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### 3485. `atomic_ref` safety should be based on operations that "potentially conflict" rather than lifetime

Section: 31.7 [atomics.ref.generic] Status: SG1 Submitter: Billy O'Neal III Opened: 2020-09-12 Last modified: 2020-09-29

Priority: 3

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Discussion:

Consider the following program:

>     #include <atomic>
>     #include <iostream>
>     #include <thread>

>     using namespace std;

>     int main() {
>       int i{500};
>       atomic_ref atom{i};
>       i += 500;
>       thread t1{[&atom] { for (int val{0}, x{0}; x < 70;) {
>         if (atom.compare_exchange_weak(val, val + 10)) { ++x; }}}};
>       thread t2{[&atom] { for (int val{0}, y{0}; y < 29;) {
>         if (atom.compare_exchange_weak(val, val + 1)) { ++y; }}}};
>       t1.join(); t2.join();
>       cout << i << endl; // 1729
>     }

Technically this program has undefined behavior. 31.7 [atomics.ref.generic] p3 says that, during the lifetime of any `atomic_ref` referring to an object, that the object may only be accessed through the `atomic_ref` instances. However, in this example the `atomic_ref` is constructed before the `i+=500` and is not destroyed before the print, even though we have a happens-before relationship between the atomic and non-atomic 'phases' of access of the value.

The user would instead have to write:

>     #include <atomic>
>     #include <iostream>
>     #include <thread>

>     using namespace std;

>     int main() {
>       int i{500};
>       i += 500;
>       {
>         atomic_ref atom{i};
>         thread t1{[&atom] { for (int val{0}, x{0}; x < 70;) {
>           if (atom.compare_exchange_weak(val, val + 10)) { ++x; }}}};
>         thread t2{[&atom] { for (int val{0}, y{0}; y < 29;) {
>           if (atom.compare_exchange_weak(val, val + 1)) { ++y; }}}};
>         t1.join(); t2.join();
>       } // destroy atom
>       cout << i << endl; // 1729
>     }

We should probably get SG1 on record clarifying whether they intend the first program to be acceptable. I can think of a reason to for `atomic_ref`'s ctor to do something (zeroing out padding), but in our implementation it does nothing. I can't think of any reason for `atomic_ref`'s dtor to do anything.

[2020-09-29; Priority to P3 after reflector discussions; Status set to "SG1"]

Proposed resolution: