Re: can I turn base object into inherited one?

From: (Pascal J. Bourguignon)
Mon, 14 Apr 2008 10:12:32 +0200
alexl <> writes:

I have 2 classes,

class base {
some virtual functions


class derived : public base {
derived(base* b);


is there a simpler way to turn a base object into derived than the

base* b = new base();

b->do some stuff

now he becomes derived

base* d = new derived(b);

There is no simplier way, in C++.

 ( You are wanting CLOS (Common Lisp Object System), where an object can
 ( change of class on the run.
 ( (defclass base () ())
 ( (defmethod do-something ((self base)) (print `(I am a base ,self)))
 ( (defclass derived (base) ())
 ( (defmethod do-something ((self derived)) (print `(I am a derived ,self)))
 ( (let ((b (make-instance 'base)))
 ( (do-something b)
 ( (change-class b 'derived)
 ( (do-something b)
 ( (values))
 ( Which prints:
 ( (I AM A BASE #<BASE #x00033477AFB8>)
 ( (I AM A DERIVED #<DERIVED #x00033477AFB8>)
 ( Note that's the same object, at the same address.


But in C++, there's no simplier way than to explicitely define a
conversion method that will instanciate a new object. To solve the
identity problem, you have to implement a 'State' design pattern.

#include <iostream>
using namespace std;

class ChangeableObject;

class Base {
    ChangeableObject* self;
    static Base* factory(ChangeableObject* mySelf){return new Base(mySelf);}
    Base(ChangeableObject* mySelf):self(mySelf){}
    virtual void doSomething(void){ cout<<"I am a base "<<self<<endl; }

class ChangeableObject {
    Base* implementation;
    typedef Base*(*Factory)(ChangeableObject*);
    ChangeableObject(){ changeClass(Base::factory); }
    void changeClass(Factory factory){ implementation=factory(this); }
    void doSomething(void){ implementation->doSomething(); };

class Derived:public Base {
    static Base* factory(ChangeableObject* mySelf){return new Derived(mySelf);}
    Derived(ChangeableObject* mySelf):Base(mySelf){}
    virtual void doSomething(void){ cout<<"I am a derived "<<self<<endl; }

int main(void){
    ChangeableObject* o=new ChangeableObject();

-*- mode: compilation; default-directory: "~/src/tests-c++/" -*-
Compilation started at Mon Apr 14 10:11:03

g++ -o state-pattern state-pattern.c++ && ./state-pattern
I am a base 0x603010
I am a derived 0x603010

Compilation finished at Mon Apr 14 10:11:03

__Pascal Bourguignon__

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