How to tell a std::priority_queue to refresh its ordering?

I have a priority queue of pointers to a struct city. I modify the objects pointed by these pointers outside the priority queue, and want to tell the priority queue to "reorder" itself according to the new values.

What should I do?

Example:

#include <iostream>
#include <queue>

using namespace std;

struct city {
    int data;
    city *previous;
};

struct Compare {
    bool operator() ( city *lhs, city *rhs )
    {
        return ( ( lhs -> data ) >= ( rhs -> data ) );
    }
};

typedef priority_queue< city *, vector< city * >, Compare > pqueue;

int main()
{
    pqueue cities;

    city *city1 = new city;
    city1 -> data = 5;
    city1 -> previous = NULL;
    cities.push( city1 );

    city *city2 = new city;
    city2 -> data = 3;
    city2 -> previous = NULL;
    cities.push( city2 );

    city1 -> data = 2;
    // Now how do I tell my priority_queue to reorder itself so that city1 is at the top now?

    cout << ( cities.top() -> data ) << "\n";
    // 3 is printed :(

    return 0;
}

Solution

This is a bit hackish, but nothing illegal about it, and it gets the job done.

std::make_heap(const_cast<city**>(&cities.top()),
               const_cast<city**>(&cities.top()) + cities.size(),
               Compare());

Update:

Do not use this hack if:

The underlying container is not vector.
The Compare functor has behavior that would cause your external copy to order differently than the copy of Compare stored inside the priority_queue.
You don't fully understand what these warnings mean.

You can always write your own container adaptor which wraps the heap algorithms. priority_queue is nothing but a simple wrapper around make/push/pop_heap.