C++ Inheritance: Public and Private

• Public and private inheritance are mechanisms in C++ that control the accessibility of base class members in a derived class.
• They determine how the inherited members can be accessed and used by the derived class and external code.

Public Inheritance

• Public inheritance is the most common and straightforward type of inheritance.
• When a class is derived using public inheritance, all public members of the base class remain public in the derived class.
• Protected members of the base class remain protected in the derived class.
• Private members of the base class are inaccessible in the derived class.
• Public inheritance establishes an "is-a" relationship between the base class and the derived class, meaning that a derived class object can be treated as a base class object.
• Example C++ program:

#include <iostream>


class Base {
public:
 int publicVar;


 Base() : publicVar(10) {}


 void publicFunc() {
 std::cout << "Public function in Base class\n";
 }


protected:
 int protectedVar;


private:
 int privateVar;
};


class PublicDerived : public Base {
public:
 void accessBaseMembers() {
 std::cout << "Public variable from Base: " << publicVar << std::endl;
 std::cout << "Protected variable from Base: " << protectedVar << std::endl;
 publicFunc();
 }
};


int main() {
 PublicDerived obj;
 obj.accessBaseMembers();
 obj.publicFunc();
 std::cout << "Public variable from Base via object: " << obj.publicVar << std::endl;
 return 0;
}

• In the PublicDerived class, publicVar and publicFunc are accessible from outside the class.
• protectedVar is accessible within PublicDerived but not from outside.
• privateVar is not accessible from PublicDerived.

Private Inheritance

• When a class is derived using private inheritance, all public and protected members of the base class become private members of the derived class.
• This means they are accessible only within the derived class and not from outside.
• Private inheritance does not establish an "is-a" relationship.
• It is typically used when the derived class wants to reuse the functionality of the base class internally without exposing the base class's interface.
• Example C++ program:

#include <iostream>


class Base {
public:
 int publicVar;


 Base() : publicVar(10) {}


 void publicFunc() {
 std::cout << "Public function in Base class\n";
 }


protected:
 int protectedVar;


private:
 int privateVar;
};


class PrivateDerived : private Base {
public:
 void accessBaseMembers() {
 std::cout << "Public variable from Base: " << publicVar << std::endl;
 std::cout << "Protected variable from Base: " << protectedVar << std::endl;
 publicFunc();
 }
};


int main() {
 PrivateDerived obj;
 obj.accessBaseMembers();
 // obj.publicFunc(); // Error: 'publicFunc' is inaccessible
 // std::cout << obj.publicVar << std::endl; // Error: 'publicVar' is inaccessible
 return 0;
}

• In the PrivateDerived class, publicVar and publicFunc are now private and not accessible from outside the class.
• The accessBaseMembers function can access them because it is a member of PrivateDerived.

Key Differences

• Accessibility: Public inheritance preserves the accessibility of base class members, while private inheritance makes them private in the derived class.
• Relationship: Public inheritance represents an "is-a" relationship, while private inheritance does not.
• Use Cases: Public inheritance is used when the derived class needs to expose the base class's interface, while private inheritance is used for implementation reuse without exposing the base class's interface.