#include <memory>

using std::shared_ptr;
using std::make_shared;

class A {
 public:
  shared_ptr<A> next;
  shared_ptr<A> prev;
};

int main(int argc, char **argv) {
  shared_ptr<A> head = make_shared<A>();
  head->next = make_shared<A>();
  head->next->prev = head;

  return 0;
}