protected: C++ access control works on per-class basis, not on per-object basis

一个很简单的问题：

//为什么BASE：：foo()中可以直接通过p访问val? 看本记录标题，这个问题困扰了很长一段时间，终于解决
class BASE

{
      private:
           int val;

      public:void foo(BASE *p)

      {

            int w = p->val;

      }  

};

 

同学参加一场笔试，抛出个问题，丫凡和我讨论了下，丫凡在stackoverflow上找到了答案……

如下内容引述自：http://stackoverflow.com/questions/6986798/subtle-c-inheritance-error-with-protected-fields                        

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Below is a subtle example of accessing an instance's protected field x. B is a subclass of A so any variable of type B is also of type A. Why can B::foo() access b's x field, but not a's x field?

class A 

{

    protected:

          int x;

};



class B : public A 

{

    protected:

      A *a;

      B *b;

    public:

      void foo() 

      {

          int u = x;     // OK : accessing inherited protected field x

          int v = b->x;  // OK : accessing b's protected field x

          int w = a->x;  // ERROR : accessing a's protected field x

      }

};

Why can the B::foo() access the members of the contained class B pointer b?

The rule is:
In C++ access control works on per-class basis, not on per-object basis.
So an instance of class B will always have access to all the members of another instance of class B.

 

 

class A 

{

    protected:

        int x;

};



class B : public A 

{

    public:

        void foo() 

        {

            int u = x;     // OK : accessing inherited protected field

        }

};

Since child is inheriting parent, child gets x. Hence you can access x directly in foo() method of child. This is the concept of protected variables. You can access protected variables of parent in child directly. Note : Here I am saying you can access x directly but not through A's object! Whats the difference ? Since, x is protected, you cannot access A's protected objects outside A. Doesnt matter where it is - If its main or Child . That's why you are not able to access in the following way.

Here comes an interesting concept. You can access a private variable of a class using its object with in the class!

class dummy 

{

    private : 

      int x;

    public:

      void foo() 

      {

          dummy *d;

          int y = d->x; // Even though x is private, still you can access x from object of d - But only with in this class. You cannot do the same outside the class. 

      }

};