I have created a simple plugin that locks down content for users not logged in and it is working fine. However any user on a multi-author site could use the same short code in his post to lock down content too. I do not want this to happen.

How may I restrict this functionality to administrators only?
This current code thows up a fatal error:Fatal error: Call to undefined function wp_get_current_user()

public function check_user_role() {
  if(current_user_can( 'activate_plugins' )) {
        return true;
    }
}

I then intended to use this method in my class constructor to determine if the add_shortcode() function should run. Any clues how I should go about implementing this shall be appreciated.