The document discusses inheritance in object-oriented programming, including defining inheritance as objects inheriting properties like data members and member functions from another class, and describing single inheritance where a class inherits from one base class and multiple inheritance where a class inherits from more than one base class. It also covers access control and how private, protected, and public members can be accessed by classes and derived classes.

1. Lecture 18
Inheritance: Base and
Derived Classes

2. Introduction
• In dictionary inheritance is defined as the action of inheriting;
the transfer of property; to receive from a predecessor.
• In programming language term, inheritance refers to objects
inheriting properties(data members & member functions) of
another class.
• Advantage: code reusability.
• Definition: Inheritance is the mechanism which allows a
class A to inherit properties of a class B. We say "A inherits
from B". Objects of class A thus have access to attributes
and methods of class B without having to redefine them.
• Definition: If class A inherits from class B then B is called
the superclass (or parent class) of A. A is called the subclass
(or child class) of B.

3. Introduction
Class Person
Data: x
Method: f1()
Class Student Class Employee
Data: y Data: z
Method: f2() Method: f3()
Class Undergraduate
Data: a
Method: f4()

4. Single Inheritance
• A class is derived from ONE base class.
Class Patient
Data: Idnum, Name
Method: SetDetails(),
DisplayDetails()
Class InPatient
Data: WardNum,
DaysinWard
Method: InSetDetails(),
InDisplayDetails()

5. Sample Program of Single Inheritance
#include <iostream.h> void InPatient::InSetdetails (int Wnum, int Dys)
class Patient { { Wardnum = Wnum;
public: Daysinward = Dys;
void Setdetails(int, char); }
void Displaydetails();
private: void InPatient :: InDisplaydetails ()
int IdNumber; char Name; }; { cout << endl << "Ward Number is "
void Patient::Setdetails (int Idnum, char << Wardnumber;
Namein) cout << endl << "Number of days in ward "
{ IdNumber = Idnum; Name = Namein; } << Daysinward;
void Patient::Displaydetails() }
{ cout << endl << IdNumber << Name; } void main()
{ InPatient p1;
class InPatient : public Patient { public: p1.Setdetails(1234, 'B');
void InSetdetails (int, int); p1.Displaydetails();
void InDisplaydetails(); p1.InSetdetails(3,14);
p1.InDisplaydetails();
private: }
int Wardnum, Daysinward; };

6. Multiple Inheritance
• A class is derived from more than one base classes.
Class Physical Class Mental
Data: Height, Data: IQ, Readingage
Weight Method: SetMental(),
Method: SetPhysical(), DisplayMental()
DisplayPhysica ()
Class Person
Data: Name
Method: SetName()

7. Sample Program of Multiple
Inheritance
#include <iostream.h> class Person : public Physical , public Mental
class Physical { { private:
private : char Name;
public:
float height, weight;
void setname()
public : { cin >> Name; }
void setphysical() };
{ cin >> height; cin >> weight; }
void displayphysical() void main ()
{ Person a1;
{ cout << height << weight; } };
a1.setname();
class Mental { a1.setphysical();
private : a1.setmental();
int IQ, Readingage; a1.displayphysical();
public : a1.displaymental();
}
void setmental()
{ cin >> IQ; cin >> Readingage; }
void displaymental()
{ cout << IQ << Readingage; } };

8. Access Control
• If a member is declared in a class C and is private, it can
only be used by the member functions in C and by the
friends of class C.
Class C Class E: friend Class C
private: int a; private: int num;
public: void Set_a() public: void Set_num()
• void Set_a() and Class E can access the private data
member, a which belongs to Class C.

9. Access Control
• If a member is declared in a class C and the member is
protected, it can only be used by the member functions in
C, friends of C and member functions and friends of
classes derived from C.
Class C Class E: friend Class C
protected: int a; private: int num;
public: void Set_a() public: void Set_num()
Class D Class F: friend Class D
private: int numD; private: int numF;
public: void Set_numD() public: void Set_numF()
• void Set_a(),Class E, Class D, and Class F can access the private
data member, a which belongs to Class C.

10. Access Control
• If a member is public it can be used everywhere without
restrictions.
Class C
public: int a;
public: void Set_a()
• int a and void Set_a can be accessed everywhere.

11. Access Control
• A derived class cannot access directly the private members
of its base class.
• However, the derived class can access the public and
protected member of its base class.
• The derived class can only access private members of the
base class only through access functions provided in the
base class’s public and protected interfaces.