Can Objective-C be given safe categories?

That was the subject of this lunchtime’s vague thinking out loud. The problems with categories are well-known: you can override the methods already declared on a class, or the methods provided in another category (and therefore another category can replace your implementations too). Your best protection is to use ugly wartifying prefixes in the hope that your bewarted method names don’t collide with everybody else’s bewarted method names.

A particular problem with categories, and one that’s been observed in the wild, is when you add a method in a category that is, at some later time, added to the original implementation of the class itself. Other consumers of the class (including the framework it’s part of) may be expecting to work with the first-party implementation, not your substitution. If the first-party method has a different binary interface to yours (e.g. one of you returns a primitive value and the other a struct), as happened to a lot of people with NSArray around the end of the 1990s, prepare to start crashinating.

Later implementations of similar features in other languages have avoided this problem by refusing to add methods that already exist, and by ensuring that even if multiple extensions define the same method they can all coexist and the client code expresses exactly which one it’s referring to. Can we add any of this safety to Objective-C?

Partially. We could design a function for adding a collection of methods from a “category” to a class at runtime, that only adds them if the class doesn’t already implement them. class_addCategory() shows what this might look like, but it only supports non-struct-returning instance methods.

If class_addCategory(target, source, NO) succeeds, then the methods you were trying to add did not exist on the target class before you called the function. However, you cannot be sure that they weren’t being added while your call was in progress, and you can’t know later that they weren’t clobbered by someone else at some point between successfully adding the methods and using them. Also, if class_addCategory() fails, you may find the only reasonable course of action is to not use the methods you were trying to add: the only thing you know about their implementation is that it either doesn’t exist or isn’t the one you were expecting. This is at odds with a hypothetical purist notion of Object-Oriented Programming where you send messages to objects and don’t care what happens as a result.

There are plenty of ways to work around the limitations of categories: composition is the most likely to succeed (more likely than subclassing, which suffers the same collision problem as a later version of the superclass might try to define a method with the same name as one you’ve chosen, which you’re now clobbering). It doesn’t let you replace methods on a class—a tool that like most in the programmer’s utility belt is both occasionally useful and occasionally abuseful.


I should point out that I’m not a fan of taking away the potentially dangerous tools. Many people who see the possibility for a language feature to be abused argue that it should never be used or that languages that don’t offer it should be preferred. This is continuum-fallacy nonsense, to which I do not subscribe. Use whatever language features help you to produce a working, comprehensible, valuable software system: put in whatever protections you want to guard against existing or likely problems.

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