dynamic memory allocation template class - c++

Question

I am attempting to create a dynamic memory for this class i have created:
#ifndef STACK_H
#define STACK_H
#include <stdexcept>
#include "Link.h"
template <class T>
struct stack{
Link<T> * head;
stack();
void push(T * data);
T* top();
T* pop();
void cleanup();
};
#endif
src file:
#include "stack.h"
#include <cstddef>
template <class T>
stack<T>::stack(){
head=nullptr;
}
driver:
#include "stack.h"
#include <iostream>
#include <memory>
int main(){
stack<double> oneStack();
stack<double> * oneStack2=new stack<double>;
}
When I compile the code I get the following error:
g++ -Wall driver.o Link.o stack.o -o driver.exe
driver.o: In function main':
driver.cpp:(.text+0x1c): undefined reference tostack::stack()'
for some reason using the new keyword is causing this error?

Answer 1

When you are writing template class, you need to define function in the same place where you declare it, so you should probably write it like this
#ifndef STACK_H
#define STACK_H
#include <stdexcept>
#include "Link.h"
template <class T>
struct stack{
Link<T> * head;
stack()
{
// Constructor code goes here
}
void push(T * data)
{
// Method code goes here
}
T* top()
{
// Method code goes here
}
T* pop()
{
// You get the idea
}
void cleanup()
{
// ...
}
};
#endif
The reason why compiler needs the template classes and functions to be declared and defined in the same place is that it actually generates a new class for every different set of templates parameters that are used in your code. So, imagine you have .h file with declaration of template class and it's methods, and you have a .cpp file where these methods are defined. The compiler tries to compile .cpp file into an .obj file that will later be used to link code into a single executable or library file. But it can't do so, because it doesn't have template parameters right now in this file, so it can't actually generate a concrete code for a concrete parameters. Something like this.
If you want a better insight, you can look here: http://isocpp.org/wiki/faq/templates#templates-defn-vs-decl