3.继承而来的同名变量与派生类中的同名变量占用不同的空间

class Base {
public:
    int a = 100;
};
class Derived : public Base {
public:
    int a = 200;
};
int main() {
    Derived d;
    cout << "sizeof(int) = " << sizeof(int) << ", ";
    cout << "sizeof(d) = " << sizeof(d) << endl;
    cout << "d.a = " << d.a << ", d.Base::a = " << d.Base::a << ", d.Derived::a = " << d.Derived::a << endl;
    cout << "d.Base::a 的地址为" << (&d.Base::a) << endl << "d.Derived::a 的地址为" << (&d.Derived::a);
    return 0;
}

输出

sizeof(int) = 4, sizeof(d) = 8
d.a = 200, d.Base::a = 100, d.Derived::a = 200
d.Base::a 的地址为0x70fdf0
d.Derived::a 的地址为0x70fdf4

所以这就产生的多继承的二义性问题

class Base {
public:
    int a = 0;
};
class Derived1 : public Base {
public:
    int b1 = 100;
};
class Derived2 : public Base {
public:
    int b2 = 200;
};
class Derived : public Derived1, public Derived2 {
public:
    int c = 200;
};
int main() {
    Derived d;
    cout << "sizeof(int) = " << sizeof(int) << ", ";
    cout << "sizeof(d) = " << sizeof(d) << endl;
    //cout << d.a << endl;  // 存在二义性
    //cout << d.b1 << endl;
    //cout << d.b2 << endl;
    //cout << d.c << endl;
    return 0;
}

结果

sizeof(int) = 4, sizeof(d) = 20

可以推断出，在派生类Derived的对象d中，保存了 2个a ，1个b1,1个b2，1个c，其中2个a分别是从类 Derived1 与 Derived2 继承来的。
此时语句 cout << d.a << endl; 存在二义性，无法编译通过。
但是 cout << d.Derived1::a << d.Derived2::a << endl; 可以通过
将 Derived1 与· Derived1 的继承修改位虚继承后，编译可以通过。