C++ Inheritance Access Control

Inheritance Access Control

• When creating a derived class from a base class then, you can use different access specifiers to inherit the data members of the base class.
• The derived class can access all the non-private members of its base class. And the base class members that are not accessible to the member functions of derived classes should be declared private in the base class.
• The access specifiers that are used are public, private and protected.
• When deriving a class from a base class, the base class may be inherited through public, private and protected inheritance.

In C++ inheritance, we can derive a child class from the base class in different access modes. For example,

class Base {
.... ... ....
};

class Derived : public Base {
.... ... ....
};

Notice the keyword public in the code

class Derived : public Base

This means that we have created a derived class from the base class in public mode. Alternatively, we can also derive classes in protected or private modes.

These 3 keywords (public, protected, and private) are known as access specifiers in C++ inheritance.

Syntax of Inheritance Access Control in C++

public, protected, and private inheritance have the following features:

1. Public inheritance: This is the most used inheritance mode. In this the protected member of Base class becomes protected members of Derived class and public becomes public.
2. Protected inheritance: In protected mode, Then both public member and protected members of the Base class will become protected in Derived class.
3. Private inheritance: In private mode, the protected and public members of Base class become private members of Derived class.

Note: private members of the base class are inaccessible to the derived class.

Syntax:-

class Base {
public:
int a;
protected:
int b;
private:
int c;
};

class PublicDerived: public Base {
// a is public
// b is protected
// c is not accessible from PublicDerived
};

class ProtectedDerived: protected Base {
// a is protected
// b is protected
// c is not accessible from ProtectedDerived
};

class PrivateDerived: private Base {
// a is private
// b is private
// c is not accessible from PrivateDerived
}

Example 1: C++ Program to demonstrate the working of public inheritance

#include <iostream>
using namespace std;

class Base {
   private:
    int pvt = 1;

   protected:
    int prot = 2;

   public:
    int pub = 3;

    // function to access private member
    int getPVT() {
        return pvt;
    }
};

class PublicDerived : public Base {
   public:
    // function to access protected member from Base
    int getProt() {
        return prot;
    }
};

int main() {
    PublicDerived object1;
    cout << "Private = " << object1.getPVT() << endl;
    cout << "Protected = " << object1.getProt() << endl;
    cout << "Public = " << object1.pub << endl;
    return 0;
}

Output

Private = 1
Protected = 2
Public = 3

Here, we have derived PublicDerived from Base in public mode.

As a result, in PublicDerived:

• prot is inherited as protected.
• pub and getPVT() are inherited as public.
• pvt is inaccessible since it is private in Base.

Since private and protected members are not accessible, we need to create public functions getPVT() and getProt() to access them:

// Error: member "Base::pvt" is inaccessible
cout << "Private = " << object1.pvt;

// Error: member "Base::prot" is inaccessible
cout << "Protected = " << object1.prot;

Accessibility in Public Inheritance

Example 2: C++ Program to demonstrate the working of protected inheritance

#include <iostream>
#include <string>
using namespace std;

class Base {
   private:
    int pvt = 1;

   protected:
    int prot = 2;

   public:
    int pub = 3;

    // function to access private member
    int getPVT() {
        return pvt;
    }
};

class ProtectedDerived : protected Base {
   public:
    // function to access protected member from Base
    int getProt() {
        return prot;
    }

    // function to access public member from Base
    int getPub() {
        return pub;
    }
};

int main() {
    ProtectedDerived object1;
    cout << "Private cannot be accessed." << endl;
    cout << "Protected = " << object1.getProt() << endl;
    cout << "Public = " << object1.getPub() << endl;
    return 0;
}

Output

Private cannot be accessed.
Protected = 2
Public = 3

Here, we have derived ProtectedDerived from Base in protected mode.

As a result, in ProtectedDerived:

• prot, pub and getPVT() are inherited as protected.
• pvt is inaccessible since it is private in Base.

As we know, protected members cannot be accessed directly.

As a result, we cannot use getPVT() from ProtectedDerived. That is also why we need to create the getPub() function in ProtectedDerived in order to access the pub variable.

// Error: member "Base::getPVT()" is inaccessible
cout << "Private = " << object1.getPVT();

// Error: member "Base::pub" is inaccessible
cout << "Public = " << object1.pub;

Accessibility in Protected Inheritance

Example 3: C++ Program to demonstrate the working of private inheritance

#include <iostream>
using namespace std;

class Base {
   private:
    int pvt = 1;

   protected:
    int prot = 2;

   public:
    int pub = 3;

    // function to access private member
    int getPVT() {
        return pvt;
    }
};

class PrivateDerived : private Base {
   public:
    // function to access protected member from Base
    int getProt() {
        return prot;
    }

    // function to access private member
    int getPub() {
        return pub;
    }
};

int main() {
    PrivateDerived object1;
    cout << "Private cannot be accessed." << endl;
    cout << "Protected = " << object1.getProt() << endl;
    cout << "Public = " << object1.getPub() << endl;
    return 0;
}

Output

Private cannot be accessed.
Protected = 2
Public = 3

Here, we have derived PrivateDerived from Base in private mode.

As a result, in PrivateDerived:

• prot, pub and getPVT() are inherited as private.
• pvt is inaccessible since it is private in Base.

As we know, private members cannot be accessed directly.

As a result, we cannot use getPVT() from PrivateDerived. That is also why we need to create the getPub() function in PrivateDerived in order to access the pub variable.

// Error: member "Base::getPVT()" is inaccessible
cout << "Private = " << object1.getPVT();

// Error: member "Base::pub" is inaccessible
cout << "Public = " << object1.pub;

Accessibility in Private Inheritance

Next Tutorial

We hope that this tutorial helped you develop better understanding of the concept of Inheritance Access Control in C++.

Keep Learning : )

In the next tutorial, you'll learn about C++ Inheritance Types.