The main() function

主函数

int main()
{
  try
  {

We need to create a server object to accept incoming client connections. The boost::asio::io_service object provides I/O services, such as sockets, that the server object will use.

我们需要创建一个服务器对象，用来接受客户端的连接。boost::asio::io_service对象提供了像sockets这样的I/O服务，这些服务都是服务器对象将要使用的。

    boost::asio::io_service io_service;
    tcp_server server(io_service);

Run the boost::asio::io_service object so that it will perform asynchronous operations on your behalf.

运行boost::asio::io_service 对象，它将执行你想要的异步操作。

    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << e.what() << std::endl;
  }

  return 0;
}

The tcp_server class

TCP服务器类

class tcp_server
{
public:

The constructor initialises an acceptor to listen on TCP port 13.

构造函数初始化一个用于监听TCP 端口13的接收器。

  tcp_server(boost::asio::io_service& io_service)
    : acceptor_(io_service, tcp::endpoint(tcp::v4(), 13))
  {
    start_accept();
  }

private:

The function start_accept() creates a socket and initiates an asynchronous accept operation to wait for a new connection.

函数start_accept()创建一个socket ，同时启动一个异步接收操作去等待一个新的连接。

  void start_accept()
  {
    tcp_connection::pointer new_connection =
      tcp_connection::create(acceptor_.io_service());

    acceptor_.async_accept(new_connection->socket(),
        boost::bind(&tcp_server::handle_accept, this, new_connection,
          boost::asio::placeholders::error));
  }

The function handle_accept() is called when the asynchronous accept operation initiated by start_accept() finishes. It services the client request, and then calls start_accept() to initiate the next accept operation.

当start_accept()启动的异步接收操作完成后，handle_accept()函数将被调用。它响应客户端的请求，然后调用start_accept()函数去启动另一个接收操作。

  void handle_accept(tcp_connection::pointer new_connection,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_connection->start();
      start_accept();
    }
  }

The tcp_connection class

TCP连接类

We will use shared_ptr and enable_shared_from_this because we want to keep the tcp_connection object alive as long as there is an operation that refers to it.

我们希望只要还有一个操作涉及tcp_connection对象，该对象就是有效的。因此我们使用shared_ptr 和enable_shared_from_this。

class tcp_connection
  : public boost::enable_shared_from_this<tcp_connection>
{
public:
  typedef boost::shared_ptr<tcp_connection> pointer;

  static pointer create(boost::asio::io_service& io_service)
  {
    return pointer(new tcp_connection(io_service));
  }

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

In the function start(), we call boost::asio::async_write() to serve the data to the client. Note that we are using boost::asio::async_write(), rather than boost::asio::ip::tcp::socket::async_write_some(), to ensure that the entire block of data is sent.

在start()函数中，我们调用boost::asio::async_write()为客户端处理数据。注意：为了确保数据被整块发送，我们使用的是boost::asio::async_write()，而不是boost::asio::ip::tcp::socket::async_write_some()。

  void start()
  {

The data to be sent is stored in the class member message_ as we need to keep the data valid until the asynchronous operation is complete.

要发送的数据保存在类成员变量message_ 中，在异步操作完成前我们需要保证数据的有效性。

 

    message_ = make_daytime_string();

 

When initiating the asynchronous operation, and if using boost::bind(), you must specify only the arguments that match the handler's parameter list. In this program, both of the argument placeholders (boost::asio::placeholders::error and boost::asio::placeholders::bytes_transferred) could potentially have been removed, since they are not being used in handle_write().

当启动一个异步操作时，如果使用boost::bind()，你只需要指定一个符合句柄参数列表签名的参数。在本例中，任何一个参数占位符（boost::asio::placeholders::error 和boost::asio::placeholders::bytes_transferred）皆可被隐式地移除，因为操作write()并没有使用它们。

    boost::asio::async_write(socket_, boost::asio::buffer(message_),
        boost::bind(&tcp_connection::handle_write, shared_from_this(),
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));

Any further actions for this client connection are now the responsibility of handle_write().

  }

private:
  tcp_connection(boost::asio::io_service& io_service)
    : socket_(io_service)
  {
  }

  void handle_write(const boost::system::error_code& /*error*/,
      size_t /*bytes_transferred*/)
  {
  }

  tcp::socket socket_;
  std::string message_;
};

Removing unused handler parameters

移除无用的操作参数

You may have noticed that the error, and bytes_transferred parameters are not used in the body of the handle_write() function. If parameters are not needed, it is possible to remove them from the function so that it looks like:

你可能已经注意到了：error和bytes_transferred 参数并没有在 handle_write() 函数体内被应用。因此，如果参数并不是必须的，我们可以移除它们，如下所示：

  void handle_write()
  {
  }

The boost::asio::async_write() call used to initiate the call can then be changed to just:

用来发起呼叫的boost::asio::async_write()函数通常可以被改写成下面这样：

  boost::asio::async_write(socket_, boost::asio::buffer(message_),
      boost::bind(&tcp_connection::handle_write, shared_from_this()));

See the full source listing

全部源码：