C++ why the assignment operator should return a const ref in order to avoid (a=b)=c

Question

I am reading a book about C++ and more precisely about the operator overloading.

The example is the following:

const Array &Array::operator=(const Array &right) { // check self-assignment // if not self- assignment do the copying return *this; //enables x=y=z } 

The explanation provided by the book about returning const ref instead of ref is to avoid assignments such as (x=y)=z. I don't understand why we should avoid this. I understand that x=y is evaluated first in this example and since it returns a const reference the =z part cannot be executed. But why?

Answer 1

There is no need to avoid this, unless the book is aimed at programmers that commonly write (x=y)=z when they mean x=y=z. In practice, nobody in their right mind writes that, so the precaution is entirely unnecessary. It also forbids some other terse constructs, such as (x=y).nonConstMember(), that hardly anyone writes but that might be useful in some contexts (although they shouldn't be over-used).

@ybungalobill is right, get a better book.

Answer 2

(x=y) means x.operator=(y), which returns the object x. Therefore, (x=y)=z means (x.operator=(y)).operator=(z). The expression in parens sets x to y and returns x, and then the outer bit sets x to z. It does not set y to z as you might expect, and as the expression x = y = z does.

This behavior is counter-intuitive (they should all be equal after the assignment, right?); returning a const reference makes it impossible and avoids the problem.