Stack on C++

template <typename T>
class stack {
private:
    class node {
    public:
        T value;
        node* prev;
        node(T value, node* prev=nullptr) {
            this->value = value;
            this->prev = prev;
        }
        node(node *nd)
        {
            value = nd->value;
            prev = nd->prev;
        }
    };

    node* top;
public:
    stack() {
        top = nullptr;
    }
    void add(T value) {
        if (top) {
            top = new node(value, top);
        } else
            top = new node(value);
    }
    node* pop() {
        node *tmp = new node(top);
        delete top;
        if (tmp->prev) {
            top = tmp->prev;
        }
        return tmp;
    }
};