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int main() { try {
Create a server object to accept incoming client requests, and run the io_context object.
asio::io_context io_context; udp_server server(io_context); io_context.run(); } catch (std::exception& e) { std::cerr << e.what() << std::endl; } return 0; }
class udp_server { public:
The constructor initialises a socket to listen on UDP port 13.
udp_server(asio::io_context& io_context) : socket_(io_context, udp::endpoint(udp::v4(), 13)) { start_receive(); } private: void start_receive() {
        The function ip::udp::socket::async_receive_from()
        will cause the application to listen in the background for a new request.
        When such a request is received, the io_context
        object will invoke the handle_receive() function with two arguments: a value of
        type std::error_code indicating whether the operation succeeded or failed,
        and a size_t value bytes_transferred specifying the number
        of bytes received.
      
socket_.async_receive_from( asio::buffer(recv_buffer_), remote_endpoint_, std::bind(&udp_server::handle_receive, this, asio::placeholders::error, asio::placeholders::bytes_transferred)); }
        The function handle_receive() will service the client request.
      
void handle_receive(const std::error_code& error, std::size_t /*bytes_transferred*/) {
        The error parameter contains
        the result of the asynchronous operation. Since we only provide the 1-byte
        recv_buffer_ to contain the
        client's request, the io_context
        object would return an error if the client sent anything larger. We can ignore
        such an error if it comes up.
      
if (!error) {
Determine what we are going to send.
std::shared_ptr<std::string> message( new std::string(make_daytime_string()));
We now call ip::udp::socket::async_send_to() to serve the data to the client.
socket_.async_send_to(asio::buffer(*message), remote_endpoint_, std::bind(&udp_server::handle_send, this, message, asio::placeholders::error, asio::placeholders::bytes_transferred));
        When initiating the asynchronous operation, and if using std::bind,
        you must specify only the arguments that match the handler's parameter list.
        In this program, both of the argument placeholders (asio::placeholders::error
        and asio::placeholders::bytes_transferred) could potentially have been removed.
      
Start listening for the next client request.
start_receive();
        Any further actions for this client request are now the responsibility of
        handle_send().
      
} }
        The function handle_send() is invoked after the service request has
        been completed.
      
void handle_send(std::shared_ptr<std::string> /*message*/, const std::error_code& /*error*/, std::size_t /*bytes_transferred*/) { } udp::socket socket_; udp::endpoint remote_endpoint_; std::array<char, 1> recv_buffer_; };
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