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4194. atomic<void*> should use generic class template

Section: 32.5.8.5 [atomics.types.pointer] Status: New Submitter: Gonzalo Brito Opened: 2025-01-16 Last modified: 2025-01-19

Priority: Not Prioritized

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Discussion:

32.5.8.5 [atomics.types.pointer] p1 states (emphasis mine):

There is a partial specialization of the atomic class template for pointers.

which requires atomic<void*> to use the atomic class template for pointers. However, the fetch_add/_sub member functions add a difference_type to a T* which requires a pointer-to-object type (these member functions are constexpr, so trying to support this seems unimplementable).

For atomic_ref, the 32.5.7.5 [atomics.ref.pointer] p1 states (emphasis mine):

There are specializations of the atomic_ref` class template for all pointer-to-object types.

which avoids this issue and applying the same form to 32.5.8.5 [atomics.types.pointer] would make atomic<void*> and atomic_ref<void*> consistent.

Technically this would be a breaking change, but all C++ standard library implementations surveyed are broken, and the proposed fix would make them compliant: see libstdc++, libc++ and MSVC STL errors here. These standard libraries require a pointer-to-object type, atomic<void*> uses the general template. Therefore, no user code seems to be impacted.

Proposed resolution:

This wording is relative to N5001.

1. Modify 32.5.8.5 [atomics.types.pointer] as indicated:

   -1- There is a partial specialization of the atomic class template for pointerspointer-to-object types. Specializations of this partial specialization are standard-layout structs. They each have a trivial destructor.