How can I check if object isn't deallocated on Objective-C?
The following standard condition checks only if object is initialized:
NSObject *objectVariable = nil;
...
if (objectVariable) {...}
You can't check after-the-fact whether an object is already deallocated or not, because it is invalid to do anything with an object that is deallocated (once an object is deallocated, any pointer that used to point to it is now an invalid pointer, and dereferencing it is undefined behavior). And since you don't know whether it is deallocated or not, and it may be deallocated, you cannot meaningfully examine it in any way. And a deallocated object may well "look like" a valid object if you try to examine it (it is undefined behavior, so any behavior is possible).
If this is an object of your own class, the obvious thing would be to print something or perform some other indicator action in the class's -dealloc method. That will tell you when it is deallocated.
Even if this is an object that is not of your own class, you may be able to either 1) add -dealloc as a category method, if the class doesn't override -dealloc, or 2) swizzle the -dealloc method, if the class does override it. Then you can print something in your version.
You can also profile it in Instruments; the Allocations instrument can tell you how many objects of a given class have been allocated, and how many are alive. If you suspect that you are losing track of objects or there is a retain cycle, the Leaks instrument may be able to tell you that.
I'm in agreement with the comments, If you're doing memory management right, there should be no need for such a check. Nor am I aware of such a check, if the address gets populated with a new object, the check would pass but could still crash your app.
My suggestions are to:
Read up on manual memory management rules, pay special attention to how blocks affect memory management, alloc/init methods, when to use assign, etc. Memory management rules should become second nature to you. Start with this Apple guide.
Run static analysis on your app and fix any memory errors. Fix all the errors really, these are bugs in your app. (CMD+Shift+B or Product->Analyze in the menu)
Reproduce the crash in Instruments using zombies. Read the retain/release report to find out where the object may have been over-released. (CMD+I or Product->Profile. Select Zombies in the window that appears)
Consider converting to ARC. Converting to ARC doesn't get you completely off the hook for understanding ObjC memory management, but it will take a lot of the burden off of you.
Related
thanks for viewing this post, it'd be great if you guys can help me out. I've been doing some objective-c and learned about the objective-c way of memory management, like making sure to call release whenever I own the object, when to call autorelease, etc. I also do not want to use ARC or the newly introduced GC because I like to manage my own memory, I plan to advance later on into iOS development, and I know it's a good practice to manage my own memory. But there's still one small detail that I seem to have hit a brick wall in. It has to do with sending objects the -retain message. I learned that sending the -retain message increments the reference count by 1. But would this be an appropriate time to send -retain? :
- (void) setName : (NSString* ) theName
{
// name is an instance variable of type NSString
[theName retain]; // Must release this
name = [theName copy]; // Must release this in dealloc
[theName release]; // decrement the reference count because of retain
}
Should I call retain here so that I own the argument temporarily and ensure it doesnt'
get released somehow before I get to use it?
Any help would be appreciated! Thanks!
No. You the object supplied as an argument to the method will generally be around until your method returns. You don't need the retain messages there. You copy the string here to keep it around after the method returns.
This is documented in Apple's Documentation on this page in the "Avoid Causing Deallocation of Objects You’re Using" Section. Specifically:
Cocoa’s ownership policy specifies that received objects should
typically remain valid throughout the scope of the calling method. It
should also be possible to return a received object from the current
scope without fear of it being released. It should not matter to your
application that the getter method of an object returns a cached
instance variable or a computed value. What matters is that the object
remains valid for the time you need it.
As an aside you really should consider using ARC. Its not good practise to manage your own memory. No matter how good one can be at managing their own memory the LLVM compiler is still better. Managing your own memory will lead to hard to troubleshoot issues caused only by yourself. It is an extra level of cognitive load that you really don't have to deal with and, when you finally let manual memory management go, you will breathe a sigh of relief at all the mental overhead you didn't even know was there.
I have an error that I can't understand, that is happening while I want to release all objects in an NSMutableDictionary.
It's happening for a custom object called body and the output is :
-[__NSTaggedDate body]: unrecognized selector sent to instance 0xffffffffffffffff
I found very poor informations about it on the Internet.
That's a private class of Apple. Errors like this usually occur when you mess up your memory management.
Why are you trying to release all objects in a dictionary? When you add an object to a dictionary (or an array), the dictionary will retain it (take ownership). And when you remove the object from the dictionary it will be released, you don't have to do that.
Did you already consider using ARC? It makes memory management a lot easier. You don't have to worry about retaining and releasing objects anymore.
It's an internal undocumented cocoa class. But you are not concerned with it as it's not really what's happening, it's a red herring that is probably happening for reasons that are complex to explain and irrelevant here.
Look at the reported address: 0xffffffffffffffff. That's a value that makes no sense. You should have got a segmentation fault, if it was not for that red herring.
You are for some reason sending the message body to an invalid pointer (maybe some corrupted data somewhere?).
Don't know this class, but it is probably a private class (my bet would be that it is a internal representation for NSDate objects that use the "tagged pointers" trick, but I'm just guessing).
Anyway your crash is happening not on an object called body, but when calling a method called body. And the crash is probably due to bad memory managment in your code that generates memory corruption
You should activate Zombies when running your app in debug to help you track over-released objects
You normally don't have to retain and release objects of an NSDictionary yourself, as container classes like NSArray and NSDictionary retain the objects they hold, and release them when the object is removed from them. So I don't see why you "want to release all objects in an NSMutableDictionary" : you only need to call removeAllObjects on that NSDictionary and you're done, no need to call release on the objects by yourself (neither do you need to call retain on the objects when adding them in the dictionary)
Whenever you try to set one data type to another data type like "if you directly assign an date component to text to an UIlabel" in this case I ll occurs
[toDateLabel setText:[tempArr lastObject]]; // Cause
[toDateLabel setText:[NSString stringWithFormat:#"%#",[tempArr lastObject]]]; // Solution
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.
In the section titled 'Memory Warnings' here http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmNibObjects.html, I don't follow why the IBOutlet is set to nil in the dealloc. If
self.anOutlet = nil
causes a crash as mentioned in the topic, why are they setting the ivar to nil?
In general, why would you set an ivar to nil in the dealloc when you are already calling release?
After a release, the pointer is essentially invalid and accessing it again may cause a crash. By setting a variable to nil after release you prevent that crash from happening. There's no harm in accessing a nil pointer.
The example you've linked to simply demonstrates why it's always a good idea to set a variable or ivar to nil after release, even when it looks like the variable/ivar won't be accessed again.
In the example, the anOutlet ivar is actually accessed by the superclass after your dealloc method, so if you don't set it to nil you will get a crash. Scenarios like that are very hard to spot just by looking at the code, so it's a good idea to nil every variable after release, even in dealloc.
Sending a message on an object that is released causes a crash, sending a message to a nil object is ignored.
Sometimes a crash is a good thing, and a quick solution would hide a deeper problem. Calling a released variable might be something you want to know about.
The book iOS Recipes refers to this issue:
Cleanup in -dealloc
In addition to releasing all relevant instance variables in the -dealloc, our examples set them to nil. This practice is one of the most hotly debated topics among Cocoa programmers, and both sides of the argu- ment hold weight. This book is not meant to participate in the debate at all: we set them to nil, but that doesn’t mean you have to. If you don’t like nil-in-dealloc, feel free to leave it out of your own code.
A quick google search found this thread:
http://www.cocoabuilder.com/archive/cocoa/204055-why-should-we-set-ivars-to-nil-in-dealloc.html
Sometimes when one property becomes invalid (set to nil) we want to make other properties invalid too. If a class invalidates a property by using self.property_name=nil, then this will send a release message, which will cause a crash in dealloc if we have already called release on that property. If the invalidation occurs in a superclass, then this error is hidden and quite nasty. So whenever a superclass might invalidate a property it may be a good idea to set it to nil rather than just deallocing.
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.