Error when compiling C++ program (with templates): Undefined symbol

Question

as a beginner, I was practicing C++ programming by building a program with one header file but I have been stuck in one problem for several hours.

My program has very simple structure:

main.cpp, which includes function, int main()

chain.hpp, which includes declaration of classes and functions

chain.cpp, which includes detailed definition of each functions

// main.cpp
#include <iostream>
#include "chain.hpp"
using namespace std;

int main(int argc, const char * argv[]) {
    // insert code here...
    Chain<int> linkedlist;
    linkedlist.insertNode(0, 123);
}

// Chain.hpp
#ifndef chain_hpp
#define chain_hpp

#include <stdio.h>

template<class T>
class ChainNode{
public:
    ChainNode<T>* next;
    T data;
    
    ChainNode(){next=NULL; data=NULL;}
    ChainNode(ChainNode<T>* next, T data);
    ~ChainNode();
private:
};

template<class T>
class Chain{
private:
    ChainNode<T>* first;
public:
    Chain(){first=NULL;}
    ~Chain();
    int indexOf(const T& element) const;
    void deleteNode(int index);
    void insertNode(int index, const T& element);
    bool isEmpty() const {return first==0;}
};

#endif /* chain_hpp */

// chain.cpp
#include "chain.hpp"

template<class T>
ChainNode<T>::ChainNode(ChainNode<T>* next, T data){
    this->next = next;
    this->data = data;
}

template<class T>
ChainNode<T>::~ChainNode(){
    delete this;
}


template<class T>
Chain<T>::~Chain(){
    while(first!=NULL){
        ChainNode<T>* next = first->next;
        delete first;
        first = next;
    }
}

template<class T>
int Chain<T>::indexOf(const T& element) const{
    ChainNode<T>* current = first;
    int index = 0;
    while(current!=NULL && current->data){
        current = current->next;
        index++;
    }
    if(current==NULL) return -1;
    return index;
}

template<class T>
void Chain<T>::deleteNode(int index){
    if(index<0) throw "Negative index cannot be input";
    if(first ==0) throw "Empty chain";
    
    ChainNode<T>* deleteNode = first;
    ChainNode<T>* beforeNode = first;
    if(index==0){
        first = first->next;
    }
    else{
        for(int i=0;i<index-1;i++){
            if(beforeNode==NULL) throw "Index is bigger than length of the chain";
            beforeNode = beforeNode->next;
        }
        deleteNode = beforeNode->next;
        beforeNode->next = beforeNode->next->next;
    }
    delete deleteNode;
}

template<class T>
void Chain<T>::insertNode(int index, const T& element){
    if(index<0) throw "Negative index cannot be input";
    if(first==0) throw "Empty chain";
    
    ChainNode<T>* newnode = new ChainNode<T>();
    if(index==0){
        newnode->next = first;
        first = newnode;
    }
    else{
        ChainNode<T>* beforenode = first;
        for(int i=0;i<index-1;i++){
            if(beforenode==NULL) throw "Index is bigger than length of the chain";
            beforenode = beforenode->next;
        }
        newnode->next = beforenode->next;
        beforenode->next = newnode;
    }
}

And error messages are as below

Undefined symbols for architecture x86_64:
  "Chain<int>::insertNode(int, int const&)", referenced from:
      _main in main.o
  "Chain<int>::~Chain()", referenced from:
      _main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I know myself that there might be further problems to be fixed, but due to this build time error, I cannot step to that level.

Since I was using Xcode for my programming, I suspected Xcode itself might be the reason of this problem, but even when I tried compiling through my terminal and g++ *.cpp command, the same error message was annoying me. Is there any expert who can help me stop wasting my time?