How to design a custom IO object for Boost.Asio

Question

I have an base class (DeviceBase) representing an embedded device, with which I want to communicate. The device can be accessed in various ways, including USB and TCP sockets. Additionally, there is a mock implementation which works on files.

Until now, I have only used synchronous read/write calls, and all the implementations are simply classes derived from the base class, overriding the read/write functions. This allows me to use polymorphic pointers and containers to provide implementation-independent access to the device to the application's logic.

Now i want to use Boost.Asio to enable simple async IO. I have found this guide http://www.highscore.de/cpp/boost/asio.html describing how to write your own extensions, but it is quite simplified, and I have stumbled upon some issues which aren't discussed there.

• Currently, all my implementations have different constructors (obviously, because of the different ways of addressing/initializing the device connection). The implementation class presented in the guide is constructed by the io_service, which means, the user can't pass arguments to the constructor.

• The example in the guide provides a template class in the form of custom_io_object<implementation>, which would prevent the usage of polymorphic pointers, because now there is no common base class to the different kinds of IO objects.

Is there any form of documentation, or any books which discuss the design of Boost.Asio extensions? Do I overlook something?

Answer 1

It sounds like an inheritance based solution may be more appropriate for you.

Here’s an example base class using boost::signals2 to signal a received message:

class Connection
{
public:

  typedef boost::signals2::signal<void (const std::vector<char>&)>
     PacketReceived;

protected:
  PacketReceived packet_received_;
  size_t max_rx_packet_size_;
  std::vector<char> read_buffer_;
  std::deque<std::vector<char> > tx_queue_;

  void read_handler(boost::system::error_code const& error,
                    size_t bytes_transferred)
  {
    if (boost::asio::error::operation_aborted != error)
    {
      if (error)
        ; // TODO handle the error, it may be a disconnect.
      else
      {
        read_buffer_.resize(bytes_transferred);
        packet_received_(read_buffer_);
        enable_reception(max_rx_packet_size_);
      }
    }
  }

  void write_handler(boost::system::error_code const& error,
                     size_t bytes_transferred)
  {
    if (boost::asio::error::operation_aborted != error)
    {
      tx_queue_.pop_front();
      if (error)
        ; // TODO handle the error, it may be a disconnect.
      else
        if (!tx_queue_.empty())
          transmit();
    }
  }

  virtual void receive() = 0;

  virtual void transmit() = 0;

  explicit Connection() :
    packet_received_(),
    max_rx_packet_size_(),
    read_buffer_(),
    tx_queue_()
  {}

public:

  virtual void close() = 0;

  virtual ~Connection()
  {}

  void connectPacketReceived(const PacketReceived::slot_type& slot)
  { packet_received_.connect(slot); }

  void enable_reception(size_t max_rx_packet_size)
  {
    max_rx_packet_size_ = max_rx_packet_size;
    receive();
  }

  const std::vector<char>& read_buffer() const
  { return read_buffer_; }

#if defined(BOOST_ASIO_HAS_MOVE)
  void send(std::vector<char>&& packet )
#else
  void send(const std::vector<char>& packet )
#endif
  {
    bool queue_empty(tx_queue_.empty());
    tx_queue_.push_back(packet);
    if (queue_empty)
      transmit();
  }
};

And here's the outline of a class to implement an SSL socket:

class SslConnection :
    public Connection,
    public boost::enable_shared_from_this<SslConnection>
{
  boost::asio::ssl::stream<boost::asio::ip::tcp::socket> ssl_socket_;

  virtual void receive()
  {
    ssl_socket_.async_read_some(boost::asio::buffer(read_buffer_),
         boost::bind(&SslConnection::read_handler, shared_from_this(),
                     boost::asio::placeholders::error,
                     boost::asio::placeholders::bytes_transferred));
  }

  virtual void transmit()
  {
    boost::asio::async_write(ssl_socket_,
                             boost::asio::buffer(tx_queue_.front()),
         boost::bind(&SslConnection::write_handler, shared_from_this(),
                     boost::asio::placeholders::error,
                     boost::asio::placeholders::bytes_transferred));
  }

  SslConnection(boost::asio::io_service&   io_service,
                boost::asio::ssl::context& ssl_context) :
    Connection(),
    ssl_socket_(io_service, ssl_context)
  {}

public:

  static boost::shared_ptr<SslConnection> create
                     (boost::asio::io_service&   io_service,
                      boost::asio::ssl::context& ssl_context)
  {
    return boost::shared_ptr<SslConnection>
        (new SslConnection(io_service, ssl_context));
  }

  virtual void close()
  {
    boost::system::error_code ignoredEc;
    ssl_socket_.lowest_layer().close(ignoredEc);
  }
};