _Base_type::__state_type __state_type;

      explicit
      future(const __state_type& __state) : _Base_type(__state) { }

    public:
      constexpr future() noexcept : _Base_type() { }

      /// Move constructor
      future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }

      // Disable copying
      future(const future&) = delete;
      future& operator=(const future&) = delete;

      future& operator=(future&& __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      _Res&
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        return this->_M_get_result()._M_get();
      }

      shared_future<_Res&> share();
    };

  /// Explicit specialization for future<void>
  template<>
    class future<void> : public __basic_future<void>
    {
      friend class promise<void>;
      template<typename> friend class packaged_task;
      template<typename _Fn, typename... _Args>
        friend future<typename result_of<_Fn(_Args...)>::type>
        async(launch, _Fn&&, _Args&&...);

      typedef __basic_future<void> _Base_type;
      typedef typename _Base_type::__state_type __state_type;

      explicit
      future(const __state_type& __state) : _Base_type(__state) { }

    public:
      constexpr future() noexcept : _Base_type() { }

      /// Move constructor
      future(future&& __uf) noexcept : _Base_type(std::move(__uf)) { }

      // Disable copying
      future(const future&) = delete;
      future& operator=(const future&) = delete;

      future& operator=(future&& __fut) noexcept
      {
        future(std::move(__fut))._M_swap(*this);
        return *this;
      }

      /// Retrieving the value
      void
      get()
      {
        typename _Base_type::_Reset __reset(*this);
        this->_M_get_result();
      }

      shared_future<void> share();
    };