c++ mqtt subscribe 代码示例

以下是一个C++ MQTT订阅的示例代码，使用Eclipse Paho C++库实现：

#include <iostream>
#include <cstring>
#include "mqtt/async_client.h"

const std::string SERVER_ADDRESS("tcp://localhost:1883");
const std::string CLIENT_ID("async_subcribe_cpp");
const std::string TOPIC("test/topic");
const int QOS = 1;

class callback : public virtual mqtt::callback
{
public:
    virtual void connection_lost(const std::string& cause) override
    {
        std::cout << "\nConnection lost" << std::endl;
        if (!cause.empty())
            std::cout << "\tcause: " << cause << std::endl;
    }

    virtual void message_arrived(mqtt::const_message_ptr msg) override
    {
        std::cout << "Message arrived" << std::endl;
        std::cout << "\ttopic: '" << msg->get_topic() << "'" << std::endl;
        std::cout << "\tpayload: '" << msg->to_string() << "'\n" << std::endl;
    }

    virtual void delivery_complete(mqtt::delivery_token_ptr token) override {}
};

int main(int argc, char* argv[])
{
    mqtt::async_client client(SERVER_ADDRESS, CLIENT_ID);

    callback cb;
    client.set_callback(cb);

    mqtt::connect_options connOpts;
    connOpts.set_keep_alive_interval(20);
    connOpts.set_clean_session(true);

    try {
        std::cout << "Connecting to the MQTT server..." << std::flush;
        client.connect(connOpts)->wait();
        std::cout << "OK" << std::endl;

        std::cout << "Subscribing to topic '" << TOPIC << "'..." << std::flush;
        client.subscribe(TOPIC, QOS)->wait();
        std::cout << "OK" << std::endl;

        while (true)
            ;

        client.unsubscribe(TOPIC)->wait();
        std::cout << "Unsubscribed" << std::endl;

        client.disconnect()->wait();
        std::cout << "Disconnected" << std::endl;
    }
    catch (const mqtt::exception& exc) {
        std::cerr << "Error: " << exc.what() << std::endl;
        return 1;
    }

    return 0;
}