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 13:33:05 -0700
-isEqual: is how Cocoa collections define equality. Saying that two
objects are "equal" means, by definition, that -[obj1 isEqual: obj2]
returns true.
On Thu, Aug 20, 2009 at 1:30 PM, Bryan Henry<email@hidden> wrote:
> Why do you say that? I haven't noticed any documented requirement that ties
> the implementation details of -hash and -isEqual together.
>
> - Bryan
>
> Sent from my iPhone
>
> On Aug 20, 2009, at 4:27 PM, Clark Cox <email@hidden> wrote:
>
>> On Thu, Aug 20, 2009 at 12:33 PM, David Duncan<email@hidden>
>> wrote:
>>>
>>> On Aug 20, 2009, at 12:00 PM, Seth Willits wrote:
>>>
>>>> Returning 0 is certainly simpler :p
>>>
>>>
>>> It is, but you can generally do better than just returning 0, usually by
>>> just extracting some bits from 'self', ala
>>>
>>> -(NSUInteger)hash
>>> {
>>> uintptr_t hash = (uintptr_t)self;
>>> return (hash >> 4);
>>> }
>>>
>>> This satisfies the condition of hash (two equal objects will have the
>>> same
>>> hash code)
>>
>> No it doesn't. Writing the hash method like that basically prevents
>> you from having an isEqual that does anything other than a pointer
>> comparison.
>>
>> --
>> Clark S. Cox III
>> email@hidden
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--
Clark S. Cox III
email@hidden
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