CODEWITHSUNDEEP
c++listpointerslinker
Lachie
Lachie

Reputation: 105

Incorrect output of Linked List (c++)

I'm trying to write a linked list of Node objects, where each Node contains: a string - data, and a pointer - next. To manage the list (e.g: add/remove nodes), I have Node objects: head, curr and temp. The code compiles fine, but the result is not what I want.

What I'm having trouble with is making the next pointer in the curr node actually point to the next node. I've been adding output statements to narrow down the error and it seems to be coming from the line: curr.setNext(n); in the addNode() function.

LinkedList.cpp:

#include <iostream>
#include <cstdlib>
#include "LinkedList.h"
#include "Node.h"

using namespace std;

LinkedList::LinkedList() {

}

void LinkedList::addNode(value_type addData) {
    Node n;
    n.setData(addData);

    if (head.getData() != "") {
        curr = head;

        while(curr.getNext() != NULL) {
            curr = *(curr.getNext());
        }

        curr.setNext(n);    //This line is not linking the nodes.
        cout << "(2) Curr is pointing to: " << curr.getNext() << " , The location of n is:" << &n << endl;
    }
    else {
        head = n;
    }
}

LinkedList.h:

#ifndef LINKEDLIST_H
#define LINKEDLIST_H
#include "Node.h"

class LinkedList {
    public:
        typedef std::string value_type;

        LinkedList();
        LinkedList(value_type setData);
        void addNode(value_type addData);

    private:
        Node head;
        Node curr;
        Node temp;

};

#endif

Node.cpp:

#include <iostream>
#include <cstdlib>
#include "Node.h"

using namespace std;

Node::Node() {
    data = "";
    next = NULL;
}

void Node::setData(value_type setData) {
    data = setData;
}

string Node::getData() { 
    return data;
}

void Node::setNext(Node n) { 
    next = &n;
    cout << "(1) Curr is pointing to: " << next << " , The location of n is:" << &n << endl;
}

Node * Node::getNext() {
    return next;
}

Node.h:

#ifndef NODE_H
#define NODE_H

class Node {
    private:    
        typedef std::string value_type;

        value_type data;
        Node* next;
        Node* prev;

    public:
        Node();

        void setData(value_type setData);
        value_type getData();
        void setNext(Node n);
        Node * getNext();
};

#endif

So when I call the function a few times with test strings and a blank list, the cout statements should be printing out the same memory address in both instances. However, this is the output:

(1) Curr is pointing to: 0x22c9a0 , The location of n is:0x22c9a0
(2) Curr is pointing to: 0x22c9a0 , The location of n is:0x22c960
(1) Curr is pointing to: 0x22c9a0 , The location of n is:0x22c9a0
(2) Curr is pointing to: 0x22c9a0 , The location of n is:0x22c960
(1) Curr is pointing to: 0x22c9a0 , The location of n is:0x22c9a0
(2) Curr is pointing to: 0x22c9a0 , The location of n is:0x22c960

So in the Node class, the memory addresses line up correctly. But as soon as it leaves that class and prints again, they are different. Does it have something to do with next and curr.getNext() being stored differently? How would I go about fixing this?

Thanks in advance!

Upvotes: 0

Views: 73

Answers (2)

R Sahu
R Sahu

Reputation: 206567

The following function is not right:

void Node::setNext(Node n) { 
    next = &n;
    cout << "(1) Curr is pointing to: " << next << " , The location of n is:" << &n << endl;
}

You are storing the address of an object that is on the stack. The pointer becomes invalid as soon as the function returns.

PS There may be other problems in your code.

Upvotes: 0

Soren
Soren

Reputation: 14688

You are making a copy of the nodes, rather than operating on the linked list;

This line copies the node

curr = *(curr.getNext());

Try instead to change Node curr; to Node *curr; and then operate on pointers instead of copies, like

curr = curr->getNext()

and 

curr->setNext(n);

You also need to make proper memory management, so code like this will not work as the Node n is a local variable will not survived beyound the scope of the function

Node n;
n.setData(addData);

Upvotes: 1

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