This is a way to get polymorphic behaviour for operator <<.

quiz_polymorphic_for_stream.cpp

#include <iostream>

struct A
{
    virtual std::ostream& put(std::ostream& o ) const { return o << 'A'; };
};

struct B : A
{
    virtual std::ostream& put(std::ostream& o ) const { return o << 'B'; };
};

std::ostream& operator <<(std::ostream& o, const A& a)
{
    return a.put(o);
}

int main () {
    B b;
    std::cout << b << std::endl ;
}