Introduction to Object-Oriented Programming (OOP)

Introduction to Classes and Objects

Classes are programmer-defined data types, a bit like a template or blueprint for their objects. It requires no memory itself. Objects are instances of Classes, and they hold the data, which will require memory.

The easiest way to understand objects is to look at real-world examples. Have a look at this real-world example of an object in OOP.

Example 1

In this example, the Class is Dog, which might even be a derived class from a base class called Animals. An object of this Dog class might be rufus. rufus contains member variables/attributes and member functions/methods. Member variables would be his height, weight, and how much food he eats, and his member functions are run(), play(), and eat(). Some of these functions will be derived from the base class, for example, eat() is something all animals do, so that would be a derived method.

Example 2

Here is another example, this time the object is a square, which is a member of the class Square.

Objects are defined just like other variables:

Members are accessed using dot operators:

Its member variable would be the length of the sides, and its member functions would be setting the length of the sides and getting the length of the sides.

Data

• side: the length of square objects side

Functions

• setSide(): function to set the length of the objects side
• getSide(): function to get/return the length of the objects side

Note that these are two different types of member functions:

• getSide() is an accessor/get/getter type, which accesses a member variable, but doesn’t change it.
• setSide() is a mutator/set/setter type, which modifies a member variable.

Let's check out the following video, where Saldina gives an excellent coding example of creating a class and some objects in C++ OOP

Assess specifiers/modifiers

We previously alluded to access specifiers, and Saldina mentioned them in the video, they are:

• private (+)
• public (-)
• protected (#)

Some things you need to know about access specifiers:

• These determine how the member variables and member functions of a class can be accessed.
• The only exception is that a class or function can be declared as a ‘friend’ of a certain class to allow access to that class’s members.
• public, private, or protected… they don’t need to be in any order.
• In class diagrams, which we will look at shortly, a class’ attributes and operations/functions are preceded with either a +, -, or a # to indicate the type of access allowed.

Have a look at the image below to bring clarity to the different specifiers.

animalCLass ✓

animalClass ✓

animalClass ✓
            ^
dogClass ✓

*friend classes/functions ✓

*friend classes/functions ✓

Classes are also defined similarly to variables.

Notice the last semi-colon, after the closing curly bracket.

Example

In this example, we are declaring a class called Square using access specifiers

Now the blueprint is set for what an object of this class should contain, as a minimum.

Watch the second part of Saldina's explanation below and then attempt the activity that follows

Class and object activity

To complete this activity:

• create a class for a combined class group of year 1 and year 2 (aged 5-7) school kids called juniors
• set up 5 class objects with their full names (fake ones), ages, and school ‘year’
• print out a list of the objects showing their names, ages, and years.

Your code might look something like this:

What are UML Class Diagrams?

Imagine you are writing a full program now. It’s quite possible that you will need to create many different classes that are all inter-related. For this, you will need to put together a diagram. We have talked about UML diagrams before, click on the link if you need to refresh your mind. Essentially, we are talking about using a universally accepted method to plan and document your object-oriented programs.

A UML class diagram is a structural type UML diagram which will display all the program’s classes including their member data (called attributes) and member functions (called operations), and the relationships between the classes.

Using this layout means it is easy to understand regardless of the programming language you will use.

Read UML class diagram concise tutorial (Develop Paper, 2021) to learn the ins and outs of creating UML class diagrams. Pay special attention to the relationships between classes, we will briefly discuss this shortly.

Classes activities

Use Visual Paradigm Online’s free class diagram builder to create a class to the given specifications. There are 5 practice activities for you to sink your teeth into.

Activity 1:Bank account

• Create a class called BankAccount
• The account can have an owner and a balance
• Use deposit() and withdrawal() to change the balance
• Need to be able to print the owner and the balance
• Once the owner is set, it can’t be changed
• Constructors can be used to initialise objects (you saw this in Saldina’s last video, however we will cover constructors in more detail later), list them at the start of the operators, with no return value.

Feedback

We hope you enjoyed your first play around with class diagram, the tools make it very easy for you, don't they?

Your class diagram should look something like this:

Activity 2: Pizza

• Create a class called Pizza
• The pizza should have a name, cost, and diameter
• It should also have a list of toppings, make this a vector of strings
• We want to add one topping at a time to the vector
• We want to print the cost of the pizza to the console
• We want to print a list of all the toppings to the console
• Create a constructor to initialise objects with the name, cost, and diameter.

Feedback

Your class diagram should look something like this:

Activity 3: Circle

• Create a class called Circle
• Circle has a radius
• We want to print the radius
• We want to set the radius
• We want to get the circumference of the Circle
• We want to get the area of the Circle
• Create two constructors:
  1. No arguments = create a Circle with radius of 1
  2. One argument = create a Circle with a radius provided by a parameter

Feedback

Your class diagram should look something like this:

Activity 4: Rectangle

• Create a class called Rectangle
• It has length measurement
• It has width measurement
• We want to print the length
• We want to print the width
• We want to assign the length
• We want to assign the width
• We want to find out the area of the Rectangle
• Create two constructors:
  1. No arguments = create Rectangle with length of 2 and width of 1
  2. Two arguments = create Rectangle object with length and width from parameters

Feedback

Your class diagram should look something like:

Activity 5: Book

• Create a class called Book
• Has author, title, cost, and number of pages
• We want to get the author name
• We want to get the title
• We want to get the cost
• We want to get the number of pages
• Create a constructor to initialise an object with: author, title, cost, and number of pages.

Feedback

Your class diagram should look something like this:

Class relationships

When you are creating a class diagram you will need to link the various classes together to show how they relate to each other. We use different kinds of arrows to show different kinds of relationships. The image above shows the types of arrows with the relationship types, and on the right, you can see the same with a short description and example of the relationship.

Multiplicity

We will add details to the arrows to define the relationship even further, to indicate how many objects can be in this class in relation to one instance of a class. This is called multiplicity. Here are a few examples.

Watch Zach from Lucidchart explain relationships between classes and how we display these in our diagrams in his UML class diagram tutorial (Lucidchart, 2017)

Here are a few examples for you with More explanation.

Example 1- Association

Example 2-Dependency

An object of the client class relies on/uses the supplier for a method. Exists if changes to one (supplier) may cause changes to the other (client), but not the other way around.

Class 1 depends on Class 2, as seen in this example, Order package uses Payment package.

Example 3: Aggregation

Aggregation is a type of association relationship, it represents a “part of” a relationship. In this example a book is part of the library. The * shows that there can be many books associated with the library, and the 1 shows that there is only one library for each book.

Objects of the library and books have separate lifetimes.

Example 4: Composition

Composition is also a type of association relationship and represents a “part of” relationship. In composition, however, the parts are destroyed when the whole is destroyed. The car registration objects live and die with the car object, they cannot stand alone.

Example 5: Inheritance/Generalisation

The inheritance relationship is often called generalisation, because the higher you go up the diagram the more general the classes get.

Each class down the diagram is more specialised than the one above, it carries all the same methods and attributes, but also has it's own making it less general than the class above. An object of the Saint Bernard class is an ‘indirect’ instance of the mountain dog class.

Inheritance represents an “as is” relationship. For example, a Newfoundland is a mountain dog.

You can think of inheritance relationships as categories and sub-categories.

You can also have multiple inheritances, for example, a Labrador retriever is a cross between a Labrador and a Golden Retriever, it will inherit the features of both the Labrador and the Golden Retriever class.

UML Class diagram activities

Use Visual Paradigm Online’s free class diagram builder to create a UML class diagram to the given specifications. There are 3 practice activities for you to sink your teeth into.

Ensure you include the classes, relationships, and multiplicity.

Activity 1 – Hotel management system

For this activity, you will need to draw a UML class diagram that represents the following requirements for a hotel management system.

• Hotel room bookings can be made by the receptionist (by phone) or by the guest (online).
• Guests can order food items, which will be cooked by the chef and delivered by the hotel attendant.
• Guests can make complaints to the manager.
• The manager is responsible for ordering stock.
• The receptionist needs to be able to print out the bill so the guest can pay it

Feedback

Have a look at this example of a simple hotel management system class diagram (Beriwal,2012)

Activity 2- Airport

For this activity, draw a UML class diagram that represents the following:

• A country can have many airports.
• A flight has an origin airport and a departure airport.
• Planes are required to make flights.
• Passengers go on flights with tickets and luggage.

Feedback

Have a look at this simple airport example (De Nicola, n.d.) and compare it to yours.

Activity 3 – Bank ATM

For this activity, draw a UML class diagram that represents the following:

• Bank, ATM, ATM transaction, withdrawal, transfer.
• Debit card, customer, account, savings account, checking account.

Feedback

Your diagram should look something like the ‘class diagram for ATM’ pictured part way down this article - UML Class diagram examples of common scenarios (EdrawMax, n.d)

There is a saying that practice makes perfect. This is often used in relation to learning new skills. The more you practice, the better you will become at the skill. Go through the practice activities below.

Practice activity 1- removing special characters

By completing this activity, you will be practising the following C++ programming topics:

1. Arrays
2. Strings
3. For Loop.

To complete this activity, you might need to do some of your own research to work out what is the best way to write a program in C++ that will remove all special characters in a string, leaving only letters from the alphabet.

In action, it should look something like this:

Enter a string: Th!sisn0t@v5r1g00d#x$mpl&
Output string: thsisntvrgdxmpl

Feedback

There are a few different ways to approach this, but whatever method you chose, you should have finished up with the ability to enter a string containing special characters, and then had the program output the string with only the alphabet characters, as illustrated below.

Practice activity 2- calculating time

By completing this activity, you will be practising the following C++ programming topics:

1. Structure
2. Function
3. Pointer to structure.

To complete this activity, you will need to write a program in C++ that will take in two amounts of time and output the difference between them in hours, minutes, and seconds.

You are aiming to see something like this on your console:

Enter bigger time length. 
Enter hours, minutes, and seconds on their own lines: 72. 
52
20
Enter smaller time length. 
Enter hours, minutes, and seconds on their own lines: 72
42
20

TIME DIFFERENCE: 72:52:20 - 72:42:20 = 0:10:0

Practice activity 3- Guess the random number

For this activity, you will need to write a game in C++ that will allow the player to make multiple guesses until they find the correct random number that was generated by the program.

Just a few notes before you begin:

This is the sort of output you are aiming to create:

• rand() does not produce a truly random number, but a pseudo-random value in a sequence of values ranging from 0 to RAND_MAX.
• It is better to use srand() to set a random starting point in the sequence.
• You initialise the random number generator using a seed. The most accepted way to do this is to use time as the seed, as the starting time of each game should be random. It will look like this: srand(time(0));
• Your program will need to include the time header: #include<time.h>.
• Now, the program will start generating numbers from a different point in the sequence each time it runs, depending on the time when it executes, making it appear less predictable than if you had simply used rand().

This is the sort of output you are aiming to create:

Guess the Random Number Game 
Enter a number between 1 and 100 : 75
Too high!

Enter a number between 1 and 100 : 35 
Too low! 

Enter a number between 1 and 100 : 55
Too low!

Enter a number between 1 and 100 : 65 
Correct! You got it in 4 guesses!