Question

In my app, I have a NSDictionary whose keys should be instances of a subclass of NSManagedObject.

The problem, however, is that NSManagedObject does not implement the NSCopying protocol which means that no Core Data objects / instances of NSManagedObject can be used as dictionary keys even though the -[hash] method works fine for them.

Was should I do?

Answer 1

There are four options:

1. Use a different object as the dictionary key instead, and lookup from that. [object objectID] or +[NSValue valueWithNonretainedObject:] seem the most obvious
2. Use CFDictionaryCreateMutable() to create a dictionary with retained keys, rather than copied, instead, and then call CFDictionarySetValue() to store the objects
3. On OS X or iOS6+, [NSMapTable mapTableWithStrongToStrongObjects] gives you a purely Objective-C equivalent to CFMutableDictionary
4. Implement NSCopying for your managed object subclass, such that it returns self (with a bumped reference count if you’re not using ARC)

Notes

+valueWithNonretainedObject: is pretty dangerous, since it’s possible to be left with a dangling pointer; likely best to avoid.

Storing object IDs is fine, apart from the fact that new objects start out life with a temporary ID. That ID then changes to a permanent one when the context is saved to disk (or -obtainPermanentIDsForObjects:… is called). Your mapping code needs to be smart enough to handle this unless it can guarantee that all incoming objects already have a permanent ID.

Implementing NSCopying like this feels a bit icky, but should work just fine. As it happens, this is exactly the approach NSURLSessionTask takes, I presume for dictionary friendliness.

Prior to OS X 10.8 Mountain Lion, it used to be possible to create a regular NSMutableDictionary and then call CFDictionarySetValue() for it. That’s no longer the case though; new dictionaries now have proper copy callbacks specified down at the CF level, rather than purely being a feature of NSMutableDictionary.