用boost.asio链接异步lambdas

一种优雅的解决方案是使用coroutines。BOOST.ASIO支持两种无stackless coroutines，它引入了一小部分伪关键字和堆叠的Coroutines，它们使用Boost.Coroutine。

无固定的Coroutines

stackless coroutines引入了一组伪关键字的预处理器宏，该预处理宏使用类似于Duff设备的技术实现开关语句。该文档详细介绍了每个关键字。

原始问题（连接 ->读取标题 ->读取主体）在用无stack coroutines实现时看起来类似于以下内容：

struct session
 : boost::asio::coroutine
{
  boost::asio::ip::tcp::socket socket_;
  std::vector<char> buffer_;
  // ...
  void operator()(boost::system::error_code ec = boost::system::error_code(),
                  std::size_t length = 0)  
  {
    // In this example we keep the error handling code in one place by
    // hoisting it outside the coroutine. An alternative approach would be to
    // check the value of ec after each yield for an asynchronous operation.
    if (ec)
    {
      print_error(ec);
      return;
    }
    // On reentering a coroutine, control jumps to the location of the last
    // yield or fork. The argument to the "reenter" pseudo-keyword can be a
    // pointer or reference to an object of type coroutine.
    reenter (this)
    {
      // Asynchronously connect. When control resumes at the following line,
      // the error and length parameters reflect the result of
      // the asynchronous operation.
      yield socket_.async_connect(endpoint_, *this);
      // Loop until an error or shutdown occurs.
      while (!shutdown_)
      {
        // Read header data. When control resumes at the following line,
        // the error and length parameters reflect the result of
        // the asynchronous operation.
        buffer_.resize(fixed_header_size);
        yield socket_.async_read(boost::asio::buffer(buffer_), *this);
        // Received data.  Extract the size of the body from the header.
        std::size_t body_size = parse_header(buffer_, length);
        // If there is no body size, then leave coroutine, as an invalid
        // header was received.
        if (!body_size) return;
        // Read body data. When control resumes at the following line,
        // the error and length parameters reflect the result of
        // the asynchronous operation.
        buffer_.resize(body_size);
        yield socket_.async_read(boost::asio::buffer(buffer_), *this);
        // Invoke the user callback to handle the body.
        body_handler_(buffer_, length);
      }
      // Initiate graceful connection closure.
      socket_.shutdown(tcp::socket::shutdown_both, ec);
    } // end reenter
  }
}

堆叠的coroutines

使用spawn()函数创建堆叠式Coroutines。最初的问题可能看起来像以下内容，以备用coroutines实现：

boost::asio::spawn(io_service, [&](boost::asio::yield_context yield)
  {
    boost::system::error_code ec;
    boost::asio::ip::tcp::socket socket(io_service);
    // Asynchronously connect and suspend the coroutine.  The coroutine will
    // be resumed automatically when the operation completes.
    socket.async_connect(endpoint, yield[ec]);
    if (ec)
    {
      print_error(ec);
      return;
    }
    // Loop until an error or shutdown occurs.
    std::vector<char> buffer;
    while (!shutdown)
    {
      // Read header data.
      buffer.resize(fixed_header_size);
      std::size_t bytes_transferred = socket.async_read(
        boost::asio::buffer(buffer), yield[ec]);
      if (ec)
      {
        print_error(ec);
        return;
      }
      // Extract the size of the body from the header.
      std::size_t body_size = parse_header(buffer, bytes_transferred);
      // If there is no body size, then leave coroutine, as an invalid header
      // was received.
      if (!body_size) return;
      // Read body data.
      buffer.resize(body_size);
      bytes_transferred =
        socket.async_read(boost::asio::buffer(buffer), yield[ec]);
      if (ec)
      {
        print_error(ec);
        return;
      }
      // Invoke the user callback to handle the body.
      body_handler_(buffer, length);
    }
    // Initiate graceful connection closure.
    socket.shutdown(tcp::socket::shutdown_both, ec);
   });