boost::asio::async_read_until problem

Question

I'm trying to modify the echo server example from boost asio and I'm running into problem when I try to use boost::asio::async_read_until. Here's the code:

    #include <cstdlib>
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>

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

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

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

  void start()
  {
    std::cout<<"starting"<<std::endl;  
  boost::asio::async_read_until(socket_, boost::asio::buffer(data_, max_length), ' ',
        boost::bind(&session::handle_read, this,
          boost::asio::placeholders::error,
          boost::asio::placeholders::bytes_transferred));
  }

  void handle_read(const boost::system::error_code& error,
      size_t bytes_transferred)
  {
    std::cout<<"handling read"<<std::endl;
    if (!error)
    {
      boost::asio::async_write(socket_,
          boost::asio::buffer(data_, bytes_transferred),
          boost::bind(&session::handle_write, this,
            boost::asio::placeholders::error));
    }
    else
    {
      delete this;
    }
  }

  void handle_write(const boost::system::error_code& error)
  {
    if (!error)
    {
    /*
      socket_.async_read_some(boost::asio::buffer(data_, max_length),
          boost::bind(&session::handle_read, this,
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));
   */ }
    else
    {
      delete this;
    }
  }

private:
  tcp::socket socket_;
  enum { max_length = 1024 };
  char data_[max_length];
};

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* 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* new_session,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_session->start();
      new_session = new session(io_service_);
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&server::handle_accept, this, new_session,
            boost::asio::placeholders::error));
    }
    else
    {
      delete new_session;
    }
  }

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

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 2)
    {
      std::cerr << "Usage: async_tcp_echo_server <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;
}

The problem is when I try to compile I get this weird error:

server.cpp: In member function ‘void session::start()’: server.cpp:27: error: no matching function for call to ‘async_read_until(boost::asio::basic_stream_socket >&, boost::asio::mutable_buffers_1, char, boost::_bi::bind_t, boost::_bi::list3, boost::arg<1> ()(), boost::arg<2> ()()> >)’

Can someone please explain what's going on? From what I can tell the arguments to async_read_until are correct.

Thanks!

Saki

Answer 1

The second argument of async_read_until should be a streambuf object into which the data will be read. To put it simple, you need to pass a boost::asio::streambuf by reference, not a boost::asio::buffer by value.

Donnenfeld

Absolute correct analysis from what I can tell without compiling – Backlog

Answer 2

There is no need to use streambufs. There are overloads that accept dynamic buffers. Dynamic is key since read_until implies the need for a buffer that can increase its size at the callee's discretion. Therefore, all you need is to replace the call to boost::asio::buffer with boost::asio::dynamic_buffer and call it on either std::string or std::vector<char>.

...

  void start()
  {
    std::cout<<"starting"<<std::endl;  
    boost::asio::async_read_until(
        socket_, boost::asio::dynamic_buffer(data_), ' ', 
        boost::bind(&session::handle_read, this,
                    boost::asio::placeholders::error,
                    boost::asio::placeholders::bytes_transferred));
  }

...

 private:
  tcp::socket socket_;
  std::string data_;
  // or
  //std::vector<char> data_;
};

I would also get rid of boost::bind since it's an overkill and use a lambda (use them as much as you can)

[this](const boost::system::error_code& error, size_t bytes_transferred) {
  handle_read(error, bytes_transferred);
}

Barrington

Finally I found my answer! Boost documentation showcases examples using raw std::string that DON'T compile, and this made the trick. – Snicker

The problem with using std::string or std::vector<char> instead of asio::streambuf is that they do not support efficient erasure from the start of the sequence (which is what you'll no doubt be doing in your handle_read() function). This means that under certain conditions they could end up doing a quadratic amount of copying. – Grimaud

Answer 3

const (in)correctness may also cause this type of error message.

Maki