基于Mosquitto的MQTT C++服务器端示例代码：

Mosquitto本身是一个MQTT代理，而不是一个MQTT服务器。如果你需要一个MQTT服务器，可以考虑使用Eclipse Mosquitto的扩展版本Mosquitto.rsmb，它提供了一个MQTT服务器实现，可以用来实现自己的MQTT服务器。

以下是一个基于Mosquitto.rsmb的MQTT C++服务器端示例代码：

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <unistd.h>
#include <cstring>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <mosquitto.h>
#include <pthread.h>

#define MQTT_PORT 1883
#define MQTT_KEEP_ALIVE 60
#define MAX_CLIENTS 1024

static void on_connect(struct mosquitto *mosq, void *userdata, int rc) {
    if (rc == 0) {
        std::cout << "Connected to MQTT broker" << std::endl;
    } else {
        std::cerr << "Failed to connect to MQTT broker: " << mosquitto_connack_string(rc) << std::endl;
    }
}

static void on_message(struct mosquitto *mosq, void *userdata, const struct mosquitto_message *msg) {
    std::cout << "Received message on topic " << msg->topic << ": " << (char *)msg->payload << std::endl;
}

class Client {
public:
    Client(int fd) : fd_(fd), mosq_(NULL) {}
    ~Client() {
        if (mosq_) {
            mosquitto_destroy(mosq_);
            mosq_ = NULL;
        }
        if (fd_ > 0) {
            close(fd_);
            fd_ = -1;
        }
    }

    int fd() const { return fd_; }

    void start() {
        mosq_ = mosquitto_new(NULL, true, this);
        if (!mosq_) {
            std::cerr << "Failed to create Mosquitto object" << std::endl;
            return;
        }

        mosquitto_connect_callback_set(mosq_, on_connect);
        mosquitto_message_callback_set(mosq_, on_message);

        int rc = mosquitto_socket(mosq_);
        if (rc == MOSQ_ERR_SUCCESS) {
            rc = mosquitto_loop_start(mosq_);
        }

        if (rc != MOSQ_ERR_SUCCESS) {
            std::cerr << "Failed to start Mosquitto loop: " << mosquitto_strerror(rc) << std::endl;
            return;
        }

        char buf[1024];
        while (true) {
            int n = recv(fd_, buf, sizeof(buf), 0);
            if (n <= 0) {
                break;
            }
            mosquitto_raw_publish(mosq_, NULL, "test_topic", n, buf, 0, false);
        }
    }

private:
    int fd_;
    struct mosquitto *mosq_;
};

void *client_handler(void *arg) {
    Client *client = (Client *)arg;
    client->start();
    delete client;
    return NULL;
}

int main(int argc, char **argv) {
    int listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (listen_fd < 0) {
        perror("Failed to create socket");
        return 1;
    }

    struct sockaddr_in addr;
    std::memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(MQTT_PORT);

    int optval = 1;
    setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval));

    if (bind(listen_fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
        perror("Failed to bind socket");
        return 1;
    }

    if (listen(listen_fd, SOMAXCONN) < 0) {
        perror("Failed to listen on socket");
        return 1;
    }

    std::cout << "Listening on port " << MQTT_PORT << std::endl;

    while (true) {
        int conn_fd = accept(listen_fd, NULL, NULL);
        if (conn_fd < 0) {
            perror("Failed to accept connection");
            continue;
        }

        std::cout << "Accepted connection from " << inet_ntoa(addr.sin_addr) << std::endl;

        pthread_t tid;
        pthread_create(&tid, NULL, client_handler, new Client(conn_fd));
        pthread_detach(tid);
    }

    close(listen_fd);

    return 0;
}

此代码使用Mosquitto.rsmb作为MQTT服务器，并在本地的1883端口监听连接。在接受到客户端连接之后，它会启动一个新线程来处理该客户端连接。在处理过程中，它会接收客户端的消息，并使用Mosquitto API将其发布到名为“test_topic”的主题上。

请注意，此代码仅供参考，并不是一个完整的MQTT服务器实现。在实际使用中，还需要考虑安全性、可靠性、性能等因素。