banana_dictionary.m

@interface Banana : NSObject
- (id)objectForKeyedSubscript:(id)key;
- (void)setObject:(id)obj forKeyedSubscript:(id <NSCopying>)key;
@end

@implementation Banana
NSMutableDictionary *_attributes;

-(id) init {
  self = [self init];
  if (self) {
    _attributes = [[NSMutableDictionary alloc] init];
  }
  return self
}

- (id)objectForKeyedSubscript:(id)key {
  return _attributes[key];
}

- (void)setObject:(id)obj forKeyedSubscript:(id <NSCopying>)key {
  _attributes[key] = obj;
}
@end

Banana *banana = [[Banana alloc] init];
banana[@"colour"] = @"Yellow";
banana[@"colour"]; // returns "Yellow"