I am getting a Segmentation Fault with my For loops - c++

Question

I keep getting a segmentation fault error in my code
Header
#ifndef DUALSTK
#define DUALSTK
#include <deque>
using namespace std;
enum stackNumber {One, Two};
template <typename T>
class dualStack {
public:
// constructor. set counts to 0
dualStack() : dualStackElements(20, 0) {}
void push(const T& item, stackNumber n);
void pop(stackNumber n);
T& top(stackNumber n);
//const T& top(stackNumber n) const;
bool empty(stackNumber n) const;
int size(stackNumber n) const;
int getCount1() const {return count1;}
int getCount2() const {return count2;}
private:
deque<T> dualStackElements;
int count1, count2;
};
#endif // DUALSTK
main.cpp
#include <iostream>
#include <deque>
#include "dualstk.h"
using namespace std;
template <typename T>
T& dualStack<T>::top(stackNumber n) {
int num = 0;
if (n == 0) {
num = count1 - 1;
return num;
}
if (n == 1) {
num = 20 - count2;
return num;
}
}
template <typename T>
void dualStack<T>::push(const T& item, stackNumber n) {
if (n == 0) {
count1++;
dualStackElements[top(One)] = item;
}
if (n == 1) {
count2++;
dualStackElements[top(Two)] = item;
}
}
template <typename T>
void dualStack<T>::pop(stackNumber n) {
int item = 0;
int item2 = 0;
if (n == 0) {
item = dualStackElements[top(One)];
cout << item << " ";
count1--;
}
if (n == 1) {
item2 = dualStackElements[top(Two)];
cout << item2 << " ";
count2--;
}
}
template <typename T>
bool dualStack<T>::empty(stackNumber n) const {
return 1;
}
int main() {
dualStack<int> stack;
for(int i = 1; i < 21; i++) {
if(i % 2 == 0) {
stack.push(i, One);
}
else {
cout << i;
stack.push(i, Two);
}
}
cout << endl;
for (int j = 0; j < 10; j++)
stack.pop(One);
cout << endl;
for (int i = 0; i < 10; i++)
stack.pop(Two);
cout << endl;
cout << stack.getCount2();
cout << stack.getCount1();
cout << endl;
cout << "\n" << stack.top(One);
cout << stack.top(Two) << endl;
return 0;
}
Whenever I have one loop for my stack.pop it will work fine
but if I do a for loop right after I use one to do stack.pop it wont work
I am not sure what I did wrong to this
for (int j = 0; j < 10; j++)
stack.pop(One);
for (int i = 0; i < 10; i++)
stack.pop(Two);

Answer 1

function top() returns reference to local variable ‘num’. Its storage is on stack so the value would be modified when the function returns and then you use it as the index into dualStackElements. This may lead to access to invalid memory address.

Answer 2

I think you are misunderstanding what this does (or I misunderstood your code):
dualStackElements[top(One)] = item;
you have not declared an array of dualStackElements
Better to declare two dequeues instead.
deque<T> dualStackElementsOne;
deque<T> dualStackElementsTwo;

Answer 3

Your Top method needs some error checking. What if count1 is 0 (or count2 > 20)? You'd return a negative value and then use it as an index to your deque. And remove the return by reference!

Answer 4

top() method returns a reference to local variable num which was freed after the method call. But different OS have different limits on accessing illegal memory, sometimes, it is just OK in Linux.
In your constructor, there is no initializing operations for fields: count1, count2. The program always returns "Segmentation fault;" if I comment out "count1=count2=0;" in my modified constructor.
In order to get the stable result for a program, you should do a clean before a make. I don't know exactly why, but it did happen in my test run.