A question regarding array

[LarryChen]

Here is the code,

#include <iostream>

using namespace std;

class B
{
public:
	virtual int getValue(){return 1;}
private:
	int _x;
};

class D : public B
{
	virtual int getValue() {return 2;}
private:
	double _y;
	int _z;
	int _w;
};

int _tmain(int argc, _TCHAR* argv[])
{	
	B* b = new D[10];

	int diff = &b[2] - &b[1];
	int x = b[2].getValue();
	return 0;
}

I have two questions here. Actually B's size is 8 bytes and D's size is 24 bytes. First question is why diff is 1 instead of 8? The second question why b[2].getValue() gives access violation exception since b points to a chuck of size 24*10. Thanks.