opEquals and the Non-Virtual Interface idiom

[Andrei Alexandrescu]

Here's an article about the perils of equals in Java (opEquals in D):

http://www.ddj.com/article/printableArticle.jhtml;jsessionid=GFKUCQH5S4IHNQE1GHOSKHWATMY32JVN?articleID=184405053&dept_url=/java/

It turns out this is a great example for NVI. In D, we could and should
do the following:

class Object {
     // Implement this
     private bool opEqualsImpl(Object rhs) {
         return false;
     }
     // Use this
     final bool opEquals(Object rhs) {
         if (this is rhs) return true;
         if (this is null || rhs is null) return false;
         return opEqualsImpl(rhs) && rhs.opEqualsImpl(this);
     }
}

I took advantage of the fact that in a final function this may be null
without an access violation. The implementation above ensures symmetry
of equality and has each class implement a simpler primitive.

What do you think?


Andrei