Why does overloaded assignment operator return reference to class?

Question

class item {
public:
    item& operator=(const item &rh) {
        ...
        ...
        return *this;
    }
};

Is the following signature wrong?

void operator=(const item &rh);

item a, b;
a = b; // equivalent to a.operator=(b); so there is no need to return this.

Answer 1

It is not "wrong", but surprising. An assignment evaluates to the target object. That's what the builtin meaning is. If you define it different for your own class, people could become confused.

Example:

int c;
while((c = getchar()) != EOF) {
  // ...
}

The assignment to c returned c itself and compared it to EOF afterwards. Users would expect your item class to behave similar.

Answer 2

The signature with void would not allow chained assignments:

a = b = c;

(Look at Johannes' answer for one more example of a usage pattern based on assignment returning the assigned value.)

That's why using such signatures is discouraged. However, if I am not mistaken, you actually can use such a signature.