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I've learned that in dealloc you do [object release]; but in viewDidUnload (in a UIViewController subclass) you do self.object = nil. What is really the difference because self.object = nil (we're assuming object is a (nonatomic, retain) property) retains nil (which does nothing) and then releases the old value and then the reference count is 0 right?
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You can validly send any message to a "nil" pointer in Objective-C. This is very different to languages like C++ where invoking a method on a "NULL" pointer will likely crash your program. Sending a message to "nil" will have only one effect: it will return a zero value. No other action will occur.
In Swift: nil is not a pointer, it's the absence of a value of a certain type. NULL and nil are equal to each other, but nil is an object value while NULL is a generic pointer value ((void*)0, to be specific). [NSNull null] is an object that's meant to stand in for nil in situations where nil isn't allowed.
It's Shorthand writing. In Objective-C, any character , numeric or boolean literal prefixed with the '@' character will evaluate to a pointer to an NSNumber object (In this case), initialized with that value. C's type suffixes may be used to control the size of numeric literals.
self.object = nil calls your setter, which will release the old value, set the member to nil, and possibly do other things (it's a method, so it could do anything). The "anything" part of that is potentially dangerous; see this question, for example.
[object release] releases the old value, but leaves the member as a now-dangling pointer, which is a good recipe for bugs. In dealloc it doesn't really matter, since the pointer itself is about to go away too, but in any other case it's a very bad idea to release a member without setting it to nil.
(As a sidenote, you should never assume that releasing an object gives it a reference count of 0. It releases your reference, but other objects may still have references to it.)
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