CodeIgniter会话数据在身份验证期间显示为null

I'm trying to send the session variable to another controller but when i use r_print() it shows nothing and there is condition applied which returns null.

<?php 
// library/login_lib.php 
class Login_lib
{
     public function logged_in()
     {
        $CI = & get_instance();
        return ($CI->session->userdata('userdata')['is_logged_in']) 
               ? $CI->session->userdata('userdata')['is_logged_in'] : Null;
     }
}
?>
<?php    
    // another file common_helper.php        
    if( !function_exists('authentication_user') )
    {
      function authentication_user()
      {
         $CI = & get_instance();

         if( !$CI->login_lib->logged_in() )
         {
              $CI->session->set_flashdata('error', 'Please login with username and password');
             if( $CI->input->is_ajax_request() )
             {
                   echo 'session_expire';
                   die();
             }
             else
             {
               redirect();
             }
         }
      }
    }

There is another controller called Dashboard where i calling the above function authentication_user(). What i want now i want to call true from the login_lib which i am not actually. I don't know why its blocking to view the Login session.

Thanks in advance

return $CI->session->userdata('is_logged_in') ?  $CI->session->userdata('is_logged_in') : Null;

syntax for getting session data in codeigniter is

$this->session->userdata('session_key');