What is the correct way of implementing this custom priority_queue

Question

I want to build a priority queue of an object having three elements having my own comparator. I tried the following code but is showing error .

---

#include<bits/stdc++.h>
using namespace std;


class triplet {
    public:    
        int data;
        int arr;
        int index;       
};

bool compare( triplet x, triplet y ){

    if(x.data < y.data){
        return true;
    }
    else{
        return false;
    }
}

int main(){

 priority_queue<triplet,vector<triplet>,compare> pq1;

}

---

getting the following error enter image description here

Answer 1

Comparison routines must return false if the two elements are equal, but your version returns true.

Try this instead

bool compare(triplet x, triplet y) {

    if (x.data < y.data) {
        return true;
    }
    else {
        return false;
    }
}

or to make it a little simpler like this

bool compare(triplet x, triplet y) {    
    return x.data < y.data;
}

But the important point is < not <=.

EDIT

Your code also uses compare incorrectly. compare is not a type, so it's not a valid argument for the priority_queue template.

The type of compare is bool (*)(triplet, triplet) so the right way to do this is to use that type in the template and pass the actual comparison function to the constructor. Like this

priority_queue<triplet, vector<triplet>, bool (*)(triplet, triplet)> pq1(compare);

BTW normally you would do this by creating a comparison functor, not a function. It's a little cleaner that way (and potentially more efficient).