从 sub-sub-class 或孙子访问基础 class

Question

我只是想弄清楚一些事情，因为在学习继承时我注意到我可以从已经派生的 class 继承，并且从最派生的 class 访问 Base class的直接会员。我认为以下说明了我的意思：

#include <iostream>

struct Base
{
    int member = 0;     // All members null
};

struct D1 : Base {};
struct D2 : Base {};
struct D3 : Base {};
struct D4 : Base {};

struct MostDerived : D1, D2, D3, D4 {};  // I know MostDerived now has 4 copies of Base


int main()
{
    MostDerived mostderived{};
    mostderived.D2::member = 2;                     // D2::Base::member = 2
    mostderived.D3::member = 3;                     // D3::Base::member = 3
    mostderived.D4::member = 4;                     // D4::Base::member = 4

    std::cout << mostderived.D1::member << '\n';        // Haven't touched D1, is still zero
    std::cout << mostderived.D1::Base::member << '\n';  // Equals 0, equivalent to above line
    std::cout << mostderived.Base::member << '\n';      // Read Base scope directly from most derived, equals zero

    mostderived.Base::member++;                         // Assign to member in Base scope directly from mostderived
                                                        // it now equals 1

    std::cout << mostderived.D1::member << '\n';        // But also does D1::member equal 1

    return 0;
}

所以通过 mostderived.Base::member++ 我改变了 mostderived.D1::member 的值。我只是想知道这样访问 Base 是否有意义，以及为什么它特别更改了 D1 的成员副本。我描绘布局的方式是 MostDerived 包含 D1、D2、D3、D4，并且每个包含一个 Base, 所以它看起来像下面这样。

Answer 1

So by doing mostderived.Base::member++ I changed the value of mostderived.D1::member.

如果'MostDerived'继承自D1-4，则

mostderived.Base::member++;
                  ^

有歧义。

因为'MostDerived'有这个层级

int Base::member
int Base::member
int Base::member
int Base::member

和Base可以替换为D1-4中的任何一个。

如果你会使用虚拟继承

struct D1 : virtual Base {};
struct D2 : virtual Base {};
struct D3 : virtual Base {};
struct D4 : virtual Base {};

这会导致'MostDerived'只包含一个基class，所以做

就不会有歧义了

mostderived.Base::member++;

并且可以通过所有 D1-4

访问此 member

mostderived.D1::member = 1;
mostderived.D4::member = 4;

当然，因为所有 D 都共享同一个变量，所以它对所有人都有相同的值，例如：

MostDerived mostderived{};
mostderived.Base::member = 1;
std::cout << mostderived.D1::member << mostderived.D2::member << mostderived.D3::member << mostderived.D4::member;

将打印 1111.

从 sub-sub-class 或孙子访问基础 class

Accessing Base class from sub-sub-class or grandchild

c++

class

inheritance

base-class