Can i copy the libconfig::Setting from one libconfig::Config to another libconfig::Config?

Question

I have a config ("/home/work/father.config") looks like:

father_config = {
  child_config = "/home/work/child.config";
  father_int = 1;
};

in /home/work/child.config:

child_config = {
  child_int =2;
};

what i want is to get father_int from child_cfg.

please see the code:

  #include <iostream>
  #include <libconfig.h++>
  
  using namespace std;
  
  int main() {
    libconfig::Config father_cfg;
    father_cfg.readFile("./father.cfg");
    const libconfig::Setting& father_setting = father_cfg.lookup("father_setting");
    const std::string& child_path = father_setting.lookup("child_path");
    libconfig::Config child_cfg;
    child_cfg.readFile(child_path);
    libconfig::Setting& child_setting = child_cfg.lookup("child_setting");
    auto & s = child_cfg.getRoot().add("father_int", libconfig::Setting::TypeGroup);// father_setting.lookup("father_int");
    s = father_setting;  // !!!!!!! this doesn't work
    int father_int = child_setting.lookup("father_int");  // the ulimate purpose is to make father int can be found in child_cfg
    cout << father_int;
  }

I know if the father_int is type int and the key-father_int wont change, i can use:

int father_int = father_setting.lookup("father_int");
child_cfg.GetRoot().add("father_int", libconfig::Setting::TypeInt) = father_int;

but i want this can be more flexible, which need to add libconfig::Setting::TypeGroup instead of libconfig::Setting::TypeInt,

Can you help on this?