listing_7.21.cpp

template<typename T>
class lock_free_queue
{
private:
    void set_new_tail(counted_node_ptr &old_tail,
                      counted_node_ptr const &new_tail)
    {
        node* const current_tail_ptr=old_tail.ptr;
        while(!tail.compare_exchange_weak(old_tail,new_tail) &&
              old_tail.ptr==current_tail_ptr);
        if(old_tail.ptr==current_tail_ptr)
            free_external_counter(old_tail);
        else
            current_tail_ptr->release_ref();
    }
public:
    void push(T new_value)
    {
        std::unique_ptr<T> new_data(new T(new_value));
        counted_node_ptr new_next;
        new_next.ptr=new node;
        new_next.external_count=1;
        counted_node_ptr old_tail=tail.load();
        for(;;)
        {
            increase_external_count(tail,old_tail);
            T* old_data=nullptr;
            if(old_tail.ptr->data.compare_exchange_strong(
                   old_data,new_data.get()))
            {
                counted_node_ptr old_next={0};
                if(!old_tail.ptr->next.compare_exchange_strong(
                       old_next,new_next))
                {
                    delete new_next.ptr;
                    new_next=old_next;
                }
                set_new_tail(old_tail, new_next);
                new_data.release();
                break;
            }
            else
            {
                counted_node_ptr old_next={0};
                if(old_tail.ptr->next.compare_exchange_strong(
                       old_next,new_next))
                {
                    old_next=new_next;
                    new_next.ptr=new node;
                }
                set_new_tail(old_tail, old_next);
            }
        }
    }
};