C ++ Связанный список с объектами, который появляется с ошибкой LNK2019

По сути, это мой проект по назначению для C ++, и я довольно новичок в C ++, и я получаю эту ошибку. Также наличие ошибки для закомментированного кода в main == не совпадает с операндом

1> Main2.obj: ошибка LNK2019: неразрешенный внешний символ «class std :: basic_ostream> & оператор __cdecl<<(класс std :: basic_ostream> &, класс Заказчик &) «(?? 6 @ YAAAV? $ Basic_ostream @ DU? $ Char_traits @ D @ std @@@ std @@ AAV01 @ AAVCustomer @@@ Z) ссылка на функцию» void __cdecl display (struct Node *) «(? Display @@ YAXPAUNode @@@ Z)

Вот код Main.cpp

    #include <iostream>
#include "account.h"#include "cheque.h"#include "customer.h"

using namespace std;

struct Node {
Node* m_next;
Customer& m_customer;
Node(Customer& customer) : m_next(0), m_customer(customer) {}
};

// only for the 1st Node
void initNode(struct Node *head,Customer n){
head->m_customer = n;
head->m_next =NULL;
}

// apending
void addNode(struct Node *head, Customer n) {
Node *newNode = new Node(n);
newNode->m_customer = n;
newNode->m_next = NULL;

Node *cur = head;
while(cur) {
if(cur->m_next == NULL) {
cur->m_next = newNode;
return;
}
cur = cur->m_next;
}
}void insertFront(struct Node **head, Customer n) {
Node *newNode = new Node(n);
newNode->m_customer = n;
newNode->m_next = *head;
*head = newNode;
}

//struct Node *searchNode(struct Node *head, Customer n) {
//Node *cur = head;
//while(cur) {
//if(cur->m_customer == n) return cur;
//cur = cur->m_next;
//}
//cout << "No Node " << n << " in list.\n";
//}

bool deleteNode(struct Node **head, Node *ptrDel) {
Node *cur = *head;
if(ptrDel == *head) {
*head = cur->m_next;
delete ptrDel;
return true;
}

while(cur) {
if(cur->m_next == ptrDel) {
cur->m_next = ptrDel->m_next;
delete ptrDel;
return true;
}
cur = cur->m_next;
}
return false;
}

void deleteLinkedList(struct Node **node)
{
struct Node *tmpNode;
while(*node) {
tmpNode = *node;
*node = tmpNode->m_next;
delete tmpNode;
}
}

void display(struct Node *head) {
Node *list = head;
while(list) {
cout << list->m_customer << " ";
list = list->m_next;
}
cout << endl;
cout << endl;
}

int main()
{
Customer test1("sdfs","sdf2","dsfpppsf","fdgdfg","fdgdsffg");

struct Node *newHead;
struct Node *head = new Node(test1);Customer test2("sdsffs","sdfhhmj2","dsfhfsf","fdgdfgs","fdsggdfg");
Customer test3("sdsdfs","sdllllf2","dsfldfgsf","fdgaghdfg","fdgbvcbdfg");
Customer test4("sdgnfgfs","ssdfsdf2","dsfhjhdsf","fdbvcgdfg","fdgsfddfg");

addNode(head,test2);
display(head);

addNode(head,test3);
display(head);

insertFront(&head,test4);
display(head);

cout << "Deleting the copied list\n";
deleteLinkedList(&newHead);
display(newHead);
return 0;
}

Customer.cpp

    #include "customer.h"
using namespace std;Customer::Customer(string init_name, string init_address, string init_telephone, string init_sex, string init_dob)
{
name = init_name;
address = init_address;
telephone = init_telephone;
sex = init_sex;
dob = init_dob;
}void Customer::showPersonDetails(void)
{
cout << "Name : "<< name << endl;
cout << "Address : "<< address << endl ;
cout << "Telephone : "<< telephone << endl;
cout << "Sex : "<< sex << endl ;
cout << "Date of Birth : "<< dob << endl;
}

и, наконец, customer.h

  #ifndef CUSTOMER_H
#define CUSTOMER_H

#include <iostream>
#include<string>

using namespace std;

class Customer
{
private:
//
// class members
//
string name;
string address;
string telephone;
string sex;
string dob;

public:

// Constructor
Customer(string init_name, string init_address, string init_telephone, string init_sex, string init_dob);

// a print method
void showPersonDetails(void);
void changeDetails(void);

// This operator is a friend of the class, but is NOT a member of the // class:
friend ostream& operator <<(ostream& s, Customer& a);

};

// This is the prototype for the overload
ostream& operator <<(ostream& s, Customer& a);

#endif