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

c++ operator-overloading assignment-operator
alampada picture alampada · Jan 16, 2011 · Viewed 13.4k times · Source

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

Asher Dunn picture Asher Dunn · Jan 16, 2011

(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.

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