Question

Xcode has issues about Autosynthesized property 'ViewController' and similary about 'window' and tells me the properties are declared in BSAppDelegate.h. I don’t know how to fix this even though several people here have tried to explain the underscore issue. When I omit any of the lines below which mention ViewController or window, my app will not compile.

//
//  BSAppDelegate.h
//

#import <UIKit/UIKit.h>

@class BSViewController;

@interface BSAppDelegate : UIResponder <UIApplicationDelegate>{
    UIWindow *window;
    BSViewController *viewController;
}


@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) BSViewController *viewController;

@end

Answer 1

When you declare properties, you don’t need to also declare the same variables in the instance variables section. In other words, this should be fine:

@interface BSAppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) BSViewController *viewController;

@end

If your app fails to compile like this, do you have a second @interface section and/or do you have @synthesize statements in the implementation file? As of Xcode 4.4 you don’t need the @synthesize statements anymore, but if you don’t explicitly synthesize the properties then Xcode will synthesize them with a preceding underscore (_window or _viewController). Here’s a link with more information: Automatic Property Synthesis With Xcode 4.4.