Question

I am learning Obj-C but still occasionally have a difficult time wrapping my head around some of the mem management stuff. I am using custom cells with a UITableView, and implemented the cellForRowAtIndexPath method where I accidentally released the cell at the end. This obviously caused problems as the cell was also getting released when the tableView was popped. This led to a crash due to releasing the cell twice – no prob, understand.

However, as I kept working, I intermingled standard and custom cells, so my method got a bit more complex. My first try was the below, which caused the same problem as the above scenario. This is where I am a bit confused – since I am not releasing “cell”, why can’t I release “customCell” after I set cell to its value?

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

     UITableViewCell *cell;

     //CATEGORY_SECTION is a constant defined elsewhere
     if (indexPath.section == CATEGORY_SECTION) {
          cell = [tableView dequeueReusableCellWithIdentifier:@"StandardCellIdentifier"];
          if (cell == nil) {
               cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"StandardCellIdentifier"] autorelease];
               cell.accessoryType = UITableViewCellAccessoryCheckmark;
          }
          cell.labelText.text = myModelObject.name;

     } else {

          cell = [tableView dequeueReusableCellWithIdentifier:@"CustomCellIdentifier"];
          MyCustomCellClass *customCell;
          if (cell == nil) {
               UIViewController *helperController = [[UIViewController alloc] initWithNibName:@"MyCustomCell" bundle:nil];
               customCell = (MyCustomCellClass *)[helperController view];


               [helperController release];
          }

     customCell.myCustomLabel.text = myModelObject.description;
     cell = customCell;
     [customCell release];
     }

     return cell;
}

I understand that when I set cell = customCell, I am assigning customCell’s memory address, not the actual object, to cell…so when I release customCell it is also in effect releasing cell? How would I actually copy customCell so I could release it? Or do I not have to release it (even though I alloc’d it) – it seems like a memory leak waiting to happen, how would you approach it?

For the record, here is the revised code I used to avoid this issue. This question isn’t so much as finding a solution (since I have, below) but understanding what is happening behind the scenes. Thanks for any guidance!

My working method:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

     //CATEGORY_SECTION is a constant defined elsewhere
     if (indexPath.section == CATEGORY_SECTION) {
          UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"StandardCellIdentifier"];
          if (cell == nil) {
               cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"StandardCellIdentifier"] autorelease];
               cell.accessoryType = UITableViewCellAccessoryCheckmark;
          }
          cell.labelText.text = myModelObject.name;

          return cell;

     } else {

          MyCustomCell *customCell = (MyCustomCell *)[tableView dequeueReusableCellWithIdentifier:@"CustomCellIdentifier"];

          if (customCell == nil) {
               UIViewController *helperController = [[UIViewController alloc] initWithNibName:@"MyCustomCell" bundle:nil];
               customCell = (MyCustomCell *)[helperController view];
               [helperController release];
          }

          customCell.myCustomLabel.text = myModelObject.description;

          return customCell;
     }
}

Answer 1

Right, you’re releasing the same memory, which both variables point to. If you were setting a class property, for example, the property would probably be marked with retain. If a property has retain it means it wants to “own” the object. You could then release it.

In your case, your cell variable doesn’t own the memory, so you can’t release it.