好了，现在我已经解决了这个问题。谢谢你的意见和建议。我已经研究了指针，我很明白。这是我的代码，我修复。它可能不是很好，但我认为它是最适合我现在。

#include <iostream>

class Node { //this class create node 
public:
    int new_data; //set default value 
    Node* next = nullptr;

};

class linked_list { //Create linked list 
public:
    Node* first; //represent the first node of linked list;
    Node* last; //represent the last node of linked list;

    linked_list() {// constructor assign default this value to NULL;
        this->first = nullptr;
        //this->last = new Node();->DO NOT DO THIS BECAUSE IT WILL CREATE MEMORY LEAK ["new" keyword it allocate memory].
    }

    void pushback(int add_data); //create abstract class for write out side class;
    void insertfromfront(int add_data);

    void printout() { //show the values of linked list;

        while (this->first != nullptr) {
            std::cout << this->first->new_data << std::endl;
            this->first = this->first->next;
        }
        
    }
};

void linked_list::pushback(int new_data) {//push from back function 

    if (this->first == nullptr) { // if 1st node is NULL do this
        Node* new_node = new Node(); /*create new_node assign value and connected to linked list*/
        new_node->new_data = new_data;
        new_node->next = nullptr;
        this->first = new_node;
        this->last = new_node;//this is for connect the node.
        /*std::cout << "an adress of value of new_node " << &new_node << std::endl;
        std::cout << "an dress of new_node " << new_node << std::endl;
        std::cout << "adress values of first " << this->first << std::endl;
        std::cout << "address value of last " << this->last << std::endl;
        std::cout << "address first " << &this->first << std::endl;
        std::cout << "address last " << &this->last << std::endl;*/
        
    }
    else {
        Node* new_node = new Node(); /*create new_node assign value and connected to linked list*/
        new_node->new_data = new_data;
        new_node->next = nullptr;
        this->last->next = new_node;//this is for connect the node
        this->last = new_node;//this put tail to the last of linked list.

    }

}

int main()
{
    linked_list a;
    std::cout << "Adress of a " << &a << std::endl;
    
    /*a.pushback(3);
    a.pushback(9);
    a.pushback(6);
    a.pushback(66);
    a.pushback(77);
    a.pushback(99);
    a.pushback(10000);
    a.pushback(10001);*/
    a.printout();
}