#include <atomic>
#include <memory>

template <typename T>
class lock_free_stack {
private:
  struct node {
    std::shared_ptr<T> data;
    node* next;
    node(T const& data_) : data(std::make_shared<T>(data_)) {}
  };
  std::atomic<node*> head;

public:
  void push(T const& data)
  {
    node* const new_node = new node(data);
    new_node->next       = head.load();
    while (!head.compare_exchange_weak(new_node->next, new_node))
      ;
  }
  std::shared_ptr<T> pop()
  {
    node* old_head = head.load();
    while (old_head && !head.compare_exchange_weak(old_head, old_head->next))
      ;
    return old_head ? old_head->data : std::shared_ptr<T>();
  }
};