原子类型可以赋给另一个原子变量的值吗？

Question

在《并发行动》中指出：

原子类型上的所有操作都定义为原子的，并且赋值 复制构造涉及两个对象。一次操作对两个 不同的对象不能是原子的。

但是我有点困惑

atomic<int> i1, i2;
i1 = 0;
i2 = 1;
i1 = i2;          // error
i1.store(i2);  // but it works, is this operation atomic?

最后一行代码完成了赋值，还涉及到两个对象

Answer 1

atomic<int> i1, i2;
i1 = 0; // this is calling int atomic<int>::operator=(int)
i2 = 1; // this is also calling int atomic<int>::operator=(int)
i1 = i2; // this is also calling 
// atomic<int>& atomic<int>::operator=(constatomic<int>&)
// (so called copy assignment operator which is explicitly deleted 
// https://en.cppreference.com/w/cpp/atomic/atomic/operator%3D)

// this is completely different beast
i1.store(i2);
// it actually does two things:
int temporary = i2.load(); // *1
i2.store(temporary); // *2
// what happens here is:
// -> i2 get implicitly converted to int (*1)
// -> i1.store gets called with that int (*2)

// each of those 2 instructions are atomic but not both of them at once
// in other words they i1.store(i2) is not atomic as a whole