为什么C ++中空类的大小不为零？

Question

我正在阅读一些c ++难题。 我已经看到了这个问题。

#include <iostream>
using namespace std;

class Empty
{};

class Derived1 : public Empty
{};

class Derived2 : virtual public Empty
{};

class Derived3 : public Empty
{    
    char c;
};

class Derived4 : virtual public Empty
{
    char c;
};

class Dummy
{
    char c;
};

int main()
{
    cout << "sizeof(Empty) " << sizeof(Empty) << endl;
    cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;
    cout << "sizeof(Derived2) " << sizeof(Derived2) << endl;
    cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;
    cout << "sizeof(Derived4) " << sizeof(Derived4) << endl;    
    cout << "sizeof(Dummy) " << sizeof(Dummy) << endl;

    return 0;
}

当我运行代码时。结果发现：

sizeof(Empty) 1
sizeof(Derived1) 1
sizeof(Derived2) 4
sizeof(Derived3) 1
sizeof(Derived4) 8
sizeof(Dummy) 1

有人可以解释为什么sizeof（Derived2）和sizeof（Derived4）分别是4和8？

感谢。