CODEWITHSUNDEEP
c++for-looplinked-liststack
PeakGen
PeakGen

Reputation: 23025

Infinite loop in Stack, implemented as "Linked List"

Please have a look at the following code

#include <iostream>

using namespace std;

class Stack
{
public:
    Stack();
    ~Stack();

    void push(void *data);
    void *pop();
    void print();

protected:
    typedef struct Element
    {
        struct Element *next;
        void *data;

    }Element;

    Element *top;
};

Stack::Stack()
{
    top = NULL;
}

Stack::~Stack()
{
    while(top)
    {
        Element *elm = top->next;
        delete top;
        top = elm;
    }
}

void Stack::push(void *data)
{
    Element *elm = new Element;
    elm->data = data;
    elm->next = top;;
    top = elm;
}

void *Stack::pop()
{
    void *data;

    if(top==NULL) return data;

    data = top->data;

    Element *elm = top;

    top = elm->next;
    delete elm;
    return data;


}

void Stack::print()
{
    Element *elm = top;

    while(elm)
    {
        cout << *(static_cast<int *>(elm->data))<<" ";
        elm = elm->next;
    }

    cout << endl;
}

int main()
{
    Stack *st = new Stack;;

    int n1 = 10;
    int n2 = 20;
    int n3 = 30;
    int n4 = 40;
    int n5 = 50;

    st->push(&n1);
    st->push(&n2);
    st->push(&n3);
    st->push(&n4);
    st->push(&n5);
    st->print();

    cout << *(static_cast<int*>(st->pop()))<< " poped\n";
    cout << *(static_cast<int*>(st->pop()))<< " poped\n";

    st->print();

    cout << endl;

    system("pasue");
    return 0;
}

Actually, I am learning this data structure to be implemented in this way, and this is my first attempt. Why it is entering to an infinite loop?

Upvotes: 1

Views: 154

Answers (2)

Moha Dehghan
Moha Dehghan

Reputation: 18443

Here is the problem:

void *Stack::pop()
{
    void *data;
    //...
}

Initialize the variable before usage:

void *Stack::pop()
{
    void *data = NULL;
    //...
}

If you put the pop method in a loop to extract all the elements, it may never return NULL because uninitialzed variables can have any value (whatever was in the memory location before).

Upvotes: 2

Grijesh Chauhan
Grijesh Chauhan

Reputation: 58271

You are returning garbage value 

void *Stack::pop()
{
    void *data;

    if(top==NULL) return data;

At this if(top==NULL) return data; data points garbage
You should return NULL 

if(top==NULL) return top;

Upvotes: 1

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