C++ private and protected virtual method

Question

It seems that it is good to make the virtual methods private in order to separate the interfaces for following two clients - 1. clients that instantiate an object and call the

Answer 1

You are exactly right:

• NVI (Non-Virtual Interface) requires that virtual methods not be public
• Calling the base class method requires that it not private

therefore protected is the obvious solution, at least in C++03. Unfortunately it means you have to trust the derived class developer not to forget to call "super".

---

In C++11, you can use final to prevent a derived class from overriding a virtual method; it means though that you are forced to introduce a new hook, example:

class Base {
public:
    void save() {
        // do something
        this->saveImpl();
        // do something
    }
private:
    virtual void saveImpl() {}
};

class Child: public Base {
private:
     virtual void saveImpl() final {
         // do something
         this->saveImpl2();
         // do something
     }
     virtual void saveImpl2() {}
};

Of course, there is the trouble of having to come up with a new name each and every time... but at least you are guaranteed that Child::saveImpl will be called because none of its children can override it.