c++ - 无法访问 protected 成员

Question

这个问题在这里已经有了答案:

关闭 11 年前.

Possible Duplicate:
cannot call base class protected functions?

我不明白以下内容，当 Derived 从 Base 继承时，它可以访问其 protected 成员，这些成员可以通过 Derived 函数访问。但是，如果 Base 类试图从 Derived 类(它本身允许访问 Base)访问它自己的成员，它就无法访问，为什么？

class Base {
protected:
    int x;
};

class Derived : Base {
public:
    void foo(Base* b);
};


void Derived::foo(Base* b) {
    b->x = 2;       // cannot access protected member,
                    // though Derived inherits from Base, why?
}

Answer 1

一个常见的误解。

在Derived::foo() 中，您可以访问Derived 类对象的 protected 基成员。但是，*b 不是 类型的 Derived。相反，它属于 Base 类型，因此它与您的类没有任何关系。

如果您将 Derived* 作为参数，那就不同了——那么您确实可以访问 protected 基成员。

---

让我们把它拼出来:

struct Derived;

struct Base
{
  int f(Derived *);
protected:
  int x;
private:
  int y;
};

struct Derived : public Base
{
  int g(Base *);
  int h(Derived *);
};

int Derived::g(Base * b)
{
   return b->x; // error, protected member of unrelated class
   return b->y; // error, private member of different class
}

int Derived::h(Derived * d)
{
  return d->x;  // OK, protected base member accessible in derived class
  return d->y;  // error, private member of different class
}

int Base::f(Derived * d)
{
  return d->x;  // OK, d converts to Base*
  return d->y;  // OK, ditto
}