需要解释才能理解一个引用的例子

Question

我正在查看的完整示例是：

#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，试图了解boost asio是如何工作的，有些地方我需要帮助。我的问题是：

1. 在这些行中：

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

   这个shared_const_buffer稍后将用于async_write，所以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？

2. 在这些行中：

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

   share_const_buffer有data_是一个共享指针，但不是它本身，buffer在async_write实际写入数据之前是如何有效的？

Answer 1

这个shared_const_buffer稍后将用于async_write()，所以我认为应该实现某种缓冲区，但我看不到任何继承签名那么定义begin()和end()就足够了吗？

shared_const_buffer类使用的缓冲区是其_data成员boost::shared_ptr<std::vector<char> >。向缓冲区公开迭代器就足以将其与async_write()一起使用。

share_const_buffer有data_是一个共享指针，但不是它自己，如何缓冲区在async_write()实际写入数据之前是否有效？

shared_const_buffer类实现了asio ConstBufferSequence类型需求

  // 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; }

因此，当调用async_write时，它被复制，文档明确地指出：

缓冲区

包含要写入的数据的一个或多个缓冲区。尽管可以根据需要复制缓冲区对象底层内存块由调用者保留，调用者必须保证它们在调用处理程序之前一直有效。

然而，底层数据不会被复制，因为它被保留在shared_ptr中。你可以看到这一点，可以通过喷洒一些调试语句来看到

--- 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

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

在另一个外壳中

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$ 

因此，实际缓冲区保持有效，直到最后一个shared_const_buffer超出范围并运行描述符