how to get all roles in wordpress

/* get all role in wordpress in associative key = role(slug) => value(Role Display name) by [email protected]  */
function get_role_names() {

global $wp_roles;

if ( ! isset( $wp_roles ) )
    $wp_roles = new WP_Roles();

return $wp_roles->get_names();
}

function get_role_names() {

global $wp_roles;

if ( ! isset( $wp_roles ) )
    $wp_roles = new WP_Roles();

return $wp_roles->get_names();
}