Object release in Objective-C: Why release an object twice in viewDidUnload and dealloc method? -

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Objective-I have a question about the release of the object. I saw some sample code in chapter 9 of "Safing iPhone 4 Development" (p. 287). The sample code releases an object twice: both viewDownload and Deloop method. Here are sample codes: 

  - (zero) viewDidUutup {self.list = zero; [ChildController release], childController = nil;} - (zero) DLock {[release list]; [Child Controller Release]; [Super Delooc];}   
  Child Controller  has been declared as an example of the  UIViewController  subclass, both of it  Why is the ViewDidInloadload  and  Deloop  method released? Since  childController  is already being released in  viewDidUnload , is it necessary to reissue it in the  dealloc  method? Based on my understanding, I will write the code: 
  
  - (zero) viewDiduude {self.list = zero; ChildController = Zero;} - (Zero) Dailyock {[Release list]; [Child Controller Release]; [Super DeLoc];}   
 Thanks, Sam   
 
 < P> The problem is that every time such as the  dealloc  method is not guaranteed to be called (check)  viewDidUnload .  
  viewDidUnload  is the reason for releasing the object in order to avoid memory leaks. Since  viewDidUnload  is said to have low memory warning, you want to clear the case to avoid troubles.  
 and  zero  will not be the cause of any problem, so it is considered safe to release calls to objects placed in their  dealloc  method Standards are set in the form of zero (such as  viewDidUnload  in your example) after being released at other locations.

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