Re: derived class and virtual function

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 17 Aug 2010 01:54:18 -0700 (PDT)

Message-ID:

<d743ff85-7926-4b90-9320-9e4dadcd66e0@z10g2000yqb.googlegroups.com>

On Aug 13, 10:09 am, "Francesco S. Carta" <entul...@gmail.com> wrote:

Francesco S. Carta <entul...@gmail.com>, on 13/08/2010 10:39:51, wrote:

subramanian10...@yahoo.com, India <subramanian10...@yahoo.com>, on
13/08/2010 01:29:13, wrote:

In Stanley Lippman's 'C++ Primer Fourth Edition', in page 564, the
following is mentioned:

"A virtual function in the derived class can return a reference or
pointer to a class that is PUBLICLY derived from the type returned by
the base class function."

I am unable to understand this sentence. Kindly explain it with
program sample.

IIUIC, that means that if you have a function with signature "virtual
Base* Clone() const;" in the base class, then the implementation of the
same function in the derived class can be: "Base* Clone() const { retur=

new Derived(*this); }" - just an example that assumes Derived is
copy-constructible, of course.

There is either something wrong in that sentence, in my understanding
or in my compiler - or my settings thereof - because it allows me to
return a pointer to Derived where Base is private:

In your understanding, I think, because...

//-------

#include <iostream>

using namespace std;

class Base {
     public:
     Base(int data = 0) : data(data) {};
     Base(const Base& base) : data(base.data) {};
     virtual Base* Clone() const {
         cout << "cloning Base" << endl;
         return new Base(*this);
     }
     int Data() const {
         return data;
     }
     private:
     int data;
};

class Derived : private Base {
     public:
     Derived(int data = 0) : Base(data) {};
     Derived(const Derived& derived) : Base(derived) {};
     Base* Clone() const {

Here, you're still returning a Base. Any conversion takes place
in Derived (where the derivation is visible).

Try changing the return type to Derived*; the compiler should
complain then.

See =A710.3/5 in the standard for details.

         cout << "cloning Derived" << endl;
         return new Derived(*this);
     }
};

--
James Kanze