I'm trying to get a friend function inside a templated class to compile, but the error message and warning I do not understand. I've made a demonstration of the issue. The error I'm getting is:
prog.cpp:8:57: error: non-class, non-variable partial specialization C operator+(const B& lhs, const C& rhs);
prog.cpp:15:59: warning: friend declaration 'C operator+(const B&, const C&)' declares a non-template function [-Wnon-template-friend] friend C operator+(const B& lhs, const C& rhs);
prog.cpp:15:59: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)
#include <iostream>
using namespace std;
template<typename A, typename B>
class C;
template<typename A, typename B>
C<A, B> operator+<A, B>(const B& lhs, const C<A, B>& rhs);
template<typename A, typename B>
struct C
{
A val_;
C operator+(const C& other) const;
friend C<A, B> operator+(const B& lhs, const C<A, B>& rhs);
};
template<typename A, typename B>
C<A, B> C<A, B>::operator+(const C<A, B>& other) const
{
C<A, B> c;
c.val_ = this->val_ + other.val_;
return c;
}
template<typename A, typename B>
C<A, B> operator+(const B& lhs, const C<A, B>& rhs)
{
C<A, B> c;
c.val_ = lhs + rhs.val_;
return c;
}
int main()
{
C<string, char> c0,c1;
c0.val_ = " C0 ";
c1.val_ = " C1 ";
cout << "Stuct:" << (c0 + c1).val_ << '\n';
cout << "Friend:" << ('~' + c1).val_ << endl;
return 0;
}
This declaration:
template<typename A, typename B>
C<A, B> operator+<A, B>(const B& lhs, const C<A, B>& rhs);
...is wrong because of the <A,B> between operator+ and (, I don't really know what you wanted to do here. You would use this form if you were to specialize a templated operator+, but you are not here, you are overloading one.
This declaration should be:
template<typename A, typename B>
C<A, B> operator+ (const B& lhs, const C<A, B>& rhs);
Then you should explicitely specify in your friend declaration that you want a specialized version by writting:
friend C<A,B> operator+<>(const B& lhs, const C<A,B>& rhs);
You need to put this before your operator+, otherwize the compiler will think this is a specialization of a non-templated function.
Anyway, if you have no real reason to put your code outside the C class, I would go @Jarod42 solution.
Your whole code should look like this:
// Declaration of struct C with delayed definition
template <typename A, typename B>
struct C;
// Initial declaration of templated operator+
template <typename A, typename B>
C<A, B> operator+ (const B&, const C<A, B>&);
// Definition of C
template <typename A, typename B>
struct C {
friend C operator+<> (const B&, const C&);
// This must be AFTER the templated operator+
C operator+ (const C&) const;
};
template<typename A, typename B>
C<A, B> C<A, B>::operator+(const C<A, B>& other) const {
}
template<typename A, typename B>
C<A, B> operator+(const B& lhs, const C<A, B>& rhs) {
}