C++ Segmentation Fault Linked List Element Addition

1
votes

I am facing a problem while coding Linked List implementation in c++. Whenever I am trying to add an element I am getting Segmentation Fault error.

 #include <iostream>


    class node{
    public:
        int data;
        class node *next;
    };
    class LinkedList {
    public:
        node *head;
        LinkedList(int num);

        void add(int number);
        void display();
    };

    const int null = 0;
    LinkedList::LinkedList(int num) {
        // TODO Auto-generated constructor stub
        std::cout<<"Constructor Called"<<std::endl;
        head->data = num;
        head->next = null;
        std::cout<<"Constructor Call completed"<<std::endl;
    }



    void LinkedList::add(int num) {
        struct node *n=new node;
        n->data = num;
        n->next = null;
        struct node *current = head;
        while (current->next != null) {
            current = current->next;
        }
        current->next = n;
    }
    void LinkedList::display() {
        std::cout<<"Display"<<std::endl;
        struct node *current = head;

        while (current!= null) {
            std::cout << current->data << "->";
            current = current->next;
        }
        std::cout << "null"<<std::endl;
    }

    int main() {
        LinkedList l(1);
        l.display();
        l.add(2);
        l.display();
        return 0;
    }

Please look into the gdb debug logs: node n is not referencing to any memory location. So it could not be initialized. Please let me know in case you require any further information.

Thanks in advance!

struct node *n=new node;

Constructor Called
Constructor Call completed

Breakpoint 1, main () at SampleCPP.cpp:60
60      l.display();
(gdb) s
LinkedList::display (this=0xbffff03c) at SampleCPP.cpp:48
48      std::cout
(gdb) n
51      while (current!= null) {
(gdb) n
52          std::cout data ";
(gdb) n
53          current = current->next;
(gdb) n
51      while (current!= null) {
(gdb) n
55      std::cout null
56  }
(gdb) n
main () at SampleCPP.cpp:61
61      l.add(2);
(gdb) s
LinkedList::add (this=0xbffff03c, num=2) at SampleCPP.cpp:38
38      struct node *n=new node;
(gdb) print n
$2 = (node *) 0x0
(gdb)
3
c++linked-list

3 Answers

4
votes

You never allocate memory for head. That's your problem.

Allocate it in the constructor:

LinkedList::LinkedList(int num) {
    std::cout<<"Constructor Called"<<std::endl;
    head = new node; // This is missing!
    head->data = num;
    head->next = null;
    std::cout<<"Constructor Call completed"<<std::endl;
}

and it would be good to free all that memory too before your program finishes..

1
votes

you are not allocating memory for head; it should be like this

LinkedList::LinkedList(int num) {
    // TODO Auto-generated constructor stub
    head=new node();
    std::cout<<"Constructor Called"<<std::endl;
    head->data = num;
    head->next = null;
    std::cout<<"Constructor Call completed"<<std::endl;
}
0
votes

With a small modification,

class node{
public:
    int data;
    class node *next;
    node(int num): data(num), next(NULL){} 
};

And 

LinkedList::LinkedList(int num): head(new node(num))  {
    // TODO Auto-generated constructor stub
    std::cout<<"Constructor Called"<<std::endl;
    //Do sth if you wish
    std::cout<<"Constructor Call completed"<<std::endl;
}

This removes some additional complexities. Change where ever you instantiate node with new constructor.