Re: Virtual Constructors

"Victor Bazarov" <>
Tue, 27 Feb 2007 11:45:53 -0500
Carl R. Davies wrote:

On Feb 27, 3:39 pm, wrote:

On Feb 27, 10:35 am, "Carl R. Davies" <> wrote:

"As we have seen (section 14.2) C++ supports virtual destructors.
Like many other object oriented languages (e.g., Java), however,
the notion of a virtual constructor is not supported. The absence
of a virtual constructor turns into a problem when only a base
class reference or pointer is available, and a copy of a derived
class object is required."

What's wrong with down casting the base class pointer and using

Bad design. If you design your class hierarchy correctly, client
code should be unaware of derived classes and work only through base
class pointers.

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();

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