How to implement a private virtual function within derived classes?

Question

Hi,

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

Polymorphism and accessibility are two separate concepts. You can always override a base class' private virtual function, but you won't be able to call the base class version from anywhere but the base class itself. Also, C++ FAQ Lite has a pretty lengthy entry on the subject.

Answer 2

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?

Answer 3

For what I understand here, you're trying to do polymorphism.

There are 4 rules that you must follow to achieve polymorphism.

1. You must inherit from the base class.
2. Your functions must have the same name in every class.
3. You need the virtual keyword in front of every function, and the override keyword at the end of the child's fonctions.
4. You have to use a pointer on the main class, and use the "new" keyword to define it as a child type.

See this awesome wiki page for code example.

Hope this is what you wanted. ^^

Yours truly,

SeargX