Bdtjtk

I've been trying to write my own implementation of linked list, but the code segfaults when I try to access an the third element or anything after it. Adding elements doesn't segfault, but accessing does. I can't find the pointer error in my get() function.

Each node in the list stores data (of Template t) and a pointer leading to the next node. I have two functions for everything- one for the first element, and one for any subsequent elements. The get() function for the subsequent elements always segfaults. I have some debug messages in the function that spit out results I can't explain. For example, if I run a get() request for the second element, an then the third, the code doesn't segfault, but it does return clearly incorrect results. Debug messages I placed indicate the segfault occurs when the second element calls the function to check the third element, if it occurs at all. Try the code with and without the line cout << newList.get(2) << endl; and you'll get very different results.

One possible cause is the pointer storage- I have the get() function output the pointer of each element (except the first) as it cycles through, and compare them to the pointers outputted by the add() function, and and pointers for element 0 and 1 match, but 2 and beyond do not match, and I can't seem to figure out why that would be.

#include <iostream>

using namespace std;







template <class T> class myLinkedList{

T data;

myLinkedList<T> *next = NULL;



public:

    myLinkedList(T input){

        data = input;



    }

    void add(T input){

        if(next == NULL){

            myLinkedList<T> newItem(input);

            next = &newItem;

            cout << "adding to list, data is " << input << ", pointer is " << next << endl;

        }else{

            myLinkedList<T> nextEntry = *next;

            nextEntry.add(input);

        }

    }





    T getData(){

        return data;

    }

    //the start  of the get function, only used by the first entry in the list

    T get(int entry){

        int currentPosition = 0;

        if(entry == currentPosition){

            return getData();

        }else{

            //defrefrence the pointer anc check the next entry

            myLinkedList<T> nextEntry = *next;

           return nextEntry.get(entry, ++currentPosition);

        }

    }



private:

    //this vesion is the hidden, private vesion only used by nodes other than the first one

    //used to keep track of position in the list

    T get(int entry, int currentPosition){

        //cout << currentPosition << endl;

        if(entry == currentPosition){

            return data;

        }else{

            //derefrence the pointer and check the next entry

            cout << next << endl;

            myLinkedList<T> nextEntry = *next;

            currentPosition++;

           T output = nextEntry.get(entry, currentPosition);

           return output;

        }



    }





};

int main(){

myLinkedList<int> newList(3);

newList.add(4);

newList.add(5);

newList.add(7);

newList.add(9);

cout << newList.get(2) << endl;

cout << newList.get(3) << endl;

return 0;

}

Results are clearly erroneous- program should spit oout two macthing sets of pointers, as well as the numbers 5 and 7 ( the list elements)

One of your main problems is here:

if(next == NULL){

    myLinkedList<T> newItem(input); // <<<<<<<<<<<<<

    next = &newItem;

    cout << "adding to list, data is " << input << ", pointer is " << next << endl;

}

you allocate an item on stack inside the if scope. Then you make next to point to this item. But... lifetime of the item is bounded by this scope. As son as you exit the scope, this item does not exist any longer. You need to allocate it dynamically by 'new' or other methods.

I had a breakthrough! Following Serge's solution was helpful, but one more change was needed- rather than create a function reference in the else block of my add function,
eg

 myLinkedList<T> nextEntry = *next;

 nextEntry.add(input)

i needed to use the pointer directly, as in

next->add(input)

I didn't know my pointer/object syntax