public function size()
    {
        $size = 0;
        if ( ! is_null($this->key_)) {
            $l = strlen($this->key_);
            $size += 1 + Protobuf::size_varint($l) + $l;
        }
        if ( ! is_null($this->value_)) {
            $l = strlen($this->value_);
            $size += 1 + Protobuf::size_varint($l) + $l;
        }

        return $size;
    }