Broken interaction between std::priority_queue and move-only types

Let's say you've got a move-only type (such as std::unique_ptr< T >
or std::packaged_task) with an associated Compare functor that
defines some kind of ordering on objects of the move-only type:

    struct Node;
    std::priority_queue<
        std::unique_ptr< Node >,
        std::vector< std::unique_ptr< Node > >,
        ... >
    > maxHeap;

You insert elements to the container by using emplace:

    std::unique_ptr< Node > node{ new Node() };
    // other operations on node ...
    maxHeap.emplace( std::move( node ) );
    // add further elements ...

At some point you want to consume the elements; you can only
move them, as the type is not copy-able. Unfortunately you are
pretty much stuck, as std::priority_queue< T > does not
provide an interface to move elements out from the container,
as it only defines:

    const value_type& top() const
    void pop()

which is insufficient to be able to move elements from the container.

You can work around he problem by applying a const_cast on top():

    std::unique_ptr< Node > node2 =
        std::move(
            const_cast< std::unique_ptr< Node >& >( maxHeap.top() )
            );

    // oops, invariant might be broken, heap property is not upheld...

    maxHeap.pop(); // ok, invariant is fixed

but that is pretty far from ideal.

So as a first step, I would like to confirm that this is indeed
an example of a broken interaction of move-only types and
std::priority_queue. Can you please comment on that?

As far as the solution is concerned, I guess providing

    value_type& top() const

would open the door wide open to mess with the invariant of the
container, so that is not a good idea.

What we really need is a new pop:

    void pop( value_type& e )

that is capable of moving the top element from the queue to 'e',
under certain circumstances. I can see some problems with strong
exception guarantees along this path, but I am fairly sure that
would be still a better option than forcing the user to
const_cast top() :)

-- Zoltan

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