felipecrv commented on code in PR #41956:
URL: https://github.com/apache/arrow/pull/41956#discussion_r1626761756


##########
cpp/src/arrow/array/array_nested.cc:
##########
@@ -847,14 +847,13 @@ Result<std::shared_ptr<Array>> 
MapArray::FromArraysInternal(
   const auto& typed_offsets = checked_cast<const OffsetArrayType&>(*offsets);
 
   BufferVector buffers;
-  int64_t null_count;
-  if (null_bitmap != nullptr) {
-    buffers = BufferVector({std::move(null_bitmap), typed_offsets.values()});
-    null_count = null_bitmap->size();
-  } else {
-    buffers = BufferVector({null_bitmap, typed_offsets.values()});
-    null_count = 0;
+  buffers.resize(2);
+  int64_t null_count = 0;
+  if (null_bitmap) {
+    buffers[0] = std::move(null_bitmap);

Review Comment:
   Change `const std::shared_ptr<Buffer>& null_bitmap` to 
`std::shared_ptr<Buffer> null_bitmap` so the `std::move` is effective. That 
would mean callers that pass null_bitmap as an `&&` don't cause a single 
reference increment on the atomic refcount integer of the shared_ptr.
   
   (I know this is not the style in the other `std::shared_ptr` parameters but 
is the direction we want to go with parameters in constructors of factory 
functions in general).



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