i think you will need to know who is logged in user through out the application what you can do is to store the user id when user log in from the response of the webservice
you can store that using following
[[NSUserDefaults standardUserDefaults] setValue:[dictData valueForKey:@"id"] forKey:@"uid"];
here dictData is my response dictionary and field is id
Further add these to your constant or in your .pch file
#define UserId [[NSUserDefaults standardUserDefaults] valueForKey:@"uid"]
and then use UserId where ever you need in the application in any viewController in any class.
When you want to provide the Logout Facility in the application give
NSString *appDomain = [[NSBundle mainBundle] bundleIdentifier];
[[NSUserDefaults standardUserDefaults] removePersistentDomainForName:appDomain];
These will remove all the keys and again when you execute the application it will go the login page.