Why does std::atomic<T>::operator= should return a value instead of reference?

Question

When using atomic type in c++

Why does std::atomic<T>::operator= should return a value instead of reference? It's not like other common assignment operator which returns a reference.

From a cppreference site I can get some hint.

Unlike most assignment operators, the assignment operators for atomic types do not return a reference to their left-hand arguments. They return a copy of the stored value instead.

I can guess that if I use reference, it has a problem of atomic things. But I'm not completely clear.

Do you have any example for the async error case ?

And if I return a value instead of a reference, why does the async error gone?

Answer 1

Do you have any example for the async error case ?

The return type of that operator is T. If it were T&, you could do something like:

atomic<int> i;
++(i = 10);

And the ++ would be a non-atomic read-increment-write, which is very wrong.

And if I return a value instead of a reference, why does the async error gone?

Since it returns a copy, not a reference, you can't do what I showed above.