empty $current_user value in plugin class

I’m getting a empty $current_user->ID value inside my plugin class.

This plugin is suppose to update a user custom field (once) before show template content, and prints its content inside a page template.

I check out the database and found a user_id=0 on wp_usermeta with my custom meta key

So I tried to store the ID to debbug and always I get a empty value

My code looks like it:

<?php
class My_class{

 var $out;

 function My_class() {
    add_action ( 'init', array( $this, 'my_function' ) );
 }

 function my_function() {

  global $current_user;
  $current_user = get_currentuserinfo();	

  // do stuffs
  // I need the current user at this point
  $this->out = $current_user->ID; 

  update_usermeta( $this->out, 'my_meta_key', $value );

  // after I'll set a cookie here
  // for while, I only want the user id

 } 

 // this will be called on template/theme
 public function show(){
   // check if the current user was found before
   echo $this->out; // it prints ""

   global $current_user;
   echo "*".$current_user->ID; // it works fine

 }
} 

$my_obj = new My_class();

?>

Is there any rule to get $current_user inside a class?