I am using ARC. I have a method that runs at the end of a game I have written which should clear up memory. There are a series of objects in an NSMutableArray, which i remove using removeObject:. I then set these objects to nil. However, using NSLog on these objects shows that they still exist. Why does setting them to nil not remove them from memory?
In ARC (automatic reference counting), setting a reference to an object to nil means two different things depending on the kind of reference you are nil-ing:
If it is a strong reference, then nil-ing it means decreasing the reference count of the referenced object;
if it is a weak reference, nil-ing it does nothing.
Thus, nil-ing can lead to different outcomes. Specifically, it is only when the reference count goes to zero that the object is deallocated. This would correspond to a case where no other object in the system is owning the first one (which means holding a strong reference to it).
So, in your case there could be either some other objects keeping a strong reference to the objects you try to nil; or, you might be nil-ing a weak reference. If you show some code, it may become clearer.
Related
During the last time I'm trying to become aware of Objective-C memory management and once I asked myself the following question:
Sinceweak reference in Objective-C let us to avoid retain cycles why not to use them all the time?
An object only stays in memory while there is at least one strong (default) reference to it. If you only use weak references, the object is immediately deallocated.
Simply said, "weak" means: This object belongs to someone else, I'm only accessing it and I don't mind if it disappears.
Sinceweak reference in Objective-C let us to avoid retain cycles why not to use them all the time?
Because an object is deallocated as soon as there are no strong references to it.
More importantly, a strong reference is often wanted in order to prevent an object's deallocation. For example, a view controller is ultimately responsible for its view -- it wouldn't make much sense to have the controller's view suddenly just disappear. So UIViewController's view property is strong.
A retain cycle happens when two objects each have strong references to the other. Neither object can be deallocated as long as the other one keeps it's strong reference. This isn't really a problem if you're aware of the situation, because you can break the retain cycle by eliminating either of the references. However, it's easy to not be aware of the cycle, or to forget to deal with it, and then you've got two objects keeping each other in memory beyond their useful lifetime. Making one of those references weak eliminates the problem, but it also means that the referenced object may be deallocated and the reference may become nil at any point. In practice, that's usually not a problem.
As far as I know, when I dereference a dead weak reference in Objective-C, I get a nil value as the result. I'm wondering if there is any way to actually tell if there was a weak value assigned to the variable once it goes away as opposed to simply having a value of nil (for instance if the reference was never assigned).
Is there perhaps a lower-level runtime function that I can use?
I've taken to using a BOOL to record when the reference is assigned, but this feels ugly to me.
As Rob said, you can't do it directly. But you can do so indirectly.
By using associated objects, you can associate a subclass of NSObject with the object being weakly referenced. In that subclass, override dealloc to notify something that the weakly referenced object is being deallocated.
As long as you make absolutely sure that the weakly referenced object's associated reference to your NSObject subclass is the only strong reference to your subclass's instance, then you've effectively created a means of receiving a notification of when the weakly referenced object is deallocated.
Yes, it is fragile. One additional strong reference to that subclass's instances and the whole thing stops working.
No, there is no way to tell if a weak reference has been set to nil because its referent has been deallocated.
The weak reference is set to nil by weak_clear_no_lock in objc-weak.mm.
In ARC, when an object is released, the pointer is set to nil.
How does the object tell all those points that it's about to be released?
Does this work for strong pointers or all types of pointers?
Based on some quick reading of what ARC required be added to the Objective-C runtime, the weak reference itself is registered with the runtime. There are a bunch of calls for setting up a weak connection, tearing it down and reassigning it. The compiler acts to decide what sort of assignment to do, much as it also has a role in automatically retaining and releasing. Per the linked document:
The runtime tracks __weak objects which holds non-null values. It is
undefined behavior to direct modify a __weak object which is being
tracked by the runtime except through an objc_storeWeak,
objc_destroyWeak, or objc_moveWeak call.
From that I'd conclude that the runtime maintains a collection of every weak pointer that currently points to a given object. When that object is deallocated it zeros out the pointers.
So there is a list, per object, that points to the relevant pointers to create a two-directional connection. How and where that's stored isn't explicit — it could be via the existing object association mechanisms, it could be a global dictionary, it could be just about anything.
In ARC (or MRC), a pointer is NOT set to nil when an object is released. In ARC, a weak object reference is set to nil when an object is deallocated, not when it is released. There is a big difference here.
I'm trying to learn/understand what happens and why when working with or creating various objects. (Hopefully to LEARN from the docs.)
I'm reading "Programming in Objective-C 2.0" (2nd edition, by Steven Kochan). On page 408, in the first paragraph is a discussion of retain counts:
Note that its reference count then goes to 2. The addObject: method does this automatically; if you check your documentation for the addObject: method, you will see this fact described there.
So I read the addObject: docs:
Inserts a given object at the end of the array.
There, the description is missing, while other items, like arrayByAddingObject:, state it:
Returns a new array that is a copy of the receiving array with a given object added to the end.
Where in the reference does it indicate that addObject: increases the retain count? Given the presence of ARC, I should still understand what these methods are doing to avoid bugs and issues. What does ARC bring to this? (Going to read that again...)
Great question, I'm glad to see someone actually reading the docs and trying to understand them!
Since you are looking for how to research answers using Apple's documentation more so than the actual answer itself, here is how I found the answer:
First I look at the class reference for addObject: which is a method of NSMutableArray and there is no mention of memory management.
Then I look at the Overview section at the top... Hmmm, still no luck.
Since the behavior might be inherited from a parent class, I look at the Inherits from section at the top of the class reference and see that NSArray is the most immediate parent. Let's check there:
Under the Overview There is one small section about retain's:
Special Considerations
In most cases your custom NSArray class should conform to Cocoa’s
object-ownership conventions. Thus you must send retain to each object
that you add to your collection and release to each object that you
remove from the collection. Of course, if the reason for subclassing
NSArray is to implement object-retention behavior different from the
norm (for example, a non-retaining array), then you can ignore this
requirement.
Okay, I'm still not happy... Where next? The parent class of NSArray is NSObject and I know that it won't be covered there in this case (from experience) so I won't bother checking that. (If the parent was another class or something that might be covered by NSObject, I would keep moving up the tree until I found something.)
The Companion Guides usually contains a lot of good information for these types of classes. Let's try the first one, Collections Programming Topics.
The first section (after Overview) is Accessing Indexes and Easily Enumerating Elements: Arrays. Sounds promising! Click on Relevant Chapters: “Arrays: Ordered Collections”
There it is under Array Fundamentals along with a link to even more information:
And when you add an object to an NSMutableArray object, the object
isn’t copied, (unless you pass YES as the argument to
initWithArray:copyItems:). Rather, an object is added directly to an
array. In a managed memory environment, an object receives a retain
message when it’s added; in a garbage collected environment, it is
strongly referenced. When an array is deallocated in a managed memory
environment, each element is sent a release message. For more
information on copying and memory management, see “Copying
Collections.”
The book must be referring to out of date documentation because you are correct it doesn't mention anything about the retain count. It does in fact retain the object though. The way you need to think of it is not in terms of retain counts (which are useless) but rather ownership. Especially so when using ARC.
When you add an object to an NSMutableArray, it is taking ownership of that object (in ARC terminology it has a strong reference to it).
"What does ARC bring to this?"
ARC does nothing different. All ARC does (besides some optimization) is add the same release, retain, and autorelease statements that you would add yourself without using ARC. All you need to care about is that once you add an object to the array, it will live at least as long as the array.
And the arrayByAddingObject: method creates a new NSArray (or NSMutableArray) containing the object you're passing, and keeps a strong reference to the passed object. The actual array object that it creates has no references yet unless you assign it to either an ivar, property, or local variable. What you assign it to determines it's lifespan.
Basically even without ARC, it's best to think of object life-cycles in terms of ownership, ARC just formalizes that. So because of that, when using the frameworks, it doesn't matter when retains happen or don't happen, you are only responsible for your objects until you pass ownership to another object and you can trust that the framework will keep the object alive as long as it needs it.
Now of course you have to intuit what constitutes ownership. For instance delegate properties are often assign, or in ARC unsafe_unretained or weak, to prevent circular retains cycles (where two objects each retain each other), though are sometimes retained/strong so you need to look into those on a case by case basis.
And also in cases like key value observing and NSNotification observing the object you are observing does not retain the observer.
But those are really exceptions to the rule. Generally you can assume a strong reference.
Regarding this sentence above: "The actual array object that it creates has no references yet unless you assign it to either an ivar, property, or local variable. What you assign it to determines it's lifespan." I'll try to explain:
When you run this piece of code: [someArray arrayByAddingObject:someObject]; you've instantiated a new NSArray or NSMutableArray object (depending on which object type someArray is) but you haven't actually assigned it to any reference. That means that if you're using ARC, it may be immediately released afterwards, or if not using ARC, it will be released when it's autoreleasepool is drained (probably on the next iteration of that thread's runloop).
Now if instead you did this: NSArray *someOtherArray = [someArray arrayByAddingObject:someObject]; you now have a reference to the newly created array, called someOtherArray. In this case, this is a local variable who's scope is only within whichever set of { } it resides (so it could be inside an if statement, a loop, or a method. Now if you do nothing else with it, it will die sometime after it's scope ends (it isn't guaranteed to die right away, but that isn't important, you just can't assume it lives longer).
Now if in your class you have an iVar (instance variable) declared in the header like NSArray *someOtherArray; (which is strong by default in ARC) and you run someOtherArray = [someArray arrayByAddingObject:someObject]; somewhere in your class, the object will live until you either remove the reference (someOtherArray = nil), you overwrite the reference (someOtherArray = someThirdArray), or the class is deallocated. If you were not using ARC, you would have to make sure to retain that to achieve the same effect (someOtherArray = [[someArray arrayByAddingObject:someObject] retain]; which is essentially what ARC is doing behind the scenes).
Or you may have a property declared instead like #property (nonatomic, strong) NSArray *someOtherArray in which self.someOtherArray = [someArray arrayByAddingObject:someObject]; would achieve the same effect but would use the proprety accessor (setSomeOtherArray:) or you could still use someOtherArray = [someArray arrayByAddingObject:someObject]; to set the iVar directly (assuming you #synthesized it).
Or assuming non-ARC, you might have declared the property like #property (nonatomic, retain) NSArray *someOtherArray in which self.someOtherArray = [someArray arrayByAddingObject:someObject]; would behave exactly as ARC would, but when setting the iVar directly you would still need to add that retain manually.
I hope that clears things up a bit, please let me know if there's anything I glossed over or left out.
As you mentioned in your comment, the key here is intuitively knowing when an object would be considered owned by another one or not. Luckily, the Cocoa frameworks follow a pretty strict set of conventions that allow you to make safe assumptions:
When setting an NSString property of a framework object (say the text property of a UILabel for example) it is always copied (if anyone knows of a counter-example, please comment or edit). So you don't have to worry about your string once you pass it. Strings are copied to prevent a mutable string from being changed after it's passed.
When setting any other property other than delegate, it's (almost?) always retained (or strong reference in ARC)
When setting delegate properties, it's (almost?) always an assign (or weak reference) to prevent circular retain cycles. (For instance, object a has a property b that is strong referenced and b has a strong referenced delegate property. You set a as the delegate for b. Now a and b are both strongly referencing each other, and neither object will ever reach a retain count of 0 and will never reach it's dealloc method to dealloc the other object. NSURLConnection is a counter-example that does strongly reference it's delegate, because it's delegate is set via a method -- see that convention below -- and it's convention to nil out or release an NSURLConnection after it completes rather than in dealloc, which will remove the circular retain)
When adding to an array or dictionary, it's always retained (or strong reference).
When calling a method and passing block(s), they are always copied to move them from the stack (where they are initially created for performance purposes) into the heap.
Methods that take in object parameters and don't return a result immediately are (always? I can't think of any that don't) either copying or retaining (strong referencing) the parameters that you pass to ensure that the method can do what it needs to with them. For instance, NSURLConnection even retains it's delegate because it's passed in via a method, whereas when setting the delegate property of other objects will not retain, as that is the convention.
It's suggested that you follow these same conventions in your own classes as well for consistency.
Also, don't forget that the headers of all classes are available to you, so you can easily see whether a property is retain or assign (or strong or weak). You can't check what methods do with their parameters, but there's no need because of the convention that parameters are owned by the receiver.
In general, you should look in the "most global" spot for information about anything in the Cocoa APIs. Since memory management is pervasive across the system APIs and the APIs are consistent in their implementation of the Cocoa memory management policy, you simply need to read and understand the Cocoa memory management guide.
Once understood, you can safely assume that all system APIs implement to that memory management policy unless explicitly documented otherwise.
Thus, for NSMutableArray's addObject: method, it would have to retain the object added to the array or else it would be in violation of that standard policy.
You'll see this throughout the documentation. This prevents every method's documentation from being a page or more long and it makes it obvious when the rare method or class implements something that is, for whatever reason (sometimes not so good), an exception to the rule.
In the "Basic Memory Management Rules" section of the memory management guide:
You can take ownership of an object using retain.
A received object is normally guaranteed to remain valid within the
method it was received in, and that method may also safely return the
object to its invoker. You use retain in two situations: (1) In the
implementation of an accessor method or an init method, to take
ownership of an object you want to store as a property value; and (2)
To prevent an object from being invalidated as a side-effect of some
other operation (as explained in “Avoid Causing Deallocation of
Objects You’re Using”).
(2) is the key; an NS{Mutable}Array must retain any added object(s) exactly because it needs to prevent the added object(s) from being invalidated due to some side-effect. To not do so would be divergent from the above rule and, thus, would be explicitly documented.
Are there any good rules to learn when I should use retain, and when assign?
Assign is for primitive values like BOOL, NSInteger or double. For objects use retain or copy, depending on if you want to keep a reference to the original object or make a copy of it.
The only common exception is weak references, where you want to keep a pointer to an object but can't retain it because of reference cycles. An example of this is the delegate pattern, where an object (for example a table view) keeps a pointer to its delegate. Since the delegate object retains the table view, having the table view retain the delegate would mean neither one will ever be released. A weak reference is used in this case instead. In this situation you would use assign when you create your property.
I would think that when working with objects you would almost always use retain instead of assign and when working with primitive types, structs, etc, you would use assign (since you can't retain non-objects). That's because you want the object with the property deciding when it is done with the object, not something else. Apple's Memory Management Guide states this:
There are times when you don’t want a
received object to be disposed of; for
example, you may need to cache the
object in an instance variable. In
this case, only you know when the
object is no longer needed, so you
need the power to ensure that the
object is not disposed of while you
are still using it. You do this with a
retain message, which stays the effect
of a pending autorelease (or preempts
a later release or autorelease
message). By retaining an object you
ensure that it won’t be deallocated
until you are done with it.
For discussion around using copy vs retain, see this SO question.
I know this was an old question, but I found these guidelines from the uber guru Matt Gallagher, super useful: http://cocoawithlove.com/2009/07/rules-to-avoid-retain-cycles.html. In my case, I had a "retain hell" of my own making for having a hard reference to a parent object.
If you intend to keep the object and use it, use retain. Otherwise, it may be released and you'll end up with errors with your code.