std::atomic<T>::exchange (3) - Linux Manuals
T exchange( T desired, std::memory_order order = std::memory_order_seq_cst ) noexcept; (since C++11)
T exchange( T desired, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept;
Atomically replaces the underlying value with desired. The operation is 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 atomic variable before the call.
atomic_exchange_explicit atomically replaces the value of the atomic object with non-atomic argument and returns the old value of the atomic