text: //
  // server.cpp
  // ~~~~~~~~~~
  //
  // Copyright (c) 2003-2017 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 <cstdlib>
  #include <iostream>
  #include <boost/aligned_storage.hpp>
  #include <boost/array.hpp>
  #include <boost/bind.hpp>
  #include <boost/enable_shared_from_this.hpp>
  #include <boost/noncopyable.hpp>
  #include <boost/shared_ptr.hpp>
  #include <boost/asio.hpp>
  
  using boost::asio::ip::tcp;
  
  // Class to manage the memory to be used for handler-based custom allocation.
  // It contains a single block of memory which may be returned for allocation
  // requests. If the memory is in use when an allocation request is made, the
  // allocator delegates allocation to the global heap.
  class handler_allocator
    : private boost::noncopyable
  {
  public:
    handler_allocator()
      : in_use_(false)
    {
    }
  
    void* allocate(std::size_t size)
    {
      if (!in_use_ && size < storage_.size)
      {
        in_use_ = true;
        return storage_.address();
      }
      else
      {
        return ::operator new(size);
      }
    }
  
    void deallocate(void* pointer)
    {
      if (pointer == storage_.address())
      {
        in_use_ = false;
      }
      else
      {
        ::operator delete(pointer);
      }
    }
  
  private:
    // Storage space used for handler-based custom memory allocation.
    boost::aligned_storage<1024> storage_;
  
    // Whether the handler-based custom allocation storage has been used.
    bool in_use_;
  };
  
  // Wrapper class template for handler objects to allow handler memory
  // allocation to be customised. Calls to operator() are forwarded to the
  // encapsulated handler.
  template <typename Handler>
  class custom_alloc_handler
  {
  public:
    custom_alloc_handler(handler_allocator& a, Handler h)
      : allocator_(a),
        handler_(h)
    {
    }
  
    template <typename Arg1>
    void operator()(Arg1 arg1)
    {
      handler_(arg1);
    }
  
    template <typename Arg1, typename Arg2>
    void operator()(Arg1 arg1, Arg2 arg2)
    {
      handler_(arg1, arg2);
    }
  
    friend void* asio_handler_allocate(std::size_t size,
        custom_alloc_handler<Handler>* this_handler)
    {
      return this_handler->allocator_.allocate(size);
    }
  
    friend void asio_handler_deallocate(void* pointer, std::size_t /*size*/,
        custom_alloc_handler<Handler>* this_handler)
    {
      this_handler->allocator_.deallocate(pointer);
    }
  
  private:
    handler_allocator& allocator_;
    Handler handler_;
  };
  
  // Helper function to wrap a handler object to add custom allocation.
  template <typename Handler>
  inline custom_alloc_handler<Handler> make_custom_alloc_handler(
      handler_allocator& a, Handler h)
  {
    return custom_alloc_handler<Handler>(a, h);
  }
  
  class session
    : public boost::enable_shared_from_this<session>
  {
  public:
    session(boost::asio::io_service& io_service)
      : socket_(io_service)
    {
    }
  
    tcp::socket& socket()
    {
      return socket_;
    }
  
    void start()
    {
      socket_.async_read_some(boost::asio::buffer(data_),
          make_custom_alloc_handler(allocator_,
            boost::bind(&session::handle_read,
              shared_from_this(),
              boost::asio::placeholders::error,
              boost::asio::placeholders::bytes_transferred)));
    }
  
    void handle_read(const boost::system::error_code& error,
        size_t bytes_transferred)
    {
      if (!error)
      {
        boost::asio::async_write(socket_,
            boost::asio::buffer(data_, bytes_transferred),
            make_custom_alloc_handler(allocator_,
              boost::bind(&session::handle_write,
                shared_from_this(),
                boost::asio::placeholders::error)));
      }
    }
  
    void handle_write(const boost::system::error_code& error)
    {
      if (!error)
      {
        socket_.async_read_some(boost::asio::buffer(data_),
            make_custom_alloc_handler(allocator_,
              boost::bind(&session::handle_read,
                shared_from_this(),
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred)));
      }
    }
  
  private:
    // The socket used to communicate with the client.
    tcp::socket socket_;
  
    // Buffer used to store data received from the client.
    boost::array<char, 1024> data_;
  
    // The allocator to use for handler-based custom memory allocation.
    handler_allocator allocator_;
  };
  
  typedef boost::shared_ptr<session> session_ptr;
  
  class server
  {
  public:
    server(boost::asio::io_service& io_service, short port)
      : io_service_(io_service),
        acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
    {
      session_ptr new_session(new session(io_service_));
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&server::handle_accept, this, new_session,
            boost::asio::placeholders::error));
    }
  
    void handle_accept(session_ptr new_session,
        const boost::system::error_code& error)
    {
      if (!error)
      {
        new_session->start();
      }
  
      new_session.reset(new session(io_service_));
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&server::handle_accept, this, new_session,
            boost::asio::placeholders::error));
    }
  
  private:
    boost::asio::io_service& io_service_;
    tcp::acceptor acceptor_;
  };
  
  int main(int argc, char* argv[])
  {
    try
    {
      if (argc != 2)
      {
        std::cerr << "Usage: server <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;
  }
  