[GitHub] [arrow] lidavidm commented on a diff in pull request #12775: ARROW-16006: [C++] Row conversion helpers and example

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

lidavidm commented on code in PR #12775:
URL: https://github.com/apache/arrow/pull/12775#discussion_r893820377


##########
docs/source/cpp/examples/row_columnar_conversion.rst:
##########
@@ -21,7 +21,98 @@
 Row to columnar conversion
 ==========================
 
+Fixed Schemas
+-------------
+
 The following example converts an array of structs to a :class:`arrow::Table`
 instance, and then converts it back to the original array of structs.
 
 .. literalinclude:: ../../../../cpp/examples/arrow/row_wise_conversion_example.cc
+
+
+Dynamic Schemas
+---------------
+
+For schemas not known at compile time, implement an :class:`arrow::ToRowConverter`
+or an :class:`arrow::FromRowConverter`.
+The following example shows how to implement conversion between `rapidjson::Document`

Review Comment:
   ```suggestion
   The following example shows how to implement conversion between ``rapidjson::Document``
   ```



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();

Review Comment:
   While minor, I wonder why `BaseListArray::values` doesn't return `const Array&` or at least `const std::shared_ptr<Array>&`…not that it should be a big deal here.



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }

Review Comment:
   and here, why not take say `const arrow::Field&` to avoid a shared_ptr copy? (though again, probably all of these don't add up to anything meaningful…)



##########
cpp/examples/arrow/CMakeLists.txt:
##########
@@ -17,6 +17,8 @@
 
 add_arrow_example(row_wise_conversion_example)
 
+add_arrow_example(rapidjson_row_converter)

Review Comment:
   wrap in `if(ARROW_JSON)`?



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();
+      auto array_len = array.value_length(i);
+
+      rapidjson::Value value;
+      value.SetArray();
+      value.Reserve(array_len, allocator);
+
+      for (int64_t j = 0; j < array_len; ++j) {
+        rapidjson::Value row_val;
+        // Must copy value to new allocator
+        row_val.CopyFrom(rows[values_i][value_field_name.c_str()], allocator);
+        value.PushBack(row_val, allocator);
+        ++values_i;
+      }
+
+      rapidjson::Value str_key(field_->name().c_str(), allocator);
+      rows_[i].AddMember(str_key, value, allocator);
+    }
+
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Field> field_;
+  std::vector<rapidjson::Document> rows_;
+};  // RowBatchBuilder
+
+class ArrowToDocumentConverter : public arrow::ToRowConverter<rapidjson::Document> {
+ public:
+  arrow::Result<std::vector<rapidjson::Document>> ConvertToVector(
+      std::shared_ptr<arrow::RecordBatch> batch) override {
+    RowBatchBuilder builder{batch->num_rows()};
+
+    for (int i = 0; i < batch->num_columns(); ++i) {
+      builder.SetField(std::move(batch->schema()->field(i)));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*batch->column(i).get(), &builder));
+    }
+
+    return std::move(builder).Rows();
+  }
+};  // ArrowToDocumentConverter
+
+/// \brief Iterator over rows values of a document for a given field
+///
+/// path and array_levels are used to address each field in a JSON document. As
+/// an example, consider this JSON document:
+/// {
+///     "x": 3,                   // path: ["x"],             array_levels: 0
+///     "files": [                // path: ["files"],         array_levels: 0
+///         {                     // path: ["files"],         array_levels: 1
+///             "path": "my_str", // path: ["files", "path"], array_levels: 1
+///             "sizes": [        // path: ["files", "size"], array_levels: 1
+///                 20,           // path: ["files", "size"], array_levels: 2
+///                 22
+///             ]
+///         }
+///     ]
+/// },
+class DocValuesIterator {
+ public:
+  /// \param rows vector of rows
+  /// \param path field names to enter
+  /// \param array_levels number of arrays to enter
+  DocValuesIterator(const std::vector<rapidjson::Document>& rows,
+                    std::vector<std::string> path, int64_t array_levels)
+      : rows(rows), path(std::move(path)), array_levels(array_levels) {}
+
+  const rapidjson::Value* NextArrayOrRow(const rapidjson::Value* value, size_t* path_i,
+                                         int64_t* arr_i) {
+    while (array_stack.size() > 0) {
+      ArrayPosition& pos = array_stack.back();
+      // Try to get next position in Array
+      if (pos.index + 1 < pos.array_node->Size()) {
+        ++pos.index;
+        value = &(*pos.array_node)[pos.index];
+        *path_i = pos.path_index;
+        *arr_i = array_stack.size();
+        return value;
+      } else {
+        array_stack.pop_back();
+      }
+    }
+    ++row_i;
+    if (row_i < rows.size()) {
+      value = static_cast<const rapidjson::Value*>(&rows[row_i]);
+    } else {
+      value = nullptr;
+    }
+    *path_i = 0;
+    *arr_i = 0;
+    return value;
+  }
+
+  arrow::Result<const rapidjson::Value*> Next() {
+    const rapidjson::Value* value = nullptr;
+    size_t path_i;
+    int64_t arr_i;
+    // Can either start at document or at last array level
+    if (array_stack.size() > 0) {
+      auto pos = array_stack.back();
+      value = pos.array_node;
+      path_i = pos.path_index;
+      arr_i = array_stack.size() - 1;
+    }
+
+    value = NextArrayOrRow(value, &path_i, &arr_i);
+
+    // Traverse to desired level (with possible backtracking as needed)
+    while (path_i < path.size() || arr_i < array_levels) {
+      if (value == nullptr) {
+        return value;
+      } else if (value->IsArray() && value->Size() > 0) {
+        ArrayPosition pos;
+        pos.array_node = value;
+        pos.path_index = path_i;
+        pos.index = 0;
+        array_stack.push_back(pos);
+
+        value = &(*value)[0];
+        ++arr_i;
+      } else if (value->IsArray()) {
+        // Empty array means we need to backtrack and go to next array or row
+        value = NextArrayOrRow(value, &path_i, &arr_i);
+      } else if (value->HasMember(path[path_i].c_str())) {
+        value = &(*value)[path[path_i].c_str()];
+        ++path_i;
+      } else {
+        return &kNullJsonSingleton;
+      }
+    }
+
+    // Return value
+    return value;
+  }
+
+ protected:
+  const std::vector<rapidjson::Document>& rows;
+  std::vector<std::string> path;
+  int64_t array_levels;
+  size_t row_i = -1;  // index of current row
+
+  // Info about array position for one array level in array stack
+  struct ArrayPosition {
+    const rapidjson::Value* array_node;
+    int64_t path_index;
+    rapidjson::SizeType index;
+  };
+  std::vector<ArrayPosition> array_stack;
+};
+
+class JsonValueConverter {
+ public:
+  explicit JsonValueConverter(const std::vector<rapidjson::Document>& rows)
+      : rows_(rows), array_levels_(0) {}
+
+  JsonValueConverter(const std::vector<rapidjson::Document>& rows,
+                     const std::vector<std::string>& root_path, int64_t array_levels)
+      : rows_(rows), root_path_(root_path), array_levels_(array_levels) {}
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, arrow::ArrayBuilder* builder) {
+    return Convert(field, field.name(), builder);
+  }
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, const std::string& field_name,
+                        arrow::ArrayBuilder* builder) {
+    field_name_ = field_name;
+    builder_ = builder;
+    ARROW_RETURN_NOT_OK(arrow::VisitTypeInline(*field.type().get(), this));
+    return arrow::Status::OK();
+  }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::DataType& type) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", type.ToString());
+  }
+
+  arrow::Status Visit(const arrow::Int64Type& type) {
+    arrow::Int64Builder* builder = static_cast<arrow::Int64Builder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        if (value->IsUint()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint()));
+        } else if (value->IsInt()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt()));
+        } else if (value->IsUint64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint64()));
+        } else if (value->IsInt64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt64()));
+        } else {
+          return arrow::Status::Invalid("Value is not an integer");
+        }
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::DoubleType& type) {
+    arrow::DoubleBuilder* builder = static_cast<arrow::DoubleBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetDouble()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringType& type) {
+    arrow::StringBuilder* builder = static_cast<arrow::StringBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetString()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::BooleanType& type) {
+    arrow::BooleanBuilder* builder = static_cast<arrow::BooleanBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetBool()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructType& type) {
+    arrow::StructBuilder* builder = static_cast<arrow::StructBuilder*>(builder_);
+
+    std::vector<std::string> child_path(root_path_);
+    if (field_name_.size() > 0) {
+      child_path.push_back(field_name_);
+    }
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_);
+
+    for (int i = 0; i < type.num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type.field(i);
+      std::shared_ptr<arrow::ArrayBuilder> child_builder = builder->child_builder(i);
+
+      ARROW_RETURN_NOT_OK(
+          child_converter.Convert(*child_field.get(), child_builder.get()));
+    }
+
+    // Make null bitmap
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+    }
+
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListType& type) {
+    arrow::ListBuilder* builder = static_cast<arrow::ListBuilder*>(builder_);
+
+    // Values and offsets needs to be interleaved in ListBuilder, so first collect the
+    // values
+    std::unique_ptr<arrow::ArrayBuilder> tmp_value_builder;
+    ARROW_ASSIGN_OR_RAISE(tmp_value_builder,
+                          arrow::MakeBuilder(builder->value_builder()->type()));
+    std::vector<std::string> child_path(root_path_);
+    child_path.push_back(field_name_);
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_ + 1);
+    ARROW_RETURN_NOT_OK(
+        child_converter.Convert(*type.value_field().get(), "", tmp_value_builder.get()));
+
+    std::shared_ptr<arrow::Array> values_array;
+    ARROW_RETURN_NOT_OK(tmp_value_builder->Finish(&values_array));
+    std::shared_ptr<arrow::ArrayData> values_data = values_array->data();
+
+    arrow::ArrayBuilder* value_builder = builder->value_builder();
+    int64_t offset = 0;
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+      if (!value->IsNull() && value->Size() > 0) {
+        ARROW_RETURN_NOT_OK(
+            value_builder->AppendArraySlice(*values_data.get(), offset, value->Size()));
+        offset += value->Size();
+      }
+    }
+
+    return arrow::Status::OK();
+  }
+
+ protected:
+  std::string field_name_;
+  arrow::ArrayBuilder* builder_;
+  const std::vector<rapidjson::Document>& rows_;
+  std::vector<std::string> root_path_;
+  int64_t array_levels_;
+
+  /// Return a flattened iterator over values at nested location
+  arrow::Iterator<const rapidjson::Value*> FieldValues() {
+    std::vector<std::string> path(root_path_);
+    if (field_name_.size() > 0) {
+      path.push_back(field_name_);
+    }
+    auto iter = DocValuesIterator(rows_, std::move(path), array_levels_);
+    auto fn = [iter]() mutable -> arrow::Result<const rapidjson::Value*> {
+      return iter.Next();
+    };
+
+    return arrow::MakeFunctionIterator(fn);
+  }
+};  // JsonValueConverter
+
+class DocumentToArrowConverter : public arrow::FromRowConverter<rapidjson::Document> {
+ public:
+  explicit DocumentToArrowConverter(arrow::MemoryPool* pool) : pool_(pool) {}
+
+  arrow::Result<std::shared_ptr<arrow::RecordBatch>> ConvertToRecordBatch(
+      const std::vector<rapidjson::Document>& rows,
+      std::shared_ptr<arrow::Schema> schema) override {
+    std::unique_ptr<arrow::RecordBatchBuilder> batch_builder;
+    ARROW_ASSIGN_OR_RAISE(batch_builder,
+                          arrow::RecordBatchBuilder::Make(schema, pool_, rows.size()));
+
+    JsonValueConverter converter(rows);
+    for (int i = 0; i < batch_builder->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> field = schema->field(i);
+      arrow::ArrayBuilder* builder = batch_builder->GetField(i);
+      ARROW_RETURN_NOT_OK(converter.Convert(*field.get(), builder));
+    }
+
+    std::shared_ptr<arrow::RecordBatch> batch;
+    ARROW_ASSIGN_OR_RAISE(batch, batch_builder->Flush());
+
+    DCHECK_OK(batch->ValidateFull());
+    return batch;
+  }
+
+ protected:
+  arrow::MemoryPool* pool_;
+};  // DocumentToArrowConverter
+
+int main(int argc, char** argv) {
+  // Get sizes
+  int32_t num_rows = argc > 1 ? std::atoi(argv[1]) : 100;
+  int32_t batch_size = argc > 2 ? std::atoi(argv[2]) : 100;
+
+  //(Doc section: Convert to Arrow)
+  // Write JSON records
+  std::vector<std::string> json_records = {
+      R"({"pk": 1, "date_created": "2020-10-01", "data": {"deleted": true, "metrics": [{"key": "x", "value": 1}]}})",
+      R"({"pk": 2, "date_created": "2020-10-03", "data": {"deleted": false, "metrics": []}})",
+      R"({"pk": 3, "date_created": "2020-10-05", "data": {"deleted": false, "metrics": [{"key": "x", "value": 33}, {"key": "x", "value": 42}]}})"};
+
+  std::vector<rapidjson::Document> records;
+  records.reserve(num_rows);
+  for (int32_t i = 0; i < num_rows; ++i) {
+    rapidjson::Document document;
+    document.Parse(json_records[i % json_records.size()].c_str());
+    records.push_back(std::move(document));
+  }
+  // for (const std::string& json : json_records) {
+  //   rapidjson::Document document;
+  //   document.Parse(json.c_str());
+  //   records.push_back(std::move(document));
+  // }
+  for (const rapidjson::Document& doc : records) {
+    rapidjson::StringBuffer sb;
+    rapidjson::Writer<rapidjson::StringBuffer> writer(sb);
+    // At batch_size >= 495, we segfault here:
+    doc.Accept(writer);
+    std::cout << sb.GetString() << std::endl;
+  }
+  auto tags_schema = arrow::list(arrow::struct_({
+      arrow::field("key", arrow::utf8()),
+      arrow::field("value", arrow::int64()),
+  }));
+  auto schema = arrow::schema(
+      {arrow::field("pk", arrow::int64()), arrow::field("date_created", arrow::utf8()),
+       arrow::field("data", arrow::struct_({arrow::field("deleted", arrow::boolean()),
+                                            arrow::field("metrics", tags_schema)}))});
+
+  // Create converter
+  DocumentToArrowConverter to_arrow_converter(arrow::default_memory_pool());
+
+  // Convert records into a table
+  arrow::Result<std::shared_ptr<arrow::Table>> table_result =
+      to_arrow_converter.ConvertToTable(records, schema);
+  std::shared_ptr<arrow::Table> table = std::move(table_result).ValueOrDie();

Review Comment:
   or, we could factor out a `Status Main()` that gets called from `int main`



##########
cpp/src/arrow/row_conversion.h:
##########
@@ -0,0 +1,192 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "arrow/record_batch.h"
+#include "arrow/table.h"
+#include "arrow/util/iterator.h"
+
+namespace arrow {
+
+/// \brief Base class for Arrow to row converter.
+///
+/// To use, specialize the conversion for a particular row type.
+///
+/// \tparam RowType type that contains a single row
+/// \tparam ContainerType optional alternative vector implementation
+///
+/// The derived conversion functions take a batch_size parameter that determines
+/// how many rows convert at once. This allows you to do the conversion in smaller
+/// batches.
+template <typename RowType, template <typename...> class ContainerType = std::vector>
+class ToRowConverter {
+ public:
+  /// \brief Convert a record batch into a vector of rows.
+  ///
+  /// Implement this method in specialized classes to derive all other methods.
+  ///
+  /// \param batch Record batch to convert
+  /// \return A vector of rows containing all data in batch
+  virtual Result<ContainerType<RowType>> ConvertToVector(
+      std::shared_ptr<RecordBatch> batch) = 0;
+
+  /// \brief Convert a record batch into a vector of rows.
+  ///
+  /// \param batch Record batch to convert
+  /// \param batch_size Number of rows to convert at once
+  /// \return A vector of rows containing all data in batch
+  Result<ContainerType<RowType>> ConvertToVector(std::shared_ptr<RecordBatch> batch,
+                                                 size_t batch_size) {
+    ARROW_ASSIGN_OR_RAISE(std::shared_ptr<Table> table,
+                          Table::FromRecordBatches({batch}));
+    return ConvertToVector(table, batch_size);

Review Comment:
   IMO, it's pointless to wrap in a table only to unwrap it right away



##########
cpp/src/arrow/row_conversion.h:
##########
@@ -0,0 +1,192 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "arrow/record_batch.h"
+#include "arrow/table.h"
+#include "arrow/util/iterator.h"
+
+namespace arrow {
+
+/// \brief Base class for Arrow to row converter.
+///
+/// To use, specialize the conversion for a particular row type.
+///
+/// \tparam RowType type that contains a single row
+/// \tparam ContainerType optional alternative vector implementation
+///
+/// The derived conversion functions take a batch_size parameter that determines
+/// how many rows convert at once. This allows you to do the conversion in smaller
+/// batches.
+template <typename RowType, template <typename...> class ContainerType = std::vector>
+class ToRowConverter {
+ public:
+  /// \brief Convert a record batch into a vector of rows.
+  ///
+  /// Implement this method in specialized classes to derive all other methods.
+  ///
+  /// \param batch Record batch to convert
+  /// \return A vector of rows containing all data in batch
+  virtual Result<ContainerType<RowType>> ConvertToVector(
+      std::shared_ptr<RecordBatch> batch) = 0;

Review Comment:
   Why not take `const RecordBatch&` or `const std::shared_ptr<RecordBatch>&`?



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();
+      auto array_len = array.value_length(i);
+
+      rapidjson::Value value;
+      value.SetArray();
+      value.Reserve(array_len, allocator);
+
+      for (int64_t j = 0; j < array_len; ++j) {
+        rapidjson::Value row_val;
+        // Must copy value to new allocator
+        row_val.CopyFrom(rows[values_i][value_field_name.c_str()], allocator);
+        value.PushBack(row_val, allocator);
+        ++values_i;
+      }
+
+      rapidjson::Value str_key(field_->name().c_str(), allocator);
+      rows_[i].AddMember(str_key, value, allocator);
+    }
+
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Field> field_;
+  std::vector<rapidjson::Document> rows_;
+};  // RowBatchBuilder
+
+class ArrowToDocumentConverter : public arrow::ToRowConverter<rapidjson::Document> {
+ public:
+  arrow::Result<std::vector<rapidjson::Document>> ConvertToVector(
+      std::shared_ptr<arrow::RecordBatch> batch) override {
+    RowBatchBuilder builder{batch->num_rows()};
+
+    for (int i = 0; i < batch->num_columns(); ++i) {
+      builder.SetField(std::move(batch->schema()->field(i)));

Review Comment:
   nit: I don't think this `std::move` does anything here



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();

Review Comment:
   nit: can this be `const&`?



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();
+      auto array_len = array.value_length(i);
+
+      rapidjson::Value value;
+      value.SetArray();
+      value.Reserve(array_len, allocator);
+
+      for (int64_t j = 0; j < array_len; ++j) {
+        rapidjson::Value row_val;
+        // Must copy value to new allocator
+        row_val.CopyFrom(rows[values_i][value_field_name.c_str()], allocator);
+        value.PushBack(row_val, allocator);
+        ++values_i;
+      }
+
+      rapidjson::Value str_key(field_->name().c_str(), allocator);
+      rows_[i].AddMember(str_key, value, allocator);
+    }
+
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Field> field_;
+  std::vector<rapidjson::Document> rows_;
+};  // RowBatchBuilder
+
+class ArrowToDocumentConverter : public arrow::ToRowConverter<rapidjson::Document> {
+ public:
+  arrow::Result<std::vector<rapidjson::Document>> ConvertToVector(
+      std::shared_ptr<arrow::RecordBatch> batch) override {
+    RowBatchBuilder builder{batch->num_rows()};
+
+    for (int i = 0; i < batch->num_columns(); ++i) {
+      builder.SetField(std::move(batch->schema()->field(i)));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*batch->column(i).get(), &builder));
+    }
+
+    return std::move(builder).Rows();
+  }
+};  // ArrowToDocumentConverter
+
+/// \brief Iterator over rows values of a document for a given field
+///
+/// path and array_levels are used to address each field in a JSON document. As
+/// an example, consider this JSON document:
+/// {
+///     "x": 3,                   // path: ["x"],             array_levels: 0
+///     "files": [                // path: ["files"],         array_levels: 0
+///         {                     // path: ["files"],         array_levels: 1
+///             "path": "my_str", // path: ["files", "path"], array_levels: 1
+///             "sizes": [        // path: ["files", "size"], array_levels: 1
+///                 20,           // path: ["files", "size"], array_levels: 2
+///                 22
+///             ]
+///         }
+///     ]
+/// },
+class DocValuesIterator {
+ public:
+  /// \param rows vector of rows
+  /// \param path field names to enter
+  /// \param array_levels number of arrays to enter
+  DocValuesIterator(const std::vector<rapidjson::Document>& rows,
+                    std::vector<std::string> path, int64_t array_levels)
+      : rows(rows), path(std::move(path)), array_levels(array_levels) {}
+
+  const rapidjson::Value* NextArrayOrRow(const rapidjson::Value* value, size_t* path_i,
+                                         int64_t* arr_i) {
+    while (array_stack.size() > 0) {
+      ArrayPosition& pos = array_stack.back();
+      // Try to get next position in Array
+      if (pos.index + 1 < pos.array_node->Size()) {
+        ++pos.index;
+        value = &(*pos.array_node)[pos.index];
+        *path_i = pos.path_index;
+        *arr_i = array_stack.size();
+        return value;
+      } else {
+        array_stack.pop_back();
+      }
+    }
+    ++row_i;
+    if (row_i < rows.size()) {
+      value = static_cast<const rapidjson::Value*>(&rows[row_i]);
+    } else {
+      value = nullptr;
+    }
+    *path_i = 0;
+    *arr_i = 0;
+    return value;
+  }
+
+  arrow::Result<const rapidjson::Value*> Next() {
+    const rapidjson::Value* value = nullptr;
+    size_t path_i;
+    int64_t arr_i;
+    // Can either start at document or at last array level
+    if (array_stack.size() > 0) {
+      auto pos = array_stack.back();
+      value = pos.array_node;
+      path_i = pos.path_index;
+      arr_i = array_stack.size() - 1;
+    }
+
+    value = NextArrayOrRow(value, &path_i, &arr_i);
+
+    // Traverse to desired level (with possible backtracking as needed)
+    while (path_i < path.size() || arr_i < array_levels) {
+      if (value == nullptr) {
+        return value;
+      } else if (value->IsArray() && value->Size() > 0) {
+        ArrayPosition pos;
+        pos.array_node = value;
+        pos.path_index = path_i;
+        pos.index = 0;
+        array_stack.push_back(pos);
+
+        value = &(*value)[0];
+        ++arr_i;
+      } else if (value->IsArray()) {
+        // Empty array means we need to backtrack and go to next array or row
+        value = NextArrayOrRow(value, &path_i, &arr_i);
+      } else if (value->HasMember(path[path_i].c_str())) {
+        value = &(*value)[path[path_i].c_str()];
+        ++path_i;
+      } else {
+        return &kNullJsonSingleton;
+      }
+    }
+
+    // Return value
+    return value;
+  }
+
+ protected:
+  const std::vector<rapidjson::Document>& rows;
+  std::vector<std::string> path;
+  int64_t array_levels;
+  size_t row_i = -1;  // index of current row
+
+  // Info about array position for one array level in array stack
+  struct ArrayPosition {
+    const rapidjson::Value* array_node;
+    int64_t path_index;
+    rapidjson::SizeType index;
+  };
+  std::vector<ArrayPosition> array_stack;
+};
+
+class JsonValueConverter {
+ public:
+  explicit JsonValueConverter(const std::vector<rapidjson::Document>& rows)
+      : rows_(rows), array_levels_(0) {}
+
+  JsonValueConverter(const std::vector<rapidjson::Document>& rows,
+                     const std::vector<std::string>& root_path, int64_t array_levels)
+      : rows_(rows), root_path_(root_path), array_levels_(array_levels) {}
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, arrow::ArrayBuilder* builder) {
+    return Convert(field, field.name(), builder);
+  }
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, const std::string& field_name,
+                        arrow::ArrayBuilder* builder) {
+    field_name_ = field_name;
+    builder_ = builder;
+    ARROW_RETURN_NOT_OK(arrow::VisitTypeInline(*field.type().get(), this));
+    return arrow::Status::OK();
+  }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::DataType& type) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", type.ToString());
+  }
+
+  arrow::Status Visit(const arrow::Int64Type& type) {
+    arrow::Int64Builder* builder = static_cast<arrow::Int64Builder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        if (value->IsUint()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint()));
+        } else if (value->IsInt()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt()));
+        } else if (value->IsUint64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint64()));
+        } else if (value->IsInt64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt64()));
+        } else {
+          return arrow::Status::Invalid("Value is not an integer");
+        }
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::DoubleType& type) {
+    arrow::DoubleBuilder* builder = static_cast<arrow::DoubleBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetDouble()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringType& type) {
+    arrow::StringBuilder* builder = static_cast<arrow::StringBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetString()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::BooleanType& type) {
+    arrow::BooleanBuilder* builder = static_cast<arrow::BooleanBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetBool()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructType& type) {
+    arrow::StructBuilder* builder = static_cast<arrow::StructBuilder*>(builder_);
+
+    std::vector<std::string> child_path(root_path_);
+    if (field_name_.size() > 0) {
+      child_path.push_back(field_name_);
+    }
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_);
+
+    for (int i = 0; i < type.num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type.field(i);
+      std::shared_ptr<arrow::ArrayBuilder> child_builder = builder->child_builder(i);
+
+      ARROW_RETURN_NOT_OK(
+          child_converter.Convert(*child_field.get(), child_builder.get()));
+    }
+
+    // Make null bitmap
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+    }
+
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListType& type) {
+    arrow::ListBuilder* builder = static_cast<arrow::ListBuilder*>(builder_);
+
+    // Values and offsets needs to be interleaved in ListBuilder, so first collect the
+    // values
+    std::unique_ptr<arrow::ArrayBuilder> tmp_value_builder;
+    ARROW_ASSIGN_OR_RAISE(tmp_value_builder,
+                          arrow::MakeBuilder(builder->value_builder()->type()));
+    std::vector<std::string> child_path(root_path_);
+    child_path.push_back(field_name_);
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_ + 1);
+    ARROW_RETURN_NOT_OK(
+        child_converter.Convert(*type.value_field().get(), "", tmp_value_builder.get()));
+
+    std::shared_ptr<arrow::Array> values_array;
+    ARROW_RETURN_NOT_OK(tmp_value_builder->Finish(&values_array));
+    std::shared_ptr<arrow::ArrayData> values_data = values_array->data();
+
+    arrow::ArrayBuilder* value_builder = builder->value_builder();
+    int64_t offset = 0;
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+      if (!value->IsNull() && value->Size() > 0) {
+        ARROW_RETURN_NOT_OK(
+            value_builder->AppendArraySlice(*values_data.get(), offset, value->Size()));
+        offset += value->Size();
+      }
+    }
+
+    return arrow::Status::OK();
+  }
+
+ protected:
+  std::string field_name_;
+  arrow::ArrayBuilder* builder_;
+  const std::vector<rapidjson::Document>& rows_;
+  std::vector<std::string> root_path_;
+  int64_t array_levels_;
+
+  /// Return a flattened iterator over values at nested location
+  arrow::Iterator<const rapidjson::Value*> FieldValues() {
+    std::vector<std::string> path(root_path_);
+    if (field_name_.size() > 0) {
+      path.push_back(field_name_);
+    }
+    auto iter = DocValuesIterator(rows_, std::move(path), array_levels_);
+    auto fn = [iter]() mutable -> arrow::Result<const rapidjson::Value*> {
+      return iter.Next();
+    };
+
+    return arrow::MakeFunctionIterator(fn);
+  }
+};  // JsonValueConverter
+
+class DocumentToArrowConverter : public arrow::FromRowConverter<rapidjson::Document> {
+ public:
+  explicit DocumentToArrowConverter(arrow::MemoryPool* pool) : pool_(pool) {}
+
+  arrow::Result<std::shared_ptr<arrow::RecordBatch>> ConvertToRecordBatch(
+      const std::vector<rapidjson::Document>& rows,
+      std::shared_ptr<arrow::Schema> schema) override {
+    std::unique_ptr<arrow::RecordBatchBuilder> batch_builder;
+    ARROW_ASSIGN_OR_RAISE(batch_builder,
+                          arrow::RecordBatchBuilder::Make(schema, pool_, rows.size()));
+
+    JsonValueConverter converter(rows);
+    for (int i = 0; i < batch_builder->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> field = schema->field(i);
+      arrow::ArrayBuilder* builder = batch_builder->GetField(i);
+      ARROW_RETURN_NOT_OK(converter.Convert(*field.get(), builder));
+    }
+
+    std::shared_ptr<arrow::RecordBatch> batch;
+    ARROW_ASSIGN_OR_RAISE(batch, batch_builder->Flush());
+
+    DCHECK_OK(batch->ValidateFull());
+    return batch;
+  }
+
+ protected:
+  arrow::MemoryPool* pool_;
+};  // DocumentToArrowConverter
+
+int main(int argc, char** argv) {
+  // Get sizes
+  int32_t num_rows = argc > 1 ? std::atoi(argv[1]) : 100;
+  int32_t batch_size = argc > 2 ? std::atoi(argv[2]) : 100;
+
+  //(Doc section: Convert to Arrow)
+  // Write JSON records
+  std::vector<std::string> json_records = {
+      R"({"pk": 1, "date_created": "2020-10-01", "data": {"deleted": true, "metrics": [{"key": "x", "value": 1}]}})",
+      R"({"pk": 2, "date_created": "2020-10-03", "data": {"deleted": false, "metrics": []}})",
+      R"({"pk": 3, "date_created": "2020-10-05", "data": {"deleted": false, "metrics": [{"key": "x", "value": 33}, {"key": "x", "value": 42}]}})"};
+
+  std::vector<rapidjson::Document> records;
+  records.reserve(num_rows);
+  for (int32_t i = 0; i < num_rows; ++i) {
+    rapidjson::Document document;
+    document.Parse(json_records[i % json_records.size()].c_str());
+    records.push_back(std::move(document));
+  }
+  // for (const std::string& json : json_records) {
+  //   rapidjson::Document document;
+  //   document.Parse(json.c_str());
+  //   records.push_back(std::move(document));
+  // }

Review Comment:
   nit: commented code?



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();
+      auto array_len = array.value_length(i);
+
+      rapidjson::Value value;
+      value.SetArray();
+      value.Reserve(array_len, allocator);
+
+      for (int64_t j = 0; j < array_len; ++j) {
+        rapidjson::Value row_val;
+        // Must copy value to new allocator
+        row_val.CopyFrom(rows[values_i][value_field_name.c_str()], allocator);
+        value.PushBack(row_val, allocator);
+        ++values_i;
+      }
+
+      rapidjson::Value str_key(field_->name().c_str(), allocator);
+      rows_[i].AddMember(str_key, value, allocator);
+    }
+
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Field> field_;
+  std::vector<rapidjson::Document> rows_;
+};  // RowBatchBuilder
+
+class ArrowToDocumentConverter : public arrow::ToRowConverter<rapidjson::Document> {
+ public:
+  arrow::Result<std::vector<rapidjson::Document>> ConvertToVector(
+      std::shared_ptr<arrow::RecordBatch> batch) override {
+    RowBatchBuilder builder{batch->num_rows()};
+
+    for (int i = 0; i < batch->num_columns(); ++i) {
+      builder.SetField(std::move(batch->schema()->field(i)));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*batch->column(i).get(), &builder));
+    }
+
+    return std::move(builder).Rows();
+  }
+};  // ArrowToDocumentConverter
+
+/// \brief Iterator over rows values of a document for a given field
+///
+/// path and array_levels are used to address each field in a JSON document. As
+/// an example, consider this JSON document:
+/// {
+///     "x": 3,                   // path: ["x"],             array_levels: 0
+///     "files": [                // path: ["files"],         array_levels: 0
+///         {                     // path: ["files"],         array_levels: 1
+///             "path": "my_str", // path: ["files", "path"], array_levels: 1
+///             "sizes": [        // path: ["files", "size"], array_levels: 1
+///                 20,           // path: ["files", "size"], array_levels: 2
+///                 22
+///             ]
+///         }
+///     ]
+/// },
+class DocValuesIterator {
+ public:
+  /// \param rows vector of rows
+  /// \param path field names to enter
+  /// \param array_levels number of arrays to enter
+  DocValuesIterator(const std::vector<rapidjson::Document>& rows,
+                    std::vector<std::string> path, int64_t array_levels)
+      : rows(rows), path(std::move(path)), array_levels(array_levels) {}
+
+  const rapidjson::Value* NextArrayOrRow(const rapidjson::Value* value, size_t* path_i,
+                                         int64_t* arr_i) {
+    while (array_stack.size() > 0) {
+      ArrayPosition& pos = array_stack.back();
+      // Try to get next position in Array
+      if (pos.index + 1 < pos.array_node->Size()) {
+        ++pos.index;
+        value = &(*pos.array_node)[pos.index];
+        *path_i = pos.path_index;
+        *arr_i = array_stack.size();
+        return value;
+      } else {
+        array_stack.pop_back();
+      }
+    }
+    ++row_i;
+    if (row_i < rows.size()) {
+      value = static_cast<const rapidjson::Value*>(&rows[row_i]);
+    } else {
+      value = nullptr;
+    }
+    *path_i = 0;
+    *arr_i = 0;
+    return value;
+  }
+
+  arrow::Result<const rapidjson::Value*> Next() {
+    const rapidjson::Value* value = nullptr;
+    size_t path_i;
+    int64_t arr_i;
+    // Can either start at document or at last array level
+    if (array_stack.size() > 0) {
+      auto pos = array_stack.back();
+      value = pos.array_node;
+      path_i = pos.path_index;
+      arr_i = array_stack.size() - 1;
+    }
+
+    value = NextArrayOrRow(value, &path_i, &arr_i);
+
+    // Traverse to desired level (with possible backtracking as needed)
+    while (path_i < path.size() || arr_i < array_levels) {
+      if (value == nullptr) {
+        return value;
+      } else if (value->IsArray() && value->Size() > 0) {
+        ArrayPosition pos;
+        pos.array_node = value;
+        pos.path_index = path_i;
+        pos.index = 0;
+        array_stack.push_back(pos);
+
+        value = &(*value)[0];
+        ++arr_i;
+      } else if (value->IsArray()) {
+        // Empty array means we need to backtrack and go to next array or row
+        value = NextArrayOrRow(value, &path_i, &arr_i);
+      } else if (value->HasMember(path[path_i].c_str())) {
+        value = &(*value)[path[path_i].c_str()];
+        ++path_i;
+      } else {
+        return &kNullJsonSingleton;
+      }
+    }
+
+    // Return value
+    return value;
+  }
+
+ protected:
+  const std::vector<rapidjson::Document>& rows;
+  std::vector<std::string> path;
+  int64_t array_levels;
+  size_t row_i = -1;  // index of current row
+
+  // Info about array position for one array level in array stack
+  struct ArrayPosition {
+    const rapidjson::Value* array_node;
+    int64_t path_index;
+    rapidjson::SizeType index;
+  };
+  std::vector<ArrayPosition> array_stack;
+};
+
+class JsonValueConverter {
+ public:
+  explicit JsonValueConverter(const std::vector<rapidjson::Document>& rows)
+      : rows_(rows), array_levels_(0) {}
+
+  JsonValueConverter(const std::vector<rapidjson::Document>& rows,
+                     const std::vector<std::string>& root_path, int64_t array_levels)
+      : rows_(rows), root_path_(root_path), array_levels_(array_levels) {}
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, arrow::ArrayBuilder* builder) {
+    return Convert(field, field.name(), builder);
+  }
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, const std::string& field_name,
+                        arrow::ArrayBuilder* builder) {
+    field_name_ = field_name;
+    builder_ = builder;
+    ARROW_RETURN_NOT_OK(arrow::VisitTypeInline(*field.type().get(), this));
+    return arrow::Status::OK();
+  }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::DataType& type) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", type.ToString());

Review Comment:
   Cannot convert to Arrow array?



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();
+      auto array_len = array.value_length(i);
+
+      rapidjson::Value value;
+      value.SetArray();
+      value.Reserve(array_len, allocator);
+
+      for (int64_t j = 0; j < array_len; ++j) {
+        rapidjson::Value row_val;
+        // Must copy value to new allocator
+        row_val.CopyFrom(rows[values_i][value_field_name.c_str()], allocator);
+        value.PushBack(row_val, allocator);
+        ++values_i;
+      }
+
+      rapidjson::Value str_key(field_->name().c_str(), allocator);
+      rows_[i].AddMember(str_key, value, allocator);
+    }
+
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Field> field_;
+  std::vector<rapidjson::Document> rows_;
+};  // RowBatchBuilder
+
+class ArrowToDocumentConverter : public arrow::ToRowConverter<rapidjson::Document> {
+ public:
+  arrow::Result<std::vector<rapidjson::Document>> ConvertToVector(
+      std::shared_ptr<arrow::RecordBatch> batch) override {
+    RowBatchBuilder builder{batch->num_rows()};
+
+    for (int i = 0; i < batch->num_columns(); ++i) {
+      builder.SetField(std::move(batch->schema()->field(i)));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*batch->column(i).get(), &builder));
+    }
+
+    return std::move(builder).Rows();
+  }
+};  // ArrowToDocumentConverter
+
+/// \brief Iterator over rows values of a document for a given field
+///
+/// path and array_levels are used to address each field in a JSON document. As
+/// an example, consider this JSON document:
+/// {
+///     "x": 3,                   // path: ["x"],             array_levels: 0
+///     "files": [                // path: ["files"],         array_levels: 0
+///         {                     // path: ["files"],         array_levels: 1
+///             "path": "my_str", // path: ["files", "path"], array_levels: 1
+///             "sizes": [        // path: ["files", "size"], array_levels: 1
+///                 20,           // path: ["files", "size"], array_levels: 2
+///                 22
+///             ]
+///         }
+///     ]
+/// },
+class DocValuesIterator {
+ public:
+  /// \param rows vector of rows
+  /// \param path field names to enter
+  /// \param array_levels number of arrays to enter
+  DocValuesIterator(const std::vector<rapidjson::Document>& rows,
+                    std::vector<std::string> path, int64_t array_levels)
+      : rows(rows), path(std::move(path)), array_levels(array_levels) {}
+
+  const rapidjson::Value* NextArrayOrRow(const rapidjson::Value* value, size_t* path_i,
+                                         int64_t* arr_i) {
+    while (array_stack.size() > 0) {
+      ArrayPosition& pos = array_stack.back();
+      // Try to get next position in Array
+      if (pos.index + 1 < pos.array_node->Size()) {
+        ++pos.index;
+        value = &(*pos.array_node)[pos.index];
+        *path_i = pos.path_index;
+        *arr_i = array_stack.size();
+        return value;
+      } else {
+        array_stack.pop_back();
+      }
+    }
+    ++row_i;
+    if (row_i < rows.size()) {
+      value = static_cast<const rapidjson::Value*>(&rows[row_i]);
+    } else {
+      value = nullptr;
+    }
+    *path_i = 0;
+    *arr_i = 0;
+    return value;
+  }
+
+  arrow::Result<const rapidjson::Value*> Next() {
+    const rapidjson::Value* value = nullptr;
+    size_t path_i;
+    int64_t arr_i;
+    // Can either start at document or at last array level
+    if (array_stack.size() > 0) {
+      auto pos = array_stack.back();
+      value = pos.array_node;
+      path_i = pos.path_index;
+      arr_i = array_stack.size() - 1;
+    }
+
+    value = NextArrayOrRow(value, &path_i, &arr_i);
+
+    // Traverse to desired level (with possible backtracking as needed)
+    while (path_i < path.size() || arr_i < array_levels) {
+      if (value == nullptr) {
+        return value;
+      } else if (value->IsArray() && value->Size() > 0) {
+        ArrayPosition pos;
+        pos.array_node = value;
+        pos.path_index = path_i;
+        pos.index = 0;
+        array_stack.push_back(pos);
+
+        value = &(*value)[0];
+        ++arr_i;
+      } else if (value->IsArray()) {
+        // Empty array means we need to backtrack and go to next array or row
+        value = NextArrayOrRow(value, &path_i, &arr_i);
+      } else if (value->HasMember(path[path_i].c_str())) {
+        value = &(*value)[path[path_i].c_str()];
+        ++path_i;
+      } else {
+        return &kNullJsonSingleton;
+      }
+    }
+
+    // Return value
+    return value;
+  }
+
+ protected:
+  const std::vector<rapidjson::Document>& rows;
+  std::vector<std::string> path;
+  int64_t array_levels;
+  size_t row_i = -1;  // index of current row
+
+  // Info about array position for one array level in array stack
+  struct ArrayPosition {
+    const rapidjson::Value* array_node;
+    int64_t path_index;
+    rapidjson::SizeType index;
+  };
+  std::vector<ArrayPosition> array_stack;
+};
+
+class JsonValueConverter {
+ public:
+  explicit JsonValueConverter(const std::vector<rapidjson::Document>& rows)
+      : rows_(rows), array_levels_(0) {}
+
+  JsonValueConverter(const std::vector<rapidjson::Document>& rows,
+                     const std::vector<std::string>& root_path, int64_t array_levels)
+      : rows_(rows), root_path_(root_path), array_levels_(array_levels) {}
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, arrow::ArrayBuilder* builder) {
+    return Convert(field, field.name(), builder);
+  }
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, const std::string& field_name,
+                        arrow::ArrayBuilder* builder) {
+    field_name_ = field_name;
+    builder_ = builder;
+    ARROW_RETURN_NOT_OK(arrow::VisitTypeInline(*field.type().get(), this));
+    return arrow::Status::OK();
+  }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::DataType& type) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", type.ToString());
+  }
+
+  arrow::Status Visit(const arrow::Int64Type& type) {
+    arrow::Int64Builder* builder = static_cast<arrow::Int64Builder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        if (value->IsUint()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint()));
+        } else if (value->IsInt()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt()));
+        } else if (value->IsUint64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint64()));
+        } else if (value->IsInt64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt64()));
+        } else {
+          return arrow::Status::Invalid("Value is not an integer");
+        }
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::DoubleType& type) {
+    arrow::DoubleBuilder* builder = static_cast<arrow::DoubleBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetDouble()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringType& type) {
+    arrow::StringBuilder* builder = static_cast<arrow::StringBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetString()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::BooleanType& type) {
+    arrow::BooleanBuilder* builder = static_cast<arrow::BooleanBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetBool()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructType& type) {
+    arrow::StructBuilder* builder = static_cast<arrow::StructBuilder*>(builder_);
+
+    std::vector<std::string> child_path(root_path_);
+    if (field_name_.size() > 0) {
+      child_path.push_back(field_name_);
+    }
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_);
+
+    for (int i = 0; i < type.num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type.field(i);
+      std::shared_ptr<arrow::ArrayBuilder> child_builder = builder->child_builder(i);
+
+      ARROW_RETURN_NOT_OK(
+          child_converter.Convert(*child_field.get(), child_builder.get()));
+    }
+
+    // Make null bitmap
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+    }
+
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListType& type) {
+    arrow::ListBuilder* builder = static_cast<arrow::ListBuilder*>(builder_);
+
+    // Values and offsets needs to be interleaved in ListBuilder, so first collect the
+    // values
+    std::unique_ptr<arrow::ArrayBuilder> tmp_value_builder;
+    ARROW_ASSIGN_OR_RAISE(tmp_value_builder,
+                          arrow::MakeBuilder(builder->value_builder()->type()));
+    std::vector<std::string> child_path(root_path_);
+    child_path.push_back(field_name_);
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_ + 1);
+    ARROW_RETURN_NOT_OK(
+        child_converter.Convert(*type.value_field().get(), "", tmp_value_builder.get()));
+
+    std::shared_ptr<arrow::Array> values_array;
+    ARROW_RETURN_NOT_OK(tmp_value_builder->Finish(&values_array));
+    std::shared_ptr<arrow::ArrayData> values_data = values_array->data();
+
+    arrow::ArrayBuilder* value_builder = builder->value_builder();
+    int64_t offset = 0;
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+      if (!value->IsNull() && value->Size() > 0) {
+        ARROW_RETURN_NOT_OK(
+            value_builder->AppendArraySlice(*values_data.get(), offset, value->Size()));
+        offset += value->Size();
+      }
+    }
+
+    return arrow::Status::OK();
+  }
+
+ protected:
+  std::string field_name_;
+  arrow::ArrayBuilder* builder_;
+  const std::vector<rapidjson::Document>& rows_;
+  std::vector<std::string> root_path_;
+  int64_t array_levels_;
+
+  /// Return a flattened iterator over values at nested location
+  arrow::Iterator<const rapidjson::Value*> FieldValues() {
+    std::vector<std::string> path(root_path_);
+    if (field_name_.size() > 0) {
+      path.push_back(field_name_);
+    }
+    auto iter = DocValuesIterator(rows_, std::move(path), array_levels_);
+    auto fn = [iter]() mutable -> arrow::Result<const rapidjson::Value*> {
+      return iter.Next();
+    };
+
+    return arrow::MakeFunctionIterator(fn);
+  }
+};  // JsonValueConverter
+
+class DocumentToArrowConverter : public arrow::FromRowConverter<rapidjson::Document> {
+ public:
+  explicit DocumentToArrowConverter(arrow::MemoryPool* pool) : pool_(pool) {}
+
+  arrow::Result<std::shared_ptr<arrow::RecordBatch>> ConvertToRecordBatch(
+      const std::vector<rapidjson::Document>& rows,
+      std::shared_ptr<arrow::Schema> schema) override {
+    std::unique_ptr<arrow::RecordBatchBuilder> batch_builder;
+    ARROW_ASSIGN_OR_RAISE(batch_builder,
+                          arrow::RecordBatchBuilder::Make(schema, pool_, rows.size()));
+
+    JsonValueConverter converter(rows);
+    for (int i = 0; i < batch_builder->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> field = schema->field(i);
+      arrow::ArrayBuilder* builder = batch_builder->GetField(i);
+      ARROW_RETURN_NOT_OK(converter.Convert(*field.get(), builder));
+    }
+
+    std::shared_ptr<arrow::RecordBatch> batch;
+    ARROW_ASSIGN_OR_RAISE(batch, batch_builder->Flush());
+
+    DCHECK_OK(batch->ValidateFull());
+    return batch;
+  }
+
+ protected:
+  arrow::MemoryPool* pool_;
+};  // DocumentToArrowConverter
+
+int main(int argc, char** argv) {
+  // Get sizes
+  int32_t num_rows = argc > 1 ? std::atoi(argv[1]) : 100;
+  int32_t batch_size = argc > 2 ? std::atoi(argv[2]) : 100;
+
+  //(Doc section: Convert to Arrow)
+  // Write JSON records
+  std::vector<std::string> json_records = {
+      R"({"pk": 1, "date_created": "2020-10-01", "data": {"deleted": true, "metrics": [{"key": "x", "value": 1}]}})",
+      R"({"pk": 2, "date_created": "2020-10-03", "data": {"deleted": false, "metrics": []}})",
+      R"({"pk": 3, "date_created": "2020-10-05", "data": {"deleted": false, "metrics": [{"key": "x", "value": 33}, {"key": "x", "value": 42}]}})"};
+
+  std::vector<rapidjson::Document> records;
+  records.reserve(num_rows);
+  for (int32_t i = 0; i < num_rows; ++i) {
+    rapidjson::Document document;
+    document.Parse(json_records[i % json_records.size()].c_str());
+    records.push_back(std::move(document));
+  }
+  // for (const std::string& json : json_records) {
+  //   rapidjson::Document document;
+  //   document.Parse(json.c_str());
+  //   records.push_back(std::move(document));
+  // }
+  for (const rapidjson::Document& doc : records) {
+    rapidjson::StringBuffer sb;
+    rapidjson::Writer<rapidjson::StringBuffer> writer(sb);
+    // At batch_size >= 495, we segfault here:
+    doc.Accept(writer);
+    std::cout << sb.GetString() << std::endl;
+  }
+  auto tags_schema = arrow::list(arrow::struct_({
+      arrow::field("key", arrow::utf8()),
+      arrow::field("value", arrow::int64()),
+  }));
+  auto schema = arrow::schema(
+      {arrow::field("pk", arrow::int64()), arrow::field("date_created", arrow::utf8()),
+       arrow::field("data", arrow::struct_({arrow::field("deleted", arrow::boolean()),
+                                            arrow::field("metrics", tags_schema)}))});
+
+  // Create converter
+  DocumentToArrowConverter to_arrow_converter(arrow::default_memory_pool());
+
+  // Convert records into a table
+  arrow::Result<std::shared_ptr<arrow::Table>> table_result =
+      to_arrow_converter.ConvertToTable(records, schema);
+  std::shared_ptr<arrow::Table> table = std::move(table_result).ValueOrDie();

Review Comment:
   in this case why not just `table = *to_arrow_converter.ConvertToTable();`?



##########
cpp/examples/arrow/rapidjson_row_converter.cc:
##########
@@ -0,0 +1,590 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include <arrow/api.h>
+#include <arrow/result.h>
+#include <arrow/table_builder.h>
+#include <arrow/type_traits.h>
+#include <arrow/util/iterator.h>
+#include <arrow/util/logging.h>
+#include <arrow/visit_array_inline.h>
+
+#include <rapidjson/document.h>
+#include <rapidjson/stringbuffer.h>
+#include <rapidjson/writer.h>
+
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+// Transforming dynamic row data into Arrow data
+// When building connectors to other data systems, it's common to receive data in
+// row-based structures. While the row_wise_conversion_example.cc shows how to
+// handle this conversion for fixed schemas, this example demonstrates how to
+// writer converters for arbitrary schemas.
+//
+// As an example, this conversion is between Arrow and rapidjson::Documents.
+//
+// We use the following helpers and patterns here:
+//  * arrow::ToRowConverter and arrow::FromRowConverter, which provide additional
+//    conversion methods given a basic converter
+//  * arrow::VisitArrayInline and arrow::VisitTypeInline for implementing a visitor
+//    pattern with Arrow to handle different array types
+//  * arrow::enable_if_primitive_ctype to create a template method that handles
+//  * conversion for Arrow types that have corresponding C types (bool, integer,
+//    float).
+
+rapidjson::Value kNullJsonSingleton = rapidjson::Value();
+
+/// \brief Builder that holds state for a single conversion.
+///
+/// Implements Visit() methods for each type of Arrow Array that set the values
+/// of the corresponding fields in each row.
+class RowBatchBuilder {
+ public:
+  explicit RowBatchBuilder(int64_t num_rows) : field_(nullptr) {
+    // Reserve all of the space required up-front to avoid unnecessary resizing
+    rows_.reserve(num_rows);
+
+    for (int64_t i = 0; i < num_rows; ++i) {
+      rows_.push_back(rapidjson::Document());
+      rows_[i].SetObject();
+    }
+  }
+
+  /// \brief Set which field to convert.
+  void SetField(std::shared_ptr<arrow::Field> field) { field_ = std::move(field); }
+
+  /// \brief Retrieve converted rows from builder.
+  std::vector<rapidjson::Document> Rows() && { return std::move(rows_); }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::Array& array) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", array.type()->ToString());
+  }
+
+  // Handles booleans, integers, floats
+  template <typename ArrayType, typename DataClass = typename ArrayType::TypeClass>
+  arrow::enable_if_primitive_ctype<DataClass, arrow::Status> Visit(
+      const ArrayType& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, array.Value(i), rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringArray& array) {
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // StringArray.Value returns a string view
+        auto value_view = array.Value(i);
+        rapidjson::Value value;
+        value.SetString(value_view.data(),
+                        static_cast<rapidjson::SizeType>(value_view.size()),
+                        rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, value, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructArray& array) {
+    const arrow::StructType* type = array.struct_type();
+
+    RowBatchBuilder child_builder(rows_.size());
+    for (int i = 0; i < type->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type->field(i);
+      child_builder.SetField(std::move(child_field));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*array.field(i).get(), &child_builder));
+    }
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    for (int64_t i = 0; i < array.length(); ++i) {
+      if (!array.IsNull(i)) {
+        rapidjson::Value str_key(field_->name().c_str(), rows_[i].GetAllocator());
+        // Must copy value to new allocator
+        rapidjson::Value row_val;
+        row_val.CopyFrom(rows[i], rows_[i].GetAllocator());
+        rows_[i].AddMember(str_key, row_val, rows_[i].GetAllocator());
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListArray& array) {
+    // First create rows from values
+    std::shared_ptr<arrow::Array> values = array.values();
+    RowBatchBuilder child_builder(values->length());
+    std::shared_ptr<arrow::Field> value_field = array.list_type()->value_field();
+    std::string value_field_name = value_field->name();
+    child_builder.SetField(value_field);
+    ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*values.get(), &child_builder));
+
+    std::vector<rapidjson::Document> rows = std::move(child_builder).Rows();
+
+    int64_t values_i = 0;
+    for (int64_t i = 0; i < array.length(); ++i) {
+      rapidjson::Document::AllocatorType& allocator = rows_[i].GetAllocator();
+      auto array_len = array.value_length(i);
+
+      rapidjson::Value value;
+      value.SetArray();
+      value.Reserve(array_len, allocator);
+
+      for (int64_t j = 0; j < array_len; ++j) {
+        rapidjson::Value row_val;
+        // Must copy value to new allocator
+        row_val.CopyFrom(rows[values_i][value_field_name.c_str()], allocator);
+        value.PushBack(row_val, allocator);
+        ++values_i;
+      }
+
+      rapidjson::Value str_key(field_->name().c_str(), allocator);
+      rows_[i].AddMember(str_key, value, allocator);
+    }
+
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Field> field_;
+  std::vector<rapidjson::Document> rows_;
+};  // RowBatchBuilder
+
+class ArrowToDocumentConverter : public arrow::ToRowConverter<rapidjson::Document> {
+ public:
+  arrow::Result<std::vector<rapidjson::Document>> ConvertToVector(
+      std::shared_ptr<arrow::RecordBatch> batch) override {
+    RowBatchBuilder builder{batch->num_rows()};
+
+    for (int i = 0; i < batch->num_columns(); ++i) {
+      builder.SetField(std::move(batch->schema()->field(i)));
+      ARROW_RETURN_NOT_OK(arrow::VisitArrayInline(*batch->column(i).get(), &builder));
+    }
+
+    return std::move(builder).Rows();
+  }
+};  // ArrowToDocumentConverter
+
+/// \brief Iterator over rows values of a document for a given field
+///
+/// path and array_levels are used to address each field in a JSON document. As
+/// an example, consider this JSON document:
+/// {
+///     "x": 3,                   // path: ["x"],             array_levels: 0
+///     "files": [                // path: ["files"],         array_levels: 0
+///         {                     // path: ["files"],         array_levels: 1
+///             "path": "my_str", // path: ["files", "path"], array_levels: 1
+///             "sizes": [        // path: ["files", "size"], array_levels: 1
+///                 20,           // path: ["files", "size"], array_levels: 2
+///                 22
+///             ]
+///         }
+///     ]
+/// },
+class DocValuesIterator {
+ public:
+  /// \param rows vector of rows
+  /// \param path field names to enter
+  /// \param array_levels number of arrays to enter
+  DocValuesIterator(const std::vector<rapidjson::Document>& rows,
+                    std::vector<std::string> path, int64_t array_levels)
+      : rows(rows), path(std::move(path)), array_levels(array_levels) {}
+
+  const rapidjson::Value* NextArrayOrRow(const rapidjson::Value* value, size_t* path_i,
+                                         int64_t* arr_i) {
+    while (array_stack.size() > 0) {
+      ArrayPosition& pos = array_stack.back();
+      // Try to get next position in Array
+      if (pos.index + 1 < pos.array_node->Size()) {
+        ++pos.index;
+        value = &(*pos.array_node)[pos.index];
+        *path_i = pos.path_index;
+        *arr_i = array_stack.size();
+        return value;
+      } else {
+        array_stack.pop_back();
+      }
+    }
+    ++row_i;
+    if (row_i < rows.size()) {
+      value = static_cast<const rapidjson::Value*>(&rows[row_i]);
+    } else {
+      value = nullptr;
+    }
+    *path_i = 0;
+    *arr_i = 0;
+    return value;
+  }
+
+  arrow::Result<const rapidjson::Value*> Next() {
+    const rapidjson::Value* value = nullptr;
+    size_t path_i;
+    int64_t arr_i;
+    // Can either start at document or at last array level
+    if (array_stack.size() > 0) {
+      auto pos = array_stack.back();
+      value = pos.array_node;
+      path_i = pos.path_index;
+      arr_i = array_stack.size() - 1;
+    }
+
+    value = NextArrayOrRow(value, &path_i, &arr_i);
+
+    // Traverse to desired level (with possible backtracking as needed)
+    while (path_i < path.size() || arr_i < array_levels) {
+      if (value == nullptr) {
+        return value;
+      } else if (value->IsArray() && value->Size() > 0) {
+        ArrayPosition pos;
+        pos.array_node = value;
+        pos.path_index = path_i;
+        pos.index = 0;
+        array_stack.push_back(pos);
+
+        value = &(*value)[0];
+        ++arr_i;
+      } else if (value->IsArray()) {
+        // Empty array means we need to backtrack and go to next array or row
+        value = NextArrayOrRow(value, &path_i, &arr_i);
+      } else if (value->HasMember(path[path_i].c_str())) {
+        value = &(*value)[path[path_i].c_str()];
+        ++path_i;
+      } else {
+        return &kNullJsonSingleton;
+      }
+    }
+
+    // Return value
+    return value;
+  }
+
+ protected:
+  const std::vector<rapidjson::Document>& rows;
+  std::vector<std::string> path;
+  int64_t array_levels;
+  size_t row_i = -1;  // index of current row
+
+  // Info about array position for one array level in array stack
+  struct ArrayPosition {
+    const rapidjson::Value* array_node;
+    int64_t path_index;
+    rapidjson::SizeType index;
+  };
+  std::vector<ArrayPosition> array_stack;
+};
+
+class JsonValueConverter {
+ public:
+  explicit JsonValueConverter(const std::vector<rapidjson::Document>& rows)
+      : rows_(rows), array_levels_(0) {}
+
+  JsonValueConverter(const std::vector<rapidjson::Document>& rows,
+                     const std::vector<std::string>& root_path, int64_t array_levels)
+      : rows_(rows), root_path_(root_path), array_levels_(array_levels) {}
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, arrow::ArrayBuilder* builder) {
+    return Convert(field, field.name(), builder);
+  }
+
+  /// \brief For field passed in, append corresponding values to builder
+  arrow::Status Convert(const arrow::Field& field, const std::string& field_name,
+                        arrow::ArrayBuilder* builder) {
+    field_name_ = field_name;
+    builder_ = builder;
+    ARROW_RETURN_NOT_OK(arrow::VisitTypeInline(*field.type().get(), this));
+    return arrow::Status::OK();
+  }
+
+  // Default implementation
+  arrow::Status Visit(const arrow::DataType& type) {
+    return arrow::Status::NotImplemented(
+        "Can not convert to json document for array of type ", type.ToString());
+  }
+
+  arrow::Status Visit(const arrow::Int64Type& type) {
+    arrow::Int64Builder* builder = static_cast<arrow::Int64Builder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        if (value->IsUint()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint()));
+        } else if (value->IsInt()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt()));
+        } else if (value->IsUint64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetUint64()));
+        } else if (value->IsInt64()) {
+          ARROW_RETURN_NOT_OK(builder->Append(value->GetInt64()));
+        } else {
+          return arrow::Status::Invalid("Value is not an integer");
+        }
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::DoubleType& type) {
+    arrow::DoubleBuilder* builder = static_cast<arrow::DoubleBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetDouble()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StringType& type) {
+    arrow::StringBuilder* builder = static_cast<arrow::StringBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetString()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::BooleanType& type) {
+    arrow::BooleanBuilder* builder = static_cast<arrow::BooleanBuilder*>(builder_);
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      if (value->IsNull()) {
+        ARROW_RETURN_NOT_OK(builder->AppendNull());
+      } else {
+        ARROW_RETURN_NOT_OK(builder->Append(value->GetBool()));
+      }
+    }
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::StructType& type) {
+    arrow::StructBuilder* builder = static_cast<arrow::StructBuilder*>(builder_);
+
+    std::vector<std::string> child_path(root_path_);
+    if (field_name_.size() > 0) {
+      child_path.push_back(field_name_);
+    }
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_);
+
+    for (int i = 0; i < type.num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> child_field = type.field(i);
+      std::shared_ptr<arrow::ArrayBuilder> child_builder = builder->child_builder(i);
+
+      ARROW_RETURN_NOT_OK(
+          child_converter.Convert(*child_field.get(), child_builder.get()));
+    }
+
+    // Make null bitmap
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+    }
+
+    return arrow::Status::OK();
+  }
+
+  arrow::Status Visit(const arrow::ListType& type) {
+    arrow::ListBuilder* builder = static_cast<arrow::ListBuilder*>(builder_);
+
+    // Values and offsets needs to be interleaved in ListBuilder, so first collect the
+    // values
+    std::unique_ptr<arrow::ArrayBuilder> tmp_value_builder;
+    ARROW_ASSIGN_OR_RAISE(tmp_value_builder,
+                          arrow::MakeBuilder(builder->value_builder()->type()));
+    std::vector<std::string> child_path(root_path_);
+    child_path.push_back(field_name_);
+    auto child_converter = JsonValueConverter(rows_, child_path, array_levels_ + 1);
+    ARROW_RETURN_NOT_OK(
+        child_converter.Convert(*type.value_field().get(), "", tmp_value_builder.get()));
+
+    std::shared_ptr<arrow::Array> values_array;
+    ARROW_RETURN_NOT_OK(tmp_value_builder->Finish(&values_array));
+    std::shared_ptr<arrow::ArrayData> values_data = values_array->data();
+
+    arrow::ArrayBuilder* value_builder = builder->value_builder();
+    int64_t offset = 0;
+    for (const auto& maybe_value : FieldValues()) {
+      ARROW_ASSIGN_OR_RAISE(auto value, maybe_value);
+      ARROW_RETURN_NOT_OK(builder->Append(!value->IsNull()));
+      if (!value->IsNull() && value->Size() > 0) {
+        ARROW_RETURN_NOT_OK(
+            value_builder->AppendArraySlice(*values_data.get(), offset, value->Size()));
+        offset += value->Size();
+      }
+    }
+
+    return arrow::Status::OK();
+  }
+
+ protected:
+  std::string field_name_;
+  arrow::ArrayBuilder* builder_;
+  const std::vector<rapidjson::Document>& rows_;
+  std::vector<std::string> root_path_;
+  int64_t array_levels_;
+
+  /// Return a flattened iterator over values at nested location
+  arrow::Iterator<const rapidjson::Value*> FieldValues() {
+    std::vector<std::string> path(root_path_);
+    if (field_name_.size() > 0) {
+      path.push_back(field_name_);
+    }
+    auto iter = DocValuesIterator(rows_, std::move(path), array_levels_);
+    auto fn = [iter]() mutable -> arrow::Result<const rapidjson::Value*> {
+      return iter.Next();
+    };
+
+    return arrow::MakeFunctionIterator(fn);
+  }
+};  // JsonValueConverter
+
+class DocumentToArrowConverter : public arrow::FromRowConverter<rapidjson::Document> {
+ public:
+  explicit DocumentToArrowConverter(arrow::MemoryPool* pool) : pool_(pool) {}
+
+  arrow::Result<std::shared_ptr<arrow::RecordBatch>> ConvertToRecordBatch(
+      const std::vector<rapidjson::Document>& rows,
+      std::shared_ptr<arrow::Schema> schema) override {
+    std::unique_ptr<arrow::RecordBatchBuilder> batch_builder;
+    ARROW_ASSIGN_OR_RAISE(batch_builder,
+                          arrow::RecordBatchBuilder::Make(schema, pool_, rows.size()));
+
+    JsonValueConverter converter(rows);
+    for (int i = 0; i < batch_builder->num_fields(); ++i) {
+      std::shared_ptr<arrow::Field> field = schema->field(i);
+      arrow::ArrayBuilder* builder = batch_builder->GetField(i);
+      ARROW_RETURN_NOT_OK(converter.Convert(*field.get(), builder));
+    }
+
+    std::shared_ptr<arrow::RecordBatch> batch;
+    ARROW_ASSIGN_OR_RAISE(batch, batch_builder->Flush());
+
+    DCHECK_OK(batch->ValidateFull());
+    return batch;
+  }
+
+ protected:
+  arrow::MemoryPool* pool_;
+};  // DocumentToArrowConverter
+
+int main(int argc, char** argv) {
+  // Get sizes
+  int32_t num_rows = argc > 1 ? std::atoi(argv[1]) : 100;
+  int32_t batch_size = argc > 2 ? std::atoi(argv[2]) : 100;
+
+  //(Doc section: Convert to Arrow)
+  // Write JSON records
+  std::vector<std::string> json_records = {
+      R"({"pk": 1, "date_created": "2020-10-01", "data": {"deleted": true, "metrics": [{"key": "x", "value": 1}]}})",
+      R"({"pk": 2, "date_created": "2020-10-03", "data": {"deleted": false, "metrics": []}})",
+      R"({"pk": 3, "date_created": "2020-10-05", "data": {"deleted": false, "metrics": [{"key": "x", "value": 33}, {"key": "x", "value": 42}]}})"};
+
+  std::vector<rapidjson::Document> records;
+  records.reserve(num_rows);
+  for (int32_t i = 0; i < num_rows; ++i) {
+    rapidjson::Document document;
+    document.Parse(json_records[i % json_records.size()].c_str());
+    records.push_back(std::move(document));
+  }
+  // for (const std::string& json : json_records) {
+  //   rapidjson::Document document;
+  //   document.Parse(json.c_str());
+  //   records.push_back(std::move(document));
+  // }
+  for (const rapidjson::Document& doc : records) {
+    rapidjson::StringBuffer sb;
+    rapidjson::Writer<rapidjson::StringBuffer> writer(sb);
+    // At batch_size >= 495, we segfault here:

Review Comment:
   why? 



##########
cpp/examples/arrow/CMakeLists.txt:
##########
@@ -17,6 +17,8 @@
 
 add_arrow_example(row_wise_conversion_example)
 
+add_arrow_example(rapidjson_row_converter)

Review Comment:
   I'm also surprised you don't have to link to rapidjson explicitly…



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