Function key_type

cpp/src/arrow/type.h:1345–1345

  std::shared_ptr<Field> key_field() const { return value_type()->field(0); }
  std::shared_ptr<DataType> key_type() const { return key_field()->type(); }

  std::shared_ptr<Field> item_field() const { return value_type()->field(1); }
  std::shared_ptr<DataType> item_type() const { return item_field()->type(); }