Module 12

1
Module 12

• Classes and Objects

2
By the end of this lecture, you should be able to

• Distinguish classes from objects
• Know why a class hierarchy is useful
• Know what overloading is
• Know what constructors and destructors do

3
Recall

• In the last module, we noted that there were
  things hard to do in a procedural paradigm
• maintain code
• support things like windows
• So why would supporting pop-up windows be so hard
  to do?

4
Object Oriented Programming Revisited

• Paradigm
• A way of thinking Assumptions Rules
• Procedural Languages
• Examples C, Pascal, Basic, Fortran
• A program is like a recipe in this paradigm
• OOP, the new paradigm
• The world is made up of objects and the
  interaction between them
• Objects can share behavior (code)

5
So what is an Object?

• Notice that when we had the stream class, we
  could build more than one stream, just like we
  can build many ints
• Each individual thing built from a class is
  called an object or instance
• Each one has its own data (its data members), and
  each has access to the member functions of the
  class

6
Classes

• What's the relationship between a Class and an
  Object?
• A class is a template for creating objects
• Class Examples Their Objects
• Blue Prints Buildings
• DNA Humans
• Sewing patterns Shirt
• Cookie Cutter Chocolate Chip Cookie

7
The Class Hierarchy

• Why have classes? Animals, mammals, and Giraffes
• We can use these to simplify programming
• Reuse code that others have written and tested
• Inherit features of complex programs
• But customize features we wish to change!
• Special names we use superclass, subclass,
  parent class, child class

8
Example of Inheritance

• You already know a little about file streams
• There is a file stream classfstream
• Two subclasses you have used ifstream, ofstream
• Each of these classes shares code inherited from
  the fstream class
• Well talk more about inheritance a bit later
  lets focus on objects a bit more

9
Building Objects With Constructors

• A constructor is any function with the same name
  as the class name
• If a constructor function is called, it returns a
  new object of the class it belongs to
• This new object includes
• memory allocated to hold its data members
• links to class functions

10
Default Constructor

• If I forget to declare a constructor for my class
  (as on the next slide), the compiler supplies a
  default constructor
• The default constructor takes no arguments, and
  has an empty function body
• The term default constructor refers to any
  constructor which does not require arguments when
  called
• Slightly different from your intuition that it is
  one I havent defined

11
Example Class With Default Constructor

• What does this classs constructor look like?
• class Square // Declaration Section of Square
• private
• int sideLength, xPosition, yPosition
• public
• void setLength( )
• int getXPos( )
• int getYPos( )

12
Using a Constructor
EXAMPLE 1 Square mySquare( ) RectangleShape
frontYard(42, 21,Yellow, 8, 10) ALTERNATE
SYNTAX // my personal preference Square
mySquare Square( ) RectangleShape frontYard
RectangleShape(42, 21,Yellow, 8,
10) DANGEROUS!! // AVOID this syntax Square
mySquare 5 // Uses constructor with one
int argument
13
Overloaded Constructors

• We say a function is overloaded when there is
  more than one function with the same name
• The way the compiler tells apart the overloaded
  functions is that each must have a unique return
  type, number of arguments, or types of
  argumentssometimes called the signature of the
  function

14
Overloading, Continued

• Some operators in C are overloaded, too
• Can you think of one? How can you prove to
  someone that it is overloaded?
• Note you can overload regular functions, too,
  not just constructors

15
Destructors

• A destructor function destroys an object
• why would we do that?
• why would the compiler do that?
• Destructor functions use the class name preceded
  by a tilde ()
• EXAMPLE Square( )
• Limit of one destructor, and it cannot return a
  value or take a parameter

16
Example With Destructor
class BuildTest public BuildTest( )
cout ltlt default contructed ltlt
endl BuildTest( ) cout ltlt default
destructed ltlt endl private int x main(
) int x 5 BuildTest a int y
4 BuildTest b // what happens here?
// what happens here?