invalid initialization of reference of type âstd::pair

#include <iostream>
#include <utility>
#include <vector>
#include <sys/types.h>
#include <algorithm>
#include <sys/wait.h>
#include <unistd.h>
 using namespace std;
void childFunction(pair<int, pair<int, int>>b);
void parentFunction(pair<int, pair<int, int>>b);
bool priority_sort(pair<int, int> &a, pair<int, int> &b);
int main() {
 pid_t PID;
 int pid = 0;
 int count = 0;
 int quantum, exec, pr;
 vector<pair<int, int>> a;
 vector<int> pid2;
 vector<int>::iterator check;
 pair<int, pair<int, int>>b;
 while (cin >> quantum) {
     while (cin >> exec >> pr) {
        a.push_back(make_pair(exec, pr));
    }
}
 for (int i = 0; i < a.size(); i++) {
     pid2.push_back(pid);
     pid++;
}
 sort(a.begin(), a.end(), priority_sort);
 cout << "Scheduling queue: ";
 for (int i = 0; i < a.size(); i++) {
     if (a[i].first > quantum) {
         a.insert(a.begin() + i + 2, make_pair(a[i].first - quantum, 
   a[i].second)); //SPLIT IF GREATER THEN QUANTUM;
         pid2.insert(pid2.begin() + i + 2, pid2[i]);
        a[i].first = a[i].first - (a[i].first - quantum);
    }
     b = make_pair(pid2[i], a[i]);
     parentFunction(b);
}
 cout << endl;
 pid = 0;
 int endtime = a.size();
 for (int i = 0; i < a.size(); i++) {
    b = make_pair(pid2[i], a[i]);
    pid++;
    if (PID = fork() == 0) {
    childFunction(b);
    exit(0);
    }
    else
    wait(&endtime);

}

}
 void childFunction(pair<int, pair<int, int>>b) {
 cout << "Process " << b.first << ": " << "exec time = " << b.second.first 
 << 
 "," << "priorty = "
    << b.second.second << endl;
 sleep(b.second.first);
 cout << "Process " << b.first << "ends." << endl;
}
  void parentFunction(pair<int, pair<int, int>>b) {
    cout << "(" << b.first << "," << b.second.first << "," <<
    b.second.second << ")" << " ";
}
 bool priority_sort(pair<int, int> &a, pair<int, int> &b)
{
 return (a.second < b.second);
}

The program is to output the childFunction everytime I fork, which works in atom on ubuntu server, and worked on visual studio without fork, but a carbon copy, however inputting it in a ssh server, I get this error:

/usr/include/c++/4.8.2/bits/stl_algo.h:2263:35: error: invalid initialization of reference of type âstd::pair&â from expression of type âconst std::pairâ while (__comp(*__first, __pivot)) ^

/usr/include/c++/4.8.2/bits/stl_algo.h:2266:34: error: invalid initialization of reference of type âstd::pair&â from expression of type âconst std::pairâ while (__comp(__pivot, *__last))

I am unable to get past this intilazation as debugging the code and reading over const haven't lead me to any proper solution. USING NAMESPACE AS IT IS REQUIRED FOR THIS PROJECT