Creating an new instance of a C++ class, results a duplicate - c++

Question

I'm new to C++. I'm trying to implement a LinkedList, for which I created two classes Node and LinkedList.
I created some test functions. One to test the Node creation and another to test the isEmpty function from LinkedList. However, when I try to test them. What's created in 'testNode()ends up being in the same Node I create insideLinkedListashead`.
This may be trivial question, however as a newcomer to C++ this concept is still no clear to me. I'd like to know why it is referring to the same instance created previously.
#include <iostream>
#include <assert.h>
using namespace std;
class Node
{
private:
int data;
int next;
public:
int getData(){return data;}
void setData(int new_data) {data = new_data;}
int getNext(){return next;}
void setNext(int new_next) {next = new_next;}
};
class LinkedList
{
Node head;
Node head2;
public:
bool isEmpty()
{
if (head.getData() == 0) {return true;}
return false;
}
};
void testNode()
{
Node aNode;
aNode.setData(15);
aNode.setNext(23);
assert (aNode.getData() == 15);
assert (aNode.getNext() == 23);
}
void testEmptyLinkedList()
{
LinkedList ll;
assert (ll.isEmpty() == true);
}

Answer 1

Initialize your data.
int data = 0;
int next = 0;
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#include <iostream>
#include <cassert>
using namespace std;
class Node {
private:
int data = 0;
int next = 0;
public:
int getData() { return data; }
void setData(int new_data) { data = new_data; }
int getNext() { return next; }
void setNext(int new_next) { next = new_next; }
};
class LinkedList {
Node head;
Node head2;
public:
bool isEmpty() {
if (head.getData() == 0) {
return true;
}
return false;
}
};
void testNode() {
Node aNode;
aNode.setData(15);
aNode.setNext(23);
assert(aNode.getData() == 15);
assert(aNode.getNext() == 23);
}
void testEmptyLinkedList() {
LinkedList ll;
assert(ll.isEmpty() == true);
}
int main() {
testEmptyLinkedList();
}

Answer 2

If your intention is to implement LinkList, each node of the list should contain the address of the next one.
So "next" shoud be declared as a pointer to a Node. Same for the first node of the list.
class Node {
private:
int data;
Node *next;
....
};
class LinkedList {
private:
Node *head;
...
};