Copy constructor for a class that contains a pointer to a base classtype (newbie)

[=?ISO-8859-1?Q?Szabolcs_Horv=E1t?=]

This is a newbie question. Suppose I have a class A (see the attached
code) which contains a pointer that can point to an object of either
type D1 or D2. How do I write a copy constructor for A, so that the
object that it references also gets copied (I don't know whether it is a
D1 or D2 object)?

Thank you for your answers in advance,
Szabolcs

----

class B {
public:
virtual void print() = 0;
};

class D1 : public B {
int i;
public:
void print() { cout << i; }
};

class D2 : public B {
double x;
public:
void print() { cout << x; }
};

class A {
B *b;
};

 

[David Harmon]

On Fri, 27 Oct 2006 21:00:34 +0200 in comp.lang.c++, Szabolcs Horvát

This is a newbie question. Suppose I have a class A (see the attached
code) which contains a pointer that can point to an object of either
type D1 or D2. How do I write a copy constructor for A, so that the
object that it references also gets copied

As your example shows, the Object-Oriented way for a pointer to
point to different classes is for them to be derived classes of a
common base. In that case, one answer is for the base to declare
virtual B* clone() = 0;
and all the derived classes to implement that virtual function and
each class return a new copy object of its own type. Your copy
constructor calls the virtual b->clone() and gets the new copy that
it needs without concern about how many derived classes there are.

 

[=?ISO-8859-1?Q?Szabolcs_Horv=E1t?=]

In that case, one answer is for the base to declare
virtual B* clone() = 0;
and all the derived classes to implement that virtual function and
each class return a new copy object of its own type. Your copy
constructor calls the virtual b->clone() and gets the new copy that
it needs without concern about how many derived classes there are.

Thanks! This solves my problem, but I thought that there is a better
solution for this. What if I cannot change B, D1 and D2 (e.g. they are
part of a library)?

Szabolcs

P.S.: Sorry for the duplicate post. First I tried to post using Google
Groups, but I got an 500 Server Error. It seems that the message has
been sent, but it took more than 40 minutes for it to arrive.

 

[Kai-Uwe Bux]

Thanks! This solves my problem, but I thought that there is a better
solution for this. What if I cannot change B, D1 and D2 (e.g. they are
part of a library)?

There are smart pointer templates with deep copy semantics that will help
you. Search the archives of this news group for copy_ptr<> or clone_ptr<>.


Best

Kai-Uwe Bux

 

[Daniel T.]

This is a newbie question. Suppose I have a class A (see the attached
code) which contains a pointer that can point to an object of either
type D1 or D2. How do I write a copy constructor for A, so that the
object that it references also gets copied (I don't know whether it is a
D1 or D2 object)?

http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.8