Singleton objective c clarification

Question

as I continue my studies the book implemented a singleton. I understood the reason why use it but I just wanted some clarification regarding the code.

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Answer 1

Apple recommends something like the following

+ (BNRItemStore *)defaultStore
{
    static BNRItemStore *defaultStore = nil;
    static dispatch_once_t done;
    dispatch_once(&done,
                  ^{ defaultStore = [BNRItemStore alloc]init];});
    return defaultStore;
}

The above code assumes ARC - If not using ARC you would have to define do nothing retain, release, autorelease, and dealloc methods.