Question

I get a linker error when I try to create an executable from the following code. I get the impression I need to place a few “typenames” around or make some forward declarations; I’ve tried a few combinations but none worked.

template<typename T>
class enabled
{
  private:
    T type_;
    friend const T& typeof(const enabled<T>& obj); // Offending line
};

template<typename T>
const T& typeof(const enabled<T>& obj) {
    return obj.type_;
}


int main()
{
    enabled<std::string> en;
    std::cout << typeof(en);

    std::cin.clear(), std::cin.get();
    return 0;
}

1>main.obj : error LNK2001: unresolved external symbol “class std::string const& __cdecl typeof(class enabled<class std::string> const&)”

Answer 1

By forward declaring and specifying that the function is templated

template<typename T> class enabled;

template<typename T>
const T& typeof(const enabled<T>& obj) {
    return obj.type_;
}

template<typename T>
class enabled
{
  private:
    T type_;
    friend const T& typeof<>(const enabled<T>& obj);
};