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Faking instance variables in Objective-C categories with Associative References

In OS X 10.6 and iOS 3.1, Apple added Associative References to the Objective-C runtime. Essentially, this means that each and every object has an optional dictionary you can add arbitrary key/value pairs to.

This is a great feature, especially considering that Objective-C has forever had a feature to add methods to existing classes: categories. Categories, however, do not permit you to add instance variables. Using associative references, it’s easy to fake ivars.

Ole Begemann

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