Why does priority_queue in STL not follow strict weak ordering?

Question

i Have been playing around with STL containers and the compare function/functors they supports, however i found priority_queue doesn't follow the usual strict weak ordering , i am trying to understand what might be the reason but not able to figure it out, any pointers would be helpful.

It also mentioned in this blog that priority_queue doesnt follow strict weak ordering. enter link description here

#include "STL.h"
#include "queue"
#include "vector"
#include "iostream"
#include "functional"
using namespace std;

typedef bool(*func)(const int& val1 , const int& val2);

bool strict_weak_order_function(const int& val1 , const int& val2){
    return val1 > val2;
}

bool comparer_function(const int& val1 , const int& val2){
    return !strict_weak_order_function(val1 , val2);
}

struct Compaper_functor{
    bool operator()(const int& val1 , const int& val2){
        return !strict_weak_order_function(val1 , val2);
    }
};


void runPriorityQueue(void){
    //priority_queue<int , vector<int> , func > pq(comparer_function);
    priority_queue<int , vector<int> , Compaper_functor > pq;
    int size;
    cin >> size;
    while(size--){
        int val;
        cin >> val;
        pq.push(val);
    }
    while(!pq.empty()){
        cout <<'\n'<< pq.top() << '\n';
        pq.pop();
    }
}

" i found priority_queue doesn't follow the usual strict weak ordering" Citation needed. — n. 'pronouns' m., Aug 31 '16 at 07:40
do you mean that the ordering is reversed? i.e. a predicate of less<> inserts lower items at the back of the priority_queue? — Richard Hodges, Aug 31 '16 at 07:42
@arup `priority_queue` does require a strict weak ordering. You need to re-evaluate your assumptions! — juanchopanza, Aug 31 '16 at 07:57
https://ideone.com/8kR9qQ , according the the functor is 5 < 4 , 5 should be placed ahead of 4, so even though the functor returns false 5 is still placed ahead of 4. This should not be happening with strict weak ordering right? — Arup, Aug 31 '16 at 07:59
@juanchopanza i am refering to this code for you: ideone.com/8kR9qQ — Arup, Aug 31 '16 at 08:13
Maybe [RTFM](http://en.cppreference.com/w/cpp/container/priority_queue). — juanchopanza, Aug 31 '16 at 08:19

score 6 · Accepted Answer · edited May 23 '17 at 11:51

The problem is that the negation of your strict_weak_order (that uses >) is <= and that is not a strict weak order. A strict weak order R has to satisfy x R x == false for all x. However, R equal to <= yields (x <= x) == true.

You need to reverse the order of arguments (which corresponds to <) instead.

bool comparer_function(const int& val1 , const int& val2){
    return strict_weak_order_function(val2 , val1);
}

struct Compaper_functor{
    bool operator()(const int& val1 , const int& val2){
        return strict_weak_order_function(val2 , val1);
    }
};

Note however that a std::priority_queue has a std::less as default comparator, but that gives a max-heap (i.e. [5, 4, 3, 2, 1] output from the same input), so to get a min-heap (i.e. with output [1, 2, 3, 4, 5] from input [5, 4, 3, 2, 1]) you need to pass std::greater, see e.g. this:

#include <queue>
#include <iostream>

int main()
{
    auto const v  = std::vector<int> { 5, 4, 3, 2, 1 };

    // prints 5 through 1
    for (auto p = std::priority_queue<int> { v.begin(), v.end()  }; !p.empty(); p.pop())
        std::cout << p.top() << ',';
    std::cout << '\n';

    // prints 1 through 5
    for (auto p = std::priority_queue<int, std::vector<int>, std::greater<int>> { v.begin(), v.end()  }; !p.empty(); p.pop())
        std::cout << p.top() << ',';
    std::cout << '\n';
}

Live Example

https://ideone.com/8kR9qQ , according the the functor is 5 < 4 , 5 should be placed ahead of 4, so even though the functor returns false 5 is still placed ahead of 4. This should not be happening with strict weak ordering right? — Arup, Aug 31 '16 at 08:01
@Arup see my expanded answer, and the link to [**the other Q&A**](http://stackoverflow.com/questions/32748069/the-reason-of-using-stdgreater-for-creating-min-heap-via-priority-queue) about why priority queue behaves slightly counter-intuitively. — TemplateRex, Aug 31 '16 at 11:02

Why does priority_queue in STL not follow strict weak ordering?

1 Answers1