C++: What's this? A reference? (idea)

Sometimes it just looks like C++ doesn't believe in C++. This is an excellent example. Well, actually this is. Whenever you're writing a non-static method, you can access the object being acted upon through this. this is a pointer to the object. It already comes initialised (because there's no other way of accessing the object, apart from using this), and you can't change its value. Such objects have a name in C++, but the name is "reference", not "pointer". So why isn't this a reference?

Let's go back to the favourite example of C++ textbooks: a complex numbers class. Ours will just have a "+" operation defined for it. For convenience, I'll just define operator+=(); it's easy to define operator+() using that (and the default copy constructor):

class Complex {
  double re, im;
public:
  Complex(double r, double i) : re(r), im(i) {}
  Complex& operator+=(const Complex& other) {
    re += other.re;
    im += other.im;
    return *this;
  }
}

This is almost the only way to use this; had this been Complex& instead of Complex* the only difference would have been return this;, which is surely clearer.

As it stands, this behaves like a reference, not like a pointer. It's not even like a constant pointer: that can be NULL (or 0, as we say in C++), but this is guaranteed to refer to a valid object. So why on earth isn't it a reference?

Because C++ doesn't believe in C++!