nsorderedset

I did some studies on this post . But none of its solution work for me. Let me explain what I did: It's actually very similar with the mentioned post. Category.h @class Category, Item; @interface Category : NSManagedObject @property (nonatomic, retain) NSString * name; @property (nonatomic, retain) NSOrderedSet *items; @property (nonatomic, retain) Category *parentcategory; @property (nonatomic, retain) NSSet *subcategories; @end @interface Category (CoreDataGeneratedAccessors) - (void)insertObject:(Item *)value inItemsAtIndex:(NSUInteger)idx; - (void)removeObjectFromItemsAtIndex:(NSUInteger

Surely an ordered set is a more-specific case of a set, so why does NSOrderedSet inherit from NSObject rather than NSSet ? I went through the interface of NSSet and you're right, ordered sets appear to satisfy the Liskov substitution principle and could therefor inherit from NSSet . There is one little method that breaks this: mutableCopy . The return value of mutableCopy must be an NSMutableSet , but NSMutableOrderedSet should inherit from NSOrderedSet . You can't have both. Let me explain with some code. First, let's look at the correct behaviour of NSSet and NSMutableSet : NSSet* immutable