When will NSautorelease release this? - cocoa

Question

Okay say I have a function like below:
-(NSNumber *)calculate{
NSNumber *myNum = [[[NSNumber alloc]initWithInt:5] autorelease];
return myNum;
}
When will myNum be released? Will whenever I call calculate, myNum will be created and added to the stack?
Also say I have a property like:
#property (nonatomic, retain) NSMutableArray *inputsArr;
and I synthesized it as:
#synthesize inputsArr = _inputsArr;
and I alloc and initiate it in the code of one of mu functions..
How would I go about releasing this memory? any guides to CoaCoa memory management...I can only find really confusing or obvious guides..
Thanks in advance

Answer 1

autorelease pools are thread local stacks -- you push and pop them. the deferred release message will be sent to the object when the pool is destroyed.
consider this:
NSAutoreleasePool * pool = [NSAutoreleasePool new];
NSNumber * n = [NSNumber numberWithDouble:1.0/17.0]; << n is autoreleased
[n self]; << OK!
[pool release]; << n is messaged release
[n self]; << BAM!
So 'when' really depends on how the autorelease pools are built -- but it's always possible to ensure your objects outlive a local pool, so this is never a restriction:
NSAutoreleasePool * pool = [NSAutoreleasePool new];
NSNumber * n = [NSNumber numberWithDouble:1.0/17.0]; << n is autoreleased
[n self]; << OK!
[n retain];
[pool release]; << n is messaged release
[n self]; << OK!
[n release];
[n self]; << BAM!
The above is what you should rely on. In some real world cases, an object may still be alive where you would expect "BAM!", but you should never rely on "well, it should have been destroyed, but it seems to work alright".

Answer 2

The NSApplication class sets up autorelease pools (instances of the NSAutoreleasePool class) during initialization and inside the event loop—specifically, within its initialization (or sharedApplication) and run methods. generally autorelease pool is popped at the end of the event loop, but this depends on you or the app.
If you are going to be using lots of temporary objects (autoreleased / from convenience methods) you may want to think about creating your own short-term autorelease pools to avoid temporary memory peaks.Autorelease objects are added to the latest autorelease pool to be created.
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; // create your own little autorelease pool
// these objects get added to the autorelease pool you created above
NSNumber *aNumber1 = [NSNumber numberWithFloat:1]; // refcount is 1, you are not owner, will be automatically released
NSNumber *aNumber2 = [NSNumber numberWithFloat:2]; // refcount is 1, you are not owner, will be automatically released
NSNumber *aNumber3 = [NSNumber numberWithFloat:3]; // refcount is 1, you are not owner, will be automatically released
NSNumber *aNumber4 = [NSNumber numberWithFloat:4]; // refcount is 1, you are not owner, will be automatically released
NSNumber *aNumber5 = [NSNumber numberWithFloat:5]; // refcount is 1, you are not owner, will be automatically released
NSNumber *aNumber6 = [NSNumber numberWithFloat:6]; // refcount is 1, you are not owner, will be automatically released
// ... do a bunch of stuff with all objects above.
...
[pool release]; // all objects added to this pool (the ones above) are released
Take a look at Memory Management with Objective C / Cocoa / iPhone.