UINavigationController navigation & “EXC_BAD_ACCESS”

[Lukonian]

Hey all,

I'm quite new to iPhone dev so I am hoping this might be resolved quite easily.

I have created a navigation based project in Xcode. The default view controller ( as setup by the navigation application template ) is a table view. I have created two sub view controllers, SubViewOneController and SubViewTwoController. The order of navigation is...

defaultViewController --> subViewOneController ---> subViewTwoController

So from the default view you click on the table cell which pushes subViewOneController to the navigation controller...

[[self navigationController] pushViewController:subViewOneController animated:YES];

Then, there is a button in subViewOne which creates and pushes subViewTwoController to the navigations controller's stack, so inside subViewOneController's implementation I have...

SubViewTwoController *subViewTwo = [[SubViewTwoController alloc] initWithNibName:@"SubViewTwoController" bundle:nil];
NSString *subViewString = [NSString stringWithFormat:@"Sub view 2"];

[[appDelegate navigationController] pushViewController:subViewTwo animated:YES];
[[subViewTwo label] setText:subViewString];

This all works until I track back through the navigation for the 2nd time. I.E.

defaultViewController --> subViewOneController ( OK ) ---> subViewTwoController ( OK )( click back button ) ---> subViewOneController ( OK ) ---> subViewTwoController ( OK )( click back button ) ---> "EXC_BAD_ACCESS”

I am not sure why this is happening. As far as I know “EXC_BAD_ACCESS” is normally caused by memory problems but UINavigationController is meant to handle that for you.

I need help to debug this; what method does the back button call? I've counted the number of controllers in navigationController.viewControllers and that has been correct. So why when I click sub view two's back button the second time do I get this error?

Thanks in anticipation
Lukonian

[kevinyide]

i had a similar problem which seem to get fixed once i released my viewController once i was done using it.
[SubViewTwo release];

Not sure though if its the same case with you.
Does the same problem happen when you navigate to your root view & then push your View One?

[[self navigationController] pushViewController:subViewOneController animated:YES];

[[appDelegate navigationController] pushViewController:subViewTwo animated:YES];

i think [self navigationController] should suffice instead of using appDelegate.

[BrianSlick]

There is nothing magical about navigation controllers when it comes to memory management. If you push a view controller, it will be retained, if you pop one, it will be released.

The symptoms you describe point to something being over-released, probably in the subViewTwoController.

[Lukonian]

Resolved...

It was down to a couple of lines of code...

NSString *displayString = [NSString stringWithFormat:@"Set by %@", [self title]];
[appDelegate setModelData:displayString];

I've come from an ActionScript background and to me, referencing displayString would prevent it from being trashed. However, in objective-c this is not the case and NSString stringWithFormat is an autorelease method so when subViewOne was trying to get the string, [appDelegate getModelData] was pointing to invalid data because garbage collection had cleared the object created with NSString stringWithFormat.

I hope you are following this.

The resolution was to change NSString stringWithFormat to this...

NSString *displayString = [[NSString alloc] initWithFormat:@"Set by %@", [self title]];
[appDelegate setModelData:displayString];

and clear any existing pointers in the setModelData method...

-(void) setModelData : (NSString *) newData {
if (modelData != nil)
{
[modelData release];
}
modelData = newData;
}


I suspect there will be more memory management lessons coming my way

[BrianSlick]

NSString *displayString = [[NSString alloc] initWithFormat:@"Set by %@", [self title]];
[appDelegate setModelData:displayString];

You are now leaking displayString.

-(void) setModelData : (NSString *) newData {
	if (modelData != nil)
	{
		[modelData release];
	}
	modelData = newData;
}

If modelData is a property, then just use the standard setter you get with synthesizing that property. If it isn't a property, it should be.

As is, you have a couple of issues here.

For one, just because modelData is not nil, does NOT inherently mean it should be released.

A good example of why not is your last line, where you simply do straight assignment, without a retain or copy.

You might get away with it once, but it should crash the second time around.

Edit: also, there is no garbage collection currently on the iPhone. The object you had previously was autoreleased, which is not the same as garbage collection.

[Lukonian]

Are you sure [modelData release] doesn't release displayString?

[BrianSlick]

Not on the first pass. And if you send in an autoreleased object, you're dead.

This is a fundamentally flawed setter you have here. For all of the good it is doing, you might as well rewrite it like this:

-(void) setModelData : (NSString *) newData
{
	[modelData release];

	modelData = newData;
}

Since sending a message to nil doesn't do anything, there is no real reason for your test. So what you have here is a method that does nothing but release old data whenever something is passed in. But you don't retain the new data. Which means next time around, when the new data is now the old data, it will be released, even though you don't want to.

Just use the standard setter until 1) you understand memory management, and/or 2) you have a need for custom behavior. Even if you are fully accounting for the bad behavior here elsewhere in code - which you obviously are not, hence your crashing - you are totally defeating the purpose of having a good setter in the first place.