text: //
  // server.cpp
  // ~~~~~~~~~~
  //
  // Copyright (c) 2003-2018 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 "asio.hpp"
  
  using 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_memory
    : private boost::noncopyable
  {
  public:
    handler_memory()
      : 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_;
  };
  
  // The allocator to be associated with the handler objects. This allocator only
  // needs to satisfy the C++11 minimal allocator requirements, plus rebind when
  // targeting C++03.
  template <typename T>
  class handler_allocator
  {
  public:
    typedef T value_type;
  
    explicit handler_allocator(handler_memory& mem)
      : memory_(mem)
    {
    }
  
    template <typename U>
    handler_allocator(const handler_allocator<U>& other)
      : memory_(other.memory_)
    {
    }
  
    template <typename U>
    struct rebind
    {
      typedef handler_allocator<U> other;
    };
  
    bool operator==(const handler_allocator& other) const
    {
      return &memory_ == &other.memory_;
    }
  
    bool operator!=(const handler_allocator& other) const
    {
      return &memory_ != &other.memory_;
    }
  
    T* allocate(std::size_t n) const
    {
      return static_cast<T*>(memory_.allocate(sizeof(T) * n));
    }
  
    void deallocate(T* p, std::size_t /*n*/) const
    {
      return memory_.deallocate(p);
    }
  
  //private:
    // The underlying memory.
    handler_memory& memory_;
  };
  
  // Wrapper class template for handler objects to allow handler memory
  // allocation to be customised. The allocator_type typedef and get_allocator()
  // member function are used by the asynchronous operations to obtain the
  // allocator. Calls to operator() are forwarded to the encapsulated handler.
  template <typename Handler>
  class custom_alloc_handler
  {
  public:
    typedef handler_allocator<Handler> allocator_type;
  
    custom_alloc_handler(handler_memory& m, Handler h)
      : memory_(m),
        handler_(h)
    {
    }
  
    allocator_type get_allocator() const
    {
      return allocator_type(memory_);
    }
  
    template <typename Arg1>
    void operator()(Arg1 arg1)
    {
      handler_(arg1);
    }
  
    template <typename Arg1, typename Arg2>
    void operator()(Arg1 arg1, Arg2 arg2)
    {
      handler_(arg1, arg2);
    }
  
  private:
    handler_memory& memory_;
    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_memory& m, Handler h)
  {
    return custom_alloc_handler<Handler>(m, h);
  }
  
  class session
    : public boost::enable_shared_from_this<session>
  {
  public:
    session(asio::io_context& io_context)
      : socket_(io_context)
    {
    }
  
    tcp::socket& socket()
    {
      return socket_;
    }
  
    void start()
    {
      socket_.async_read_some(asio::buffer(data_),
          make_custom_alloc_handler(handler_memory_,
            boost::bind(&session::handle_read,
              shared_from_this(),
              asio::placeholders::error,
              asio::placeholders::bytes_transferred)));
    }
  
    void handle_read(const asio::error_code& error,
        size_t bytes_transferred)
    {
      if (!error)
      {
        asio::async_write(socket_,
            asio::buffer(data_, bytes_transferred),
            make_custom_alloc_handler(handler_memory_,
              boost::bind(&session::handle_write,
                shared_from_this(),
                asio::placeholders::error)));
      }
    }
  
    void handle_write(const asio::error_code& error)
    {
      if (!error)
      {
        socket_.async_read_some(asio::buffer(data_),
            make_custom_alloc_handler(handler_memory_,
              boost::bind(&session::handle_read,
                shared_from_this(),
                asio::placeholders::error,
                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 memory to use for handler-based custom memory allocation.
    handler_memory handler_memory_;
  };
  
  typedef boost::shared_ptr<session> session_ptr;
  
  class server
  {
  public:
    server(asio::io_context& io_context, short port)
      : io_context_(io_context),
        acceptor_(io_context, tcp::endpoint(tcp::v4(), port))
    {
      session_ptr new_session(new session(io_context_));
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&server::handle_accept, this, new_session,
            asio::placeholders::error));
    }
  
    void handle_accept(session_ptr new_session,
        const asio::error_code& error)
    {
      if (!error)
      {
        new_session->start();
      }
  
      new_session.reset(new session(io_context_));
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&server::handle_accept, this, new_session,
            asio::placeholders::error));
    }
  
  private:
    asio::io_context& io_context_;
    tcp::acceptor acceptor_;
  };
  
  int main(int argc, char* argv[])
  {
    try
    {
      if (argc != 2)
      {
        std::cerr << "Usage: server <port>\n";
        return 1;
      }
  
      asio::io_context io_context;
  
      using namespace std; // For atoi.
      server s(io_context, atoi(argv[1]));
  
      io_context.run();
    }
    catch (std::exception& e)
    {
      std::cerr << "Exception: " << e.what() << "\n";
    }
  
    return 0;
  }
  