CODEWITHSUNDEEP
iosobjective-ccastingappdelegate
user2502232
user2502232

Reputation: 11

Objective C Why do I need to Type Cast in this situation?

I have a storyboard with the rootViewController class set to CoursesTableViewController.

Why in the appDelegate do I need to use a typecast as in this example...

- (BOOL)application:(UIApplication *)application 
        didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    CoursesTableViewController *cvtc 
       = (CoursesTableViewController *)self.window.rootViewController;

why can't I just do this...

- (BOOL)application:(UIApplication *)application 
        didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    CoursesTableViewController *cvtc = self.window.rootViewController;

Upvotes: 0

Views: 443

Answers (3)

Rui Peres
Rui Peres

Reputation: 25917

You can actually do that. It will just warn you about that, because is expecting one thing and he is seeing another. When you are casting is just way of letting the compiler know that it's ok what you are doing.

Its type:

enter image description here

Upvotes: 0

Sam Aleksov
Sam Aleksov

Reputation: 1201

rootViewController is a property of UIWindow of type UIViewController.

You need to cast it to your controller type, if you want to interface with properties and selectors specific to your type.

Upvotes: 0

Ramy Al Zuhouri
Ramy Al Zuhouri

Reputation: 21996

What you're doing is called downcasting. You need to do that because polymorphism allows to assign a pointer to an object of a certain class, to a pointer to an object of a subclass of it's class, but not the viceversa (because it's not said that the pointer will actually point to an object that belongs to that subclass, while instead in the opposite case this is guaranteed).

This is the UIWindow property you're calling: 

@property(nonatomic, retain) UIViewController *rootViewController;

Like you see it's of type UIViewController,and CoursesTableViewController is a subclass of UIViewController.

PS: When I say it's needed I mean due to to avoid the compiler warning.

Upvotes: 2

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