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:
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; }.
When you wish to create a more local pool, as #mattjgalloway has shown in his answer.
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.
#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.
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 :).
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.
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.
Quoted from https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmAutoreleasePools.html:
Autorelease Pool Blocks and Threads
Each thread in a Cocoa application maintains its own stack of
autorelease pool blocks. If you are writing a Foundation-only program
or if you detach a thread, you need to create your own autorelease
pool block.
If your application or thread is long-lived and potentially generates
a lot of autoreleased objects, you should use autorelease pool blocks
(like AppKit and UIKit do on the main thread); otherwise, autoreleased
objects accumulate and your memory footprint grows. If your detached
thread does not make Cocoa calls, you do not need to use an
autorelease pool block.
Note: If you create secondary threads using the POSIX thread APIs
instead of NSThread, you cannot use Cocoa unless Cocoa is in
multithreading mode. Cocoa enters multithreading mode only after
detaching its first NSThread object. To use Cocoa on secondary POSIX
threads, your application must first detach at least one NSThread
object, which can immediately exit. You can test whether Cocoa is in
multithreading mode with the NSThread class method isMultiThreaded.
...
In Automatic Reference Counting, or ARC, the system uses the same
reference counting system as MRR, but it insertsthe appropriate memory
management method callsfor you at compile-time. You are strongly
encouraged to use ARC for new projects. If you use ARC, there is
typically no need to understand the underlying implementation
described in this document, although it may in some situations be
helpful. For more about ARC, see Transitioning to ARC Release Notes.
TL;DR
Why is #autoreleasepool still needed with ARC?
#autoreleasepool is used by Objective-C and Swift to work with autorelese inside
When you work with pure Swift and allocate Swift objects - ARC handles it
But if you decide call/use Foundation/Legacy Objective-C code(NSData, Data) which uses autorelese inside then #autoreleasepool in a rescue
//Swift
let imageData = try! Data(contentsOf: url)
//Data init uses Objective-C code with [NSData dataWithContentsOfURL] which uses `autorelese`
Long answer
MRC, ARC, GC
Manual Reference Counting(MRC) or Manual Retain-Release(MRR) as a developer you are responsible for counting references on objects manually
Automatic Reference Counting(ARC) was introduced in iOS v5.0 and OS X Mountain Lion with xCode v4.2
Garbage Collection(GC) was available for Mac OS and was deprecated in OS X Mountain Lion. Must Move to ARC
Reference count in MRC and ARC
//MRC
NSLog(#"Retain Count: %d", [variable retainCount]);
//ARC
NSLog(#"Retain Count: %ld", CFGetRetainCount((__bridge CFTypeRef) variable));
Every object in heap has an integer value which indicates how many references are pointed out on it. When it equals to 0 object is deallocated by system
Allocating object
Working with Reference count
Deallocating object. deinit is called when retainCount == 0
MRC
A *a1 = [[A alloc] init]; //this A object retainCount = 1
A *a2 = a1;
[a2 retain]; //this A object retainCount = 2
// a1, a2 -> object in heap with retainCount
Correct way to release an object:
release If only this - dangling pointer. Because it still can point on the object in heap and it is possible to send a message
= nil If only this - memory leak. deinit will not be called
A *a = [[A alloc] init]; //++retainCount = 1
[a release]; //--retainCount = 0
a = nil; //guarantees that even somebody else has a reference to the object, and we try to send some message thought variable `a` this message will be just skipped
Working with Reference count(Object owner rules):
(0 -> 1) alloc, new, copy, mutableCopy
(+1) retain You are able to own an object as many times as you need(you can call retain several times)
(-1) release If you an owner you must release it. If you release more than retainCount it will be 0
(-1) autorelease Adds an object, which should be released, to autorelease pool. This pool will be processed at the end of RunLoop iteration cycle(it means when all tasks will be finished on the stack)[About] and after that release will be applied for all objects in the pool
(-1) #autoreleasepool Forces process an autorelease pool at the end of block. It is used when you deal with autorelease in a loop and want to clear resources ASAP. If you don't do it your memory footprint will be constantly increasing
autorelease is used in method calls when you allocate a new object there and return it
- (B *)foo {
B *b1 = [[B alloc] init]; //retainCount = 1
//fix - correct way - add it to fix wrong way
//[b1 autorelease];
//wrong way(without fix)
return b;
}
- (void)testFoo {
B *b2 = [a foo];
[b2 retain]; //retainCount = 2
//some logic
[b2 release]; //retainCount = 1
//Memory Leak
}
#autoreleasepool example
- (void)testFoo {
for(i=0; i<100; i++) {
B *b2 = [a foo];
//process b2
}
}
ARC
One of biggest advantage of ARC is that it automatically insert retain, release, autorelease under the hood in Compile Time and as developer you should not take care of it anymore
Enable/Disable ARC
//enable
-fobjc-arc
//disable
-fno-objc-arc
Variants from more to less priority
//1. local file - most priority
Build Phases -> Compile Sources -> Compiler Flags(Select files -> Enter)
//2. global
Build Settings -> Other C Flags(OTHER_CFLAGS)
//3. global
Build Settings -> Objective-C Automatic Reference Counting(CLANG_ENABLE_OBJC_ARC)
Check if ARC is enabled/disabled
Preprocessor __has_feature function is used
__has_feature(objc_arc)
Compile time
// error if ARC is Off. Force to enable ARC
#if ! __has_feature(objc_arc)
#error Please enable ARC for this file
#endif
//or
// error if ARC is On. Force to disable ARC
#if __has_feature(objc_arc)
#error Please disable ARC for this file
#endif
Runtime
#if __has_feature(objc_arc)
// ARC is On
NSLog(#"ARC on");
#else
// ARC is Off
NSLog(#"ARC off");
#endif
Reverse engineering(for Objective-C)
//ARC is enabled
otool -I -v <binary_path> | grep "<mrc_message>"
//e.g.
otool -I -v "/Users/alex/ARC_experiments.app/ARC_experiments" | grep "_objc_release"
//result
0x00000001000080e0 748 _objc_release
//<mrc_message>
_objc_retain
_objc_release
_objc_autoreleaseReturnValue
_objc_retainAutoreleaseReturnValue
_objc_retainAutoreleasedReturnValue
_objc_storeStrong
Tool to Migrate Objective-C MRC to ARC
ARC generates errors where you should manually remove retain, release, autorelease and others issues
Edit -> Convert -> To Objective-C ARC...
New Xcode with MRC
If you enable MRC you get next errors(warnings)(but the build will be successful)
//release/retain/autorelease/retainCount
'release' is unavailable: not available in automatic reference counting mode
ARC forbids explicit message send of 'release'
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.
According to the leaks tool, the following code results in a memory leak:
- (NSString *)emojiWithCode:(int)code {
int sym = EMOJI_CODE_TO_SYMBOL(code);
__block __weak NSString *result = [[NSString alloc] initWithBytes:&sym length:sizeof(sym) encoding:NSUTF8StringEncoding];
return result;
}
I tried to free the sym with free(&sym) but at that time the following error comes up:
malloc: *** error for object 0x27d2e60c: pointer being freed was not allocated *** set a breakpoint in malloc_error_break to debug
How is this causing a leak if it is not allocated?
How can I free this memory correctly and solve the issue?
EMOJI_CODE_TO_SYMBOL is this
#define EMOJI_CODE_TO_SYMBOL(x) ((((0x808080F0 | (x & 0x3F000) >> 4) | (x & 0xFC0) << 10) | (x & 0x1C0000) << 18) | (x & 0x3F) << 24);
I have attached the leaks tool screenshot as well.
Instruments has no way of knowing why this object leaked, so it's only showing you the line of code where the leaked object was initially allocated. This line of code (once you remove the __block and __weak) is fine, so you have to dig in further to find out why this object leaked.
Bottom line, you don't have to do anything with this line of code, but rather you need to figure out where this string was subsequently used in order to identify why it leaked. Most likely, the object that used this string itself leaked (e.g. a strong reference cycle or something like a repeating timer that is holding on to the leaked object). And because that object leaked, this string also leaked. So, take your analysis up a level, and look where this string was used and figure out why it didn't get released.
I'd suggest you check out WWDC 2012 video iOS App Performance: Memory for a tutorial on how to use Instruments to guide your research to identify the ultimate source of the leak.
Also, it's often worth doing a static analysis (choose "Analyze" on the Xcode "Product" menu), as in some situations it can identify sources of problems. Make sure the static analyzer gives you a clean bill of health before diving into Instruments. This is admittedly more useful in non-ARC code, but sometimes it can identify issues.
I try to better understand memory management in Objective C (without ARC).
Currently create simple program to try it in use.
My code
...
//create some object
RetainTracker * rt = [RetainTracker new]; //RC=1
NSLog(#"Just created - %lu", (unsigned long)[rt retainCount]);
[rt retain]; // RC=2
NSLog(#"%lu", (unsigned long)[rt retainCount]);
[rt release]; //RC=1
NSLog(#"%lu", (unsigned long)[rt retainCount]);
[rt release]; //RC=0 -> call to dealloc
//call to object rt again after it was deallocated and get RC=1
NSLog(#"%lu", (unsigned long)[rt retainCount]); //think that here must be exception ?
Result :
So, on result we can see that reference counter still equal to 1 after I release last reference and for object rt.
I go deeper, and investigate with Instruments "life" of this object, got next
As I understand:
step 0 - just create - RC=1;
step 1 - for code [rt retain] - RC=2;
step 3 - for code [rt release] - RC=1;
step 4 - for code [rt release] - RC=0;
So, object must be deallocated, but if I call to [rt retainCount] after step 4 I got that RC for this object still equal to 1. Why? Maybe I make somewhere mistake or miss something?
After the object is deallocated, using the pointer to the object results in undefined behavior. That is, it can print 1, 42, your program could crash, Hello Kitty could pop up on your computer; anything can happen. Any behavior is consistent with undefined behavior, even behavior that seems to indicate some other thing is happening.
Besides the fact that it's undefined behavior, what you see is very likely based on what you did. After an object is deallocated, its memory is marked as available for use, but the bytes that constituted the object in memory still remain there. So in the immediate time being, it is likely that it hasn't been overwritten with other stuff, and the memory will still "look" like the object that was deallocated, and sending messages to it (though undefined behavior) will likely succeed, because it simply uses the parts of the object that are still in the right place in memory.
As regarding the retain count, you are assuming that when you release it, and the retain count (however it is stored in memory) will always be decremented, so that if before the release it is 1, afterwards it should be 0. However, they don't have to do this when the retain count is 1, because they know that when you release an object with retain count 1, the object will be deallocated, after which you are not supposed to use the object anyway, so they might as well skip decrementing it in this case, because there's no point in updating a variable that won't be used anymore.
Objects are deallocated when the pool is drained when their retain count is 0. The object may still be in memory after you release it. Try accessing it after the next drain and see what happens.
Update:
Yes. My mistake. For some reason I missed the fact that it was not autoreleased. However I was looking into the retainCount method and found this in the documentation: "Do not use this method. (required)".
And follows: "... it is very unlikely that you can get useful information from this method".
It seems that the retainCount method may not reliably give you the retain count of an object. However, still curious that you can still even send that message to that object which is supposed to have been freed.
I am using Zombies to try and get rid of an EXC_BAD_ACCESS error.
In Zombies, I got this message when the app crashes -
An Objective-C message was sent to a deallocated object (zombie) at
address: 0x8955310.
My question is what do I do next to solve the problem ?
Turn on malloc stack logging and zombies for your scheme in Xcode, and run the app in the simulator. Xcode should enter the debugger when the message is sent to the zombie. Run this command at the debugger prompt:
info malloc 0x8955310
(Substitute the actual address of the zombie!) You'll get stack traces from when that address was allocated and freed.
Most likely you have created an object, released it and later sent it a message.
To make sure this won't happen, a safe practice would be to set your object to nil once you are done using it
Consider:
NSMutableArray *a = [NSmutableArray array];
[a dealloc];
[a do_something_weird];
Your app is likely crash (won't always crash) in response to this message, as after release, you don't own this memory, and it may be used by some other object.
If you change this sequence to
NSMutableArray *a = [NSmutableArray array];
[a dealloc];
a=nil;
[a do_something_weird];
Exactly nothing will happen. This is a safe practice to follow when you are sure you're done using the object.
You also may want to consider using the Automatic Reference Counting feature, which helps a lot with memory management.
I was getting a segfault 11 memory access error in the IOS Simulator, but it disappears when I comment out the release in the code below.
// get get the question number
NSString *text = [attributeDict valueForKey:XML_TAG_QUESTION_ATTRIBUTE_NUMBER];
question.number = [text intValue];
//[text release]; <==== no more segfault 11 when this is commented out.
My question is, since I am receiving an instance of NS String returned by the NSXMLParser implementation, isn't the reference count increased and should I not be releasing it?
Here's the rule: Always NARC on your memory management.
If you call:
(N)ew
(A)lloc
(R)etain or
(C)opy...
You need to release. If not, you got it through a convenience method and it's autoreleased.
In the case of containers of other objects, the container has the objects retained, and you don't need to worry about it until you release the container.
No it should not.
Read the memory management programming guide : http://developer.apple.com/library/mac/#documentation/cocoa/conceptual/MemoryMgmt/MemoryMgmt.html