是的，这是预期的行为。可以在派生类中访问基类的受保护的成员，但是仅通过是派生类（或当前派生类的进一步派生类）（包括 this ）类型的对象。这意味着您无法通过指向基类的指针访问受保护的成员。

Yes, this is expected behavior. Protected members of base class can be accessed in the derived class, but only via an object of a type that is the derived class (or the further derived class of current derived class) (including this). That means you can't access protected members via a pointer to base class.

只能访问基类的受保护成员

A protected member of a class Base can only be accessed

1）...

2）由Base派生的任何类的成员，但仅当
对从Base（包括
this）派生的类型的对象进行操作

2) by the members of any class derived from Base, but only when operating on an object of a type that is derived from Base (including this)