  HashCrosserV2(
      const std::vector<std::unique_ptr<ColumnInterface<int64>>>& columns,
      const int64 num_buckets, const uint64 hash_key_unused,
      const tstring k_feature_separator_unused)
      : columns_(columns), num_buckets_(num_buckets) {}
  HashCrosserV2(
      const std::vector<std::unique_ptr<ColumnInterface<int64>>>& columns,
      const int64 num_buckets, const uint64 hash_key_unused,
      const tstring k_feature_separator_unused)
      : columns_(columns), num_buckets_(num_buckets) {}