ProblemSolving OO Concepts SessionIIIv1.0

8/13/2019 ProblemSolving OO Concepts SessionIIIv1.0

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INTERNAL

April 06, 2009

Problem Solving

Inheritance


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OOP Orientation

INTERNAL- 2 -

Problem Context

Connection manager

A local grocery shop in our society is an authorized distributor for GSM mobile

connectionsfor Talk the Talk" service provider. The shop sells new connections,

provide easy recharge, topupsand monthly payments. The shopkeeper has got hispersonal accountwhich keeps track of the balance amount for easy recharge and

topups. The shop also maintains an inventory of new connections and the connections

sold in the locality for audits and reports. Talk the Talk offers various plans for

convenience of usage in terms of charge per pulse rate.

Mr X and Mr Y are two friends decide to take new connections, visits the shop. Mr Y

decides to take prepaid connectionand Mr X gets convinced for a postpaid

connection. Mr Y gets initially recharged with INR 200 and a topup of INR 20 and Mr X

was charged INR 500 for connection.

The usage of Mr Y is limited by the amount available on last topup and will have to put

topups till validity of the card. Incase validity of the connection expires he needs to

recharge again. The amount deducted as per the usage and the pulse rate plan.

Mr X is charged monthly with a rental of INR 200 in addition with a charge the total usage

(duration recorded) and the pulse rate plan. If not paid the connection is deactivated till

payment.


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OOP Orientation

INTERNAL- 3 -

PrePaid Connection

Mr Y

INR 0.02 / secINR 0.01 / sec

Mr XConnection# : 81XXXXXX10

Validity Status: Valid

Recharge: INR 200

Top Up: INR 20

Connection# : 96XXXXXX06

Validity Status: Valid

Monthly Rent: INR 200

Recorded Pulse : TBD

PlanPlan

PostPaid Connection

Connection

Visualizationof the scenario


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OOP Orientation

INTERNAL- 4 -

Layers of System in real world

The Real World user Interactions

Layer1:Interaction with Mr X and Mr Y and they speak

Mr Xand Mr Yget new connections (96XXXXXX06 & 81XXXXXX10)and activated

with respective plans (INR 0.01/sec and INR0.02/sec)

Layer2 :Interaction with Service Provider and he says

Talk the Talk provides Prepaidconnections which needs recharge and topup as

per assigned Plan.

Talk the Talk provides Postpaidconnections which needs monthly rental and usage

payment as per assigned Plan.

Layer3:Interaction with Shopkeeper and he says

Shopkeeper sells new Connections to Customers and manages inventory and records

for Customers, Connections, Plans.

Each user interaction layer describes the system at different level of abstractions:

Mr Xand Mr Y [Layer 1] -> Customer [Layer 3]

96XXXXXX06 [Layer 1] -> Prepaid [Layer 2]81XXXXXX10 [layer 1] -> Postpaid [layer 2]

INR 0.01/sec & INR0.02/sec [layer 1] -> Plan [Layer2 & Layer3]

Prepaid & Postpaid [Layer2] -> Connection [Layer3]


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OOP Orientation

INTERNAL- 5 -

Inheritance

Inheritance is to visualize the system in different layers of abstraction

Each layer of abstraction is taken from different stakeholders involved in that

system

The system can be designed at each layer by leveraging abstraction(knowing and doing responsibilities)

Example :

Shopkeeper can maintain inventory of Connections.

The connections can be Prepaid or Postpaid, and can be further be

elaborated as the actual connections.

The approach of inheritance is bottom up.

The bottom most layer is concreteand the top most layer is abstract.

The concrete layer reuses and refinesthe abstraction [knowing and

doing responsibilities]of abstract layer.

Since abstract classes cannot have definitions, we cannot create its

instances*, the instances can be only for concrete classes.

The abstractions can be realized through the objects of concrete classes,

only if it is an IS_A type of relationship.


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OOP Orientation

INTERNAL- 6 -

Inheritance

In a is-a relationship child inherits abstractions of the parent

In a is-a class hierarchy

A child class of one parent can be the parent of another child

Common features should be put as high in the hierarchy as is reasonable.

The is-a relationship is transitiveExample: 96XXXXXX06 is a PostPaid Connection, where PostPaid Connection is a

Connection, thus 96XXXXXX06 is a Connection


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OOP Orientation

INTERNAL- 7 - 7

Its the generalization/specialization relationship between classes.

The inheriting class inherits all the methods and fields of the class it inherits

from.

The code of the inheriting class consists only of the changes and additions

to the base class.

If class B inherits from class A, then objects of class B are also of type A. It

must make sense to call the methods of class A on an object of class B.

Inheritance


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OOP Orientation

INTERNAL- 8 -

Class Relationship

Number : stringactiveStaus : bool

Connection

calculateTotalAmount :double

getPulseRate : double

recharges : Recharge []

topups : Topup []

PrePaid


calculateTopupAmount : double

calculaterechargeAmount : double

callRecords : Duartion []

monthlyRental : double

billingAmount : Rental []

PostPaid


calculateTotalRental :double

calculateBillingAmount : double

Plan

pulseRate : double

getPulseRate : double

C


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OOP Orientation

INTERNAL

Derived Classes

A derived class inherits properties of its base class / classes which includes its data

members and member functions.

The inherited base class function can be redefined in the derived class. New members can be added to the derived class.

Members of the base can be referred to as if they were members of derived class.

D i d Cl ( d )


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OOP Orientation

INTERNAL- 10 -

class Connection {

//Attributes defines the state:

string number;

protected:bool activeStatus;

public:

Connection(string number) {

this->number = number;

this->activeStatus = false;

}// A constructor that initializes the fieldsdouble getPulseRate(){

//method definition

}

};

class Prepaid : public Connection {

//Attributes

//Methods define Behaviours of Objects

public:

Prepaid(string number) : Connection(number){

}

string getNumber(){ return number;} // Gives error as number is private in super class

bool getStatus(){ return activeStatus;}

};

Invokes the constructor

of Connection

Derived Classes (contd.)


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OOP Orientation

INTERNAL

Private Members of the base class are not accessible to the derived class.

Protected members of the base class can be used in the member functionsof the derived class.

Public members of the base class are accessible to the member of thederived class butmay or may not be a accessible to the external users.

What is not inherited? Base class constructor

Base class destructor

Friends

Derived Classes (Contd)


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OOP Orientation

INTERNAL

Multiple Inheritance

A Research Scientist in a University is an Employee of the University as he assists in teaching and getpaid for it as well as a Student of the University attending lectures and sessions on various courses.

Thus a class may be derived from any number of base classes

class Employee {};

class Student {};

class ResearchScientist : public Employee, public Student

{};

A class cannot be specified as a direct base class of a derived class more than once.

For e.g.

class A{};

class B{};

class C{};class D : public A,public B public A // error

{};

Indirect inheritance more than once is allowed.

class L{};

class A ; public L

{};class B ; public L

{};

class C : public A, public B

{};

The object of C will have two sub-objects of class L.

L L

A B

C


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OOP Orientation

INTERNAL

Virtual base class

The problem of multiple copies of base class in child class is solved by making the base class virtual

base class.

Grand parent

Parent1 Parent2

Child


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OOP Orientation

INTERNAL

Virtual base class

class A //Grand parent

{

};

class B: virtual public A //Parent 1

{

..

};class C: virtual public A //Parent2

{

------\

};

class D: public B, public C

{

//Only cone copy of A will be inherited}


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OOP Orientation

INTERNAL

Public and private inheritance

When a class is inherited by a derived class, the private members of the base classare not inherited.

If a class inherits from the base class using public, the public data remains publicand protected data becomes protected.

If the derived class inherits the base class using protected, all the public data andprotected data of base class become protected in child class.

If a class inherits from the base class using private, all the public data andprotected data become private.


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OOP Orientation

INTERNAL

Exampleclass employee

{

char *name;

short age;

short department;float salary;

public;

void print _ Name ();

};

/* Public inheritance for class Manager*/

class Manager : public employee {

int level;

public ;

void print _ level ();

};

class Branch_Manager : public Manager

{

char * branch;public :

void print();

};

void Branch_Manager :: print (){

print_Name ();

print_level ();

printf (%s,branch);

}

/*Private inheritance for Manager in thesame example*/

class Manager : employee

{int level;

public :

void print ();

};

void Branch Manger :: Print()

{

print Name(); // error privateinheritance

print_level ();

printf (%s,branch);

}


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OOP Orientation

INTERNAL

Public and private inheritance

Good practices:

Public inheritance signifies is a relation ship and it is an instance of interface inheritance.

Private inheritance on the other hand is an implementation inheritance.


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OOP Orientation

INTERNAL

Static Members are not inherited

Class A

{

public:static a;

}

Class B : public A{};

Class C : public A{};

Class D : public B,public A{};

D pd;

pd.a = 1; // OK a is a static Hence, no ambiguity

Static members are not inherited.


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OOP Orientation

INTERNAL

Constructor rule for inheritanceConstructor of Base cannot be inherited.

For a class derived from another class that has constructors in it :

The constructors of the derived class must pass parameters to anyone, of the base class constructors, if no defaultconstructor is defined.

class Employee{

char *name;

int age;

public;

Employee( ){

name = Mr X;

age = 20;

}

Employee(char *name, int age){this->name = name;

this->age = age;

}

void printName ();

};

class Manager : public Employee {

int level;

public ;

Manager(){level = 2;

} // Default Constructor of Employee class is called first

Manager(char *name, int age, int level) : Employee(name,age) //Invoking parameterized constructor of Employee

{

this->level = level;

}

void print _ level ();

};


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OOP Orientation

INTERNAL

Multiple Inheritance

Class A

{

public :

A (int);

};Class B

{

public :

B (char *);

};

Class C : public A, public B

{

Public :C(int, char *);

};C :: C (int c, char *s) : A ( c ),B(s)

{}


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OOP Orientation

INTERNAL

Destructor rule for InheritanceDestructors of the base classes cannot be inherited.

Destructors for derived classes

- Should only do clean up required for itself

- Should not worry about clean up related to inherited properties

Destructor of base class is automatically called after the destructor derived class.e.g.,

class Employee

{

protected :

char *name;

public :

Employee (char *s) (name = strdup (s);}

~ Employee () {if (name) free (name);}

};

class Manager : public Employee

{

char *dname;

public :

Manager (char *s) : Employee(s) { dname = new char [20]; strcpy (dname ,OOP;}~ Manager() {if dname) delete dname;}

};

main ()

{

Manager Tom (string)

// calls constructor of Employeeand then the constructor of Manager

// calls destructor of Manager then the destructor of Employee}

D t t l f I h it (C td)


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OOP Orientation

INTERNAL

class employee{

char *name;short dept;public :employee (char *nam,short dpt){

name = strdup (nam);dept = dpt;

}~employee () {free (name);}

};class manager : public employee{

char * area_name;public :manager (char *nam, short dpt,char *amam);~manager();

};manager :: manager (char *nam, short dpt,char *amam) : employee (nam, dpt)

{area_name = strdup (amam);

}Manager :: ~manager (){

free (area_name);}

Destructor rule for Inheritance (Contd)


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OOP Orientation

INTERNAL

Copy Constructors

The copy Constructor of a derived class calls the copy constructor of the base class first and then the

copy constructor of its own class. If the derived class does not have a user-defined copy constructor

and base class has a user-defined copy constructor, the base class user-defined copy constructor is

called first and then the default copy constructor derived class.

Class A

{

public :

A (A&); // user-defined copy constructor

A (int);

};

Class B:public

{

public :

B (int);

};

B b(3); //call B :: B(int)

B bb =b; //calls the copy construtor A :: A (A&) and then the

//default copy constructor of B ie B :: B(B&)


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OOP Orientation

INTERNAL

Inheritance and Pointers

class A{

int a;

public:

func(){cout


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OOP OrientationINTERNAL

Virtual functions

class Animal{

public:

virtual void whoAreYou(){cout


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Virtual functions

If a base class contains a virtual functions f which is re-defined in the derived class, then a call f() foran object of derived class invokes derived::f() even if access is through the a pointer of baseclass.

Run time polymorphism is achieved using virtual functions.

Vi t l f ti


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Virtual functions

class A

{

public:

virtual void f(){cout


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Access control for virtual functions

The access controls of a virtual function are determined by its declaration and not affected by rules fora function that later override it!!

class A{

public:

virtual void who(){cout


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Explicit scope resolution

An inheriting class that redefines virtual function can call the function of its parent class by calling itexplicitly using scope resolution operator.

class A{

public:

virtual void who(){cout


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Happy Learning!

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ProblemSolving OO Concepts SessionIIIv1.0