Why myInstance = nil instead of self.myInstance = nil? - ios

Why would I use (inside my dealloc method)?
[myInstance release] instead of [self.myInstance release]
myInstance = nil instead of self.myInstance = nil
Although we use self.myInstance = [[[AClass alloc] init] autorelease] instead of myInstance = [[[AClass alloc] init] autorelease]?
Those practices are from numerous examples I see on the web.

1) [myInstance release] instead of [self.myInstance release]
prefer the former.
the returned value of self.myInstance is defined by implementation when a subclass has overridden the method myInstance. you're not interested in the behaviour of the interface of a constructed object during dealloc (since a subclass may override and return something other than your ivar).
what you are interested in dealloc is releasing the references you own before your object is destroyed. if the subclass has overridden myInstance, then it could:
a) return an ivar (declared in the subclass) that's already been released
or
b) the implementation of the override may return a newly created autoreleased object
either a or b could lead to an over-release and a crash (assuming everything else is correctly retained/released). this also suggests why you should assign nil to the ivar after releasing it.
this is also a classic example of how to trigger object resurrection. object resurrection occurs when an implementation of the getter/setter you call recreates its state after it's already been deallocated. the least offensive side-effect would cause a harmless leak.
2) myInstance = nil instead of self.myInstance = nil
again, prefer the former.
a formal response would look much like the response to #1 -- the rationale, side-effects and dangers apply here as well.
the safest way to handle this is to access the ivar directly:
[myInstance release], myInstance = nil;
because there may be really nasty side-effects (crashes, leaks, resurrection) which may be difficult to reproduce.
these dangers may be easily avoided and your code will be far easier to maintain. on the other hand, if people encounter the side-effects when using your programs, they will probably avoid (re)using it wherever they can.
good luck

Calling self.myInstance = uses the auto generated setter method. Calling [self.myInstance release]; calls release on the object returned by your getter method. It all depends on how your properties were set up (retain, assign?). There is no necessarily right or wrong answer to your question, since it all depends on the property in question. I suggest you read up on Objective C properties to get a better feel for this kind of thing.
And, unless myInstance was declared with assign, you wouldn't want to call self.myInstance = [[AClass alloc] init] You'd be much better off with self.myInstance = [[[AClass alloc] init] autorelease]

Note that using
myInstance = nil
instead of
self.myInstance = nil
Is incorrect (in the context of say a viewDidUnload method in a UIViewController subclass) if myInstance is a retain property, since if myInstance points to an object, it will be leaked!

This depends on a property that you defined in interface. For example if you define retain property:
#property (nonatomic, retain) NSObject *property;
then you may use just self.property = nil; in dealloc method, because it equals to:
[property release]; // releases previous property
property = [nil retain]; // [nil retain] returns just nil
The very same thing with self.property = [[A alloc] init];. This equals to
[property release]; // releases previous property
property = [[[A alloc] init] retain];
in case of property = [[A alloc] init]; property won't be retained.
Here's a full properties guide form Apple.

Actually using
self.myInstance = [[AClass alloc] init];
will lead in a memory leak, cause self.myInstance is using setter methods which leads in retain +1 along with alloc/init retain +1. So you'll get a retain count +2;

... = self.myInstance
and
self.myInstance = ...
are actually subroutine or method calls to getters and setters, which depending on how you define these subroutines, or have Objective C Properties create them, could do almost anything.
If the case of retain properties, the subroutines might play with the retain counts. If you do your own getters and setters, you could have them control the lights in your house, turning them on for none zero sets and turning the lights out when setting something to zero or nil. There doesn't even need to be a backing variable named "instance" which could be set by:
instance = ...

Related

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.

ARC weak ivar released before being returned - when building for release, not debug

I have a class that creates an object lazily and stores it as a weak property. Other classes may request this object, but must obviously keep a strong reference to it to keep the object from being deallocated:
// .h
#interface ObjectManager
#property(nonatomic, weak, readonly) NSObject *theObject;
#end
// .m
#interface ObjectManager ()
#property(nonatomic, weak, readwrite) NSObject *theObject;
#end
#implementation ObjectManager
- (NSObject *)theObject
{
if (!_theObject) {
_theObject = [[NSObject alloc] init];
// Perform further setup of _theObject...
}
return _theObject;
}
#end
When the scheme is Xcode is set to build for Debug, things work just fine - an object can call objectManagerInstance.theObject and get back theObject.
When the scheme is set to build for Release, theObject returns nil:
// Build for Debug:
NSObject *object = objectManagerInstance.theObject;
// object is now pointing to theObject.
// Build for Release:
NSObject *object = objectManagerInstance.theObject;
// object is now `nil`.
My guess is that the compiler is optimising my code by seeing that _theObject is not used further in the accessor method itself, so the weak variable is being set to nil before returning. It seems that I would have to create a strong reference before actually returning the variable, which I can only think to do using a block, but would be messy and I'd rather avoid it!
Is there some kind of keyword I can use with the return type to stop the ivar from being nilled so soon?
Most likely, DEBUG builds cause the object to sit in the autorelease pool long enough to cause it to "work" whereas a RELEASE build causes the optimizer to do a bit more control flow analysis which subsequently eliminates the autorelease chatter.
Frankly, that the compiler isn't spewing a warning in the release build saying that the code can never work is a bug (please file it as you have a great, concise, example)!
You'll need to maintain a strong reference somewhere to the object until whatever needs a strong reference has an opportunity to take a reference.
I'm wondering if something like this might work:
- (NSObject *)theObject
{
NSObject *strongObject;
if (!_theObject) {
strongObject = [[NSObject alloc] init];
_theObject = strongObject;
// Perform further setup of _theObject...
} else {
strongObject = _theObject;
}
return strongObject;
}
I.e. the above would be more akin to a factory method that returns an autoreleased object while also maintaining a weak reference internally. But the optimizer might be too clever by half and break the above, too.
You're being bitten by the optimizer.
Since _theObject is a weak reference, the system is free to get rid of it, and zero out your weak reference, whenever it's not retained. But it's not required to do it right away.
In your lazy instantiator, the newly-created object is never retained. The optimizer sees this, and says "Wow! I can zero this reference at any time! Why don't I do it...right now!" And before you know it, you're returning nil.
What you want to do is assign the lazily-instantiated object to a local variable, for an implicitly strong reference that lasts for the scope of the function. You also want to tell the compiler that you really do want the full scope, using the objc_precise_lifetime annotation.
For details from the standard, see this page.

Objective C does self point to parent after doing self = [super init]?

When I have code like the following:
self = [super init]
does self point to super? If so, why would you want this? if my instance object has the variable "someVal", I won't be able to get to it by doing [self someVal]. correct?
How then would I get to the instance variable's using self when self points to super?
does self point to super?
It's really the other way around. super is really the same as self, except that it tells the compiler to start looking for method implementations starting with the superclass rather than the class itself. You can check this by logging the value of super and the value of self; you'll find that they both point to the same address.
When you create an object, you do this:
Foo *f = [[Foo alloc] init];
The alloc allocates the memory that will become the object you're creating, but until that memory is initialized it's just a chunk of memory -- not a valid object. If Foo is a subclass of Bar and Bar is a subclass of NSObject, then by convention Foo's initializer will call Bar's, and Bar's will call NSObject's, so that the initialization proceeds in order: first the memory is initialized by NSObjects' -init, and Bar's init receives the returned value and assigns it to self. It then proceeds to do any Bar-specific initialization, and returns self. Foo's -init then assigns the returned value to self again and finally does any Foo-specific initialization.
All that assigning to self might seem both redundant and confusing. It's really just a convention, but the purpose is to allow the superclass's initializer to return some object other than the one that was allocated, including nil. So, for example, if the initialization of Bar failed for some reason, -[Bar init] could return nil. The possibility that nil might be returned from [super init] is the reason we put the self = [super init] assignment inside a conditional: if the assigned value is nil, the initialization part is skipped and nil is returned. It's also possible that -[Bar init] could return a pointer to an object other than the one that was allocated, such as when an object similar to the one being created already exists and can be reused.
Most of the time, the pointer you get back from -init will be the same one that you got from +alloc, so you could write this:
Foo *f = [Foo alloc];
[f init];
If you write that, however, you're making an assumption that the initializers of your class and all the classes that it inherits from will always return the same object, and will never return nil. By doing that you're breaking the convention and severely hamstringing yourself and whoever wrote the classes from which Foo inherits -- they'll break your code if they return a different object in a future release of the class. Also, it'll look like you don't know what you're doing.
does self point to super?
This question doesn't make sense in an instance method, since there super is not really a concrete/actual pointer-to-instance, it just indicates that the implementation of the superclass must be called. And since in the case of most objects (except class clusters) all methods return self;, then the answer is no: the actual pointer to the instance doesn't change magically by itself.
For the record: the exception is manifested by class clusters (most Cocoa container classes, for example, NSString, NSArray, etc.). These classes often have an initializer method that returns a different instance than the one that was originally allocated, and of which the class is a concrete subclass of the class of the original self. For example, an implementation of the NSString initializer could be:
#implementation NSString
- (NSString *)init
{
[self release];
self = [[__NSCFString alloc] init];
return self;
}
#end
The reason for this is that optimizing for different types of initialization can be achieved this way.
Self is always pointing to one instance. When you use super you are referencing parent methods not a parent instance.
self means current class' instance.
self = [super init] means self is getting the value returned by [super init].

Memory management and properties (init/dealloc)

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

EXC_BAD_ACCESS error when changing views with PresentModalViewController

I'm trying to switch views in my app using this chunk of code:
self->variable1 = [[NSNumber alloc] initWithInt:0];
self->variable2 = [[NSMutableArray arrayWithCapacity:1];
self->variable3 = [[NSMutableArray arrayWithCapacity:1];
[self presentModalViewController:titleScreen animated:YES];
If I comment out all of the allocated variable lines, the code works fine. If it leave just 1 line in the code crashes with the "EXC_BAD_ACCESS" error. Why is this happening? The variables aren't being used at all, just declared for later use. I'm not getting any compile errors on the lines either. What am I doing wrong?
UPDATE:
Thank you everyone for the help. I change the way I declare my variables to #property/#synth to clean up my code, but it didn't fix the problem. After a long time of fiddling I fixed it. I changed the code from this:
self.variable1 = [[NSNumber alloc] initWithInt:0];
to this:
self.variable1 = [NSNumber alloc];
[self.variable1 initWithInt:0];
and it worked! Can someone explain why this worked and the first line didn't?
Update:
Thank you Peter Hosey for showing me my evil ways. This time I'm pretty sure it's fixed. I was storing my variable Releases in
-(void)release
I didn't realize xCode will release when it needs to. I moved all the variable releases to
-(void)Destroy
so I can release everything on MY command. Now the code works. Thanks again!
I suggest that you declare variable1, variable2, and variable3 as properties, not instance variables. Then, use self.variable1, self.variable2, and self.variable3 to access them.
The dot syntax (self.variable1, etc.) uses the memory management policy you declared on each property; the arrow syntax (self->variable1, etc.) will access the variables directly. The crash is because you created two arrays in away that doesn't leave you owning them, and then did not assign the arrays to a property that would retain them.
You may also want to upgrade your project to use ARC. Then there is no memory-management difference; assigning to the instance variables rather than the properties will not cause the object to be prematurely released, because ARC considers instance variables to be ownerships by default. You may still want to switch to using properties after you switch to ARC, but not to prevent a crash.
In response to your edit:
I change the way I declare my variables to #property/#synth to clean up my code, but it didn't fix the problem.
Then something else was wrong.
You never did say much about the problem itself. You said you got an EXC_BAD_ACCESS, but not what statement triggered the crash or on what grounds you blamed it on the code you showed.
I changed the code from this:
self.variable1 = [[NSNumber alloc] initWithInt:0];
That's the correct code, though. That's what you should be using.
to this:
self.variable1 = [NSNumber alloc];
[self.variable1 initWithInt:0];
Noooo! That code is wrong, wrong, wrong, on multiple levels.
init methods (including initWithWhatever: methods) are not guaranteed to return the same object you sent the message to. NSNumber's initWithInt: very probably doesn't.
That object creates an uninitialized NSNumber object and assigns that to the property. Then it sends initWithInt: to that object, which will return an initialized object, which can be and very probably will be a different object. Now you are holding an uninitialized object (which you will try to use later) and have dropped the initialized object on the floor.
Never, ever, ever send alloc and init(With…) in separate expressions. Always send them in the same expression. No exceptions. Otherwise, you risk holding the uninitialized object rather than the initialized object. In your case (with NSNumbers), that is almost certainly what will happen.
What you should be doing is declaring and synthesizing a strong property that owns the NSNumber object, and creating the NSNumber object in a single statement: either [[NSNumber alloc] initWithInt:] or [NSNumber numberWithInt:]. If you're not using ARC, you'll want the latter, since the property will retain the object. If you are using ARC, they're effectively equivalent.
And if you get a crash with that code, then something else is wrong, so please tell us—either in this question or in a new question—about the crash so we can help you find the true cause of it.
variable2 and variable3 are being autoreleased before you actually access them (presumably) later after presenting the modal view.
At the very least change the lines to:
self->variable2 = [[NSMutableArray arrayWithCapacity:1] retain];
self->variable3 = [[NSMutableArray arrayWithCapacity:1] retain];
or
self->variable2 = [[NSMutableArray alloc] initWithCapacity:1];
self->variable3 = [[NSMutableArray alloc] initWithCapacity:1];
variable1 should be fine.
Best would be to use #property and #synthesize so you can use dot notation:
.h
#interface MyClass : SuperClass
#property (nonatomic,retain) NSMutableArray *variable2;
#property (nonatomic,retain) NSMutableArray *variable3;
#end
.m
#implementation MyClass
#synthesize variable2,varible3;
- (void)foo {
self.variable2 = [NSMutableArray arrayWithCapacity:1];
self.variable3 = [NSMutableArray arrayWithCapacity:1];
}
#end
By default, all instance variables in objective-c have protected scope. So unless you have explicitly declared them public in your interface file as:
#interface MYClass {
#public
NSNumber *variable1;
NSMutableArray *variable2;
NSMutableArray *variable3;
}
//...
#end
then they will not be accessible using the struct dereferencing operator. This is likely the cause of those EXC_BAD_ACCESS errors.

Resources