C++ :: Stack Implementation Of Linked Lists

I have this code that I need to memorize for my final. Memorizing code is easy for me, but I'm trying pretty hard to fundamentally understand the functions, and what they are doing (even using pen and paper, to draw and trace).For example, in the push function below, I understand everything, except why I'm setting ptr = p. I feel like p should be equal to NULL, then the next node I push should be equal to p, etc.

Stack & Stack::push(double x)
{
Node * p = NULL;
try
{
p = new Node;
}

[code].....

Also, are LL Queues that hard to implement once you can do them w/stacks - That will probably be something I have to code for my final, as well. Below is the full code for my Stack class.

#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;
class Stack
}

[code]....