When a project is set to utilize ARC, the compiler flag -fobjc-arc is by default set for every Objective-C source file. ARC can be disabled for particular classes through the compiler flag, -fno-obj-arc. In Xcode, go to the target Build Phases tab, open the Compile Sources group, and you will be able to see the source file list. When you double-click on the file where you want to set it, a pop-up panel will appear. In that panel, get in the -fno-obj-arc flag and click on Done to finish.
If ARC was not enabled when the project was created, then to enable it, follow this process:
Open the project.
Go to Edit | Refactor | Convert to Objective-C ARC.
If there is no problem and it's ready to convert, it will check your code.
By default, all newly created Objective-C projects in Xcode 5 are enabled with ARC. However, if you need to disable it, follow these steps:
Select Project.
Select Targets.
From the right panel, go to Build Settings.
Select Automatic Reference Counting.
Select...