Does ARC set its reference type instance properties to nil before deallocation?

Question

This question occured to me while reading this.

My question is in reference to the image below:

Once john is set to nil, Person instance no longer has any more strong reference and hence will be deallocated. But Apartment has two strong references and one of which is by the property on Person instance that would be soon deallocated. I believe, this strong reference continue to remain after deallocation and goes out of reach by the code.

So, setting unit14A to nil will remove only one strong reference to Apartment instance and it should not be deallocated as there would be one more strong reference due to the above case.

But, as the document says Apartment instance promptly got deallocated. To me, this can only happen if at the time of Person instance deallocation it sets its apartment property to nil, by that removing that strong reference on Apartment instance. But I couldn't find any documentation to verify this.

So, How does the Apartment instance get deallocated? What happened to the strong reference from the Person instance apartment property?

Can someone help me to understand this?

Answer 1

Objective-C objects are reference counted, meaning that for each object, the system keeps track of how many other objects hold a reference to it. This is object's reference count. Two special messages, retain and release, are used to maintain the reference count behind the scene. Once reference count goes down to zero, the system deallocates the object.

ARC provides "magic" to make reference counting work in a declarative way. The compiler knows every strong reference in your code, so when you do this

myStrongRef = nil;

the compiler quietly inserts a call to release in front of the assignment:

[myStrongRef release];
myStrongRef = nil;

To me [deallocation of Apartment] can only happen if at the time of Person instance deallocation it sets its apartment property to nil, by that removing that strong reference on Apartment instance.

Setting a strong reference to nil one way of breaking a strong reference. It is sufficient, but it isn't necessary. The important thing about setting a strong reference to nil is not the act of setting itself, but what happens immediately before it: the instance referred to by the strong reference gets a release message, instructing it to decrement its reference count. That is precisely what ARC does behind the scene for you: it sends the release message to Apartment, without setting Person's reference to nil.

How does the Apartment instance get deallocated? What happened to the strong reference from the Person instance apartment property?

Once strong reference from Person has sent its release message to Apartment, that strong reference disappears. The actual pointer may be set to Apartment's address, but nobody cares about it, because the Person itself is unreachable.