On Sep 18, 2008, at 7:12 PM, Keith Duncan wrote:
if [objectA isEqual:objectB] returns true then [objectA hash] ==
[objectB hash] must be true
the object's hash cannot be derived from any mutable properties of
the object
So, with both those points in mine how exactly does one implement it.
I think that for all practical purposes, the answer is that you just
aren't allowed to mutate objects that are in collections. The
documentation for -hash gives the example that "a mutable dictionary
can be put in a hash table but you must not change it while it is in
there".
One counterexample is internal storage that isn't integral to the
object's value. For example, an object may cache a value in some
internal storage while it's in a collection, but that cached value
can't participate in either -isEqual: or -hash. That is, the cached
value is incidental and doesn't change the object's externally visible
value.
Another counterexample is an object which doesn't provide value
semantics. That is, its -isEqual: only tests identify (pointer
comparison) and whose hash is purely based on the object pointer.
Like the default NSObject implementation. Such an object never
compares equal to any other object. It may also be safely mutated
within a collection. (However, such an object would be a poor
candidate for a dictionary key, for example. Dictionary keys are
copied, and since the copy can never equal the original, you'd never
be able to look up the value using the original key. It'd be fine in
a set, though.)
Cheers,
Ken
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