...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
int main() { try {
Create a server object to accept incoming client requests, and run the io_context object.
boost::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(boost::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 boost::system::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( boost::asio::buffer(recv_buffer_), remote_endpoint_, std::bind(&udp_server::handle_receive, this, boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred)); }
The function handle_receive()
will service the client request.
void handle_receive(const boost::system::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(boost::asio::buffer(*message), remote_endpoint_, std::bind(&udp_server::handle_send, this, message, boost::asio::placeholders::error, boost::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 (boost::asio::placeholders::error
and boost::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 boost::system::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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