boost::asio::async_read 绑定编译错误

Question

我不知道为什么会出现这个错误：

/usr/local/include/boost/asio/impl/read.hpp: In member function ‘void boost::asio::detail::read_op<AsyncReadStream, boost::asio::mutable_buffers_1, CompletionCondition, ReadHandler>::operator()(const boost::system::error_code&, size_t, int) [with AsyncReadStream = boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, CompletionCondition = boost::asio::detail::transfer_at_least_t, ReadHandler = boost::function<void ()(long unsigned int)>]’:
/usr/local/include/boost/asio/impl/read.hpp:263:   instantiated from ‘void boost::asio::async_read(AsyncReadStream&, const MutableBufferSequence&, CompletionCondition, ReadHandler) [with AsyncReadStream = boost::asio::basic_stream_socket<boost::asio::ip::tcp, boost::asio::stream_socket_service<boost::asio::ip::tcp> >, MutableBufferSequence = boost::asio::mutable_buffers_1, CompletionCondition = boost::asio::detail::transfer_at_least_t, ReadHandler = boost::function<void ()(long unsigned int)>]’
src/communicator/protocol/Command.cc:34:   instantiated from here
/usr/local/include/boost/asio/impl/read.hpp:215: error: no match for call to ‘(boost::function<void ()(long unsigned int)>) (const boost::system::error_code&, const long unsigned int&)’
/usr/local/include/boost/function/function_template.hpp:1007: note: candidates are: typename boost::function1<R, T1>::result_type boost::function1<R, T1>::operator()(T0) const [with R = void, T0 = long unsigned int]
make: *** [src/communicator/protocol/Command.o] Error 1

这是我的课： 命令.hh

namespace communicator {                                              
  namespace protocol {                                                
    namespace in {                                                    
      class Command : public boost::enable_shared_from_this<Command> {
      public:                                                         
        ~Command();                                                   

        typedef boost::shared_ptr<Command>      pointer;              

        void got_newline();                                           

      protected:                                                      
        Command(tcp::socket& socket, structure::Client& client) :     
          m_socket(socket), m_client(client)  {};

        void endParsing();

        tcp::socket&            m_socket;                             

        structure::Client&      m_client;                             
        char                    m_newline[2];                          
      private:                             

      };                                                              
    }
}   

Command.cc：

namespace   communicator {
    namespace protocol {
    namespace in {

      void Command::endParsing()  {
        boost::function<void()> cb = boost::bind(&Command::got_newline,
                                          shared_from_this());
        boost::asio::async_read(m_socket,
                                boost::asio::buffer(m_newline, 2),
                                boost::asio::transfer_at_least(2),
**ERROR POINTING THIS LINE**                                    cb);
      }

      void Command::got_newline()  {
        if (m_newline[0] == '\r' && m_newline[1] == '\n') {
           std::cout << "End" << std::endl;
          }
      }

    }
  }
}

检查代码块上的“** Error pointing this line**”，这是它有问题的地方......不知道为什么，一次又一次地打破我的头......

感谢您的帮助

为了清楚起见，我删除了一些代码，如果您有任何问题，请不要犹豫

Answer 1

您的完成处理程序签名不正确，请考虑此示例

#include <boost/asio.hpp>

#include <boost/function.hpp>
#include <boost/bind.hpp>

void
foo()
{

}

int
main()
{
    boost::asio::io_service io_service;
    boost::asio::ip::tcp::socket socket( io_service );

    char buf[2];

    // this compiles file
    boost::asio::async_read(
            socket,
            boost::asio::buffer(buf),
            boost::asio::transfer_at_least(2),
            boost::bind( &foo )
            );

    // this does not
    boost::function<void()> cb = boost::bind( &foo );
    boost::asio::async_read(
            socket,
            boost::asio::buffer(buf),
            boost::asio::transfer_at_least(2),
            cb
            );

}

boost::bind 足够聪明，不会将error 或bytes_transferred 参数传递给绑定的函数指针。 Asio 库的作者有一个detailed blog post 关于在库中使用绑定。值得一读。

Answer 2

async_* 操作需要不同的回调函数签名：

void handler(
  const boost::system::error_code& error, // Result of operation.
  std::size_t bytes_transferred           // Number of bytes read.
); 

请更深入地查看文档，了解如何编写和调用此类回调处理程序的一些示例。