On 20 Aug 2009, at 22:16, Clark Cox wrote:
On Thu, Aug 20, 2009 at 2:14 PM, Kyle Sluder<[email protected]>
wrote:
On Thu, Aug 20, 2009 at 2:01 PM, Clark Cox<[email protected]>
wrote:
Yes, and two different objects will have different pointer values.
If
the hash is based on the pointer values, then two different objects
cannot have the same hash, regardless of whether or not they are
equal. Hence, that implementation of hash is broken for any object
that does anything other than a pointer comparison in -isEqual:.
I thought we were talking about -hash just returning zero?
--Kyle Sluder
No, we were talking about:
OK, so this discussion is getting a little crazy :-)
The -hash method is important for objects that are used as keys in
associative collections. So the worry about the hash value changing
when an object's properties are altered is a bit of a red herring,
because you *aren't supposed to change keys in associative
collections*. If you do that in just about *any* implementation, ObjC
or otherwise, you'll get undefined behaviour.
So, in practice, it's perfectly safe in 99.9% of cases to base your
hash off your object's properties. In the specific case when you're
mutating objects that are keys in associative collections
(NSDictionary and NSSet being the primary examples, along with their
CoreFoundation counterparts), if you change a property that affects
*either* -isEqualTo: *or* -hash, you need to remove the object before
mutating it and then add it back again afterwards.
The one other thing I'll note is that occasionally you might be using
an object as a key where only *some* of its properties contribute
towards -hash and -isEqualTo:. In that case, you can safely change
any of the other non-contributing properties.
Kind regards,
Alastair.
--
http://alastairs-place.net
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