text: //
  // handler_allocator.cpp
  // ~~~~~~~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2008 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)
  //
  
  #ifndef HANDLER_ALLOCATOR_HPP
  #define HANDLER_ALLOCATOR_HPP
  
  #include "asio.hpp"
  #include <boost/aligned_storage.hpp>
  
  // 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);
  }
  
  #endif // HANDLER_ALLOCATOR_HPP
  