how to define a constructor outside [duplicate]

This question already has an answer here:

• Why can templates only be implemented in the header file? 12 answers

A.h:

template<typename T>
class A{
public:
    A(int n);
    int a;
};

A.cpp:

#include "header.h"
template <typename T>
A<T>::A(int n){
    this->a = n;
}

main:

int main ()
{
    A<int> m(5);
    return 0;
};

and i got err undefined reference to `A::A(int)',code seems correct to me, any idea why this happen? thx a lot.