Passing an array from shortcode-function to filter-function

Okay, it turns out that I was missing a tiny little number in the add_filter command. The line:

add_filter( 'the_content', array( &$this, 'after_content' ));

should be:

add_filter( 'the_content', array( &$this, 'after_content' ), 12);

And now all works. The array is accessible inside the filter-function. This number is a priority variable: https://developer.wordpress.org/reference/functions/add_filter/, but if anyone will add a comment on the usefulness of it, please do so.