CODEWITHSUNDEEP
c++classinheritanceprotected
saumitra mallick
saumitra mallick

Reputation: 405

Access declaration of a protected member of base class as public in derived class C++

When I declare a member protected in base class and inherit as private in derived class , access to the member is not allowed 

class base{
protected:
int a;
};
class derived : public base
{
protected:
int b;
public:
derived():base(){ a=0; b=0;}
void show(){cout<<"a= "<<a<<"\tb= "<<b;}
};
int main ()
{
derived d;
d.a=10;    //error: 'int base::a' is protected within this context
d.show();
}

But when I write the derived class, to grant public access for 'a' (protected in base)

class derived : public base 
{
protected:
int b;
public:
base::a;
};

int main ()
{
derived d; 
d.a=20;   // no error 
}

Now I can change value of 'a' in main() without any error.

I read in c++ complete reference book, granting access will restore access rights ,but one can not raise or lower the access status.

Can anyone tell me why I'm able to access protected member of base class ,inherited privately , and later given public access like a public variable of derived class ( doesn't it violate encapsulation ,ie Protected member should be restored as protected). Please guide me if my understanding is incorrect

Upvotes: 2

Views: 2150

Answers (2)

Christian Hackl
Christian Hackl

Reputation: 27528

protected doesn't live up to its name. It just doesn't protect much; it's not very far from public.

The moment base says that a is protected, the field's fate is sealed - it's now practically open to the rest of the world. All you have to do is create a derived class, access the member variable and expose is to the outside world - which is what you did with derived when you wrote base::a (by the way, you normally add using before that).

You could even do that with a normal function, after all:

class base {
protected:
    int a;
};

class derived : public base 
{
protected:
    int b;
public:
    int& GotYou()
    {
        return a;
    }
};


int main ()
{
    derived d; 
    d.GotYou() = 20; // no error 
}

If you want protection, use private.

By the way, the following line probably gives you a wrong impression of how C++ works:

derived():base(){ a=0; b=0;}

What happens here is that base is constructed, a is default-initialised to an indeterminate value, then b is default-initialised to an indeterminate value and then two assignments are made. I recommend some research on initialisation lists.

Upvotes: 2

Ai Da
Ai Da

Reputation: 17

The following links will help you:

Difference between private, public, and protected inheritance

http://www.geeksforgeeks.org/inheritance-in-c/

Upvotes: 1

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