opEquals and the Non-Virtual Interface idiom

[Yigal Chripun]

On 27/09/2009 19:01, Andrei Alexandrescu wrote:
> 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
>

This is a smalltalk idea -

method:
   ^ self method

by sending a message to self without implementing it you make the method 
abstract.
of course you can add pre/post code.

I also seen this with the keyword inner:

class Foo {
   void bar() {
       ... setup
       inner();
       ...tear down
   }
}