c++ boost: how to implement session per thread

I want to implement there is one thread for each connected session in a server. However, the handle accept callback is called in the same thread.I am not quite familiar the threading model in asio. Is there website mentioned about it?

Edit: I try to write a chat room server based on the example from boost. as you can see the same thread is used to handle the connection request. So I don't know how to make it into separated thread.

output:

[00410190] main Thread Start

[00410860] Thread Start

[00410898] Thread Start

[00410190] start accept

[00410898] handle accept

[00410898] start accept

[00410898] handle accept

[00410898] start accept

code:

#include "stdafx.h"
#include <algorithm>
#include <cstdlib>
#include <deque>
#include <iostream>
#include <list>
#include <set>
#include <boost/bind.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/asio.hpp>
#include "chat_message.h"
#include <boost/thread.hpp>
#include <boost/thread/mutex.hpp>

using boost::asio::ip::tcp;

//----------------------------------------------------------------------

typedef std::deque<chat_message> ch开发者_如何学运维at_message_queue;
boost::mutex global_stream_lock;
//----------------------------------------------------------------------

class chat_participant
{
public:
  virtual ~chat_participant() {}
  virtual void deliver(const chat_message& msg) = 0;
};

typedef boost::shared_ptr<chat_participant> chat_participant_ptr;

//----------------------------------------------------------------------

class chat_room
{
public:
  void join(chat_participant_ptr participant)
  {
    participants_.insert(participant);
    std::for_each(recent_msgs_.begin(), recent_msgs_.end(),
        boost::bind(&chat_participant::deliver, participant, _1));
  }

  void leave(chat_participant_ptr participant)
  {
    participants_.erase(participant);
  }

  void deliver(const chat_message& msg)
  {
    recent_msgs_.push_back(msg);
    while (recent_msgs_.size() > max_recent_msgs)
      recent_msgs_.pop_front();

    std::for_each(participants_.begin(), participants_.end(),
        boost::bind(&chat_participant::deliver, _1, boost::ref(msg)));
  }

private:
  std::set<chat_participant_ptr> participants_;
  enum { max_recent_msgs = 100 };
  chat_message_queue recent_msgs_;
};

//----------------------------------------------------------------------

class chat_session
  : public chat_participant,
    public boost::enable_shared_from_this<chat_session>
{
public:
  chat_session(boost::asio::io_service& io_service, chat_room& room)
    : socket_(io_service),
      room_(room)
  {
  }

  tcp::socket& socket()
  {
    return socket_;
  }

  void start()
  {
    room_.join(shared_from_this());
    boost::asio::async_read(socket_,
        boost::asio::buffer(read_msg_.data(), chat_message::header_length),
        boost::bind(
          &chat_session::handle_read_header, shared_from_this(),
          boost::asio::placeholders::error));
  }

  void deliver(const chat_message& msg)
  {
    bool write_in_progress = !write_msgs_.empty();
    write_msgs_.push_back(msg);
    if (!write_in_progress)
    {
      boost::asio::async_write(socket_,
          boost::asio::buffer(write_msgs_.front().data(),
            write_msgs_.front().length()),
          boost::bind(&chat_session::handle_write, shared_from_this(),
            boost::asio::placeholders::error));
    }
  }

  void handle_read_header(const boost::system::error_code& error)
  {
    if (!error && read_msg_.decode_header())
    {
      boost::asio::async_read(socket_,
          boost::asio::buffer(read_msg_.body(), read_msg_.body_length()),
          boost::bind(&chat_session::handle_read_body, shared_from_this(),
            boost::asio::placeholders::error));
    }
    else
    {
      room_.leave(shared_from_this());
    }
  }

  void handle_read_body(const boost::system::error_code& error)
  {
    if (!error)
    {
      room_.deliver(read_msg_);
      std::string s(read_msg_.data(), read_msg_.length());
      std::cout << s<< std::endl;
      boost::asio::async_read(socket_,
          boost::asio::buffer(read_msg_.data(), chat_message::header_length),
          boost::bind(&chat_session::handle_read_header, shared_from_this(),
            boost::asio::placeholders::error));
    }
    else
    {
      room_.leave(shared_from_this());
    }
  }

  void handle_write(const boost::system::error_code& error)
  {
    if (!error)
    {
      write_msgs_.pop_front();
      if (!write_msgs_.empty())
      {
        boost::asio::async_write(socket_,
            boost::asio::buffer(write_msgs_.front().data(),
              write_msgs_.front().length()),
            boost::bind(&chat_session::handle_write, shared_from_this(),
              boost::asio::placeholders::error));
      }
    }
    else
    {
      room_.leave(shared_from_this());
    }
  }

private:
  tcp::socket socket_;
  chat_room& room_;
  chat_message read_msg_;
  chat_message_queue write_msgs_;
};

typedef boost::shared_ptr<chat_session> chat_session_ptr;

//----------------------------------------------------------------------

class chat_server
{
public:
  chat_server(boost::asio::io_service& io_service,
      const tcp::endpoint& endpoint)
    : io_service_(io_service),
      acceptor_(io_service, endpoint)
  {
    start_accept();
  }

  void start_accept()
  {
       global_stream_lock.lock();
        std::cout << "[" << boost::this_thread::get_id()
                << "] start accept" << std::endl;
        global_stream_lock.unlock();
    chat_session_ptr new_session(new chat_session(io_service_, room_));
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&chat_server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

  void handle_accept(chat_session_ptr session,
      const boost::system::error_code& error)
  {
      global_stream_lock.lock();
        std::cout << "[" << boost::this_thread::get_id()
                << "] handle accept" << std::endl;
        global_stream_lock.unlock();

    if (!error)
    {
      session->start();
    }

    start_accept();
  }

private:
  boost::asio::io_service& io_service_;
  tcp::acceptor acceptor_;
  chat_room room_;
};

typedef boost::shared_ptr<chat_server> chat_server_ptr;
typedef std::list<chat_server_ptr> chat_server_list;


void WorkerThread( boost::shared_ptr< boost::asio::io_service > io_service )
{
        global_stream_lock.lock();
        std::cout << "[" << boost::this_thread::get_id()
                << "] Thread Start" << std::endl;
        global_stream_lock.unlock();

        io_service->run();

        global_stream_lock.lock();
        std::cout << "[" << boost::this_thread::get_id()
                << "] Thread Finish" << std::endl;
        global_stream_lock.unlock();
}

//----------------------------------------------------------------------

int _tmain(int argc, char* argv[])
{
  try
  {
    if (argc < 2)
    {
      std::cerr << "Usage: chat_server <port> [<port> ...]\n";
      return 1;
    }

    boost::shared_ptr< boost::asio::io_service > io_service( 
                new boost::asio::io_service
        );

    boost::shared_ptr< boost::asio::io_service::work > work(
                new boost::asio::io_service::work( *io_service )
        );

    //boost::asio::io_service::strand strand( *io_service );

    global_stream_lock.lock();
    std::cout << "[" << boost::this_thread::get_id()
             << "] main Thread Start" << std::endl;
    global_stream_lock.unlock();

    boost::thread_group worker_threads;
    for( int x = 0; x < 2; ++x )
    {
        worker_threads.create_thread( boost::bind( &WorkerThread,
                                                   io_service ) );
    }

    chat_server_list servers;
    for (int i = 1; i < argc; ++i)
        std::cout<<argv[i] << std::endl;

    for (int i = 1; i < argc; ++i)
    {
      using namespace std; // For atoi.
      tcp::endpoint endpoint(tcp::v4(), atoi(argv[i]));
      chat_server_ptr server(new chat_server(*io_service, endpoint));
      servers.push_back(server);
    }

    work.reset();

    worker_threads.join_all();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}

This boost asio example shows an HTTP server using a single io_service and a thread pool to handle incoming sessions. It might help.