std::queue and std::deque cleanup

Question

Suppose we have a situation where we need FIFO data structure. For example, consume some events in the order they came in.

Additionally, we need to clear the entire queue from time to time.

std::queue seems like the perfect fit for doing that, but unfortunately it lacks a function for clearing the container.

So at this point, we have 2 alternatives:

std::queue

• we asked the STL lib what we need. Granted, the STL lib will give us more: it will give us an std::deque disguised as a std::queue
• we got back only a part from what we need, namely the pop front and push back but without clear
• we will have to "emulate" clear somehow, without the naive way of looping and popping

std::deque

• we asked the STL lib what we need
• we got back what we asked for, but we've got too much: we also got push front and pop back

Overall, we either received too few or too much, never exactly what we really wanted.

Here is the thing that took me by surprise, while I was trying to provide clear functionality for using with std::queue which is a member var of my object

struct message
{
};
struct consumer
{
    std::queue<message> _pending;

    void clear_ver_1()
    {
       auto will_be_deleted_when_out_of_scope = std::move(_pending);
    }

    void clear_ver_2()
    {
        std::queue<message> will_be_deleted_when_out_of_scope;
        _pending.swap(will_be_deleted_when_out_of_scope);
    }
};

I've read the specs and I can not say for sure if clear_ver_1 will leave the _pending in a valid but unspecified state or not. See the string example there.

I'm quite surprised that the specs are so vague about this topic. Am I not looking in the right place?

Thank you all!

Update

It seems there is a non-ignorable difference between assigning and clearing. Internally, queue and deque are almost the same (one is using the other one)

Answer 1

The usage of std::move

std::move isn't meant to be used that way. You should only use std::move, well, to move an object to somewhere else in the program when you won't be using it anymore. As you said, it is then left in a valid but unspecified state:

• Valid because it is completely safe for destruction;
• Unspecified because you should not be accessing that object anymore.

std::queue vs std::deque

If you only plan to use FIFO functionality, I would recommend using std::queue. It really makes it clear that you will only use std::deque as a FIFO data structure — clarity is kind of the only reason for std::queue to exist in the first place.

Clearing a std::queue

You can just assign it to an empty std::queue, or as you did it, swap it for for an empty one.

// ...

struct consumer
{
    std::queue<message> _pending;

    void clearQueue()
    {
        _pending = {};
    }
};

Edited according to DevSolar's comment

Answer 2

we got back what we asked for, but we've got too much: we also got push front and pop back

You got exactly what you asked for, a dequeue is a data structure that allows efficient inserts and deletes at either end point. It might not be the data structure for you, but that's your fault for choosing it.

we will have to "emulate" clear somehow, without the naive way of looping and popping

For the record, popping is extremely cheap in terms of performance, it simply decrements a number. A pop in a while loop translates to decrementing an integer until 0, which unless you have a lot of numbers is very fast. In fact, it's probably much faster than allocating memory, which brings us to:

The STL way of clearing these collection classes is to swap them with an empty collection (which is what you figured out on your own) or to just straight up re-allocate them in place (apple's answer). Both of those will (probably, the standard is vague about this point) allocate memory, which is a very expensive linear operation.

You have all the pieces to do it, though I'd suggest profiling to see which way is faster if it really matters to you. Personally I just pop the queue in a loop, it leaves the allocated memory in place for the next time I need to push more, so it saves on potentially multiple allocations and re-allocations (when compared to resetting the queue), depending on the amount of data you have.

Answer 3

To clear the queue, you can also simply write

_pending = {};

---

Note: the first method may not work and should not be rely on, since moved-form object are in valid but unspecified state.