equality of two objects in Objective C

Question

I am reading the Programming with Objective-C . In the section of Determining Equality of Objects , it says the following words:

- When dealing with objects, the == operator is used to test whether two separate pointers are pointing to the same object:

if (firstPerson == secondPerson) {
    // firstPerson is the same object as secondPerson
}

- If you need to test whether two objects represent the same data, you need to call a method like isEqual:, available from NSObject:

if ([firstPerson isEqual:secondPerson]) {
    // firstPerson is identical to secondPerson
}

I get confused about the differences between == and isEqual with the above explanation, does it mean firstPerson == secondPerson is an alternative of [firstPerson isEqual:secondPerson] ?

Answer 1

NSString *string1 = [[NSString alloc] initWithString:@"some string"];
NSString *string2 = [[NSString alloc] initWithString:@"some string"];
NSString *string3 = string2;

BOOL equal1 = (string1 == string2); // NO

BOOL equal2 = [string1 isEqual:string2]; // YES

BOOL equal3 = (string2 == string3); // YES

BOOL equal4 = [string2 isEqualToString:string3]; // YES

Answer 2

The definition of == is correct, it checks to see that they're pointing to the actual same pointer/memory address (ie. 0xffffff)

The key to understanding what you're asking is to think about what you mean by the word "equal". "equal" typically means, from the developer's point of view, that "these two objects contain the same data in the fields that I require for all practical purposes". You can have two user objects each with the same userID property but different times in lastUpdated - would you consider them equal? Depends on your use case. Most likely you would say yes because they're the same user. They were updated from the server at different times, so some fields differ, but for your implementation, they're equal.

In the case above, are they the same object? Definitely not. They point to different memory addresses. So == would be NO, whereas if you wrote your isEqual: method to check just the userID property, it would return YES

The definition of isEqual: is entirely up to the author of the class. isEqual: can be written to use == if you wanted. All you have to do, in your class, is to override the isEqual: method which is defined by the NSObject protocol.

If you have a custom object, use isEqual: to define what your definition of equal is. In the example of a user object, you might define:

- (BOOL)isEqual:(id)otherObject {
    if ([otherObject isKindOfClass:[self class]]) {
        MyClass *otherObjectAfterCast = (MyClass*)otherObject;
        
        if ([otherObjectAfterCast.userID isEqualToString:self.userID])
            return YES;
    }
    
    return NO;
}

Technically you'd probably want to use caseInsensitiveCompare: or something like that but you get the drift...

isEqual: can also be used to trigger other methods - in the case of NSString - calling isEqual: when both operands are strings results in a call to isEqualToString: - which is why the documentation recommends calling isEqualToString: if you know they're both strings, since it's a bit faster.

So, isEqual: is whatever you make of it, or whatever the class author has defined it to be.

This is also a pretty clear definition in the docs (for once lol): NSObject Protocol Reference

Hope this helps! Let me know if you need any further clarification.

Answer 3

The simple version is this.

== tells you if the pointers are the same object or not.

The isEqual: family of methods do something different. They tell you if the objects at the other end of the pointers are effectively the same based on some criteria such as the properties or ivars holding equal values or whatever logic is implemented in the method used. They may or may not be the exact same object.