Public, secure, private

Question

Take a look at this code:

#include <iostream>

using namespace std;

class A
{
private:
    int privatefield;
protected:
    int protectedfield;
public:
    int publicfield;
};

class B: private A
{
private:
    A a;
public:
    void test()
    {
        cout << this->publicfield << this->protectedfield << endl;
    }
    void test2()
    {
        cout << a.publicfield << a.protectedfield << endl;
    }
};

int main()
{
    B b;
    b.test();
    b.test2();
    return 0;
}

B has access to this -> protected field, but does not have a.protectedfield. What for? However, B is a subclass of A.

Answer 1

B has access only to protected fields in itself or to other objects of type B (or, possibly, obtained from B if it sees them as Bs).

B does not have access to the protected fields of any other unrelated objects in the same inheritance tree.

Apple does not have access to the internal components of Orange, even if they are both fruits.

class Fruit
{
    protected: int sweetness;
};

class Apple: public Fruit
{
    public: Apple() { this->sweetness = 100; }
};

class Orange: public Fruit
{
public:
    void evil_function(Fruit& f)
    {
        f.sweetness = -100;  //doesn't compile!!
    }
};

int main()
{
    Apple apple;
    Orange orange;
    orange.evil_function(apple);
}

Answer 2

this- > protectedfield: B enherits of A, , protectedfield , .

a.protectedfield: a B, , . B , A .

Answer 3

Allows to break all code into small parts. Copy and paste this two code and try to compile !!!!

#include <iostream>
using namespace std;
class A
{
private:
    int privatefield;
protected:
    int protectedfield;
public:
    int publicfield;
};

int main()
{
    A a;
    cout<<a.publicfield;
    cout<<a.privatefield;/////not possible ! private data can not be seen by an object of that class
    cout<<a.protectedfield;////again not possible. protected data is like privete data except it can be inherited by another.If inherited as private then they are private,if as protected then protected and if as public then also protected.
}

Now B inherits class A as private

#include <iostream>

using namespace std;

class A
{
private:
    int privatefield;
protected:
    int protectedfield;
public:
    int publicfield;
};

class B: private A
{
private:
    A a;
public:
    void test()
    {
        cout << this->publicfield << this->protectedfield << endl;
    }
    void test2()
    {
        cout << a.publicfield << endl;
    }
};
int main()
{
    /*Now B will have both public and protected data as private!!!!
    That means
     B now looks like this class


     Class B
     {  

        private:
        int protectedfield;
        int publicfield;
     }

     As we have discussed private/protected data can not be accessed by object of the class
     so you you can not do things like this

     B b;
     b.protectedfield; or b.publicfield;

     */
    B b;
    b.privatefield;////Error !!!
    b.protectedfield/////error!!!!
}

Thank!