Question

Is it a legitimate practice to rely on a deterministic dealloc (ex: for clean-up)?

Since ARC, and even manual reference counting, is inherently deterministic, I was wondering what other people thought about relying on dealloc getting called immediately (relatively, considering autoreleasepool).

In other modern programming languages, like C#, a dispose-like pattern is employed when you need deterministic clean-up. And I would imagine Obj-C with garbage collection encourages this behavior, as well.

So, with that said, an example would be a UIViewController which cancels outstanding operations in dealloc, rather than trying to program around the sometimes frustrating semantics of viewDidDisappear.

Another example would be a stream object that implicitly opens and closes in init and dealloc, respectively, rather than requiring open or close to be called.

Since Apple has deprecated GC, I would imagine that these sorts of patterns won’t be broken anytime soon, and they are incredibly handy, though I can’t find any documentation on whether this should be encouraged.

Answer 1

You are absolutely correct, you can rely on dealloc being called relatively soon after the last reference is released (manually or though ARC, it does not matter). Unlike GC where the finalizer is called when the system has some free time, or in some cases is never called, dealloc gets called very reliably. Apple allows and even encourages using this pattern by suggesting that we should perform all of our resource clean-up tasks inside dealloc.

This does not mean, however, that you should rely on dealloc exclusively. For example, take a look at the NSStream class: it offers you an explicit close method, letting you force closing the stream at will, without waiting for the call of dealloc to happen. This is a very good pattern to follow in case the resource is very expensive (file handles, semaphores, etc.): the primary mechanism for releasing these resources should be a separate close method. The dealloc method should release the resource as well, but it should also issue a warning, informing your that you missed a call of close.