stack using list

#include <iostream>
#include <stdio.h>
#include <stdlib.h>


struct ListNode {
    struct ListNode *nxt;
    int elem;
};

class Stack {
public:
    Stack() {
        stBuf = nullptr;
    }

    void push(int elem) {
        ListNode *newLN = (ListNode*)malloc(sizeof(ListNode));
        if (newLN == nullptr) {
            perror("");
            _free();
            exit(1);
        }
        newLN->elem = elem;
        newLN->nxt = stBuf;
        stBuf = newLN;
        size++;
    }

    int getSize() {
        return size;
    }

    int getTop() {
        return stBuf->elem;
    }

    int pop() {
        if (!size) {
            exit(-1);
        }
        ListNode *topElem = stBuf;
        int elem = stBuf->elem;
        stBuf = topElem->nxt;
        free(topElem);
        size--;
        return elem;
    }

    ~Stack() {
        _free();
    }

private:
    ListNode *stBuf;
    int size = 0;

    void _free() {
        while (size) {
            pop();
        }
    }
};

int main() {
    using std::cout;
    using std::endl;
    Stack st;
    st.push(2);
    st.push(4);
    st.pop();
    cout << st.getSize() <<  ' ' << st.getTop() << endl;
    return 0;
}