Segmentation fault in C++ with std::cin while macro is defined

Question

I am trying to solve a problem related to stack data structure, I have an implementation of a stack, and a main method that uses it, this is a for-learning question as i am a beginner, can you guys tell me, why i get this error?:

    GDB trace:
Reading symbols from solution...done.
[New LWP 24202]
Core was generated by `solution'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  main () at solution.cc:70
70      cin >> N;
#0  main () at solution.cc:70

my code is the following:

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

#define MAX_SIZE 5000000

class Stack
{
private:
    int A[MAX_SIZE];  // array to store the stack
    int top;   // variable to mark the top index of stack. 
public:
    // constructor
    Stack()
    {
        top = -1; // for empty array, set top = -1
    }

    // Push operation to insert an element on top of stack. 
    void Push(int x) 
    {
      if(top == MAX_SIZE -1) { // overflow case. 
            printf("Error: stack overflow\n");
            return;
        }
        A[++top] = x;
    }

    // Pop operation to remove an element from top of stack.
    void Pop() 
    {
        if(top == -1) { // If stack is empty, pop should throw error. 
            printf("Error: No element to pop\n");
            return;
        }
        top--;
    }

    // Top operation to return element at top of stack. 
    int Top() 
    {
        return A[top];
    }

    // This function will return 1 (true) if stack is empty, 0 (false) otherwise
    int IsEmpty()
    {
        if(top == -1) return 1;
        return 0;
    }

    // ONLY FOR TESTING - NOT A VALID OPERATION WITH STACK 
    // This function is just to test the implementation of stack. 
    // This will print all the elements in the stack at any stage. 
    void Print() {
        int i;
        printf("Stack: ");
        for(i = 0;i<=top;i++)
            printf("%d ",A[i]);
        printf("\n");
    }
};

int main() {

    int N;
    cin >> N;

    Stack S1;
    Stack S2;


    for(int i = 0; i < N; i++)
    {
        int q;
        cin >> q;

        if(q == 1)
        {
            int x;
            cin >> x;

            if(S1.IsEmpty() || S2.IsEmpty())
            {
                S1.Push(x);
                S2.Push(x);
            }
            else
            {
                S1.Push(x);
                if(x >= S2.Top()) S2.Push(x);                
            }
        }

        if(q==2)
        {
            if(S1.Top() == S2.Top())
            {
                S1.Pop();
                S2.Pop();
            }else
            {
                S1.Pop();
            }
        }

        if(q==3)
        {
            cout << S2.Top() << endl;
        }
    }


    return 0;
}

if i set MAX_SIZE variable to a lower number the code runs well, i want to know why is that the case, how std::cin and macros interact??, i am a beginner, sorry if this is a simple question it is the first time that i am asking in stackoverflow,

Answer 1

MAX_SIZE is far too big. MAX_SIZE determines the size of your Stack objects. As the total size of local variables in a function is limited to a few megabytes (depending on the platform), you simply exceed this size.

In your case you have two local Stack objects in main ( S1 and S2 ), each of them taking roughly 20 Mbytes (assuming sizeof int is 4).

This is totally unrelated to cin though.

Answer 2

Your Stack objects are allocated on the stack.

By default the stack is limited to something like 1-8 MB per thread depending on your platform.

Each of your stack objects takes up 20 MB so you are running out of stack space. To fix this change your code to:

std::unique_ptr<Stack> S1(new Stack());
std::unique_ptr<Stack> S2(new Stack());

This will allocate your objects on the heap which is only limited by the size of the available memory and swap space on your machine.