C++ Pop Function Linked List

Question

I am writing a program that implements stacks as linked lists. The program complies but when I run it, it crashes. I ran the debugger and says unhandled exception when it gets inside the Pop() function and to the line "topPtr = topPtr->next". I was wondering if anyone noticed something in there that is causing this error. I attached the portion of main and the pop function that I believe i sbeing affected. thanks

template<class ItemType>
struct NodeType
{ 
   ItemType info;
   NodeType* next;
};

template<class ItemType>
class Stack
{ 
private:
   int stacklength;
   NodeType<ItemType>* topPtr; // It points to a singly-linked list
public: 
    void Pop(ItemType &x);

template<class ItemType>
void Stack<ItemType>::Pop(ItemType &x)
{
    NodeType<ItemType>* tempPtr;
    tempPtr = topPtr;
    topPtr = topPtr->next;
    delete tempPtr;
    stacklength--;
}

int main()
{
Stack <int> IntStack;
int x;
IntStack.Pop(x);
}

That would happen if topPtr is nullptr or uninitialized. So 1: you need to make sure you initialize topPtr = nullptr; in the constructor and 2: you need to check stack depth on Pop (can't pop from an empty stack!). — crashmstr
– crashmstr, Commented Dec 2, 2015 at 20:16

crashmstr · Accepted Answer · 2015-12-04 13:04:57Z

1

First off, you don't initialize your pointers.

template<class ItemType>
struct NodeType
{ 
    //...
    NodeType() : next(nullptr) {} ///Initialize next so we can check for null
};

template<class ItemType>
class Stack
{ 
public:
    Stack() : topPtr(nullptr), stacklength(0) { } ///initialize
    //...

Then, in your Pop, you need to check for an empty stack (you can't pop if there are no elements).

template<class ItemType>
void Stack<ItemType>::Pop(ItemType &x)
{
    if (!topPtr)
    {
        //Here, we need to decide how to handle this.
        //One way would be to throw an exception,
        //another way would be to change the method signature
        //and return a bool.
    }
    ///...
}

answered Dec 4, 2015 at 13:04

crashmstr

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