[GitHub] [arrow] fsaintjacques commented on a change in pull request #7244: PARQUET-1855: [C++] Improve parquet *MetaData documentation

[GitBox <gi...@apache.org>]

fsaintjacques commented on a change in pull request #7244:
URL: https://github.com/apache/arrow/pull/7244#discussion_r428879824



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File path: cpp/src/parquet/metadata.h
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@@ -178,27 +179,42 @@ class PARQUET_EXPORT ColumnChunkMetaData {
   std::unique_ptr<ColumnChunkMetaDataImpl> impl_;
 };
 
+/// \brief RowGroupMetaData is a proxy around format::RowGroupMetaData.
 class PARQUET_EXPORT RowGroupMetaData {
  public:
-  // API convenience to get a MetaData accessor
+  /// \brief Create a RowGroupMetaData from a serialized thrift message.
   static std::unique_ptr<RowGroupMetaData> Make(
       const void* metadata, const SchemaDescriptor* schema,
       const ApplicationVersion* writer_version = NULLPTR,
       std::shared_ptr<InternalFileDecryptor> file_decryptor = NULLPTR);
 
   ~RowGroupMetaData();
 
-  // row-group metadata
+  /// \brief The number of columns in this row group. The order must match the
+  /// parent's column ordering.
   int num_columns() const;
+
+  /// \brief Return the ColumnChunkMetaData of the corresponding column ordinal.
+  ///
+  /// WARNING, the returned object references memory location in it's parent
+  /// (RowGroupMetaData) object. Hence, the parent must outlive the returned
+  /// object.
+  ///
+  /// \param[i] index of the ColumnChunkMetaData to retrieve.
+  ///
+  /// \throws ParquetException if the index is out of bound.
+  std::unique_ptr<ColumnChunkMetaData> ColumnChunk(int i) const;

Review comment:
       @wesm I believe we should make a breaking change and return either a const-ref or const-ptr (like SchemaDescriptor) and hold the corresponding ColumnChunks in a private vector.




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