PHP Helper Class to create shortcodes

Question

I created a helper class to extend other classes for shortcodes.

When I try to use $this in the add_shortcode() function I'm getting this error:

Using $this when not in object context

How can I solve this?

Code below:

Parent Class

//shortcode-helper.php
abstract class ShortcodeHelper {

    private $post_name_id;
    private $short_code_name;

    /**
     * ShortcodeHelper constructor.
     *
     * @param $post_name_id
     * @param $short_code_name
     */
    protected function __construct( $post_name_id, $short_code_name ) {
        $this->post_name_id      = $post_name_id;
        $this->short_code_name   = $short_code_name;
    }

    /**
     * Sets shortcode for the ShortcodeHelper
     */
    public function register() {
        add_shortcode( $this->short_code_name, $this->do_frontend_shortcode() );
    }

    protected function do_frontend_shortcode() {
        echo 'Test! '.$this->metabox_id;
    }
}

Child Class

//test-form.php
    class TestForm extends ShortcodeHelper {

        /**
         * TestForm constructor.
         */
        public function __construct() {
            parent::__construct(
                //post_name_id
                'about_us',
                //short_code_name
                'cmb-about-form'
            );
        }
    }

    new TestForm();

Answer 1

If you are targeting class method you need to pass an array where first argument is class instance and second is method name as string.

Your add_shortcode line should look like this:

add_shortcode($this->short_code_name, array($this, 'do_frontend_shortcode'));