How to implement a private virtual function within derived classes?

Question

I know why I want to use private virtual functions, but how exactly can I implement them?

For example:

class Base{
[...]
private:
 virtual void func() = 0;
[...]
};

class Derived1: public Base{
  void func() 
  { //short implementation is ok here
  }
};

class Derived2: public Base{
  void func(); //long implementation elsewhere (in cpp file)
};

[...] 

void Derived2::func()
{ //long implementation 
}

The first version is ok but not always possible. Isn't the second version simply name hiding? How do you define the Base::func() of Derived2, if you cannot do it within the class declaration of Dereived2?

Thanks

Answer 1

How do you define the Base::func() of Derived2, if you cannot do it within the class declaration of Dereived2?

You don't define "Base::func() of Derived2" (whatever this might be), you define Derived2::func(). This compiles just fine for me:

#include <iostream>

class Base{
private:
 virtual void foo() = 0;
public:
  void bar() {foo();}
};

class Derived: public Base{
  void foo();
};

void Derived::foo()
{
   std::cout << "inside of 'Derived1::foo()'\n";
}

int main()
{
  Derived d;
  Base& b = d;
  b.bar();
  return 0;
}

What's your problem with it?