why say 'exchange' is a 'read-modify-write' operation?

[Wu zhen hai]

why say 'exchange' is a 'read-modify-write' operation?

[kevinkjt2000]

The entire operation is atomic (an atomic read-modify-write operation): the value is not affected by other threads between the instant its value is read (to be returned) and the moment it is modified by this function.

http://www.cplusplus.com/reference/atomic/atomic/exchange/