What's the deal with [[UIApplication sharedApplication] delegate]?

Question

I am using [[UIApplication sharedApplication] delegate] to share a variable across several classes. I set the value in the AppDelegate. I am able to NSLog it from the myAppDelegate.m and see the value. Then I try to NSLog the value when one of my Tabs loads, and it crashes:

myAppDelegate *app = (myAppDelegate *)[[UIApplication sharedApplication] delegate];
NSLog(@"Value:%@ ", app.delegateVar); // <--- Causes Crash

Basically seems like it is creating a new instance of app.delegateVar ?

delegateVar is defined in myAppDelegate.h and then it myAppDelegate.m I do this:

  - (void)applicationDidFinishLaunching:(UIApplication *)application {

        ...

        [delegateVar release];
        delegateVar = [NSString stringWithFormat:@"Test Value"];
        NSLog(@"%@",delegateVar);

    ...
    }

Answer 1

One possibility is that delegateVar is being released prematurely.

For example, maybe the delegateVar property is not set up with the retain option, you are explicitly calling [delegateVar release], or you are bypassing the setter (and its retain semantics) by assigning directly to it (delegateVar = instead of self.delegateVar =).

In any case, look at the code that creates, assigns, and releases delegateVar.

---

Update:

Bingo. This is your problem right here:

    [delegateVar release];
    delegateVar = [NSString stringWithFormat:@"Test Value"];
    NSLog(@"%@",delegateVar);

You are assigning an autoreleased value (from +NSString stringWithFormat:) to delegateVar, and are not doing anything to retain it. That means that as soon as applicationDidFinishLaunching: returns, delegateVar is automatically released (and becomes invalid).

If delegateVar is a property with the "retain" option defined, you should be doing this as:

self.delegateVar = [NSString stringWithFormat:@"Test Value"];

You don't need to release delegateVar before assigning to it (using self.delegateVar =), because the setter will release the old value as needed. But you do need to release it in your dealloc method.