#include <bits/stdc++.h>

using namespace std;

bool comp(const int &a , const int&b)
{
    return ((a > 0 and b > 0) ? true : ((a == 0) ? false : true));
}


int32_t main()
{
    vector <int> a = {1 , 0 , 0 , 3 , 12};
    sort(a.begin() , a.end() , comp);
    for(int &x : a)
        cout << x << ", " ;
    cout << endl;
    return 0;
}

The aim is to move all the zeroes to the back keeping the relative order of all other non-zero elements intact. I thought the comparator would do the job as it changes the relative order of two elements a and b if a is equal to 0 and not in any other case, and print the final output as :

0, 0, 1, 3, 12,

However, it printed this :

12, 3, 1, 0, 0,

Why is the comparator failing?

THIS REALLY HELPED : Custom Comparators