writing a library that uses other libraries

[eluser]C. Jiménez[/eluser]
You can't call $this->session because it doesn't exist.

You have referenced your CI objec in $CI :
$CI->load->library('session');

so use $CI instead $this

Code:

class Loginclass {

    public function logged_in()
    {
        $CI =& get_instance();
        $CI->load->library('session');
        
        //$username = $this->session->userdata('username');
        //$logged_in = $this->session->userdata('logged_in');
        $username = $CI->session->userdata('username');
        $logged_in = $CI->session->userdata('logged_in');
      
        if(empty($username)){$value = FALSE;}
        elseif(!is_numeric($username)){$value = FALSE;}
        elseif($logged_in !== TRUE){$value = FALSE;}
        else{$value = TRUE;}
        return $value;
    }
}

if you want to use your $CI object in more than one method I reccomend you to save it in a private var of your class and fill it with your referenced CI object on class's constructor.

Hope it helps!