std::atomic_ref::fetch_or

From cppreference.com
< cpplrm; | atomiclrm; | atomic ref
T fetch_or( T arg,
std::memory_order order = std::memory_order_seq_cst ) const noexcept;
(since C++20)
(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.

Parameters

arg - the other argument of bitwise OR
order - memory order constraints to enforce

Return value

The value of the referenced object, immediately preceding the effects of this function.