Showing off vtables for stackoverflow answer.

vtablefun.cpp

#include <iostream>

using namespace std;

class A
{
public:
	int a1;
	int a2;
	virtual void A1() { cout << "A->A1" << endl; }
	virtual void DoSomething() { cout << "DoSomething" << endl; };
};

class B : public A
{
public:
	int b1;
	int b2;
	virtual void A1() { cout << "B->A1" << endl; }
	virtual void B1() { cout << "B->B1" << endl; }
};

class C : public A
{
public:
	int c1;		 
	int c2;		 
	virtual void A1() { cout << "C->A1" << endl; }
	virtual void C1() { cout << "C->C1" << endl; }
};

void somefunc(void* thiz)
{
	cout << "somefunc" << endl;
}

int main(int argc, char** argv)
{
	A* a = new A();
	B* b = new B();
	C* c = new C();

	//first lets show the size of the types
	cout << sizeof(A) << ", " << sizeof(B) << ", " << sizeof(C) << endl;

	//from wikipedia we know the class layout for B and C will consist of four
	//integers and a vtable pointer so a total of 20 bytes (assuming 32bit machine)

	c->A1(); //Will print C->A1()
	//Lets lets move the vtable pointer of B and put it on C
	int* cVtablePtr = (int*)((int*)c)[0];
	((int*)c)[0] = ((int*)b)[0];
	c->A1();
	//and restore
	((int*)c)[0] = (int)cVtablePtr;
	c->A1();

	c->DoSomething();
	//So that was the whole vtable what if you wanted to change just one function in the vtable
	//cVtablePtr[0] is C->A1() and cVtablePtr[1] is A->DoSomething()
	void* doSomethingPtr = (void*)cVtablePtr[1];
	cout << &cVtablePtr[1] << endl;
	int k;
	cin >> k;
	cVtablePtr[1] = (int)somefunc;

	c->DoSomething();

	return 0;
}