protected inheritance in C++

To C++ Expert :

 I have a code looks like the follows :

class Base{
  public:
    Base(){}
    virtual ~Base(){}
} ;

class Derived: protected Base{
  public:
    virtual ~Derived(){}
} ;

int main()
{
    Base* pb = new Derived ;
    return 0 ;
}

However, it won't compile .... the error message says
 " fields of `Base' are inaccessible in `Derived' due to  
   private inheritance " ...

Therefore, I have a few questions here :
1. I use a protected inheritance ..... why it complains
   about the private inheritance ?
2. Why a pointer of the base type can not point to the
    class of derived type ? Is that just because it is a
    protected inheritance ? If so, why ?

 Thanks a lot !
 
meow .......
LVL 1
meow00Asked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Mustak_ShaikhCommented:
Whenever you do a protected inheritence all the public base class members become protected derived class members and all protected base class members become private derived class members.
A pointer of the base type can indeed point to the class of derived type , there is no problem as long as if you public inheritence. because when you do public inheritence base class interface is a subset of derived class's interface , so in that case there is no problem of pointing to derived class's object with base object pointer.
Even when we do public inheritence , and when we make base pointer pointing to derived class's object , basically this pointer pointer points to common data members and functions i.e. the common interface.
as this case is not possible with niether protected nor private inheritence.

let me know if need more info on this.

Mustak_Shaikh

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
meow00Author Commented:
got a general idea now ... but would u mind explaining a bit more
about the "base class interface" and "derived class interface" ?
do they mean data members and member functions ? or they mean something more than that ? thanks !
tinchosCommented:
Yes, I believe that Mustak refered to all the members and methods in the class.

I believe he meant the following.

class Base
{
public:
             int getX();
             int getY();

             int x;
             int y;
}

class Derived
{
             int getW();
             int getX();

             int w;
             int z;
}

in this case the base class interface would be

             int getX();
             int getY();

             int x;
             int y;

and the derived interface

             int getX();
             int getY();

             int x;
             int y;

             int getW();
             int getX();

             int w;
             int z;

Determine the Perfect Price for Your IT Services

Do you wonder if your IT business is truly profitable or if you should raise your prices? Learn how to calculate your overhead burden with our free interactive tool and use it to determine the right price for your IT services. Download your free eBook now!

Mustak_ShaikhCommented:
tinchos,

yes you are correct.
continuing with your answer:
in this scenario if we see the comman interface, then
that interface is having following member function only:
int getX()
so if you declare this
Base *ptrbase = new derived;
this ptrbase will only point to getX() only.
now coming back to your problem, your common
interface is having no data member or funciton at all,
since base class interface and derived class interface are disjoint set,
so you can't do this:
Base *ptrbase = new derived;

regards,
Mustak_Shaikh
Mustak_ShaikhCommented:
tenchos,

i think in your example you have forgotted to include public:
word in Dervied class defination, since by default scope
is private and in that your derived class's interface is null or empty.
Mustak_ShaikhCommented:
All,
there is a small correction about my last comment
where i said only getX() function will be there in common interface
part, but rather all the public data members and function will be there in it.
i.e.
  int getX();
  int getY();

  int x;
  int y;


Regards,
Mustak_Shaikh
tinchosCommented:
Yeap, you're right

I forgot the public keyword in the derived class

It would be like

class Base
{
public:
             int getX();
             int getY();

             int x;
             int y;
}

class Derived
{
public:
             int getW();
             int getX();

             int w;
             int z;
}
meow00Author Commented:
I see .... so is that true that :
when a pointer of base type points to a derived object:
it can only points to the "public , common" members in the derived class. If there is no "public, common" members in the derived class, we can not use a base type pointer to point to a derived object ?
Thanks  a lot !
DexstarCommented:
meow00:

> 1. I use a protected inheritance ..... why it complains
>    about the private inheritance ?

Just to reiterate what was said before:  When you use protected inheritance, everything about the base class is PRIVATE to everything outside of the derived class.  That's why it says "private".

> 2. Why a pointer of the base type can not point to the
>     class of derived type ? Is that just because it is a
>     protected inheritance ? If so, why ?

"Protected" and "Private" inheritance basically HIDE the fact that the Derived class is based on Base class.  When you hide that fact, the compiler will no longer let you treat pointers to Derived as pointers to Base.  Make it public inheritance, then it will let you.

Hope that helps,
Dex*
Scarecrow248Commented:
 Hi there!
  Sorry guys but i don't agree that "When you do a protected inheritence  all protected base class members become private derived class members."
  As i know
+In (protected) derived class the public and protected parts of Base are protected but
+the private parts are still private
 (if you don't belive me, please check atutorial at :
http://www.parashift.com/C++-faq-lite)
 I also think that somewhere in his program mewoo have tried to access private data or methods of Base from Derived.
ScareCrow
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Visual Basic Classic

From novice to tech pro — start learning today.