LWG 3661 broke atomic<shared_ptr<T>> a; a = nullptr;
Zachary Wassall

Created on 2023-02-22.00:00:00 last changed 1 week ago


Date: 2023-03-22.22:32:33

Proposed resolution:

This wording is relative to N4928.

  1. Modify [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;
        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).

Date: 2023-03-15.00:00:00

[ 2023-03-22; Reflector poll ]

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

Date: 2023-02-15.00:00:00

[ 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.

Date: 2023-02-22.00:00:00

LWG 3661, "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, 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;
Date User Action Args
2023-03-22 22:32:33adminsetmessages: + msg13468
2023-03-22 22:32:33adminsetstatus: new -> ready
2023-02-25 15:13:55adminsetmessages: + msg13433
2023-02-25 15:13:55adminsetmessages: + msg13432
2023-02-22 00:00:00admincreate