Overloading = operator in c++

Question

I've just begun getting into classes in c++ and I was working on a class that defines 3D vectors. I was asked to overload the = operator and my professor suggested to implement it like this:

Vector3D& Vector3D::operator=(const Vector3D &rhs){
    vec[0] = rhs[0];
    vec[1] = rhs[1];
    vec[2] = rhs[2];
    return *this
}

I'm confused as to where the reference is returned and why do we use the "This" pointer. I asked my professor about this and he told me that it's necessary when we try to do succesive assignments like this:

Vector3D a;
Vector3D b;
Vector3D c=a=b;

Still, I don't understand why its necessary to have a return value as we have already updated the values before.

It's just a convention. You *can* write a `operator=` that returns `void` (according to the language standard), but it'll confuse people, because we all expect it to return a reference to the left-hand side. Also, not sure if you've figured this out yet, but just to save you the trouble, go read [Rule of Three](https://stackoverflow.com/questions/4172722/what-is-the-rule-of-three). You're implementing one of the three (five, if targeting C++11 or newer) operations, so you need to implement the other two (four). — Silvio Mayolo, Mar 07 '22 at 01:29
@SilvioMayolo I think that over ten years should have been enough to assume C++11 unless specifically stated otherwise. Actually, I would assume C++17 by now, mimimum. We are not doing ourselves a favor using a >20 years old version of the language as baseline still. — DevSolar, Mar 07 '22 at 01:41

score 2 · Answer 1 · edited Mar 07 '22 at 01:47

An assignment in C++ returns the object assigned to, and it is good practice to not change this kind of behavior when you are overloading an operator (*). This allows to e.g. check the value assigned, or assigning the same value to multiple objects.

Inside a member function, this is a pointer to the current object. So *this is the current (assigned-to) object, and the return type Vector3D& makes it clear that we are returning by reference, not by value.

(*) You cannot actually emulate the short-circuit evaluation of the || and && operators, which is why you should not normally overload them.

score 2 · Answer 2 · edited Mar 07 '22 at 04:35

This (I know its slightly different, I wanted to make the chaining clear);

 Vector3D a;
 Vector3D b;
 Vector3D c;
 c=a=b; <<<<=====

is converted by the compiler to this:

 c.operator=(a.operator=(b));

This only works because operator= returns something that can be used as input to the next operator=.

You ask "why return *this;".

Well, the function has to return a reference to this invoked object, the only thing you have is this. You can't return that (excuse the pun) because its not a reference, it's a pointer, so you have to do *this to actually get the object it points to. The compiler sees you have declared the return type as Vector3D& so it knows to, in fact, return a reference, not the object itself.

Overloading = operator in c++

2 Answers2