Objective-C Literals: Negative Array Subscripts

NSArray+NegativeIndexes.m

//
//  NSArray+NegativeIndexes.m
//  Allow Negative Array Literals
//
//  Created by Matthew Robinson on 25th October 2012.
//  Copyright (c) 2012 Matthew Robinson. All rights reserved.
//
//  WARNING: This is a proof of concept and should not be used
//           in production code.  This could break at anytime.

#import <Foundation/Foundation.h>

#if __has_feature(objc_subscripting)

#import <objc/runtime.h>

@implementation NSArray (NegativeIndexes)

+ (void)load {
    method_exchangeImplementations(
        class_getInstanceMethod(self, @selector(objectAtIndexedSubscript:)),
        class_getInstanceMethod(self, @selector(BLC_objectAtIndexedSubscript:))
    );
}

- (id)BLC_objectAtIndexedSubscript:(NSInteger)idx {
    if (idx < 0) {
        idx = [self count] + idx;
    }

    return [self BLC_objectAtIndexedSubscript:idx];
}

@end

@implementation NSMutableArray (NegativeIndexes)

+ (void)load {
    method_exchangeImplementations(
        class_getInstanceMethod(self, @selector(setObject:atIndexedSubscript:)),
        class_getInstanceMethod(self, @selector(BLC_setObject:atIndexedSubscript:))
    );
}

- (void)BLC_setObject:(id)anObject atIndexedSubscript:(NSInteger)idx {
    if (idx < 0) {
        idx = [self count] + idx;
    }

    [self BLC_setObject:anObject atIndexedSubscript:idx];
}

@end

#endif

If the index is negative it is used as an offset from the end of the array instead of the beginning. For example, the last element of the array will have an index of -1, the second last element has an index of -2. This is a common feature in scripting languages.

My code doesn't contain any error handling as that will be handled by the original swizzled methods.

This really shouldn't be used in production code as I can't even think of all the things that could go wrong with it.

Great stuff. This inspired me to https://gist.github.com/mickeyl/d266e567da2e3af2ac0b