Question

How do I fix an "Undefined Reference" error?

I'm just starting out with c++ after learning a great deal of Java.
I think the error has something to do with the very first part of my program:

#include <iostream>
struct Stack;
void initialize(Stack);
bool push(Stack,int);
bool isEmpty(Stack);
int pop(Stack);
using namespace std;

this is followed by a main method and the methods listed above.
Is there anything wrong with the code snippet above or would the problem be somewhere else in my program?