Ch.1 oop introduction, classes and objects

Object Oriented Programming
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lecture notes
introduction

Background
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 In a Procedural Programming (PP), program is written in a step by
step approach. At the end of the program or subroutine, tasks get
executed in a sequential manner
 PP focuses on breaking down a programming task into a collection of
variables, data structures and subroutines where the later two can
act on as many variables declared in the program.
 Variables can easily be modified by any member of a program
 Object Oriented Programming (OOP) focuses on breaking down a
task into units known as objects where each one includes its own
variables (data) and subroutines (methods).
 PP uses subroutines and other members to act on program data
structures whereas in OOP, object subroutines act on object data

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 Nomenclature varies between the two paradigms but have the same
semantics
 In OOP, each object is capable of receiving messages, processing
data and sending messages to other objects
PP OOP
variable attribute
function method
argument message
module object

Advantages of OOP
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4 OOP simulates real world objects thus providing easy understanding
and visualization of problems as well as the designs of the solutions.
Complexity of problems is reduced, program structures become clear.
 Easy modification (maintenance) of the system. This is because objects
are interacting through public interfaces allowing modifications in their
implementations without affecting the overall performances of the
system provided that modifications do not affect their functionalities.
 Modularity allows scalability of the system. More functionalities of the
system can be easily added at any time if modeled in form of objects.
 Reusability of code is practiced at highest level with OOP. Objects can be
reused as many times as needed and can also be used to solve similar
problems.
 Security of the program is much enhanced due to limitations of object
data from being accessed by other objects.

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concepts
object & class

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 Using OOP ;
o The overall program is made up of lots of different self-contained
components (objects),
o each object has a specific role in the program
o all objects can talk to each other in predefined ways.
Overview of Object and Class
 Object is the smallest element of a program designed to simulate a
real world object presented by the problem.
 A given problem can be broken down into unlimited number of
objects.
 Each object is designed and coded independently according to what
they actually represented in a real world.
 To deliver the overall program task, objects communicate through
messages.

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 Objects have a standard structure; must have attributes and methods
(functions).
 Attributes/variables/data define specifications of objects while
methods define the functionalities offered by objects. Methods are
said to explain behaviors of objects.
 Methods make use of attributes of the same object to define different
behaviors of the object.
 For example
 Dog
 Attributes – name, age, colour, breed, etc.
 Behaviour – eat, go for a walk, wiggle tail, etc.
 Car
 Attributes – make, engine size, colour, gear system, speed, etc
 Behaviour – change gear, change speed, stop, etc.
 Person
 ???

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 Class is an abstract of objects or can be also defined as a collection of
objects possessing similar general properties.
 Classes have similar structure as that of objects, the difference
between them being the degree of specification.
 Attributes of objects carry specific values while those of classes are
assigned to either general or default ones.
 Several objects can belong to the same class.

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Classes are like “templates” for a particular set of objects

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 A class is a generic template for a set of objects with similar features.
 Instance of a class = object
 If class is the general (generic) representation of an object, an
instance is its concrete representation.
 Another way of distinguishing classes from objects is:
 A class exists at design time, i.e. when we are writing OO code to
solve problems.
 An object exists at runtime when the program that we have just
coded is running.

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 Example – address book. For each person we want to store:
 Name
 Age
 Address
 Phone number
 We can write a class called Person which will represent the abstract
concept of a Person.
 We will then be able to create as much Person objects as we like in
order to model our address book.
 All of these objects will be an instance of our Person class.

Class Structure
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12//basic class construct
class ClassName {
//Attributes (identity section)
//Methods (behavior section)
}

Attributes
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 The type of the attributes could be
 Any of the primitive types – int, double, boolean, …
 Previously defined classes in the C++ libraries e.g String
 Previously user-defined classes
 We can use as many attributes as we wish
 more attributes = more complex class
 Example:
Person – name, age, address, DOB, phoneNo, ppsNo, …

Methods
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 Methods (subroutines –typically functions)
 a way of dividing larger programs into many smaller more
manageable segments of code each having it’s own specific task
 methods performs tasks independently of each other
 allows us to modularise the program
 Advantages
 More manageable programs
 Software reusability

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 Each method has
 parameters (arguments)
 The parameters are local variables (accessible only within the
method)
 return type
 returns a value (or void)

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How to define class in C++
Alternative 1: define methods implementations within the class
class Rectangle {
float width, height, area;
public:
void set_values (float a, float b) {
width = a; height = b;
}
void calcArea () {
area = width * height;
}
float getArea() {
return area;
}
};

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Alternative 2: define methods implementations outside the class
class Rectangle {
public:
void set_values (float, float);
void calArea () ;
float getArea();
};
void Rectangle::set_values (float a, float b) {
width = a; height = b; }
void Rectangle::calArea () {
area= width * height; }
float getArea() {
return area; }

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Most classes define a set of “set” and “get” methods to access and
modify the class variables (accessor and mutator methods)
class Person {
//attributes
string name;
int age;
string address;
string phoneNo;
// methods
public:
void setDetails (string newName, int newAge, string newAddress,
string newPhoneNo) {
name = newName; age = newAge;
address=newAdress;phoneNo=newPhoneNo;
}

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string getName () {
return name;
}
string getAddress () {
return address;
}
};
How are we going to return age and phoneNo?

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 Incorporating a class in a program
Example 1
//The program gets user’s values of width and height, calculates
rectangle //area then displays the area
#include <iostream>
using namespace std;
class Rectangle {
public:
void calcArea () {
area = width * height; }
float getArea() {
return area; }
};

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int main() {
float w,h;
//creating an object of Rectangle
Rectangle rect;
cout<<“Enter Rectangle width:”;
cin>>w;
cout<<“Enter Rectangle height:”;
cin>>h;
//Assigning user inputs to attributes
rect.set_values(w,h);
rect.calArea();
cout<<“Area of the Rectangle is:” << rect.getArea();
system(“pause”);
return 0;
}

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Example 2
#include <iostream>
#include <string>
class Person {
string name, address, phoneNo;
int age;
public:
void setDetails (string newName, int newAge, string newAddress,
string newPhoneNo) {
name = newName; age = newAge;
address=newAdress;phoneNo=newPhoneNo;
}
string getName () {
return name; }
string getAddress () {
return address; }

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int getAge () {
return age; }
string getPhone () {
return phoneNo; }
};
int main() {
string newName, newAddress,NewPhoneNo;
int newAge;
Person p;
cout<<“Enter Name:”;
cin>> newName;
cout<<“Enter Address:”;
cin>>newAddress;
cout<<“Enter Age:”;
cin>>newAge;

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cout<<“Enter Phone No:”;
cin>>newPhoneNo;
p.setDetails(newName, newAge, newAddress, newPhoneNo);
cout<<“Name of the person is:” << p.getName();
cout<<“n Address is:” << p.getAddress();
cout<<“n Age is:” << p.getAge();
cout<<“n Phone No is:” << p.getPhone();
return 0;
}

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Example 3
What if we have more than one rectangle? Lets say two
#include <iostream>
class Rectangle {
public:
void calcArea () {
float getArea() {
return area; }
};

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int main() {
float w,h;
Rectangle rect1, rect2;
cout<<“Enter width of Rectangle 1:”;
cin>>w;
cout<<“Enter height of Rectangle 1 :”;
cin>>h;
rect1.set_values(w,h);
rect1.calArea();
cout<<“Enter width of Rectangle 2:”;
cin>>w;
cout<<“Enter height of Rectangle 2 :”;
cin>>h;
rect2.set_values(w,h);
rect2.calArea();

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cout<<“Area of the Rectangle 1 is:” << rect1.getArea();
cout<<“Area of the Rectangle 2 is:” << rect2.getArea();
return 0;
}

Encapsulation
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 This refers to the hiding of class data and its implementation details
from being exposed to objects of other classes and making data
available only through defined methods (interface).
 This aims at avoiding accidental or deliberate damage of either class
data or implementations.
 Allows easy modifications and testing of internal structure of a class
without affecting how users use the class as long as class outputs are
retained.
 Encapsulation is achieved through access modifiers which commonly
are private , public and protected.

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Public;
 Accessible anywhere with elements from either the same class or not.
Private;
 Accessible to only elements within the same class.
 Protected will be discussed in inheritance.

Constructors
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 When object of a class is created, compiler calls for constructor for
that class. If no one was defined by programmer, compiler invokes a
default constructor it has created which only allocates memory for
the object but does not initialize attributes.
 The purpose of a user-defined constructor is to initialize attributes of
an object which later can be changed to the desired specifics of that
object. Initialization avoids risks of the program in assigning garbage
(or inconsistent) values to attributes which might develop serious
bugs.
 Constructor is closely similar to function with the exceptions that no
return type nor return statement. It is defined as just another
method of the class.
 Name of the constructor should be the same as that of the class

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 There are two types of user defined constructors; constructor with no
parameters and the one with parameters.
 The first one, attributes values are determined within the class
whereas in the second one, values are determined outside the class
and therefore need to be passed as parameters.
 Constructor is called when new object is created.
 Consider the class below demonstrating the two constructors ( but
only one should be used in practical cases)

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Example 1
class Point {
int x,y;
public:
Point() { // constructor with no parameter
x=0;
y=0;
}
Point(int new_x,int new_y) { // constructor with parameters
x=new_x;
y=new_y;
}
int getX() {
return x; }
int getY() {
return y; }
};

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......in the main function
Point p; //parameterless constructor is called
Point q(10,20); //constructor with no parameter is called

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Example 2
……………
Class Rectangle {
public:
Rectangle() { //user defined constructor with no parameters
width = 0;
height =0;
area=0; }
void calArea () {
area = width * height;
}

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float getArea () {
return area; }
};
int main () {
int w,h;
Rectangle rect;
cout <<“Specify width and height”;
cin>>w;
cin>>h;
rect.set_values (w,h);
rect.calArea();
cout <<“n Area is:”;
cout <<rect.getArea();
….

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Example 3
……………
Class Rectangle {
float width, heigth, area;
public:
Rectangle(float p, float q) { //user defined constructor with
parameters
width = p;
heigth =q; }
void calArea () {
float getArea () {
return area; }
};

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int main () {
float w,h;
Rectangle rect(10.0,20.0);
cout <<“Specify width and height”;
cin>>w;
cin>>h;
rect.set_values (w,h);
rect.calArea();
cout <<“n Area is:”;
cout <<rect.getArea();
….

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Example 4
#include <iostream>
class Rectangle {
float width, height
public:
Rectangle (float a, float b) {
width=a; height=b;
}
float getArea() {
return (width * height);
}
};

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int main () {
Rectangle recta (3.0,4.0);
Rectangle rectb (5.0,6.0);
cout << “recta area is: “ << recta.getArea() << endl;
cout << “rectb area is: “ << rectb.getArea() << endl;
…......

Array of objects
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 Objects can be regarded as any other data types and therefore can be
saved in arrays.
Example 1
#include <iostream>
class Rectangle {
int width;
int height;
public:
Rectangle() {
width = height = 0; }
void set(int w, int h) {
width = w;
height = h; }
int area() {
return (width * height); }
};

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int itsAge;
int itsWeight;
};
int main()
{
Rectangle p[3];
p[0].set(3, 4);
p[1].set(10, 8);
p[2].set(5, 6);
for(int i=0; i < 3; i++) {
cout << "Area is " << p[i].area() << endl;
}
return 0;
}
 Note the declaration of arrays of object with defined max number of
objects to be stored.

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Example 2
#include <iostream>
class MyClass
{
public:
MyClass() {
itsAge = 1;
itsWeight=5;
}
int GetAge() {
return itsAge;
}
int GetWeight() {
return itsWeight;
}
void SetAge(int age) {
itsAge = age;
}

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private:
int itsAge;
int itsWeight;
};
int main()
{
MyClass myObject[5];
int i;
for (i = 0; i < 5; i++) {
myObject[i].SetAge(2*i +1); }
for (i = 0; i < 5; i++) {
cout << " #" << i+1<< ": " << myObject[i].GetAge() << endl;
}
return 0;
}

Ch.1 oop introduction, classes and objects

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