Question

In objective-c manual it is written that something like

return [[[SomeClass alloc] init] autorelease];

can be done and then release is not necessary at any point, even not in the function that received this object. Who owns this object then? when will it be released?

Answer 1

The current NSAutoreleasePool does and will take care of the releasing when drained.

IBAction calls get wrapped into an NSAutoreleasePool that gets drained after the call.

For all non-IBAction calls the following would apply:

Say, you have these methods:

- (id)foo {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    SomeClass *bar = [self bar]; //bar still exists here
    //do other stuff
    //bar still exists here
    //do other stuff
    //bar still exists here
    [pool drain]; //bar gets released right here!
    //bar has been released
}

- (id)bar {
    return [[[SomeClass alloc] init] autorelease]; //bar will still be around after the return
}

Consider another scenario:

- (void)foo {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    //do other stuff
    [self bar];
    //do other stuff
    [pool drain]; //the string from blee would be released right here;
}

- (void)bar {
    [self baz];
}

- (void)baz {
    NSString *string = [self blee];
}

- (id)blee {
    return [NSString string]; //autoreleased
}

As you can see the autoreleased string object did not even have to be used in or get returned to the scope in which the pool was created.

NSAutoreleasePools exist on a stack (one per thread) and autoreleased objects get owned by the pool that’s topmost at the time of the call to autorelease.

---

Update:
If you are dealing with a tight loop and want to keep the memory moderately low while not slowing down your loop, consider doing something like this:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
for (NSUInteger i = 0; i < 1000000000; i++) {
    [NSString stringWithString:@"foo"];
    if (i % 1000 == 0) {
        [pool drain];
        pool = [[NSAutoreleasePool alloc] init];
    }
}
[pool drain];

However NSAutoreleasePool is highly optimized, so one pool per iteration usually isn’t much of a problem.

---

To fully understand how NSAutoreleasePools work read this excellent article by Mike Ash