Boost ASIO TCP/IP asynchronous server

Having seen how simple is creating a synchronous ASIO TCP/IP server, let's see now how to create an asynchronous one.

Main

The code for this example is divided in two classes, Server and Connection, described below. The example main function instantiates an ASIO io_context, uses it to instantiate a Server object, and then run() the I/O context.

namespace ba = boost::asio;
// ...

ba::io_context io;
Server server(io);
io.run();

Connection

The lower details of our code are here. Connection has as private data member an ASIO TCP/IP socket object on which we are going to write data to the client. Since we want to perform the write asynchronously, we use the ASIO async_write() function. This leads us to ensure that the current connection object is still alive when the write would actually be performed. To do that we'll pass to async_write() an instance of the connection object itself. To avoid a nightmarish memory management, we'll wrap it in a shared_ptr. However, to to that, we need to create a shared smart pointer from this, and to do that we have to enable the feature explicitly, deriving our class from the standard enable_shared_from_this:

class Connection : public std::enable_shared_from_this<Connection>
{
private:
 tcp::socket socket_;
 std::string message_{ "Async hello from ASIO " };
 static int counter_;

// ...

The Connection ctor creates its member socket using the passed ASIO I/O context, and sets the message that we'll send to the client. Notice that the message has to be a Connection data member because we have to guarantee its liveliness until the asynchronous write is performed.

Connection(ba::io_context& io) : socket_(io)
{
 message_ += std::to_string(counter_++);
}

However, the ctor is private. The only way we want to let a Connection user to create an instance of this class is by wrapping it in a smart pointer, for the reason we described above, so, we have this static public method:

static std::shared_ptr<Connection> create(ba::io_context& io)
{
 return std::shared_ptr<Connection>(new Connection(io));
}

The writing is performed by this public method:

void start()
{
 ba::async_write(socket_, ba::buffer(message_),
  std::bind(&Connection::write, shared_from_this(), std::placeholders::_1, std::placeholders::_2));
}

ASIO async_write() requires an AsyncWriteStream, our socket, a ConstBufferSequence, that we create on the fly from our message, and a WriteHandler. This last parameter represent a function in which we can perform any further action after the normal write to socket as been done and before the connection to the client is closed. A free function with two parameters, a constant reference to a Boost error_code and a size_t, is expected, but bind() is here a helpful friend. I use both parameters, but we could easily get rid of them. More importantly, notice the use of shared_from_this(). Even if we don't want do anything in the WriteHandler, it is vital that the connection is kept alive till the end of writing. Keeping the "this" reference active here does the trick.

Server

In the core of our server there is an ASIO TCP/IP acceptor, that is initialized by the ctor, and used by the Server start() function to accept - asynchronously - a connection from a client on a Connection object.

using ba::ip::tcp;
const int HELLO_PORT = 50013;
// ...

class Server
{
private:
 tcp::acceptor acceptor_;
// ...
public:
 Server(ba::io_context& io) : acceptor_(io, tcp::endpoint(tcp::v4(), HELLO_PORT))
 {
  start();
 }
// ...

The ctor calls the Server private method start(), that creates a new connection on the ASIO I/O context received from the main and stored in the acceptor. The socket owned by the connection is used in the async_accept() call on the acceptor, so that the server would wait for a client connection on it.

void start()
{
 ba::io_context& io = acceptor_.get_executor().context();
 std::shared_ptr<Connection> connection = Connection::create(io);
 tcp::socket& socket = connection->socket();
 acceptor_.async_accept(socket, std::bind(&Server::handle, this, connection, std::placeholders::_1));
}

As second parameter, async_accept() expects an ASIO AcceptHandler, a void free function that gets in input a constant reference to a boost system error code, we bind it to call the following Server private method:

void handle(std::shared_ptr<Connection> connection, const bs::error_code& ec)
{
 if (!ec)
 {
  connection->start();
 }
 start();
}

If the handshake with the client worked fine, we use the connection to write - asynchronously - through the socket. Then we call again Server start(), to prepare the server to accept another connection.

This is more or less all. You could see the full C++ code on GitHub.

I have tested this server using the client described in the previous post. I found interesting adding here and there sleeps and printing to console to better observe how the process work. For more fun I'd suggest you to run more clients and let ASIO I/O control to run on a few threads, as shown in the strand example. The code is based on the official ASIO tutorial, Daytime.3 example.