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3633. Atomics are copy constructible and copy assignable from volatile atomics

Section: 33.5 [atomics] Status: New Submitter: Wesley Maxey Opened: 2021-11-05 Last modified: 2022-01-29

Priority: 3

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Discussion:

The specification of atomic and atomic<T*> (in 33.5.8.1 [atomics.types.generic.general] and 33.5.8.5 [atomics.types.pointer]) explicitly deletes the following functions:

atomic(const atomic&) = delete;
atomic& operator=(const atomic&) = delete;
atomic& operator=(const atomic&) volatile = delete;

The intent is to make atomic objects not copyable, so that initializing an atomic object from another atomic, or assigning an atomic object with a value from another atomic, must be an explicit operation.

We also explicitly support volatile objects of types that are specializations of std::atomic; some of the functions that are vital for the support of volatile atomics are the following conversion operators:

operator T() const volatile noexcept; // for non-pointers
operator T*() const volatile noexcept; // for pointers

The presence of this conversion operator means that all the statements in the following piece of code compile successfully today, despite the deleted functions mentioned earlier:

volatile std::atomic<int> a;
volatile std::atomic<int> b(a);
std::atomic<int> c(a);
b = a;
c = a;

However, if a is not a volatile object, none of the last four lines compile.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.
This PR would allow

atomic<int> x, y = std::move(x);

Proposed resolution:

This wording is relative to N4901.

1. Modify 33.5.8.1 [atomics.types.generic.general], class template atomic synopsis, as indicated:

   […]
   atomic(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) volatile = delete;
   […]
   

2. Modify 33.5.8.3 [atomics.types.int], class template atomic<integral> specialization synopsis, as indicated:

   […]
   atomic(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) volatile = delete;
   […]
   

3. Modify 33.5.8.4 [atomics.types.float], class template atomic<floating-point> specialization synopsis, as indicated:

   […]
   atomic(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) volatile = delete;
   […]
   

4. Modify 33.5.8.5 [atomics.types.pointer], class template atomic<T*> partial specialization synopsis, as indicated:

   […]
   atomic(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) = delete;
   atomic& operator=(const volatile atomic&) volatile = delete;
   […]