Using UITouch as key in NSMutableDictionary?

Question

I'm storing some information about touches, and am using the fact that the touch objects are the same ones for the same touch. Now I wanted to make this more efficient by using the UITouch as the key in a NSMutableDictionary for faster lookup - but that doesn't work. Apparently, the dictionary tries to copy the object, but it throws an exception in some copyWithZone method.
I guess I could encapsulate the hash value in a simple class to use as the key, but I wonder if there is a better way to do this? Also, why does NSMutableDictionary try to copy the object that's used the key? Why doesn't it simply store the pointer?

Answer 1

From the +iPhone Application Programming Guide+
In your event-handling code you can store relevant bits of touch state for later comparison with the mutated UITouch instance. As an example, say you want to compare the final location of each touch with its original location. In the touchesBegan:withEvent: method, you can obtain the original location of each touch from the locationInView: method and store those in a CFDictionary object using the addresses of the UITouch objects as keys. Then, in the touchesEnded:withEvent: method you can use the address of each passed-in UITouch object to obtain the object’s original location and compare that with its current location. *(You should use a CFDictionary object rather than an NSDictionary object; the latter copies its keys, but the UITouch class does not adopt the NSCopying protocol, which is required for object copying.)*
http://developer.apple.com/iPhone/library/documentation/iPhone/Conceptual/iPhone OSProgrammingGuide/EventHandling/EventHandling.html#//apple_ref/doc/uid/TP400070 72-CH9-SW17