objc_msgsend when setting cell.imageView.image

Question

Hi, this code works fine until I start scrolling:

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

static NSString *CellIdentifier = @"cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier] autorelease];
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;
}

Route *r = [data routeForDay:day index:indexPath.row];
cell.textLabel.text = r.name;
cell.imageView.image = r.image;

return cell;
}

It works perfectly for every row until I scroll down, when it crashes with objc_msgsend on cell.imageView.image = r.image; I've confirmed that nothing is nil, I've even checked the retainCount for everything involved. I'm completely at a loss, any ideas? Thanks.

Edit:

I solved the problem, but do not understand how the change in code makes the bug go away, so I'd appreciate a hint if anyone knows.

This is how the image was initially created, in the Route init method. image became deallocated when the table was scrolled, in my tableview controller.

NSString *imagePath = [[NSBundle mainBundle] pathForResource:[dict valueForKey:@"image"] ofType:@"png"];
image = [UIImage imageWithContentsOfFile:imagePath];

When I changed the second line to

image = [[UIImage alloc] initWithContentsOfFile:imagePath];

it worked fine.

I'm just a little confused and unhappy.

Answer 1

Your bug is most definitely a retain/release problem. Try to turn on zombies. If you don’t know how to do that, see below and read this tech note.

I assume one of the objects pointed to by data, r, or r.image is not retained properly.

One more thing: don’t look at retainCount before understanding memory management (and especially autorelease pools) in depth. Otherwise you’ll only be confused by the returned value.

How to enable zombies:

Choose Project > Edit Active Executable to open the executable Info window.
Click Arguments.
Click the add (+) button in the “Variables to be set in the environment” section.
Enter NSZombieEnabled in the Name column and YES in the Value column.
Make sure that the checkmark for the NSZombieEnabled entry is selected.

Edit:

Congratulations on finding the bug. The only pice of the puzzle you’re still missing is understanding Cocoa’s memory management. I recommend that you read the official documentation, as it is concise and easy to read.

In short: imageWithContentsOfFile: returns an autoreleased object, while initWithContentsOfFile: returns a retained object. But again: read the docs, or you’ll continue having memory errors.