Simple yet efficient integer-to-object dictionaries?

Sometimes I need an NSDictionary-like object that maps a few integers (non-negative) to objects. At first glance, NSMutableArray is great for this, provided that the indexes aren't too high so I came up with a quick category to allow holes in an array:

@implementation NSArray (FoundationAdditions)

-(id) objectAtCheckedIndex:(NSUInteger) index {
    if(index >= self.count) {
        return nil;
    } else {
        id result =  [self objectAtIndex:index];
        return result == [NSNull null] ? nil : result;
    }
}

@end


@implementation NSMutableArray (FoundationAdditions)

-(void) setObject:(id) object atCheckedIndex:(NSUInteger) index {
    NSNull* null = [NSNull null];
    if (!object) {
        object = null;
    }

    NSUInteger count = self.count;
    if (index < count) {
        [self replaceObjectAtIndex:index withObject:object];
    } else {
        if (index > count) {
            NSUInteger delta = index - count;
            for (NSUInteger i=0; i<delta;i++) {
                [self addObject:null];
            }
        }
        [self addObject:object];
    }
}

@end

Is there a better/easier/simpler way to do this? Preferably using something that's pre-existing in the Cocoa stack. Yes I know STL has some pretty good containers in this area, but mixing-in C++ just for this is overkill.