Вывод данных в поток у наследованного класса

#include <iostream>
#include <fstream>
using namespace std;
 
class a
{
       private:
            int number1;    
       public:
            a(){number1 = 10;};
           ~a(){};
           friend ostream &operator<<(ostream &, a &);
};
 
class b: public a
{
       private: 
               int number2;
    public:
        b():a(){number2 =20;};
        ~b(){};
};
 
ostream &operator<<(ostream &obj, a &tmp)
{
    cout<<tmp.number1;
    return obj;
}
 
void main()
{
    a n1;
    b n2;
    cout<<n1<<endl;
       //cout<<n2<<endl; как реализовать?
}

class base
{
   int val = 10;
public:
   friend std::ostream& operator << (std::ostream& os, const base& b)
   {
      return os << "base: " << b.val << "\n";
   }
   virtual ~base() {}
};
 
class derived : public base
{
   int num = 20;
public:
   friend std::ostream& operator << (std::ostream& os, const derived& d)
   {
      return os << static_cast<base>(d) << "derived: " << d.num << "\n";
   }
};
 
 
int main()
{
   base b;
   derived d;
   std::cout << b;
   std::cout << d;
 
   return 0;
}

class base
{
   int val = 10;
public:
   virtual std::ostream& print(std::ostream& os) const
   {
      return os << "base: " << val << "\n";
   }
   virtual ~base() {}
};
 
class derived : public base
{
   int num = 20;
public:
   virtual std::ostream& print(std::ostream& os) const
   {
      return base::print(os) << "deerived: " << num << "\n";
   }
};
 
std::ostream& operator << (std::ostream& os, const base& b)
{
   return b.print(os);
}