private inheritance?

Usually composition are used instead of private inheritance, suck like

class Y
{
   private:
   X x;
   public:
   ...
}

private inheritance is useful if you want to hide part of the functionality of the base class:

class Pet {
public:
   char eat() const { return 'a'; }
   int speak() const { return 2; }
   float sleep() const { return 3.0; }
   float sleep(int) const { return 4.0; }
};
class Goldfish : Pet { // Private inheritance
public:
   Pet::eat; // Name publicizes member
   Pet::sleep; // Both overloaded members exposed
};
int main() {
   Goldfish bob;
   bob.eat();
   bob.sleep();
   bob.sleep(1);
   //! bob.speak();// Error: private member function
} 

--Thinkinginc++