Apply suggestions from code review

Co-authored-by: Sutou Kouhei <[email protected]>

Author: Arkur745

cpp/src/arrow/array/array_struct_test.cc

@@ -751,7 +751,7 @@ TEST(TestFieldRef, GetChildren) {
 TEST(TestStructBuilderUnsafe, UnsafeAppend) {
   auto int_type = int32();
   auto str_type = utf8();
-  auto struct_type = struct_({field("a", int_type), field("b", str_type)});
+  auto struct_type = struct_({field("int", int_type), field("str", str_type)});
   auto pool = default_memory_pool();
   std::shared_ptr<Array> final_array;
   auto int_builder = std::make_shared<Int32Builder>(pool);
@@ -768,13 +768,13 @@ TEST(TestStructBuilderUnsafe, UnsafeAppend) {
   ASSERT_OK(int_builder->Append(2));
   ASSERT_OK(str_builder->Append("arrow"));
 
-  ASSERT_OK(builder.Finish(&final_array));
+  ASSERT_OK_AND_ASSIGN(auto final_array, builder.Finish());
   ASSERT_EQ(2, final_array->length());
   ASSERT_EQ(0, final_array->null_count());
-  auto expected_json = R"([{"a": 1, "b": "hello"}, {"a": 2, "b": "arrow"}])";
 
+  auto expected_json = R"([{"a": 1, "b": "hello"}, {"a": 2, "b": "arrow"}])";
   auto expected_array = ArrayFromJSON(struct_type, expected_json);
-  AssertArraysEqual(*final_array, *expected_array);
+  AssertArraysEqual(*expected_array, *final_array);
 }
 
 TEST(TestStructBuilderUnsafe, UnsafeAppendNull) {

cpp/src/arrow/array/builder_nested.h

@@ -814,13 +814,10 @@ class ARROW_EXPORT StructBuilder : public ArrayBuilder {
   /// builders to maintain a consistent state.
   /// param length The number of null slots to append.
   Status UnsafeAppendNulls(int64_t length) {
-    for (int64_t i = 0; i < length; ++i) {
-      UnsafeAppend(false);
-    }
-
     for (const auto& child : children_) {
       ARROW_RETURN_NOT_OK(child->AppendEmptyValues(length));
     }
+    UnsafeAppendToBitmap(length, false);
     return Status::OK();
   }