function __construct( $attributes, $content = null, $form = null ) {
    $attributes = shortcode_atts(
      array(
        'label'       => null,
        'type'        => 'text',
        'required'    => false,
        'options'     => array(),
        'id'          => null,
        'default'     => null,
        'values'      => null,
        'placeholder' => null,
        'class'       => null,
      ), $attributes, 'contact-field'
    );

    // special default for subject field
    if ( 'subject' == $attributes['type'] && is_null( $attributes['default'] ) && ! is_null( $form ) ) {
      $attributes['default'] = $form->get_attribute( 'subject' );
    }

    // allow required=1 or required=true
    if ( '1' == $attributes['required'] || 'true' == strtolower( $attributes['required'] ) ) {
      $attributes['required'] = true;
    } else {
      $attributes['required'] = false;
    }

    // parse out comma-separated options list (for selects, radios, and checkbox-multiples)
    if ( ! empty( $attributes['options'] ) && is_string( $attributes['options'] ) ) {
      $attributes['options'] = array_map( 'trim', explode( ',', $attributes['options'] ) );

      if ( ! empty( $attributes['values'] ) && is_string( $attributes['values'] ) ) {
        $attributes['values'] = array_map( 'trim', explode( ',', $attributes['values'] ) );
      }
    }

    if ( $form ) {
      // make a unique field ID based on the label, with an incrementing number if needed to avoid clashes
      $form_id = $form->get_attribute( 'id' );
      $id      = isset( $attributes['id'] ) ? $attributes['id'] : false;

      $unescaped_label = $this->unesc_attr( $attributes['label'] );
      $unescaped_label = str_replace( '%', '-', $unescaped_label ); // jQuery doesn't like % in IDs?
      $unescaped_label = preg_replace( '/[^a-zA-Z0-9.-_:]/', '', $unescaped_label );

      if ( empty( $id ) ) {
        $id        = sanitize_title_with_dashes( 'g' . $form_id . '-' . $unescaped_label );
        $i         = 0;
        $max_tries = 99;
        while ( isset( $form->fields[ $id ] ) ) {
          $i++;
          $id = sanitize_title_with_dashes( 'g' . $form_id . '-' . $unescaped_label . '-' . $i );

          if ( $i > $max_tries ) {
            break;
          }
        }
      }

      $attributes['id'] = $id;
    }

    parent::__construct( $attributes, $content );

    // Store parent form
    $this->form = $form;
  }