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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.
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(asio::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(acceptor_.get_executor().context());
acceptor_.async_accept(new_connection->socket(),
boost::bind(&tcp_server::handle_accept, this, new_connection,
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 asio::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(asio::io_context& io_context)
{
return pointer(new tcp_connection(io_context));
}
tcp::socket& socket()
{
return socket_;
}
In the function start(), we call asio::async_write() to serve
the data to the client. Note that we are using 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 (asio::placeholders::error
and asio::placeholders::bytes_transferred) could potentially have been removed,
since they are not being used in handle_write().
asio::async_write(socket_, asio::buffer(message_),
boost::bind(&tcp_connection::handle_write, shared_from_this(),
asio::placeholders::error,
asio::placeholders::bytes_transferred));
Any further actions for this client connection are now the responsibility
of handle_write().
}
private:
tcp_connection(asio::io_context& io_context)
: socket_(io_context)
{
}
void handle_write(const asio::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 asio::async_write() call used to initiate the call can then be changed to just:
asio::async_write(socket_, asio::buffer(message_),
boost::bind(&tcp_connection::handle_write, shared_from_this()));
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