Why would you set-to-nil AND release a property of a UIViewController?

Question

There's something I don't get in Apple documentation. Here's an extract from the -(void)viewDidUnload of the UIViewController class :

your dealloc method should release each object but should also set the reference to that object to nil before calling super.

If you use a retain and a synthetize for a xxx attribute in your code, why do most of Apple examples do a set-to-nil in viewDidUnload :

self.xxx = nil;

but recommands to do both a set-to-nil AND a release in the dealloc :

[xxx release];
self.xxx = nil;

Why is the set-to-nil not enough for the dealloc ?

ps : i know my question is very similar to this one "Why release a property that you've already set to nil?" but it's not strictly the same

Answer 1

[xxx release];
self.xxx = nil;

This is wrong as xxx will get released twice, recommended way is release iVar and set it to nil, not using property:

[xxx release];
xxx = nil;

The reason for not just using

self.xxx = nil;

is that calling setter method may have some side effects that may cause problems in dealloc method (ie use some other iVar that may be already deallocated)