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3508. atomic_ref<cv T> is not well-specified

Section: 32.5.7.1 [atomics.ref.generic.general] Status: Open Submitter: Casey Carter Opened: 2020-12-02 Last modified: 2024-06-28

Priority: 2

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Discussion:

atomic_ref<T> requires only that its template parameter T is trivially copyable, which is not sufficient to implement many of the class template's member functions. Consider, for example:

int main() {
  static const int x = 42;
  std::atomic_ref<const int> meow{x};
  meow.store(0);
  return meow.load();
}

Since const int is trivially copyable, this is a well-formed C++20 program that returns 0. That said, the store call that mutates the value of a constant object is (a) not implementable with library technology, and (b) pure madness that violates the language semantics. atomic_ref::store should presumably require is_copy_assignable_v<T>, and other operations need to have their requirements audited as well. (Related: LWG 3012⁽ⁱ⁾ made similar changes to atomic.)

As a related issue, volatile atomic<T> recently had its member functions deprecated when they are not lock-free. Presumably atomic_ref<volatile T> should require that atomic operations on T be lock-free for consistency.

Jonathan:

The last point is related to LWG 3417⁽ⁱ⁾ (another case where we screwed up the volatile deprecations).

[2020-12-19; Reflector prioritization]

Set priority to 2 during reflector discussions.

[2024-06; Related to issue 4069⁽ⁱ⁾.]

[St. Louis 2024-06-28; SG1 feedback]

SG1 forwarded P3323R0 to LEWG to resolve LWG issues 3508⁽ⁱ⁾ and 4069⁽ⁱ⁾.

Proposed resolution: