Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!haven!purdue!bu-cs!att!cbnews!trm
From: trm@cbnews.ATT.COM (Tom R. Mueller)
Newsgroups: comp.lang.c++
Subject: Re: Question: Object transformations in C++
Message-ID: <7748@cbnews.ATT.COM>
Date: 26 Jun 89 14:29:50 GMT
References: <4413@druco.ATT.COM>
Reply-To: trm@cbnews.ATT.COM (Tom R. Mueller)
Distribution: comp
Organization: AT&T Bell Laboratories
Lines: 52


Here's a portable way to change the vtbl entry.  This example transforms 
an object from a base class to a derived class.  It uses forwarding
functions in the base class to redirect calls through the vtbl.  Note that
if you're calling functions in a derived class using d.func(), the
vtbl isn't referenced so changing it won't help.

#include <stream.h>

class Base
{
public:
        Base() { cout << "Constructing Base.\n"; }
        void makeMeADerived();
        virtual void f()
        {
	                cout << "Called Base::f()\n";
	                this->f();
        }
        void *operator new(long) { return ::new Base; };
        void *operator new(long, void *where)
        {
                return where;
        }
        void operator delete(void *) { }
};

class Derived : public Base
{
public:
        Derived() { cout << "Constructing Derived.\n"; }
        void f() { cout << "Called Derived::f()\n"; }
};

void Base::makeMeADerived()
{
        Derived *d = new(this) Derived;
}

main()
{
        Base b;

        b.makeMeADerived();
        b.f();                  // Calls Derived::f (indirectly via Base)
        (&b)->f();              // Calls Derived::f (directly via vtbl)
}

-- 
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