Memory management with pushViewController

Question

I'm still not that great with memory management techniques, and wondered if someone could explain this weird behaviour to me. Consider these 3 pieces of code which I have been testing:

DofView* dofView = [[DofView alloc] initWithNibName:@"DofView" bundle:nil];
NSLog(@"dof retain count = %d", [dofView retainCount]); 

This logs: retain count = 1. This is fine.

DofView* dofView = [[DofView alloc] initWithNibName:@"DofView" bundle:nil];
[dofView release];
NSLog(@"dof retain count = %d", [dofView retainCount]); 

This logs: retain count = 1. Shouldn't it be 0??

DofView* dofView = [[DofView alloc] initWithNibName:@"DofView" bundle:nil];
[self.navigationController pushViewController:dofView animated:YES];
NSLog(@"dof retian count = %d", [dofView retainCount]);

This logs: retain count = 5. I have NO idea why its five?

Cany anyone shed any light on this at all? Im concerned that I'm eating up memory every time I'm creating a new view.

Thanks!

Answer 1

Do not rely on retainCount for memory analysis. Go through the reference documents on Memory Management for further info

Answer 2

Looking at retain count is strongly discouraged, it won't give you any valuable information. If you want to know if something is getting properly released you should put a breakpoint or a log-entry in the dealloc method of your class - when dealloc is called the object is very soon to be released. Apart from the instruments app, this is what I use to find retain-cycles.

Answer 3

When to use -retainCount?

retaincount seems to be useless

Answer 4

It's important to know release happens immediately (autorelease takes a while).

So why does your object still have a retain count of 1?

Because it's been deallocated - it's not your object anymore, it's just some free memory!

Try doing this:

NSObject* o = [[NSObject alloc] init];
NSLog(@"retain count = %d", [o retainCount]); 
[o retain];
NSLog(@"retain count = %d", [o retainCount]); 
[o release];
NSLog(@"retain count = %d", [o retainCount]); 
[o release];
NSLog(@"retain count = %d", [o retainCount]); 

You get the output

retain count = 1
retain count = 2
retain count = 1
retain count = 1

You might expect the last line to say 0, not 1. However, it won't bother decrementing the retain count if it's going to be released - what would be the point!

Actually, the fact that the last line outputs anything at all without crashing is lucky - because the object has been deallocated, there's nothing stopping that memory being used by something else - it's effectively just random data now. This is exactly the kind of bug that causes EXC_BAD_ACCESS crashes :)

In fact, you get a compiler analyser warning on the final NSLog because you're passing a message to an object that doesn't exist any more :)

---

As for the retain count of 5 - I can only echo the other answers - you don't know what's going on inside objects - just because you only called retain once doesn't mean that no-one else called retain :)

As long as you release for every retain you make, you're doing the right thing.

If you're worried about leaks, use the profiler and check!