Cocoa's NSDictionary: why are keys copied?

Question

All objects used as keys in NS(Mutable)Dictionaries must support the NSCopying protocol, and those objects are copied when they\'re used in the dictionary.

I frequently

Answer 1

Since iOS 6 if you want to use pointers as keys, you can use the NSMapTable object, see http://nshipster.com/nshashtable-and-nsmaptable/

You can specify whether keys and/or values are stongly or weakly held:

NSMapTable *mapTable = [NSMapTable mapTableWithKeyOptions:NSMapTableStrongMemory
                                         valueOptions:NSMapTableWeakMemory];

Another option that could be appropriate sometimes is to use NSCache, which holds keys strongly and is actually thread-safe.