This document provides an overview of the structure of C++ programs, highlighting the importance of objects and classes as fundamental components. It explains attributes, behaviors, and variables, illustrating how C++ allows for object-oriented programming. The document further details the lifecycle of variables, user input, and basic arithmetic operations within coding examples.

1. The Structure of a
Program
Michael Heron

2. Introduction
• Computer programs in C++ are broken up into several parts.
• For various reasons, which we will discuss as we go through the
module.
• In this lecture, we’re going to look at the big picture of what
parts make up a program.
• We’ll get to the detail in later lectures.
• Some of this will be quite abstract at the moment.

3. A C++ Program
• A properly designed C++ program consists of:
• One or more objects.
• These objects are defined by classes.
• Each object consists of:
• Attributes
• Behaviours
• Each behaviour consists of:
• Variables
• Selection structures
• Repetition structures
• Data structures

4. Objects and Classes
• Conceptually the most difficult thing to visualise.
• So don’t worry about it too much for now.
• C++ is an object oriented programming language.
• Except, not really.
• We will be looking at writing object oriented programs.
• This is a framework common to many programming languages.
• Java and C# being two very popular examples.

5. The Structure of an Object
• We’ll begin closer to home.
• With the idea of an object.
• An object is an instance of a class.
• A class is like a blueprint telling the object what structure it has.
• A class defines:
• Attributes
• Behaviours
• The object defines
• State

6. Eh?
• There exists, somewhere, a blueprint for a chair.
• Blueprints for the chairs on which you are sitting.
• The blueprint defines what the chair looks like.
• It defines the structure of the chair
• It defines the relationship of the legs to the seat
• This blueprint would be the class.
• The specific chairs on which you are sitting would be objects.

7. Uh…
• The blueprint tells us how the chair is supposed to behave and
what information about that chair may be mutable.
• The colour of the chair
• The material of the chair
• The size of the chair
• The class says:
• A chair has a colour, material, and size
• The object says:
• I am blue, made of leather, and is medium sized.

8. Okay!
• We’ll come back to this subject later.
• Because regardless of what people may tell you, object
orientation does not come naturally to most people.
• Suffice to say that an object is one of the building blocks of an
object oriented program.
• Java and C# require you to use objects.
• C++ lets you use them if you like.

9. Attributes
• Attributes are things that an object will have.
• Usually things that are mutable (they can change).
• Consider a human face.
• It has eyes
• It has a nose
• It has a mouth
• These are modeled in a computer program as variables.
• A class is thus a collection of variables.

10. Behaviours
• As well as these variables, a class contains behaviours for
acting upon those variables.
• If the class is a human face:
• Attributes: Eyes, Mouth, Nose
• Behaviours: blink, smile, sniff
• These are modeled in a computer program as functions.
• Also called methods.
• Two names for the same thing, we’ll use these interchangeably.

11. Behaviours
• Behaviours can be broken down into further parts.
• Behaviours may have variables of their own.
• Temporary variables that only exist as long as the method is
executing.
• Behaviours will usually incorporate programming structures.
• Some structures allow the programmer to choose between courses
of actions.
• Some structures allow the programmer to repeat blocks of code.

12. My First Program
• The simplest program you can write in C++ is the following:
#include <iostream>
int main() {
cout << "Hello Dundee University!" << endl;
return 1;
}

13. The Program
• The first part is known as a preprocessor directive.
• More on this later.
• Main is the starting point of the program.
• It’s a function.
• C++ will look for this when you tell it to compile and execute a
program.
• cout is a special keyword in C++
• It refers to the standard output device.

14. The Program
• The << symbols are known as operators.
• These are a big part of C++, so we’ll come back to these.
• In this case, they mean ‘send the follow text to the standard
output device’
• endl is a special keyword.
• It means ‘a line end symbol’
• Return means very little in this context.
• We’ll come back to it in future lectures.

15. Experimentation
• The key to successful programming is experimentation.
• You have to be willing to play around with code to see what happens.
• One of the questions I am asked most often is ‘what happens when I
do this?’
• You don’t need to ask me.
• Make a backup of your code, and try it yourself.
• Your code is just plain text
• You can save it in notepad

16. A Second C++ Program
#include <iostream>
int main() {
int age;
cout << "How old are you?" << endl;
cin >> age;
cout << "You are " << age << " years old" << endl;
}

17. New Features
• A variable!
• int age
• User input
• cin is keyboard input
• Notice that the >> goes the other way
• Display of the contents of a variable to the user
• Sending age to the standard output

18. Variables
• Variables are little boxes we use to hold data when we don’t
know in advance what’s going to go in them.
• Common when dealing with user input.
• Variables have two parts to them.
• A name
• A type
• The type we have used here is int
• Short for integer, which is a whole number.

19. Variables
• A variable has three parts to its lifecycle.
• It is created.
• Or declared.
• It is manipulated
• It has values assigned to it.
• We can set the variable to have certain values.
• age = 100;
• It has the value queried.
• It is destroyed.
• This is done automatically for this variable.
• Not so automatically for others.

20. Variables
• C++ offers support for different kinds of variable.
• float
• Floating point numbers
• char
• Single alphanumeric characters
• bool
• True or false

21. Variables
• C++ also offers support for strings.
• But not natively.
• String support can be enabled by adding the following to your
program after the #include:
• using namespace std;
• From that point on, the string data type is available to you.

22. A Third Program
#include <iostream>
using namespace std;
int main() {
string name;
cout << "What is your name?" << endl;
cin >> name;
cout << "Your name is " << name << endl;
}

23. Arithmetic Operators
• For the numeric data types, we can perform simple arithmetic on them
using the arithmetic operators.
• +
• Addition
• -
• Subtraction
• *
• Multiplication
• /
• Division
• %
• Modulo division

24. A Fourth Program
#include <iostream>
using namespace std;
int main() {
int num1, num2;
int answer;
cout << "What is your first number?" << endl;
cin >> num1;
cout << "What is your second number?" << endl;
cin >> num2;
answer = num1 + num2;
cout << "Your answer is " << answer << endl;
}

25. Summary
• C++ Programs are made up of many parts
• Objects, classes
• Variables and Structures
• We’ve looked at one of these today in any depth.
• The variable.
• Variables are the building blocks of a program.
• They let us deal with ambiguity, the unknown, and complexity.