implicit change of message type on callback function

edit

From a quick glance at the ROS API, I don't think there is a way to do it implicitly. I guess its more of a C++ function redefinition functionaility than a ROS functionaility.

You could create a class that inherits from the class outside ROS.

class ClassInsideROS: public ClassOutsideROS {
public:
    ClassInsideROS{
      // Parent Constructor goes here
      }

    void new_callback(std_msgs::String::ConstPtr msg)  {
        std::string msg_string;
        // Perform Type Conversion from std_msgs::String::ConstPtr to
        old_callback(msg_string)
     }
};

 void main() {
   // Register Callback
   ros::NodeHandle n;
   ClassInsideROSobj;
   ros::Subscriber sub = n.subscribe(topic, 1000, &ClassInsideROS::new_callback, &obj);
}

But if all you need is the member function and no other properties of the class, you could just create a function.

ClassOutsideROS obj_;

void new_callback(std_msgs::String::ConstPtr msg)  {  
   std::string msg_string;
   // Perform Type Conversion from std_msgs::String::ConstPtr to
   obj_.callback(msg_string) 
}

void main() {
  // Register Callback
   ros::NodeHandle n;
   InsideROS obj;
   ros::Subscriber sub = n.subscribe(topic, 1000, &ClassOutsideROS::new_callback, &obj);
}