C++ 带有出现LNK2019错误的对象的链接列表

基本上，这是我C++的作业项目，我对C++很陌生，我遇到了这个错误。main == 中的注释代码也有错误，这不是匹配的操作数

1>Main2.obj ： 错误 LNK2019： 未解析的外部符号 "class std：：basic_ostream> & __cdecl 运算符<<（class std：：basic_ostream> &，class Customer &）" （？？6@YAAAV？$basic_ostream@DU？$char_traits@D@std@@@std@@AAV01@AAVCustomer@@@Z） 在函数"void __cdecl display（struct Node *）" （？display@@YAXPAUNode@@@Z） 中引用

这是代码主.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 listn";
deleteLinkedList(&newHead);
display(newHead);
return 0;
     }

客户.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;
           }

最后是客户。

  #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