c++空的class的大小

#include<iostream>#include<vector>using namespace std;class A{//virtual void func(){};};class B{int b;};class C{int c;virtual void func(){};};int main(){cout<<sizeof(A)<<endl;//output 1cout<<sizeof(B)<<endl;//output 4cout<<sizeof(C)<<endl;//output 8cout<<sizeof(int)<<endl;A a1,a2,a3;cout<<&a1<<" "<<&a2<<" "<<&a3<<endl;//0018FECF 0018FEC3 0018FEB7B b1, b2, b3;cout<<&b1<<" "<<&b2<<" "<<&b3<<endl;//0018FEA8 0018FE9C 0018FE90C c1, c2, c3;cout<<&c1<<" "<<&c2<<" "<<&c3<<endl;//0018FE80 0018FE70 0018FE60return 0;}

在VS202中，每个A和B占据的空间是12，而每个C占据的空间是16.这是为什么？