Re: Virtual Constructors

From:

"Carl R. Davies" <nwsgrp1@googlemail.com>

Newsgroups:

comp.lang.c++

Date:

27 Feb 2007 08:54:48 -0800

Message-ID:

<1172595288.158478.251750@h3g2000cwc.googlegroups.com>

On Feb 27, 4:45 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

Carl R. Davies wrote:

Ok, is it because the client code might not know what the derived
class is at time of writing so it's unable to down cast?

1. Framework code declares classes Base and Derived " class Derived :
public Base "
2. So in the client code:

Derived *ptr = new Derived();

Then calls a function that only knows Base:

void func( Base *inPtr )
{
// Can't down cast because doesn't know existence of Derived
// So how does Clone() help here?
}

Apologies for being persistent, I would like to know so I can
recognise the need for "Prototype Pattern" in the future.

class clonable {
    virtual clonable* clone() const = 0;

};

class Base : public clonable {
    ...
    Base* clone() const { return new Base(*this); }

};

class Derived : public Base {
    ...
    Derived* clone() const { return new Derived(*this); }

};

void func(Base *inPtr)
{
     Base *newOne = inPtr->clone();

}

Thanks Victor, I now understand what it does and importantly "Why?".
Cheers everyone.