为什么人们使用原子交换而不是存储？

Question

That is C++

atomic_int turn(0)

turn.exchange(1);
turn.store(1);

this two code works same way, right? But I checked many examples about lock use 1st one.

Why?

Answer 1

Both statements store a value into the atomic variable. The exchange call returns the previously held value, which, in this case, is discarded. However, this does not mean the two calls are exactly the same. The exchange operation is a read-modify-write operation, while the store operation is only a write. That means that if we have:

// thread 1
turn.store(0);  // A

// thread 2
turn.exchange(1);  // B

and if thread 2 reads the value stored by thread 1, then line A synchronizes with line B. If thread 2 were to use store rather than exchange , there would be no such synchronization.