Question

I am struggling to figure if whether adding an already existing object to a set in Cocoa actually replaces the object or simply ignores addObject: if there is a duplicate. I am using a custom object that is considered the same as another object if a specific field is equal.

I am overriding both isEqual: and hash methods and containsObject: does return true when I call it, but I would like to update the set with the new object and for some reason it doesn’t work if I call addObject:.

If it does ignore it, what would be the best data structure to use in place of NSMutableSet in order to have the desired effect?

Answer 1

NSMutableSet‘s -addObject: method will not add an object which passes the -member: test, as such it doesn’t do anything.

If you still want to update the set with the new object regardless, you can use an NSMutableArray, on which you’d call -replaceObjectAtIndex:withObject::

@interface NSMutableArray(JRAdditions) 
- (void)addUniqueObject:(id)object;
@end

@implementation NSMutableArray(JRAdditions) 
- (void)addUniqueObject:(id)object {
    NSUInteger existingIndex = [self indexOfObject:object];
    if(existingIndex == NSNotFound) {
       [self addObject:object];
       return;
    }
    [self replaceObjectAtIndex:existingIndex withObject:object];
}
@end