`<memory>`: Potential ABI issue on `constexpr`-compatibility of `(in)out_ptr_t`
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Description
WG21-P3037R4 "constexpr std::shared_ptr" has been recently forwarded to LWG for C++26. The paper currently also makes (in)out_ptr_t constexpr, including their operator void**() conversion functions.
It seems that the non-trivial changes, probably including changing the type of some data members, are necessary for constexpr-compatibility (e.g. avoiding UB, reinterpret_cast, and bit_cast involving pointers). Before C++23 ABI freeze of MSVC STL, we might need to determine and switch to use a suitable implementation strategy for (in)out_ptr_t to avoid undesired ABI breakage.
IMO we might need to switch to use union to store raw pointers (along with void* when necessary), while using std::is_within_lifetime to detect the active member during constant evaluation (although the compiler support from MSVC is not yet available now).
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
No files, tests, or entry points are named. Start by reading WG21-P3037R4 and the current (in)out_ptr_t implementation, then assess the constexpr requirements and ABI impact described in the issue. Done means selecting and implementing an ABI-safe strategy, with appropriate validation for constexpr compatibility.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100