So I just found out that ARC is only supported for devices running iOS 4.0+.
How does my app (in the store) work on devices running iOS 3.0 then (definitely using ARC)?
I'm so confused.
Solution from below: As long as I don't use zeroing weak references it seems that using ARC still works with iOS 3.x, but Apple just has not guaranteed the result.
ARC is mostly a compile time feature, the compiler inserts the appropiate release calls. AFAIK only zeroing weak references need runtime support. So as longs as you don't use those you're on thin ice, but it can work.
Because ARC works at compile time so you can link with SDK 5.0 and support iOS 4.0 if you want. The app will crash on 4.0 if you use any function that's been intruduced on 5.0.
来源：https://stackoverflow.com/questions/9646607/how-does-my-arc-app-work-in-ios-3-x