[GitHub] [arrow] bkietz commented on a change in pull request #8806: ARROW-10776: [C++] Allow STL iteration over concrete primitive arrays

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

bkietz commented on a change in pull request #8806:
URL: https://github.com/apache/arrow/pull/8806#discussion_r539405203



##########
File path: cpp/src/arrow/stl_iterator_test.cc
##########
@@ -0,0 +1,263 @@
+// 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 <algorithm>
+#include <cstdint>
+
+#include <gtest/gtest.h>
+
+#include "arrow/stl.h"
+#include "arrow/stl_iterator.h"
+#include "arrow/testing/gtest_util.h"
+#include "arrow/type.h"
+#include "arrow/util/checked_cast.h"
+
+namespace arrow {
+
+using internal::checked_cast;
+using internal::checked_pointer_cast;
+using util::nullopt;
+using util::optional;
+
+namespace stl {
+
+template <typename T>
+void AssertOptionalEquals(optional<T> value, T expected) {
+  ASSERT_TRUE(value.has_value());
+  ASSERT_EQ(*value, expected);
+}
+
+template <typename T>
+void AssertOptionalEmpty(optional<T> value) {
+  ASSERT_FALSE(value.has_value());
+}
+
+TEST(ArrayIterator, Basics) {
+  auto array =
+      checked_pointer_cast<Int32Array>(ArrayFromJSON(int32(), "[4, 5, null, 6]"));
+
+  ArrayIterator<Int32Array> it(*array);
+  optional<int32_t> v = *it;
+  AssertOptionalEquals(v, 4);
+  AssertOptionalEquals(it[0], 4);
+  ++it;
+  AssertOptionalEquals(it[0], 5);
+  AssertOptionalEquals(*it, 5);
+  AssertOptionalEmpty(it[1]);

Review comment:
       Since this will be used in testing we'll need to reiterate AssertOptionalEquals and AssertOptionalEmpty everywhere, which is a pain. Instead, please use:
   ```suggestion
     EXPECT_EQ(*it, 5);
     EXPECT_EQ(it[1], nullopt);
   ```
   
   And add `void PrintTo(const optional<T>&, std::ostream*)`. This will make equality checks easier to write and will also allow gtest to format containers of optional usefully. I think we'd even be able to write:
   
   ```
   EXPECT_THAT(*array, ElementsAre(1, 2, nullopt, 4));
   ``` 

##########
File path: cpp/src/arrow/stl_iterator.h
##########
@@ -0,0 +1,143 @@
+// 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 <cstddef>
+#include <iterator>
+#include <utility>
+
+#include "arrow/type_fwd.h"
+#include "arrow/util/macros.h"
+#include "arrow/util/optional.h"
+
+namespace arrow {
+namespace stl {
+
+namespace detail {
+
+template <typename ArrayType>
+struct DefaultValueAccessor {
+  using ValueType = decltype(std::declval<ArrayType>().GetView(0));
+
+  ValueType operator()(const ArrayType& array, int64_t index) {

Review comment:
       ```suggestion
     using ViewType = decltype(std::declval<ArrayType>().GetView(0));
   
     ViewType operator()(const ArrayType& array, int64_t index) {
   ```

##########
File path: cpp/src/arrow/stl_iterator.h
##########
@@ -0,0 +1,143 @@
+// 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 <cstddef>
+#include <iterator>
+#include <utility>
+
+#include "arrow/type_fwd.h"
+#include "arrow/util/macros.h"
+#include "arrow/util/optional.h"
+
+namespace arrow {
+namespace stl {
+
+namespace detail {
+
+template <typename ArrayType>
+struct DefaultValueAccessor {
+  using ValueType = decltype(std::declval<ArrayType>().GetView(0));
+
+  ValueType operator()(const ArrayType& array, int64_t index) {
+    return array.GetView(index);
+  }
+};
+
+}  // namespace detail
+
+template <typename ArrayType,
+          typename ValueAccessor = detail::DefaultValueAccessor<ArrayType>>
+class ArrayIterator {
+ public:
+  using value_type =
+      arrow::util::optional<decltype(ValueAccessor()(std::declval<ArrayType>(), 0))>;

Review comment:
       ```suggestion
         arrow::util::optional<typename ValueAccessor::ViewType>;
   ```




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