是否NSArray：lastObject返回一個自動釋放對象？

Question

我正在研究一個iPhone項目，我想從NSMutableArray中檢索一個對象，從該數組中刪除該對象，然後再使用它。代碼如下所示：

NSMutableArray * array; 
// fill the array 
NSObject * obj = [array lastObject]; 
[array removeLastObject]; 
// do something with obj (in the same function) 

數組是唯一的實體在被訪問的對象上有一個保留。這是安全的嗎？我會假設這隻會在lastObject自動釋放對象的時候起作用，這是我無法弄清楚的。

Answer 1

爲什麼不叫

[array removeLastObject] 

在你的函數結束了嗎？這是少一個版本/保留。它可能會使您的代碼更易讀/更少混亂。

備案Apple documentation：

Like NSArray, instances of NSMutableArray maintain strong references to their contents. If you do not use garbage collection, when you add an object to an array, the object receives a retain message. When an object is removed from a mutable array, it receives a release message. If there are no further references to the object, this means that the object is deallocated. If your program keeps a reference to such an object, the reference will become invalid unless you send the object a retain message before it’s removed from the array. For example, if anObject is not retained before it is removed from the array, the third statement below could result in a runtime error:

id anObject = [[anArray objectAtIndex:0] retain]; 
[anArray removeObjectAtIndex:0]; 
[anObject someMessage]; 

Answer 2

NSObject * obj = [array lastObject]; // Just returns an object, its retain count not changed 
[array removeLastObject]; // object receives release message 

所以，如果你的陣列是保留您的對象，然後取出它的保留計數後的唯一實體變爲零，它應該得到釋放。要使用該對象，你應該保留它，並釋放不需要的時候安全地工作：

NSObject * obj = [[array lastObject] retain]; 
[array removeLastObject]; 
// do something with obj (in the same function) 
[obj release]; 

Answer 3

值得一提這裏的Cocoa Memory Management Rules。如果您在非GC環境中工作，例如iPhone，它總是值得一提的。

基本規則狀態：

You take ownership of an object if you create it using a method whose name begins with 「alloc」 or 「new」 or contains 「copy」 (for example, alloc, newObject, or mutableCopy), or if you send it a retain message. You are responsible for relinquishing ownership of objects you own using release or autorelease. Any other time you receive an object, you must not release it.

顯然沒有用ALLOC，新...什麼的含副本獲得的obj。所以你不擁有它。

在你的代碼中，推論的第二個也是相關的：

A received object is normally guaranteed to remain valid within the method it was received in (exceptions include multithreaded applications and some Distributed Objects situations, although you must also take care if you modify an object from which you received another object). That method may also safely return the object to its invoker

我已經加粗的相關部分。您已經修改了從中接收對象的數組，因此它可能會消失。您可以在從陣列中刪除對象之前保留該對象，或者在完成對象後執行刪除操作。

其實，你可能是安全，即使在你的問題的代碼，因爲NSMutableArray中的實現的objectAtIndex的：可能做挽留隨後在退貨項目自動釋放。在多線程環境中，這將是唯一的方法來確保對象保持足夠長的時間以將其返回給調用者。但是，我沒有看到NSMutableArray的代碼，所以不要依賴它。

ETA：剛剛在看線程編程指南，看來NSMutableAtrray不會保留，autorelease。