std::atomic_ref<T>::exchange (3) - Linux Manuals
T exchange( T desired, std::memory_order order = std::memory_order_seq_cst ) const noexcept; (since C++20)
Atomically replaces the value of the referenced object with desired. The operation is a read-modify-write operation. Memory is affected according to the value of order.
desired - value to assign
order - memory order constraints to enforce
The value of the referenced object before the call.