In article <39E58D13.358BC7FD@tre.ntc.nokia.com>,
  ext-pratik.khasnabis@nokia.com wrote:

[...]
> Please comment about the behaviour of this code.

> =======================CODE================================
> #include <iostream>
>
> using namespace std;
> class Base
> {
> public:
>  Base():a(0){}
>  int a;
> };
>
> class PubD : public Base
> {
> };
>
> class PrivD : private Base
> {
> };
>
> class ProtD : protected Base
> {
> };
>
> class PubProtD : public ProtD
> {
>  void myfunc(PubD *pD1 , PrivD *pD2 , ProtD *pD3) {
>
>  Base *pB;
>
>  pB = pD1;
>  pB->a = 2;
>
>  //pB = pD2;
>  //pD2->a = 2;
>  pB = pD3;    //--------------------- >> error in this line ???
>  //pD3->a = 2;
>
> }
> };
>
> int main() {
> return 0;
> }

Gcc and Borland are correct (even though gcc's error message is
misleading), Comeau, HP aCC, and you are wrong. Protected inheritance
means that the derived classes can access public and protected members
of the base class *in its own object*.

/Niklas Mellin



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In article <8s7als$ouf$1@nnrp1.deja.com>,
  Niklas Mellin <nimel@my-deja.com> wrote:
> In article <39E58D13.358BC7FD@tre.ntc.nokia.com>,
>   ext-pratik.khasnabis@nokia.com wrote:
>
> [...]
> > Please comment about the behaviour of this code.
>
> > =======================CODE================================
> > #include <iostream>
> >
> > using namespace std;
> > class Base
> > {
> > public:
> >  Base():a(0){}
> >  int a;
> > };
> >
> > class PubD : public Base
> > {
> > };
> >
> > class PrivD : private Base
> > {
> > };
> >
> > class ProtD : protected Base
> > {
> > };
> >
> > class PubProtD : public ProtD
> > {
> >  void myfunc(PubD *pD1 , PrivD *pD2 , ProtD *pD3) {
> >
> >  Base *pB;
> >
> >  pB = pD1;
> >  pB->a = 2;
> >
> >  //pB = pD2;
> >  //pD2->a = 2;
> >  pB = pD3;    //--------------------- >> error in this line ???
> >  //pD3->a = 2;
> >
> > }
> > };
> >
> > int main() {
> > return 0;
> > }
>
> Gcc and Borland are correct (even though gcc's error message is
> misleading), Comeau, HP aCC, and you are wrong. Protected inheritance
> means that the derived classes can access public and protected members
> of the base class *in its own object*.

Well, that's not quite the rule (see 11.5: it has to be via a
pointer to its own class or one derived from it, but it could
be in any object, not just *this).  However, I think the
conclusion is right.  The applicable description is in 11.2p4.
The question of whether PubProtD::my_func can convert pD3 to
Base* is equivalent to whether it could access an invented
public member of Base via pD3.  The answer is "no" -- a public
member of Base is a protected member in ProtD, and ProtD is
not PubProtD nor a class derived from it.  Thus the invented
member is inaccessible and the conversion is not permitted.

--
William M. Miller, wmm@fastdial.net
Vignette Corporation (www.vignette.com)


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Hi
I was following the example in Bjarne's book (C.11.2). I am including
the minimal compilable version of my code. I tried four different
compilers. Stroustrup means something like this. Because class Base is a
protected base of ProtD , members and friends  of ProtD's derived
classes (eg PubProtD) can (impicitly) convert a ProtD* to Base* , just
as they can access public and protected members of class Base.
If there is a bug in the Borland and GNU compiler it is serious indeed
as concept of protected is fundamental to C++.In fact gcc's error
message says ProtD has a private inheritance which is totally
incomprehensible. In fact I was thinking some kind of protected
inheritance for a curious design problem. Now this result got me
thinking.
Please comment about the behaviour of this code.
Pratik
=======================CODE================================
#include <iostream>

using namespace std;
class Base
{
public:
 Base():a(0){}
 int a;
};

class PubD : public Base
{
};

class PrivD : private Base
{
};

class ProtD : protected Base
{
};

class PubProtD : public ProtD
{
 void myfunc(PubD *pD1 , PrivD *pD2 , ProtD *pD3) {

 Base *pB;

 pB = pD1;
 pB->a = 2;

 //pB = pD2;
 //pD2->a = 2;
 pB = pD3;    //--------------------- >> error in this line ???
 //pD3->a = 2;

}
};

int main() {
return 0;
}



===============================RESULTS=======================
Comeau Compiler 4.2.44 (on the web):-
Compile succeeded

HP aCC 3.13 :-
OK

gcc 2.95.2:-
In method `void PubProtD::myfunc(PubD *, PrivD *, ProtD *)':
accesstobase.cc: fields of `Base' are inaccessible in `ProtD' due to
private inheritance

Borland5.5 bcc32:-
accesstobase.cpp : Cannot convert 'ProtD *' to 'Base *' in function
PubProtD::myfunc(PubD *,PrivD *,ProtD *)

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pratik khasnabis <pratik.khasnabis@tre.ntc.nokia.com> writes:

[snip]
> gcc 2.95.2:-
> In method `void PubProtD::myfunc(PubD *, PrivD *, ProtD *)':
> accesstobase.cc: fields of `Base' are inaccessible in `ProtD' due to
> private inheritance

I believe gcc's behavior in this example is a bug. You can report it
  by following the instructions at gcc.gnu.org/bugs.html

>
> Borland5.5 bcc32:-
> accesstobase.cpp : Cannot convert 'ProtD *' to 'Base *' in function
> PubProtD::myfunc(PubD *,PrivD *,ProtD *)

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