Getting a unique_ptr out of a priority queue

I am maintaining a set of unique_ptr instances in a priority_queue. At some point, I want to get the first element and remove it from the queue. However, this always produces a compiler error. See sample code below.

int main ()
{
  std::priority_queue<std::unique_ptr<int>> queue;
  queue.push(std::unique_ptr<int>(new int(42)));

  std::unique_ptr<int> myInt = std::move(queue.top());
  return 1;
}

This produces the following compiler error (gcc 4.8.0):

uptrtest.cpp: In function ‘int main()’: uptrtest.cpp:6:53: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = int; _Dp = std::default_delete<int>]’    std::unique_ptr<int> myInt = std::move(queue.top());
                                                     ^ In file included from /usr/include/c++/4.8/memory:81:0,
                 from uptrtest.cpp:1: /usr/include/c++/4.8/bits/unique_ptr.h:273:7: error: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^

Changing the code to use queue like in this question fixes the issue and the code compiles just fine.

Is there no way to keep unique_ptrs in a priority_queue or am I missing something?

Solution 1:

std::priority_queue::top() returns a const reference so you can't move it. Looking at the public interface of priority_queue there is no method to get a non-const reference that you can move (which is mandatory for unique_ptr, it has no copy constructor).

Solution: replace unique_ptr with shared_ptr to be able to copy them (and not just move them).

Or, of course, use another kind of container altogether (but if you chose priority_queue in the first place, this is probably not acceptable for you).

You could also maybe use a "protected member hack" to access the protected member c (the underlying container) but I wouldn't recommend it, this is quite dirty and quite probably UB.

Solution 2:

I agree, this is incredibly annoying. Why does it let me std::move elements into the queue, then give me no way of moving them out? We no longer have a copy of the original, so I need a non-const object when I do a top() and pop().

Solution: extend std::priority_queue, adding a method pop_top() that does both at once. This should preserve any ordering of the queue. It does depend on c++11 though. The following implementation only works for gcc compilers.

template<typename _Tp, typename _Sequence = std::vector<_Tp>,
    typename _Compare = std::less<typename _Sequence::value_type> >
class priority_queue: std::priority_queue<_Tp, _Sequence, _Compare> {
public:
    typedef typename _Sequence::value_type value_type;
public:

#if __cplusplus < 201103L
explicit
priority_queue(const _Compare& __x = _Compare(),
        const _Sequence& __s = _Sequence()) : 
        std::priority_queue(__x, __s) {}
#else
explicit 
priority_queue(const _Compare& __x, const _Sequence& __s) :
        std::priority_queue<_Tp, _Sequence, _Compare>(__x, __s) {}

explicit 
priority_queue(const _Compare& __x = _Compare(), _Sequence&& __s =
        _Sequence()) :
        std::priority_queue<_Tp, _Sequence, _Compare>(__x, std::move(__s)) {}
#endif

using std::priority_queue<_Tp, _Sequence, _Compare>::empty;
using std::priority_queue<_Tp, _Sequence, _Compare>::size;
using std::priority_queue<_Tp, _Sequence, _Compare>::top;
using std::priority_queue<_Tp, _Sequence, _Compare>::push;
using std::priority_queue<_Tp, _Sequence, _Compare>::pop;

#if __cplusplus >= 201103L

using std::priority_queue<_Tp, _Sequence, _Compare>::emplace;
using std::priority_queue<_Tp, _Sequence, _Compare>::swap;

/**
 *  @brief  Removes and returns the first element.
 */
value_type pop_top() {
    __glibcxx_requires_nonempty();

    // arrange so that back contains desired
    std::pop_heap(this->c.begin(), this->c.end(), this->comp);
    value_type top = std::move(this->c.back());
    this->c.pop_back();
    return top;
}

#endif

};