bkietz commented on code in PR #36018:
URL: https://github.com/apache/arrow/pull/36018#discussion_r1254369336
##########
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{};
Review Comment:
I was using `new` to explicitly create the UnionScratchSpace. The
reinterpret_cast would probably be fine, but technically doesn't implicitly
start the lifetime of the UnionScratchSpace object until c++20 where it's [part
of the object model](https://eel.is/c++draft/basic.memobj#intro.object-10) that
implicit lifetime types may be used this way. Would you prefer:
```suggestion
auto* union_scratch_space = [&] {
UnionScratchSpace union_scratch_space{...initializers...};
std::memcpy(checked_cast<const UnionScalar&>(value).scratch_space_,
&union_scratch_space,
sizeof(UnionScratchSpace));
return reinterpret_cast<UnionScratchSpace*>(checked_cast<const
UnionScalar&>(value).scratch_space_);
}();
```
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