Question

I have a few basic questions regarding the syntax and usage of properties in Objective C:

Assume the following declaration in the header:

@interface TestObject : NSObject {

    NSArray *myArray;
}

@property (nonatomic, retain) NSArray *myArray;

In the implementation, can I:

1. List item
2. Use myArray and self.myArray interchangeably for setting and getting purposes?
3. Is self.myArray = nil equivalent to [myArray release]?
   If so, Is there ever a reason to use self.myArray = nil rather than [myArray release]?

Answer 1

1. myArray and self.myArray are actually different. myArray is accessing the variable directly, whereas self.myArray (which is equivalent to [self myArray]) is calling an accessor method. Most people agree that you should use self.myArray (or [self myArray]) all the time, and never use myArray directly. This is because the accessor might have side effects; for example, KVO won’t work if you set your variable directly, and memory management won’t be handled for you.

2. Your property is declared with retain, so self.myArray = anArray (which is the same as [self setMyArray:anArray]) does the following:

   1. Retain anArray, which will soon be the new myArray.
   2. Release the old myArray, which will soon no longer be myArray.
   3. Change the pointer myArray so it now points to anArray.

Therefore, when you do self.myArray = nil, one of the steps (#2) is indeed release the old array. (And since the new one is nil, we don’t have to worry about its memory management, even though we retained it.) So yes, self.myArray = nil is a valid way of releasing myArray.

HOWEVER, if you’re talking about releasing myArray in dealloc, it’s generally a good idea to use [myArray release], because calling self.myArray = nil will have side effects if any other objects are observing myArray through KVO. So while it does follow memory management standards, it’s not a good idea to write your dealloc method using self.myArray = nil.