Why is @autoreleasepool still needed with ARC?

Question

For the most part with ARC (Automatic Reference Counting), we don't need to think about memory management at all with Objective-C objects. It is not permitted to create NSAutoreleasePools anymore, however there is a new syntax:

@autoreleasepool {
    …
}

My question is, why would I ever need this when I'm not supposed to be manually releasing/autoreleasing ?

---

EDIT: To sum up what I got out of all the anwers and comments succinctly:

New Syntax:

@autoreleasepool { … } is new syntax for

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
…
[pool drain];

More importantly:

• ARC uses autorelease as well as release.
• It needs an auto release pool in place to do so.
• ARC doesn't create the auto release pool for you. However:
  • The main thread of every Cocoa app already has an autorelease pool in it.
• There are two occasions when you might want to make use of @autoreleasepool:
  1. When you are in a secondary thread and there is no auto release pool, you must make your own to prevent leaks, such as myRunLoop(…) { @autoreleasepool { … } return success; }.
  2. When you wish to create a more local pool, as @mattjgalloway has shown in his answer.

Answer 1

@autoreleasepool doesn't autorelease anything. It creates an autorelease pool, so that when the end of block is reached, any objects that were autoreleased by ARC while the block was active will be sent release messages. Apple's Advanced Memory Management Programming Guide explains it thus:

At the end of the autorelease pool block, objects that received an autorelease message within the block are sent a release message—an object receives a release message for each time it was sent an autorelease message within the block.

Answer 2

People often misunderstand ARC for some kind of garbage collection or the like. The truth is that, after some time people at Apple (thanks to llvm and clang projects) realized that Objective-C's memory administration (all the retains and releases, etc.) can be fully automatized at compile time. This is, just by reading the code, even before it is run! :)

In order to do so there is only one condition: We MUST follow the rules, otherwise the compiler would not be able to automate the process at compile time. So, to ensure that we never break the rules, we are not allowed to explicitly write release, retain, etc. Those calls are Automatically injected into our code by the compiler. Hence internally we still have autoreleases, retain, release, etc. It is just we don't need to write them anymore.

The A of ARC is automatic at compile time, which is much better than at run time like garbage collection.

We still have @autoreleasepool{...} because having it does not break any of the rules, we are free create/drain our pool anytime we need it :).

Answer 3

It's because you still need to provide the compiler with hints about when it is safe for autoreleased objects to go out of scope.

Answer 4

ARC doesn't get rid of retains, releases and autoreleases, it just adds in the required ones for you. So there are still calls to retain, there are still calls to release, there are still calls to autorelease and there are still auto release pools.

One of the other changes they made with the new Clang 3.0 compiler and ARC is that they replaced NSAutoReleasePool with the @autoreleasepool compiler directive. NSAutoReleasePool was always a bit of a special "object" anyway and they made it so that the syntax of using one is not confused with an object so that it's generally a bit more simple.

So basically, you need @autoreleasepool because there are still auto release pools to worry about. You just don't need to worry about adding in autorelease calls.

An example of using an auto release pool:

- (void)useALoadOfNumbers {
    for (int j = 0; j < 10000; ++j) {
        @autoreleasepool {
            for (int i = 0; i < 10000; ++i) {
                NSNumber *number = [NSNumber numberWithInt:(i+j)];
                NSLog(@"number = %p", number);
            }
        }
    }
}

A hugely contrived example, sure, but if you didn't have the @autoreleasepool inside the outer for-loop then you'd be releasing 100000000 objects later on rather than 10000 each time round the outer for-loop.

Update: Also see this answer - https://stackoverflow.com/a/7950636/1068248 - for why @autoreleasepool is nothing to do with ARC.

Update: I took a look into the internals of what's going on here and wrote it up on my blog. If you take a look there then you will see exactly what ARC is doing and how the new style @autoreleasepool and how it introduces a scope is used by the compiler to infer information about what retains, releases & autoreleases are required.

Answer 5

There seems to be a lot of confusion on this topic (and at least 80 people who probably are now confused about this and think they need to sprinkle @autoreleasepool around their code).

If a project (including its dependencies) exclusively uses ARC, then @autoreleasepool never needs to be used and will do nothing useful. ARC will handle releasing objects at the correct time. For example:

@interface Testing: NSObject
+ (void) test;
@end

@implementation Testing
- (void) dealloc { NSLog(@"dealloc"); }

+ (void) test
{
    while(true) NSLog(@"p = %p", [Testing new]);
}
@end

displays:

p = 0x17696f80
dealloc
p = 0x17570a90
dealloc

Each Testing object is deallocated as soon as the value goes out of scope, without waiting for an autorelease pool to be exited. (The same thing happens with the NSNumber example; this just lets us observe the dealloc.) ARC does not use autorelease.

The reason @autoreleasepool is still allowed is for mixed ARC and non-ARC projects, which haven't yet completely transitioned to ARC.

If you call into non-ARC code, it may return an autoreleased object. In that case, the above loop would leak, since the current autorelease pool will never be exited. That's where you'd want to put an @autoreleasepool around the code block.

But if you've completely made the ARC transition, then forget about autoreleasepool.

Answer 6

Autorelease pools are required for returning newly created objects from a method. E.g. consider this piece of code:

- (NSString *)messageOfTheDay {
    return [[NSString alloc] initWithFormat:@"Hello %@!", self.username];
}

The string created in the method will have a retain count of one. Now who shall balance that retain count with a release?

The method itself? Not possible, it has to return the created object, so it must not release it prior to returning.

The caller of the method? The caller does not expect to retrieve an object that needs releasing, the method name does not imply that a new object is created, it only says that an object is returned and this returned object may be a new one requiring a release but it may as well be an existing one that doesn't. What the method does return may even depend on some internal state, so the the caller cannot know if it has to release that object and it shouldn't have to care.

If the caller had to always release all returned object by convention, then every object not newly created would always have to be retained prior to returning it from a method and it would have to be released by the caller once it goes out of scope, unless it is returned again. This would be highly inefficient in many cases as one can completely avoid altering retain counts in many cases if the caller will not always release the returned object.

That's why there are autorelease pools, so the first method will in fact become

- (NSString *)messageOfTheDay {
    NSString * res = [[NSString alloc] initWithFormat:@"Hello %@!", self.username];
    return [res autorelease];
}

Calling autorelease on an object adds it to the autorelease pool, but what does that really mean, adding an object to the autorelease pool? Well, it means telling your system "I want you to to release that object for me but at some later time, not now; it has a retain count that needs to be balanced by a release otherwise memory will leak but I cannot do that myself right now, as I need the object to stay alive beyond my current scope and my caller won't do it for me either, it has no knowledge that this needs to be done. So add it to your pool and once you clean up that pool, also clean up my object for me."

With ARC the compiler decides for you when to retain an object, when to release an object and when to add it to an autorelease pool but it still requires the presence of autorelease pools to be able to return newly created objects from methods without leaking memory. Apple has just made some nifty optimizations to the generated code which will sometimes eliminate autorelease pools during runtime. These optimizations require that both, the caller and the callee are using ARC (remember mixing ARC and non-ARC is legal and also officially supported) and if that is actually the case can only be known at runtime.

Consider this ARC Code:

// Callee
- (SomeObject *)getSomeObject {
    return [[SomeObject alloc] init];
}

// Caller
SomeObject * obj = [self getSomeObject];
[obj doStuff];

The code that the system generates, can either behave like the following code (that is the safe version that allows you to freely mix ARC and non-ARC code):

// Callee
- (SomeObject *)getSomeObject {
    return [[[SomeObject alloc] init] autorelease];
}

// Caller
SomeObject * obj = [[self getSomeObject] retain];
[obj doStuff];
[obj release];

(Note the retain/release in the caller is just a defensive safety retain, it's not strictly required, the code would be perfectly correct without it)

Or it can behave like this code, in case that both are detected to use ARC at runtime:

// Callee
- (SomeObject *)getSomeObject {
    return [[SomeObject alloc] init];
}

// Caller
SomeObject * obj = [self getSomeObject];
[obj doStuff];
[obj release];

As you can see, Apple eliminates the atuorelease, thus also the delayed object release when the pool is destroyed, as well as the safety retain. To learn more about how that is possible and what's really going on behind the scenes, check out this blog post.

Now to the actual question: Why would one use @autoreleasepool?

For most developers, there's only one reason left today for using this construct in their code and that is to keep the memory footprint small where applicable. E.g. consider this loop:

for (int i = 0; i < 1000000; i++) {
    // ... code ...
    TempObject * to = [TempObject tempObjectForData:...];
    // ... do something with to ...
}

Assume that every call to tempObjectForData may create a new TempObject that is returned autorelease. The for-loop will create one million of these temp objects which are all collected in the current autoreleasepool and only once that pool is destroyed, all the temp objects are destroyed as well. Until that happens, you have one million of these temp objects in memory.

If you write the code like this instead:

for (int i = 0; i < 1000000; i++) @autoreleasepool {
    // ... code ...
    TempObject * to = [TempObject tempObjectForData:...];
    // ... do something with to ...
}

Then a new pool is created every time the for-loop runs and is destroyed at the end of each loop iteration. That way at most one temp object is hanging around in memory at any time despite the loop running one million times.

In the past you often had to also manage autoreleasepools yourself when managing threads (e.g. using NSThread) as only the main thread automatically has an autorelease pool for a Cocoa/UIKit app. Yet this is pretty much legacy today as today you probably wouldn't use threads to begin with. You'd use GCD DispatchQueue's or NSOperationQueue's and these two both do manage a top level autorelease pool for you, created before running a block/task and destroyed once done with it.