Comparing objects in Obj-C

Question

How does one compare objects in Objective-C?

Is it as simple as == ?

I want to check an array for an object and if it doesnt exist add it to the array otherwise, remove it from the array.

Answer 1

Comparing objects in Objective-C works much the same as in Java or other object-oriented languages:

• == compares the object reference; in Objective-C, whether they occupy the same memory address.
• isEqual:, a method defined on NSObject, checks whether two objects are "the same." You can override this method to provide your own equality checking for your objects.

So generally to do what you want, you would do:

if(![myArray containsObject:anObject]) {
    [myArray addObject:anObject];
}

This works because the Objective-C array type, NSArray, has a method called containsObject: which sends the isEqual: message to every object it contains with your object as the argument. It does not use == unless the implementation of isEqual: relies on ==.

If you're working entirely with objects that you implement, remember you can override isEqual: to provide your own equality checking. Usually this is done by comparing fields of your objects.

Answer 2

Every Objective-C object has a method called isEqual:.

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/isEqual:

So you would want to override this for your custom object types.

One particular important note in the documentation:

If two objects are equal, they must have the same hash value. This last point is particularly important if you define isEqual: in a subclass and intend to put instances of that subclass into a collection. Make sure you also define hash in your subclass.

Answer 3

== will compare the pointer, you need to override

- (BOOL)isEqual:(id)anObject

Answer 4

Implement isEqual: and hash

Per the Apple documentation on NSObject you need to implement isEqual: and hash at a minimum. Below you'll find one way to implement object equality, of course how to implement hash enters the land of serious debate here on StackOverflow, but this will work. General rule - you need to define what constitutes object equality and for each unique object they should have a unique hash. It is best practice to add an object specific equality method as well, for example NSString has isEqualToString:.

- (BOOL)isEqual:(id)object
{
    BOOL result = NO;

    if ([object isKindOfClass:[self class]]) {
        CLPObject *otherObject = object;
        result = [self.name isEqualToString:[otherObject name]] &&
        [self.shortName isEqualToString:[otherObject shortName]] &&
        [self.identifier isEqualToString:[otherObject identifier]] &&
        self.boardingAllowed == [otherObject isBoardingAllowed];
    }

    return result;
}

- (NSUInteger)hash
{
    NSUInteger result = 1;
    NSUInteger prime = 31;

    result = prime * result + [_name hash];
    result = prime * result + [_shortName hash];
    result = prime * result + [_identifier hash];
    result = prime * result + _boardingAllowed;

    return result;
}