add_action 'init' from inside a class for custom post types

Question

I'm trying to create archive pages for my custom post type. I create custom post types from within custom classes:

functions.php

include_once(get_template_directory() . '/admin/admin_init.php');

admin_init.php

include_once(ADMIN_CPT_DIR . '/CPT.php');

CPT.php

class CPT
{
    protected $cpt_name;

    public function __construct($cpt_name)
    {
        $this->cpt_name = $cpt_name;
        add_action('init', array( $this, 'cpt_init' ) );
    }

    public function cpt_init()
   {
        $labels = array(
        'name' => __('custom_name', 'textDomain'),
        'singular_name' => __('custom_name', 'textDomain')
         );

        $args = array(
        'labels' => $labels,
        'public' => true,
        'has_archive' => true,
        'publicly_queryable' => true,
        'exclude_from_search' => false,
        'show_ui' => true,
        'show_in_menu' => true,
        'has_archive' => true,
        'query_var' => true,
        'rewrite' => array('slug' => 'custom_name'),
        'supports' =>array('title','editor', 'custom-fields','thumbnail')
    );

    register_post_type( $this->cpt_name, $args );
   }
}

if( is_admin() )
    $cpt = new CPT('cpt_child');

The custom post type is registered and all works fine, but I always get a 404 when trying to access the archive page (archive-custom_name.php).

If I take the inner part of the cpt_init - method and put it into the functions.php like so:

add_action( 'init', 'create_post_type' );

    function create_post_type()
    {
        $labels = array(
        'name' => __('mycpt', 'synTh_textDomain'),
        'singular_name' => __('mycpt', 'synTh_textDomain')
        );

        $args = array(
        'labels' => $labels,
        'public' => true,
        'has_archive' => true,
        'publicly_queryable' => true,
        'exclude_from_search' => false,
        'show_ui' => true,
        'show_in_menu' => true,
        'has_archive' => true,
        'query_var' => true,
        'rewrite' => array('slug' => 'boom'),
        'supports' =>array('title','editor', 'custom-fields','thumbnail')
        );  

        register_post_type( 'mycpt', $args );
    }

everything works as expected. There is an archive-boom.php, which I can access via: www.domain.com/boom/

I flushed the permalinks settings and I have them set to postname.

Is there a difference, when attaching to a hook from inside a class??

Answer 1

You only create an instance of your class on admin requests, so your CPT doesn't exist on front end requests. Instead of:

if( is_admin() )
    $cpt = new CPT('cpt_child');

create the instance on all requests:

$cpt = new CPT('cpt_child');