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Introduction to Object-OrientedProgramming

TCP1201 OOPDS 2

Lecture 1

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To understand object & class To understand abstraction in C++ To understand encapsulation in C++ To categorize similar objects To categorize objects by Composition To understand object behaviors To construct UML Class Diagram To differentiate between Procedural

Programming and Object-Oriented Programming

Learning Objectives

What is an “object”?

An object is a computer representation of real-world person, place, event or anything in the problem that we are solving.

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What is an “object”? An object consists of:

– attributes/data/states/fields/variables (typically noun), e.g. name, date, balance, size, mark, etc.

– behaviors/procedures/methods/operations/functions (typically verb), e.g. eat, drive, set, get, push, etc.

We refer to a group of similar objects with the same attributes and behaviors as a class.

Same attribute does not mean same value for the attribute, e.g. 2 students have attribute 'name', but the name of each student can be different.

Object and ClassClass: Student.Attributes: name, student_ID.Behavior/method: do_homeworks().Objects of Student class: steve, victor.

Object and ClassSteve and victor share the same attributes (name & student_id), but each object has its own value for the attributes.

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Object and Class We refer to a group of similar objects with

the same attributes and behaviors as a class.

Thus, we can define a class as:– “a set of objects comprised of the

same attributes and behaviors”. Thus, we can define object as “a

particular instance of a class”.

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Object-Oriented Programming

The idea is to design a program to solve a problem by combining both the data and procedures (member functions) that operate on that data into a singe unit called object.

Popular OOP languages: C++, Java, C#, VB, etc.

C++ in TCP1201 focuses on OOP.

Data

Member Function

Data

Member Function

Data

Member Function

Object1 Object3

Object2

Messages

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Steps to Use OOP1. Identify the objects in the problem. 2. Identify the data/attributes and

operations/methods in each object.3. Determine how the objects interact with

one another (messaging).

Data

Member Function

Data

Member Function

Data

Member Function

Object1 Object3

Object2

Messages

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4 Principles of OOP

1. Abstraction – identify the properties that are important to the user in the problem, and create representations that are similar to its original meaning.

2. Encapsulation – combine data and its operations in a single unit, and hide the implementation from user.

3. Inheritance – create new classes from existing classes (Lecture 3).

4. Polymorphism – use the same expression to denote different operations (Lecture 4).

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OOP Principle – Abstraction

Abstraction is the process of identifying important logical properties (object, attributes, methods) that simplifies the modeling and working of the problem.

If you are designing a mobile phone, example properties that are important to the user are the screen, keypad, UI, cover, etc., but not the internal working of the processor, how the screen is actually rendered, etc.

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OOP in Practice – Abstraction

Assume that we are developing simple system to keep track of subjects registered by student. It won't be difficult to identify the following Student class, its attributes and behaviors/methods.– We keep the class simple for the purpose of

discussion.

Attributesidsubjects

Methodsshow_subjectsregister_subjectwithdraw_subject

ClassStudent

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OOP in Practice – Abstraction

Based on what we collected and consider appropriate data type, the following class definition can be declared:

The next step is to consider encapsulation – the 2nd OOP principle.

class Student { int id; vector<string> subjects; void show_subjects(); void register_subject(); void withdraw_subject();};

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OOP Principle – Encapsulation

Encapsulation is the idea that the internal workings of an object can be hidden from the outside world.

Encapsulation is performed in 2 ways:1. We encapsulate how attributes are

accessed via access privileges (private, public, protected, friend).

2. We encapsulate how methods are implemented by separating interface and implementation into different files.

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Access Privileges in C++ Access privileges allow us to restrict the

access to an object’s members. 4 types of access privileges in C++:1. private2. public3. protected (Lecture 3 Inheritance)4. friend (Lecture 5 Operator Overloading)

private and public Access Privileges

private members Not accessible from

anywhere except for the object itself.

We generally declare attributes as private.

By default, all members of a C++ class are declared as private if unspecified.

public members Accessible from

anywhere including outside the class.

We generally declare methods as public.

By default, all members of a C++ struct are declared as public if unspecified.

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OOP Principle – Encapsulation

To maintain data integrity, attribute is usually set hidden/private inside a class to prevent direct modification from outside the class.

To access or modify an object' attribute from outside the class, we provide public get or set methods.

Get method – a method that returns the value of an attribute but does not modify it.

Set method – a method that modifies the value of an attribute.

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OOP in Practice – Encapsulation

Since all our methods are meant to be used from outside the class, we should declare them as public.

The next step is to consider where to place the implementation.

class Student { // private by default int id; vector<string> subjects; public: // public from now on int getId(); // get method for id void setId (int id); // set method for id void show_subjects(); void register_subject(); void withdraw_subject();};

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Placing Implementation There are 2 ways to place the

implementation (method body) in C++.

class Student { int id; ... int getId(); //prototype void setId (int id); //prototype ...}; // End of class

int Student::getId() { return id;}void Student::setId (int id) { this->id = id;}

class Student { int id; ... int getId() { return id; } void setId (int id) { this->id = id; } ...}; // End of class

Inside of class declarationOutside of class declaration

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Placing Implementation Outside of Class Declaration "::" is a scope resolution operator. "Student::" indicates that the function is a

method of Student class, not a global function.class Student { int id; ... int getId(); //prototype void setId (int id); //prototype ...}; // End of class

int Student::getId() { return id;}void Student::setId (int id) { this->id = id;}

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The this Pointer

We use this pointer to refer to a member of a class.

class Student { int id; ... void setId (int id); //prototype ...}; // End of class...void Student::setId (int id) { this->id = id;}

id here is function parameter, not attribute

id here is attribute

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The this Pointer We use this pointer to refer to a member of a

class.

class Student { int id; ... void setId (int id); //prototype ...}; // End of class...void Student::setId (int id) { id = id; // Wrong, "parameter = parameter". // Attribute id is not updated. this->id = id; // Correct, "attribute = parameter". Student::id = id; // Correct, "attribute = parameter".}

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Maintaining Data Integrity with Encapsulation

By declaring attributes as private and providing public set methods, we can prevent erroneous data being entered into the object.

...// Ensure id entered is within range.void Student::setId (int id) { if (id < 1000000000 || id > 9999999999) { cout << "Error: Id out of range.\n"; this->id = 0; } else this->id = id;}

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Placing Interface and Implementation in Separate

Files To bring encapsulation to the next level, we

separate the interfaces and its implementations into different files.

For every class:– Place its interface in a .hpp file – called header file, e.g.

Student.hpp.– Place its implementation in a .cpp file – called source

file, e.g. Student.cpp.– In every cpp that needs to refer to the class, "#include"

the hpp.

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HPP Header File The use of #ifndef, #define & #endif is to

prevent multiple inclusion.// Student.hpp header file#ifndef STUDENT_HPP // Prevent multiple inclusion.#define STUDENT_HPP#include <iostream>#include <string>using namespace std;

class Student { int id; vector<string> subjects; public: int getId(); void setId (int id); void show_subjects(); void register_subject(); void withdraw_subject();}; // End of class#endif

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CPP Source File

Every cpp source file that needs to refer to the class should "#include" the hpp.

// Student.cpp source file#include "Student.hpp" // Use "", not <>.

int Student::getId() { return id; }void Student::setId (int id) { this->id = id; }void Student::show_subjects() { ... }void Student::register_subject() { ... }void Student::withdraw_subject() { ... }

// main.cpp source file#include "Student.hpp" // Use "", not <>.

int main() { Student s; s.register_subject(); ...}

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Why Encapsulation?1. To maintain data integrity

– By limiting direct access to attributes, we prevent class users from providing invalid data to the object.

2. To reduce the need for class users to worry about the implementation– By separating the interface from the

implementation (shorter class declaration), class users can focus on using the class instead of being bothered by the implementation of the class.

3. To improve program maintenance– Separating the interface from the

implementation enables us to change the implementation without the class users ever being aware (provided that the interface remains the same).

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Categorizing Objects There might be many objects. 2 common ways to categorize objects are:1. Categorize similar objects that have the

same attributes and behaviors, e.g. the student example.– We have covered this one up until now.

2. Categorize objects by composition, that is when an attribute of a class is a class by itself, e.g. a faculty has many students.

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Categorizing by Composition When categorizing objects by composition, an

attribute of a class is a class by itself. Is also referred to as a “has-a” relationship. Example 1: A faculty has many students (both

faculty and student are classes). Example 2: Typical corporate organization:

– 3 classes can be identified: Dept, Staff, Data.– A Dept has Staff and Data.

Personnel Data

PersonnelStaff

Finance Data

Finance Staff

Sales Data

Sales Staff

Sales Dept. Finance Dept.

Personnel Dept.

Messages

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Categorizing by Composition Sample code for the composition involving faculty

and student.

class Student { // Student is a class. ...}; // End of class

class Faculty { // Faculty is a class. string name; vector<Student> students1; // Composition, attribute is // a class by itself. //vector<Student*> students2; // Also composition. //Student students3[10]; // Also composition. //Student* students3; // Also composition. ...};

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UML Class Diagram UML is a formal notation to describe

models (representations of things/objects) in sofware development.

Class Diagram is one type of many diagrams in UML.

Class Diagram contains 2 elements:– Classes represent objects with common

attributes, operations, and associations.– Associations represent relationships that

relate two or more classes.

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Class Diagram

Must have 3 sections. Class name is placed at the top

section. Attributes are placed at the

middle section. Behaviors are placed at the

bottom section. '-' denotes private access

privilege. '+' denotes public access

privilege. '[*]' denotes "many" (array,

vector).

Student

-id:int-subjects:string[*]

+getId():int+setId(id:int):void+show_subjects():void+register_subject():void+withdraw_subject():void

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Class Diagram

Associations shows the relationship between instances of classes, e.g. a faculty has one or more students, a student belongs to exactly one faculty only.

Faculty

-name:string-students:Student[*]

+intake()

Association

1 1..n

'n' means many

Student

-id: int-subjects:string[*]

+getId():int+setId(id:int):void+show_subjects():void+register_subject():void+withdraw_subject():void

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Revisiting Procedural Programming

C++ in TCP1101 is taught as a procedural programming language.

In procedural programming, the idea is to design a program to solve a problem by concentrating on the procedures first and data second. This approach is known as top-down design.

Procedures and data are 2 separate units that relate primarily via function parameter.

Main ProgramData

Function1 Function2 Function3

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Problems with Procedural Programming

Unrestricted access to global data and procedures.

Poor modeling of the real world (data and procedures are separated). Not the way that humans

naturally think about a situation. Poor code reusability.

struct Student { int id; vector<string> subjects;};

void register_subject (Student& a) { ... }

void withdraw_subject (Student& s){ ... }

int main() { Student s; register_subject (s); withdraw_subject (s);}

Why Object-Oriented Programming? Solve the problems of procedural programming. Restrict access to data and procedures (via

encapsulation). Better modeling of real world objects (via

abstraction).– Data and procedures are combined in a single unit.– Easier to understand, correct, and modify.

Better code reusability – existing objects can be reused to create new objects via inheritance (Lecture 3).

More useful for development of large and complex systems.

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Converting Procedural Program to OOP

1. Identify the variables and the global functions that use the variables as parameters, create a class and include the variables and the global functions as class members.

2. Make all attributes private. Const attributes can opt for public.

3. For methods that use the class as parameter(s), remove ONE such parameter. Then update the method body to refer to the member instead of the removed parameter.

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// Procedural version#include <iostream>#include <string>#include <cmath>using namespace std;struct Point { int x, y; // public by default};

Point readPoint();void printPoint (Point);double distPoint (Point, Point);

int main() { Point p1 = readPoint(); cout << "p1 = "; printPoint (p1); cout << endl; Point p2 = readPoint(); cout << "p2 = "; printPoint (p2); cout << endl << endl; double d = distPoint (p1, p2); cout << "Distance p1 to p2 = " << d << endl;}

Point readPoint() { Point p; cout << "Enter point x y : "; cin >> p.x >> p.y; return p;}

void printPoint (Point p) { cout << "(x = " << p.x << ", y = "<< p.y << ")" << endl;}

double distPoint (Point p, Point q) { double dx = p.x - q.x; double dy = p.y - q.y; double dsquared = dx*dx + dy*dy; return sqrt (dsquared);}

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// OOP version#include <iostream>#include <string>#include <cmath>using namespace std;

class Point { int x, y; // private by default public: void readPoint(); void printPoint(); double distPoint (Point q);};

void Point::readPoint() { cout << "Enter point x y : "; cin >> x >> y;}void Point::printPoint() { cout << "(x = " << x << ", y = " << y << ")" << endl; }

double Point::distPoint (Point q) { double dx = x - q.x; double dy = y - q.y; double dsquared = dx*dx + dy*dy; return sqrt(dsquared);}

int main() { Point p1, p2;

p1.readPoint(); cout << "p1 = "; p1.printPoint(); cout << endl; p2.readPoint(); cout << "p2 = "; p2.printPoint(); cout << endl; double d = p1.distPoint (p2); cout << "Distance p1 to p2 = " << d << endl;}