Why operator= returns reference not const reference

Question

The original question is related to overloading operator= and I like to share my findings as it was nontrivial for me to find them. I cannot imagine reasonable example to us

Answer 1

If we consider three auto_ptr a, b and c, the operator = has to return a non-const reference so that you can do multiple assigns since assigning the pointer to another modifies the first.

so if we have a = b = c, the following happens: c is assigned to b (c is modified to point to null), the operator returns a reference to b the reference returned by (b = c) is assigned to a, it is thus modified to point to null, which is only possible if the reference is non-const.