#include <iostream>

using namespace std;

class base
{
   friend class derived; //<======= note this line
   private:
      base()
      {
         cout << "base: ctor()\n";
      }
};

class derived: public base
{
   public:
      derived()
      {
         cout << "derived: ctor()\n";
      }
};

int main()
{
   derived d;
}