4.关于虚继承的size问题

虚继承时，共同基类中的变量只会保存一份，但是会有一个虚基类指针（具体内容不讨论 参考 ）
为了避免对齐带来的干扰，把 int 换成 long long

继承关系如下

全部是空类的情况

class Base {
public:
    //long long a = 0;
};
class Derived1 : virtual public Base {
public:
    //long long b1 = 100;
};
class Derived2 : virtual public Base {
public:
    //long long b2 = 200;
};
class Derived : public Derived1, public Derived2 {
public:
    //long long d = 200;
};
int main() {
    cout << sizeof(Base) << endl;
    cout << sizeof(Derived1) << endl;
    cout << sizeof(Derived2) << endl;
    cout << sizeof(Derived) << endl;
    return 0;
}

结果

其中 Base 类是空类，但是为了让每一个类的实例都占用一个独特的内存空间，所以会由编译器隐含的分配一个char。
Derived1 与 Derived2 各有一个虚基类指针，古大小为8字节， Derived 的大小为16字节，即为两个继承来的虚基类指针。

Base非空的情况

class Base {
public:
    long long a = 0;
};
class Derived1 : virtual public Base {
public:
    //long long b1 = 100;
};
class Derived2 : virtual public Base {
public:
    //long long b2 = 200;
};
class Derived : public Derived1, public Derived2 {
public:
    //long long d = 200;
};
int main() {
    cout << "sizeof(Base) = " << sizeof(Base) << endl;
    cout << "sizeof(Derived1) = " << sizeof(Derived1) << endl;
    cout << "sizeof(Derived2) = " << sizeof(Derived2) << endl;
    cout << "sizeof(Derived) = " << sizeof(Derived) << endl;
    return 0;
}

结果

可见 Base 中的变量 a 在 Derived 中只保留了一份

全部非空

class Base {
public:
    long long a = 0;
};
class Derived1 : virtual public Base {
public:
    long long b1 = 100;
};
class Derived2 : virtual public Base {
public:
    long long b2 = 200;
};
class Derived : public Derived1, public Derived2 {
public:
    long long d = 200;
};
int main() {
    cout << "sizeof(Base) = " << sizeof(Base) << endl;
    cout << "sizeof(Derived1) = " << sizeof(Derived1) << endl;
    cout << "sizeof(Derived2) = " << sizeof(Derived2) << endl;
    cout << "sizeof(Derived) = " << sizeof(Derived) << endl;
    return 0;
}

结果

计算公式为

sizeof(Derived) = (sizeof(Derived1) - sizeof(Base))
                + (sizeof(Derived2) - sizeof(Base))
                + sizeof(Base)
                + (Derived类内定义的变量的size)