problem with template in extern .h and .cpp files

From:

Stefan Bollmann <earl_of_oelde@web.de>

Newsgroups:

comp.lang.c++

Date:

Mon, 19 Nov 2007 05:38:43 -0800 (PST)

Message-ID:

<dcbab79a-798a-4471-a4b8-8c5895d701da@a28g2000hsc.googlegroups.com>

Hi,

i wonder about a problem with functions i cannot put out of my
main.cpp file into one *.h and *.cpp. When I run this little
testprogram:

#include <iostream>
#include <vector>
//#include"potential.h"
using namespace std;

template<class T>void test(T t);

template<class T>void test(T t)
{
    cout << t << endl;
}

int main ()
{
    cout << "Il sole splende!" << endl;
    vector<vector<double> >A(2);
    for (int i=0;i<A.size();i++)
        A[i].resize(4,2);
    test((double)5.);

    return 0;
}

everything is well.
If i compile the following code everythings fine at first, too, but
running it produces a crash.
Code:

--- main.cpp ---

#include <iostream>
#include <vector>
#include"potential.h"
using namespace std;

int main ()
{
    cout << "Il sole splende!" << endl;
    vector<vector<double> >A(2);
    for (int i=0;i<A.size();i++)
        A[i].resize(4,2);
    test((double)5.);

    return 0;
}

--- potential.h ---

#include <iostream>
using namespace std;

template<class T>void test(T t);

--- potential.cpp ---

#include "potential.h"

template<class T>void test(T t)
{
    cout << t << endl;
}

console:

myPC>g++ main.cpp potential.h potential.cpp
/usr/bin/ld: Undefined symbols:
void test<double>(double)
collect2: ld returned 1 exit status

I wonder about it, because if i don't use templates everything is fine
again:

--- potential.h ---

#include <iostream>
using namespace std;

void test(double t);

-- potential.cpp ---

#include "potential.h"

void test(double t)
{
    cout << t << endl;
}

and the main.cpp like the second example.
What am I doing/thinking wrong?

Thanks for reading ;-) ,
Stefan