CODEWITHSUNDEEP
objective-cautomatic-ref-counting
J Shapiro
J Shapiro

Reputation: 3861

ARC and __unsafe_unretained

I think I have a pretty good understanding of ARC and the proper use cases for selecting an appropriate lifetime qualifiers (__strong, __weak, __unsafe_unretained, and __autoreleasing). However, in my testing, I've found one example that doesn't make sense to me.

As I understand it, both __weak and __unsafe_unretained do not add a retain count. Therefore, if there are no other __strong pointers to the object, it is instantly deallocated (with immutable strings being an exception to this rule). The only difference in this process is that __weak pointers are set to nil, and __unsafe_unretained pointers are left alone.

If I create a __weak pointer to a simple, custom object (composed of one NSString property), I see the expected (null) value when trying to access a property:

Test * __weak myTest = [[Test alloc] init];
myTest.myVal = @"Hi!";
NSLog(@"Value: %@", myTest.myVal); // Prints Value: (null)

Similarly, I would expect the __unsafe_unretained lifetime qualifier to cause a crash, due to the resulting dangling pointer. However, it doesn't. In this next test, I see the actual value:

Test * __unsafe_unretained myTest = [[Test alloc] init];
myTest.myVal = @"Hi!";
NSLog(@"Value: %@", myTest.myVal); // Prints Value: Hi!

Why doesn't the __unsafe_unretained object become deallocated?

[EDIT]: The object is being deallocated... if I try to substitute lines 2 - 3 with NSLog(@"%@", myTest); the app crashes (and an overridden dealloc in Test is being called immediately after the first line). I know that immutable strings will continue to be available even with __unsafe_unretained, and that a direct pointer to the NSString would work. I am just surprised that I could set a property on a deallocated object (line 2), and that it could later be dereferenced from a pointer to the deallocated object it belonged to (line 3)! If anyone could explain that, it would definitely answer my question.

Upvotes: 2

Views: 2513

Answers (5)

Vadym
Vadym

Reputation: 9

Nothing strange there… You need to have at least 1 strong reference to object to keep it alive.

Test * anTest = [[Test alloc] init];
Test * __weak myTest = anTest;
myTest.myVal = @"Hi!";
NSLog(@"Value: %@", myTest.myVal); // Prints Value: (Hi)

Upvotes: 0

CRD
CRD

Reputation: 53000

I am just surprised that I could set a property on a deallocated object (line 2), and that it could later be dereferenced from a pointer to the deallocated object it belonged to (line 3)! If anyone could explain that, it would definitely answer my question.

When the object is deallocated it is not zeroed. As you have a pointer to the deallocated object and the property value is stored at some offset to that pointer it is possible that storing and retrieving that property value will succeed after deallocation, it is also quite possible that everything will blow up for some reason or other.

That your code works is quite fragile, try debugging it with "Show Disassembly While Debugging" and stepping through, you'll probably hit an access violation, or take down Xcode itself...

You should never be surprised that strange things happen in C, Objective-C, C++ or any of the family; instead reserve your surprise for so few strange things happening!

Upvotes: 3

Mario
Mario

Reputation: 4520

Because the constant string in objc is a constant pointer to a heap address and the address is still valid.

edited after comment:

Maybe because the memory at the test objects address hasn't been overwritten and still contains that object? Speculating....

Upvotes: 2

bbum
bbum

Reputation: 162712

@"hi!" produces a static global constant string instance that is, effectively, a singleton. Thus, it'll never be deallocated because it wasn't really allocated in the first place (at least, it really isn't a normal heap allocation).

Anytime you want to explore object lifespan issues, always use a subclass of NSObject both to guarantee behavior and to make it easy to drop in logging hooks by overriding behavior.

Upvotes: 1

Darren
Darren

Reputation: 25619

You can see when Test is deallocated by implementing its -dealloc method and adding some simple logging.

However, even if Test is deallocated immediately, the memory it occupied in RAM may remain unchanged at the time you call myVal.

Upvotes: 1

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