C++ Public, Protected and Private 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.

public, protected and private inheritance in C++

• public inheritance makes public members of the base class public in the derived class, and the protected members of the base class remain protected in the derived class.
• protected inheritance makes the public and protected members of the base class protected in the derived class.
• private inheritance makes the public and protected members of the base class private in the derived class.

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

class Base {
  public:
    int x;
  protected:
    int y;
  private:
    int z;
};

class PublicDerived: public Base {
  // x is public
  // y is protected
  // z is not accessible from PublicDerived
};

class ProtectedDerived: protected Base {
  // x is protected
  // y is protected
  // z is not accessible from ProtectedDerived
};

class PrivateDerived: private Base {
  // x is private
  // y is private
  // z is not accessible from PrivateDerived
};

Example 1: C++ public Inheritance

// 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 from main(), 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;

Notice that the getPVT() function has been defined inside Base. But the getProt() function has been defined inside PublicDerived.

This is because pvt, which is private in Base, is inaccessible to PublicDerived.

However, prot is accessible to PublicDerived due to public inheritance. So, getProt() can access the protected variable from within PublicDerived.

Accessibility in public Inheritance

Example 2: C++ protected Inheritance

// C++ program to demonstrate the working of protected 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 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 directly accessed from outside the class. 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++ private Inheritance

// 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 directly accessed from outside the class. 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