How do I share an object between UIViewControllers on iPhone?

One option you have is to declare your date model as instance variables of your app delegate (as mentioned by other commenters).

Instead of referencing the app delegate as suggested by nevan an alternative is to add a property to your view controller classes (A and B) for your data model.

Say you wanted to share a data model object between your view controllers you can add a property to each:

@interface AViewController : UIViewController {
    MyDataModel *model;
}

@property (nonatomic, retain) MyDataModel *model;

@end

@interface BViewController : UIViewController {
    MyDataModel *model;
}

@property (nonatomic, retain) MyDataModel *model;

@end

When you initialise your view controller you can then set this property to the object context initialised previously.

You have mentioned a tab bar controller. If your view controllers are wired through IB all you have to do is to set these parameters in your application delegate applicationDidFinishLaunching: method, before the tab bar controller is displayed:

@interface MyAppDelegate : NSObject <UIApplicationDelegate, UITabBarControllerDelegate>
{

    MyDataModel *model;
    AViewController *aViewController;
    BViewController *bViewController;
    ...
}

@property (retain) IBOutlet AViewController *aViewController;
@property (retain) IBOutlet BViewController *aViewController;

@end

@implementation MyAppDelegate

...

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

    aViewController.model = model;

    bViewController.model = model;

    [window addSubview:tabBarController.view];
    [window makeKeyAndVisible];
}

Don’t forget to release the model in your view controller’s dealloc method.

The alternative is to use a singleton object. An simple singleton example:

@interface MyDataModel : NSObject
{
}

+ (MyDataModel *) sharedDataModel;

@end

@implementation MyDataModel

static MyDataModel *sharedDataModel = nil;

+ (MyDataModel *) sharedDataModel
{

    @synchronized(self)
    {
        if (sharedDataModel == nil)
        {
            sharedDataModel = [[MyDataModel alloc] init];
        }
    }
    return sharedDataModel;
}

@end

You can access this data model from all your view controllers with something similar to the following:

MyDataModel *model = [MyDataModel sharedDataModel];

See also this stack overflow discussion about singletons.