Thanks for your reply!
I tried to load an image at get time.
But the exception occurred.
Debugger pointed at dataloader/base.h file line.
 optional<BatchType> next() {
    if (options_.workers > 0) {
      while (optional<Result> result = this->pop_result()) {
        if (result->exception) {
          **throw WorkerException(result->exception);**
        } else if (result->batch) {
          prefetch(1);
          return std::move(result->batch);
        }
      }
    } else if (auto batch_request = get_batch_request()) {
      return this->main_thread_dataset_->get_batch(std::move(*batch_request));
    }
    return nullopt;
  }

I am stucked …