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I am currently working at a little programming project. After Deutsche Telekom's website turned out to be too braindead to analyse our phone bill properly, I decided to have my own little program for this task. While the main work is done and I am just adding some nice touches, there was a bit of pain on the way. Pain in the form of things to learn about the NSDictionary class.
The first thing is: You can use any object as a key for your entries in the dictionary, not only strings. A silly coincidence with my variable names and a couple of typos in #define statements got me into quite some trouble there that cost a lot of time tracking down and sorting out.
The second thing is: If you use your own classes as keys, you should RTFM properly, keep in mind that the keys are copied and and implement the -copyWithZone: and -hash methods. Once this is done, everything works like a charm.