对空类型求sizeof

#include <iostream>
using namespace std;

class Empty1
{

};

class Empty2
{
public:
    Empty2(){};
    ~Empty2(){};
};

class Empty3
{
public:
    Empty3(){};
    virtual ~Empty3(){};
};

int main()
{
    cout<<sizeof(Empty1)<<endl; //1
    cout<<sizeof(Empty2)<<endl; //1
    cout<<sizeof(Empty3)<<endl; //4
    return 0;
}