I'm still not 100% with ARC internals.
A few examples that I would like clarified:
What happens with someObjectToReturn = [[SomeClass alloc] init]; that is allocated inside a method and returned? If someObjectToReturnis assigned to e.g. someObject, when will someObjectToReturn be released? When someObject is nilled or does someObjectToReturn still need to be added to an autorelease pool?
Another case is if an array of objects is nilled, are all the objects released?
Lastly, where is a good place to release a copied block object? Or will the block still be released after execution? If the copied block is added to an autorelease pool, when would one drain that pool?
Lets say there are 10 blocks, each added to an autorelease pool, draining it too soon would result in the executing blocks to die. Can a copied block be nilled from within itself, such as dispatching the nilling of the block back to the main thread?
You seem to be overcomplicating things. Under ARC, you can't release objects, or drain autorelease pools, so there is no "right" time to do this. Don't think about retain and release and autorelease (except in tight loops), think about object ownership.
Your specific questions:
The object will be released when the variable that the return value is assigned to goes out of scope or is set to nil. You don't have to worry about it.
Yes, all the contents of an array are released when the array is set to nil, you don't have to worry about it
You can set the value holding the copied block to nil when you're done, or it will disappear when the object owning the copied block goes. You can't drain an autorelease pool under ARC, you can wrap statements in an #autoreleasepool{} block, but this just captures everything in the block - you don't "add" things to the pool or "drain" the pool.
Related
I am not clear about how to use autorelease;
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
RetainTracker* tracker = [RetainTracker new];
[tracker retain];
[tracker retain];
[tracker autorelease];
[pool release];
Is there any memory leak in the above code?
I know the autorelease just puts tracker into NSAutoreleasePool, without modifying the reference count for tracker. When i call [pool release], the object receives one message release, then the reference count of tracker is 2. So the compiler can't call the dealloc function of object tracker, so there is a memory leak.
So i get this: we shoul call retain and release same times, is it right?
If you're new to Objective-c, you should be using Automatic Reference Counting. Almost everyone should. But for the sake of understanding:
An object has a reference count of how many objects have a reference to it. When it drops to zero, the object is not referenced by anything and is deallocated. Every method in Objective-C is responsible for releasing any object it retains. The problem is, if a factory method's job is to create an object, it's breaking the rules by not releasing an object it retained (in this example it was retained because it was created). If the method called release on the object right before returning it, it would be returning the address to an object that is already gone. So, there is autorelease, which temporarily delays the release until the calling method can retain the object the called method created.
In your example above, if you got rid of the 2 retains, and only called autorelease, then the object would get released and deallocated when the autorelease pool is released and (right before deallocation) is drained, meaning it calls release on all objects in its pool, tracker being one of them because calling autorelease on tracker added it to the pool.
new is equivalent to doing an "alloc" & "init", so retain count on tracker on that line is 1.
Then you increment the retain count 2 more times to give a total retain count of 3.
Autorelease sets the tracker to be released when the pool is released (and the retain count is decremented as well). But it's already retained 3 times, so it doesn't really get released.
And yeah, there is a memory leak with the retain count being greater than 0 and no reference to "tracker" outside of that method (that is, unless you're using "tracker" in an instance variable).
It's very good to know the basics of memory management; but if you want to save yourself a ton of headache, just do what everyone else here seems to be saying and simply enable ARC.
EDIT: And to finish your question, you should make sure every call to retain is balanced with a release. You also do a "new", which also increments the retain count so you need to "release" because of that as well.
What prevents the autoreleased instance of NSArray from being released before it's used in the method it's returned to?
- (NSArray *) f {
return [NSArray array];
}
- (void) g {
NSArray *a = [self f];
// do something with a
}
What prevents the autoreleased instance of NSArray from being released
before it's used in the method it's returned to?
Autorelease pools are per thread. That is, an autoreleased object is, effectively, a delayed call to release that happens on a per thread basis.
Thus, if there is no call to drain between the caller and the callee, there is no way the autoreleased object will be released because of a pool drain (barring serious threading stupidity).
In general, pools are drained at very well specified times, like Kevin said:
• the run loop will drain the pool at the bottom of each pass through the loop
• dispatched blocks via GCD will execute within a pool context that'll be drained "every now and then" (implementation detail). NSOperationQueue behaves similarly
• other threads are responsible for doing it themselves
Thus, in your simple example, the code execution is quite linear and, by inspection, there can be no drain in that thread of execution between the return and the use in the caller.
(Note that this is also one of the reasons why retainCount is useless; it neither accounts for any "delayed release" calls caused by autorelease nor does it quantify any per-thread retains. In reality, you should think of retains/releases as being entirely per-thread; if you retain an object on a thread you should release it on the same thread unless you are explicitly transferring ownership from one thread to another.)
Autoreleased objects are only released when their surrounding pool is drained. This is not some magical process that happens behind the scenes. It's a deterministic event that happens at well-defined times. In general, the pool is drained when control returns to the run loop. If you establish your own nested pool, then you control when that drains.
The autorelease pool is drained during an iteration of the event loop. Since it's that drain that causes release messages to be sent to the autoreleased objects, they are still valid in the calling method.
Does autorelease guaranty that at the end of blocks the object will get released?
Or is it better to manually release objects?
It guarantees it will be released sometime after the block executes, not necessarily immediately after. It's up to the runtime to determine exactly when.
It's not big deal unless you're doing something with a lot of autoreleased variables, like creating them in a big loop, or if you're creating large autoreleased objects, like UIImages. In these cases, you should manually release when you're through, otherwise autorelease is perfectly acceptable.
If an object is autoreleased, you MUST not manually release it (unless it is retained of course). The NSAutoRelease pool which is part of the UIKit event handler will release it for you. If you were to manually release the object, the pool may cause a crash or other undefined behavior as the object will be doubly-released.
If there are cases where you generate a lot of objects or use a lot of memory in objects, you can pre-emptively autorelease them (perhaps in your loop) by creating your own NSAutoReleasePool - pools can be nested.
It's better to release objects rather than autorelease, unless of course you have an explicit reason to use autorelease, for example use autorelease when returning an object the method retained and you can't avoid it.
Basically autorelease should be used as an excuse to completely avoid memory management. You want to release objects as soon as you possible can. Autorelease just says the object will be released some time in the future.
Is there a way to change an autoreleased object to one that is non-autoreleased?
NSCoder's decodeObjectForKey returns an autoreleased object, which messes with a couple memory systems in my app. How can I change its returned value to a non-autoreleased object?
I know I can run retain on it, but if I release it, it'll still remain autoreleased. Whereas, I want to manage the memory myself.
Set up an autorelease pool before you retrieve the autoreleased object. It will be in that autorelease pool. Retain the object. Now release the autorelease pool. Your object is no longer in an autorelease pool. It's retained, and it's now your responsibility to release it.
I still have some unclear understand about release and autorelease. What are the difference between both of them? I have this code. For facebook connection. I crash it sometimes when I go to Facebook login, I doubting maybe it is because I don't release the object nicely.? Thanks for any helps
if (_session.isConnected) {
[_session logout];
} else {
FBLoginDialog* dialog = [[[FBLoginDialog alloc] initWithSession:_session] autorelease];
[dialog show];
}
The Memory Management Programming Guide for Cocoa will soon be your best friend. In brief, object instances in Cocoa are memory managed using reference counting (unless, of course you're using garbage collection on OS X). An object indicates that it wants to 'retain' an ownership interest in an other instance--keep it from being deallocated--by sending it a -retain message. An object indicates that it wants to release that interest by sending the other instance a -release message. If the number of objects that have 'retained' and ownership interest in an object drops to 0 (i.e. when the last of the owning instances sends a -release message), the instance with a 0 retain count is deallocated.
It's sometimes convenient to say "I want this instance to be released some time in the future". That's the purpose of -autorelease. Sending an -autorelease message adds the receiver to the current NSAutoreleasePool. When that pool is drained, it sends a -release message to all the instances in the pool. An NSAutoreleasePool is automatically created at the start of each iteration of each thread's run loop and drained at the end of that iteration. Thus, you can do something like this in a method:
- (id)myMethod {
return [[[MyObject alloc] init] autorelease];
}
The caller of this method will get back an instance that they can -retain if they wish to keep it. If they don't retain it, it will stick around at least until the enclosing autorelease pool is drained:
- (void)someOtherMethod {
...
id instance = [obj myMethod];
... // do more with instance, knowing that it won't be dealloc'd until after someOtherMethod returns
}
Releasing means you release that right away.
Autoreleasing means you want the variable to be released on the next autorelease pool.
You use autorelease when you want to keep retaining the variable but don't want to create a memory leak. You use release when you don't need the variable anymore.
Sample:
- (NSNumber *)return5 {
NSNumber * result = [[NSNumber alloc]initWitnInt: 5];
[result autorelease];
return result;
}
Why do we use autorelease there?
If we use [result release] instead, variable result will be destroyed AT that time. Which means that the returned value will be garbage.
If we do not release at all, variable result will be hold FOREVER incurring memory leak.
We can tell every caller to the function to release result but that would be a headache and prone to error.
So we use autorelease. We mark the variable to be released on the next autorelease pool. Basically we mark the variable to be released near the alloc. Hence the mantra alloc is paired with release in the same function holds all the time.
Actually, you'll do fine changing all release into autorelease. Your memory use won't be efficient, however, the effect is minimal. All variables, in all programming language is effectively autoreleased.
Anyway, use ARC.
background discussion:
objective-c is reference counted, so objects are deleted when the reference count reaches 0. release reduces the reference-count immediately, autorelease reduces it when the autorelease-pool is popped
when to use:
use autorelease when allocating the object if
you do not need it after the current function
it will be retiained by some other objet/function and will be released by a later by the retaining code
when the logic of the current function is tricky, so you would have to send release in a dozen different places before doing a return
use "manual" release
to revert a previous retain (in case you are implementing a library)
if you need precise control of freeing objects (e.g. they use lots of memory or the autorelease pool will not be popped for some time)
but really my freand:
read the Memory Management Programming Guide for Cocoa as suggested by Barry and run your code with instruments (zombies and leaks) often to catch any and almost all memory management errors.
Erik
According to the Memory Management Programming Guide for Cocoa:
The autorelease method, defined by
NSObject, marks the receiver for later
release. By autoreleasing an
object—that is, by sending it an
autorelease message—you declare that
you don't want to own the object
beyond the scope in which you sent
autorelease.
Also:
The autorelease method thus allows
every object to use other objects
without worrying about disposing of
them.