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Generate a UUID string with ARC enabled

Before ios 6:
- (NSString *)uuidString {
    // Returns a UUID

    CFUUIDRef uuid = CFUUIDCreate(kCFAllocatorDefault);
    NSString *uuidString = (__bridge_transfer NSString *)CFUUIDCreateString(kCFAllocatorDefault, uuid);
    CFRelease(uuid);

    return uuidString;
}

If you are on OS X 10.8 or iOS 6 you can use the new NSUUID class to generate a string UUID, without having to go to Core Foundation:

NSString *uuidString = [[NSUUID UUID] UUIDString];

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