NSUserDefaults behaving erratically - ios

Question

Can I know how exactly NSUserDefaults works?
I'm using it to maintain user info like username.
In one controller I set :
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
[prefs setObject:#"xyz" forKey:#"username"];
and in another one I retrieve it as :
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
username = [prefs stringForKey:#"username"];
It works sometimes, but sometimes the setobject doesn't set anything ( username = [prefs stringForKey:#"username"]; gives me nil. Sometimes it works fine. I thought this was a persistent storage so I'm not sure what's happening. This is in simulator as I haven't got the chance to test it on a phone yet.

Answer 1

This is what Mac Developer Library is saying about NSUserDefaults
The NSUserDefaults class provides a programmatic interface for interacting with the defaults system. The defaults system allows an application to customize its behavior to match a user’s preferences. For example, you can allow users to determine what units of measurement your application displays or how often documents are automatically saved. Applications record such preferences by assigning values to a set of parameters in a user’s defaults database. The parameters are referred to as defaults since they’re commonly used to determine an application’s default state at startup or the way it acts by default.
At runtime, you use an NSUserDefaults object to read the defaults that your application uses from a user’s defaults database. NSUserDefaults caches the information to avoid having to open the user’s defaults database each time you need a default value. The synchronize method, which is automatically invoked at periodic intervals, keeps the in-memory cache in sync with a user’s defaults database.
The NSUserDefaults class provides convenience methods for accessing common types such as floats, doubles, integers, Booleans, and URLs. A default object must be a property list, that is, an instance of (or for collections a combination of instances of): NSData, NSString, NSNumber, NSDate, NSArray, or NSDictionary. If you want to store any other type of object, you should typically archive it to create an instance of NSData. For more details, see Preferences and Settings Programming Guide.
Values returned from NSUserDefaults are immutable, even if you set a mutable object as the value. For example, if you set a mutable string as the value for "MyStringDefault", the string you later retrieve using stringForKey: will be immutable.
A defaults database is created automatically for each user. The NSUserDefaults class does not currently support per-host preferences. To do this, you must use the CFPreferences API (see Preferences Utilities Reference). However, NSUserDefaults correctly reads per-host preferences, so you can safely mix CFPreferences code with NSUserDefaults code.
If your application supports managed environments, you can use an NSUserDefaults object to determine which preferences are managed by an administrator for the benefit of the user. Managed environments correspond to computer labs or classrooms where an administrator or teacher may want to configure the systems in a particular way. In these situations, the teacher can establish a set of default preferences and force those preferences on users. If a preference is managed in this manner, applications should prevent users from editing that preference by disabling any appropriate controls.

Answer 2

Would u mind trying this.
[[NSUserDefaults standardUserDefaults] setObject:#"xyz" forKey:#"username"];
than doing like the way u are doing .
And Check the Value by using
NSLog(#"%#",[NSUserDefaults standardUserDefaults] objectForKey:#"username"]);