Question

I just downloaded the newest iOS SDK (4.3) and noticed that when I start a Window Based Application, the UIWindow is not declared in the header file, it is only mentioned as a property.

#import <UIKit/UIKit.h>

@interface GleekAppDelegate : NSObject <UIApplicationDelegate> {
    IBOutlet UILabel *label;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@end

I would expect, and remember from older SDK’s, that the above code should be

#import <UIKit/UIKit.h>

@interface GleekAppDelegate : NSObject <UIApplicationDelegate> {
    IBOutlet UILabel *label;

    UIWindow *window;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@end

Is that just a new feature of the SDK?

Thanks

Answer 1

The new Objective-C runtime has the ability to synthesize ivars without explicitly declaring them. From Runtime Difference in The Objective-C Programming Language:

In general the behavior of properties
is identical on both modern and legacy
runtimes (see “Runtime Versions and
Platforms” in Objective-C Runtime
Programming Guide). There is one key
difference: the modern runtime
supports instance variable synthesis
whereas the legacy runtime does not.

…

With the modern runtime, if you do not
provide an instance variable, the
compiler adds one for you.

From Runtime Versions and Platforms in Objective-C Runtime Programming Guide:

• Phone applications and 64-bit programs on Mac OS X v10.5 and later
  use the modern version of the runtime.

• Other programs (32-bit programs on Mac OS X desktop) use the legacy
  version of the runtime.

Also have a look at this questions:

• Objective C: Why do we declare ivars in the .h member area if @property seems to do it automatically?

• What is the underlying mechanism for ivar synthesis in the modern Objective C runtime

• Automatic iVars with @synthesize