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.

Answer 2

The operator= has a return value for allow multiple assignments in a single line:

Object a, b;
// Short form
a = b = <value>;
// Equivalent long form
a = <value>;
b = <value>;

For accomplish this behavior, on may consider std::atomic<T>::operator= to return T , T & or std::atomic<T> & :

• T

  This is how the operator is actually implemented.

• T &

  Like a pointer, a reference should point to object of type T. But there is no T object stored inside an atomic object!

  So, there is nothing suitable for being returned as T & .

• std::atomic<T> &

  With such return type one cannot write

  std::atomic<uint32_t> a, b; a = b = 5;

  b = 5 returns the type std::atomic<T> & , and one cannot assign the atomic variable a to that type because corresponded operator= is marked as deleted (assigning one atomic variable to another is prohibited).

As one can see, the only possible way for allow multiple assignments is returning T , as it is actually implemented.