Function should return reference or object?

Question

Let's discuss these two functions:

1. complex& operator+=(const T& val);
2. complex operator+(const T& val);

Where "complex" is a name of a class that implements for example complex variable.

So first operator returnes reference in order to be possible to write a+=b+=c ( which is equivalent to b=b+c; a=a+b;).

Second operator returnes and objec(NOT A REFERENCE), be we still able to write a=b+c+d.

Who could explain me this nuance? What is the difference between returning reference or object?

Answer 1

The assignment operators support multiple application to the same object:

(a += b) += c;

Which will add both b and c to a. For this to work, a += b must return a reference to a. The addition operator, however, doesn't require this, since the expression b + c + d has no side-effect. Only the final assignment to a has a side-effect.

Answer 2

In 1, a+=b, the += operator modifies a. Therefore it can return a reference to itself, because a itself is the correct result of the operation.

However, in 2. a new object is required because a+b returns something that is not a, so returning a reference to a wouldn't be correct.