I've go a situatiion in Objective-C where I'm trying to access an object's variable through another object. The classes (simplified):

A.h

@interface A : NSObject {  
  NSMutableArray *someStuff;  
}  

@property (nonatomic, retain) NSMutableArray *someStuff;    

@end 

A.m

@implementation A  

@synthesize someStuff;  

//  blah, blah, blah  

Then, because I'm doing an iPhone app, there is an app delegate that contains a variable of this object type:

AppDelegate.h

@interface AppDelegate : NSObject <UIApplicationDelegate> {  

   A *aPtr;  
}  

@property (nonato开发者_开发问答mic, retain) A *aPtr;  
@end  

AppDelegate.m

@implementation AppDelegate  

@synthesize aPtr;  
// blah, blah, blah  

Then, in another class (in this a view controller), I'm trying to access 'someStuff' in this manner:

AViewController.m

AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];  

NSMutableArray *someArray = appDelegate.aPtr.someStuff;  

So, the problem is that this blows up in fine fashion. I think I'm too much of a Java junkie to understand why this won't work. Can anyone elighten me?

Many thanks,

Craig

You need to initialize this in this way

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

NSMutableArray *someArray = appDelegate.someArray;

This will resolve your problem.....

Craig,

appDelegate.aPtr will return null. as it is not initialized yet, and when you try to access some member of a null object,(in java NULLPointerException).Its behavior is as expected.(this blows up in fine fashion).

Thanks,

Ravin is correct. The class definition defines the iVars and properties for a class so you have defined an iVar aPtr that references an object of type A. However, you have not allocated and initialized this object.

An example using the default initialization would be `aPtr = [[A alloc] init]'.

This all sounds suspicious since in ObjC you are allowed to send messages to nil without a problem and properties are just diguised methods. For example you can

view = nil;
view.hidden = NO;

and it doesn't blow up, it just does nothing.

So since appDelegate.aPtr.someStuff is just

[[appDelegate aPtr] someStuff];

and [appDelegate aPtr] does nothing and returns nil so it should be safe to call [[appDelegate aPtr] someStuff] without a problem but also without any results.

So while it is a problem with using objects that hadn't been initialized (which most often should be done in a designated constructor of the appropriate object), since you don't get results that you expect, in my undestanding of "sending message to nil" in ObjC it shouldn't blow up. If it is then either I am missing the point or something other causes the problem and not this call.

EDIT

just checked: if not initialized at all it works as I explained: ObjC allows messages to be sent to nil:

A *aPtr = appDelegate.aPtr;
NSMutableArray *someArray = aPtr.someStuff;
NSLog(@"%@", someArray);

or

NSMutableArray *someArray = appDelegate.aPtr.someStuff;
NSLog(@"%@", someArray);

both don't break and print null.

If you initialize A properly but not initialize someStuff in A it still doesn't break but print null. The problem might be that you initialize aPtr to a different class than A, in which case you get unrecognized selector exception (you should be able to see it in the error log) and program crash.