private_virtual.cpp

// You CAN call private virtual functions via public base interface

#include <iostream>

class Base {
  public:
    virtual void f() { std::cout << "Base::f()" << std::endl; }
    virtual ~Base() = default;
};

class Derived : public Base {
  private:
    virtual void f() override { std::cout << "Derived::f()" << std::endl; }
};
int main() {
    Base* base = new Derived;
    base->f();
}