How do you properly override isEqual: in Objective-C? The “catch” seems to be that if two objects are equal (as determined by the isEqual: method), they must have the same hash value.
The Introspection section of the Cocoa Fundamentals Guide does have an example on how to override isEqual:, copied as follows, for a class named MyWidget:
- (BOOL)isEqual:(id)other {
if (other == self)
return YES;
if (!other || ![other isKindOfClass:[self class]])
return NO;
return [self isEqualToWidget:other];
}
- (BOOL)isEqualToWidget:(MyWidget *)aWidget {
if (self == aWidget)
return YES;
if (![(id)[self name] isEqual:[aWidget name]])
return NO;
if (![[self data] isEqualToData:[aWidget data]])
return NO;
return YES;
}
It checks pointer equality, then class equality, and finally compares the objects using isEqualToWidget:, which only checks the name and data properties. What the example doesn’t show is how to override hash.
Let’s assume there are other properties that do not affect equality, say age. Shouldn’t the hash method be overridden such that only name and data affect the hash? And if so, how would you do that? Just add the hashes of name and data? For example:
- (NSUInteger)hash {
NSUInteger hash = 0;
hash += [[self name] hash];
hash += [[self data] hash];
return hash;
}
Is that sufficient? Is there a better technique? What if you have primitives, like int? Convert them to NSNumber to get their hash? Or structs like NSRect?
(Brain fart: Originally wrote “bitwise OR” them together with |=. Meant add.)
