Priority Queues in c++

Question

Is there a way to iterate over a priority queue in c++ ? My understanding is that they are more or less immutable and the only manipulation of the container is to the top element. I would like to be able to print out the contents of a priority queue but am unsure of how to even approach the problem.

Answer 1

The underlying container is a protected data member named c (see here for further details). Therefore you can always inherit from a std::priority_queue and export a couple of iterators over that container (if available).
As a minimal, working example:

#include<queue>
#include<iostream>

struct MyPriorityQueue: std::priority_queue<int> {
    auto begin() const { return c.begin(); }
    auto end() const { return c.end(); }
};

int main() {
    MyPriorityQueue pq;
    pq.push(0);
    pq.push(1);
    for(auto &v: pq) {
        std::cout << v << std::endl;
    }
}

Note: inheriting from data structures in the std:: namespace is usually discouraged.
That being said, it works at least.

---

The code above works in C++14.
Below a slightly modified version that works also in C++11 as requested in the comments:

#include<queue>
#include<iostream>

struct MyPriorityQueue: std::priority_queue<int> {
    decltype(c.begin()) begin() const { return c.begin(); }
    decltype(c.end()) end() const { return c.end(); }
};

int main() {
    MyPriorityQueue pq;
    pq.push(0);
    pq.push(1);
    for(auto &v: pq) {
        std::cout << v << std::endl;
    }
}