How to call a function inside class without $this?

I want to hook a form onto an external function. The hook load my form by the name:

add_action(media_uploader, 'the_hook');
function the hook(){
//.....
return wp_iframe( 'media_upload_potato_form', $errors );
}
function media_upload_potato_form(){ }// my form

If I use the above functions in a class as showing below, it gets error:

return wp_iframe($this->media_upload_potato_form, $errors);

How can I hook up correctly?