“Pro Objective-C Design Patterns for iOS” subclassing Singleton confusion

Question

I did not understand a question, I test the following two methods to create a subclass instance, the results were running well. For example SingletonSon: Singleton, sub-class without any modification, when you call [SingletonSon sharedInstance] or [SingletonSon alloc] returns an instance is SingletonSon instead of Singleton. Contrary to the results with the original content in book, the original said:if not modified subclass Singleton, always returns an instance of Singleton.

    +(Singleton *) sharedInstance  
    {  
       if(sharedSingleton==nil)  
       {  
          sharedSingleton=[[super allocWithZone:NULL] init];  
       }  
       return sharedSingleton;  
    }

    +(Singleton *) sharedInstance  
    {  
       if(sharedSingleton==nil)  
       {  
          sharedSingleton=[NSAllocateObject([self class],0,NULL) init];  
       }  
       return sharedSingleton;  
    }

I am a Chinese student, my English is not very good, hope forgive me. Looking forward to your answer.

Answer 1

Well, I'd remove the "Pro", because the code isn't thread safe at all. Here is the generally accepted pattern to create a singleton:

+(Singleton *)sharedSingleton {

    static dispatch_once_t once;
    static Singleton *sharedSingleton;
    dispatch_once(&once, ^{
        sharedSingleton = [[self alloc] init];
    });
    return sharedSingleton;
}