Re: Virtual constructor?

From:

"ThosRTanner" <ttanner2@bloomberg.net>

Newsgroups:

comp.lang.c++.moderated

Date:

3 Jun 2006 10:01:57 -0400

Message-ID:

<1149253982.019969.179490@i40g2000cwc.googlegroups.com>

Gene Bushuyev wrote:

"Azumanga" <4zumanga@gmail.com> wrote in message
news:1149031273.468412.11660@v35g2000cwv.googlegroups.com...

Imagine I have the following code:

struct A { .. };
struct B : public A { .. };
struct C : public A { .. };

[...]

A* copy_object(A* obj)
{
if(B* b_obj = dynamic_cast<B*>(obj))
{ return new B(*b_obj); }

This code is an absolute anathema to object-oriented programming. Each code that
relies on switching on types is not only error-prone, it breaks the foundational
open-close design principle. Your function must know about every possible
derivative of A, changing every time whenever the classes are changed or new
classes added.

or:

struct A
{ virtual A* clone() { return new A(*this);} ... };

struct B : public A
{ virtual B* clone() { return new B(*this);} ... };

This is a typical way of cloning objects (except it should be const,) which is
sometimes called "virtual constructor." There is nothing error-prone about it,
the only inconvenience is typing virtually the same code in every class.

Isn't this one of those places where you use the Curiously Recurring
Template pattern

template <class C> cloneable { virtual C* clone() return new C(*this) }

and then do
struct B : public A, cloneable<B> { .... }

I haven't used the pattern that much but I think it works something
like that.

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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