I have a basic question here.
I know that dealloc will be called when the object's reference count becomes zero,and dealloc releases all the resources hold by the object or frees memory, right?
The object reference count becomes zero if we send release message to that object right?.
Lets consider the following object with its property created as,
#property (retain) NSString* myString;//reference count 1
and dealloc
[myString release];//reference count 0
[super dealloc];
I am not releasing the myString object any where except in dealloc.
My question is who is making myString object reference count to zero so that dealloc will be called?
Please clarify my doubt.
Anything that maintains ownership of the object is responsible for releasing it. For example if the code you posted is the only thing that maintains ownership of the NSString stored in myString then when you call release the reference count will be decreased and the object will likely be deallocated (String literals are different). Now if you passed myString around or something else requested myString and retains it then that code is also responsible for releasing it which may be before or after you release it in the dealloc method.
I recommend referring to the documentation for reinforcing this concept.
Let's say your property myString is within your class MyClass. Each instance of your class is created and (presumably!) released. When that instance is [released], your dealloc method of MyClass is called. Your dealloc in turn calls release on your properties, which in turn invokes their dealloc, and so on until you reach the 'bottom' of this stack of objects using objects.
Your object will only be free'ed if the retain/release pairs match up. If you follow the cocoa memory management (see apple docs - advances memory management programming guide)
rules, you are the only 'owner' at that point - and running dealloc zap's the last reference away - free'ing the object.
Related
I have created an object using alloc/init method, and after I release it -dealloc should be called immediately as per documentation. I set a breakpoint on -dealloc method but it isn't hit, and my -dealloc method is not called.
Please tell me what is the reason behind that, and what is use of dealloc method in objective c ?
The -dealloc method is not always called when you expect it to be called. The runtime might also have issued a -retain on your object for internal reasons.
It's also possible that you have (directly or indirectly) caused an extra -retain to be issued. If the retains/allocs and releases are not balanced, you'll never see -dealloc called. It helps to turn on the Static Analyzer, to make sure your calls balance.
Just follow the memory management rules, don't second guess the runtime, and let the memory management system do its job.
The answers to When does dealloc method call? may help you understand what you're seeing.
because it still has reference. that means its reference count not reached to zero. i don't know your code, where it is referencing. but it is not calling that means somehow still it has reference. it may be because of strong relationship or parent-child relationship
all Objective-C objects are allocated on the heap, so they must
therefore be deallocated somewhere if you are not to run out of
resources.
This gave way to the reference counting method, which is still used
today: each object keeps count of any references held to it. If you
receive an object and you want to keep it, you retain that object,
incrementing its reference count. When you are done with it, you
release it, which decrements its reference count. Once that count
reaches zero, it is inferred that no one is referencing the object and
it is automatically deallocated using the -dealloc method.
Additionally, an object could be told to “release at some point in the
(hopefully) near future” using autorelease pools. The idea is that
somewhere on the stack (typically at the start of a thread or while
responding to input events) an autorelease pool is created and pushed
onto a stack. Any object can then be sent an -autorelease message, and
it is assigned to that pool.
When the pool object is deallocated, it simply sends a -release
message to all its assigned objects. That way, any objects that are no
longer used (i.e. they haven’t been explicitly retained) are then
deallocated.
The dealloc is called (at more cases) whenever your object is released. You can't directly call this method.
#interface myViewController:UIViewController
{
NSString *myStr;
}
#end
Here the dealloc method in the #implementation of myViewController will be called (at most cases) when the myViewController object is released, not when myStr is released.
Although you don't have to use if you ARC.
I have this in my .h file:
#property (nonatomic,retain) CCTexture2D *tempScreenshot;
Since it says "retain", I'm assuming that whatever value is stored in tempScreenshot is retained.
Do I have to release tempScreenshot in the dealloc method of my class?
Also, if I use the set method to give a new value to tempScreenshot, this new value will be retained, right? What happens to the old value?
Since you're saying "retain" I am assuming this is manual memory management code. If at all possible, you should switch to automatic reference counting (ARC). It dramatically simplifies this work and is faster than the equivalent manual code. Other than support for older OSes, there is almost never a reason to do manual memory management anymore.
Since it says "retain", I'm assuming that whatever value is stored in tempScreenshot is retained.
Whenever you call [self setTempScreenshot:], the accessor will retain the parameter and release the old value. Assigning the ivar directly will not give you any of this. You should always use the accessor except in init and dealloc.
Do I have to release tempScreenshot in the dealloc method of my class?
Yes.
Also, if I use the set method to give a new value to tempScreenshot, this new value will be retained, right? What happens to the old value?
It will be released as long as you use the accessor.
The retain keyword marks the property as retain upon assignment, meaning that on a set, retain will be called on the new value, and release will be called on the old value.
You will need to release the object on dealloc.
In your dealloc method, do: self.tempScreenshot = nil;
This will cause the object to be released. It will then be deallocated assuming nothing else has retained it.
If you were to write your own setter method to retain an object it would look like this.
//Assume an instance variabled name obj for this example
- (void)setAndRetainObjectExample:(NSObject *)newObj {
[obj release];
obj = newObj;
[obj retain];
}
Using the synthesized methods is much cleaner, but looking at it this way may clear up any confusion.
Is it good practice for an object in Objective-C to commit suicide? That is, for an object to declare [self dealloc] where -dealloc permits an orderly wind down as usual? What are the principal risks?
As it happens I have a specific example, a custom timer object that extends NSObject and comprises an NSTimer instance and an NSUInteger which is set to limit the number of times the timer fires. When time is up the object tells the timer to -invalidate and then commits suicide by calling its -dealloc method. As this is automatic we have no worries about having to track the object or crucially knowing when is the correct moment to deallocate it.
For a more detailed explanation see my post over here.
You shouldn't be calling -dealloc. Instead call [self release] so the reference count goes to 0 and let the system call -dealloc.
Is it good practice for an object in Objective-C to commit suicide? That is, for an object to declare [self dealloc] where -dealloc permits an orderly wind down as usual? What are the principal risks?
No.
The only time you should ever write a call to dealloc is to send dealloc to the super object in the dealloc method of one of your classes. No exceptions.
If you try to send dealloc to an object at any other time, you risk leaving other objects with dangling pointers. Don't do it.
Should you ever send release to self? That is a different matter, but you should still follow the Memory Management Rules. If you have sent retain to self, then at some point you will need to send release to self. There is one exception which is in init, if initialisation fails you have to release self and return nil (I guess you could claim that alloc has sent retain to self).
I am still new to this Memory Management stuff (Garbage Collector took care of everything in Java), but as far as I understand if you allocate memory for an object then you have to release that memory back to the computer as soon as you are finished with your object.
myObject = [Object alloc];
and
[myObject release];
Right now I just have 3 parts in my Objective-C .m file: #Interface, #Implementation and main. I released my object at the end of the program next to these guys:
[pool drain];
return 0;
But what if this program were to be a lot more complicated, would it be okay to release myObject at the end of the program?
I guess a better question would be when do I release an object's allocated memory? How do I know where to place [myObject release];?
This is probably a little over-simplified, but in general, you are going to want to release it where you declared it.
If you declare an object INSIDE a particular method call, then by definition, you will be done with that object (or at least that handle to that object) at the end of that method call... release it then.
If you declare an object as an instance variable, then by definition you will be done with it when that instance is destroyed... release it in the dealloc method of that class.
Keep in mind that "release" does not equal "destroy." When passing objects around in your application, it may make sense to have more than one handle to that object stored in different places... in that case "release" means "I'm done with this object, but someone else may still be using it." Deallocation only occurs when the number of "handles" (retain count) reaches zero.
Apple has some fantastic documentation on memory management, I would check it out at developer.apple.com.
You essentially have three kinds of objects, each with a different pattern.
Transients Objects
In general, you should autorelease transient objects. These are objects that are allocated locally and do not need to exist beyond the method in which they are called. Or they are passed around from method to method.
Chain of Ownership
When one object exists as an instance field inside another, you should release the "owned" (or "child") object when the "owner" (or "parent") object goes out of existence. This is done in the dealloc method of the parent object:
- (void) dealloc {
[child release]; // child was declared as an instance variable
[super dealloc];
}
Lifetime of the Program
When an object is intended to exist for the lifetime of the program, it usually isn't necessary to call release at all, unless some kind of resource cleanup needs to occur. You can put this in applicationWillTerminate:, which you can look up in Apple's documentation.
(You should probably avoid having such objects, but that is a discussion for another question.)
You have to think in terms of ownership. When you take ownership of an object by calling alloc, new or retain, you're also responsible for releasing it, either by calling autorelease when you return an owned object to the caller, or by calling release.
A general rule is:
Local variable: release it within the same method. When you want to return it to the caller, use autorelease
Class member: release it in the dealloc method
If we write the following code:
ExplorerAppDelegate * appDelegate = (ExplorerAppDelegate *)[[UIApplication sharedApplication] delegate];
This makes a reference to the original delegate pointer, but:
Does it increase the reference count?
Do we have to explicitly call as [ExplorerAppDelegate retain] right after, or not at all?
What's happening, exactly?
After we've used this, we should also do a [ExplorerAppDelegate release] in the dealloc method, right?
No, it does not increase the retain count.
The convention in Objective-C is that objects you are given should be memory managed by yourself - but in the case of obtaining a shared common resource like the app delegate, the memory is maintained elsewhere and of course (with this being the app delegate) you know that it will always be "alive" as long as your class is... so there is no need to retain the reference.
In most uses of delegates, instead of fetching a delegate you are given one, and that reference is not retained either. In that case whoever gave you the delegate is also responsive for clearing out the delegate link before the delegate is released.
The reason you don't want to generally retain delegate references is that it can prevent some objects from being deallocated, for instance if one class is a delegate of a class that ues the other class as a delegate.
The reference count will not be increased
You should retain it if you want to be sure that it isn't deallocated while you have a pointer to it
You should only release it if you retained it
So basically, if you're only using the object in a single function, you probably don't need retain or release it. If it exists when you get it, then it's (probably) not going to be deallocated by the end of the function. If you're keeping it around, in an ivar (member variable) for example, then you should retain it and release it later.
See the "Weak References to Objects" in Memory Management Programming Guide for Cocoa for the official answer. Pointers to delegates are one of the possible exception cases to the memory management rules.