Inheritance. Reference Text: Chapter11, Big C++

Transcription

1 Inheritance Reference Text: Chapter11, Big C++

2 Class relationships Recall your Lab05; You had a Point class, you had a curve class, what was a relationship between? Curve has points? Consider Another example: Branch, Customer, and Account classes in a banking system Relationships, these classes typically have is- Accounts are owned by customers, there can be more than one customer owning an account A branch contains n number of account

3 Classes relationships This is typically aggregation/composition relationship between classes, and is detected by presence of has-a type of relationship Curve has-a number of points Branch has-a number of BankAccounts BankAccount has-a customer(s)

4 Class relationships Consider DAIICTians There are Students TA Employee Faculty Staff What kind of relationship you see here?

5 Class relationships If try to phrase out this relationship, we can say like this TA is-a (is-also-a) Student Faculty is-a (is-also-a) Employee Staff is-a (is-also-a) Employee On above all, everyone is-a (is-also-a) DAIICTian

6 Class Relationships We shall talk more about class relationships, sometime later This type of (is-a) relationship between classes is basis of Inheritance in Object Oriented Software Design

7 Inheritance Inheritance is a mechanism that allows you to create new class by reusing existing class. If a new class needs to be implemented and a class representing a more general concept is already available, then the new class can inherit from the existing class. The existing, more general class is called the base class, and the more specialized class that inherits from the base class is called the derived class.

8 Inheritance For example suppose you already have a class Employee, and need to create a more specific class Manager Manager could be derived class of Employee Any doubt, how manager is more specialized class and Employee is more general class? Some more example, TA could be derived class of Student class Student could be Derived class of DAIICTian Faculty could be derived class of Employee

9 How do we do it in C++? Define new class Manager as derived of Employee classclass Manager : public Employee { public: new member functions private: }; new data members New thing you see is : public Employee

10 Define Manager as derived class of Employee class

11 As a user what extension you have got in Manager class? You can construct Object of Manager class, as Manager m( ABC, 45000, Marketing ); You can call methods defined in Employee class, for example m.setsalary(48000); m.getname(); Note that manager class does not define these methods.

12 As a user what extension you have got in Manager class? Of course you can invoke methods defined in Manager class m.getdepartment();

13 Implementation of Manager class Constructor Manager::Manager(string name, double salary, string dept) { } : Employee(name, salary) department = dept; Note calling of Base class constructor (blue)

14 Construct a manager object, and see how, constructor of base class is called while constructing object of derived class Manager m( XYZ, 45000, Marketing ); Compare it with the employee object Employee e( XYZ, 45000);

15 Some Important questions to be understood When Manager class is derived from Employee, considering the example given here, understanding following questions is important What has been inherited to Manager class from Employee class What members of Employee can be accessed in Methods of Manager class

16 What has been inherited to Manager class from Employee class Everything A manager object will have all data of employee, plus its own All member function functions of employee class can be invoked through

17 What members of Employee can be accessed in Methods of Manager class Private data members of Employee class can not be accessed in member functions of Manager class?

18 Redefine getsalary method for Manager class Derive classes can redefine member functions of base class. If a derives class defines a function with a same signature of the base class then it is redefinition Suppose Managers have a incentive data manager, and getsalary method should return sum of salary what every employee gets plus manager s incentive;

19 Redefine getsalary method for Manager class Below is how you may attempt to implement it double Manager::getSalary() { return salary + incentive; } Where incentive is additional private data member of Manager class and salary is private data member of Employee class. But this is wrong, you can not access private data of base class

20 Redefine getsalary method for Manager class You need to call public method of base class to access salary- Manager::getSalary() { return Employee::getSalary() + incentive; } And, this is how you call method of base class in derived class; note that Employee:: is necessary here, it would refer to Manager::getSalary, otherwise

21 Redefining function in derived class Now you have two versions of getsalary(), one in employee and other in Manager Guessing, which one should be bound to which definition should be straight forward, I guess- If you have employee object e, then e.getsalary would be bound to Employee::getSalary(), and When you have Manager object m, then m.getsalary would be bound to Manager::getSalary()

22 Protected access In case of problem raised in accessing private members of base class in derived, C++ allows you to have class members as protected. Protected members can be accessed in derived class, but remain private for other classes or to any public function

23 By making it protected, salary can be accessed in any of member function of Manager class (or any other class derived from Employee)

24 Your getsalary method can be redefined like this now Below is how you may attempt to implement it double Manager::getSalary() { } return salary + incentive; Where incentive is private data member of Manager class and salary is protected data member of Employee class.

25 Thanks