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diff --git a/src/arrow/cpp/src/arrow/array/builder_union.h b/src/arrow/cpp/src/arrow/array/builder_union.h
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+++ b/src/arrow/cpp/src/arrow/array/builder_union.h
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+// 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.
+
+#pragma once
+
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "arrow/array/array_nested.h"
+#include "arrow/array/builder_base.h"
+#include "arrow/array/data.h"
+#include "arrow/buffer_builder.h"
+#include "arrow/memory_pool.h"
+#include "arrow/status.h"
+#include "arrow/type.h"
+#include "arrow/util/visibility.h"
+
+namespace arrow {
+
+/// \brief Base class for union array builds.
+///
+/// Note that while we subclass ArrayBuilder, as union types do not have a
+/// validity bitmap, the bitmap builder member of ArrayBuilder is not used.
+class ARROW_EXPORT BasicUnionBuilder : public ArrayBuilder {
+ public:
+  Status FinishInternal(std::shared_ptr<ArrayData>* out) override;
+
+  /// \cond FALSE
+  using ArrayBuilder::Finish;
+  /// \endcond
+
+  Status Finish(std::shared_ptr<UnionArray>* out) { return FinishTyped(out); }
+
+  /// \brief Make a new child builder available to the UnionArray
+  ///
+  /// \param[in] new_child the child builder
+  /// \param[in] field_name the name of the field in the union array type
+  /// if type inference is used
+  /// \return child index, which is the "type" argument that needs
+  /// to be passed to the "Append" method to add a new element to
+  /// the union array.
+  int8_t AppendChild(const std::shared_ptr<ArrayBuilder>& new_child,
+                     const std::string& field_name = "");
+
+  std::shared_ptr<DataType> type() const override;
+
+  int64_t length() const override { return types_builder_.length(); }
+
+ protected:
+  BasicUnionBuilder(MemoryPool* pool,
+                    const std::vector<std::shared_ptr<ArrayBuilder>>& children,
+                    const std::shared_ptr<DataType>& type);
+
+  int8_t NextTypeId();
+
+  std::vector<std::shared_ptr<Field>> child_fields_;
+  std::vector<int8_t> type_codes_;
+  UnionMode::type mode_;
+
+  std::vector<ArrayBuilder*> type_id_to_children_;
+  std::vector<int> type_id_to_child_id_;
+  // for all type_id < dense_type_id_, type_id_to_children_[type_id] != nullptr
+  int8_t dense_type_id_ = 0;
+  TypedBufferBuilder<int8_t> types_builder_;
+};
+
+/// \class DenseUnionBuilder
+///
+/// This API is EXPERIMENTAL.
+class ARROW_EXPORT DenseUnionBuilder : public BasicUnionBuilder {
+ public:
+  /// Use this constructor to initialize the UnionBuilder with no child builders,
+  /// allowing type to be inferred. You will need to call AppendChild for each of the
+  /// children builders you want to use.
+  explicit DenseUnionBuilder(MemoryPool* pool)
+      : BasicUnionBuilder(pool, {}, dense_union(FieldVector{})), offsets_builder_(pool) {}
+
+  /// Use this constructor to specify the type explicitly.
+  /// You can still add child builders to the union after using this constructor
+  DenseUnionBuilder(MemoryPool* pool,
+                    const std::vector<std::shared_ptr<ArrayBuilder>>& children,
+                    const std::shared_ptr<DataType>& type)
+      : BasicUnionBuilder(pool, children, type), offsets_builder_(pool) {}
+
+  Status AppendNull() final {
+    const int8_t first_child_code = type_codes_[0];
+    ArrayBuilder* child_builder = type_id_to_children_[first_child_code];
+    ARROW_RETURN_NOT_OK(types_builder_.Append(first_child_code));
+    ARROW_RETURN_NOT_OK(
+        offsets_builder_.Append(static_cast<int32_t>(child_builder->length())));
+    // Append a null arbitrarily to the first child
+    return child_builder->AppendNull();
+  }
+
+  Status AppendNulls(int64_t length) final {
+    const int8_t first_child_code = type_codes_[0];
+    ArrayBuilder* child_builder = type_id_to_children_[first_child_code];
+    ARROW_RETURN_NOT_OK(types_builder_.Append(length, first_child_code));
+    ARROW_RETURN_NOT_OK(
+        offsets_builder_.Append(length, static_cast<int32_t>(child_builder->length())));
+    // Append just a single null to the first child
+    return child_builder->AppendNull();
+  }
+
+  Status AppendEmptyValue() final {
+    const int8_t first_child_code = type_codes_[0];
+    ArrayBuilder* child_builder = type_id_to_children_[first_child_code];
+    ARROW_RETURN_NOT_OK(types_builder_.Append(first_child_code));
+    ARROW_RETURN_NOT_OK(
+        offsets_builder_.Append(static_cast<int32_t>(child_builder->length())));
+    // Append an empty value arbitrarily to the first child
+    return child_builder->AppendEmptyValue();
+  }
+
+  Status AppendEmptyValues(int64_t length) final {
+    const int8_t first_child_code = type_codes_[0];
+    ArrayBuilder* child_builder = type_id_to_children_[first_child_code];
+    ARROW_RETURN_NOT_OK(types_builder_.Append(length, first_child_code));
+    ARROW_RETURN_NOT_OK(
+        offsets_builder_.Append(length, static_cast<int32_t>(child_builder->length())));
+    // Append just a single empty value to the first child
+    return child_builder->AppendEmptyValue();
+  }
+
+  /// \brief Append an element to the UnionArray. This must be followed
+  ///        by an append to the appropriate child builder.
+  ///
+  /// \param[in] next_type type_id of the child to which the next value will be appended.
+  ///
+  /// The corresponding child builder must be appended to independently after this method
+  /// is called.
+  Status Append(int8_t next_type) {
+    ARROW_RETURN_NOT_OK(types_builder_.Append(next_type));
+    if (type_id_to_children_[next_type]->length() == kListMaximumElements) {
+      return Status::CapacityError(
+          "a dense UnionArray cannot contain more than 2^31 - 1 elements from a single "
+          "child");
+    }
+    auto offset = static_cast<int32_t>(type_id_to_children_[next_type]->length());
+    return offsets_builder_.Append(offset);
+  }
+
+  Status AppendArraySlice(const ArrayData& array, int64_t offset,
+                          int64_t length) override;
+
+  Status FinishInternal(std::shared_ptr<ArrayData>* out) override;
+
+ private:
+  TypedBufferBuilder<int32_t> offsets_builder_;
+};
+
+/// \class SparseUnionBuilder
+///
+/// This API is EXPERIMENTAL.
+class ARROW_EXPORT SparseUnionBuilder : public BasicUnionBuilder {
+ public:
+  /// Use this constructor to initialize the UnionBuilder with no child builders,
+  /// allowing type to be inferred. You will need to call AppendChild for each of the
+  /// children builders you want to use.
+  explicit SparseUnionBuilder(MemoryPool* pool)
+      : BasicUnionBuilder(pool, {}, sparse_union(FieldVector{})) {}
+
+  /// Use this constructor to specify the type explicitly.
+  /// You can still add child builders to the union after using this constructor
+  SparseUnionBuilder(MemoryPool* pool,
+                     const std::vector<std::shared_ptr<ArrayBuilder>>& children,
+                     const std::shared_ptr<DataType>& type)
+      : BasicUnionBuilder(pool, children, type) {}
+
+  /// \brief Append a null value.
+  ///
+  /// A null is appended to the first child, empty values to the other children.
+  Status AppendNull() final {
+    const auto first_child_code = type_codes_[0];
+    ARROW_RETURN_NOT_OK(types_builder_.Append(first_child_code));
+    ARROW_RETURN_NOT_OK(type_id_to_children_[first_child_code]->AppendNull());
+    for (int i = 1; i < static_cast<int>(type_codes_.size()); ++i) {
+      ARROW_RETURN_NOT_OK(type_id_to_children_[type_codes_[i]]->AppendEmptyValue());
+    }
+    return Status::OK();
+  }
+
+  /// \brief Append multiple null values.
+  ///
+  /// Nulls are appended to the first child, empty values to the other children.
+  Status AppendNulls(int64_t length) final {
+    const auto first_child_code = type_codes_[0];
+    ARROW_RETURN_NOT_OK(types_builder_.Append(length, first_child_code));
+    ARROW_RETURN_NOT_OK(type_id_to_children_[first_child_code]->AppendNulls(length));
+    for (int i = 1; i < static_cast<int>(type_codes_.size()); ++i) {
+      ARROW_RETURN_NOT_OK(
+          type_id_to_children_[type_codes_[i]]->AppendEmptyValues(length));
+    }
+    return Status::OK();
+  }
+
+  Status AppendEmptyValue() final {
+    ARROW_RETURN_NOT_OK(types_builder_.Append(type_codes_[0]));
+    for (int8_t code : type_codes_) {
+      ARROW_RETURN_NOT_OK(type_id_to_children_[code]->AppendEmptyValue());
+    }
+    return Status::OK();
+  }
+
+  Status AppendEmptyValues(int64_t length) final {
+    ARROW_RETURN_NOT_OK(types_builder_.Append(length, type_codes_[0]));
+    for (int8_t code : type_codes_) {
+      ARROW_RETURN_NOT_OK(type_id_to_children_[code]->AppendEmptyValues(length));
+    }
+    return Status::OK();
+  }
+
+  /// \brief Append an element to the UnionArray. This must be followed
+  ///        by an append to the appropriate child builder.
+  ///
+  /// \param[in] next_type type_id of the child to which the next value will be appended.
+  ///
+  /// The corresponding child builder must be appended to independently after this method
+  /// is called, and all other child builders must have null or empty value appended.
+  Status Append(int8_t next_type) { return types_builder_.Append(next_type); }
+
+  Status AppendArraySlice(const ArrayData& array, int64_t offset,
+                          int64_t length) override;
+};
+
+}  // namespace arrow