c++ - 多重继承后对类布局进行假设是否安全？

Question

据我所知，多重继承情况下的顺序与被称为构造函数的顺序相同。这个问题是关于内存布局的吗？
考虑以下代码段:

#include <iostream>

class A {
    public:
    void seta() {a = 1;}
    int a;
};

class B {
    public:
    void setb() {b = 2;}
    int b;
};

class C : public A, public B {
};

class D : public B, public A {
};

int main() {
    C c;
    c.a = 1;
    c.b = 2;
    void * p1 = (void *)(&c);
    //if layout is as inheritance order, always displays C.a: 1 C.b: 1?
    std::cout << "C.a: " << ((A*)(p1))->a << " C.b: " << ((B*)(p1))->b << "\n";
    D d;
    d.a = 1;
    d.b = 2;
    void * p2 = (void *)(&d);
    //if layout is as inheritance order, always displays D.a: 2 D.b: 2?
    std::cout << "D.a: " << ((A*)(p2))->a << " D.b: " << ((B*)(p2))->b << "\n";
    return 0;
}

在评论中依靠这两个假设是否安全？
就内存布局而言，由于继承顺序，C和D对象等效于:

class C {
    public:
    int a;
    int b;
};

class D {
    public:
    int b;
    int a;
};

？还是取决于实现方式，然后依靠它是不安全的？

Answer 1

在n4659草案中，[class.derived]在§5中明确指出:

The order in which the base class subobjects are allocated in the most derived object (4.5) is unspecified.

可以说，除非有特定的实现指定，否则您不应该依赖基类子对象的顺序。