error LNK2019

[Stiltzkin]

Hey,

I keep getting this error 2019 in my project. I suspect it has something to do with my operator<< functions,
since it only pops up each time I call the <<operator in my main function, but I can't seem to find what's wrong.

My header :

#ifndef H_Herexamen
#define H_Herexamen

#include <cstdlib>
#include <string>
#include <iostream>
#include <map>
#include <vector>

template <class T> class dataMap {
private:
	std::map<int, T> data;

public:
	dataMap(){};
	~dataMap(){};
	dataMap(const dataMap& d);
	
	std::map<int, T> getMap();
	void addElement(int i, T t);
	void removeElement(int i);
	T operator[](int i);
	template <class T> friend std::ostream& operator<<(std::ostream& output, const dataMap<T>& operand);
};

template <class K> class nodeList {
private:
	std::vector<K> nodes;

public:
	nodeList(){};
	~nodeList(){};
	nodeList(const nodeList& n);
	
	std::vector<K> getList();
	void addNode(K k);

	template <class K> friend std::ostream& operator<<(std::ostream& output, const nodeList<K>& operand);
};


#endif H_Herexamen 

My main function

#include <cstdlib>
#include <string>
#include <iostream>
#include <map>
#include <vector>

#include "Herexamen.h"

using namespace std;

//dataMap functions

template <class T> map<int, T> dataMap<T>::getMap(){
	return data;
}

template <class T> dataMap<T>:: dataMap(const dataMap<T>& d){
	this.data = d.data;
}

template <class T> void dataMap<T>::addElement(int i, T t){
	data.insert(pair<int, T>(i, t));
}

template <class T> void dataMap<T>::removeElement(int i){
	map<int, T>::iterator it = data.find(i);
	data.erase(it);
}

template <class T> T dataMap<T>::operator[](int i) {
	return data[i];
}

template <class T> ostream& operator<<(ostream& output, dataMap<T>& operand) {
	int i=0;
	map<int, T> temp = operand.getMap();
	map<int, T>::iterator it = temp.begin();
	for(i=0;i<temp.size();i++) {
		if(it!=NULL) output<<it->first<<it->second<<endl;
		it++;
	}
	return output;
}

//nodeList functions

template <class K> vector<K> nodeList<K>::getList(){
	return nodes;
}
template <class K> nodeList<K>::nodeList(const nodeList<K>& n){
	nodes = n.nodes;
}

template <class K> void nodeList<K>::addNode(K k){
	nodes.push_back(k);
}

template <class K> ostream& operator<<(ostream& output, nodeList<K>& operand){
	int i=0;
	vector<K> temp = operand.getList();
	for(i=0;i<temp.size();i++) {
		output<<temp[i]<<'\t';
	}
	return output;
}

//main function

int main(int argc, string argv) {
return 0;}

The error :
Error 2 error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> >
& __cdecl operator<<<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >
(class std::basic_ostream<char,struct std::char_traits<char> > &,class nodeList<class std::basic_string
<char,struct std::char_traits<char>,class std::allocator<char> > > const &)" (??$?6V?$basic_string
@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV
?$nodeList@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@@@Z) referenced in function _main Herexamen.obj

[helios]

Template functions have to be declared and defined in the same file.

[Stiltzkin]

Well, that explains why I had this problem in a lot of my projects, thanks!