iPhone Memory Management and Releasing

Question

Here's a common practice I see often (including from a very popular iPhone developer book)

In the .h file:

@interface SomeViewController : UIViewController
{
  UIImageView *imgView;
}

Somewhere in the .m file:

imgView = [[UIImageView alloc] initWithFrame:[[UIScreen mainScreen]
applicationFrame]];
[imgView setImage:[UIImage imageNamed:@"someimage.png"]];
[self addSubview:imgView];
[imgView release];

And later, we see this...

- (void) dealloc
{
  [imgView release];
  [super dealloc];

} 

Since imgView has a matching alloc and release, is the release of imgView in dealloc necessary?

Where is the imgView retained by the addSubview call accounted for?

Answer 1

The code is incorrect. You'll end up releasing imgView after it's been deallocated.

In your .m file, you:

1. alloc it --> you own it
2. add it as a subview --> you and the UIView owns it
3. release it --> you don't own it

Then in dealloc, you release imgView even though, as we established at step 3 above, you don't own it. When you call [super dealloc], the view will release all of its subviews, and I imagine you'll get an exception.

If you want to keep an ivar of imgView, I suggest not calling release after you add it as a subview, and keep your dealloc the same. That way, even if imgView is at some point removed from the view hierarchy, you'll still have a valid reference to it.