需要解释以理解asio REFERENCE_COUNTED示例

Question

我正在全面查看的示例是：

我正在全面查看的示例是：

#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/shared_ptr.hpp>
#include <iostream>
#include <vector>

using boost::asio::ip::tcp;

// A reference-counted non-modifiable buffer class.
class shared_const_buffer
{
public:
  // Construct from a std::string.
  explicit shared_const_buffer(const std::string& data)
    : data_(new std::vector<char>(data.begin(), data.end())),
      buffer_(boost::asio::buffer(*data_))
  {
  }
  // Implement the ConstBufferSequence requirements.
  typedef boost::asio::const_buffer value_type;
  typedef const boost::asio::const_buffer* const_iterator;
  const boost::asio::const_buffer* begin() const { return &buffer_; }
  const boost::asio::const_buffer* end() const { return &buffer_ + 1; }

private:
  boost::shared_ptr<std::vector<char> > data_;
  boost::asio::const_buffer buffer_;
};

class session
  : public boost::enable_shared_from_this<session>
{
public:
  session(boost::asio::io_service& io_service)
    : socket_(io_service)
  {
  }

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

  void start()
  {
    using namespace std; // For time_t, time and ctime.
    time_t now = time(0);
    shared_const_buffer buffer(ctime(&now));
    boost::asio::async_write(socket_, buffer,
        boost::bind(&session::handle_write, shared_from_this()));
  }

  void handle_write()
  {
  }

private:
  // The socket used to communicate with the client.
  tcp::socket socket_;
};

typedef boost::shared_ptr<session> session_ptr;

class server
{
public:
  server(boost::asio::io_service& io_service, short port)
    : io_service_(io_service),
      acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
  {
    session_ptr new_session(new session(io_service_));
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

  void handle_accept(session_ptr new_session,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_session->start();
    }

    new_session.reset(new session(io_service_));
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

private:
  boost::asio::io_service& io_service_;
  tcp::acceptor acceptor_;
};

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 2)
    {
      std::cerr << "Usage: reference_counted <port>\n";
      return 1;
    }

    boost::asio::io_service io_service;

    using namespace std; // For atoi.
    server s(io_service, atoi(argv[1]));

    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}

我是一名Java程序员，正在尝试理解boost asio的工作原理，有些问题需要帮助。我的问题如下：

1. In these lines:

   const boost::asio::const_buffer* begin() const { return &buffer_; }
   const boost::asio::const_buffer* end() const { return &buffer_ + 1; }
   

   this shared_const_buffer is use to async_write later, so I think it should implement some kind of buffer but I don't see any inherit signature. So define begin() and end() are enough?

2. And in these lines:

   shared_const_buffer buffer(ctime(&now));
   boost::asio::async_write(socket_, buffer, 
                            boost::bind(&session::handle_write, 
                                        shared_from_this()));    
   

   share_const_buffer has data_ is a shared pointer, but not itself, how is buffer valid until async_write actually writes the data?

- secmask

Answer 1

这个shared_const_buffer被用于稍后的async_write()，因此我认为它应该实现某种缓冲区，但我没有看到任何继承签名。那么定义begin()和end()就足够了吗？ shared_const_buffer类使用其_data成员作为缓冲区，它是一个boost::shared_ptr<std::vector<char> >。向缓冲区公开迭代器已足以与async_write()一起使用。 share_const_buffer有一个data_共享指针，但本身不是，那么在async_write()实际写入数据之前，缓冲区如何有效？ shared_const_buffer类实现了asio ConstBufferSequence类型要求。

  // Implement the ConstBufferSequence requirements.
  typedef boost::asio::const_buffer value_type;
  typedef const boost::asio::const_buffer* const_iterator;
  const boost::asio::const_buffer* begin() const { return &buffer_; }
  const boost::asio::const_buffer* end() const { return &buffer_ + 1; }

在调用 async_write 时，其实是被复制的。这个文档明确指出了：

buffers

包含要写入数据的一个或多个缓冲区。 虽然会根据需要复制缓冲区对象，但底层内存块的所有权仍由调用方保留，必须保证它们在处理程序被调用之前保持有效。

然而，底层数据并没有被复制，因为它在 shared_ptr 中保留。您可以通过添加一些调试语句来查看此情况。

--- reference_counted.cpp   2012-02-19 08:30:32.000000000 -0600
+++ reference_counted_good.cpp  2012-02-19 08:26:27.000000000 -0600
@@ -26,9 +26,7 @@
     : data_(new std::vector<char>(data.begin(), data.end())),
       buffer_(boost::asio::buffer(*data_))
   {
-      std::cout << "shared_const_buffer()" << std::endl;
   }
-  ~shared_const_buffer() { std::cout << "~shared_const_buffer() buffer use count: " << data_.use_count() << std::endl; }

   // Implement the ConstBufferSequence requirements.
   typedef boost::asio::const_buffer value_type;
@@ -66,7 +64,6 @@

   void handle_write()
   {
-      std::cout << "handle_write" << std::endl;
   }

 private:

并运行它

Sam-Millers-MacBook-Pro:stackoverflow samm$ ./a.out 1234
shared_const_buffer()
~shared_const_buffer() buffer use count: 8
~shared_const_buffer() buffer use count: 7
~shared_const_buffer() buffer use count: 6
~shared_const_buffer() buffer use count: 5
~shared_const_buffer() buffer use count: 4
~shared_const_buffer() buffer use count: 3
~shared_const_buffer() buffer use count: 3
~shared_const_buffer() buffer use count: 2
handle_write
~shared_const_buffer() buffer use count: 2
~shared_const_buffer() buffer use count: 1

在另一个终端窗口中。

Sam-Millers-MacBook-Pro:stackoverflow samm$ telnet localhost 1234
Trying ::1...
telnet: connect to address ::1: Connection refused
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Sun Feb 19 08:22:56 2012
Connection closed by foreign host.
Sam-Millers-MacBook-Pro:stackoverflow samm$ 

所以实际缓冲区会一直有效，直到最后一个shared_const_buffer超出范围并运行描述符。

- Sam Miller

---

在Java世界中，shared_const_buffer需要实现一种缓冲区，以便可以将其转换为基本类型，并调用接口方法来获取缓冲区。在C++中，shared_const_buffer是如何工作的？ - secmask

我创建了另一个测试，C++ 不需要类型检查，这就是为什么 shared_const_buffer 不需要显式实现接口的原因。 - secmask

@secmask 我不建议以Java的方式来处理C++编程项目。 - Sam Miller