define non-template function outside a templated class - c++

Question

I was learning about non-template friend function & template friend function to a templated class. So I tried the code below :
#include <iostream>
template<typename T>
class cl
{
private :
T val;
public:
cl()= default;
explicit cl(T v) : val(std::move(v)) {}
friend void non_template_friend(cl m);
};
template <typename T>
void non_template_friend(cl<T> m) { std::cout << m.val << std::endl;}
int main()
{
cl<int> c(10);
non_template_friend(c);
return 0;
}
so when I compile I got : undefined reference tonon_template_friend(cl)' ` So to resolve that I have to move the friend function definition inside the class definition like so :
template<typename T>
class cl
{
private :
T val;
public:
cl()= default;
explicit cl(T v) : val(std::move(v)) {}
friend void non_template_friend(cl m) { std::cout << m.val << std::endl;}
};
But I was wondering,is there any trick to do to be able to define the friend fuinction outside the class definition ?
Thank you.

Answer 1

If you want the definition outside the class, it needs to be a template function since it must be able to accept any kind of c1.
You can achieve this by forward declaring both the class and the function.
#include <iostream>
template<typename T>
class cl;
template<typename T>
void non_template_friend(cl<T> m);
template<typename T>
class cl
{
private :
T val;
public:
cl()= default;
explicit cl(T v) : val(std::move(v)) {}
friend void non_template_friend<T>(cl m); //Now we can refer to a specific instatiation of the template here
};
template <typename T>
void non_template_friend(cl<T> m) { std::cout << m.val << std::endl;}
int main()
{
cl<int> c(10);
non_template_friend(c);
return 0;
}

Answer 2

In the first program
template<typename T>
class cl
{
private :
T val;
public:
cl()= default;
explicit cl(T v) : val(std::move(v)) {}
friend void non_template_friend(cl m);
};
template <typename T>
void non_template_friend(cl<T> m) { std::cout << m.val << std::endl;}
You declared a non-template friend funcrion but then you declared and defined a template function with the same name/
A non-template friend function declaration means thatf you have to provide a set of overloaded non-template functions for specializations of the template class/
Here is a demonstrative program.
#include <iostream>
template<typename T>
class cl
{
private :
T val;
public:
cl()= default;
explicit cl(T v) : val(std::move(v)) {}
friend void non_template_friend(cl m) ;
};
void non_template_friend( cl<int> m) { std::cout << m.val << std::endl;}
void non_template_friend( cl<double> m) { std::cout << m.val << std::endl;}
int main()
{
cl<int> c(10);
non_template_friend(c);
cl<double> c2( 20.2 );
non_template_friend(c2);
return 0;
}
The program output is
10
20.2
That is in a template class a non-template friend function declaration is in fact declares a set of overloaded non-template functions.