Unresolved external symbols for cout operator overload - c++

Question

I'm getting "Unresolved external symbols" for the following code:
template <int n> class Fibo
{
private:
int pre, cur, next, tmp;
public:
Fibo() : pre(1), cur(1), next(2), tmp(0) {}
int get()
{
if(n == 0 || n == 1) return 1;
for(int i = 2; i < n; ++i)
{
next = pre + cur;
tmp = cur;
cur = next;
pre = tmp;
} return pre + cur;
}
friend ostream& operator<< (ostream& out, Fibo<n>& f);
};
template<int n>
ostream& operator<< (ostream& out, Fibo<n>& f)
{
out << f.get();
return out;
}
int main()
{
Fibo<5> a;
cout << a << endl;
cin.get();
}
I get this compilation error when I'm trying to print a:
cout << a << endl;. When I'm printing "normaly", i.e. cout << a.get() << endl, no errors popped.
I know that Unresolved external symbols error are associated with declared function that didn't implemented. is that the case in my code? I can't find it.

Answer 1

There are a couple ways to handle this:
template <int n> class Fibo ; // Forward declaration
// Definition, you could also forward declare here and define after the class
template<int n>
ostream& operator<< (ostream& out, Fibo<n>& f)
{
out << f.get();
return out;
}
template <int n> class Fibo
{
// Don't forget <> to tell its a template
friend ostream& operator<< <> (ostream& out, Fibo<n>& f);
};
The above way is verbose, but you could also define the friend in-class:
template <int n> class Fibo
{
// Note this is not a template
friend ostream& operator<< (ostream& out, Fibo& f)
{
out << f.get();
return out;
}
};
The in-class definition will define a friend function for each instantiation, i.e. Fibo<1>, Fibo<2>, etc.

Answer 2

GCC compiler messages:
test.C:22:57: warning: friend declaration ‘std::ostream& operator<<(std::ostream&,
Fibo<n>&)’ declares a non-template function [-Wnon-template-friend]
test.C:22:57: note: (if this is not what you intended, make sure the function
template has already been declared and add <> after the function name here)
Pretty self-explanatory I think.