how do i find the id of login page in wordpress code example

Example: how to display user id from a function on a wordpress page

function get_current_user_id() {
    if ( ! function_exists( 'wp_get_current_user' ) ) {
        return 0;
    }
    $user = wp_get_current_user();
    return ( isset( $user->ID ) ? (int) $user->ID : 0 );
}