Method print_is_pending

examples/async.cc:127–131

  int32_t get_value_to_print() { return _value_to_print.get_future().get(); }

  bool print_is_pending() const {
    return _print_finished_future.valid() &&
           _print_finished_future.wait_for(std::chrono::seconds(0)) !=
               std::future_status::ready;
  }
  void wait_for_print_result() const { _print_finished_future.wait(); }
  void get_print_result() { _print_finished_future.get(); }
  void set_print_success() { _print_finished.set_value(); }