std::atomic_ref<T>::fetch_or (3) - Linux Manuals
T fetch_or( T arg, (since C++20)
std::memory_order order = std::memory_order_seq_cst ) const noexcept; (member only of atomic_ref<Integral> template specialization)
Atomically replaces the current value of the referenced object with the result of bitwise OR of the value and arg. This operation is a read-modify-write operation. Memory is affected according to the value of order.
arg - the other argument of bitwise OR
order - memory order constraints to enforce
The value of the referenced object, immediately preceding the effects of this function.