c++ - 为什么使用类模板时出现未定义的引用错误？

Question

这两个类都在头文件中，这不是问题。出于某种原因，我的程序告诉我有一个未定义的引用错误。我不完全确定为什么会这样。

我有 2 个类的一个实现文件。这可能是错误的来源吗？

这是我的第一堂课

#ifndef NODE_H
#define NODE_H
#include <iostream>
#include <string>

using namespace std;

// Node class
template <class T>
class Node 
{
    private:
        int ID;
        string name;
        T test1, test2, test3;
        static const int counter = 0;
        Node* next;
  public:
    Node() 
    {
        ID = NULL;
        name = NULL;
        test1 = NULL;
        test2 = NULL;
        test3 = NULL;
    }
    ~Node(){}
    void SetID(); //giving a value for employee number
    void SetName();   //giving a value for yearly salary
    void SetTests();
    void SetNext(Node*);
};
#endif

这是我的第二节课

#include <iostream>
#include <string>
#ifndef LIST_H
#define LIST_H
#include "Node.h"

using namespace std;

template <class T>
class List 
{
    Node<T>* head; //creating the head
                // head is an object that stores the address of the first node
    string dash;
  public:
    // constructor that initializes every list to null
    List() 
    { 
        dash.assign(80,'-'); // variable used for aesthetics
        head = NULL;    
    }

    ~List(){}
    // prtototype of the list member functions
    void Add();
};
#endif

这是我的实现文件

#include <iostream>
#include <fstream>
#include "Node.h"
#include "List.h"

using namespace std;

template <class T>
void List<T> :: Add ()
{
    // Create a new node
    Node<T>* newNode = new Node<T>();
    newNode->SetID(); //giving a value for employee number
    newNode->SetName();   //giving a value for yearly salary
    newNode->SetTests();
    newNode->SetNext(NULL);              //setting the next node equal to NULL

    /*If(!isEmpty())
    {*/

   // Adding values to the elements of the head when head is empty
    if(head == NULL)
      head = newNode;

    //Inserting Elements at the beginning of the list
    else
    {
       newNode->SetNext(head); //make newNode -> point to the head
       head = newNode;     //make the head newNode
     }

}

这是主文件

#include "Node.h"
#include "List.h"
#include <iostream>
#include <cstdlib>

using namespace std;
////////////////////////////////////////////////////////////////////////////////
                                  /* Main*/
////////////////////////////////////////////////////////////////////////////////
template <class T>
void menu(List<T> &);


int main()
{
    // New list
    List<int> list;
    menu(list); //List function call
    return 0;
}

///////////////////////////////////////////////////////////////////////////////
                    /*This function dispalys a Menu for the user to
                      pick the desired action to the linked list.
                      It uses the member functions of the class List, 2 loops
                      and a switch statement to execute these actions */
///////////////////////////////////////////////////////////////////////////////
template <class T>
void menu(List<T> &list)
{   char choice;

    do{ 
        system("CLS"); // use #include <cstdlib> to be able to use this
        system("CLS") clears the window
        cout << "\t\t\tWelcome User\n";
        cout << "\t\t\tMain Menu\n\n";
        cout << "\t(A)dd\n";
        cout << "\t(L)ocation\n";
        cout << "\t(R)emoves\n";
        cout << "\t(C)lear\n";
        cout << "\t(P)eek\n";
        cout << "\t(A)verage\n";
        cout << "\t(G)rade\n";
        cout << "\t(E)xit\n";
        cout << "\t\tWhat would you like me to do? ";
        cin >>choice;
        choice = toupper(choice); // chaging the character to capital
        //validation of entered choice
        while (choice != 'A' && choice != 'L' && choice != 'R' && choice != 'C'
               && choice != 'P' && choice != 'G' && choice != 'E' )
        {
            cout << "\n\t\tSorry invalid input\n\t\tPlease enter one of the ";
            cout << "choices below : ";
            cin >> choice;
        }
        cin.ignore();
         while (choice != 'A' && choice != 'L' && choice != 'R' && choice != 'C'
               && choice != 'P' && choice != 'G' && choice != 'E' )
        //switch statement used 
        switch (choice)
        { case 'A': //To insert 
            list.Add();
            cout << endl; 
            cin.get();
            cin.ignore();
            break;} 
    }while (choice != 'E'); //Runing the program until E is entered
}

我不明白为什么我有一个参考错误，因为我确保我有警卫，并包含所有头文件。

是什么给了我未定义的参考错误？

(.text$_Z4menuIiEvR4ListIT_E[_Z4menuIiEvR4ListIT_E]+0x199): undefined reference to `List<int>::Add()'
C:\Users\Documents\collect2.exe [Error] ld returned 1 exit status
C:\Users\Documents\Makefile.win recipe for target '"Last' failed