I updated my Xcode to 4.2 version, which includes the ARC technology. It seems to be a good thing, but if I enable ARC and edit my code according to Apple's suggestions, will my app build&run even on 4.3.x devices? Or only for iOS 5 ones?
Joshua Weinberg
ARC applications will run on OS 4.0 (4.3?) and up. On 4.3 you will lose the zeroing-weak-reference feature though. So if you rely on __weak, you shouldn't expect to work properly on < 5.0.
来源：https://stackoverflow.com/questions/7768861/xcode-4-2-with-arc-will-my-code-run-even-on-ios-devices-with-firmware-older-tha