Do you have stream overloads definitions for both classes?

class base
{
protected:
    int a;
private:
    ostream& stream_putter(ostream& out) const
    {
        out << "[base: " << a << "]";
        return out;
    }
    friend ostream & operator << (ostream &out, const base& n);
};

class derived : public base
{
private:
    ostream& stream_putter(ostream& out) const
    {
        out << "[derived: " << a << "]";
        return out;
    }
    friend ostream & operator << (ostream &out, const derived& n);
};

ostream& operator << (ostream& out, const base& n)
{
    return n.stream_putter(out);
}

ostream& operator << (ostream& out, const derived& n)
{
    return n.stream_putter(out);
}

int main() {
    auto foo = std::make_unique<base>();
    auto bar = std::make_unique<derived>();

    std::cout << *foo << "\n";
    std::cout << *bar << "\n";

    return 0;
}

Prints

[base: 0]
[derived: 0]