访问受保护的继承成员，带有指向基类的指针 [英] access protected inherited member with pointer to base class

问题描述

有人可以解释为什么此代码无效。

  A类
 {
 public ：
 A（void）{} 
 virtual〜A（void）{} 
 protected：
 A * 
}; 
 
 class B：public A 
 {
 public：
 B（void）{parent = new B;} 
〜B parent;} 
 protected：
 int a; 
}; 
 
 class C：public B 
 {
 public：
 C（void）{} 
 virtual〜C（void）{} 
 void f（void）{（（B *）parent）→a; } 
}; 
  

C isn'能够访问 B 的成员

如果我转换 parent 到 C * 代替 B * 但我不希望用户承担任何不必要的风险。是否有更清洁的方法访问 a ？

谢谢。

C 类的对象，您可以访问 B 的受保护成员c $ c>，但只有当它们是 C （也许你的，也许不是）的某个对象的一部分。换句话说，要从 C 访问 a ，您需要一个指针（或引用）到 C 。这是 protected 修饰符的含义。

这样做的原因如下。 （（B *）parent）指针可能指向 B 的一些其他子类，完全不同于 C ，并且该子类可能具有 a 成员无法访问。

Can someone explain why this code doesn't work.

class A
{
public:
    A(void){}
    virtual ~A(void){}
protected:
    A* parent;
};

class B : public A
{
public:
    B(void){parent = new B;}
    ~B(void){delete parent;}
protected:
    int a;
};

class C : public B
{
public:
    C(void){}
    virtual ~C(void){}
    void f(void){ ((B*)parent)->a; }
};

How is it possible that C isn't able to access members of B?

If I convert parent to a C* in stead of a B* it works fine. But I don't want users to take any unnecessary risks. Is there a cleaner way access a?

Thanks.

解决方案

From an object of the C class, you can access protected members of B, but only if they're part of some object of class C (maybe yours, maybe not). In other words, to access a from C, you need a pointer (or a reference) to C. This is what the protected modifier means.

The reason for this is the following. The ((B*)parent) pointer may point to some other subclass of B, completely different from C, and that subclass may have the a member inaccessible.

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