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http://www.boost.org/doc/libs/1_36_0/doc/html/boost_asio/example/echo/async_tcp_echo_server.cpp

从代码中我们可以看到，首先是session类的定义，session表示一个会话，可以代表某一个客户端socket对象，而server对象表示为所有session提供服务，最后在main里面定义server对象，并传入io_server参数，启动server.

// // async_tcp_echo_server.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) // #include <cstdlib> #include <iostream> //包含Boost 函数绑定 #include <boost/bind.hpp> //包含Boost ASIO库头文件 #include <boost/asio.hpp> //使用命名空间boost::asio::ip::tcp using boost::asio::ip::tcp; //定义Session类表示每个客户端连接信息 class session { public: //构造函数，使用传入的io_service初始化socket_对象   session(boost::asio::io_service& io_service)     : socket_(io_service)   {   } //获取socket对象   tcp::socket& socket()   {     return socket_;   } //启动处理   void start()   { //调用socket_的异步读取数据函数async_read_some，将数据读入data_数组中     socket_.async_read_some(boost::asio::buffer(data_, max_length), //调用boost bind将回调函数绑定为session::handle_read         boost::bind(&session::handle_read, this,           boost::asio::placeholders::error,           boost::asio::placeholders::bytes_transferred));   } //处理数据函数handle_read   void handle_read(const boost::system::error_code& error,       size_t bytes_transferred)   { //如果非error 进行处理     if (!error)     { //异步将data_数组数据，长度为bytes_transferred写入到socket_中，并绑定函数session::handle_write       boost::asio::async_write(socket_,           boost::asio::buffer(data_, bytes_transferred),           boost::bind(&session::handle_write, this,             boost::asio::placeholders::error));     } //否则清除资源     else     {       delete this;     }   } //函数handle_write   void handle_write(const boost::system::error_code& error)   {     if (!error)     { //继续读取数据       socket_.async_read_some(boost::asio::buffer(data_, max_length),           boost::bind(&session::handle_read, this,             boost::asio::placeholders::error,             boost::asio::placeholders::bytes_transferred));     }     else     {       delete this;     }   } private: //socket对象   tcp::socket socket_;   enum { max_length = 1024 }; //存储数据空间数组   char data_[max_length]; }; //定义server对象 class server { public: //构造函数  通过传入的io_service初始化内部对象io_service_ //并初始化acceptor_对象   server(boost::asio::io_service& io_service, short port)     : io_service_(io_service),       acceptor_(io_service, tcp::endpoint(tcp::v4(), port))   { //定义新的session对象     session* new_session = new session(io_service_); //调用acceptor_异步连接该session,并绑定处理accept函数handle_accept     acceptor_.async_accept(new_session->socket(),         boost::bind(&server::handle_accept, this, new_session,           boost::asio::placeholders::error));   } //处理accept函数   void handle_accept(session* new_session,       const boost::system::error_code& error)   {     if (!error)     { //调用new_session的启动函数       new_session->start(); //初始化new_session对象       new_session = new session(io_service_);       acceptor_.async_accept(new_session->socket(),           boost::bind(&server::handle_accept, this, new_session,             boost::asio::placeholders::error));     }     else     {       delete new_session;     }   } private: //定义io_service对象   boost::asio::io_service& io_service_; //定义acceptor对象   tcp::acceptor acceptor_; }; int main(int argc, char* argv[]) {   try   {     if (argc != 2)     {       std::cerr << "Usage: async_tcp_echo_server <port>\n";       return 1;     } //定义io_service     boost::asio::io_service io_service;     using namespace std; // For atoi. //定义server 对象s     server s(io_service, atoi(argv[1])); //启动     io_service.run();   }   catch (std::exception& e)   {     std::cerr << "Exception: " << e.what() << "\n";   }   return 0; }