throw my_exception() << "The error " << error_code << " happened because of " << reasons;

class my_exception
{
   public:
      template <typename T>
      my_exception & operator << (const T & value)
      {
         // put the value somewhere
         return *this;
      }
      // etc.
};

std::stringstream s;
s << "The error " << error_code << " happened because of " << reasons;
throw my_exception(s.str());