Why does operator = return *this?

Question

Say I want to override the operator = so I can do something like

Poly p1;  // an object representing a polynomial
Poly p2;  // another object of the         


        

Answer 1

p2 = p1 is a shorthand for p2.operator=(p1). It is just calling your operator= function, which is returning a reference to p2, which you are then ignoring. To make this clear, let's call it assign instead of operator=:

Poly& Poly::assign(const Poly &source) {
    .
    .
    .
    return *this;
}

Now instead of p2 = p1, you would write

p2.assign(p1);

In this case, the result of calling assign is being ignored, but you don't have to ignore it. For example, you could write:

p3.assign(p2.assign(p1));

Using operator= instead of assign, this becomes

p3 = (p2 = p1);

but since assignment is right-associative, this can also be written as

p3 = p2 = p1;

This form of being able to do multiple assignments at once originally comes from C and has been preserved in C++ through the convention of returning *this in operator=().