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;
}

通过打印可知道

(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

当c为空的时候,c不占内存,看最底层的B类他继承了A类的大小,额外自己的成员变量极为40/8  =5个