Question

Possible Duplicate:
Why can templates only be implemented in the header file?

I’ve struggled with this for a while, and I’ve taken a look to several questions here, but being new to C++ I haven’t been able to understand where I am wrong.

Here is the code, I took it from this page and tried to make it work, but so far I haven’t been lucky:

stack.h

#ifndef STACK_H
#define STACK_H
template <class T>
class Stack {
  public:
    Stack(int n);
    ~Stack() { delete[] s; };
  private:
    T* s;
    int _top;
    int _size;
};
#endif // STACK_H

stack.cpp

#include "stack.h"
template <class T>
Stack<T>::Stack(int n) {
  _size = n;
  _top = -1;
  s = new T[_size];
}

main.cpp

#include <iostream>
#include "stack.h"
using namespace std;
int main() {
  Stack<int> s(10); // undefined reference to `Stack<int>::Stack(int)'
  return 0;
}

When I compile (gcc 4.5.2) I get one error: undefined reference to Stack<int>::Stack(int). I’ve tried several things but without any real knowledge to support what I do. I will be really thankful if somebody can explain me what’s going on.

Answer 1

You can only have a template class definition in a cpp file if it’s a specialized definition – i.e. you know what T is.

Other than that, and your case belongs here, all definitions of your template class have to go in the header file. The compiler has to know about these each time a new instance is declared or defined because the type, and thus the behavior, changes. Definitions in a cpp file would mean a different translation unit, so the compiler couldn’t possibly know about the behavior for every single T you try to apply to your template.