#include <memory>
template <typename T>
class queue {
private:
  struct node {
    std::shared_ptr<T> data;
    std::unique_ptr<node> next;
  };

  std::unique_ptr<node> head;
  node* tail;

public:
  queue() : head(new node), tail(head.get()) {}

  queue(const queue& other) = delete;
  queue& operator=(const queue& other) = delete;

  std::shared_ptr<T> try_pop()
  {
    if (head.get() == tail) {
      return std::shared_ptr<T>();
    }
    std::shared_ptr<T> const res(head->data);
    std::unique_ptr<node> const old_head = std::move(head);
    head                                 = std::move(old_head->next);
    return res;
  }

  void push(T new_value)
  {
    std::shared_ptr<T> new_data(std::make_shared<T>(std::move(new_value)));
    std::unique_ptr<node> p(new node);
    tail->data           = new_data;
    node* const new_tail = p.get();
    tail->next           = std::move(p);
    tail                 = new_tail;
  }
};