Register a sidebar for a page that has a page template selected

functions.php

add_action('widgets_init', 'gpb_register_sidebar');
/**
* Dynamically register sidebar for pages that use specified template
* 
*/
function gpb_register_sidebar() {
    global $post;
    global $wp_registered_sidebars;

	// Find all pages that use template with dynamic sidebar
    $query = new WP_Query(array(
        'meta_key' => '_wp_page_template',
        'meta_value' => '^widget-template',
	'meta_compare' => 'REGEXP',
        'post_type' => 'page',
        'nopaging' => true,
        'orderby' => 'title',
        'order' => 'ASC',
    ));

    while ($query->have_posts()) {
        $query->the_post();

        $id = "gpb-{$post->ID}-sidebar";

        if (!isset($wp_registered_sidebars[$id])) {
            register_sidebar( array(
                'name' => $post->post_title . ' Sidebar',
                'id' => $id,
                'description' => 'Sidebar widgets for ' . $post->post_title,
                'before_widget' => '<div class="content-widget widget-area"><div class="wrap"><div id="%1$s" class="%2$s">',
                'after_widget' => '</div></div></div>',
                'before_title' => '<h4 class="widget-title" itemprop="headline">',
                'after_title' => '</h4>',
            ) );
        }
    }
    wp_reset_postdata();
}