Question

I have a base Class akin to the code below. I’m attempting to overload << to use with cout.
However, g++ is saying:

base.h:24: warning: friend declaration ‘std::ostream& operator<<(std::ostream&, Base<T>*)’ declares a non-template function
base.h:24: warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning

I’ve tried adding <> after << in the class declaration / prototype. However, then I get it does not match any template declaration. I’ve been attempting to have the operator definition fully templated (which I want), but I’ve only been able to get it to work with the following code, with the operator manually instantiated.

base.h

template <typename T>
class Base {
  public:
    friend ostream& operator << (ostream &out, Base<T> *e);
};

base.cpp

ostream& operator<< (ostream &out, Base<int> *e) {
    out << e->data;
return out;
}

I want to just have this or similar in the header, base.h:

template <typename T>
class Base {
  public:
    friend ostream& operator << (ostream &out, Base<T> *e);
};

template <typename T>
ostream& operator<< (ostream &out, Base<T> *e) {
    out << e->data;
return out;
}

I’ve read elsewhere online that putting <> between << and () in the prototype should fix this, but it doesn’t. Can I get this into a single function template?

Answer 1

It sounds like you want to change:

friend ostream& operator << (ostream& out, const Base<T>& e);

To:

template<class T>
friend ostream& operator << (ostream& out, const Base<T>& e);