ARC(Automatic Reference Counting) in iOS 5

since: 2011/11/12
update: 2011/11/12
references:
Beginning ARC in iOS 5 Tutorial Part 1 | Ray Wenderlich


   1. 當在專案中使用 ARC 時, 將不再使用 retain, release 和 autorelease;
      因為啟用 ARC 功能後, 編譯器將會為你處理這些問題.

   2. "strong" 指標: 可保持讓物件存活; 預設所有的 instance variables 與 local variables
       都是 strong pointers.

   3. "weak" 指標: 仍然會指向物件, 但是沒有擁有權.
       a. 當 weak pointer 指向的物件被釋放後, weak pointer 本身變成 nil, 稱為:
           "zeroing" weak pointer; 因而不會產生 "dangling pointers" 或 "zombies" 的問題.

       b. weak pointers 的常用時機: 當二個物件處於 parent-child 關係.
          => parent 物件有一個 strong pointer 指向 child 物件, 因此擁有 child 物件;
              但是為了防止擁有權的無盡循環, child 物件只有一個 weak pointer 指回
              parent 物件.
          ex: delegate pattern
              (1). view controller: may own a UITableView through a strong pointer.
              (2). The table view's data source and delegate pointers point back
                    at the view controller, but are weak.

   4. assigning retained object to weak variable;
      object will be released after assignment.
      ex:
      __weak NSString *str = [[NSString alloc] initWithFormat:...];
      NSLog(@"%@", str); // will output "(null)"

   5. ARC only works on Objective-C objects. If your app uses Core Foundation
      or malloc() and free(), then you're still responsible for doing the memory
      management there.