控制对班级成员的访问

Question

class A
{
   private:
           int x;
   public:
           virtual void show()
           {
               cout<<"X: "<<x;
           }
};

class B: public A
{
    public:
           virtual void show()
           {
               cout<<"In class B\n";
               A::show();
           }
};

My question is about accessibility from a member function. In this example, can we say that member function of B (show() of B) can access X in the class A.

Answer 1

Since x is declared private in A , nothing in B can directly access it. Of course, it can be accessed indirectly -- B can call A::show() which can access x . But B::show() cannot access x , nor can anything else in B .

Answer 2

You are not really accessing Ax , you are accessing A.show() .

To answer your question, B::show() is not accessing the private member x in A .

The reason for this is, class A can change the function A::show() and do something else, and B can just call A::show() .

This is the main point of encapsulation. You can tell A to do things (like show() ), but A decides how to do them.

Answer 3

1. A::show() can access any member variables in it's own class, class A.
2. Member functions in class B can access any public or protected member variables in class A.
   and
3. Member functions in class B can access any public of protected member functions in class A.

2 and 3 are possible because the "Most accessible level" of A is defined as public.

Class B: public A

In your code, specifically, the accessibility level of int x; doesn't matter, because it is never directly accessed from class B.