I'm using RestKit to develop a RESTful application. I have a wrapper object that actually handles requests and even acts as the delegate for the RKObjectManager. I am experiencing an issue related to how ARC handles instance variables and retaining them, and it is clear to me that I do not understand how ARC works.
So when I do this, it fails (with error related to message sent to deallocated instance)
MyTestClient *testClient = [[MyTestClient alloc] init];
but when I declare a property and do this, all is fine:
self.testClient = [[MyTestClient alloc] init];
From what I understand, under ARC, an instance variable is always strong by default, but it's lifecycle is the scope of the method in which it's declared.
Since I can't do [testClient retain], is my only option to make it a property?
Your variables are strong by default and will be retained within their scope. So in the first example, the object will only be retained until the end of the function. In the second, since you have an instance variable, it will be retained until the owning object is deallocated (presumably long enough for you in this case). Your best option is to make it a property, but you could also just make it an instance variable. It will do the same thing for you in this case.
A simple way to think about it is you need to have a strong pointer to an object until you no longer need it around.
The equivalent to retain in ARC would be to have a pointer pointing to it. As long as you have something pointing to the object, that object will be kept alive. Hence you can use properties, instance variables, even instances pointing to those objects.
Like wbyoung said. You are getting the error because testClient is only alive inside where you initiated it, this being viewdidload, or init, or w/e.
Related
Let's say I have a method that returns an object pointer
-(MyObj *) returnMyObj {
MyObj *obj = [MyObj alloc] init];
return obj;
}
If I call this function without assigning the pointer like this
Scenario 1
[self returnMyObj];
and if I call this function with assignment to a pointer like this
Scenario 2
MyObj* obj = [self returnMyObj];
Compiler can release the object at the end of the returnMyObj method call in the Scenario 1 but it cant do the same in Scenario 2.How does ARC decide if it needs to retain the created object at the end of method invocation or not in both cases?
Here is what the ARC article in the documentation has to say:
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To make this possible, whenever you assign a class instance to a property, constant, or variable, that property, constant, or variable makes a strong reference to the instance. The reference is called a “strong” reference because it keeps a firm hold on that instance, and does not allow it to be deallocated for as long as that strong reference remains.
ARC decides which objects will be kept in memory and which will be deallocated by counting the strong references to the objects.
In your second scenario you are creating a strong reference to the MyObj instance and ARC will not deallocate it while the object is in use. In this case it will be deallocated when the method that uses this object finishes.
The optimizer may absolutely release the object in scenario 2 at the end of the statement if obj is not referenced later in the block.
The whole point is that the optimizer can see when the pointer is referenced, and may release it as soon as the last reference is completed. obj does not have precise lifetime semantics, so it only extends the life of the object until the last time obj is referenced, not when obj goes out of scope.
In scenario 1, the reference is not assigned to any variable, so it clearly has no later references, and may be immediately released. That said, it may not immediately release the object, since returnMyObj does not transfer ownership to the caller (due to its name). So the object may not actually be released until the autorelease pool drains.
I have seen report that if a delegate uses assign instead of weak, the app crashes. Why?
Example: RestKit
With ARC a weak ivar will be automatically set to nil when the ivar object is deallocated. That means that if your delegate is destroyed and you try to message the delegate you'll just message nil, which has no effect. If the delegate ivar was merely assign then you would message some chunk of memory that no longer contained a valid object.
The only difference between assign and weak is that weak does extra work to avoid some common crashing bugs. The drawback, however, is that assign has much better performance than weak.
Specifically, when an object is released, any weak property pointing to it will be set to nil. Any assign property pointing to it will be left pointing at the object that is no-longer used.
And some other object is likely to be placed at the same location in memory as the old object, so suddenly instead of an instance of MyDelegate you might have a UIImage object in the same location in memory, or perhaps a float value, or anything at all.
So you should always use weak, for everything. But if you run into performance problems, check if weak is the cause, and consider switching to assign after learning how to avoid those crashing bugs.
For a delegate you should pretty much always use weak. Normally you'd only pick assign if you are dealing with millions of objects. Typically that doesn't happen with delegates.
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.
I have member variables in my custom UIViewController that are defined as 'assign' (not 'retain') like this:
#property (nonatomic, assign) UIButton* mSkipButton;
In my loadView method, I set the var, for instance self.mSkipButton, to an autoreleased alloc of the variable type. I then attach it to my controller's view essentially having the view reference count and release it as needed.
This concerns me, however, that I have the pointer stored in my member var and that it could be referencing released memory if the count decrements at some point. Is it better practice to instead declare the variable as 'retain' and then in the viewDidUnload method release the member var (or just set it to nil to release and make sure i don't have an address in there)?
Alternatively, could I simply set the member var to nil in viewDidUnload and not make it a retained variable?
Is it better practice to instead declare the variable as 'retain' and then in the viewDidUnload...?
Yes, use retain -- good instinct. In viewDidUnload, you'd typically just set it to nil via the ivar's setter: self.ivar = nil;
I find it easier to be aware of and manage object codependencies explicitly, than to deal with issues related to using assign. You can completely avoid the issues of holding an unmanaged reference.
Arguments can be made that assign would usually be fine here (and it is in some cases), but using assign can complicate object graphs and ownership for anyone working with the class. As program complexity grows (and the libraries you depend on change), it becomes increasingly difficult to track lifetimes of unmanaged references. Things tend to break, or operate differently on different hardware and software combinations. Attempting to manage the lifetime of an unmanaged object over a complex program or in a concurrent context is self abuse. Guaranteeing defined and predictable behavior/operation reduces bug counts.
That's a property, not a "member var" (known in Objective-C as an instance variable or ivar.)
The semantics of a property depend on how that property is going to be used. Generally speaking, you'll want your properties to be retained for the lifetime of your object. If the property is a connected IBOutlet, this will be done for you by the NIB loader; otherwise, you must be explicit and use the retain or copy attribute on the property.
For objects that are expected to own your object, a property should always be marked assign to avoid a retain loop. For example, an object usually owns any object for which it acts as a delegate (usually, but not always--every CS rule has an exception.)
I'm getting weird EXC_BAD_ACCESS errors. I thought that I didn't need to retain objects if I was didn't need them after the method exited, but I want to double-check that. In the following, do I need to retain "tData"?
-(void)myMethod:(UITouch*)touch{
TouchData *tData = (TouchData *)CFDictionaryGetValue(datasByUITouch, touch);
[tData doSomething];
}
I think according to The Rules, what you're doing there doesn't require you to retain the tData object that scope. You're not doing an init, alloc, new or copy, instead you're using one of TouchData's methods to return some data, meaning that the method of TouchData that you're accessing is responsible for releasing the object.
Is the object being returned a CoreFoundation object? If so, see this from the Apple Docs (http://developer.apple.com/mac/library/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148-SW1)
If you receive an object from any Core
Foundation function other than a
creation or copy function—such as a
Get function—you do not own it and
cannot be certain of the object’s life
span. If you want to ensure that such
an object is not disposed of while you
are using it, you must claim ownership
(with the CFRetain function). You are
then responsible for relinquishing
ownership when you have finished with
it.
Right, no need to retain in this method. Looks like touch was released before this method was called.
How was the CFDictionary created? If it wasn't created with instructions to retain values, then the dictionary won't retain touch. If that's the case, pass kCFTypeDictionaryValueCallBacks (CFValueCallBacks) as the valueCallBacks argument of the CFDictionaryCreate (or CFDictionaryCreateMutable).