Method set_async

extern/boost/boost/thread/future.hpp:1775–1778

todo this function must be private and friendship provided to the internal users.

        // todo this function must be private and friendship provided to the internal users.
        void set_async()
        {
          this->future_->set_async();
        }
        // todo this function must be private and friendship provided to the internal users.
        void set_deferred()
        {