Method operator()

cpp/src/arrow/util/async_util.h:404–422

          : state_holder(std::move(state_holder)),
            task_completion(std::move(task_completion)) {}
      void operator()(const Result<T>& maybe_item) {
        if (!maybe_item.ok()) {
          task_completion.MarkFinished(maybe_item.status());
          return;
        }
        const auto& item = *maybe_item;
        if (IsIterationEnd(item)) {
          task_completion.MarkFinished();
          return;
        }
        Status visit_st = state_holder->visitor(item);
        if (!visit_st.ok()) {
          task_completion.MarkFinished(std::move(visit_st));
          return;
        }
        state_holder->task_group->AddTask(
            std::make_unique<SubmitTask>(std::move(state_holder)));
        task_completion.MarkFinished();
      }
      std::unique_ptr<State> state_holder;
      Future<> task_completion;
    };