How do I replace weak references when using ARC and targeting iOS 4.0?

I've begun developing my first iOS app with Xcode 4.2, and was targeting iOS 5.0 with a "utility application" template (the one that comes with a FlipsideViewController).

I read that since ARC is a compile-time feature, it should be compatible with iOS 4 as well, so I attempted to target my app to 4.3, and try compiling it. When I do so, I get this error:

FlipsideViewController.m: error: Automatic Reference Counting Issue: The current deployment target does not support automated __weak references

It is referencing this line:

@synthesize delegate = _delegate;

That variable is declared as:

@property (weak, nonatomic) IBOutlet id <FlipsideViewControllerDelegate> delegate;

I understand that "weak references" are not supported in iOS 4, but I don't really understand why I would want to use a weak reference to begin with, nor can I figure out how I would rewrite things to avoid using it, while still taking advantage of ARC (after all, it's supposed to work with iOS 4 AND 5 right?)

To target the older OS, you can use unsafe_unretained instead of weak in your property declaration, and it should mostly work the same way. weak references nil themselves when their target goes away, but unsafe_unretained leaves open the possibility that the object you're linking to could turn into a dangling pointer when it is deallocated. The latter is the same behavior as if you had used assign as a property declaration in manual memory management.

You do this to avoid retain cycles, which I mention in my answer here. You don't want to have a strong pointer to something that might have a strong pointer back to the original object. Then nothing would get released properly.

If only using weak references for additional safety, manually call the new runtime functions if they're available and fallback to simple assignment on __unsafe_unretained variables if not.

ZWRCompatibility.h will simplify this somewhat.

Thanks to Mike Ash's compatibility library PLWeakCompatibilty, you can now simply use __weak on iOS 4.x, as well.

It's incredibly easy to configure and requires no additional consideration or effort over 5.x.