cons lists in C++

1_ConsNode.cpp

template<typename T>
struct ConsNode {
public:
  ConsNode(T car=T(), ConsList<T> cdr=ConsList<T>()):_car(car),_cdr(cdr) {}

private:
  T _car;
  ConsList<T> _cdr;
  friend T car<>(const ConsList<T> &l);
  friend const ConsList<T>& cdr<>(const ConsList<T> &l);	
};

2_ConsList.cpp

template <typename T>
using ConsList= std::shared_ptr<ConsNode <T> >;

3_cons.cpp

template<typename T>
ConsList<T> cons(T car, const ConsList<T>& cdr=ConsList<T>()) {
  return std::make_shared<ConsNode<T> > (car,cdr);
}

car_cdr_isEmpty.cpp

template<typename T>
T car(const ConsList<T>&l)
{
  return l->_car;
}

template<typename T>
const ConsList<T>& cdr(const ConsList<T>&l)
{
  return l->_cdr;
}

template<class T>
bool isEmpty(const ConsList<T>&l)
{
  return !l;
}

main.cpp

int main(void)
{
  ConsList<int> l=cons(3,cons(4,cons(7)));
  ConsList<int> l2=cons(9,cons(5,l)); // l2 shares the last 3 elements with l
  ConsList<int> l3=cons(11,cdr(l2)); // l3 shares with l2, and also with l
  cout << car(l) << " " << car(cdr(l)) << endl;
  cout << len(l2) << " " << sum(l2) << endl;
  cout << l2 << endl;
  cout << l3 << endl;
}

operator.cpp

template<class T>
std::ostream& operator<<(std::ostream& o, const ConsList<T>&l)
{
  if(!isEmpty(l))
    o << car(l) << " " << cdr(l);
  return o;
}

operator<<.cpp

template<class T>
std::ostream& operator<<(std::ostream& o, const ConsList<T>&l)
{
  if(l)
    o << car(l) << " " << cdr(l);
  return o;
}

other_functions.cpp

template<typename T>
unsigned len(const ConsList<T>&l)
{
  if(!isEmpty(l))
    return 0;
  return 1+len(cdr(l));
}

template<typename T>
T sum(const ConsList<T>& l)
{
  if(isEmpty(l))
    return 0;
  return car(l)+sum(cdr(l));
}