Re: Question about using copy constructor of parent class?

From:

Rolf Magnus <ramagnus@t-online.de>

Newsgroups:

comp.lang.c++

Date:

Fri, 21 Jul 2006 11:57:34 +0200

Message-ID:

<e9q8ec$hbu$03$1@news.t-online.com>

flamexx7 wrote:

In my book there is a question about "properly" creating a copy
constructor of child class during inheritance. Is it ok to use upcasting
here ?

No need to cast.

I've used upcasting and it seems to work fine.

#include <iostream>
using namespace std;
class A{
      int a;
      public:
             A(int temp=0):a(temp){}
             A(A& right):a(right.a){}
             };
class B:public A{
      int b;
      public:
             B(int temp=0):b(temp){}
             B(B& right){A(*this);
                  b=right.b;
              }

This constructor probably won't do what you want. The

A(*this);

creates a new temprary object of type A and immediately afterwards, destroys
it. The A part of your B object gets default initialized. It has nothing at
all to do with the A in your constructor body. I'll repeat myself: You can
only initialize the base class parts of your object in the initializer
list. In the constructor body, the initialization is finished.

             };
int main(){
    B b;
    B b1=b;
    cin.get();
}

You tricked yourself, because the initialization you were trying to do just
happens to do the same as the default initialization - they both set the
int member to 0.