Re: how to use private inheritance

i read book <effective c++>,it tell me that public inheritance means
is-a ,and private inheritance means is-implemented-in-terms-of.
but today i am puzzled by some strange codes.

the following program can not pass compiling , bailing :
g++ d.cpp -o d
d.cpp: In function `int main()':
d.cpp:27: error: `a' is an inaccessible base of `b'

it is obviously okay to understand,because private inheritance is-
implemented-in-terms-of.

#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.

#include <iostream>
using namespace std;

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

};

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

};

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

};

int main ()
{
        b bb;
        bb.go();
  return 0;

}

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 ?
thanks.