Why it is required to return *this when this is still being passed?

Question

I have written following class which has overloaded assignment operator. As shown in example everywhere I returned *this from assignment operator.

class Sample
{
    int *p;
    int q;

public:

    Sample()
    {
        cout<<"Constructor called"<<endl;

        p = new int;
        q = 0;
    }


    Sample& Sample::operator =(const Sample &rhs)
    {
        cout<<"Assignment Operator"<<endl;

        if(this != &rhs)
        {
            delete p;

            p = new int;
            *p = *(rhs.p);
        }

        return *this;
    }

    void display()
    {

        cout<<"p = "<<p<<"     q = "<<q<<endl;
    }
};

When I call assignment operator like a = b, it goes like, a.operator=(b);.

Now I am calling a's operator function, this is already being passed with operator = then why it is required to return it from assignment operator?

Answer 1

You have to return *this (and also by reference) if you want to support chaining of assignment. For e.g

Class A
{
};

A x,y,z,w;
x = y = z = w; //For this you are returning *this.

EDITED FOR MORE CLARIFICATION:- ( In response to your comment )

Let's suppose you're not returning anything from your assignment operator then expression will be evaluated as follows:-

x=y=z  =>   x=(y=z)

Above will result into a call to

y.operator(z)

as assignment operator is right associative. After this next call would be to

x.operator ( value returned from y=z) ).

If you don't return any value chaining would fail.

Hope I am clear