What classes can be used as a key in NSDictionary?

Can we use only NSString objects as key in an NSDictionary? How do we know which objec开发者_如何学Pythonts can be used and which cannot?

From the documentation:

In general, a key can be any object (provided that it conforms to the NSCopying protocol), but note that when using key-value coding the key must be a string (see “Key-Value Coding Fundamentals”).

So you can use anything copyable besides strings, but they'll be problematic with KVC. I just use strings for keys to keep things safe, consistent and simple.

You can use anything that conforms to NSCopying. That is, you can use id - type objects, as long as they conform to NSCoding protocol.

In instances where the key is NSString, then isEqualToString: is called for retrieval. Otherwise, isEqual: is called on the object to determine whether the key matches the requested key.

The key (and value for that matter) cannot be nil or NULL. They can, however, be [NSNull null].