c++ - 访问控制和静态成员

Question

我正在阅读有关静态成员函数的访问说明符的内容，并以来自 MSDN 的这个示例结束:我认为它有错误。

// access_control.cpp  
class Base  
{  
 public:  
  int Print();             // Nonstatic member.  
   static int CountOf();    // Static member.  
};  


class Derived1 : private Base  
{  
};  

class Derived2 : public Derived1  
{  
  int ShowCount();    // Nonstatic member.  
};  

int Derived2::ShowCount()  
{  
  // Call static member function CountOf explicitly.  
  int cCount = Base::CountOf();     // OK.  

  // Call static member function CountOf using pointer.  
   cCount = this->CountOf();  // C2247. Conversion of  
                           //  Derived2 * to Base * not  
                           //  permitted.  
   return cCount;  
}  

所以它说这条线是好的，但它不是:

int cCount = Base::CountOf();     // OK.

这是错误消息:

error C2247: 'Base::CountOf' not accessible because 'Derived1' uses 'private' to inherit from 'Base'

那么错误真的存在还是与我在编译时必须添加的某些选项有关？

谢谢

Answer 1

在 C++17 标准部分 [class.access.base]/3 中有一个与您的代码类似的示例:

A member of a private base class might be inaccessible as an inherited member name, but accessible directly. Because of the rules on pointer conversions (7.11) and explicit casts (8.4), a conversion from a pointer to a derived class to a pointer to an inaccessible base class might be ill-formed if an implicit conversion is used, but well-formed if an explicit cast is used. For example,

class B {
public:
    int mi; // non-static member
    static int si; // static member
};
class D : private B {
};
class DD : public D {
    void f();
};

void DD::f() {
    mi = 3; // error: mi is private in D
    si = 3; // error: si is private in D
    ::B b;
    b.mi = 3; // OK (b.mi is different from this->mi)
    b.si = 3; // OK (b.si is different from this->si)
    ::B::si = 3; // OK
    ::B* bp1 = this; // error: B is a private base class
    ::B* bp2 = (::B*)this; // OK with cast
    bp2->mi = 3; // OK: access through a pointer to B.
}

---

您的代码与此不同，因为您执行的是 Base::CountOf()。这仍然是一个错误，因为访问是在 Derived2 的上下文中检查的，并且没有对基类的私有(private)成员的访问权限。

但是 ::Base::CountOf() 是正确的，因为在全局命名空间中检查访问。

您可以按照与标准示例类似的方式修复示例的第二部分:(::Base *)this)->CountOf()。