クラスに [], {} を実装する - Qiita

// id obj = custom[1] - (id)objectAtIndexedSubscript:(NSUInteger)idx; // custom[0] = @10 - (void)setObject:(id)obj atIndexedSubscript:(NSUInteger)idx; - (id)objectAtIndexedSubscript:(NSUInteger)idx { return object; } - (void)setObject:(id)obj atIndexedSubscript:(NSUInteger)idx { ... }