Speicherreferenz verändert sich unabsichtlich

#include <iostream>
using namespace std;

class Element_stack {
private:
    int wert;
    Element_stack* next_pls;
public:
    Element_stack(int var) { wert= var; next_pls= NULL; }
    Element_stack() {}
    Element_stack* get_next() {
        return next_pls;
    }
    void set_next(Element_stack* n) {
        next_pls= n;
    }
    int get_value() {
        return wert;
    }
    void set_value(int v) {
        wert= v;
    }
};

class Stack {
private:
    Element_stack* head;
public:
    
    Stack() { head = NULL; }
    void top() {
        if(head == NULL)
            cout<<"\nStack is empty";
        else
            cout<<"\ntop element: "<<(*head).get_value();
    }
    void push(int val) {
        Element_stack new_elem(val);
        if(head == NULL)
            head = &new_elem;
        else {
            new_elem.set_next(head);
            head = &new_elem;
        }
    }
    void pumped_it_up() {
        if(head == NULL)
            cout<<"\nStack ist leer, hier gibt es nichts zu löschen";
        else {
            cout<<"\nerstes Element wird gelöscht mit dem Wert: "<<(*head).get_value();
            head = (*head).get_next();
        }
    }
};

// stack test

int main() {
    
    Stack my_stack;
    int opt=100;
    int ins=0;

    my_stack.push(2);    my_stack.top();
    my_stack.pop();        my_stack.top();
    my_stack.push(3);    my_stack.top();
    my_stack.push(4);    my_stack.top();
    my_stack.push(5);    my_stack.top();
    my_stack.pop();        my_stack.top();
    my_stack.pop();        my_stack.top();
    my_stack.pop();        my_stack.top();
}