Downcasting with private inheritance

[n4nature]

Can anybody tell me what is the prob here?

#include <iostream>
using namespace std;

class Animal
{
public:
	virtual void speak()
	{
		cout << "Animal speaks" << endl;
	}
};

class Dog : private Animal
{
public:
	virtual void speak()
	{
		cout << "Dog barks" << endl;
	}
};

void main()
{
	Animal* pA = new Animal;
	Dog* pD = new Dog;

	pA = pD;

	pA->speak();
}

The error is:
error C2243: 'type cast' : conversion from 'Dog *' to 'Animal *' exists, but is inaccessible

[Bazzy]

To do what you are trying to do, you need public inheritance or the pointers will be incompatible

[n4nature]

Bazzy, I know public inheritance will work. I read somewhere that private inheritance is preferred over composition, when virtual functions in the base class needs to be overriden in derived classes, and that's what I am doing here. Well, can you tell me how could I implement the statement above in bold? Thanks.

[Bazzy]

You want polymorphism, private inheritance has different purposes:
http://www.parashift.com/c++-faq-lite/private-inheritance.html#faq-24.2