Doubly linked list implementation

[khansubhan95]

I have been trying to implement a barebones version of a doubly linked list(which supports only insert and printing all nodes in it).However upon executing the following code,I get a segmentation fault error.I tried debugging the code.The code fails when the constructor for Doubly_Linked_List class is executed.The code is given below,which was executed on Ubuntu 14.04 using g++.What am I doing wrong?

#include <iostream>
using namespace std;

class Node
{
private:
	int key;
	Node *prev;
	Node *next;
public:
	Node()
	{}
	void set_key(int x)
	{ key=x; }
	void set_prev(Node *x)
	{ prev=x; }
	void set_next(Node *x)
	{ next=x; }
	int get_key()
	{ return this->key; }
	Node *get_prev()
	{ return this->prev; }
	Node *get_next()
	{ return this->next; }
};

class Doubly_Linked_List
{
private:
	Node *head;
public:
	Doubly_Linked_List()
	{
		cout<<"Reached here"<<endl;
		head->set_key(0);
		head->set_prev(NULL);
		head->set_next(NULL);
	}
	void insert(Node x)
	{
		x.set_next(head->get_next());
		if(head->get_next()!=NULL)
			(head->get_next())->set_prev(&x);
		head->set_next(&x);
		x.set_prev(NULL);
	}
	void print_nodes()
	{
		Node *x=head->get_next();
		while(x!=NULL)
		{
			cout<<x->get_key()<<endl;
			x=x->get_next();
		}
	}
};

int main()
{
	int n,j;
	Doubly_Linked_List l;
	cout<<"Enter number of elements"<<endl;
	for(int i=0;i<n;i++)
	{
		cin>>j;
		Node x;
		x.set_key(j);
		l.insert(x);
	}
	l.print_nodes();
	return 0;
}

[AbstractionAnon]

Line 30: head is an uninitialized pointer (garbage).

Line 35-37: You're trying to store member variables using an uninitialized pointer. This is the cause of your crash.

Line 39: next is passed by value. You're going to change the value in a local copy. You want to pass Node by reference if you want to change it.

Line 46: x goes out of scope. References to x are no longer valid.

Line 60: n is uninitialized

Line 63: Your termination condition references an uninitialized variable (n).

Line 69: x goes out of scope for each iteration.

[khansubhan95]

Thanks for the quick reply.Followed the flaws you pointed out and got correct code

#include <iostream>
using namespace std;

class Node
{
private:
	int key;
	Node *prev;
	Node *next;
public:
	Node()
	{}
	void set_key(int x)
	{ key=x; }
	void set_prev(Node *x)
	{ prev=x; }
	void set_next(Node *x)
	{ next=x; }
	int get_key()
	{ return this->key; }
	Node *get_prev()
	{ return this->prev; }
	Node *get_next()
	{ return this->next; }
};

class Doubly_Linked_List
{
private:
	Node *head=new Node();
public:
	Doubly_Linked_List()
	{
		head->set_key(0);
		head->set_prev(NULL);
		head->set_next(NULL);
	}
	void insert(int x)
	{
		// Insertion done at front of the list
		Node *n=new Node();
		n->set_key(x);
		n->set_next(head->get_next());
		if(head->get_next()!=NULL)
			(head->get_next())->set_prev(n);
		head->set_next(n);
		n->set_prev(NULL);
	}
	void print_nodes()
	{
		Node *x=head->get_next();
		while(x!=NULL)
		{
			cout<<x->get_key()<<endl;
			x=x->get_next();
		}
	}
};

int main()
{
	int n,x;
	Doubly_Linked_List l;
	cout<<"Enter number of elements"<<endl;
	cin>>n;
	cout<<"Enter the elements:"<<endl;
	for(int i=0;i<n;i++)
	{
		cin>>x;
		l.insert(x);
	}
	cout<<"List traversal"<<endl;
	l.print_nodes();
	return 0;
}