#include <memory>

template <typename T>
class queue {
private:
  struct node {
    T data;
    std::unique_ptr<node> next;

    node(T data_) : data(std::move(data_)) {}
  };

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

public:
  queue() : tail(nullptr) {}

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

  std::shared_ptr<T> try_pop()
  {
    if (!head) {
      return std::shared_ptr<T>();
    }
    std::shared_ptr<T> const res(std::make_shared<T>(std::move(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::unique_ptr<node> p(new node(std::move(new_value)));
    node* const new_tail = p.get();
    if (tail) {
      tail->next = std::move(p);
    }
    else {
      head = std::move(p);
    }
    tail = new_tail;
  }
};