class list_iterator {
        element* data;
    public:
        list_iterator(element* _data) : data(_data) {}
        list_iterator& operator++ (){
            if (data == nullptr) {
                throw new exception();
            }
            element* temp = data;
            data = data->next;
            return list_iterator(temp);
        }
        int operator* () {
            return data->data;
        }
        friend bool operator== (const list_iterator left, const list_iterator right) {}
    };

...

list_iterator begin () {
    return list_iterator(head);
}
list_iterator end () {
    return list_iterator(nullptr);
}