Why doesn't NSMutableSet remove an object contained in it?

Question

I have an NSMutableSet set that contains custom made objects who are a subclass of SKNode . I am making a game where these objects are added and removed from the NSMutableSet. I am adding and removing from the main thread, so threading isn't an issue. For some reason sometimes an object isn't removed because it can't be found. The following method returns NO :

[self.set containsObject:object]

I looked into the this problem and printed the address and hash number of the object and all the objects in the NSMutableSet , and sure enough it appears in the set.

What could be the reason that the object isn't found if the address and hash numbers equal? I understand that the containsObject method uses the isEqual which compares these two values.

Answer 1

To use objects as elements of NSSet, keys of NSDictionary etc. they need to implement the hash method and the isEqual: method. If you don't have your own implementation, hash returns the object pointer and isEqual compares object pointers and everything will work just fine.

If you implemented hash and isEqual: yourself, you must make sure of two things: 1. The hash value and the result of isEqual most not change while the object is in the set (changing an NSMutableString that is stored in an NSSet would be a very bad idea). 2. hash and isEqual: must be consistent: Two objects that compare equal must return the same hash value.

Answer 2

The result of hash must be equal and the result of isEqual: must be YES . Just matching the hash is not sufficient. Have you checked isEqual: ? The default isEqual: compares object identity, not hash . hash is allowed to be used by collections to speed up comparisons, but it is only an optimization. If two objects return YES for isEqual: they must also return the same hash , but the converse is not true.