On Thu, Aug 20, 2009 at 9:09 PM, Jeff Laing<[email protected]> wrote:
>> Core Data doesn't use random objects as keys in dictionaries or sets
>> for this reason. It's not that we don't trust you, but ... to
>> prevent misunderstandings, all NSManagedObject subclasses are
>> forbidden from overriding -hash and -isEqual.
>
> I have to admit, I didn't know this bit but I see it in the developer library
> along with a bunch of others.
>
>> If you need to know whether or not another object has put your object
>> into an NSDictionary, you're probably doing something wrong. Do you
>> have a specific concern about Core Data using your objects ?
>
> No, I guess the point I was trying to make was that this discussion seemed to
> have touched on "if you put your objects into an NSSet then you'll need to be
> more careful about the implementation of -hash, etc". I was trying to point
> out that just because my application code doesn't go anywhere near NSSet, its
> conceivable (to me) that Core Data (for example) might be storing dirty
> objects in an NSSet "behind your back". So you can't "not implement -hash,
> etc properly" and hope everything will work.
The solution is simple:
If you implement -hash, you must implement -isEqual:
If you implement -isEqual:, you must implement -hash
If -isEqual: returns true for two given objects, those objects must
return the same value for -hash
If you don't implement either, NSObject's implementations are fine
also. i.e. NSObjects implementations are basically:
-(NSUInteger)hash { return (NSUInteger) self; }
-(BOOL)isEqual:(id)obj { return self == obj; }
--
Clark S. Cox III
[email protected]
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