Adding an autoreleased object to NSMutableArray

Question

Here's some code that throws an exception because of some of the memory management I'm doing:

My Header file:

@property(nonatomic, retain) NSMutableArray *holderArray;

My Implementation file:

@synthesize holderArray

-(void) viewDidLoad{
    holderArray = [[NSMutableArray alloc] init];
    [self addSampleObject];
}

-(void) addSampleObject{
    [holderArray addObject:[self createSampleObject]];
}

-(ModelObject *) createSampleObject{
    ModelObject *mObj = [[ModelObject alloc] init];
    // Set a few properties
    return [mObj autorelease];
}

What should be happening in this context? createSampleObject autoreleases the object at some point, while [holderArray addObject] increments the reference count for that object by 1. From what I understand, it should all be good.

But when a getter accesses that element, I'm getting the following exception. It disappears if I remove the autorelease stmt when creating the model object -

#0  0x012525a8 in objc_exception_throw ()
#1  0x010f36e5 in -[__NSArrayM objectAtIndex:] ()

Am I missing something fundamental about how autorelease works?

Thanks,
Teja.

EDIT: Actually you're right, there's a completely unrelated piece of my code that was throwing this error. I thought the only thing that I've changed from a working one is adding the autorelease statement.

Thanks!

Answer 1

I think your error might be in the getter - that looks more like an index out of bounds issue than a memory management one.

Your understanding of memory management seems to be correct, the only thing I would note is that the use of the word "create" in a method suggests that the returned object is not autoreleased. Not sure what your actual method names are like, I know this is just sample code.

Answer 2

Autoreleased objects are guaranteed to remain accessible until the end of the function in which they are autoreleased. Consider the following code:

-(ModelObject *) createSampleObject{
  ModelObject *mObj = [[ModelObject alloc] init];
  //Set a few properties
  return [mObj autorelease];
}

You could have done this with the same results:

-(ModelObject *) createSampleObject{
  ModelObject *mObj = [[[ModelObject alloc] init] autorelease];
  // Set a few properties
  return mObj;
}

I don't have enough of your code to tell you exactly what is wrong, but I recommend following the stack trace up until you see your code that you wrote. The debugger may highlight the line that is the exact problem.