Public and private inheritance in C++

Question

As we know from the literature for the public inheritance the object of child class (sub-class) also can be considered as the object of base class (super-class). Why the object of the sub-class can’t be considered as an object of super-class, when the inheritance is protected or private?

Answer 1

public inheritance serves the purpose of the is-a relationship. That is:

class A {};
class B : public A {};

Class B is a version of class A.

private inheritance serves the purpose of the has-a relationship. You can write almost any class using private inheritance using a container model instead:

class A {};
class B : private A {};

can be rewritten (and more often than not, should be rewritten for clarity):

class A {};
class B
{
private:
    A a;
};

protected inheritance is similar to private, but in reality should almost never be used (Scott Meyers and Herb Sutter both give reasons for this in their respective books).