c++ derived class with private access specifier

Question

I have a derived class (class B) from a base class (class A). Class A has a protected virtual function foo() which I want to override and use it as private in derived class.

Class A{
  protected:
   virtual void foo() = 0;
}

I am wondering whether the following

Class B: public Class A
  private:
    virtual void foo();

and

Class B: private Class A
  private:
    virtual void foo();

are the same.

Why would you want to do that? – molbdnilo Oct 14 '13 at 13:18 — molbdnilo, Oct 14 '13 at 13:18

juanchopanza · Accepted Answer · 2013-10-14T13:16:40.400

1

They are not the same. In the first example, B is-an-A, in the second it isn't. So in the first case you can have code such as

void foo(const A& a);

which accepts A and B as arguments. With private inheritance, you could not do that. For example,

A a;
B b;
foo(a); // OK with private and public inheritance
foo(b); // OK only with public inheritance, i.e. B is an A.

edited Oct 14 '13 at 13:16

answered Oct 14 '13 at 13:07

juanchopanza

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I think in the second case `B` is-an-`A` too, although you can not address `B`'s object by `A`'s reference (or pointer). – masoud Oct 14 '13 at 13:10
@MM. I would say it *has-an-A*, but I realise maybe there is no consensus about this. – juanchopanza Oct 14 '13 at 13:14
@juanchopanza Can you give an example and do you know any place which explains this more detailed? I did not get it completely. – Avb Avb Oct 14 '13 at 13:15
@AvbAvb I added a little example that illustrates what I am referring to. – juanchopanza Oct 14 '13 at 13:17

score 0 · Answer 2 · answered Oct 14 '13 at 13:11

0

No, your two cases are not the same.

In the second case class B can't be casted to class A, because a private base class is hidden. in this aspect would get the same behavior as if class A would be a member of class B.

answered Oct 14 '13 at 13:11

villekulla

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score 0 · Answer 3 · answered Oct 14 '13 at 13:38

0

No both are not same In public inheritance class A's foo() will be protected member in class B In private inheritance class B can only access the public members of the class A so there will not present any foo() of class A in class B.

answered Oct 14 '13 at 13:38

Atul

143
10

c++ derived class with private access specifier

3 Answers3