"Pro Objective-C Design Patterns for iOS" subclassing Singleton confusion -

i did not understand question, test following 2 methods create subclass instance, results running well. example singletonson: singleton, sub-class without modification, when call [singletonson sharedinstance] or [singletonson alloc] returns instance singletonson instead of singleton. contrary results original content in book, original said:if not modified subclass singleton, returns 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 chinese student, english not good, hope forgive me. looking forward answer.

well, i'd remove "pro", because code isn't thread safe @ all. here accepted pattern create singleton:

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