template selection in custom post types in wordpress

cptTemplates.php

<?php
function cptTemplateMetaBox() {
  $post_types = get_post_types();
  foreach ( $post_types as $post_type ) {
    add_meta_box( 'cpt-template-meta-box', 'Template', 'cptTemplateMetaBoxMarkup', $post_type, 'side', 'high' );
  }
}

add_action( 'add_meta_boxes', 'cptTemplateMetaBox' );

function cptTemplateMetaBoxMarkup( $post ) {
  wp_nonce_field( basename(__FILE__), 'cpt_template_meta_nonce' );
  $current_template = get_post_meta( $post->ID, 'cpt_page_template', true);
  $template_options = get_page_templates();
  $box_label = '<label for="cpt_page_template">Page Template</label>';
  $box_select = '<select name="cpt_page_template">';
  $box_default_option = '<option value="">Default Template</option>';
  $box_options = '';
  foreach (  $template_options as $name=>$file ) {
      if ( $current_template == $file ) {
          $box_options .= '<option value="' . $file . '" selected="selected">' . $name . '</option>';
      } else {
          $box_options .= '<option value="' . $file . '">' . $name . '</option>';
      }
  }
  echo $box_label;
  echo $box_select;
  echo $box_default_option;
  echo $box_options;
  echo '</select>';
}

function cptTemplateMetaBoxSave( $post_id ) {
  $current_nonce = $_POST['cpt_template_meta_nonce'];
  $is_autosaving = wp_is_post_autosave( $post_id );
  $is_revision   = wp_is_post_revision( $post_id );
  $valid_nonce   = ( isset( $current_nonce ) && wp_verify_nonce( $current_nonce, basename( __FILE__ ) ) ) ? 'true' : 'false';
  if ( $is_autosaving || $is_revision || !$valid_nonce ) {
      return;
  }
  $cpt_page_template = $_POST['cpt_page_template'];
  update_post_meta( $post_id, 'cpt_page_template', $cpt_page_template );
}

add_action( 'save_post', 'cptTemplateMetaBoxSave' );


function loadCptPostTemplate() {
  $query_object = get_queried_object();
  $page_template = get_post_meta( $query_object->ID, 'cpt_page_template', true );
  $my_post_type = get_post_type($query_object->ID);
  $default_templates    = array();
  $default_templates[]  = 'single-{$object->post_type}-{$object->post_name}.php';
  $default_templates[]  = 'single-{$object->post_type}.php';
  $default_templates[]  = 'single.php';
  if ( $query_object->post_type == $my_post_type ) {
      // if the page_template isn't empty, set it as the default_template
      if ( !empty( $page_template ) ) {
          $default_templates = $page_template;
      }
  }
  $new_template = locate_template( $default_templates, false );
  return $new_template;
}

add_filter( 'single_template', 'loadCptPostTemplate' );