苹果文档Transitioning to ARC Release Notes里描述了好几种处理arc和非arc情况的方法，我遇到的情况是需要在一个非arc的工程里载入一个同步twitter的库，而该库是使用arc模式的文件。为了使两者兼容，我采用如下方法，顺利通过编译：
Use Compiler Flags to Enable and Disable ARC
You enable ARC using a new -fobjc-arc compiler flag. You can also choose to use ARC on a per-file basis if it’s more convenient for you to use manual reference counting for some files. For projects that employ ARC as the default approach, you can disable ARC for a specific file using a new-fno-objc-arc compiler flag for that file.
ARC is supported in Xcode 4.2 for Mac OS X v10.6 and v10.7 (64-bit applications) and for iOS 4 and iOS 5. Weak references are not supported in Mac OS X v10.6 and iOS 4. There is no ARC support in Xcode 4.1 and earlier.
操作截图：
这种在xcode中设置文件的编译方式的方法，觉得还是挺方便的。
其他方式可以查阅文档的该部分，就不列举了。
转自:http://blog.csdn.net/likendsl/article/details/7711887