overloaded << operator function : unresolved external symbol error - c++

Question

I've overloaded << operator to print the address in pointer member of a class. However, it throws the following error (using Visual Studio 2017). Using a normal class method does the job.
Any leads?
Error:
error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl IntroSmartPointers::operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class IntroSmartPointers::AutoPtr<class IntroSmartPointers::Resource> const &)" (??6IntroSmartPointers##YAAEAV?$basic_ostream#DU?$char_traits#D#std###std##AEAV12#AEBV?$AutoPtr#VResource#IntroSmartPointers###0##Z) referenced in function "void __cdecl IntroSmartPointers::main(void)" (?main#IntroSmartPointers##YAXXZ)
Code:
#include <iostream>
using namespace std;
namespace SmartPointers {
template<class T>
class AutoPtr {
private:
T* m_ptr;
public:
// constructor
AutoPtr(T* ptr = nullptr)
: m_ptr(ptr)
{}
// destructor
~AutoPtr() {
delete m_ptr;
}
friend ostream& operator<<(ostream& out, const AutoPtr<T>& ptr);
};
template<class T>
ostream& operator<<(ostream& out, const AutoPtr<T>& ptr) {
out << ptr.m_ptr;
}
class Resource {
public:
Resource() { cout << "Resource(): acquired\n"; }
~Resource() { cout << "~Resource: destroying...\n"; }
void sayHi() {
cout << "Hi\n";
}
};
}
void main() {
SmartPointers::AutoPtr<Resource> ptr{ new Resource{} };
cout << ptr << endl;
}

Answer 1

Your friend declares non-template operator.
In this case, you can define a friend function inside of the class body, or try the following:
template<typename T>
class AutoPtr {
// ...
template<typename U>
friend ostream& operator<<(ostream& out, const AutoPtr<U>& ptr);
};
template<class T>
ostream& operator<<(ostream& out, const AutoPtr<T>& ptr) {
out << ptr.m_ptr;
return out; // You missed this part too
}
see Template friend operators section in https://en.cppreference.com/w/cpp/language/friend
And main should return int instead of void.