...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 {
We need to create a server object to accept incoming client connections. The io_context object provides I/O services, such as sockets, that the server object will use.
boost::asio::io_context io_context; tcp_server server(io_context);
Run the io_context object so that it will perform asynchronous operations on your behalf.
io_context.run(); } catch (std::exception& e) { std::cerr << e.what() << std::endl; } return 0; }
class tcp_server { public:
The constructor initialises an acceptor to listen on TCP port 13.
tcp_server(boost::asio::io_context& io_context) : io_context_(io_context), acceptor_(io_context, tcp::endpoint(tcp::v4(), 13)) { start_accept(); } private:
The function start_accept()
creates a socket and initiates an
asynchronous accept operation to wait for a new connection.
void start_accept() { tcp_connection::pointer new_connection = tcp_connection::create(io_context_); acceptor_.async_accept(new_connection->socket(), boost::bind(&tcp_server::handle_accept, this, new_connection, boost::asio::placeholders::error)); }
The function handle_accept()
is called when the asynchronous
accept operation initiated by start_accept()
finishes. It services
the client request, and then calls start_accept()
to initiate
the next accept operation.
void handle_accept(tcp_connection::pointer new_connection, const boost::system::error_code& error) { if (!error) { new_connection->start(); } start_accept(); }
We will use shared_ptr
and enable_shared_from_this
because we want to keep the tcp_connection
object alive as long
as there is an operation that refers to it.
class tcp_connection : public boost::enable_shared_from_this<tcp_connection> { public: typedef boost::shared_ptr<tcp_connection> pointer; static pointer create(boost::asio::io_context& io_context) { return pointer(new tcp_connection(io_context)); } tcp::socket& socket() { return socket_; }
In the function start()
, we call boost::asio::async_write()
to serve the data to the client. Note that we are using boost::asio::async_write(),
rather than ip::tcp::socket::async_write_some(),
to ensure that the entire block of data is sent.
void start() {
The data to be sent is stored in the class member message_
as
we need to keep the data valid until the asynchronous operation is complete.
message_ = make_daytime_string();
When initiating the asynchronous operation, and if using boost::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, since they are not being used in handle_write()
.
boost::asio::async_write(socket_, boost::asio::buffer(message_), boost::bind(&tcp_connection::handle_write, shared_from_this(), boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred));
Any further actions for this client connection are now the responsibility
of handle_write()
.
} private: tcp_connection(boost::asio::io_context& io_context) : socket_(io_context) { } void handle_write(const boost::system::error_code& /*error*/, size_t /*bytes_transferred*/) { } tcp::socket socket_; std::string message_; };
You may have noticed that the error
, and bytes_transferred
parameters are not used in the body of the handle_write()
function.
If parameters are not needed, it is possible to remove them from the function
so that it looks like:
void handle_write() { }
The boost::asio::async_write() call used to initiate the call can then be changed to just:
boost::asio::async_write(socket_, boost::asio::buffer(message_), boost::bind(&tcp_connection::handle_write, shared_from_this()));
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