IPhone programming starter questin with retain/release/object access

Question

Hi there,

I'm pretty new in IPhone Development, so maybe you can help me with this problem:

I'm having an object "preferences" that holds a single NSString: (All code simplified...)

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>


@interface Preferences : NSObject {
 NSString *username;

}

-(void)loadPreferences;

@property(nonatomic,retain) NSString *username;

@end

The implementation looks like this:

#import "Preferences.h"
@implementation Preferenes

@synthesize username;

-(void)loadPreferences{
     username=@"MyUser";
}

-(void) dealloc {
 [username release];
}
@end

Next, I'm using a retain reference to this object in my main delegate:

#import <UIKit/UIKit.h>

@interface MainDelegate : NSObject <UIApplicationDelegate> {

   ...

  Preferences *prefs;
}

@property(retain,nonatomic) Preferences *prefs;

Implementation:

#import "MainDelegate.h"
#import "Preferences.h";

@implementation MainDelegate

@synthesize prefs;

- (void)applicationDidFinishLaunching:(UIApplication *)application {    
     prefs=[[Preferences alloc] init];
     NSLog(@"Username: %@",prefs.username); // THIS WORKS 
}

So I can access prefs.username in the Main Delegate.

Now I want to access the preferences reference from my MainDelegate.

I'm trying to do this this way:

MainDelegate *delegate = (MainDelegate*) [[UIApplication sharedApplication] delegate];
Preferences *prefs=delegate.prefs;
// This works...

NSLog(@"The Username is: %@",prefs.username);
// CRASH

When I'm trying to access the "username" object within the preferences object, the programm crashes with only showing the GNU Licence Information.

I think there is something going wrong that maybe an object is already released when I try to access it...

Can you help me out ?

Thanks !

Answer 1

You may have issues with assigning your property. This line does not use your property setter that handles retaining the value for you

username=@"MyUser";

You want to do:

self.username = @"MyUser";

Which actually compiles to

[self setUsername:@"MyUser"];

This calls the method created by @synthesize username; which retains that string for you. So what may be happening is you are assigning an autoreleased string to your variable, which gets released, and you later try and access it, and it explodes.

If you declare a property with retain then it's best to do your assigning and releasing most of the time through that synthesized property. It makes memory management much simpler.

---

This line, hoever, works fine:

prefs=[[Preferences alloc] init];

This is because you are directly setting the instance variable to an object with a retain count of 1, since it was just initialized. But I usually prefer to use the property setter to keep things clearer and more consistent.

self.prefs = [[[Preferences alloc] init] autorelease];

This init's it (+1) it's retained by the property setter (+1) and is autoreleased later (-1). Leaving a retain count of 1, which represents being owned by the app delegate object.

In my experience, using the retained property setters really helps keep memory management bugs down to a minimum.

Answer 2

Thank you so much !

When I use self.username=@"MyUser" everthing works perfect. I didn't realize that the property will be skipped without using the self.

Great help !!