Question

I am trying to implement a simple singly linked list of integers which are to be sorted upon insertion in Visual Studio c++ 2010 express.

The problem is that when I create a new node and call the .getValue() function on it, the correct number is returned, however somehow that is being lost when I try calling getValue() on a node already in the list. The node might not be inserted into the list correctly, however I can’t find why that would be the case. Some other value which looks like a reference value or something is displayed instead of the correct value.

I added current to the watch window when debugging but was still unable to see any of my variables other than the give value to be inserted. I am new to visual studio so I’m not sure if I’m missing something there. Here is my code:

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

//namespace Linked{
//The first two constructors would be the first in the linked list.
Node::Node(void){
    value = 0;
    next = 0;
}
Node::Node(int setValue){
    value = setValue;
    next = 0;
}
Node::Node(int setValue,Node *nextNode){
    value = setValue;
    next = nextNode;
}
Node * Node::getNext(){
    return next;
}
void Node::setNext(Node newNext){
    next = &newNext;
}
int Node::getValue(){
    return value;
}
bool Node::isEqual(Node check){
    return value==check.getValue()&&next == check.getNext();
}

/*
int main(){
    int firstInt, secondInt;
    std::cin>>firstInt;
    Node first = Node(firstInt);
    std::cout<<"Enter second int: ";
    std::cin>>secondInt;
    Node second = Node(secondInt, &first);
    std::cout<<"Second: "<<second.getValue()<<"\nFirst: "<<(*second.getNext()).getValue();

    system("pause");
}*/

Here is the linked list:

    //LinkedList.cpp

    LinkedList::LinkedList(void)
    {
        head = 0;
        size = 0;
    }

    LinkedList::LinkedList(int value)
    {
        head = &Node(value);
        size = 1;
    }

    void LinkedList::insert(int value){
        if(head == 0){

            Node newNode = Node(value);
            head = &newNode;
            std::cout<<"Adding "<<(*head).getValue()<<" as head.\n";
        }else{
            std::cout<<"Adding ";
            Node current = *head;
            int numChecked = 0;
            while(size<=numChecked && (((*current.getNext()).getValue())<value)){
                current = (*(current.getNext()));
                numChecked++;
            }

            if(current.isEqual(*head)&&current.getValue()<value){
                Node newNode = Node(value, &current);
                std::cout<<newNode.getValue()<<" before the head: "<<current.getValue()<<"\n";
            }else{
                Node newNode = Node(value,current.getNext());
                current.setNext(newNode);
                std::cout<<newNode.getValue()<<" after "<<current.getValue()<<"\n";
            }

        }
        size++;
    }
    void LinkedList::remove(int){

    }
    void LinkedList::print(){
        Node current = *head;
        std::cout<<current.getValue()<<" is the head";
        int numPrinted = 0;
        while(numPrinted<(size-1)){
            std::cout<<(current.getValue())<<", ";
            current = (*(current.getNext()));
            numPrinted++;
        }
    }
    int main(){
        int a[5] = {30,20,25,13,2};
        LinkedList myList = LinkedList();
        int i;
        for(i = 0 ; i<5 ; i++){
            myList.insert(a[i]);
        }
        myList.print();
        system("pause");
    }

Any guidance would be greatly appreciated!

Answer 1

When you create nodes in insert, you’re allocating them off the stack, which means that they’ll be lost after the function returns.

Get them off the heap with:

Node * newNode=new Node(value);

When you use:

Node newNode=Node(value);

You’re allocating that object on the stack, which means that pointers:

&newNode

to it are only valid until that function returns. If you use heap memory this is no longer an issue, but it does mean that you have to implement a destructor for your list which goes through and deletes each node.