what does @autoreleasepool do?

Question

I am using ARC and i know the autoreleasepool sends release to all elements inside it. Consider the following example.

-(NSString*)someMethod{

    NSString *string1=@"sample text inside string 1"; // string1 add to main autorelease pool
    NSString *string2=[[NSString alloc] init];        // string2 retain count is 1

    string2=@"sample text inside string 2";           // just assigning

    return string1;

}

-(void)viewDidLoad{

    NSString *returnedString=[self someMethod];

}

1. so my question is how do i completely release string2 in someMethod?

2. How to completely release string1 from viewDidLoad method completely?

Note: I am using ARC

if i use @autoreleasepool in someMethod then i think it will destroy both string1 and string2. wont it?

And what will be the effect of using @autoreleasepool in someMethod

-(NSString*)someMethod{

    @autoreleasepool{

    NSString *string1=@"sample text inside string 1"; // string1 add to main autorelease pool
    NSString *string2=[[NSString alloc] init];        // string2 retain count is 1

    string2=@"sample text inside string 2";           // just assigning

    }
    return string1;

}

It's worth noting that in all the hundreds of questions like this that come up, no useful example can ever be done using literal strings. They are compile-time constants and as such don't get treated with standard memory management. — jrturton, May 01 '13 at 11:01
Possible duplicate of [Objective-C: Why is autorelease (@autoreleasepool) still needed with ARC?](http://stackoverflow.com/questions/9086913/objective-c-why-is-autorelease-autoreleasepool-still-needed-with-arc) — Igor, Jan 24 '17 at 13:01

score 2 · Answer 1 · answered May 01 '13 at 09:30

2

You can't release any object, instance if you are using ARC. It is handled by complier itself when you use ARC.

So, even string will be released the same way.

answered May 01 '13 at 09:30

Anil

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what if i use string2=nil; in someMethod and then in viewDidload set returnedString=nil; will that work? – Muhammad Nasir May 01 '13 at 09:32

score 2 · Accepted Answer · edited May 23 '17 at 10:29

2

First of all,

NSString *string2=[[NSString alloc] init];
string2=@"sample text inside string 2";

string2 is not an object itself. It is a pointer to an object. So here you create an object, get a pointer to it, and then immediately replace it with a pointer to another string. If you didn't use ARC you would leak memory here. Take a look at @Anoop Vaidya's answer for more details about it.

As for your questions: when you use ARC, the compiler automatically tracks objects and releases them when they are no longer used. So you don't have to release it manually. You could assign a pointer to nil, but as soon as I remember it is not necessary.

The code from your second example won't compile, because string1 is declared inside the autorelease scope. And actually this @autorelease here doesn't make any difference since you don't create autoreleased objects inside it.

This question contains more details about using autorelease pool with ARC. And Apple's official documentation is probably the best I've ever seen, so please feel free to have a look at their Advanced memory management programming guide. It isn't too long, and it explains everything in details :)

edited May 23 '17 at 10:29

Community

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answered May 01 '13 at 10:18

FreeNickname

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`string2`'s retain count is not 1 in that example. – bbum May 01 '13 at 13:43
I copied this code the question and didn't modify the comment. But why isn't it 1? If we use [[NSString alloc] init] without ARC we get a string with retainCount == 1. Does ARC change this behavior? I'm talking about the first line of code since the comment is in the first line. For now I will remove the comment from the code. Anyway thank you for helping to find the truth :) – FreeNickname May 01 '13 at 16:59
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It has nothing to do with ARC. It is entirely an internal implementation detail and indicative of exactly how useless `retainCount` is. `[[NSString alloc] init]` returns a singleton whose retain count is `UINT_MAX`. You can-- and should-- consider the retain count to be `+1` (a delta of +1) and you need to balance that with a `release`, but the absolute retain count's value is useless. – bbum May 01 '13 at 17:02
Now I get it. Thank you for your comments! – FreeNickname May 01 '13 at 17:05

Anoop Vaidya · Answer 3 · 2013-05-01T09:34:48.220

0

NSString *string2=[[NSString alloc] init];        // string2 retain count is 1
string2=@"sample text inside string 2";

Bad way to doing.

Instead do this way

NSString *string2=[[NSString alloc] initWithString:@"sample text inside string 2"];

Or even

NSString *string2=@"sample text inside string 2";

string1 is in autoreleased mode, you dont have to release it.

As you are using ARC(Automatic Reference Counting). You never release object. The compiler takes care of it.

edited May 01 '13 at 09:34

answered May 01 '13 at 09:22

Anoop Vaidya

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what about string1 how can i release that? – Muhammad Nasir May 01 '13 at 09:25
@MuhammadNasir: I wrote, You are not required to release it., that is in autorelease mode. It will be released at the end of autorelease pool. – Anoop Vaidya May 01 '13 at 09:28
@MuhammadNasir: which string? – Anoop Vaidya May 01 '13 at 09:28
Just to make my question clear. I want string2 to be released in someMethod and string1 to be released in viewDidLoad. How would i do that? – Muhammad Nasir May 01 '13 at 09:30
The object will be released once there are no more references to it. You can attempt to make it quicker by assigning it to `nil` but it is not a guarantee. The bottom line is that the compiler knows better than you. – borrrden May 01 '13 at 09:37
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And as in your question title, you never use create/retain in ARC. – Anoop Vaidya May 01 '13 at 09:38

score 0 · Answer 4 · answered May 01 '13 at 09:23

0

You don't need to release it ,it will automatically done by ARC. it will give an error if you try to release any object while using ARC.

answered May 01 '13 at 09:23

Suraj Gupta

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I know it will be taken care of when the pool is drained. My question is how do i release it immediately. eg i want to release string2 in the same method. so how do i do that – Muhammad Nasir May 01 '13 at 09:38

what does @autoreleasepool do?

4 Answers4