int stack for ddd

struct Node{
    int data;
    Node* next = NULL;
};

class Stack{
private:
    Node* begin = NULL;
    int __size = 0;
public:
    void push(int a){
        if(begin == NULL){
            begin = new Node;
            begin -> data = a;
        }
        else{
            Node* temp = new Node;
            temp -> data = a;
            temp -> next = begin;
            begin = temp;
        }
    }
    int top(){
        return begin -> data;
    }
    int pop(){
        if(begin == NULL){
            cerr << "Stack is empty but pop() founded";
            return 0;
        }
        Node* mark = begin;
        int temp = begin -> data;
        begin = begin -> next;
        delete mark;
        return temp;
    }
    bool empty(){
        return begin == NULL;
    }
    int size(){
        int i = 0;
        Node* temp = begin;
        while(temp != NULL){
            i++;
            temp = temp -> next;
        }
    }
    void clear(){
        Node* temp;
        while(begin != NULL){
            temp = begin;
            begin = begin -> next;
            delete temp;
        }
    }
};