Memory Management in Objective-C, initializing a new array to an autoreleased array from another function

Question

I'm trying to understand memory management better. If I have a function that returns an autorelease NSArray like this

// DataClass
    - (NSArray *)getData {
       NSMutableArray *array = [[[NSMutableArray alloc] init] autorelease];
       // do some stuff to get data from sqlite
       return array;
    }

then in another class file, I want to use this getData. I have a property

@property (nonatomic, retain) NSArray *myData;

- viewDidLoad {
    NSMutableArray *data = [[NSMutableArray alloc] init];
    data = [DataClass getData];
    self.myData = data;
    [data release];
}

Why do I get a bad access error in this case? I know it's because of [data release], but I thought that since the getData method returns an autorelease NSArray, and because I initialize a new NSMutableArray with alloc/init, then I'd need to release it? Or is what happening is even though I initialize data with alloc/init, I then am not even using it because with the data=[DataClass getData] statement, I point to a different NSArray, and then try to release that already autoreleased NSArray from getData, and then the NSMutableArray data is still floating around in memory somewhere? Thanks in advance.

Answer 1

Your guess is correct. :-)

First of all, as written, in viewDidLoad you leak the data you alloc/init on line 1 when you assign data the result of calling getData on line 2.

And then, you are correct, that data at this point is pointing at an autoreleased object. So calling release on it is a bad thing.

Frankly, in your viewDidLoad method, you really don't need the call to getData at all. The complete and appropriate sequence for this kind of operation is:

NSMutableArray *array = [[NSMutableArray alloc] init];
// here is where you could fill array with your data, or call a method to 
// which you pass array to be filled.
self.data = array;
[array release];

assuming that the data property is defined with retain. By using the self. prefix, you get a retain done for you.

Does this help?

Answer 2

There are two possible problems with your code. The first is certain:

 NSMutableArray *data = [[NSMutableArray alloc] init];
 data = [DataClass getData];

This is not the correct pattern for initializing objects. Let's assume a class method called getData exists, and does what you want (which may not be the case, more on that later). The first line would allocate memory for a new array, and return a pointer to it. The second line would then immediately reassign that pointer to point to a different (autoreleased) array, leaking the first array in the process.

Secondly, your getData method has the following signature:

- (NSArray *)getData;

Implying that it is an instance method, meaning you send it to an instance of the DataClass. However, your code that calls the method does so by sending the message to the class itself, not an instance of it. This may just be a typo in your posting, but better safe than sorry.