public function is($klass)
    {
        $pool = \Google\Protobuf\Internal\DescriptorPool::getGeneratedPool();
        $desc = $pool->getDescriptorByClassName($klass);
        $fully_qualifed_name = $desc->getFullName();
        $type_url = GPBUtil::TYPE_URL_PREFIX.substr(
            $fully_qualifed_name, 1, strlen($fully_qualifed_name));
        return $this->type_url === $type_url;
    }