[jira] [Updated] (ARROW-13084) [C++] Avoid construction of unaligned pointers

["Ben Kietzman (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-13084?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Ben Kietzman updated ARROW-13084:
---------------------------------
    Labels: beginner  (was: )

> [C++] Avoid construction of unaligned pointers
> ----------------------------------------------
>
>                 Key: ARROW-13084
>                 URL: https://issues.apache.org/jira/browse/ARROW-13084
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: C++
>            Reporter: Ben Kietzman
>            Priority: Major
>              Labels: beginner
>
> IMHO it's a foot gun to allow construction of pointers which are not aligned. For example, see https://github.com/apache/arrow/pull/10489 where {{const int64_t* key_left_ptr}} was dereferenced without being wrapped in SafeLoad, resulting in undefined behavior. Unaligned pointers are convenient because they apply the correct multiple of {{sizeof(T)}} to integer arithmetic, but there's no way to warn at the point of access that they must be wrapped in SafeLoad.
> I propose we remove the overload of SafeLoad which accesses an unaligned pointer and replace it with an indexed overload of SafeLoadAs. This will avoid boilerplate of multiplying by {{sizeof(T)}} but will make clear with typing that access requires SafeLoadAs:
> {code}
> template <typename T>
> T SafeLoadAs(const void* buf, size_t index) {
>   T value;
>   std::memcpy(&value, reinterpret_cast<const T*>(buf) + index, sizeof(value));
>   return value;
> }
> // ...
>     const void* key_left_ptr = ...;
>     auto key_left = SafeLoadAs<uint64_t>(key_left_ptr, istripe);
> {code}



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