Question

I’m getting an error on a when saving changes to the managed object context, but I have a problem with my error handler: the error is nil, and thus gives me no useful information. I have two versions of the error handler. This one was generated by Xcode, and it works (i.e., the log message contains useful error info):

AppDelegate.c

- (void)saveContext
{
NSError *error = nil;
NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
if (managedObjectContext != nil)
{
    if ([managedObjectContext hasChanges] && ![managedObjectContext save:&error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. 
         */
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    } 
}
}

But I want to be able to pass success/failure (eventually, for now I’m just aborting) + the error information back to the caller, so I have this, which does not work (error is nil, and so provides no useful information about the error).

Database.h

+ (BOOL) commit:(NSError **)error;

Database.c

+ (BOOL) commit:(NSError **)error {


AppDelegate *appDelegate = (AppDelegate*) [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *managedObjectContext = appDelegate.managedObjectContext;

if (managedObjectContext != nil)
{
    if ([managedObjectContext hasChanges] && ![managedObjectContext save:error])
    {
        /*
         Replace this implementation with code to handle the error appropriately.

         abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
         */
        if (error == nil) {
            NSLog(@"Unresolved error");
            abort();
        } else {
            NSLog(@"Unresolved error %@, %@", *error, [*error userInfo]);
            abort();
            //return FALSE;
        }
    }
    return TRUE;
}
return FALSE;
}

I’m pretty sure my problem is with the pointers, and getting lost in the layers of redirection.

[EDIT:]
The code that calls commit:

[Database commit:nil];

I wonder if I need to add something like this to the start of the commit method, but I’m not sure about the pointers:

if (error == nil) {
    error = [[NSError alloc] init];
}

Answer 1

If commit: is called with nil (from your comment) then this is probably the error. You have to call your function with the address of an error variable:

NSError *error = nil;
if (![Database commit:&error]) {
    // commit failed, "error" contains error message.
}

and of course return NO instead of abort() in the error case inside the commit method.

There is no need to allocate the error message in your commit method. [managedObjectContext save:error] will do that if the save fails.