boost::asio::async_read bind compilation error

Question

I can't figure out why I get this error :

/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

Here my class : Command.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;
          }
      }

    }
  }
}

Check for the "** Error pointing this line**" on the Code block, this is where it has an issue... not sure why, broke my head again and again...

Thank for the help

I have removed some code for clarity purpose, if you have any question, don't hesitate

Answer 1

Your completion handler signature is not correct, consider this example

#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 is smart enough to not pass the error or bytes_transferred parameters to your bound function pointer. The author of the Asio library has a detailed blog post about using bind with the library. It is worth the read.

Answer 2

The async_* operations requires a different signature for the callback function:

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

Please have a deeper look at the documentation for some examples how to write and invoke such a callback handler.