Question

The following program successfully builds up a stack , but the 2 operations pop and stack top are giving exception and wrong result respectively. Here is the program :

// Working with stack using Array

/*
 * @author Suhail Gupta
 *
 */
#include <iostream>

using namespace std;

char choice;
int *ptrToArray; // this will be a pointer to the array alias stack that we'll make
int stackSize;
int currentStackSize = 0;
int *firstElement = NULL; // this pointer stores the address of the first element in the stack. It initially points to NULL
void push();
void pop();
void stackTop();

int main() {

cout << "Enter the size of stack : ";
cin >> stackSize;
ptrToArray = new int[stackSize];

do {
     push();
    cout << "Push elements ? y/n ";
    cin >> choice;
} while( choice == 'y');

cout << endl << "pop elements ? y/n ";
cin >> choice;
if( choice == 'y') {
    pop();
}

cout << endl << "know the stack top ? y/n ";
cin >> choice;
if( choice == 'y') {
    stackTop();
}
 }

void push() {

if( currentStackSize == stackSize) {        // check before pushing if the stack is full
    cout << endl << "Stack-Overflow !";
} else {
    int numberToPush;
    cout << endl << "Enter the number to be pushed : ";
    cin >> numberToPush;
    firstElement = &numberToPush; // Store the address of the Last number inserted.This is the address of the first element in the stack
    ptrToArray[currentStackSize] = numberToPush;
    cout << "The element you just inserted : " << ptrToArray[currentStackSize] << endl;
    currentStackSize++;     
 }    
}

void pop() {
if( stackSize == 0 ) {
    cout << "Stack Underflow !";
} else {
    delete firstElement; // delete the memory allocated to the first element
    firstElement = &ptrToArray[currentStackSize-1];
    currentStackSize--;
 }
}

void stackTop() {
if( firstElement == NULL) {
    cout << endl << "Stack Underflow !" << endl;
} else {
    cout << "The first element in the stack is : " << *firstElement;
  }
}

The pushing operation works fine.But if i call the pop function the following message is displayed :

Second question :

Now just comment the statement where pop function gets called . The result that i get when trying to know the first element on the stack by calling the function stackTop() is The first element in the stack is : -858993460 . This is some garbage number that i didn’t enter while making up the stack. Why the statement *firstElement gives the wrong result then ?

Answer 1

int *firstElement = NULL; // Global pointer variable
void push() {
 // ...    
    else {
        // ..
        int numberToPush;  // Resides on stack.
        // ....
        firstElement = &numberToPush; 
        // ...

    } // numberToPush cease to exist from this point.
}

numberToPush is a local variable and have a blocked scope to else part and you are taking the reference of it.This is resulting you the garbage.

Edit: You need to understand that storage duration of global variables are different to that of local. Just taking the reference of local to a global variable doesn’t increase the life time of a local variable. As soon as it’s scope finishes, the variable ceases to exist.