Seg Fault at return statement in function

Question

My program is supposed to convert a prompt from infix to postfix. So far, through a debugger and various other methods, I have located the exact point at which I segfault, but don't understand why.

Here's my code:

Here's itop.h:

using namespace std;
#include <cstdlib>
#include <iostream>

class sNode{
    public:
       char data;
       sNode *next;
};

class stack{
    public:
        sNode *head;
        void push (char);
        sNode pop();
        int rank(char);
        stack()
        {
        cout << "Initiliazing stack." << endl;
        }
};

This is my itop.cpp file:

    #include "itop.h"

    void stack::push (char a)
    {
            // cout << "Pushing " << a << endl;
            sNode *sn;
            sn = new sNode;
            sn->data = a;
            sn->next = head;
            head = sn;
    }

    sNode stack::pop()
    {
            // cout << "Popping stack." << endl;
            sNode *sn;
            sn = head;
            head = head->next;
            return *sn;
    }

    int stack::rank(char x)
    {
            int num = 0;
            // cout << "Checking rank." << endl;
            if(x == '\0')
            {
                    num = 1;
                    // cout << "Checking for null" << endl;
                    return num;
            }
            else if(x == '+' || x == '-')
            {
                    num = 2;
                    // cout << "Checking if + or -" << endl;
                    return num;
                    // cout << "After return." << endl;
            }
            else if(x == '*' || x == '/')
            {
                    num = 3;
                    // cout << "Checking for * or /" << endl;
                    return num;
            }
            else
                    cout << "Error! Input not valid!" << endl;
    }

And here's main.cpp:

using namespace std;
#include <iostream>
#include <cstdlib>
#include <cstring>
#include "itop.h"

int main()
{
char *temp1;            //Instantiating variables.
char *temp2;
temp1 = new char[20];
temp2 = new char [20];
stack s;
do              //Checking commands.
{
    cout << "infix_to_postfix> ";
    cin >> temp1;
    if(strcmp(temp1, "quit") == 0)
    {
        return 0;
    }
    if(strcmp(temp1, "convert") != 0)
    {
        cout << "Error! Invalid command." << endl;
    }
    cin >> temp2;
    if(strcmp(temp1, "convert") == 0)
    {
        for(int i=0; i<sizeof(temp2); i++)
        {
            if(isdigit(temp2[i]))
            {
                cout << atoi(&temp2[i]);
            }
            else if(s.rank(temp2[i]) < s.rank(s.head->data))
            {
                sNode temp = s.pop();
                cout << temp.data;
            }
            else
            {
                s.push(temp2[i]);
            }
        }
    }
    else
    {
        cout << "Error! Command not supported." << endl;
    }
}while(strcmp(temp1, "quit") != 0);

return 0;
}

The function is called at

else if(s.rank(temp2[i]) < s.rank(s.head->data))

And the problem is in here:

            else if(x == '+' || x == '-')
            {
                    num = 2;
                    // cout << "Checking if + or -" << endl;
                    return num;
                    // cout << "After return." << endl;
            }

Specifically right before return num, I get "Segmentation fault (core dumped)" error message. I have used gdb and all I know is that right after "Checking if + or -" I see "$1 = 2". I'm not quite sure what that means, but it is what I want to return.

Thank you for your help.

Answer 1

Fix 1:Write a proper constructor.

    stack()
    {
    head=NULL;
    cout << "Initiliazing stack." << endl;
    } 

Fix 2:Write an extra method to check if stack is empty.

int stack::empty()
{
    if(head == NULL)
      return true;
    else
      return false;
}

Fix 3:Check if stack empty before using the stack data.

else if(!s.empty() && s.rank(temp2[i]) < s.rank(s.head->data))
{
 ...
}

Fix 4: Fix the rest of the code logic.