iPhone Autoreleasepool and allocations

Question

I've been reading about autoreleasepool but there is a point which is a bit unclear to me. I have some functionality using threads that required seperate memory managment using autoreleasepool.

In the following example is correct

-(void) doSomething {

   NSAutorelease *pool = [[NSAutorelasepool alloc] init];

   NSString *myString = @"Hello";

   [pool release];
}

Is this correct?

-(void) doSomething {

   NSAutorelease *pool = [[NSAutorelasepool alloc] init];

   NSString *myString = [[NSString alloc] initWithString:@"Hello"];

   [pool release];
}

or this?

-(void) doSomething {

   NSAutorelease *pool = [[NSAutorelasepool alloc] init];

   NSString *myString = [[NSString alloc] initWithString:@"Hello"];

   [myString release];
   [pool release];
}

My question is owned objects created in the scope of the autorelease pool need to be relased specifically or are the taken care of when the autorelasepool is been released?

Teo

Answer 1

Autorelease pool handles the autoreleased objects. If you own an object (via alloc or copy or retain) then you must release it. So your 2nd example is not correct. As you have allocated the string, you own it and you must release it.

An autorelease pool is created for the main thread. (You can look into the main function if you want). Every thread need its own autorelease pool to manage autoreleased objects. That's why if you create another thread then you must create an autorelease pool for that thread. Even if you don't create autoreleased object in the thread, you should create this as the library calls in that thread may create autoreleased objects. Even if you are sure that no library calls are making autoreleased objects then you also should create them as that is the best practice, specially if you are working on big project which is developed and maintained by multiple people.

Answer 2

You only need to create your own autorelease pool when you are creating a bunch of autoreleased objects you want to garbage collect immediately. However, you are correct in that you don't want to reference any "autoreleased" objects you create after you release the pool. Autoreleased objects (which you don't retain) are destroyed when the pool is drained.

Since none of the objects in your example are autoreleased, creating your own autorelease pool is essentially a no-op.

Answer 3

Neither of your examples needs an autorelease pool. Autorelease pools only take care of autoreleased objects:

NSArray *foo = [NSArray array];
NSObject *bar = [[[NSObject alloc] init] autorelease];

Your first string is initialized using a string literal and therefore is probably special with respect to memory management (maybe someone else knows more). Your second string leaks, the pool does not make a difference. Your third string is released correctly, again the pool does not make a difference.

This is where you would need a pool:

- (void) someMethodThatRunsOnAThread {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSString *foo = [@"foo" uppercaseString];
    [pool drain];
}

Here the foo string would leak if the pool wasn’t there. Note that I’m calling drain instead of release on the pool – on iOS there’s not a difference, but in garbage-collected environments the two differ, so it’s probably better to get in the habit of calling the right one.

Also note that you may need a pool even though you don’t autorelease any objects yourself, there could be many memory operations done somewhere in the code you’re calling in your method.

Answer 4

Think that this should be something like this:

-(void) doSomething {

   NSAutorelease *pool = [[NSAutorelasepool alloc] init];


   NSString *myString = [[[NSString alloc] initWithString:@"Hello"] autorelease];

   // or create string like this (automatically autoreleased)
   NSString *myString = [NSString stringWithString:@"Hello"];

   [pool release];
}

You must send autorelease message, to objects inside autorelease pool. They will be released when release message is sent to pool.