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comp.lang.c++

initialization failure

thomas

10/1/2008 8:19:00 AM

Hi,

------------code--------------------------------------
#include<iostream>
using namespace std;

class B{
int y;
public:
B(int b=3):y(b){}
int getInt(){
return y;
}
};

class A{
int x;
public:
A(const int &x_){ //Line 1
x=x_;
}
A(B *b){
A(b->getInt()); //Line 2
}
int getInt(){
return x;
}
};

int main(){
B *b = new B;
A *a = new A(b);
cout<<a->getInt()<<endl;
}
---------------------------------------------------------
I expected that in line 2, "b->getInt()" is 3, and by calling
construction function of Line 1, the printed result should be 2;

But it's 0, can anyone tell me the reason? Thanks in advance.
5 Answers

thomas

10/1/2008 8:31:00 AM

0

On Oct 1, 4:19 pm, thomas <FreshTho...@gmail.com> wrote:
> Hi,
>
> ------------code--------------------------------------
> #include<iostream>
> using namespace std;
>
> class B{
>         int y;
> public:
>         B(int b=3):y(b){}
>         int getInt(){
>                 return y;
>         }
>
> };
>
> class A{
>         int x;
> public:
>         A(const int &x_){          //Line 1
>                 x=x_;
>         }
>         A(B *b){
>                 A(b->getInt());        //Line  2
>         }
>         int getInt(){
>                 return x;
>         }
>
> };
>
> int main(){
>         B *b = new B;
>         A *a = new A(b);
>         cout<<a->getInt()<<endl;}
>
> ---------------------------------------------------------
> I expected that in line 2, "b->getInt()" is 3, and by calling
> construction function of Line 1, the printed result should be 2;
>
> But it's 0, can anyone tell me the reason? Thanks in advance.

ok, got it, cannot call constructor in a constructor.

sonison.james

10/1/2008 8:40:00 AM

0

On Oct 1, 4:19 pm, thomas <FreshTho...@gmail.com> wrote:
>         A(B *b){
>                 A(b->getInt());        //Line  2
>         }

> ---------------------------------------------------------
> I expected that in line 2, "b->getInt()" is 3, and by calling
> construction function of Line 1, the printed result should be 2;
>
> But it's 0, can anyone tell me the reason? Thanks in advance.

This is because in Line 2, x member of the current object is not being
initialized, instead a new A object is created and it's x is
initialized. You need to simply initialize x of the current object.

A(B *b)
{
// probably have a check to see that b is not NULL before
accessing b ptr
x = b->getInt();
}



Thanks and regards
Sonison James

puzzlecracker

10/1/2008 1:07:00 PM

0


> ok, got it, cannot call constructor in a constructor.

sure you can, in your case, as someone already mention, you're
creating a new object.

Here is how you can call another ctor

class A{
A(){}
A():this(){

}

};

Obnoxious User

10/1/2008 3:03:00 PM

0

On Wed, 01 Oct 2008 06:07:10 -0700, puzzlecracker wrote:

>> ok, got it, cannot call constructor in a constructor.
>
> sure you can, in your case, as someone already mention, you're creating
> a new object.
>
> Here is how you can call another ctor
>
> class A{
> A(){}
> A():this(){
>
> }
>
> };

No you can't.

http://www.parashift.com/c++-faq-lite/ctors.htm...

--
OU
Remember 18th of June 2008, Democracy died that afternoon.
http://frapedia.se/wiki/Information_...

puzzlecracker

10/2/2008 3:27:00 AM

0

On Oct 1, 11:03 am, Obnoxious User <O...@127.0.0.1> wrote:
> On Wed, 01 Oct 2008 06:07:10 -0700, puzzlecracker wrote:
> >> ok, got it, cannot call constructor in a constructor.
>
> > sure you can, in your case, as someone already mention, you're creating
> > a new object.
>
> > Here is how you can call another ctor
>
> > class A{
> > A(){}
> > A():this(){
>
> > }
>
> > };
>
> No you can't.
>
> http://www.parashift.com/c++-faq-lite/ctors.htm...
>
> --
> OU
> Remember 18th of June 2008, Democracy died that afternoon.http://frapedia.se/wiki/Information_...

Opps, I was referring to calling the base class ctor. Sorry!