Is releasing memory of Objective-c 2.0 properties required?

Question

Something I have been wondering about properties for a while. When you are using properties, do you need to override the release message to ensure the properties are released properties?

i.e. is the following (fictitious) example sufficient?

@interface MyList : NSObject {
NSString* operation;
NSString* link;
}
@property (retain) NSString* operation;
@property (retain) NSString* link;
@end

@implementation MyList
@synthesize operation,link;
@end

Answer 1

No, you override the -dealloc method. And yes, if you don't release your properties (or rather, the backing ivars), you will leak. So in your @implementation here you should have something like

- (void)dealloc {
    [operation release];
    [link release];
    [super dealloc];
}

Answer 2

You should always release the backing variables in dealloc:

- (void) dealloc {
   [operation release];
   [link release];

   [super dealloc];
}

Another way:

- (void) dealloc {
   self.operation = nil;
   self.link = nil;

   [super dealloc];
}

That's not the preferred way of releasing the objects, but in case you're using synthesized backing variables, it's the only way to do it.

NOTE: to make it clear why this works, let's look at the synthesized implementation of the setter for link property, and what happens when it is set to nil:

- (void) setLink:(MyClass *) value {
   [value retain]; // calls [nil retain], which does nothing
   [link release]; // releases the backing variable (ivar)
   link = value;   // sets the backing variable (ivar) to nil
}

So the net effect is that it will release the ivar.

Answer 3

In non-GC applications, yes. It is usual to assign nil instead of releasing the ivars. My best experience is to release ivars initialized with init and assign nil to properties with retain and copy mode.

In your case I would assign nil

- (void) dealloc {
   self.operation = nil;
   self.link = nil;
   [super dealloc];
}

Answer 4

Synthesizing a property only creates getter and setter methods, and therefor won't release the ivar when the object is deallocated. You need to release the ivar yourself.

Answer 5

The best way to do this is:

- (void)dealloc {
    [operation release], operation = nil;
    [link release], link = nil;

    [super dealloc];
}

It would indeed be more convenient to use the generated setter methods

self.operation = nil;

but that is frowned upon. You don't always know which thread an object is deallocated on. Thus using an accessor may cause problems by triggering KVO notifications.

The catch here is that you need to adapt your dealloc to match the object management policy defined in your @property. E.g. don't go releasing a iVar backing an (assign) property.