Michael Chermside wrote:
> Andrew Koenig writes:
>
>>The reason for that, I think, is that there are really three kinds of
>>"equality" that make sense, and only two of them are reasonably available:
>>
>> Value equality (==, which might be user-defined, because
>> the concept of "value" can be user-defined);
And there might be more than one user.
>> Object identity ("is")
>>
>> Object equivalence (no easy way of expressing it today)
>>
>>By "object equivalence", I mean mutual substitutability--which is the same
>>as identity for mutable objects, but not for immutable ones.
But this mutual substitutability is context dependant.
"is" guarantees that both objects are mutual substitutable in
*any* context. "==" defines the mutual substitutability for the
"default" context. Any other context is application dependant,
so the comparison function should be defined by the application.
Bye,
Walter Dörwald
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