Question

I got nabbed by the following bug again and would like some clarification to exactly why it is a bug.

I have a simple UITableView that loads some data:

// myclass.h
@property (nonatomic, retain) NSArray *myData

// myclass.m
@synthesize myData;

- (void) viewDidLoad {
  ...
  myData = someDataSource // note the lack of self
}

- (UITableViewCell *) cellForRowAtIndexPath ... {
   ...
   cell.textLabel.text = [self.myData objectAtIndex:indexPath.row];  // EXC_BAD_ACCESS
}

The table first loads fine, but when scrolling up enough that one of the cells is totally out of the view I then get the EXC_BAD_ACCESS error.

Am I missing something in regards to @property retain. My understanding is that it releases anything that the pointer was previously pointing to before the reassignment. If I am correct then why would not using self. cause any problems?

Thanks for the help.

**** Update

Why is is that in all the examples that I have checked with to the release of objects within the dealloc method without the self?

- (void) dealloc { 
  [someArray release]; 
  [someTableView release];
}

Answer 1

If you don’t use self., you are directly assigning to the instance variable myData, which has nothing to do with the property myData.

self.myData is just syntactic sugar for [self myData] or [self setMyData:newValue], and the synthesized property just creates the -myData and -setMyData: methods.

The instance variable is just a variable, nothing more. While it may have the same name, assigning to it or reading from it it is just like accessing any variable: nothing is retained, released, or in other ways modified besides the assignment.