c++ 父类 子类 虚表占用内存空间情况

#include <iostream>
using namespace std;
class C {};
class A:public C {
private:
long a;
long b;
long c;
public:
A(long a,long b,long c){
this->a=a;this->b=b;this->c=c;
}
virtual void A_func(){};
// void A_func();
};
class B:public A{
private:
long d;
public:
B(long a1,long a2,long a3,long a4):A(a1,a2,a3){
d=a4;
}
void B_func();
};
int main() {
A x(1,2,3);
B y(1,2,3,4);
return 0;
}

(gdb) p x
$1 = (A) {<C> = {<No data fields>}, _vptr.A = 0x1004030d0 <vtable for A+16>, a = 1, b = 2, c = 3}
(gdb) p y
$2 = (B) {<A> = {<C> = {<No data fields>}, _vptr.A = 0x1004030f0 <vtable for B+16>, a = 1, b = 2, c = 3}, d = 4}
(gdb) p sizeof(x)
$3 = 32
(gdb) p sizeof(y)
$4 = 40