I'm getting a a bit annoyed about some objects being autoreleased without me knowing. It's probably a good thing that they are, but if they are, I want to know. The documentation doesn't say which methods autorelease objects, so I usually test my way forward, which in my opinion is silly. For example, [NSDate date] autoreleases the object, and so does [NSArray arrayWithObjects:...]. How do you know without the documentation telling you?
I'm starting to see a pattern though that methods like these, the ones that create objects with a static function, always returns the autoreleased object. Is this always true?
Basically, if you init, copy, or retain an object you are responsible for releasing it. If you don't, you are not responsible for releasing it.
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/MemoryMgmt.html
Many classes provide methods of the
form +className... that you can use to
obtain a new instance of the class.
Often referred to as “convenience
constructors”, these methods create a
new instance of the class, initialize
it, and return it for you to use.
Although you might think you are
responsible for releasing objects
created in this manner, that is not
the case according to the ownership
policy set out earlier. Because the
class creates the new object, it is
responsible for disposing of the new
object.
The method signature itself tells you. The pattern to methods with signatures like "classNameWithData:data1:data2" is to return an alloc/init/autorelease instance of that thing. They are conveniences there so that you don't have to do it.
If you do not want an autorelease version of something, then do not instantiate them that way, and use the proper alloc/init, and release when you are done. This method is much more explicit, and a bit more error prone because if an exception is thrown your release block could get missed, but that is the price you pay for having it that way.
Related
First off, I'm a Objective-C newbie. :)
I've learned that anything that starts with alloc, new, and copy, gives the caller ownership of the returned object. Does this also apply to class methods? I'm guessing it does, and a recent crash in one of my unit tests seems to confirm it. Apple's Advanced Memory Management Programming Guide doesn't say anything whether there's a difference between class and instance methods.
Update
What I mean that it also applies to class methods is really the "inverse". For instance, NSDecimalNumber has a class method called +decimalNumberWithDecimal:. It seems to return an auto released object (if I release it explicitly, a crash occurs shortly after that). In hindsight, the answer to my question is obvious, as Apple's guide refers to new and alloc as ownership-giving methods, and they're all class methods. Sorry for taking up your valuable time. :)
Also alloc and release. init does not indicate ownership, you are likely mixing that up with alloc. You can memorise it easily with the mnemonic NARC.
If you are naming any class methods init, copy or retain, you should stop that. Those are methods that only make sense in the context of instances. alloc and new are class methods and should only be used in that context. Don't name instance methods alloc or new.
The reason why the guide doesn't say that it applies to both instance methods and class methods is that the methods in question are clearly a mixture of both, so it's obviously the case.
Yes this applies to class methods since alloc and new are class methods which return ownership to the caller. The prefix of copy or mutableCopy should be used for instance methods returning ownership.
Edit For The Update:
You are correct a method like +decimalNumberWithDecimal: is expected to return an autoreleased object, therefore there is no reason to release it. If however they decided to name the method +newNumberWithDecimal: then you would have ownership of the returned object and need to release it. Clang static analyzer will actually complain if you prefix a method with new and return an autoreleased object.
Actually, this is almost correct. alloc, new, and copy give you ownership of the returned object. These are class methods. Other class methods should return an autoreleased object. Instance methods should also return an autoreleased object.
init does not effect ownership and should be use in conjunction with alloc as follows.
[[SomeCoolClass alloc] init]
new is usually the same thing as the above and is sometimes described as "almost deprecated" because it is a throwback to the NeXT days when the allocation and initialization were done in the same step and could not be plot apart as we do today with alloc and init.
Release does not effect ownership, but should only be used on object you already own. Otherwise a crash is likely to occur.
Retain also does not affect ownership, but should only be used on an object you already own. Otherwise the object may not be deallocated when it should be. The result could be a crash, but it can also be very very strange behavior that is difficult to troubleshoot because messages may be sent to the original object that was supposed to have been deallocated instead of a new object pointed to at the same address.
How would you write a unit test—using OCUnit, for instance—to ensure that objects are being released/retained properly in Cocoa/Objective-C?
A naïve way to do this would be to check the value of retainCount, but of course you should never use retainCount. Can you simply check whether an object's reference is assigned a value of nil to indicate that it has been released? Also, what guarantees do you have about the timing at which objects are actually deallocated?
I'm hoping for a concise solution of only a few lines of code, as I will probably use this extensively. There may actually be two answers: one that uses the autorelease pool, and another that does not.
To clarify, I'm not looking for a way to comprehensively test every object that I create. It's impossible to unit test any behavior comprehensively, let alone memory management. At the very least, though, it would be nice to check the behavior of released objects for regression testing (and ensure that the same memory-related bug doesn't happen twice).
About the Answers
I accepted BJ Homer's answer because I found it to be the easiest, most concise way of accomplishing what I had in mind, given the caveat that the weak pointers provided with Automatic Reference Counting aren't available in production versions of XCode (prior to 4.2?) as of July 23rd, 2011. I was also impressed to learn that
ARC can be enabled on a per-file basis; it does not require that your
entire project use it. You could compile your unit tests with ARC and
leave your main project on manual retain-release, and this test would
still work.
That being said, for a far more detailed exploration of the potential issues involved with unit testing memory management in Objective-C, I highly recommend Peter Hosey's in-depth response.
Can you simply check whether an object's reference is assigned a value of nil to indicate that it has been released?
No, because sending a release message to an object and assigning nil to a variable are two different and unrelated things.
The closest you can get is that assigning anything to a strong/retaining or copying property, which translates to an accessor message, causes the previous value of the property to be released (which is done by the setter). Even so, watching the value of the property—using KVO, say—does not mean you will know when the object is released; most especially, when the owning object is deallocated, you will not get a notification when it sends release directly to the owned object. You will also get a warning message in your console (because the owning object died while you were observing it), and you do not want noisy warning messages from a unit test. Plus, you would have to specifically observe every property of every object to pull this off—miss one, and you may be missing a bug.
A release message to an object has no effect on any variables that point to that object. Neither does deallocation.
This changes slightly under ARC: Weak-referencing variables will be automatically assigned nil when the referenced object goes away. That doesn't help you much, though, because strongly-referencing variables, by definition, will not: If there's a strong reference to the object, the object won't (well, shouldn't) go away, because the strong reference will (should) keep it alive. An object dying before it should is one of the problems you're looking for, not something you'll want to use as a tool.
You could theoretically create a weak reference to every object you create, but you would have to refer to every object specifically, creating a variable for it manually in your code. As you can imagine, a tremendous pain and certain to miss objects.
Also, what guarantees do you have about the timing at which objects are actually released?
An object is released by sending it a release message, so the object is released when it receives that message.
Perhaps you meant “deallocated”. Releasing merely brings it closer to that point; an object can be released many times and still have a long life ahead of it if each release merely balanced out a previous retain.
An object is deallocated when it is released for the last time. This happens immediately. The infamous retainCount doesn't even go down to 0, as many a clever person who tried to write while ([obj retainCount] > 0) [obj release]; has found out.
There may actually be two answers: one that uses the autorelease pool, and another that does not.
A solution that uses the autorelease pool only works for objects that are autoreleased; by definition, objects not autoreleased do not go into the pool. It is entirely valid, and occasionally desirable, to never autorelease certain objects (particularly those you create many thousands of). Moreover, you can't look into the pool to see what's in it and what's not, or attempt to poke each object to see if it's dead.
How would you write a unit test—using OCUnit, for instance—to ensure that objects are being released/retained properly in Cocoa/Objective-C?
The best you could do is to set NSZombieEnabled to YES in setUp and restore its previous value in tearDown. This will catch over-releases/under-retains, but not leaks of any kind.
Even if you could write a unit test that thoroughly tests memory management, it would still be imperfect because it can only test the testable code—model objects and maybe certain controllers. You could still have leaks and crashes in your application caused by view code, nib-borne references and certain options (“Release When Closed” comes to mind), and so on.
There's no out-of-application test you can write that will ensure that your application is memory-bug-free.
That said, a test like you're imagining, if it were self-contained and automatic, would be pretty cool, even if it couldn't test everything. So I hope that I'm wrong and there is a way.
If you can use the newly-introduced Automatic Reference Counting (not yet available in production versions of Xcode, but documented here), then you could use weak pointers to test whether anything was over-retained.
- (void)testMemory {
__weak id testingPointer = nil;
id someObject = // some object with a 'foo' property
#autoreleasepool {
// Point the weak pointer to the thing we expect to be dealloc'd
// when we're done.
id theFoo = [someObject theFoo];
testingPointer = theFoo;
[someObject setTheFoo:somethingElse];
// At this point, we still have a reference to 'theFoo',
// so 'testingPointer' is still valid. We need to nil it out.
STAssertNotNil(testingPointer, #"This will never happen, since we're still holding it.")
theFoo = nil;
}
// Now the last strong reference to 'theFoo' should be gone, so 'testingPointer' will revert to nil
STAssertNil(testingPointer, #"Something didn't release %# when it should have", testingPointer);
}
Note that this works under ARC because of this change to the language semantics:
A retainable object pointer is either a null pointer or a pointer to a valid object.
Thus, the act of setting a pointer to nil is guaranteed to release the object it points to, and there's no way (under ARC) to release an object without removing a pointer to it.
One thing to note is that ARC can be enabled on a per-file basis; it does not require that your entire project use it. You could compile your unit tests with ARC and leave your main project on manual retain-release, and this test would still work.
The above does not detect over-releasing, but that's fairly easy to catch with NSZombieEnabled anyway.
If ARC is simply not an option, you may be able to do something similar with Mike Ash's MAZeroingWeakRef. I haven't used it much, but it seems to provide similar functionality to __weak pointers in a backwards-compatible way.
this is possibly not what you're looking for, but as a thought experiment I wondered if this might do something close to what you want: what if you created a mechanism to track the retain/release behavior for particular objects you wanted to test. Work it something like this:
create an override of NSObject dealloc
create a CFMutableSetRef and set up a custom retain/release functions to do nothing
make a unit test routine like registerForRRTracking: (id) object
make a unit test routine like clearRRTrackingReportingLeaks: (BOOL) report that will report any object in the set at that point in time.
call [tracker clearRRTrackignReportingLeaks: NO]; at the start of your unit test
call the register method in your unit test for every object you want to track and it'll be removed automatically on dealloc.
At the end of your test call the [tracker clearRRTrackingReportingLeaks: YES]; and it'll list all the objects that were not disposed of properly.
you could override NSObject alloc as well and just track everything but I imagine your set would get overly large (!!!).
Even better would be to put the CFMutableSetRef in a separate process and thus not have it impact your program runtime memory footprint overly much. Adds the complexity and runtime hit of inter-process communication though. Could use a private heap ( or zone - do those still exist?) to isolate it to a lesser degree.
I am using a shared instance of a singleton class in a function, do I need to do a retain on the object? A few examples I have seen do this: AVAudioSession *session = [[ AVAudioSession sharedInstance] retain]; while a few simply do: AVAudioSession *session = [ AVAudioSession sharedInstance];. I am sure there is a rule somewhere which should tell me what to do but I just don't know it.
Simply treat it as if it were any other object. Sometimes a singleton class will override memory management methods to ensure that it doesn't get deallocated, but there is no way to know whether or not a shared object has this. With shared objects and singletons which don't, if the first gets deallocated a new one will be created as needed. Therefore, if you need to ensure that a pointer is valid in a future method, retain the shared object when you get it. If you don't need to keep it, you don't need to do anything as the sharedInstance method will autorelease it.
You're right, there are rules that tell you what to do. They're the same rules you use everywhere else in Cocoa Touch. Clients of a singleton shouldn't care that the object they're using is a singleton, and they definitely shouldn't rely on the singleton's single-ness to avoid the usual memory management conventions.
Yes, if you know that an object is a singleton and you know that it won't be deallocated, retaining it might seem redundant. On the other hand, retaining the singleton when you acquire it and releasing it when you're done better documents the programmer's intent and avoids the need to explain to anyone (including the future you) why it's okay not to retain this particular object. Most importantly, it prevents your code from depending on an implementation detail of some other part of your code.
you do'nt need to retain the sharedInstances because it's implemented as the singleton class.
As is common knowledge, calls to alloc/copy/retain in Objective-C imply ownership and need to be balanced by a call to autorelease/release. How do you succinctly describe where this should happen? The word "succinct" is key. I can usually use intuition to guide me, but would like an explicit principle in case intuition fails and that can be use in discussions.
Properties simplify the matter (the rule is auto-/release happens in -dealloc and setters), but sometimes properties aren't a viable option (e.g. not everyone uses ObjC 2.0).
Sometimes the release should be in the same block. Other times the alloc/copy/retain happens in one method, which has a corresponding method where the release should occur (e.g. -init and -dealloc). It's this pairing of methods (where a method may be paired with itself) that seems to be key, but how can that be put into words? Also, what cases does the method-pairing notion miss? It doesn't seem to cover where you release properties, as setters are self-paired and -dealloc releases objects that aren't alloc/copy/retained in -init.
It feels like the object model is involved with my difficulty. There doesn't seem to be an element of the model that I can attach retain/release pairing to. Methods transform objects from valid state to valid state and send messages to other objects. The only natural pairings I see are object creation/destruction and method enter/exit.
Background:
This question was inspired by: "NSMutableDictionary does not get added into NSMutableArray". The asker of that question was releasing objects, but in such a way that might cause memory leaks. The alloc/copy/retain calls were generally balanced by releases, but in such a way that could cause memory leaks. The class was a delegate; some members were created in a delegate method (-parser:didStartElement:...) and released in -dealloc rather than in the corresponding (-parser:didEndElement:...) method. In this instance, properties seemed a good solution, but the question still remained of how to handle releasing when properties weren't involved.
Properties simplify the matter (the rule is auto-/release happens in -dealloc and setters), but sometimes properties aren't a viable option (e.g. not everyone uses ObjC 2.0).
This is a misunderstanding of the history of properties. While properties are new, accessors have always been a key part of ObjC. Properties just made it easier to write accessors. If you always use accessors, and you should, than most of these questions go away.
Before we had properties, we used Xcode's built-in accessor-writer (in the Script>Code menu), or with useful tools like Accessorizer to simplify the job (Accessorizer still simplifies property code). Or we just typed a lot of getters and setters by hand.
The question isn't where it should happen, it's when.
Release or autorelease an object if you have created it with +alloc, +new or -copy, or if you have sent it a -retain message.
Send -release when you don't care if the object continues to exist. Send -autorelease if you want to return it from the method you're in, but you don't care what happens to it after that.
I wouldn't say that dealloc is where you would call autorelease. And unless your object, whatever it may be, is linked to the life of a class, it doesn't necessarily need to be kept around for a retain in dealloc.
Here are my rules of thumb. You may do things in other ways.
I use release if the life of the
object I am using is limited to the
routine I am in now. Thus the object
gets created and released in that
routine. This is also the preferred
way if I am creating a lot of objects
in a routine, such as in a loop, and
I might want to release each object
before the next one is created in the
loop.
If the object I created in a method
needs to be passed back to the
caller, but I assume that the use of
the object will be transient and
limited to this run of the runloop, I
use autorelease. Here, I am trying to mimic many of Apple's convenience routines. (Want a quick string to use for a short period? Here you go, don't worry about owning it and it will get disposed appropriately.)
If I believe the object is to be kept
on a semi-permanent basis (like
longer than this run of the runloop),
I use create/new/copy in my method
name so the caller knows that they
are the owner of the object and will
have to release the object.
Any objects that are created by a
class and kept as a property with
retain (whether through the property
declaration or not), I release those
in dealloc (or in viewDidUnload as
appropriate).
Try not to let all this memory management overwhelm you. It is a lot easier than it sounds, and looking at a bunch of Apple's samples, and writing your own (and suffering bugs) will make you understand it better.
Actually, if you use a method with "new", "create", "alloc" or "copy" in it's name, then you are responsible for releasing the object that is returned to you. BUT: Why do these methods make an call to -autorelease? Wouldn't this result in "over-releasing" the object?
Lets say I get that object from such a method, and then I call -release on that. The object is in the Autorelease Pool and has a reference count of 0. What would happen next, when the Autorelease Pool is released?
Convenience constructors don't have any of those words in their names (e.g., [NSMutableArray array]), so by convention they return autoreleased objects. Methods that do have one of the words that indicate they return a retained object don't call autorelease, so that part of the premise of your question is incorrect.
If you release objects returned from a convenience constructor yourself (without having explicitly retained them) then they will be over-released, and your application will probably crash.
Sorry #all! I've been sitting too long in the bright sun, I think. Of course these convenience methods have ownership of the objects they create, because they call -alloc at some point at the beginning. And the -alloc Method itself doesn't call -alloc, since it's -alloc already. That's it.
Thought about deleting the question, but maybe someone else was sitting in the sun for too long, too, just like me. So this might help.