How to implement an interface class using the non-virtual interface idiom in C++?

Question

In C++ an interface can be implemented by a class whose methods are pure virtual.

Such a class could be part of a library to describe what methods an object should

Answer 1

Commonly, the reason for using the NVI (sometimes also called "Template Method") is that derived classes should only change a part of the base class' behavior. So what you do is this:

class base {
  public:
    void f()
    {
      // do something derived classes shouldn't interfere with          
      vf();
      // do something derived classes shouldn't interfere with          
      vg();
      // do something derived classes shouldn't interfere with          
      vh();
      // do something derived classes shouldn't interfere with          
    }
  private:
    virtual void vf(); // might be pure virtual, too
    virtual void vg(); // might be pure virtual, too
    virtual void vh(); // might be pure virtual, too
};

Derived classes can then plug into f() at the spots they are meant to and change aspects of f()'s behavior, without messing up its fundamental algorithm.