Question

Possible Duplicate:
Dealloc method in iOS and setting objects to nil

As to property dealloc in object c, i have seen different kinds of forms. Which of following is best/correct

//Kind1:
- (void)dealloc
{
    [_property release];
    [super dealloc];
}

//Kind2:
- (void)dealloc
{
    self.property = nil;
    [super dealloc];
}

//Kind3:
- (void)dealloc
{
    [_property release]; _property = nil;
    [super dealloc];
}

Answer 1

Your second option is inadvisable. Property setters may have side-effects and generally aren’t implemented with the consideration that the object may be half torn down already. In general you should avoid any method calls to self (or super) from dealloc, other than [super dealloc]. You should also avoid non-trivial calls (i.e. anything but release) on other objects, as you may have circular references that can lead back to references to your half-deallocated object.

The first option is perfectly sufficient. Note that if you use ARC it is redundant. Using ARC is your best bet, invariably, as it’s not only safer but faster.

The third option is controversial. Some people argue that it makes your program more resilient to errors (since references after dealloc may end at the zeroed instance variable, rather than bouncing through it and seg faulting, or worse). They also argue that it ensures that if you do run a method which tries to use the value, it’ll probably fail gracefully (again, rather than dereferencing what is a dangling pointer at that point). But if you subscribe to my first point on avoiding this to begin with, it becomes somewhat moot. And my experience is that relying on that behaviour is a bad idea – even if it’s a clean design to begin with, it’s easy to forget about it and end up changing the code some time later, in a way that breaks it.

And those that dislike the third option also argue that it hides errors. Some go as far as explicitly overwriting not with nil but rather an obviously bogus value (e.g. 0x0badf00d) to make it clearer if and when a dangling pointer is dereferenced.