pitrou commented on code in PR #36018:
URL: https://github.com/apache/arrow/pull/36018#discussion_r1253129019
##########
cpp/src/arrow/array/data.cc:
##########
@@ -384,26 +382,31 @@ void ArraySpan::FillFromScalar(const Scalar& value) {
this->child_data[i].FillFromScalar(*scalar.value[i]);
}
} else if (is_union(type_id)) {
+ // Dense union needs scratch space to store both offsets and a type code
+ struct UnionScratchSpace {
+ alignas(int64_t) uint8_t type_code;
+ alignas(int64_t) uint8_t offsets[sizeof(int32_t) * 2];
+ };
+ static_assert(sizeof(UnionScratchSpace) <=
sizeof(UnionScalar::scratch_space_));
+ auto* union_scratch_space =
+ new (checked_cast<const UnionScalar&>(value).scratch_space_)
UnionScratchSpace{};
+
// First buffer is kept null since unions have no validity vector
this->buffers[0] = {};
- this->buffers[1].data = reinterpret_cast<uint8_t*>(this->scratch_space);
+ this->buffers[1].data = &union_scratch_space->type_code;
this->buffers[1].size = 1;
- int8_t* type_codes = reinterpret_cast<int8_t*>(this->scratch_space);
- type_codes[0] = checked_cast<const UnionScalar&>(value).type_code;
+ new (&union_scratch_space->type_code)
+ int8_t{checked_cast<const UnionScalar&>(value).type_code};
Review Comment:
I don't understand why this isn't simply:
```suggestion
union_scratch_space->type_code = checked_cast<const
UnionScalar&>(value).type_code;
```
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