在C++中使用librdkafka库可以很方便地发送JSON数据到Kafka。下面是一个简单的示例代码：

#include <librdkafka/rdkafkacpp.h>#include <iostream>#include <string>int main() {std::string brokers = "localhost:9092";std::string topic = "test_topic";RdKafka::Conf *conf = RdKafka::Conf::create(RdKafka::Conf::CONF_GLOBAL);RdKafka::Conf *tconf = RdKafka::Conf::create(RdKafka::Conf::CONF_TOPIC);conf->set("metadata.broker.list", brokers, errstr);RdKafka::Producer *producer = RdKafka::Producer::create(conf, errstr);if (!producer) {std::cerr << "Failed to create producer: " << errstr << std::endl;return 1;}RdKafka::Topic *kafka_topic = RdKafka::Topic::create(producer, topic, tconf, errstr);if (!kafka_topic) {std::cerr << "Failed to create topic: " << errstr << std::endl;return 1;}std::string json_data = "{\"key\": \"value\"}";RdKafka::ErrorCode resp = producer->produce(kafka_topic, RdKafka::Topic::PARTITION_UA, RdKafka::Producer::RK_MSG_COPY,const_cast<char *>(json_data.c_str()), json_data.size(), NULL, NULL);if (resp != RdKafka::ERR_NO_ERROR) {std::cerr << "Failed to produce message: " << RdKafka::err2str(resp) << std::endl;return 1;}producer->flush(1000);delete kafka_topic;delete producer;return 0;}

在这个示例中，我们首先创建一个生产者和一个主题对象，然后使用produce方法发送一个JSON数据到Kafka主题。在实际使用中，你可能需要根据你的需求调整代码，并添加错误处理逻辑。