Question

I have a database model class that is a NSObject. I have a set of these objects in a NSMutableArray. I use indexOfObject: to find a match. Problem is the model object’s memory address changes. So I am overriding the hash method to return the model’s row ID. This however does not fix it. I also have to override the isEqual: method to compare the value of the hash method.

What does the isEqual: method use to determine equality by default?

I’m assuming it uses the memory address. After reading the isEqual: documentation I thought it used the value from the hash method. Obviously, that is not the case as my attempt to override that value did not solve my initial problem.

Answer 1

As you’ve correctly guessed, NSObject‘s default isEqual: behaviour is comparing the memory address of the object. Strangely, this is not presently documented in the NSObject Class Reference, but it is documented in the Introspection documentation, which states:

The default NSObject implementation of isEqual: simply checks for pointer equality.

Of course, as you are doubtless aware, subclasses of NSObject can override isEqual: to behave differently. For example, NSString‘s isEqual: method, when passed another NSString, will first check the address and then check for an exact literal match between the strings.