Object Oriented Programming with C++/ Session 5/ 1 of 41 Inheritance Session 5 Object Oriented Programming with C++/ Session 5 / 2 of 41 Session Objectives  Describe Single Inheritance  Describe Base class and Derived class  Access Base class members and use pointers in classes  Describe types of inheritance  Describe Constructors and Destructors under inheritance  Describe how to call Member Functions of the Base Class and Derived Class  Describe Container Classes Object Oriented Programming with C++/ Session 5 / 3 of 41 Single Inheritance  To maintain and reuse class objects easily, we need to be able to relate classes of similar nature to another.  Single inheritance is the process of creating new classes from an existing base class.  For example let us consider a program in which we are dealing with people employed in an organisation. Object Oriented Programming with C++/ Session 5 / 4 of 41 Single Inheritance (Contd.)  Each of the subclasses is considered to be derived from the class Employee. The class Employee is called the base class and the newly created class is called the derived class. Employee Director Manager Secretary Clerk Object Oriented Programming with C++/ Session 5 / 5 of 41 Single Inheritance (Contd.)  In a class hierarchy, the derived classes inherit the methods and variables of the base class.  They can also have properties and methods of their own. Class: Manager m_name, m_age, m_Emp_id, m_salary m_department perks, no_of_employees reporting Class: Employee Name, Age, Emp_id Salary, Department Object Oriented Programming with C++/ Session 5 / 6 of 41 Advantages  Most important advantage: reusability of code. • Once a base class has been created it can be adapted to work in different situations.  Result of reusability of code is the development of class libraries. • A class library consists of data and methods encapsulated in a class. • Deriving a class from an existing one allows redefining a member function of the base class and also adding new members to the derived class. • The base class remains unchanged in the process. Object Oriented Programming with C++/ Session 5 / 7 of 41 Base Class and Derived Class  Derivation can be represented graphically with an arrow from the derived class to the base class.  The arrow pointing towards the base class signifies that the derived class refers to the functions and data in the base class, while the base class has no access to the derived class. Employee Manager Base class Derived class Derived from Object Oriented Programming with C++/ Session 5 / 8 of 41 Base Class and Derived Class  Declaration of a singly derived class is similar to that of any ordinary class.  We also have to give the name of the base class. For example, class Manager : public Employee  Any class can be used as a base class.  A base class can be classified into two types: • direct base • indirect base Object Oriented Programming with C++/ Session 5 / 9 of 41 Direct and Indirect Base  A base class is called direct if it is mentioned in the base list. For example: class A { }; class B : public A { }; // where class A is a direct class.  An indirect class can be written as: class A { }; class B : public A { }; class C : public B { }; //Can be extended to an arbitrary number of levels. Object Oriented Programming with C++/ Session 5 / 10 of 41 Accessibility  Accessibility: Knowing when a member function or data member of a base class can be used by objects of the derived class. • Class members can always be accessed by member functions within their own class, whether the members are private or public. • Objects defined outside the class can access class members only if the members are public. [...]... access level, as the base class (private for private base, protected for protected base) Object Oriented Multi-level inheritance Public, private or protected will affect the access the derived class functions have over the members of the base classes in a multi-level inheritance Object Oriented Example for multi-level inheritance In the following code the class B derives privately from class A and class... Yes Yes Yes No Accessible from objects outside the class Yes No No Object Oriented Example class Employee{ private: int privA; protected: int protA; public: int pubA; }; //base class Object Oriented Example (Contd.) //derived class class Manager : public Employee{ public: void fn() { int a; a = privA; //error:not accessible a = protA; //valid a = pubA; //valid } }; Object Oriented Example (Contd.) void... used as a Manager* Object Oriented Example of pointers void main() { Manager mgr; Employee* emp = &mgr; //valid:every Manager is an Employee Employee eml; Manager* man = &eml; //error: not every Employee is a Manager } • An object of a derived class can be treated as an object of its base class when manipulated through pointers However, the opposite is not true Object Oriented Types of Inheritance A derived... members; they cannot access the private members of the base class • In conformance with the object- oriented concept of information hiding No access to some of the class members Those members can be put in the private section Allow controlled access by providing some protected members Inheritance does not work in reverse Object Oriented Access rules for Base class members Access specifier Accessible Accessible... protected: int a; public: Base(){a = 0;} //default constructor Base(int c){ a = c;}//one-arg constructor }; class Derived{ public: Derived(): Base(){}//default constructor Derived(int c): Base(c){} //constructor with one-arg Object Oriented }; Constructors (Contd.) When you declare an object of the derived class, with the statement Derived obj; it will cause the constructor of the base class to be called... same name, it must use the scope resolution operator Object Oriented Calling Member Functions (Contd.) A function in the base class can be invoked using the objects of the base class as well as the derived class If a function exists in the derived class and not in the base class, it can be invoked only with the objects of the derived class Object Oriented Calling Member Functions (Contd.) class Base{... declared with one of the specifiers i.e., public, private and protected The keyword public in the class declaration of the derived class specifies that objects of the derived class are able to access public member functions of the base class With the keyword private in the derived class declaration, objects of the derived class in main() cannot even access public member functions of the base class Object Oriented. .. }; class B : private A{ public : int b; void func_b() { int x,y; x=a; // valid y=b; // valid } Object Oriented }; Example (Contd.) class C : public B { public : void func_c() {int x,y; x=a; // not valid y=b; // valid } }; Object Oriented Constructors under inheritance The constructor of the base part of an object is first called, then the appropriate constructor of the derived class is called class... class Employee Object Oriented Protected Access Specifier The protected section is like the private section in terms of scope and access • Protected members can be accessed only by members of that class • Protected members cannot be accessed by objects or functions from outside the class, such as main() • The difference between private and protected appears only in derived classes Object Oriented Accessing... creating the class, private is assumed Object Oriented Types of Inheritance (contd.) Functions in the derived classes can access protected and public members in the base class Objects of the derived classes outside the class or in main() cannot access private or protected members of the base class The difference is between publicly and privately derived classes • Objects of the class B, which is publicly . Object Oriented Programming with C++/ Session 5/ 1 of 41 Inheritance Session 5 Object Oriented Programming with C++/ Session 5 / 2 of 41 Session Objectives. Employee Director Manager Secretary Clerk Object Oriented Programming with C++/ Session 5 / 5 of 41 Single Inheritance (Contd.)  In a class hierarchy,