How to iterate over a priority_queue?

Can I traverse a standard priority_queue or standard queue in c++ with an iterator (like a vector)? I don't want to use pop because it cause my queue to be dequeued.

Thanks for any help

Solution 1:

priority_queue doesn't allow iteration through all the members, presumably because it would be too easy in invalidate the priority ordering of the queue (by modifying the elements you traverse) or maybe it's a "not my job" rationale.

The official work-around is to use a vector instead and manage the priority-ness yourself with make_heap, push_heap and pop_heap. Another work-around, in @Richard's answer, is to use a class derived from priority_queue and access the underlying storage which has protected visibility.

Solution 2:

You can do it like this - bam! Notice that items are not necessarily in "sorted" order while they are in the queue, at least with regards to a straight-forward iteration of the container.

#include <queue>
#include <cstdlib>
#include <iostream>
using namespace std;

template <class T, class S, class C>
S& Container(priority_queue<T, S, C>& q) {
    struct HackedQueue : private priority_queue<T, S, C> {
        static S& Container(priority_queue<T, S, C>& q) {
            return q.*&HackedQueue::c;
        }
    };
    return HackedQueue::Container(q);
}

int main()
{
    priority_queue<int> pq;
    vector<int> &tasks = Container(pq);

    cout<<"Putting numbers into the queue"<<endl;
    for(int i=0;i<20;i++){
        int temp=rand();
        cout<<temp<<endl;
        pq.push(temp);
    }

    cout<<endl<<"Reading numbers in the queue"<<endl;
    for(vector<int>::iterator i=tasks.begin();i!=tasks.end();i++)
        cout<<*i<<endl;

    cout<<endl<<"Taking numbers out of the queue"<<endl;
    while(!pq.empty()){
        int temp=pq.top();
        pq.pop();
        cout<<temp<<endl;
    }

    return 0;
}

Solution 3:

A queue purposefully provides a limited interface, which excludes iteration. But since a queue uses a deque as the underlying container, why not use a deque directly?

#include <iostream>
#include <queue>
using namespace std;

int main() {
  deque<int> q;
  q.push_back(1);
  q.push_back(2);
  q.push_back(3);
  for(deque<int>::iterator it = q.begin(); it != q.end(); ++it)
    cout << *it << endl;
}

Similar answer for a priority queue: no, you cannot. In this case though, a vector is used by default. In neither case can you access the underlying container to iterate over them. See this question for further reading.

Solution 4:

#include <queue>
#include <iostream>

int main() {
    std::priority_queue<int> pq;

    pq.push_back(1);
    pq.push_back(2);
    pq.push_back(3);

    std::priority_queue<int> temp = pq;

    while (!temp.empty()) {
        std::cout << temp.top() << std::endl;
        temp.pop();
    }

    return 0;
}