Re: Question about using copy constructor of parent class?

From:

"joosteto@gmail.com" <joosteto@gmail.com>

Newsgroups:

comp.lang.c++

Date:

21 Jul 2006 02:32:08 -0700

Message-ID:

<1153474328.614012.244330@h48g2000cwc.googlegroups.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 ? 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;
              }
             };
int main(){
    B b;
    B b1=b;
    cin.get();
}

It compiles, but, although I don't know what you want it to do, it
doesn't look like it does what I think you want it to do.

In the copy constructor of B, you initialize A with *this, before B is
initialised. So, if you in main() do:

int main(){
  B b(10);
  B b1=b;

you'll get a B1 who's a is uninitialised (that's what it looks like to
me. I just compiled it with g++-3.4.4, and that shows that b1.a
initialised to whatever default value I put in A::A(int temp=xxx)
constructor.

#include <iostream>
using namespace std;
class A{
  int a;
public:
  A(int temp=123):a(temp){}
  A(A& right):a(right.a){}
  void show(){
    cout<<"A: a="<<a<<endl;
  }
};
class B:public A{
  int b;
public:
  B(int temp=124):b(temp){}
  B(B& right){A(*this);
  b=right.b;
  }
  void show(){
    A::show();
    cout<<"B: b="<<b<<endl;
  }
};
int main(){
    B b(10);
    b.show();
    B b1=b;
    b1.show();
}

Output:

A: a=123
B: b=10
A: a=123
B: b=10

Oh, and BTW, why didn't you do as suggested, make the ref arguments
const?