Re: how to use private inheritance

From:

Markus Moll <markus.moll@esat.kuleuven.ac.be>

Newsgroups:

comp.lang.c++

Date:

Fri, 22 Aug 2008 13:36:07 +0200

Message-ID:

<48aea4a8$0$1078$9b4e6d93@newsspool3.arcor-online.net>

Hi

zhangyefei.yefei@gmail.com wrote:

#include <iostream>
using namespace std;

class a
{
public:
        virtual void doit() {cout<<"a\n";};
};

class b: private a
{
public:
        void doit() {cout<<"b\n";}

};
class c
{
public:
        void set(a * pa) { m_a =pa;m_a->doit();};
        a * m_a;

};

int main ()
{
        c cc;
        cc.set(new b);
  return 0;
}

but when i change source code sightly ,still with private
inheritance, everything is ok,this surprise me.

[...]

class b: a
{
public:
void doit() {cout<<"b\n";}
void go() { c cc;cc.set(this);};

};

[...]

the above two program seem same to me,but the results arte complete
different.
why ? can anyone do me a favor of giving any hints ?

The question is who is converting the pointer to b to a pointer to a.
In the first program, the function main is trying to do so, in the second,
it's a member function of b. Just like private members can only be accessed
by members of the same class, private base-classes can only be accessed by
members of the class. This means that any member of b is allowed to convert
a pointer to b to a pointer to a.

Markus