Linker Error for templated Class with friend functions - c++

Question

I'm trying to recreat a stack with a forward_list. However, i use friend functions to overload the + and << operator.
#pragma once
#include <forward_list>
template <class T> class Stack;
template <class T>
Stack<T> operator+(const Stack<T> &a, const Stack<T> &b){
//implementation
}
template <class T>
std::ostream &operator<<(std::ostream &output, Stack<T> &s)
{
//implementation
}
template <class T>
class Stack
{
friend Stack<T> operator+(const Stack<T> &a, const Stack<T> &b);
friend std::ostream &operator<<(std::ostream &output, Stack<T> &s);
std::forward_list<T> l;
public:
//Some public functions
};
For both of the friend functions i get an linker Error , when i try to call them in my main like:
int main(){
Stack<int> st;
st.push(4);
Stack<int> st2;
st2.push(8);
cout<<st + st2<<endl;
return 0;
}
And these are the errors:
error LNK2019: unresolved external symbol "class Stack<int> __cdecl operator+(class Stack<int> const &,class Stack<int> const &)" (??H#YA?AV?$Stack#H##ABV0#0#Z) referenced in function _main
error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class Stack<int> &)" (??6#YAAAV?$basic_ostream#DU?$char_traits#D#std###std##AAV01#AAV?$Stack#H###Z) referenced in function _main
Thanks in advance.

Answer 1

Your template friend declarations within the Stack class are not quite correct. You need to declare like this:
template<class T>
friend Stack<T> operator+(const Stack<T> &a, const Stack<T> &b);
template<class T>
friend std::ostream &operator<<(std::ostream &output, Stack<T> &s);
Since you are using MSVC, please see this Microsoft documentation for further reference.