Re: Relationship between -isEqual and -hash
Re: Relationship between -isEqual and -hash
- Subject: Re: Relationship between -isEqual and -hash
- From: "Jeffrey J. Early" <email@hidden>
- Date: Sat, 20 Jan 2007 16:15:38 -0800
- Thread-topic: Relationship between -isEqual and -hash
on 1/19/07 1:52 PM, John Stiles at email@hidden wrote:
> On Jan 19, 2007, at 1:46 PM, Jeffrey J. Early wrote:
>
>> In my code I have a dictionary that I want to behave in a
>> particular way. I'm using objects as keys where if two objects are
>> 'close enough', they will return the same value from the
>> dictionary, even if these objects aren't equal in the -isEqual:
>> sense. So when objects are 'close enough', I have them return the
>> same hash value.
>
> This should be all right. If any outside code is assuming that equal
> hashes means perfect equality, that code is broken.
It turns out that while it is true that equal hashes doesn't imply -isEqual:
must be true (thanks for your answer), my attempted application of this was
flawed.
NSDictionary is too clever -- after it finds the keyed-object in its hash
table using the hash value, it *then* runs an -isEqual on the two objects. I
probably should have realized this... Usage of -hash is just an optimization
in this case, -isEqual is really what it's trying to get at.
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