The main() function

主函數

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.

我們需要創建一個服務器對象，用來接受客戶端的連接。boost::asio::io_service對象提供了像sockets這樣的I/O服務，這些服務都是服務器對象將要使用的。

    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.

運行boost::asio::io_service 對象，它將執行你想要的異步操作。

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

  return 0;
}

The tcp_server class

TCP服務器類

class tcp_server
{
public:

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

構造函數初始化一個用于監聽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.

函數start_accept()創建一個socket ，同時啟動一個異步接收操作去等待一個新的連接。

  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.

當start_accept()啟動的異步接收操作完成后，handle_accept()函數將被調用。它響應客戶端的請求，然后調用start_accept()函數去啟動另一個接收操作。

  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連接類

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.

我們希望只要還有一個操作涉及tcp_connection對象，該對象就是有效的。因此我們使用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()函數中，我們調用boost::asio::async_write()為客戶端處理數據。注意：為了確保數據被整塊發送，我們使用的是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.

要發送的數據保存在類成員變量message_ 中，在異步操作完成前我們需要保證數據的有效性。

 

    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().

當啟動一個異步操作時，如果使用boost::bind()，你只需要指定一個符合句柄參數列表簽名的參數。在本例中，任何一個參數占位符（boost::asio::placeholders::error 和boost::asio::placeholders::bytes_transferred）皆可被隱式地移除，因為操作write()并沒有使用它們。

    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

移除無用的操作參數

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:

你可能已經注意到了：error和bytes_transferred 參數并沒有在 handle_write() 函數體內被應用。因此，如果參數并不是必須的，我們可以移除它們，如下所示：

  void handle_write()
  {
  }

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

用來發起呼叫的boost::asio::async_write()函數通常可以被改寫成下面這樣：

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

See the full source listing

全部源碼：