how can we make a virtual function a friend of another class?

Question

I have the below program where Derived class is inherited from Base class.

class Base
{   
    int p_var;
public:
    virtual void function()
    {
        cout << "Function() of class Base"<<endl;
    }
};
class Derived: public Base
{
    public:
    void function()
    {
        cout << "Function() of class Derived"<<endl;
    }
};
class Another
{
    int a_var;
    friend void function();
};
void function(Base& base, Another& anot)
{
    anot.a_var=base.p_var;
}
int main()
{
    Base *bptr,bobj;
    Derived dobj;
    bptr=&bobj;
    bptr->function();
    bptr=&dobj;
    bptr->function();
    cin.get();
    return 0;
}

In the above example I want to make "function()" a virtual member function of a "Base" class a friend to "Another" class. how should the syntax be?