[Dev-C++] templates + linker error

[Jorge G. <jor...@gm...>]

i have 3 source  calc.h calc.cpp and principal.cpp (i compilated it with
devcpp)

i have this error:
In function `main':
[Linker error] undefined reference to `calc<double>::multiply(double,
double)'
ld returned 1 exit status
[Build Error] ["Proyecto] Error 1

somebody could help me please :)

/********calc.h******/
#ifndef _CALC_H
#define _CALC_H

template <class A_Type> class calc
{
public:
A_Type multiply(A_Type x, A_Type y);
A_Type add(A_Type x, A_Type y);

};

#endif

/****calc.cpp******/
#include "calc.h"
#include <iostream>
using namespace std;

template <class A_Type> A_Type calc<A_Type>::multiply(A_Type x,A_Type y)
{
return x*y;
}
template <class A_Type> A_Type calc<A_Type>::add(A_Type x, A_Type y)
{
return x+y;
}


/******principal.cpp*****/
#include "calc.h"
#include <iostream>
#include <cstdlib>

using namespace std;

int main()
{
calc <double> a_calc_class;
double a, b;
a=4.3;
b=3.4;
a_calc_class.multiply(a,b);

system("PAUSE");
return EXIT_SUCCESS;
}
/***************************/