繼承中成員變量初始化

下列輸出結構是什麼？

#include<iostream>
using namespace std;

class A
{
        public:
                int m_a;
                A():m_a(0){}
};

class B : public A
{
        public:
                int m_a;
                B():m_a(2){}
};

int main(int argc,char *argv[])
{
        A *a=new A;
        cout<<a->m_a<<endl;
        cout<<sizeof(class A)<<endl;
        A *b = new B();
        cout<<b->m_a<<endl;
        cout<<sizeof(class B)<<endl;

        B *c = new B;
        cout<<c->m_a<<endl;
}

g++編譯運行後輸出如下：

0
4
0
8
2

對第三個輸出0不是很懂，求解釋~