friend operator << and private access of template class member

Question

After reading this answer , I made some tests and I came up with the following code:

#include <iostream>

template <typename T> 
class Test {
    int val{42};

    friend std::ostream & operator << (std::ostream & flux, Test const & instance) {
        return flux << Test<char>{}.val;
    }
};

int main() {
    std::cout << Test<int>{};
    return 0;
}

Using Test<int> in main() , I thought that operator<< couldn't access Test<char>{}.val . But to my surprise, GCC compiled fine.

Then, I tested with Clang and Visual Studio, and both gave me a private member error, as expected (see GCC/Clang demo ).

Which compiler is right?

Also, I'm wondering in which case one would need the extrovert version mentioned in the link? Or private access to Test<U> from within operator<< of Test<T> (when T != U )? Do you have any practical examples?

Answer 1

I believe Clang/VS are using the correct behavior, because if you create a new instance (as in your example) instead of using the friended instance from the parameters I believe it resets the access to be as if it's from outside.

As for the extra question, I can't think of a practical example the extrovert, but I imagine it's there for the same reason as many things in C++, to let you do it that way if you want to. Maybe use it to convert from one type to another? not sure, sorry^^;

PS sorry for the original confusion.