How to move elements out of STL priority queue

Question

C++'s STL priority queue have a void pop() method, and a const ref top() method. Thus, if you want to move elements out of the queue, you have to do something like this:

T moved = std::move(const_cast<T&>(myQueue.top())));
myQeue.pop();

This effectively casts the top to not a constant, so that it can be moved (rather than copied). I don't like this code, because the forced move may invalidate the invariants of the priority queue, which should not matter because of the pop, but things could go wrong.

Is there a better way to accomplish the pop/move? Why is there no T&& top_and_pop() function?

Answer 1

std::priority_queue is basically a thin layer on top of the heap algorithms. You can easily create your own priority queue with:

• std::vector
• std::push_heap
• std::pop_heap

Using these building blocks, the implementation is trivial, and you can easily implement a moving pop operation. The following listing contains a minimal, working implementation:

template <typename Type, typename Compare = std::less<Type>>
class queue
{
private:
    std::vector<Type> _elements;
    Compare _compare;
public:
    explicit queue(const Compare& compare = Compare())
        : _compare{compare}
    { }
    void push(Type element)
    {
        _elements.push_back(std::move(element));
        std::push_heap(_elements.begin(), _elements.end(), _compare);
    }
    Type pop()
    {
        std::pop_heap(_elements.begin(), _elements.end(), _compare);
        Type result = std::move(_elements.back());
        _elements.pop_back();
        return std::move(result);
    }
};