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### 3661. `constinit atomic<shared_ptr<T>> a(nullptr);` should work

Section: 31.8.7.2 [util.smartptr.atomic.shared] Status: WP Submitter: Jonathan Wakely Opened: 2022-01-21 Last modified: 2022-02-10

Priority: Not Prioritized

View all issues with WP status.

Discussion:

All the following are valid except for the last line:

>     constinit int i1{};
>     constinit std::atomic<int> a1{};
>     constinit int i2{0};
>     constinit std::atomic<int> a2{0};
>     constinit std::shared_ptr<int> i3{};
>     constinit std::atomic<std::shared_ptr<int>> a3{};
>     constinit std::shared_ptr<int> i4{nullptr};
>     constinit std::atomic<std::shared_ptr<int>> a4{nullptr}; // error

The initializer for `a4` will create a `shared_ptr<int>` temporary (using the same constructor as `i4`) but then try to use `atomic(shared_ptr<int>)` which is not `constexpr`.

This is an unnecessary inconsistency in the API for `atomic<shared_ptr<T>>` that can easily be fixed. The proposed resolution has been implemented in libstdc++.

There is no need to also change `atomic<weak_ptr<T>>` because `weak_ptr` doesn't have a constructor taking `nullptr`.

[2022-01-30; Reflector poll]

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

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4901.

1. Modify 31.8.7.2 [util.smartptr.atomic.shared], class template partial specialization `atomic<shared_ptr<T>>` synopsis, as indicated:

   >     […]
   >     constexpr atomic() noexcept;
   >     constexpr atomic(nullptr_t) noexcept : atomic() { }
   >     atomic(shared_ptr<T> desired) noexcept;
   >     atomic(const atomic&) = delete;
   >     void operator=(const atomic&) = delete;
   >     […]