How does the NSAutoreleasePool autorelease pool work?

Question

As I understand it, anything created with an alloc, new, or copy needs to be manually released. For example:

int main(void) {
    NSString *string;
    string = [[NSString alloc] init];
    /* use the string */
    [string release];
}

My question, though, is wouldn't this be just as valid?:

int main(void) {
    NSAutoreleasePool *pool;
    pool = [[NSAutoreleasePool alloc] init];
    NSString *string;
    string = [[[NSString alloc] init] autorelease];
    /* use the string */
    [pool drain];
}

Answer 1

Yes, you're second code snippit is perfectly valid.

Every time -autorelease is sent to an object, it is added to the outer-most autorelease pool. When the pool is drained, it simply sends -release to all the objects in the pool.

Autorelease pools are simply a convenience that allows you to defer sending -release until "later". That "later" can happen in several places, but the most common in Cocoa GUI apps is at the end of the current run loop cycle.

Answer 2

Yes and no. You would end up releasing the string memory but leaking the NSAutoReleasePool object into memory by using drain instead of release if you ran this under a garbage collected (not memory managed) environment.

drain

In a garbage collected environment, triggers garbage collection if memory allocated since last collection is greater than the current threshold; otherwise behaves as release.

Otherwise, it's similar, yes. I should point out that your statement "anything called with new, alloc or init" should not include "init", because "init" doesn't allocate memory, it only sets up the object (constructor fashion). If you received an alloc'd object and your function only called init as such, you would not release it:

- (void)func:(NSObject*)allocd_but_not_init
{
    [allocd_but_not_init init];
}

That does not consume any more memory than it you already started with (assuming init doesn't instantiate objects, but you're not responsible for those anyway).

Answer 3

No, you're wrong. The documentation states clearly that under non-GC, -drain is equivalent to -release, meaning the NSAutoreleasePool will not be leaked.

Answer 4

NSAutoreleasePool: drain vs. release

Since the function of drain and release seem to be causing confusion, it may be worth clarifying here (although this is covered in the documentation...).

Strictly speaking, from the big picture perspective drain is not equivalent to release:

In a reference-counted environment, drain does perform the same operations as release, so the two are in that sense equivalent. To emphasise, this means you do not leak a pool if you use drain rather than release.

In a garbage-collected environment, release is a no-op. Thus it has no effect. drain, on the other hand, contains a hint to the collector that it should "collect if needed". Thus in a garbage-collected environment, using drain helps the system balance collection sweeps.

Answer 5

yeah your code is perfect, if you'd be using garbage collecting it would be enough to just set the string to nil when you are done with it. Garbage collecting is not good for the performance of your app so I would not recommend using it :P

Answer 6

I found this link gave the best explanation on when and how to use NSAutoReleasePool: http://www.cocoadev.com/index.pl?AutoReleasePool