g++ Compiling/linking errors for the simple code.

[kikia]

I have three files.
main.cc

#include "Vector.h"

int main() {
	Vector<double> d(4, 0.2);
	return 0;
}

Vector.h

#ifndef _VECTOR_H_
#define _VECTOR_H_

template <class T>
class Vector {
private:
	int nn;	// size of array. upper index is nn-1
	T *v;
public:
	Vector(int n, const T &a);	//initialize to constant value
	~Vector();
};

#endif 

Vector.cc

#include <iomanip>
#include "Vector.h"

template <class T>
Vector<T>::Vector(int n, const T& a) : nn(n), v(n>0 ? new T[n] : NULL) {
	for(int i=0; i<n; i++) v[i] = a;
}

template <class T>
Vector<T>::~Vector() {
	if (v != NULL) delete[] (v);
}

I compiled it with g++ in linux as follows.
g++ main.cc Vector.cc -o main

Here's the output.

main.cc:(.text+0x2c): undefined reference to `Vector<double>::Vector(int, double const&)'
main.cc:(.text+0x3d): undefined reference to `Vector<double>::~Vector()'
collect2: ld returned 1 exit status

Can anyone fix it? Thank you very much.

[helios]

Template functions have to be defined in the same same file as they are declared or they'll produce linker errors. Alternatively, if your compiler supports the export keyword (and it almost certainly doesn't), you can use it to put the definition in a different file.
If one day you're bored out of your mind, open the standard <vector> header. You'll notice that includes the entire definition of the std::vector class.