Re: derived class and virtual function

From:
"Francesco S. Carta" <entuland@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 13 Aug 2010 11:09:51 +0200
Message-ID:
<4c650bdd$0$30893$5fc30a8@news.tiscali.it>
Francesco S. Carta <entuland@gmail.com>, on 13/08/2010 10:39:51, wrote:

subramanian100in@yahoo.com, India <subramanian100in@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 { return
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:

//-------

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

int main() {

// fails, of course
// Base* pb = new Derived(42);

     Derived d(42);

     // calls Derived::Clone()
     Base* pb2 = d.Clone();

     // calls Derived::Clone()
     Base* pb3 = pb2->Clone();

     // prints 42
     cout << pb3->Data() << endl;
}

//-------

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