Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!mtuxo!mtgzz!drutx!druco!jrb
From: jrb@druco.ATT.COM (John Behrs)
Newsgroups: comp.lang.c++
Subject: Question: Object transformations in C++
Message-ID: <4413@druco.ATT.COM>
Date: 20 Jun 89 17:19:19 GMT
Distribution: comp
Organization: The Right Choice
Lines: 57

How can an instantiation of one derived class transform into a
different derived class of the same base class?  The derived classes
have different virtual functions but have no additional private data,
so what I really would like to do is just change the "virtual" type field.
Below is some code that I hope illustrates the problem.

//
// Class transformations
//
#include <stream.h>

class base {
	int base_a;
	int base_b;
public:
	base(int a, int b) { base_a = a; base_b = b; }
	int a() { return base_a; }
	int b() { return base_b; }
	virtual void func() {}
};

class x:public base {
public:
	x(int a, int b) : (a, b) {}
	void func() { cout << "X"; }
};

class y:public base {
public:
	y(int a, int b) : (a, b) {}
	void func() { cout << "Y"; }
};

main()
{
	class x x1(5,10);
	x1.func();	// prints 'X'
//
//	Assuming class x and class y to have identical data structure,
//	convert x1 to be of class y
//
//	(I'd much rather change just the virtual type field,
//	but I don't know how.)
//
	{
	class y y1(x1.a(), x1.b());
	x1 = *(class x *)&y1;
	}
//
//	x1 is now of class y
//
	x1.func();	// prints 'Y'
}
-- 
"What's the point of being alive,           | John Behrs
if you're not going to communicate?"        | druco!jrb
-- Kurt Vonnegut, Bluebeard.                | (303) 538-3539