一道关于继承类的sizeof大小问题

#include <iostream>

using namespace std;

struct A{

virtual int fun(); //4 byte

char x; //1 byte

};

struct B:public A{

short myfun(); //2 byte 不占类B空间

union{

unsigned short m;

unsigned int c;

}xx; //4 byte

int (__stdcall *funs)(); //4 byte

};

union{

unsigned short m;

unsigned int c;

}xx;

short myfun();

int (__stdcall *funs)(); //4 byte

struct C{

short myfun(); //2 byte

union{

unsigned short m;

unsigned int c;

}xx; //4 byte

int (__stdcall *funs)(); //4 byte

};

int main()

{

//cout<<endl;
//cout<<offsetof( B , x )<<endl;
//cout<<offsetof( B , xx )<<endl;
//cout<<offsetof( B , funs )<<endl;

cout < < sizeof(myfun()) < < endl;//2

cout < < sizeof(( *funs)()) < < endl;//4

cout < < sizeof(xx) < < endl;//4

cout < < sizeof(A) < < endl;//8

cout < < sizeof(B) < < endl;//16

cout < < sizeof(C) < < endl;//8

return 0;

}

//为啥sizeof(B)是16而不是18？？offsetof可以看偏移

//看深入探索C++对象模型，类中非虚函数不占空间