C++多级继承中对象d的数据成员访问权限验证咨询

我编写了一段C++多级继承代码，希望验证main函数中定义的对象d在继承链中对各数据成员的访问权限。以下是我总结的各层级成员访问权限，需确认是否符合对象d的实际访问情况：

各层级成员访问权限总结 #

结构体A #

• a1: public
• a2: protected
• a3: private

结构体B #

• A::a1: private
• A::a2: private
• A::a3: NOT accessible
• b1: public
• b2: protected
• b3: private

结构体C #

• A::a1: NOT accessible
• A::a2: NOT accessible
• A::a3: NOT accessible
• B::b1: protected
• B::b2: protected
• B::b3: NOT accessible
• c1: public
• c2: protected
• c3: private

结构体D #

• A::a1: NOT accessible
• A::a2: NOT accessible
• A::a3: NOT accessible
• B::b3: NOT accessible
• B::b1: protected
• B::b2: protected
• C::c1: public
• C::c2: protected
• C::c3: NOT accessible
• d1: public
• d2: protected
• d3: private

验证代码 #

struct A
{
    A() {}
public:
    int a1;
protected:
    int a2;
private:
    int a3;
};


struct B: private A
{
    B() :
        A() {}
public:
    int b1;
protected:
    int b2;
private:
    int b3;
};


struct C : protected B
{
    C() :
        B() {}
public:
    int c1;
protected:
    int c2;
private:
    int c3;
};


struct D : public C
{
    D() :
        C() {}
public:
    int d1;
protected:
    int d2;
private:
    int d3;
};

int main()
{
    const D d;

}

内容的提问来源于stack exchange，提问作者sam