CODEWITHSUNDEEP
c++segmentation-faultsingly-linked-list
erip
erip

Reputation: 16935

Segfault when appending node to linked list

I've been trying to rewrite some basic data structures using C++ to refresh my memory on some of the basics of OOP, but I've run into a silly problem already.

I'm trying to build a singly linked list, append the strings "Hello" and "World" to the list, and then view all of the contents within the list. This is a very easy task, but I'm getting a segmentation fault when I run the following code:

driver.cc

#include <iostream>
#include <string>
#include "SinglyLinkedList.h"

int main() 
{
    SLL<std::string> List;
    List.Append("Hello");
    List.Append("World");

    List.visitAll(std::cout);

    return 0;
}

Node.h

#ifndef NODE_H
#define NODE_H

template <class T>
class Node {
    public:
        Node<T>() {} 
        Node<T>(T init) { data = init; next = nullptr; }

        void setData(T newData) { data = newData; }
        void setNext(Node<T> *nextNode) { next = nextNode; }

        const T getData() { return data; }
        Node<T> *getNext() { return next; }
    private:
        T data;
        Node<T> *next;
};

#endif

SinglyLinkedList.h

#ifndef SINGLY_LINKEDLIST_H
#define SINGLY_LINKEDLIST_H

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

template <class T>
class SLL {
    public:
        SLL<T>() { head = nullptr; size = 0; }
        ~SLL<T>() {}
        void Append(T added);
        void Delete(T deleted);
        void visitAll(std::ostream &outs);
    private:
        Node<T> *head;
        long size;
};

template <class T>
void SLL<T>::Append(T added) 
{
    Node<T> *newNode = new Node<T>(added);

    Node<T> *temp = head;

    if(temp != nullptr) {
        while(temp != nullptr) {
            temp = temp->getNext();
        }

        temp->setNext(newNode); // seg fault here
    } 
    else {
        head = newNode;
    }
}

template <class T>
void SLL<T>::visitAll(std::ostream &outs)
{
    Node<T> *temp = head;

    while(temp)
    {
        outs << temp->getData() << std::endl;
        temp=temp->getNext();
    }
}

#endif

Just debugging by hand, I create a new node with data = "Hello" and next = nullptr. This gets appended by the else in the void SLL<T>::Append method because temp == nullptr. However, on the second Append, the while loop runs once, then crashes when calling the setter of the Node class. I cannot figure out why this is the case.

I'm expecting to see 

Hello
World

Am I being too tunnel-visioned? This is pretty silly. Sorry if it's too basic for SO...

Thanks, erip

Upvotes: 1

Views: 149

Answers (2)

R Sahu
R Sahu

Reputation: 206607

while(temp != nullptr) {
    temp = temp->getNext();
}

temp->setNext(newNode); // seg fault here

That's because you are breaking out of the while loop when temp == nullptr.

Use:

while(temp->getNext() != nullptr) {
    temp = temp->getNext();
}

temp->setNext(newNode);

Upvotes: 3

riodoro1
riodoro1

Reputation: 1256

Your while loop in Append ends with temp being a null pointer, therefore no temp->setNext()

Upvotes: 1

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