I'm just beginning to have a look at Objective-C and Cocoa with a view to playing with the iPhone SDK. I'm reasonably comfortable with C's malloc and free concept, but Cocoa's references counting scheme has me rather confused. I'm told it's very elegant once you understand it, but I'm just not over the hump yet.
How do release, retain and autorelease work and what are the conventions about their use?
(Or failing that, what did you read which helped you get it?)
Let's start with retain and release; autorelease is really just a special case once you understand the basic concepts.
In Cocoa, each object keeps track of how many times it is being referenced (specifically, the NSObject base class implements this). By calling retain on an object, you are telling it that you want to up its reference count by one. By calling release, you tell the object you are letting go of it, and its reference count is decremented. If, after calling release, the reference count is now zero, then that object's memory is freed by the system.
The basic way this differs from malloc and free is that any given object doesn't need to worry about other parts of the system crashing because you've freed memory they were using. Assuming everyone is playing along and retaining/releasing according to the rules, when one piece of code retains and then releases the object, any other piece of code also referencing the object will be unaffected.
What can sometimes be confusing is knowing the circumstances under which you should call retain and release. My general rule of thumb is that if I want to hang on to an object for some length of time (if it's a member variable in a class, for instance), then I need to make sure the object's reference count knows about me. As described above, an object's reference count is incremented by calling retain. By convention, it is also incremented (set to 1, really) when the object is created with an "init" method. In either of these cases, it is my responsibility to call release on the object when I'm done with it. If I don't, there will be a memory leak.
Example of object creation:
NSString* s = [[NSString alloc] init]; // Ref count is 1
[s retain]; // Ref count is 2 - silly
// to do this after init
[s release]; // Ref count is back to 1
[s release]; // Ref count is 0, object is freed
Now for autorelease. Autorelease is used as a convenient (and sometimes necessary) way to tell the system to free this object up after a little while. From a plumbing perspective, when autorelease is called, the current thread's NSAutoreleasePool is alerted of the call. The NSAutoreleasePool now knows that once it gets an opportunity (after the current iteration of the event loop), it can call release on the object. From our perspective as programmers, it takes care of calling release for us, so we don't have to (and in fact, we shouldn't).
What's important to note is that (again, by convention) all object creation class methods return an autoreleased object. For example, in the following example, the variable "s" has a reference count of 1, but after the event loop completes, it will be destroyed.
NSString* s = [NSString stringWithString:#"Hello World"];
If you want to hang onto that string, you'd need to call retain explicitly, and then explicitly release it when you're done.
Consider the following (very contrived) bit of code, and you'll see a situation where autorelease is required:
- (NSString*)createHelloWorldString
{
NSString* s = [[NSString alloc] initWithString:#"Hello World"];
// Now what? We want to return s, but we've upped its reference count.
// The caller shouldn't be responsible for releasing it, since we're the
// ones that created it. If we call release, however, the reference
// count will hit zero and bad memory will be returned to the caller.
// The answer is to call autorelease before returning the string. By
// explicitly calling autorelease, we pass the responsibility for
// releasing the string on to the thread's NSAutoreleasePool, which will
// happen at some later time. The consequence is that the returned string
// will still be valid for the caller of this function.
return [s autorelease];
}
I realize all of this is a bit confusing - at some point, though, it will click. Here are a few references to get you going:
Apple's introduction to memory management.
Cocoa Programming for Mac OS X (4th Edition), by Aaron Hillegas - a very well written book with lots of great examples. It reads like a tutorial.
If you're truly diving in, you could head to Big Nerd Ranch. This is a training facility run by Aaron Hillegas - the author of the book mentioned above. I attended the Intro to Cocoa course there several years ago, and it was a great way to learn.
If you understand the process of retain/release then there are two golden rules that are "duh" obvious to established Cocoa programmers, but unfortunately are rarely spelled out this clearly for newcomers.
If a function which returns an object has alloc, create or copy in its name then the object is yours. You must call [object release] when you are finished with it. Or CFRelease(object), if it's a Core-Foundation object.
If it does NOT have one of these words in its name then the object belongs to someone else. You must call [object retain] if you wish to keep the object after the end of your function.
You would be well served to also follow this convention in functions you create yourself.
(Nitpickers: Yes, there are unfortunately a few API calls that are exceptions to these rules but they are rare).
If you're writing code for the desktop and you can target Mac OS X 10.5, you should at least look into using Objective-C garbage collection. It really will simplify most of your development — that's why Apple put all the effort into creating it in the first place, and making it perform well.
As for the memory management rules when not using GC:
If you create a new object using +alloc/+allocWithZone:, +new, -copy or -mutableCopy or if you -retain an object, you are taking ownership of it and must ensure it is sent -release.
If you receive an object in any other way, you are not the owner of it and should not ensure it is sent -release.
If you want to make sure an object is sent -release you can either send that yourself, or you can send the object -autorelease and the current autorelease pool will send it -release (once per received -autorelease) when the pool is drained.
Typically -autorelease is used as a way of ensuring that objects live for the length of the current event, but are cleaned up afterwards, as there is an autorelease pool that surrounds Cocoa's event processing. In Cocoa, it is far more common to return objects to a caller that are autoreleased than it is to return objets that the caller itself needs to release.
Objective-C uses Reference Counting, which means each Object has a reference count. When an object is created, it has a reference count of "1". Simply speaking, when an object is referred to (ie, stored somewhere), it gets "retained" which means its reference count is increased by one. When an object is no longer needed, it is "released" which means its reference count is decreased by one.
When an object's reference count is 0, the object is freed. This is basic reference counting.
For some languages, references are automatically increased and decreased, but objective-c is not one of those languages. Thus the programmer is responsible for retaining and releasing.
A typical way to write a method is:
id myVar = [someObject someMessage];
.... do something ....;
[myVar release];
return someValue;
The problem of needing to remember to release any acquired resources inside of code is both tedious and error-prone. Objective-C introduces another concept aimed at making this much easier: Autorelease Pools. Autorelease pools are special objects that are installed on each thread. They are a fairly simple class, if you look up NSAutoreleasePool.
When an object gets an "autorelease" message sent to it, the object will look for any autorelease pools sitting on the stack for this current thread. It will add the object to the list as an object to send a "release" message to at some point in the future, which is generally when the pool itself is released.
Taking the code above, you can rewrite it to be shorter and easier to read by saying:
id myVar = [[someObject someMessage] autorelease];
... do something ...;
return someValue;
Because the object is autoreleased, we no longer need to explicitly call "release" on it. This is because we know some autorelease pool will do it for us later.
Hopefully this helps. The Wikipedia article is pretty good about reference counting. More information about autorelease pools can be found here. Also note that if you are building for Mac OS X 10.5 and later, you can tell Xcode to build with garbage collection enabled, allowing you to completely ignore retain/release/autorelease.
Joshua (#6591) - The Garbage collection stuff in Mac OS X 10.5 seems pretty cool, but isn't available for the iPhone (or if you want your app to run on pre-10.5 versions of Mac OS X).
Also, if you're writing a library or something that might be reused, using the GC mode locks anyone using the code into also using the GC mode, so as I understand it, anyone trying to write widely reusable code tends to go for managing memory manually.
As ever, when people start trying to re-word the reference material they almost invariably get something wrong or provide an incomplete description.
Apple provides a complete description of Cocoa's memory management system in Memory Management Programming Guide for Cocoa, at the end of which there is a brief but accurate summary of the Memory Management Rules.
I'll not add to the specific of retain/release other than you might want to think about dropping $50 and getting the Hillegass book, but I would strongly suggest getting into using the Instruments tools very early in the development of your application (even your first one!). To do so, Run->Start with performance tools. I'd start with Leaks which is just one of many of the instruments available but will help to show you when you've forgot to release. It's quit daunting how much information you'll be presented with. But check out this tutorial to get up and going fast:
COCOA TUTORIAL: FIXING MEMORY LEAKS WITH INSTRUMENTS
Actually trying to force leaks might be a better way of, in turn, learning how to prevent them! Good luck ;)
Matt Dillard wrote:
return [[s autorelease] release];
Autorelease does not retain the object. Autorelease simply puts it in queue to be released later. You do not want to have a release statement there.
My usual collection of Cocoa memory management articles:
cocoa memory management
There's a free screencast available from the iDeveloperTV Network
Memory Management in Objective-C
NilObject's answer is a good start. Here's some supplemental info pertaining to manual memory management (required on the iPhone).
If you personally alloc/init an object, it comes with a reference count of 1. You are responsible for cleaning up after it when it's no longer needed, either by calling [foo release] or [foo autorelease]. release cleans it up right away, whereas autorelease adds the object to the autorelease pool, which will automatically release it at a later time.
autorelease is primarily for when you have a method that needs to return the object in question (so you can't manually release it, else you'll be returning a nil object) but you don't want to hold on to it, either.
If you acquire an object where you did not call alloc/init to get it -- for example:
foo = [NSString stringWithString:#"hello"];
but you want to hang on to this object, you need to call [foo retain]. Otherwise, it's possible it will get autoreleased and you'll be holding on to a nil reference (as it would in the above stringWithString example). When you no longer need it, call [foo release].
The answers above give clear restatements of what the documentation says; the problem most new people run into is the undocumented cases. For example:
Autorelease: docs say it will trigger a release "at some point in the future." WHEN?! Basically, you can count on the object being around until you exit your code back into the system event loop. The system MAY release the object any time after the current event cycle. (I think Matt said that, earlier.)
Static strings: NSString *foo = #"bar"; -- do you have to retain or release that? No. How about
-(void)getBar {
return #"bar";
}
...
NSString *foo = [self getBar]; // still no need to retain or release
The Creation Rule: If you created it, you own it, and are expected to release it.
In general, the way new Cocoa programmers get messed up is by not understanding which routines return an object with a retainCount > 0.
Here is a snippet from Very Simple Rules For Memory Management In Cocoa:
Retention Count rules
Within a given block, the use of -copy, -alloc and -retain should equal the use of -release and -autorelease.
Objects created using convenience constructors (e.g. NSString's stringWithString) are considered autoreleased.
Implement a -dealloc method to release the instancevariables you own
The 1st bullet says: if you called alloc (or new fooCopy), you need to call release on that object.
The 2nd bullet says: if you use a convenience constructor and you need the object to hang around (as with an image to be drawn later), you need to retain (and then later release) it.
The 3rd should be self-explanatory.
Lots of good information on cocoadev too:
MemoryManagement
RulesOfThumb
As several people mentioned already, Apple's Intro to Memory Management is by far the best place to start.
One useful link I haven't seen mentioned yet is Practical Memory Management. You'll find it in the middle of Apple's docs if you read through them, but it's worth direct linking. It's a brilliant executive summary of the memory management rules with examples and common mistakes (basically what other answers here are trying to explain, but not as well).
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 a problem with retainCount
NSLog(#"%i", [self.albumReceiver retainCount]);
self.albumReceiver = [[[FacebookAlbumsDelegateReceiver alloc] init: self] autorelease];
NSLog(#"%i", [self.albumReceiver retainCount]);
The retain count on the first line is 0 but when it gets to third line is 3.
The property on self.albumReceiver is retain property... But as far as i can see it should be 2 where other later retain count should went to 1 since it was autorelease later.
NSLog(#"%i", [self.albumReceiver retainCount]);
albumReceiver = [[[FacebookAlbumsDelegateReceiver alloc] init: self];
NSLog(#"%i", [self.albumReceiver retainCount]);
the retain count start with 0 and in this case the second retain count print 2....
Can somebody give some idea of how this retain and release work....
I thought without 'self' keyword, it will ignore the setter call is it? But if i put autorelease on the second example, i will have error.
Firstly, the retain count is an implementation detail that you should not be overly concerned with. You should really only care about ownership.
Having said that, the explanation for what you are seeingis as follows:
The retain count on the first line is 0
That's because self.albumReceiver is nil at that point. On real objects, you will never see a retain count of 0 (in the current implementation of Foundation).
when it gets to third line is 3
Normally you would expect the retain count to be 2, +1 for the alloc and +1 for assignment to a retain property. However, the init: method might cause some other object to retain it, so might the setter for the property. Another object observing the property might choose to retain the object too. In short, unless you know the exact implementation of all the methods involved and you know for sure nothing is using KVO on albumReceiver, all you can say about the retain count is that self has ownership of it (NB ownership is not exclusive, other things may also have ownership). This is why you shouldn't pay too much attention to retain counts.
Can somebody give some idea of how this retain and release work
Yes. You need to think in terms of ownership. Certain methods give you an object that you own. These are any method starting with alloc, any method starting with new, any method starting with copy or mutableCopy and -retain. If you receive an object in any other way, you do not own it. That includes receiving them as a return result of a method, ass parameters of a method or as by reference parameters of a method or as global or static variables.
If you own an object, you must relinquish ownership by either releasing or autoreleasing it, or it will leak. If you do not own an object, you must not release or autorelease it.
I find that it is best to think of "you" in the above as meaning "the scope in which the object reference was declared".
Anyway, I recommend you to read Apple's Memory Management Rules for the definitive explanation and not to trust answers here. If you look at the original edit for this answer, you'll see that I got the rules slightly wrong because they have been tightened up since I last read them.
Do not use retainCount. (For situations where it’s appropriate to use it, see this website.) Further reading: blog post by bbum, previous SO thread one, thread two. Also note that retainCount is deprecated in recent SDKs. It’s a good idea to pay attention to the deprecation warnings, and even better idea to turn all warnings into errors.
It is generally a bad idea to pay any attention to the retainCount of an object in Objective-C because it is usually impossible to know which secret parts of the frameworks feel a need to retain the object.
In the case you cite where you are adding the object to the autorelease pool, the autorelease pool will presumably be retaining the object until it comes time to flush the pool (during the runloop). This probably explains why the retain count is higher for the autoreleased object.
Note the use of the words "presumably" and "probably" in the above paragraph. I have no idea if this is actually what is happening inside the Cocoa/Cocoa Touch frameworks. This is the problem with using retainCount, you have no way of knowing what the retain count should be at any moment.
If you retain the object (or create it with a method name that contains alloc, copy, mutableCopy or new), you release it. The frameworks are free to also retain the object and they will release it when they are ready. When the retain count reaches zero it will be dealloced.
Update: Having looked at the GNUStep source code for NSObject and NSAutoreleasePool my possible explanation above probably isn't what is happening. However, I have no way to check for sure because I can't see Apple's implementation of these two objects. Even more reason not to trust retainCount or any inferences from it.
The only rule that really exists in environments with manual memory management is
If you use alloc, copy, mutableCopy, new
release it. Otherwise DON'T. retainCounts don't really work for debugging. See here
Can someone explain the differences between free(), release, autorelease and dealloc?
free() is a C function that you use to dispose of no longer needed memory you obtained through malloc() or other function that is documented to require free() to deallocate its memory e.g. strdup().
-dealloc is an Objective-C selector that is sent by the Objective-C runtime to an object when the object is no longer owned by any part of the application.
-release is the selector you send to an object to indicate that you are relinquishing ownership of that object. Once an object is not owned by anybody, it is sent the -dealloc message by the runtime.
-autorelease is a selector you send to an object to indicate you are relinquishing ownership of the object. However if nobody now owns the object the final -dealloc will be deferred until some unspecified later point. In fact, what you are really doing is transferring your ownership to an autorelease pool which will then release it when it is itself released (or drained).
You must never send -dealloc to an object except to super in the object's own -dealloc method.
Out of the 4 methods you've mentioned you'll typically only use release and autorelease in objective-c. free maybe used when you've used c and malloc otherwise you should use release and autorelease.
dealloc shouldn't be called by any of your code as it can interfere with the retain/release cycle that objective-c uses to keep track of resources. Using dealloc will almost certainly result in crashes at runtime if you ever use it.
The ultimate resource of the use of the release and autorelease is the Apple memory management documentation
free is the opposite of malloc and is used in C. You'll likely not use it very much programming in Objective C
If you own an object, you release it when you're done with it
You can also autorelease an object. This automatically releases it at the end of the current run loop iteration
When the reference count on an object drops to zero, the runtime calls dealloc. You should not call this method yourself
You "own" an object if you alloc, new, retain or copy it.
Apple provide some good documentation on this.
I want to ask about the iPhone application and objective C question. In the implementation program, there are function called 'dealloc', does this function only be called one time by the application?
For example, if I alloc a object and retain it 2 times, the retains count is 2 and I never use 'release' in the program, unless in the dealloc. Will the object be removed from the memory, or the objective will be removed from the memory. Thank you.
In the implementation program, there are function called 'dealloc', does this function only be called one time by the application?
Yes. -dealloc destroys the object. Trying to send any message to it again, including -dealloc is an error.
if I alloc a object and retain it 2 times, the retains count is 2
Careful. The retain count is at least 3. Other things than your code might retain the object. It's better not to worry to much about retain counts and only think in terms of ownership.
Each alloc, new, copy or retain is an claim of ownership. The object's dealloc method will only be called when all claims of ownership have been relinquished. A claim of ownership is relinquished by sending -release. So if you never release an object except in its own dealloc, you'll never release it.
dealloc is called once by the system when the object is destroyed (when its reference count reaches 0). If you have member variables in your class that you alloc in your init function, you must release them in your dealloc function.
If you give someone a pointer to one of those member objects and they retain it, the member could survive the release in your dealloc, but by sending a retain message they are taking responsibility for sending a release message later, ensuring its eventual destruction.