Question

I’m stuck with the following bit of code.

NSString *gridRef = [[NSString alloc] initWithFormat: @"%@", [converter LatLongToOSGrid: latLong]];
NSLog(@"Grid Ref: %@", gridRef);
self.answerLabel.text = [[NSString alloc] initWithFormat: @"%@", gridRef];

When I log gridRef, it displays the correct result. However, the line setting answerLabel.text causes an EXC_BAD_ACCESS error and the program crashes. IB is connected to the correct label, what is the problem?

Thanks

---

I’ve updated the code as follows:

    - (IBAction)convertLatLong {
    NSArray *latLong = [[NSArray alloc] initWithObjects: latTextField.text, longTextField.text, nil];

    GridRefsConverter *converter = [[GridRefsConverter alloc] init];

    NSString *gridRef = [[NSString alloc] initWithFormat: @"%@", [converter LatLongToOSGrid: latLong]];
    NSLog(@"Grid Ref: %@", gridRef);
    NSLog(@"Label: %@", self.answerLabel.text);
    answerLabel.text = @"Yippy";
    self.answerLabel.text = gridRef;

    [gridRef release];
    [converter release];
    [latLong release];
}

answerLabel is initialised through @property @synthesize when the view controller is pushed onto the stack. (I don’t know how it gets init’d apart from it’s one of the magical things IB does for you. Or so I assume. I’ve used exactly the same method in other view controllers and have not had this issue.

---

I’ve found the culprits – the question is, how do I go about releasing them?

NSString *eString = [[NSString alloc] initWithFormat: @"%f", e];
NSString *nString = [[NSString alloc] initWithFormat: @"%f", n];
eString = [eString stringByPaddingToLength: (digits/2) withString: @"0" startingAtIndex: 0];
nString = [nString stringByPaddingToLength: (digits/2) withString: @"0" startingAtIndex: 0];
NSString *theGridRef = [letterPair stringByAppendingString: eString];
theGridRef = [theGridRef stringByAppendingString: nString];
[eString release];
[nString release];

return theGridRef;

and:

NSArray *gridRef = [[NSArray alloc] init];
gridRef = [gridRef arrayByAddingObject: [NSNumber numberWithDouble: E]];
gridRef = [gridRef arrayByAddingObject: [NSNumber numberWithDouble: N]];
gridRef = [gridRef arrayByAddingObject: [NSNumber numberWithInteger: 8]];

NSString *theGridRef = [[NSString alloc] initWithFormat: @"%@", [self gridRefNumberToLetter: gridRef]];

[gridRef release];
[theGridRef autorelease];

return theGridRef;

}

Answer 1

You should enable zombie detection by setting the environment variable NSZombieEnabled to YES, so you can see which object causes the bad access (don’t forget to remove this again when you found the bug).

Also you can use Instruments to find the location where the object actually gets released. For this start a new Instruments session and use the “Allocations” instrument. In the instrument settings check “Enable NSZombie detection” and “Record reference counts”. When running the session you will break where the error occurs and you see a record of all retains/releases.

One place where you can have a quick look if your object is incorrectly freed is in the -viewDidUnload method, where you should release the outlet and set it to nil. If you forget the latter and you access the outlet somehow, it will result in a EXC_BAD_ACCESS.

Edited to match your update:

The problem is that you are assigning eString (and nString) a new string which was alloc/init-ed. Then you override those in the next statements, because -stringByPaddingToLength: (as well as all the other -stringBy... methods) return a new and autoreleased string object. So you lost the reference to the old string which means that there is a memory leak. Additionally at the end you release the already autoreleased objects explicitly which causes your bad access.

Instead you should create autoreleased strings from the beginning ([NSString stringWithFormat:…]) and don’t release them at the end.