opEquals and the Non-Virtual Interface idiom

[Andrei Alexandrescu]

grauzone wrote:
> Andrei Alexandrescu wrote:
>> 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?
> 
> Eh, now after all this discussion, we're going to allow even "this" to 
> be null? That seems like a backstep...

Good point.

> Implementing opEquals as a global/static function, that calls the actual 
> Object.opEquals virtual method would be so much more straight forward.

It's also less safe because people could call the incomplete primitive 
by hand. With NVI in place nobody outside object.d could ever call 
opEqualsImpl.

> PS: I agree about the NVI thing. If you'd go to extend the language for 
> "NVI", couldn't we just introduce a second type of virtual function that 
> works this way:
> 1. the super class' implementation is _always_ called first
> 2. the super class function can decide to "call down" to the sub class' 
> implementation of the same method
> => no extra do<something> method needed, and the code is (possibly) 
> clearer.

Do you know of a precedent for this? One thing that NVI has going for it 
is that it's quite established - it seems to be solidly planted in 
programmers' lore. I was surprised to find 2.8M Google matches for the 
exact string "Non-Virtual Interface".


Andrei