Question

Imagine I have a class used to represent some trivial numerical data, like a vector. I want this class to be immutable in terms of its members, but still support correct copy-assignment behaviour.

This is what the shell of this class might look like:

class Vector {
  public:
  Vector(float _x, float _y);

  private:
  const float x;
  const float y;
};

I would like to be able to assign a new value to a vector member within (for example) a class. An on-screen entity with a position, perhaps, represented using a non-const member of type Vector.

This ‘entity’ must be mutable itself (perhaps supporting movement), e.g. the internal Vector member used to store its position must change. Rather than modifying this member directly, I would rather replace it with an entirely new Vector instance, initialised with the modified values.

In a language such as C#, I would simply compute the new values for X and Y based on the current Vector instance, and assign a new Vector object initialised with these values to the member, discarding the previous value and letting the GC do the rest.

// Inside some class (C#/Java type)...
Vector position;
...
// Example function:
void Move(float _dX, float _dY) {
  this.position = new Vector(_dX + position.X, _dY + position.Y);
}

This is where my limited ability with C++ comes into play, because I am unsure of how to implement operator= in such a way that instances of this immutable type contained in dynamic memory do not simply ‘disappear’ and cause a leak.

So, I suppose, what I am asking is this:

Vector& operator=(const Vector& _rhs);
...
// In implementation file...
Vector& Vector::operator=(const Vector& _rhs) {
  // WHAT ON EARTH GOES IN HERE?!?!?
}

The Vector case is something I invented to get my point across, I can think of many sorts of types that should be immutable at the member level.

Am I wasting my time attempting to model immutable types?

Is there a better solution?

Answer 1

When you type

Vector position;

in a class definition, you define an instance of a Vector class that will stay here forever. If that one is immutable, you’re screwed.

In C#/Java, there is no way to do this. Instead you define references.

If you want to do the same thing in C++, you need pointers or auto_pointers

Vector* position;

That way, you can change the instance to which position is pointing.

You shouldn’t define the assignment operator of a immutable class. If you really need to, maybe you should make Vector mutable, and use const Vector when you need an immutable Vector. You will just need to add const qualifiers to method that are allowed with a const Vector.

eg:

class Vector {
    private:
    float x;
    float y;
    public:
    // allowed for const Vector
    // because of the const before the '{'
    float norm() const {
        return hypot(x,y);
    }

    Vector& operator=(const Vector& o) {
        x=o.x;
        y=o.y;
        return *this;
    }
};