text: //
  // third_party_lib.cpp
  // ~~~~~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2011 Christopher M. Kohlhoff (chris at kohlhoff dot com)
  //
  // Distributed under the Boost Software License, Version 1.0. (See accompanying
  // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  //
  
  #include <boost/asio.hpp>
  #include <boost/array.hpp>
  #include <boost/bind.hpp>
  #include <boost/shared_ptr.hpp>
  #include <boost/enable_shared_from_this.hpp>
  #include <iostream>
  
  using boost::asio::ip::tcp;
  
  namespace third_party_lib {
  
  // Simulation of a third party library that wants to perform read and write
  // operations directly on a socket. It needs to be polled to determine whether
  // it requires a read or write operation, and notified when the socket is ready
  // for reading or writing.
  class session
  {
  public:
    session(tcp::socket& socket)
      : socket_(socket),
        state_(reading)
    {
    }
  
    // Returns true if the third party library wants to be notified when the
    // socket is ready for reading.
    bool want_read() const
    {
      return state_ == reading;
    }
  
    // Notify that third party library that it should perform its read operation.
    void do_read(boost::system::error_code& ec)
    {
      if (std::size_t len = socket_.read_some(boost::asio::buffer(data_), ec))
      {
        write_buffer_ = boost::asio::buffer(data_, len);
        state_ = writing;
      }
    }
  
    // Returns true if the third party library wants to be notified when the
    // socket is ready for writing.
    bool want_write() const
    {
      return state_ == writing;
    }
  
    // Notify that third party library that it should perform its write operation.
    void do_write(boost::system::error_code& ec)
    {
      if (std::size_t len = socket_.write_some(
            boost::asio::buffer(write_buffer_), ec))
      {
        write_buffer_ = write_buffer_ + len;
        state_ = boost::asio::buffer_size(write_buffer_) > 0 ? writing : reading;
      }
    }
  
  private:
    tcp::socket& socket_;
    enum { reading, writing } state_;
    boost::array<char, 128> data_;
    boost::asio::const_buffer write_buffer_;
  };
  
  } // namespace third_party_lib
  
  // The glue between asio's sockets and the third party library.
  class connection
    : public boost::enable_shared_from_this<connection>
  {
  public:
    typedef boost::shared_ptr<connection> pointer;
  
    static pointer create(boost::asio::io_service& io_service)
    {
      return pointer(new connection(io_service));
    }
  
    tcp::socket& socket()
    {
      return socket_;
    }
  
    void start()
    {
      // Put the socket into non-blocking mode.
      socket_.non_blocking(true);
  
      start_operations();
    }
  
  private:
    connection(boost::asio::io_service& io_service)
      : socket_(io_service),
        session_impl_(socket_),
        read_in_progress_(false),
        write_in_progress_(false)
    {
    }
  
    void start_operations()
    {
      // Start a read operation if the third party library wants one.
      if (session_impl_.want_read() && !read_in_progress_)
      {
        read_in_progress_ = true;
        socket_.async_read_some(
            boost::asio::null_buffers(),
            boost::bind(&connection::handle_read,
              shared_from_this(),
              boost::asio::placeholders::error));
      }
  
      // Start a write operation if the third party library wants one.
      if (session_impl_.want_write() && !write_in_progress_)
      {
        write_in_progress_ = true;
        socket_.async_write_some(
            boost::asio::null_buffers(),
            boost::bind(&connection::handle_write,
              shared_from_this(),
              boost::asio::placeholders::error));
      }
    }
  
    void handle_read(boost::system::error_code ec)
    {
      read_in_progress_ = false;
  
      // Notify third party library that it can perform a read.
      if (!ec)
        session_impl_.do_read(ec);
  
      // The third party library successfully performed a read on the socket.
      // Start new read or write operations based on what it now wants.
      if (!ec || ec == boost::asio::error::would_block)
        start_operations();
  
      // Otherwise, an error occurred. Closing the socket cancels any outstanding
      // asynchronous read or write operations. The connection object will be
      // destroyed automatically once those outstanding operations complete.
      else
        socket_.close();
    }
  
    void handle_write(boost::system::error_code ec)
    {
      write_in_progress_ = false;
  
      // Notify third party library that it can perform a write.
      if (!ec)
        session_impl_.do_write(ec);
  
      // The third party library successfully performed a write on the socket.
      // Start new read or write operations based on what it now wants.
      if (!ec || ec == boost::asio::error::would_block)
        start_operations();
  
      // Otherwise, an error occurred. Closing the socket cancels any outstanding
      // asynchronous read or write operations. The connection object will be
      // destroyed automatically once those outstanding operations complete.
      else
        socket_.close();
    }
  
  private:
    tcp::socket socket_;
    third_party_lib::session session_impl_;
    bool read_in_progress_;
    bool write_in_progress_;
  };
  
  class server
  {
  public:
    server(boost::asio::io_service& io_service, unsigned short port)
      : acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
    {
      start_accept();
    }
  
  private:
    void start_accept()
    {
      connection::pointer new_connection =
        connection::create(acceptor_.get_io_service());
  
      acceptor_.async_accept(new_connection->socket(),
          boost::bind(&server::handle_accept, this, new_connection,
            boost::asio::placeholders::error));
    }
  
    void handle_accept(connection::pointer new_connection,
        const boost::system::error_code& error)
    {
      if (!error)
      {
        new_connection->start();
      }
  
      start_accept();
    }
  
    tcp::acceptor acceptor_;
  };
  
  int main(int argc, char* argv[])
  {
    try
    {
      if (argc != 2)
      {
        std::cerr << "Usage: third_party_lib <port>\n";
        return 1;
      }
  
      boost::asio::io_service io_service;
  
      using namespace std; // For atoi.
      server s(io_service, atoi(argv[1]));
  
      io_service.run();
    }
    catch (std::exception& e)
    {
      std::cerr << "Exception: " << e.what() << "\n";
    }
  
    return 0;
  }
  