Question

I’m using xCode 4.2, and going through the book “Programming in Objective C 2.0”

There is an exercise that involves implementing isEqual: method from the NSObject class.

The book says that the isEqual: method is triggered when the removeObjectIdenticalTo: method of the NSArray class is called. removeObjectIdenticalTo: ends up sending isEqual: message to all array members.

When I’m trying to implement this isEqual: method in my class AddressCard, and I use removeObjectIdenticalTo:, passing an object that is an instance of AddressCard class, however, my isEqual: method is not getting called. Although if I just use the isEqual: method on an instance of AddressCard explicitly, it does work.

Here is my isEqual: method from the AddressCard.m file

-(BOOL)isEqual: (id)object {
    NSLog(@"I got called");
    return NO;
}

When this code is fired in AddressBook.m

-(void) removeCard: (AddressCard *) theCard {
    [book removeObjectIdenticalTo: theCard];
}

the isEqual: method listed above is not called.

I feel like I’m missing something important, but from all I’ve read and I know, I think that isEqual: MUST be called, unless removeObjectIdenticalTo: does not involve it anymore.

I have all respective methods defined in .h files too.

Answer 1

It sounds like the book is in error.

NSMutableArray and other collections have some pairs of similar methods like removeObject: and removeObjectIdenticalTo: with important differences in their meanings.

The first tests whether the values of the objects in the collection are the same as the value of the passed-in object. In order to do this, an array calls isEqual: on each object in the array. This allows every class to define for itself what “equality” to another object means, as you are doing for your AddressCard class. Details about removeObject: can be found in its documentation:

This method uses indexOfObject: to locate matches […] Thus, matches are determined on the basis of an object’s response to the isEqual: message.

The ...IdenticalTo: methods deal with the actual identity in memory of the objects. They check whether the address of the passed-in object is the same as that of any in the collection. An array can find the address of the object without calling any methods on that object; isEqual: is not used here. Again, this is stated in the docs:

This method uses the indexOfObjectIdenticalTo: method to locate matches […] Thus, matches are determined using object addresses.

Generally speaking, you will want removeObject: much more often than removeObjectIdenticalTo:, since its meaning most closely matches the action you wish to perform: select the object whose value is the same as the supplied object’s.