felipecrv commented on code in PR #41956:
URL: https://github.com/apache/arrow/pull/41956#discussion_r1637316138
##########
cpp/src/arrow/array/array_nested.cc:
##########
@@ -847,30 +847,32 @@ 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);
+ null_count = kUnknownNullCount;
}
+ buffers[1] = typed_offsets.values();
return std::make_shared<MapArray>(type, offsets->length() - 1,
std::move(buffers), keys,
items, /*null_count=*/null_count,
offsets->offset());
}
-Result<std::shared_ptr<Array>> MapArray::FromArrays(
- const std::shared_ptr<Array>& offsets, const std::shared_ptr<Array>& keys,
- const std::shared_ptr<Array>& items, MemoryPool* pool,
- const std::shared_ptr<Buffer>& null_bitmap) {
+Result<std::shared_ptr<Array>> MapArray::FromArrays(const
std::shared_ptr<Array>& offsets,
+ const
std::shared_ptr<Array>& keys,
+ const
std::shared_ptr<Array>& items,
+ MemoryPool* pool,
+ std::shared_ptr<Buffer>
null_bitmap) {
return FromArraysInternal(std::make_shared<MapType>(keys->type(),
items->type()),
offsets, keys, items, pool, null_bitmap);
}
-Result<std::shared_ptr<Array>> MapArray::FromArrays(
- std::shared_ptr<DataType> type, const std::shared_ptr<Array>& offsets,
- const std::shared_ptr<Array>& keys, const std::shared_ptr<Array>& items,
- MemoryPool* pool, const std::shared_ptr<Buffer>& null_bitmap) {
+Result<std::shared_ptr<Array>> MapArray::FromArrays(std::shared_ptr<DataType>
type,
+ const
std::shared_ptr<Array>& offsets,
+ const
std::shared_ptr<Array>& keys,
+ const
std::shared_ptr<Array>& items,
+ MemoryPool* pool,
+ std::shared_ptr<Buffer>
null_bitmap) {
Review Comment:
`move` is tricky 😬 and misnamed. It doesn't really move anything
(shockingly), it casts an l-value into an r-value. l-values are expressions
that can be on the left of an assignment and r-values only the right.
`l-value = r-value;`
Concretely:
```cpp
str = std::string{"abc"};
// std::string{"abc"} will construct a temp string
// which will be moved into str because it's an r-value
// But if you were to assign into `str` from an l-value called `abc`...
auto abc = std::string{"abc"};
// ^ l-value ^ r-value
str = abc;
// ^ l-value ^ an l-value as well
// after `str = abc`, `str` becomes a copy of abc and
// abc remains allocated, but if you do
str = std::move(abc);
// ^ l-value ^ r-value
// then by using `std::move` you're casting `abc` so that
// the operator `=` can consume it's contents. So the move
// only really happens at the moment the functions implementing
// = runs.
```
This is why you have to keep `std::move`ing the values until they reach a
point where they are stored on something (an l-value).
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