#include <atomic>
#include <memory>

template<typename T>
class lock_free_stack
{
private:
    std::atomic<unsigned> threads_in_pop;
    void try_reclaim(node* old_head);
public:
    std::shared_ptr<T> pop()
    {
        ++threads_in_pop;
        node* old_head=head.load();
        while(old_head &&
              !head.compare_exchange_weak(old_head,old_head->next));
        std::shared_ptr<T> res;
        if(old_head)
        {
            res.swap(old_head->data);
        }
        try_reclaim(old_head);
        return res;
    }
};