On Aug 20, 2009, at 9:44 PM, Quincey Morris wrote:
The keys in a dictionary (or other keyed collection, like a map table) need to be immutable objects, but that's unrelated to the hash. Mutability in the keys would be bad, hash or no hash.
What do you mean by immutable? You can put a "mutable" object in a hashing collection as long as its -hash and -isEqual: do not depend on mutable state. For instance, if you configure a CFDictionary to use pointer equality and hash for its keys, you can use an NSMutableString as a key.
[...]
Separately, collections use isEqual: on the object values to determine equality, when they care. (I doubt that NSDictionary cares about object value equality, but NSSet certainly does, as does NSArray, when asked to compare objects.)Separately, collections may (and presumably sometimes do) use *object* hash values to distribute objects in memory. (Obviously, NSSet does, and for all we know NSDictionary does too, to distribute object values that stored under the same key hash value -- though it would be an implementation detail.)
I use NSMutableStrings as values (not keys) in NSDictionary and expect that to work, even though hash/isEqual: of those strings is certainly changing while in the collection. CFDictionary at least doesn't include a hash function in its value callback, although it does require an equality function. From my quick look at the source, the only place that equality callback is used is in testing CFEqual(dict1, dict2), CFDictionaryGetCountOfValue(), and CFDictionaryContainsValue().
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