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3893. LWG 3661 broke atomic<shared_ptr<T>> a; a = nullptr;

Section: 32.5.8.7.2 [util.smartptr.atomic.shared] Status: WP Submitter: Zachary Wassall Opened: 2023-02-22 Last modified: 2023-11-22

Priority: Not Prioritized

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Discussion:

LWG 3661⁽ⁱ⁾, "constinit atomic<shared_ptr<T>> a(nullptr); should work" added the following to atomic<shared_ptr<T>>:

constexpr atomic(nullptr_t) noexcept : atomic() { }

I believe that doing so broke the following example:

atomic<shared_ptr<T>> a;
a = nullptr;

For reference, atomic<shared_ptr<T>> provides two assignment operator overloads:

void operator=(const atomic&) = delete;          // #1
void operator=(shared_ptr<T> desired) noexcept;  // #2

Prior to LWG 3661⁽ⁱ⁾, the assignment in the example unambiguously matches #2. #1 is not viable because nullptr_t is not convertible to atomic<shared_ptr<T>>. After LWG 3611, #1 is viable and the assignment is ambiguous between #1 and #2.

I believe this could be remedied easily enough by adding an assignment operator to match the added nullptr_t constructor:

void operator=(nullptr_t) noexcept;

[2023-02-25; Daniel comments and provides wording]

The suggested delegation below to store(nullptr) is not ambiguous and calls the constructor shared_ptr(nullptr_t), which is guaranteed to be no-throwing.

[2023-03-22; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4928.

1. Modify 32.5.8.7.2 [util.smartptr.atomic.shared] as indicated:

   namespace std {
     template<class T> struct atomic<shared_ptr<T>> {
       […]
       constexpr atomic() noexcept;
       constexpr atomic(nullptr_t) noexcept : atomic() { }
       atomic(shared_ptr<T> desired) noexcept;
       atomic(const atomic&) = delete;
       void operator=(const atomic&) = delete;
       […]
       void operator=(shared_ptr<T> desired) noexcept;
       void operator=(nullptr_t) noexcept;
       […]
     private:
       shared_ptr<T> p; // exposition only
     };
   }
   

   […]

   void operator=(shared_ptr<T> desired) noexcept;
   

   -5- Effects: Equivalent to store(desired).

   void operator=(nullptr_t) noexcept;
   

   -?- Effects: Equivalent to store(nullptr).