Classes

1
Classes
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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

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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
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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
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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)
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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"

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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"

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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

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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)

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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

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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

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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

• class fraction
• public
• // Constructors
• fraction()
• fraction(int)
• fraction(int, int)
• fraction multiply(const fraction f)
• fraction divide(const fraction f)
• fraction add(const fraction f)
• fraction subtract(const fraction f)
• void readFrac()
• void displayFrac() const
• int getNum() const
• int getDenom() const
• ...
• private // Data members (attributes)

14
Classes

• The promise of Object Oriented Programming is
  that we can define common behavior once, and
  inherit that behavior in subclasses