Hi,
     I see in the code some functions are passed
things like boost::asio::placeholders::bytes_transferred.
But I didn't understand what is going on, what are their aim?

I found they are defined in placeholders.hpp as

boost::arg<2>& bytes_transferred
    = boost::asio::placeholders::detail::placeholder<2>::get();

//and the get function as
template <int Number>
struct placeholder
{
    static boost::arg<Number>& get()
    {
      static boost::arg<Number> result;
      return result;
    }
};

TIA,
Best regards,
Ozgur (Oscar) Ozturk
www.DrOzgur.com
+1 (614) 805-4370