Can a virtual function be a friend of another class?

Question

Everywhere in theory, it is written that "A virtual function can be declared as a friend of another class" , but practically on implementing it, the compiler throws an error saying "virtual functions cannot be friends".

Here is a C++ program to illustrate this:

#include <iostream>
using namespace std;

class extra;
class base {
public:
    virtual friend  void print(extra e);

    void show()
    {
        cout << "show base class" << endl;
    }
};

class derived : public base {
public:
    void print()
    {
        cout << "print derived class" << endl;
    }

    void show()
    {
        cout << "show derived class" << endl;
    }
};

class extra
{
    int k;
public:
    extra()
    {
        k=1;
    }
    friend void print(extra e);
};

void print(extra e)
{
    cout<<"virtual friend function "<<e.k;
}

int main()
{    
    base* bptr;
    extra e;
    derived d;
    bptr = &d;

    // virtual function, binded at runtime
    bptr->print(e);

    // Non-virtual function, binded at compile time
    bptr->show();
    print(e);
}

Output screen:

Answer 1

When you write

virtual friend void print(extra e);

C++ interprets this to mean “there is a free function named print , which isn't a member function of the current class, and it's virtual.” That combination can't happen, since virtual functions must be member functions of a class.

What you can do is take an existing virtual function defined in another class and make it a friend of the class. So, for example, if there's a virtual function OtherClass::myFn , you could write

friend void OtherClass::myFn();

to say “that particular virtual function is a friend of me.” As a note, though, this just makes OtherClass::myFn a friend of the class; any overrides of OtherClass::myFn won't be friends of the class, since friendship isn't be inherited.