类的大小

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C/C++ code

#include<iostream>using namespace std;class Base{public:     Base()     {         x=0;     }    int x;};class Derived1:virtual public Base{public:    Derived1()    {        x=10;    }};class Derived2:virtual public Base{public:    Derived2()    {        x=20;    }};class Derived:public Derived1,public Derived2 { };int main(){    Derived  obj;    cout<<"sizeof(Base)="<<sizeof(Base)<<endl;         cout<<"sizeof(Derived1)="<<sizeof(Derived1)<<endl;    cout<<"sizeof(Derived2)="<<sizeof(Derived2)<<endl;         cout<<"sizeof(Derived)="<<sizeof(Derived)<<endl;    cout<<"obj.x="<<obj.x<<endl;    return 0;}

结果：
sizeof(Base)=4
sizeof(Derived1)=8
sizeof(Derived2)=8
sizeof(Derived)=12
obj.x=20
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问题1：虚拟继承到底是怎样继承父类的数据成员的？？为何sizeof(Base)=4，
而sizeof(Derived1)=8，sizeof(Derived)=12

问题2：为何obj.x的值是20 而不是10

问题1：虚拟继承到底是怎样继承父类的数据成员的？？为何sizeof(Base)=4，
而sizeof(Derived1)=8，sizeof(Derived)=12

Base因为int x大小为4.
Derived1为8因为int x为4+虚基类指针4
Derived为12是因为int x为4+Derived1虚基类指针4+Derived的虚基类指针

问题2：为何obj.x的值……

正是这样，补充一下，他调用的顺序是先Derived1(),x=10;接着Derived2(),x=20;依次从继承列表顺序依次调用