How can I store a boost::bind object as a class member?

Question

I'm writing an application that uses boost::asio. Asio's async_receive (or async_read) is invariably shown using a boost::bind object given for callback:

boost::asio::async_read(socket_,
                        boost::asio::buffer(read_msg_.data(),
                                            chat_message::header_length),
                        boost::bind(&chat_session::handle_read_header,
                                    shared_from_this(),
                                    boost::asio::placeholders::error));

That's perfectly nice, but I'd like not to have to recreate the bind object after each call to the callback. Instead, I'd like to create the object, say, in the constructor of my class, and give it to async_receive.

The problem is, I don't know how to declare that object as a class member. All I know is auto, and it obviously won't work as a class member.

class Whatever
{
public:
    Whatever()
    {
        functor = boost::bind(&Whatever::Callback);
    }
private:
    void Callback()
    {
        boost::asio::async_read(socket_,
                        boost::asio::buffer(read_msg_.data(),
                                            chat_message::header_length),
                        functor);
    }

    ?? functor; // How do I declare this?
    ...
};

Note: This may very well be premature optimization, but I'd still like to know how to declare a bind object without auto.

Answer 1

Use boost::function:

class Whatever
{
public:
    Whatever()
    {
        functor = boost::bind(
            &chat_session::handle_read_header,
            shared_from_this(),
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred
        );
        boost::asio::async_read(
            socket_,
            boost::asio::buffer(
               read_msg_.data(),
               chat_message::header_length
            ),
            functor
        );
    }
private:
    boost::function<void(const error_code, const size_t)> functor;
};

... or something like that.