Classes

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Classes

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This is a common idiom: we are using the preprocessor to protect ourselves from ... You will see this idiom in most professional header files. Header file ... – PowerPoint PPT presentation

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Title: Classes

1
Classes
2
Intro to Class

What is a Class?
A class defines a new type
State plus Behavior (methods)
Have we seen classes before?
String Class
What State did a string hold?
Contents of the string
What behaviors did it support?
substr(), length(), find()
An Object is an instance of a Class
My string name is an object. It is an instance
of class string
Today we will define new types

3
Example of an Object

Class is Sprite
Instance is Scratchy
State includes position, direction, costumes,
sounds
Behaviors are kept in scripts

4
Classes vs Structs

Both Classes and Structs store state in fields
(month, day, year below)
Struct does not have methods
class Date
private
int month
int day
int year
public
Date(int m, int d, int y) // Constructor
month m
day d
year y

By default, everything in a class is private Use
the keyword public to mark methods you want to
use outside the class
Constructors are used to initialize objects
5
Classes Date

class Date
int month, day, year
public
Date(int m, int d, int y) // Constructor
month m
day d
year y
void print()
cout ltlt month ltlt '/' ltlt day ltlt '/' ltlt
year
int main()
Date today(4, 2, 2008)
today.print()

A constructor is a method used to initialize a
new instance of a class Name of constructor is
the same as class name Date in this case
Don't forget
6
Instance methods
Instance methods act on an instance of the class.
Instance methods have one fewer parameter than
the equivalent function working on structs
compare print below

class Date
int month, day, year
public
...
void print()
cout ltlt month ltlt '/' ltlt day ltlt '/' ltlt
year
int main()
Date today Date(4, 2, 2008)
today.print()

struct date int month int day int
year void printDate(const date d)
cout ltlt d.month ltlt '/' ltlt d.day ltlt '/' ltlt
d.year int main() date today ...
printDate(today)
7
Classes Date
Instance method equal compares two Dates, but
only takes one parameter

class Date
int month, day, year
public
...
bool equal(Date other)
if (month ! other.month) // If my
month ! your month...
...
int main()
Date today Date(4, 2, 2008)
Date yesterday Date(4, 1, 2008)
today.print()
yesterday.print()
if (today.equal(yesterday))
cout ltlt "Equal\n"

8
Classes Date
Instance method greater compares two Dates, but
only takes one parameter By default, everything
in a class is private Use the keyword public to
mark methods you want to use outside the class

class Date
int month, day, year
public
...
bool greater(Date other)
if (year gt other.year) // If my
year gt your year...
...
int main()
Date today Date(4, 2, 2008)
Date yesterday Date(4, 1, 2008)
today.print()
yesterday.print()
if (today.greater(yesterday))
cout ltlt "Equal\n"

9
Class fraction

class fraction // Defined in KFC
private
// Data members (attributes)
int num // private data field
int denom // private data field
public
// Member functions
// Constructors
fraction(int n, int d)
num n
denom d

10
Class fraction

// Multiply fractions
fraction multiply(const fraction f)
fraction temp(num f.num, denom
f.denom)
return temp
// Add fractions
fraction add(const fraction f)
fraction temp(num f.denom f.num
denom,
denom f.denom)
return temp
...
int main()
fraction first(1, 2) // We are calling the
Constructor
fraction second(2, 3)

11
Including a header file

The textbook introduces a class Fraction
In their example, they use an Include file
// File Fraction.cpp
// Fraction class implementation
include "C\CPlus ver 4\sourcecodefromAW\fraction
.h"
include ltiostreamgt
using namespace std
// Member functions
// Constructors
fractionfraction()
num 0
denom 0

12
Header file
This is a common idiom we are using the
preprocessor to protect ourselves from multiple
inclusions of a header file You will see this
idiom in most professional header files

ifndef FRACTION_H
define FRACTION_H
class fraction
public
// Member functions
// Constructors
fraction()
fraction(int)
fraction(int, int)
// Set numerator and denominator
void setNum(int)
void setDenom(int)
...
endif // FRACTION_H

13
Header file
The header file defines the interface of a
class Other programs can include the header file,
much as we include ltstringgt or ltiomanipgt Allows
the compiler to check usage