protected 是指派生类型可以访问它自己的基类的成员而不是任何随机对象 ^* 。在你的情况下，你在尝试修改不受你控制的 b 的成员（即你可以设置 this-> a ，但不是 ba ）

The meaning of protected is that the derived type will have access to that member of it's own base and not of any random object^*. In your case, you care trying to modify b's member which is outside of your control (i.e. you can set this->a, but not b.a)

感兴趣，但更好的解决方案是重构代码，而不依赖于hack。例如，您可以在 A 中提供一个构造函数，它以 A * 作为参数（此构造函数应为public ），然后在 B 的初始化器列表中初始化它：

There is a hack to get this to work if you are interested, but a better solution would be to refactor the code and not depend on hacks. You could, for example, provide a constructor in A that takes an A* as argument (this constructor should be public) and then initialize it in the initializer list of B:

A::A( A* p ) : a(p) {}
B::B() : b(&b) {}

^* protected 您自己类型的任何实例中的基础成员或派生自您自己的类型。

^*protected grants you access to the base member in any instance of your own type or derived from your own type.