C++ 上有点生锈了。类继承

发布于 2024-12-12 04:43:10 字数 761 浏览 0 评论 0原文

子类访问受保护成员对象的规则是什么？我以为我理解了它们，但我的代码不同意。

我有一个基类，定义为

class Datum {
public:
   Datum(Datum *r, Datum *l) : right(r), left(l) {}

protected:
   Datum *right, *left;
};

我的 Datum 子类，如下所示：

class Column: public Datum
{
public:
   Column(Datum *r, Datum *l, string n, int s): Datum(r,l), name(n), size(s) {}

   void cover() {
      right->left = left;
      left->right = right;
   }

protected:
   string name;
   int size;
};

当我使用 G++ v.4.5.1 进行编译时，我收到指向 cover 中的两行的错误消息：

error: Datum* Datum::right is protected
error:  within this context
error: Datum* Datum::left is protected
error:  within this context

显然，使该部分公开会导致错误消失离开。当该部分受到保护时，为什么它们还在那里？

原文

What are the rules for a subclass accessing protected member objects? I thought I understood them but my code disagrees.

I have a base class, defined as

class Datum {
public:
   Datum(Datum *r, Datum *l) : right(r), left(l) {}

protected:
   Datum *right, *left;
};

I subclass Datum as follows:

class Column: public Datum
{
public:
   Column(Datum *r, Datum *l, string n, int s): Datum(r,l), name(n), size(s) {}

   void cover() {
      right->left = left;
      left->right = right;
   }

protected:
   string name;
   int size;
};

When I compile, using G++ v.4.5.1, I get the error messages pointing to the two lines in cover:

error: Datum* Datum::right is protected
error:  within this context
error: Datum* Datum::left is protected
error:  within this context

Obviously, making the section public causes the errors to go away. Why are they there when the section is protected?

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香橙ぽ 2024-12-19 04:43:10

问题是 Datum::Datum *right 受到保护。指针 right 是可访问/可分配的，因为基类是公开继承的。但是 right 指向的对象无法按照您在成员函数 Column::cover() 中尝试的方式获得访问权限，因为 right< 指向的对象/code> 与派生类的当前范围没有直接关系。

 void cover() {
      // Not posssible right->left = left;

      // But -
       right = new Datum(); // Assuming the class has default constructor.
 }

Datum::Datum *left 也存在类似的问题。

The problem is Datum::Datum *right is protected. Pointer right is accessible / assignable because the base class is inherited publicly. But the object pointed by right has no access previliges the way you are trying to in the member function Column::cover() because the object pointed by right has no direct relation in the current scope of the derived class.

 void cover() {
      // Not posssible right->left = left;

      // But -
       right = new Datum(); // Assuming the class has default constructor.
 }

Similar is the problem for the Datum::Datum *left too.

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