boost::ref and boost::asio completion handlers, pass-by-reference

Question

m_io_service.post(boost::ref(i));

I have this call in a section of code, the underlying type i is definetly a callable (because removing the boost::ref leads to pass by value, which works fine), however clang tells me that :

/opt/dev_64_swat/proto-rpc2/dependencies/boost/include/boost/asio/handler_invoke_hook.hpp:64:3: error: type 'boost::reference_wrapper<rubble::rpc::TcpFrontEndConnectionInvoker>' does not provide a call operator

How do I pass by reference, I have objects that outlive the asynchronous calls, and they would be more elegant (less boost::shared_ptr<..> 's as members) if I could pass them by reference.

-- edit --

I have grepped through the example directory of asio, and boost::ref is not demonstrated for completion handlers. So I guess I am out of luck here. Is there a reason why the handlers don't have version accepting ref's ?

-- edit 2 : what I Looks like(don't bother looking at this unless you are suspicious of the implementation). --

namespace rubble { namespace rpc {
  struct InProcessInvoker : public InvokerBase
  {
    struct notification_object_
    {
      typedef notification_object_ * ptr;

      notification_object_()
      {
        reset();
      }
      void reset()
      {
        ready = false;
      }
      bool ready;
      boost::mutex mutex;
      boost::condition_variable cond;
    };

    InProcessInvoker(BackEnd & b_in)
      : b(b_in),
        notification_object(new notification_object_())
    {
      b.connect(m_client_data);
    }

    ~InProcessInvoker()
    {
      if( m_client_data.unique() )
      {
        b.disconect(m_client_data);
        delete notification_object;
      }
    }

    bool is_useable()
    {
      return b.is_useable();
    }

    void reset()
    {
      notification_object->reset();
      m_client_data->request().Clear();
      m_client_data->response().Clear();
      m_client_data->error_code().clear();
      BOOST_ASSERT_MSG( m_client_data->is_rpc_active() == false,
        "THE FLAG THAT REPRESENTS ACTIVE "
        "RPC SHOULD NOT BE SET WHEN RESETING AN OBJECT FOR RPC");
    }

    void invoke()
    {
      b.invoke(*this);
    }

    void operator() ()
    {
      service->dispatch(*client_cookie,*m_client_data);
      b.end_rpc(m_client_data.get());

      boost::lock_guard<boost::mutex> lock(notification_object->mutex);
      notification_object->ready=true;
      notification_object->cond.notify_one();
    }

    void after_post()
    {
      boost::unique_lock<boost::mutex> lock(notification_object->mutex);
      if(!notification_object->ready)
        notification_object->cond.wait(lock);
    }

    notification_object_::ptr notification_object;
    BackEnd & b;
  };

} }

Answer 1

boost::ref doesn't provide an overload of operator(). Hence, the return cannot be used directly as a callback. There are 2 options:

1. C++03: Use boost::bind to wrap the ref, and it will do what you want

   m_io_service.post(boost::bind<ReturnType>(boost::ref(i)))

   Note that you have to specify the return type unless the original functor i has a typedef for result_type

2. C++11: Use std::ref instead, which does provide an operator() which passes through to the contained reference

   m_io_service.post(std::ref(i))

Answer 2

Seems boost::ref is not intended for such usage. boost::ref provides wrapper so it's questionable what would be more effective, to pass by value or by boost::ref, mostly depends on your callable object copy constructor. As a workaround you can use boost::bind:

m_io_service.post(boost::bind(&Callable::operator(), &i));