Intro to Classes
Chapter 18 AND 19
Agenda
Classes – getting the Real World onto
the Virtual World 
Defining a Class – Data and functions
Our first C++ Class
Why do we need Classes and Objects?
Summary
The Real World
How do you look at things in the real
world ?
Objects
Look at a car
Wheels
Chassis
Steering
Doors
Color
Model
The Car as an object
Define it
4 Wheels
Metal Chassis
Can move left, right, forward and back
2 Doors
Bright Red Color
BMW Z3
The Virtual World
Why make any difference in the Virtual
World ?
With C++ Classes and Objects this can
be a reality
Solve problems as you visualize them
Even Simpler:
How about representing a balloon?
Define it:
radius  we’ll just use this
shape
color
can inflate it
can pop it
radius
Agenda
Classes – getting the Real World onto the
Virtual World
Defining a Class – Data and Functions 
Our first C++ Class
Why do we need Classes and Objects?
Summary
How would you define it?
Balloon
initialize
inflate
pop
display
radius
Data and Functions Associated
Balloon
initialize
inflate
pop
display
radius
Class Name
Data and Functions Associated
Balloon
initialize
inflate
pop
display
radius
Functions
Data and Functions Associated
Balloon
initialize
inflate
pop
display
radius
A class is a schematic
for a hypothetical balloon
Attributes
(the data)
An Actual Balloon has attributes
Balloon
initialize
inflate
pop
display
radius = 5
We can use the radius to indicate
whether balloon has been popped
(by setting it to -1)
Attribute
(the size of the
balloon)
Agenda
Classes – getting the Real World onto
the Virtual World
Defining a Class – Data and functions
Our first C++ Class 
Why do we need Classes and Objects?
Summary
Modeling a Balloon in C++
class Balloon
{
public:
void initialize(int initRad);
void inflate(int howMuch);
void pop( );
Member Functions
void display();
private:
int radius;
};
Member Data
Modeling a Balloon in C++
class Balloon
{
public:
void initialize(int initRad);
void inflate(int howMuch);
void pop( );
Users of Balloon
void display();
can access
private:
int radius;
};
Users of Balloon
can’t access
The C++ Class
class Balloon
{
public:
void initialize(int initRad);
void inflate(int howMuch);
void pop( );
void display();
private:
int radius;
};
A class defines the data and the functions that operate
on the data
A class is like defining your own data type, with
associated functions which can act on objects of your
type.
Using a class -- objects
When you declare a variable of your new
type, it’s called an object
Balloon hotAir;
hotAir is an object
Balloon is a class
Balloon bal, weather;
More objects
You can use the public functions
defined for the class
Balloon bal;
bal.initialize(5);
bal.inflate(15);
bal.pop();
Classes have Access Control
Unlike struct, you can’t access the data
directly (it’s private)
Balloon hotAir;
hotAir.radius=10;
ILLEGAL
You have to use the functions already
defined in the class
Balloon hotAir;
hotAir.initialize(10);
LEGAL
Why the extra restrictions?
For many objects it’s too dangerous to allow
ignorant (or malicious) users the ability to modify
the data in an un-authorized manner
Like encasing a complicated device (your iPod) in a
protective package—opening package voids the
warranty
You can only play, download, select song (functions)
We put “walls” around the object so it acts more
thing-like…that’s why the keyword private
Implement the initialize and
inflate functions
void Balloon::initialize(int initRad)
{
This says it is a
radius = initRad;
member function of
Class Balloon
}
void Balloon::inflate(int howMuch)
{
radius = radius + howMuch;
Notice how the
}
parameter modifies
the member data
Implement the pop and display
functions
void Balloon::pop()
{
cout<<"Pop!“<<endl;
radius = -1;
}
A “sentinel” value
Meaning it’s popped
void Balloon::display()
{
cout<<"("<<radius<<")"<<endl;
}
A ‘client’ program is one that uses a class
int main()
{
Balloon myBalloon;
myBalloon.initialize(3);
cout<<"myBalloon currently has Radius ";
myBalloon.display();
cout<<"\nInflating myBalloon by 8 \n";
myBalloon.inflate(8);
cout<<"Now myBalloon has Radius ";
myBalloon.display();
cout<<"\nPopping myBalloon \n";
myBalloon.pop();
cout<<"myBalloon currently has Radius ";
myBalloon.display();
}
Results when executing previous program:
myBalloon currently has Radius (3)
Inflating myBalloon by 8
Now myBalloon has Radius (11)
Popping myBalloon
Pop!
myBalloon currently has Radius (-1)
Improvements to Balloon functions
We can model the balloon better:
If the balloon is already popped, you can’t inflate it
If you inflate to a radius over 25, balloon pops
void Balloon::inflate(int howMuch)
{
if (radius >= 0)
radius = radius + howMuch;
if (radius > 25)
pop();
}
Invokes a different member function
Voila – You have your first class !
Remember – the definition is called a
class
An instance of a class is called an object
Example:
• int y;
• Here int is the type– is analogous to a class
• y is the instance of the type and is analogous to
an object
Classes and Objects
Data type
(int)
x
y
int x, y, z;
z
Classes and Objects
The Class
(Balloon)
bal
hotAir
weather
Balloon bal, hotAir, weather;
Each object can have its own attributes
Agenda
Classes – getting the Real World onto the
Virtual World
Defining a Class – Data and functions
Our first C++ Class
Why do we need Classes and Objects?
Summary
Why Classes and Objects ?
It may seem overwhelming or
unnecessary at first
As the complexity/size of your code
increases, classes will help you
modularize your code
Helps you visualize and solve problems
better
Brings more order into your code
Agenda
Classes – getting the Real World onto
the Virtual World
Defining a Class – Data and functions
Our first C++ Class
Why do we need Classes and Objects?
Summary
The full Balloon class definition
class Balloon
{
public:
void initialize(int initRad);
void inflate(int howMuch);
void pop( );
void display();
void input(); //reads data like (10) from keybd
float volume(); // returns volume of balloon
private:
int radius;
};
You will also work with a (buggy) Time class
class Time
{
public:
// for public or client use
void set(int h, int m, char mrd);
void advance(int h, int m);
void display();
private: // for internal use
int hr, min; // the hour and minute
char merid; // meridian: a(m) or p(m)
void validate();
};
And complete the implementation of an
Accounts class (don’t use on Project 2)
class Account
{
public:
// for public or client use
void initialize(string newName, int newID,
int newPIN, float newBalance );
void deposit(float money);
void withdraw(int money);
void display();
private: // for internal use
string name;
int userID, PIN;
float balance;
};
Don’t you feel a bit more
‘Class’y ?
Classes are fundamental to the
Java Programming language
and further programming in C++