请用C++，基于UDP编写一个包含Server/Client的网络应用程序

在C++中，我们可以使用`socket API`（如`boost.asio`库）来基于UDP协议创建服务器和客户端的网络应用。这里给出一个简单的示例，首先我们创建一个基础的UDP服务器接收并回应数据，然后再构建一个客户端发送数据到服务器。

**服务器端（server.cpp）**:

#include <boost/asio.hpp>
using boost::asio::ip::udp;

class Server {
public:
    void start(int port) {
        boost::asio::io_service io_service;
        udp::endpoint endpoint(udp::v4(), port);

        udp::socket socket(io_service, udp::v4());
        socket.open(endpoint.protocol());

        for (;;) {
            boost::array<char, 1024> buffer;
            size_t bytes_received = 0;

            try {
                boost::system::error_code ignored_error;
                bytes_received = socket.receive_from(buffer, endpoint, ignored_error);

                std::string message(buffer.data(), bytes_received);
                std::cout << "Received from client: " << message << "\n";

                // 发送响应
                boost::array<char, 1024> response("ACK");
                socket.send_to(response, endpoint);
            }
            catch (const boost::system::system_error& e) {
                if (e.code() != boost::asio::error::operation_aborted)
                    std::cerr << "Error: " << e.what() << '\n';
            }
        }
    }
};

int main() {
    Server server;
    server.start(12345); // 使用端口号12345
    return 0;
}

**客户端（client.cpp）**:

#include <boost/asio.hpp>
using boost::asio::ip::udp;

class Client {
public:
    void connectToServer(const std::string& server_ip, int server_port) {
        boost::asio::io_service io_service;
        udp::resolver resolver(io_service);
        udp::resolver::query query(server_ip, std::to_string(server_port));
        udp::endpoint endpoint = *resolver.resolve(query);

        udp::socket socket(io_service, udp::v4());
        socket.open(endpoint.protocol());

        boost::array<char, 1024> request("Hello Server!");
        socket.send_to(request, endpoint);
        
        boost::array<char, 1024> buffer;
        size_t bytes_received = 0;
        while ((bytes_received = socket.receive_from(buffer, endpoint)) > 0) {
            std::string message(buffer.data(), bytes_received);
            std::cout << "Received from server: " << message << "\n";
        }
    }
};

int main() {
    Client client;
    client.connectToServer("localhost", 12345); // 连接到本地服务器
    return 0;
}

运行时，客户端会向服务器发送消息"Hello Server!"，然后打印接收到的任何回复。