Persistent Unique Identifier (GUID, UUID, UDID)

Implementation.m

/*
   UUIDs (Universally Unique Identifiers), also known as GUIDs (Globally Unique Identifiers)
   or IIDs (Interface Identifiers), are 128-bit values guaranteed to be unique. A UUID is
   made unique over both space and time by combining a value unique to the computer on which
   it was generated—usually the Ethernet hardware address—and a value representing the number
   of 100-nanosecond intervals since October 15, 1582 at 00:00:00.
*/

#define AppDelegateDefaultsKeyPersistentUniqueIdentifier @"AppDelegateDefaultsKeyPersistentUniqueIdentifier"

- (NSString*)persistentUniqueIdentifier
{
   // try to read unique identifier from user defaults
   NSString* result = [[NSUserDefaults standardUserDefaults] objectForKey:AppDelegateDefaultsKeyPersistentUniqueIdentifier];
   
   if( result == nil )
   {
      // create new unique identifier
      CFUUIDRef uuid = CFUUIDCreate( nil );
      result = [(NSString*)CFUUIDCreateString( nil, uuid ) autorelease];
      CFRelease( uuid );
      
      // store new unique identifier in user defaults and synchronize them
      [[NSUserDefaults standardUserDefaults] setObject:result forKey:AppDelegateDefaultsKeyPersistentUniqueIdentifier];
      [[NSUserDefaults standardUserDefaults] synchronize];
   }
   
   return result;
}