Re: When do I need to override hash?
Re: When do I need to override hash?
- Subject: Re: When do I need to override hash?
- From: Clark Cox <email@hidden>
- Date: Thu, 20 Aug 2009 14:01:28 -0700
On Thu, Aug 20, 2009 at 1:47 PM, Shawn Erickson<email@hidden> wrote:
> On Thu, Aug 20, 2009 at 1:37 PM, Kyle Sluder<email@hidden> wrote:
>
>> P: Two objects are equal.
>> Q: They have the same hash.
>>
>> P -> Q.
>>
>> Note that Q does not imply P.
>
> Or said another way...
>
> If the hash of ObjectA is equal to the hash of ObjectB then ObjectA
> _could_ be equal to ObjectB. If their hash differ they cannot be
> equal. The use of a hash to do a quick reject of equality is common
> for stl collections, java collections, Cocoa collections, etc.
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:.
--
Clark S. Cox III
email@hidden
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