带ARC的Xcode 4.2：即使在固件版本低于5.0的iOS设备上，我的代码也能运行吗？
问题描述：
我将Xcode更新为4.2版本，其中包括ARC技术。这似乎是一件好事，但如果我根据Apple的建议启用ARC并编辑我的代码，我的应用程序是否可以在4.3.x设备上构建和运行？或仅适用于iOS 5的？
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?
答
ARC应用程序将在OS 4.0（4.3？）及更高版本上运行。在4.3上你将失去归零 - 弱参考功能。所以如果你依赖__weak，你不应该期望在< 5.0。
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.