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Sorry if this becomes a repost. I posted this to comp.lang.c++.moderated 1
hour ago still it does not show up there so posting this here.

Hi,
Consider class A & B both of which implement a copy constructor.
class B inherits from A.
When copy constructor of B is called, first the constructor of A gets called
My question is that, why the copy constructor for A not called.
Is there a way to have it call the copy constructor of class A?

#include <iostream>
using namespace std;


class A
{
public:
A() { cout << "A constructor called\n";};
A(A& a) { cout << "Copy A constructor called\n";};
~A() { cout << "Destructor A called\n";};
};
class B: public A
{
public:
B() { cout << "B constructor called\n";};
B(B& b) { cout << "Copy B constructor called\n"; };
~B() { cout << "Destructor B called\n";};
};
int main(void)
{
B b;
B bb = B(b);
return 0;
}

Kench wrote:
> Sorry if this becomes a repost. I posted this to comp.lang.c++.moderated 1
> hour ago still it does not show up there so posting this here.

It takes moderators up to a day to get through all posts in a moderated
newsgroup, usually. You should not expect a chat room performance from
the Usenet.

> Consider class A & B both of which implement a copy constructor.
> class B inherits from A.
> When copy constructor of B is called, first the constructor of A gets called

Not unless you specifically omit initialising the base class object.

> My question is that, why the copy constructor for A not called.

You didn't initialise the base class. It calls the default c-tor.

> Is there a way to have it call the copy constructor of class A?

Of course. See below.

>
> #include <iostream>
> using namespace std;
>
>
> class A
> {
> public:
> A() { cout << "A constructor called\n";};

First of all, lose all the semicolons after function bodies. It
just looks so amateurish and sloppy...

> A(A& a) { cout << "Copy A constructor called\n";};
> ~A() { cout << "Destructor A called\n";};
> };
> class B: public A
> {
> public:
> B() { cout << "B constructor called\n";};
> B(B& b) { cout << "Copy B constructor called\n"; };

You need to say

B(B& b) : A(b) { ...

to make sure that the 'A' part is copy-constructed.

> ~B() { cout << "Destructor B called\n";};
> };
> int main(void)
> {
> B b;
> B bb = B(b);
> return 0;
> }

Victor

Kench wrote:

> Consider class A & B both of which implement a copy constructor.
> class B inherits from A.
> When copy constructor of B is called, first the constructor of A gets called
> My question is that, why the copy constructor for A not called.
> Is there a way to have it call the copy constructor of class A?
>
> #include <iostream>
> using namespace std;
>
>
> class A
> {
> public:
> A() { cout << "A constructor called\n";};
> A(A& a) { cout << "Copy A constructor called\n";};
> ~A() { cout << "Destructor A called\n";};
> };
> class B: public A
> {
> public:
> B() { cout << "B constructor called\n";};
> B(B& b) { cout << "Copy B constructor called\n"; };

B(B& b) : A(b) { cout << "Copy B constructor called\n"; };

> ~B() { cout << "Destructor B called\n";};
> };
> int main(void)
> {
> B b;
> B bb = B(b);
> return 0;
> }

mark

Mark A. Gibbs wrote:

>> int main(void)
>> {
>> B b;
>> B bb = B(b);
>> return 0;
>> }

Incidently, the "B bb = B(b)" is a little ugly (and tough to say :/). Go
with "B bb = b", or better yet "B bb(b)".

mark

"Victor Bazarov" <> wrote in message
news:h6qvc.889$...
> Kench wrote:
> > Sorry if this becomes a repost. I posted this to comp.lang.c++.moderated
1
> > hour ago still it does not show up there so posting this here.
>
> It takes moderators up to a day to get through all posts in a moderated
> newsgroup, usually. You should not expect a chat room performance from
> the Usenet.
>
> > Consider class A & B both of which implement a copy constructor.
> > class B inherits from A.
> > When copy constructor of B is called, first the constructor of A gets
called
>
> Not unless you specifically omit initialising the base class object.
>
> > My question is that, why the copy constructor for A not called.
>
> You didn't initialise the base class. It calls the default c-tor.
>
> > Is there a way to have it call the copy constructor of class A?
>
> Of course. See below.
>
> >
> > #include <iostream>
> > using namespace std;
> >
> >
> > class A
> > {
> > public:
> > A() { cout << "A constructor called\n";};
>
> First of all, lose all the semicolons after function bodies. It
> just looks so amateurish and sloppy...
>
> > A(A& a) { cout << "Copy A constructor called\n";};
> > ~A() { cout << "Destructor A called\n";};
> > };
> > class B: public A
> > {
> > public:
> > B() { cout << "B constructor called\n";};
> > B(B& b) { cout << "Copy B constructor called\n"; };
>
> You need to say
>
> B(B& b) : A(b) { ...
>
> to make sure that the 'A' part is copy-constructed.
>
> > ~B() { cout << "Destructor B called\n";};
> > };
> > int main(void)
> > {
> > B b;
> > B bb = B(b);
> > return 0;
> > }
>
> Victor

that one worked..thanks!