c++ - 截止时间计时器到期，现在怎么办？

Question

我正在查看 http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/example/timeouts/async_tcp_client.cpp 中的 asio 示例

这是我真正难以理解的地方:

1. 为什么 handle_read 会再次回调 start_read？
2. 计时器到期时会发生什么？我看到没有提供给计时器的回调例程。

void start_read()
{ // Set a deadline for the read operation. deadline_.expires_from_now(boost::posix_time::seconds(30));

// Start an asynchronous operation to read a newline-delimited message.
boost::asio::async_read_until(socket_, input_buffer_, '\n',
    boost::bind(&client::handle_read, this, _1));   

}

void handle_read(const boost::system::error_code& ec)
{ if (stopped_) return;

if (!ec)
{
  // Extract the newline-delimited message from the buffer.
  std::string line;
  std::istream is(&input_buffer_);
  std::getline(is, line);

  // Empty messages are heartbeats and so ignored.
  if (!line.empty())
  {
    std::cout << "Received: " << line << "\n";
  }

  start_read();
}
else
{
  std::cout << "Error on receive: " << ec.message() << "\n";

  stop();
}   

}

Answer 1

Why does handle_read call back start_read again?

如果没有，客户端只会读取套接字一次，然后再也不会读取。因此，在成功读取后，客户端希望再次尝试读取。这有助于永久读取套接字。

What happens when the timer expires? I see no callback routine provided to the timer.

代码在源文件的顶部:

deadline_.async_wait(boost::bind(&client::check_deadline, this));

如果截止日期已过，check_deadline() 函数将关闭套接字。