Autorelease pools in Objective-C - release main AutoreleasePool?

Question

By my understanding, when an object is sent an autorelease message, if no autorelease pools exist other than the one in main.m, the object gets placed in the one in main.m. Assuming this is correct, I have a couple of questions:

1) Do all autoreleased objects stay in that pool until the termination of the app?

2) If 1 is true, does creating an autoreleased object without a local autorelease pool (therefore placing that object in the main.m pool) keep that object in memory until termination of the app or a memory warning is received?

3) When is the main.m autorelease pool drained, other than when the app receives a memory warning or the application is terminated?

For example, in a cellForRowAtIndexPath delegate method such as this:

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Foobar"];
if (cell == nil) {
    // No cell to reuse => create a new one
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"Foobar"] autorelease];

    // lots of custom stuff
}

return cell;

when are the cells actually released? They have to be autoreleased, because you can't release them before you return them, and you can't release them after, because they are out of scope. By my current understanding, the cells are placed in the topmost autorelease pool, and released when that pool is drained/released. In this case, that would be the only autorelease pool in the application; the one in main.

4) The problem with this it that even when I am done with those cells and the view controller has been released, the cells stay in memory, yes? If this is not the case, could someone provide an explanation of how the memory management in this situation really works? Thanks!

Note: I have looked at Apple's documentation, but it talks mostly about when to use your own local autorelease pools, but not much about how they actually function.

Answer 1

1) Do all autoreleased objects stay in that pool until the termination of the app?

Autoreleased objects by definition are owned by their autorelease pool until that pool is drained. When you send -autorelease to an object, that object is added to a list of objects that the pool will release later. Autorelease pools are organized in a stack, with the pool at the top of the stack being the pool to which any objects sent -autorelease are added. The pool created in main() is generally not the one at the top of the stack. For example, the run loop will create an autorelease pool at the beginning of each iteration.

2) If 1 is true, does creating an autoreleased object without a local autorelease pool (therefore placing that object in the main.m pool) keep that object in memory until termination of the app or a memory warning is received?

It would if the pool created in main() were the topmost pool, but as described above, that usually won't be the case.

3) When is the main.m autorelease pool drained, other than when the app receives a memory warning or the application is terminated?

There's no difference between the pool created in main() and any other autorelease pool. They're all drained when the pool is released, or at the end of the block if you used the @autorelease directive.

Answer 2

From the documentation:

The Application Kit creates an autorelease pool on the main thread at the beginning of every cycle of the event loop, and drains it at the end, thereby releasing any autoreleased objects generated while processing an event. If you use the Application Kit, you therefore typically don’t have to create your own pools. If your application creates a lot of temporary autoreleased objects within the event loop, however, it may be beneficial to create “local” autorelease pools to help to minimize the peak memory footprint.

So, autoreleased objects in the default pool will only survive the duration of the current event.