c++ - Getting a unique_ptr out of a priority queue

Question

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?

Answer 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.