David Stes stes@mundivia.es wrote:
Pascal Bourguignon wrote:
A relation noted with '=' should be an equivalence relation (thus should be symetric).
But how does it relate to classes ? I rather would expect = to be mostly symmetric for instances of a given class but not necessarily if you mix instances of different classes.
This imply that the implementor must ensure that its implementations of equivalence relations are actually symetric (and transitive and reflexive).
You cannot merely say that instances are of different classes, because everything is an object, that is, if two instances do not belong to the exact same sub-class, the have at least one common super-class: Object.
Last time I checked, equality was defined at the Object class level, and in a way that did ensure the implementation has every characteristic of equivalence relations.
If you don't want to break your system, then when you override this exact operator, you must ensure that it is done with the same semantics.
The direct relations between equivalence relations and classes is given (quite informaly, I agree) in the comment of #= in Object:
= anObject "Answer whether the receiver and the argument represent the same object. [...]"
"x is the same object than y" is obviouly an equivalence relation (that's why it's been named #=, by the way).
Therefore if you override it in a subclass in such a way that it is no more an equivalence relation (rendering it asymetric, for example), then you can only expect chaos from your image.
__Pascal Bourguignon__