intro to classes chapter 18 and 19. agenda classes – getting the real world onto the virtual world...

Intro to ClassesChapter 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 carWheelsChassisSteeringDoorsColorModel

The Car as an object

Define it4 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 AssociatedClass NameBalloon

initialize

inflate

pop

display

radius

Data and Functions Associated

Functions

Balloon

initialize

inflate

pop

display

radius

Data and Functions Associated

Attributes

(the data)

Balloon

initialize

inflate

pop

display

radius

A class is a schematic

for a hypothetical balloon

An Actual Balloon has attributes

Attribute

(the size of the balloon)

Balloon

initialize

inflate

pop

display

radius = 5

We can use the radius to indicate

whether balloon has been popped

(by setting it to -1)

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( );

void display();

private:

int radius;

}; Member Data

Member Functions

Modeling a Balloon in C++class Balloon

{

public:

void initialize(int initRad);

void inflate(int howMuch);

void pop( );

void display();

private:

int radius;

}; Users of Balloon

can’t access

Users of Balloon

can access

The C++ Classclass 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;

Balloon bal, weather;

hotAir is an object

Balloon is a class

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)

You have to use the functions already defined in the class

Balloon hotAir;

hotAir.radius=10; ILLEGAL

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 mannerLike 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)

{

radius = initRad;

}

void Balloon::inflate(int howMuch)

{

radius = radius + howMuch;

}

This says it is a member function of Class Balloon

Notice how the parameter modifies

the member data

Implement the pop and display functions

void Balloon::pop()

{

cout<<"Pop!“<<endl;

radius = -1;

}

void Balloon::display()

{

cout<<"("<<radius<<")"<<endl;

}

A “sentinel” value

Meaning it’s popped

A ‘client’ program is one that uses a classint 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 functionsWe 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 objectExample: •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 z

int x, y, 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 firstAs the complexity/size of your code increases, classes will help you modularize your codeHelps you visualize and solve problems betterBrings 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 definitionclass Balloon{public:void initialize(int initRad);void inflate(int howMuch); void pop( );void display();void input(); //reads data like (10) from keybdfloat 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 usestring 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++