Question

I have the following code where I am passing the value of buttonString (an NSString) to another view controller. the app crashes when buttonString = imageName; is set and then called on the other view controller.

in the .h file of the FirstViewController before the @interface:

extern NSString* buttonString;

in the .m file of the FirstViewController before the implementation:

NSString* buttonString = nil;

and here’s the code:

NSString *imageName = [NSString stringWithFormat:@"myImage_%d.png", selectNum + 1 ];
NSLog(@"imageName is %@", imageName);

[buttonImage setImage:[ImgUtil image:imageName]];

buttonString = imageName;

NSLog(@"buttonString %@", buttonString);

the problem arises in the SecondViewController when this is called:

NSLog(@"button Message is %@", buttonString);

if I remove the buttonString = imageName; from the FirstViewController then the app doesn’t crash.

thanks for any help.

Answer 1

This is happening because buttonString is pointing to a released object.

From your code segment, imageName is an autoreleased NSString. When you say buttonString = imageName; it just points buttonString to the same address as imageName. After this method is over, imageName gets released and so does the memory to which buttonString was pointing too.

This is why the app crashes when you refer to buttonString in NSLog.

Ideally, you shouldn’t be sharing data using global variables like this in Objective C. But if you just want to make this work you need to allocate memory to buttonString when it’s used first. Make sure you do release it when it is of no use later.
NOTE: It still will be a bad way to do things.