指针计算问题

#include "stdafx.h"
#include <vector>
using namespace std;
class A
{
public:
A()
{
a = 3;
b = 4;
}
int a;
int b;
};


vector<A>  m_vecA;


int _tmain(int argc, _TCHAR* argv[])
{   A a1, a2, a3, a4, a5;
m_vecA.push_back(a1);
    m_vecA.push_back(a2);
m_vecA.push_back(a3);
m_vecA.push_back(a4);
m_vecA.push_back(a5);
vector<A>::iterator iter1 = m_vecA.begin();
iter1++;
iter1++;
iter1++;
    vector<A>::iterator iter2 = iter1;
//vector<A>::iterator iter2 = m_vecA.end();
int index = &(*iter1) - &m_vecA[0];
return 0;

}

结果为三。