On Xcode4.2 memory management, clarification needed

Question

Assuming a new project with ARC enabled is created

@autoreleasepool {
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
}

and the following declaration

@property (nonatomic) NSString *classDescription;

Why would

@synthesize classDescription;

error out with

"Existing ivar 'classDescription' for unsafe_unretained propery 'classDescription must be _unsafe_unretained"

I thought that with Version 4.2 of Xcode, no retains, releases are needed. Is this not correct?

Please clarify

Answer 1

You need to make it a strong property so that ARC knows it should retain the object.

Answer 2

No retains/releases are needed if you are using Automatic Reference Counting. Is this an existing project or new project? New projects should have ARC enabled by default.

Answer 3

If your project was made before Xcode 4.2, then you can enable it by going to Build Settings, search for Objective-C++ Automatic Reference Counting and tick the box.