Segmentation Fault Passing Reference as Function Parameter - c++

Question

With this block of code, I'm getting a segmentation fault as I try to pass the stack references to the transferStacks() method. Any help on understanding why this is would be helpful!
I could just get rid of the helper method and it should work, but I'm trying to understand conceptually.
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <stack>
using namespace std;
void transferStacks(stack<int> & s1, stack<int> & s2){
if (s1.empty()){
for (int i = 0; i < s2.size(); i++){
int element = s2.top();
s1.push(element);
s2.pop();
}
}
}
int main() {
/* Enter your code here. Read input from STDIN. Print output to STDOUT */
int queries = 0;
cin>>queries;
stack <int> newestOnTop;
stack <int> oldestOnTop;
while (queries!=0){
int type = 0;
cin >> type;
int input = 0;
if (type == 1){ //enqueue
cin>>input;
newestOnTop.push(input);
}
else if (type == 2){ //dequeue
transferStacks(newestOnTop, oldestOnTop);
oldestOnTop.pop();
}
else if (type == 3){ //peek
transferStacks(newestOnTop, oldestOnTop);
cout<<oldestOnTop.top()<<endl;
}
queries--;
}
return 0;
}
Segmentation Fault

Answer 1

You appear to believe that this code will copy s2 to s1:
for (int i = 0; i < s2.size(); i++){
int element = s2.top();
s1.push(element);
s2.pop();
}
But it will not: if before the loop s2 contains 3 elements, only the first 2 will be copied (and generally, only the first half will be copied).
In addition, your transfer function transfers from s2 to s1, but the way you call it implies that you intended the opposite: to transfer from s1 to s2. Current code would leave oldestOnTop empty, which will then result in a crash when you use oldestOnTop.top() or oldestOnTop.pop().