Using Xcode 4.2, how can one enable/disable "Automatic Reference Counting"?
ANSWERED Under Build Settings, flip "yes" and "no" depending whether you want ARC enabled.
Globally:
Go to "Build Settings", look for "Apple LLVM compiler 3.0 - Language". Set the field "Objective-C Automatic Reference Counting" to "No".
For individual files:
Go to "Build Phases", select the file, double-click the "Compiler Flags" column and put "-fno-objc-arc" in it.
To enable ARC per-file, you can also set your "Compiler Flags" under "Build Phases" to -fobjc-arc. Make sure your compiler is set to Apple LLVM compiler 4.x and not LLVM GCC 4.2.
In XCode 4.6 this property has now changed to CLANG_ENABLE_OBJC_ARC
来源：https://stackoverflow.com/questions/7778356/how-to-enable-disable-automatic-reference-counting