Using std::move on a dereferenced shared_ptr in a concurrent queue

Question

Below is the code for a thread-safe queue in Anthony Williams' book C++ concurrency in action that I gathered in a .h file.

#ifndef THREADSAFE_QUEUE_H
#define THREADSAFE_QUEUE_H

// Anthony Williams' fine-grained lock-based thread-safe queue.

#include <mutex>                // for std::mutex
#include <condition_variable>   // for std::condition_variable
#include <memory>               // for std::shaerd_ptr and std::unique_ptr
#include <utility>              // for std::move


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

    std::mutex head_mutex;
    std::unique_ptr<node> head;
    std::mutex tail_mutex;
    node* tail;
    std::condition_variable data_cond;
public:
    threadsafe_queue():
        head(new node), tail(head.get())
    {}
    threadsafe_queue(const threadsafe_queue& other)=delete;
    threadsafe_queue& operator=(const threadsafe_queue& other)=delete;

    std::shared_ptr<T> try_pop();
    bool try_pop(T& value);
    std::shared_ptr<T> wait_and_pop();
    void wait_and_pop(T& value);
    void push(T new_value);
    void empty();

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> 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()!=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();
    }

    std::unique_ptr<node> try_pop_head()
    {
        std::unique_lock<std::mutex> head_lock(head_mutex);
        if(head.get()==get_tail())
        {
            return std::unique_ptr<node>();
        }
        return pop_head();
    }

    std::unique_ptr<node> try_pop_head(T& value)
    {
        std::unique_lock<std::mutex> head_lock(head_mutex);
        if(head.get()==get_tail())
        {
            return std::unique_ptr<node>();
        }
        value=std::move(*head->data);
        return pop_head();
    }

};

/*
 * PUBLIC INTERFACE
 */

// try pop.
template <typename T>
std::shared_ptr<T> threadsafe_queue<T>::try_pop()
{
    std::unique_ptr<node> const old_head=try_pop_head();
    return old_head?old_head->data:std::shared_ptr<T>();
}

template <typename T>
bool threadsafe_queue<T>::try_pop(T& value)
{
    std::unique_ptr<node> const old_head=try_pop_head(value);
    return old_head;
}

// wait and pop.
template <typename T>
std::shared_ptr<T> threadsafe_queue<T>::wait_and_pop()
{
    std::unique_ptr<node> const old_head=wait_pop_head();
    return old_head->data;
}

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

// push.
template <typename T>
void threadsafe_queue<T>::push(T new_value)
{
    std::shared_ptr<T> new_data(
            std::make_shared<T>(std::move(new_value)));
    std::unique_ptr<node> p(new node);
    {
        std::lock_guard<std::mutex> tail_lock(tail_mutex);
        tail->data=new_data;
        node* const new_tail=p.get();
        tail->next=std::move(p);
        tail=new_tail;
    }
    data_cond.notify_one();
}

// empty.
template <typename T>
void threadsafe_queue<T>::empty()
{
    std::lock_guard<std::mutex> head_lock(head_mutex);
    return (head.get()==get_tail());
}

#endif

There is one thing in the code I cannot reason about and it has appeared in two points. In wait_pop_head(T& value) and try_pop_head(T& value), there is value=std::move(*head->data);. Basically to assign the dereferencing result of a shared_ptr to a reference, it passes it through std::move. I appreciate if you let me know why it should be done like this? Why shouldn't value=*head->data; be used instead?

Another question that came up in the comments is why should std::shared_ptr be used instead of std::unique_ptr?

Answer 1

To the first one, if T has allocated dynamical memory, then the move can reduce much resource. And in the case, the funcation is named *_pop_head, that means the data of the head node should be treated useless after the calling. So using move to transfer the ownership of T's dynamical memory to the target T object is reasonable.

To the second one, the only reason I can think is that if we want to have a top member function, we can hold the data of head node in more than one place.

Answer 2

If type T defines a move assignment operator, then supplying an rvalue reference obtained via the call to std::move() will allow use of that more efficient operator, as opposed to forcing use a copying assignment operator.

If type T doesn't define a move assignment operator, it's fine to supply a T&& to its assignment operator, which will likely demand a parameter of type const T& or maybe just T. In either case, an argument of type T&& can be converted to the parameter type.

As far the use of std::shared_ptr rather than std::unique_ptr, I don't see why it's necessary either. It looks like it would be possible to return a std::unique_ptr from try_pop() and wait_and_pop(), moving the pointer out of the threadsafe_queue::node instance that's about to be destroyed. The only argument I can come up with is to allow callers of either of the "pop" functions to wind up with a sharable reference, thinking that to be the more flexible option. My cursory reading of the implementation doesn't show any case where two threadsafe_queue::node instances would ever point to the same "data" value, so I can't find any internal reason for that design choice.