why my templates work if I put all of them in main otherwise not

Question

Just for practice purpose I write a generic template class which do sorts for a given comparison function.

#include <vector>
#include <iostream>
#include <fstream>
#include "Student.h"
using namespace std;
template <class T, class F>
class myVec {
public:
    myVec(vector<T> a);
    void sort();

private:
    vector<T> w;
    int size;
};
template<class T, class F>
myVec<T,F>::myVec(vector<T> a){
    w = a;
    size = a.size();
}


template<class T, class F>
void myVec<T,F>::sort(){
    for(int i=0; i<size; i++){
        for(int j=i+1; j<size; j++){
            if(F(w.get(i), w.get(j))>0){
                iter_swap(w.begin()+i-1, w.begin()+j-1);
            }
        }
    }
}




int cmpStudent(Student stu1, Student stu2){
    int score1 = stu1.getScore();
    int score2 = stu2.getScore();
    if(score1 == score2) return 0;
    if(score1<score2) return -1;
    return 1;
}

int main(){
    typedef int(*fn)(Student, Student);
    vector<Student> students;
    string fileName = "mydata.data";
    ifstream ifs;
    ifs.open(fileName.c_str());
    string name;
    int score;
    while(ifs>>name>>score){
        Student student(name,score);
        students.insert(students.end(), student);
    }
    myVec<Student, fn> myvec(students);


}

IT works well.

But if I put myVec as a header file and myVec.cpp. include "myVec.h", then the compiler report error on

myVec<Student, fn> myvec(students);

IT bothers me for a long time, why it works when I put all the materials in main