listing_6.9.cpp

template<typename T>
class threadsafe_queue
{
private:
    node* get_tail()
    {
        std::lock_guard<std::mutex> tail_lock(tail_mutex);
        return tail;
    }

    std::unique_ptr<node> pop_head()
    {
        std::unique_ptr<node> const old_head=std::move(head);
        head=std::move(old_head->next);
        return old_head;
    }

    std::unique_lock<std::mutex> wait_for_data()
    {
        std::unique_lock<std::mutex> head_lock(head_mutex);
        data_cond.wait(head_lock,[&]{return head!=get_tail();});
        return std::move(head_lock);
    }

    std::unique_ptr<node> wait_pop_head()
    {
        std::unique_lock<std::mutex> head_lock(wait_for_data());
        return pop_head();
    }

    std::unique_ptr<node> wait_pop_head(T& value)
    {
        std::unique_lock<std::mutex> head_lock(wait_for_data());
        value=std::move(*head->data);
        return pop_head();
    }
        
public:
    std::shared_ptr<T> wait_and_pop()
    {
        std::unique_ptr<node> const old_head=wait_pop_head();
        return old_head->data;
    }

    void wait_and_pop(T& value)
    {
        std::unique_ptr<node> const old_head=wait_pop_head(value);
    }
};