Memory management and properties (init/dealloc) - ios

Until yesterday I thought I understood how properties memory management works, but then I ran an "Analize" task with XCode and got plenty of "This object is not own here". Here is a simple example that describes my problem :
MyObservingObject.h:
#interface MyObservingObject : NSObject
#property(nonatomic, retain) NSMutableDictionary *observedDictionary;
-(id)initWithDictCapacity:(int)capacity;
#end
MyObservingObject.m:
#synthesize observedDictionary;
-(id)initWithDictCapacity:(int)capacity {
self = [super init];
if (self) {
self.observedDictionary = [[[NSMutableDictionary alloc] initWithCapacity:capacity] autorelease];
}
return self;
}
- (void)dealloc {
// The following line makes the Analize action say :
// "Incorrect decrement of the reference count of an object that is not owned at this point by the caller"
[self.observedDictionary release], self.observedDictionary=nil;
[super dealloc];
}
What I don't understand is Why should I leave this property without calling release on it? My #property is set as retain (copy does the same), so when I'm doing self.myRetainProperty = X, then X got its retain count increased (it's owned by self), didn't it ?

You should let the setter do the releasing for you, so remove the call to release in dealloc:
- (void)dealloc {
self.observedDictionary=nil;
[super dealloc];
}
This is because the setter will be synthensized to something like:
- (void)setObject:(id)object
{
[object retain];
[_object release];
_object = object;
}
Which will work as desired when you pass in nil.

It did get increased, but when you set it to nil, the setter method first releases the backing instance variable, and only then does it retain and assign the new value. Thus setting the property to nil is enough, setting the ivar to nil leaks memory, though.
For your better understanding: the typical implementation of an autogenerated retaining setter is equivalent to something like
- (void)setFoo:(id)foo
{
if (_foo != foo) {
[_foo release];
_foo = [foo retain];
}
}
Also note that, as a consequence, you should never release properties like this. If you do so, the backing ivar may be deallocated, and messaging it (release by the accessor when setting the property to nil afterwards) can crash.

You don't need to do
[self.observedDictionary release]
before
self.observedDictionary=nil;
This is enough, because this is a property, and it will automatically send release to previous value
self.observedDictionary=nil;

The reason for the compiler warning is because of the way you are retrieving the object.
By calling
[self.observedDictionary release];
you are in fact going through the accessor method defined as
- (NSDictionary *)observedDictionary;
This returns your object but due to the naming of observedDictionary the compiler assumes that there is no transfer of ownership e.g. the callee will not have to release this object unless they take a further retain. It is because of this that the compiler thinks you are going to do an overrelease by releasing an object that you don't actually own.
More specifically the convention for method names that transfer ownership is for them to start with copy, mutableCopy, alloc or new.
Some examples
Here I have used a name that does not imply transfer for ownership so I get a warning
- (id)object;
{
return [[NSObject alloc] init];
}
//=> Object leaked: allocated object is returned from a method whose name ('object') does not start with 'copy', 'mutableCopy', 'alloc' or 'new'. This violates the naming convention rules given in the Memory Management Guide for Cocoa
Fix 1: (don't transfer ownership)
- (id)object;
{
return [[[NSObject alloc] init] autorelease];
}
Fix 2: (make the name more appropriate)
- (id)newObject;
{
return [[NSObject alloc] init];
}
With this knowledge we can of naming convention we can see that the below is wrong because we do not own the returned object
[self.object release]; //=> Produced warnings
And to show a final example - releasing an object that implies ownership transfer with it's name
[self.newObject release]; //=> No Warning

Related

How can I reference __weak self in dealloc method

I have a method called in various places called "cancelAllPendingDownloads"
This is a general method that cancels various jobs and updates internal counters.
Problem happens when it is called within the dealloc method
-(void)dealloc
{
[self cancelAllPendingDownloads]; // want to cancel some jobs
}
-(void)cancelAllPendingDownloads // updates some internals
{
__weak __typeof__(self) weakSelf = self; // This line gets a EXC_BAD_INSTRUCTION error in runtime
for(Download *dl in self.downloads)
{
dl.completionHandler = ^{ // want to replace the previous block
weakSelf.dlcounter--;
}
[dl cancel];
}
}
Not sure why it fails in the dealloc method as "self" still exists
When I change the code to
__typeof__(self) strongSelf = self; //everything works fine
__weak __typeof__(self) weakSelf = strongSelf; (or "self") BAD_INSTRUCTION error
The error happens on the second line
Just to make the "you are not supposed" or "You can't" part of the other good answers
more precise:
The runtime function for storing a weak reference is objc_storeWeak(), and the
Clang/ARC documentation states:
id objc_storeWeak(id *object, id value);
...
If value is a null pointer or the object to which it points has begun
deallocation, object is assigned null and unregistered as a __weak
object. Otherwise, object is registered as a __weak object or has its
registration updated to point to value.
Since the self object has already begun deallocation, weakSelf should be set to NULL
(and therefore is not of any use).
However, there seems to be a bug (as discussed here http://www.cocoabuilder.com/archive/cocoa/312530-cannot-form-weak-reference-to.html)
that objc_storeWeak() crashes in this case, instead of returning NULL.
If an object is in dealloc state, you are not supposed to create any new references to it. Consider the object as already destroyed. Don't use it in a callback/delegate any more.
Note that dlcounter won't ever be read. Just cancel the connections without reading the results.
TL;DR
- How can I reference __weak self in dealloc method?
- Don't reference it.
You can't initialize a week (or a strong) reference to self in the dealloc method and use it elsewhere - it's too late, the object will be inevitably destroyed.
However, you might try this:
-(void)dealloc
{
NSArray* localDownloads = self.downloads;
for(Download* dl in localDownloads) {
[dl cancel];
}
}
It should be clear that there are better places to invoke cancellation, for example, in a view controller, you may override viewWillDisappear:.
I am assuming you are using ARC for your project.
Straight from Apple:
Apple Talked about Weak and Strong
__strong is the default. An object remains “alive” as long as
there is a strong pointer to it.
__weak specifies a reference that does not keep the referenced object alive.
A weak reference is set to nil when there are no strong references to the object.
This is an Article Explaining Dealloc:
Dealloc Method Explained and More
This method will be called after the final release of the object
but before it is deallocated or any of its instance variables are destroyed.
The superclass’s implementation of dealloc will be called automatically when
the method returns.
After this being pointed out... I highly recommend you revise your code design because there is no reason for you to call a weak typeof(self) to solve your problem of cancelling those downloads at dealloc or any type of deallocing that involves _weak_typeof__self for that matter.
What I can recommend though is that that class that you are trying to cancel those downloads frin, make it keep track of those downloads with a Download UniqueID and just stop them or delete them at dealloc. Its simpler and easier to manage rather than that wierd call to __weak self and all that code you are doing.
In short: you can use a __strong reference to self in dealloc instead of __weak for your purposes but if and only if that strong reference won't outlive the end of dealloc. Otherwise, I would advise using __unsafe_unretained, which is still unsafe if it outlives the dealloc but is clearer to read.
Longer: I had a similar situation where the object (view controller) during dealloc should unsubscribe from notifications. That's a custom notifications system and unsubscribing requires creating an object with a reference to the entity that's being unsubscribed.
I ended up with the same situation: in dealloc there's no way to create that object because it required a weak reference which caused a crash (here's some stupid demo code, not something you would have in production):
#interface Dummy : NSObject
#property(nonatomic, weak) id weakProperty;
#property(nonatomic, strong) id strongProperty;
#property(nonatomic, unsafe_unretained) id unsafeProperty;
- (instancetype)initWithWeakStuff:(id)stuff;
- (instancetype)initWithStrongStuff:(id)stuff;
- (instancetype)initWithUnsafeStuff:(id)stuff;
#end
#implementation Dummy
- (instancetype)initWithWeakStuff:(id)stuff {
self = [super init];
if (self) {
_weakProperty = stuff;
}
return self;
}
- (instancetype)initWithStrongStuff:(id)stuff {
self = [super init];
if (self) {
_strongProperty = stuff;
}
return self;
}
- (instancetype)initWithUnsafeStuff:(id)stuff {
self = [super init];
if (self) {
_unsafeProperty = stuff;
}
return self;
}
- (void)dealloc {
}
#end
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
- (void)dealloc {
Dummy *dummy = [[Dummy alloc] initWithStrongStuff:self];
[[NSNotificationCenter defaultCenter]
postNotificationName:#"some notification"
object:dummy]; // do something with it
}
#end
If, on the other hand, the reference was strong, all seems to work well (during dealloc). The problem would arise if that newly created object would outlive self:
- (void)dealloc {
Dummy *dummy = [[Dummy alloc] initWithStrongStuff:self];
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter]
postNotificationName:#"some notification"
object:dummy]; // do something with it
}); //Crash at the end of the block during dummy's dealloc
}
This would mean that whenever the dummy object would need to dealloc it would try to decrease the ref count of its strongProperty. And at that point the ViewController has been deallocated and released already.
However, IMHO the "safest" way to proceed is to use unsafe_unretained in this case. Technically it's the same as using assign: pointer will be assigned regardless of memory management and that reference will not need to be released when it goes out of scope. But using unsafe_unretained tells the readers of your code (or future you) that you were aware of the risk and there must have been a reason to do what you did.

A retained property should always be released in dealloc function, is this right?

I used to do this till once I found the retain count of one of my retained propery is zero before dealloc function. (This situation is normal or abnormal?)
NOTE: It's a RC condition, not ARC.
For example, I got 4 retained properties below, should they always be released in dealloc function?
If not, how could I know when to release, and when not to release? Manually judge the retainCount?
#property (nonatomic, retain) NSString *fileName;
#property (nonatomic, retain) UIImage *fullSizeImage;
#property (nonatomic, retain) UIImage *thumbnailImage;
#property (nonatomic, retain) UIImageView *checkedImageView;
- (void)dealloc {
[checkedImageView release];
checkedImageView = nil;
[fileName release];
fileName = nil;
[fullSizeImage release];
fullSizeImage = nil;
[thumbnailImage release];
thumbnailImage = nil;
[super dealloc];
}
Well, if the question is "always?", then Wain is almost right...
a SHORT answer is YES...
because in general, when someone set-up a property, it means he's going to use it as a property, that is he uses its setter method to initialize it.
BUT (LONG answer): NO, NOT ALWAYS:
what if you, somewhere in your code, initialize the private var associated to the property without it's setter method? Keep in mind that a property is not a var, but just a useful way to get methods from Xcode to get and set a var associated to it.
in other words, when you write in .h:
#property (nonatomic, retain) NSString *fileName;
and in .m:
#synthesize fileName;
you are declaring a var called fileName and are asking xcode to create 2 (invisible) methods for you:
a setter, used to set a new retained value in fileName:
-(void)setFileName:(NSString *)newString{
if (fileName == newString) {
return;
}
NSString *oldString = fileName;
fileName = [newString retain];
[oldString release];
}
and a getter, used to get the value of fileName:
-(NSString)fileName{
return fileName
}
so, when you somewhere in your code use:
self.fileName = #"ciao";
you are using the property setter method, exactly as if you'd call it directly (and you can do it, the invisible method setFileName: really exist):
[self setFileName:#"ciao"];
doing so, as you can see in the setter method, from now on fileName is retained, and so you should release it in dealloc.
BUT, to answer your question:
if you use the dot rule to set a new string in your var, ok, everything is fine,
but you may decide to set it in the standard way, somewhere, maybe just for mistake:
fileName = #"ciao";
// code
fileName = #"Hallo";
// code
fileName = #"Bye";
this way you are not using the property setter method, but you are using the var directly, and so fileName is not retained, and if you try to release it, well you may get a crash...
PS:
Manually judge the retainCount?
no, never do that
Yes, they should always be released in dealloc. If you get to dealloc and something is already released and not set to nil then you did something wrong with your memory management elsewhere in the app.
Technically in dealloc you don't need to set to nil after releasing but setting to nil after releasing is a generally good idea.
Your dealloc is unnecessarily calling the getter for each property and then immediately releasing it. Just assign nil to release the properties:
- (void)dealloc {
self.checkedImageView = nil;
self.fileName = nil;
self.fullSizeImage = nil;
self.thumbnailImage = nil;
[super dealloc];
}
Although if you are following the current trend of letting clang auto-generate your backing instance variables, then this is better, as it won't cause KVO side-effects:
- (void)dealloc {
[_checkedImageView release];
[_fileName release];
[_fullSizeImage release];
[_thumbnailImage release];
[super dealloc];
}
Yes, they should normally all be released. If you have a retain count of zero, that usually means you've made a mistake somewhere in your memory management code.
You ask: If not, how could I know when to release, and when not to release? Manually judge the retainCount?
Possibly, but you could also let Xcode help you, using static analysis. Go to Product -> Analyze. It will quite often help you find erroneous releases, etc.
When to release? Quite obviously, if your object was holding a reference to another object, and your object goes away, then it should stop holding a reference to the other object. Why would you even look at the retain count? Retain count is about other people holding on the same object, but they are none of your business. They should know what they are doing. So you release the object. You do your job; everyone else has to do theirs. The easiest way, as others said, is to assign
self.someproperty = nil;
If your object was the only one holding a reference, that other object will go away. If others held a reference, it won't go away. Just as everyone would expect. The "release" method should be the only one ever caring about what the retain count of an object is.

[CFString retain]: message sent to deallocated

I enabled zombies with my Xcode to find if my process crashes to a memory leak. Here is a code snippet:
- (NSString *)facVersion
{
return facVersion;
}
- (void) setFacVersion:(NSString*)_facVersion
{
if(facVersion != nil) [facVersion release];
facVersion = [_facVersion retain];
}
Now when I call
NSLog(#"%#", facVersion);
[self setFacVersion:facVersion];
the code crashes with the message
[CFString retain]: message sent to deallocated
Do you know what the problem is?
This is a typical problem with badly written setters. When the object itself is the last owner of the backing ivar of the property, assigning the property to itself causes a release effectively deallocating the object, then a retain on the same deallocated object. You can fix this in two ways. Either check for the to-be-assigned object not being the same as the current value of the property, or retain first and release only after. All in all, solution one:
- (void) setFacVersion:(NSString*)_facVersion
{
if (facVersion == _facVersion) return;
[facVersion release];
facVersion = [_facVersion retain];
}
Soltion two:
- (void) setFacVersion:(NSString*)_facVersion
{
[_facVersion retain];
[facVersion release];
facVersion = _facVersion;
}
By the way, checking for an object not being nil before releasing it is superfluous. Objective-C is not Java.
It's supposed to be if (facVersion != _facVersion) in the setter. Otherwise if you set the same object again, your setter will release it (resulting in the object being deallocated) and you can't use it (retain) after that:
- (void) setFacVersion:(NSString*)_facVersion
{
if(facVersion != _facVersion) {
[facVersion release];
facVersion = [_facVersion retain];
}
}
Also, did you know about Automatic Reference Counting (ARC)?
Automatic Reference Counting (ARC) is a compiler feature that provides
automatic memory management of Objective-C objects. Rather than having
to think about about retain and release operations, ARC allows you to
concentrate on the interesting code, the object graphs, and the
relationships between objects in your application.
If you pass the same object to setFacVersion as already set, it will first release it and then try to retain released (and deallocated) object.
You're releasing it before you call retain.
Unless you're comparing the pointers before releasing and retaining, you should use something like:
- (void) setFacVersion:(NSString*)_facVersion
[_facVersion retain];
[facVersion release];
facVersion = _facVersion;
}
That way, if the pointers are the same, as you demonstrated, you won't get a crash.
Or just let the compiler synthesize the setter for you.

What are the pros and cons of these different dealloc strategies?

I've seen several different approaches to memory management in iOS as regards releasing properties. After some debate with colleagues, the pros and cons have become muddled in my head.
I'm hoping to get a summary of pros and cons that will allow myself and others to easily choose a default approach while still understanding when to make exceptions. Here are the 3 variations I've seen:
Assume #property (nonatomic, retain) MyObject *foo;
// Release-only. Seems to be the favored approach in Apple's sample code.
- (void)dealloc {
[foo release];
[super dealloc];
}
// Property accessor set to nil.
- (void)dealloc {
self.foo = nil;
[super dealloc];
}
// Release, then nil.
- (void)dealloc {
[foo release];
foo = nil;
[super dealloc];
}
If you have a different variation to add, comment here and I'll edit the op.
Versions (1): is the best. Each of the others have attributes that may be harmful.
Version (2): It is generally advised not to use accessors in dealloc (or init). The reasoning behind this is that the object is in the process of being torn-down (or created) and is in an inconsistent state. This is especially true if you are writing a library where someone else may later override an accessor unaware that it may be called when the object is in an inconsistent state. (Of course even Apple sometimes breaks this rule -[UIView initWithFrame:] calls -[UIView setFrame:] if the argument is not CGRectZero which can make for fun debugging.
Version (3); Setting the ivar to nil serves no useful purpose, indeed it may mask an error and make debugging more difficult. To see why this is true, consider the following piece of code, assume myObject has a version (3) dealloc.
FastMovingTrain* train = [[FastMoving alloc] init];
MyObject* myObject = [[MyObject alloc] init];
myObject.foo = train;
[train release];
// my myObject.foo is the only thing retaining train
...
....
[myObject release];
// Because of version (3) dealloc if myObject
// points to the dealloced memory this line
// will silently fail...
[myObject.foo applyBrakes];
Interestingly enough this code provides an opportunity to demonstrate when setting a variable to nil after a release does make sense. The code can be made more resilient by modifying it as follows.
FastMovingTrain* train = [[FastMoving alloc] init];
MyObject* myObject = [[MyObject alloc] init];
myObject.foo = train;
[train release];
// my myObject.foo is the only thing retaining train
...
....
[myObject release];
myObject = nil;
// This assertion will fail.
NSAssert(myObject, #"myObject must not be nil");
[myObject.foo applyBrakes];
Just my $0.02.

View controller / memory management

i'm a little bit confused with memory management in view controllers.
Lets say i have header file like this:
#interface MyController : UIViewController {
NSMutableArray *data;
}
#property (nonatomic, retain) NSMutableArray *data;
#end
and .m file looks like that:
#implementation MyController
#synthesize data;
- (void)dealloc
{
[self.data release];
[super dealloc];
}
- (void)viewDidLoad
{
[super viewDidLoad];
if (self.data == nil)
self.data = [[NSMutableArray alloc] init];
}
- (void)viewDidUnload
{
[super viewDidUnload];
[self.data release];
self.data = nil;
}
Is that ok from the correct memory management point of view? Will that work after dealloc via Memory Warning? How You do that in your apps?
Thanks for your answers ;)
While the alloc-retain calls balance out in viewDidLoad and viewDidUnload and should prove no problem memory-wise, it would be cleaner to take ownership only once and relinquishing it once rather than twice.
- (void)viewDidLoad
{
[super viewDidLoad];
if (self.data == nil)
self.data = [NSMutableArray array];
}
and
- (void)viewDidUnload
{
[super viewDidUnload];
self.data = nil;
}
You are not guaranteed that viewDidUnload will ever get called. Unlike init/dealloc, which get called in pairs, viewDidUnload is undeterministically called. viewDidUnload is only called if there is a low memory situation and your view is not the active view.
Depending on how your model is created and the implications of it remaining in memory, it may make more sense for you not to get rid of it. An example of this may be that recreating that data may involve an expensive web service call. It therefore would be a bad user experience to have to wait for that data to get recreated. If it must absolutely go, a better strategy may be to cache the data to disk so that you can easily reconstruct it.
viewDidUnload should only contain cleaning up your IBOutlets and flushing easily recreatable data.
These lines from -viewDidUnload both release data:
[self.data release];
self.data = nil;
Since you're using the property setter in the second line, and data is a retained property, the setter will release data. This is an over-release, and it'll cause a crash either right away or later, depending on whether other objects also retain that object. To fix, simply delete the first line and rely on the setter to do the right thing.
The -dealloc method, on the other hand, shouldn't use the setter as it does now. You should change:
[self.data release];
to:
[data release];
data = nil; // this line isn't strictly necessary, but often considered good form
The reasoning here is that it's conceivable that this class could be subclassed, and someone might override the property setter in such a way that it has some side effects that could cause problems when the object is being deallocated. You should access the ivar directly -- notice that I left off the "self." so that we're dealing with the ivar and not the property accessor. (-init and -dealloc are the only places where you have to worry about that; use the property accessors everywhere else.)

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