get ConstPtr for msg field

I have a callback function defined as on_string_msg(const std_msgs::String::ConstPtr& msg). I have both std_msgs::String and a self-defined msg which wraps a std_msgs::String field. myMsg.msg { std_msgs/String subField; } How can I get std_msgs::String::ConstPtr for subField in the myMsg callback function on_myMsg(const myMsg::ConstPtr& msg) so that I can reuse the function on_string_msg(const std_msgs::String::ConstPtr& msg) with the subField? Basically, I would like to do on_myMsg(const myMsg::ConstPtr& msg) { on_string_msg(msg->subFieldConstPtr); // How to achieve this? }