Where's the difference between setObject:forKey: and setValue:forKey: in NSMutableDictionary?

setValue:forKey: is part of the NSKeyValueCoding protocol, which among other things, lets you access object properties from the likes of Interface Builder. setValue:forKey: is implemented in classes other than NSDictionary.

setObject:forKey: is NSMutableDictionary's reason to exist. Its signature happens to be quite similar to setValue:forKey:, but is more generic (e.g. any key type). It's somewhat of a coincidence that the signatures are so similar.

What adds to the confusion is that NSMutableDictionary's implementation of setValue:forKey: is equivalent to setObject:forKey: in most cases. In other classes, setValue:forKey: changes member variables. In NSMutableDictionary, it changes dictionary entries, unless you prefix the key with a '@' character -- in which case it modifies member variables.

So, in a nutshell, use setObject:forKey: when you need to work with dictionary keys and values, and setValue:forKey: in the rarer cases where you need to tackle KVP.

EDIT: and oh, it looks like this has been asked and answered before: Difference between objectForKey and valueForKey?

Another difference is that if you give a nil value to setValue:forKey:, it removes the key from the dictionary if it exists, otherwise does nothing. But if you give a nil value to setObject:forKey:, it raises an exception.

anObject — The value for key. The object receives a retain message before being added to the NSDictionary. This value must not be nil.

aKey — The key for value. The key is copied (using copyWithZone:; keys must conform to the NSCopying protocol). The key must not be nil.

value — The value for key.

key — The key for value. Note that when using key-value coding, the key must be a string (see “Key-Value Coding Fundamentals”).