Simple Classes in C

1
Simple Classes in C

• CSCI 293
• September 12, 2005

2
Today's Objectives

• Terminology
• Scope of Variables and Methods
  public, private, and more
• Constructors, Initializers, and Destructors
• Instance Variables and
  Class (static) Variables

3
Terminology of C Classes

• an object is a particular instance of a class
• class elephant, object Dumbo
• Classes contain
• methods - functions
• fields - member variables
• properties - methods that looks like fields to
  users of the method

4
Limiting Scope of Fields

• Given a class named "list" with a variable named
  "size"
• public int size
• everyone can see size
• private int size
• only list objects can see size, default is
  private
• protected int size
• list objects and objects derived from list can
  see size
• internal and protected internal

5
Constructors

• Just like C
• public class list
• private int size // variable
• public list () // constructor
• size 0
• Constructors are optional in C
• this constructor sets size0, which is the
  default anyway

6
Initializers

• Since constructors are optional
• public class list
• private int maxsize 10 // set default
• public list ()
• maxsize 10 // redundant
• public list (int newmax)
• maxsize newmax // override default

7
Creating an Object

• Don't forget to call new()
• class MainProgram
• static void Main()
• list mylist1 new list()
• list mylist2 new list(50)
• ...

8
Passing Parameters

• void find_max (int size, ref int max)
• search the array and set max
• ...
• // pass 10 by value and new_max by reference
• find_max (10, ref new_max)

9
"this" this object

• public class list
• private int size
• ...
• public set_size (int size)
• this.size size

10
Instance Variables vs Class Variables

• Sometimes, you need a variable that belongs to
  the entire class, not one variable for each
  object.
• public class WarningBox
• private string message
• private static int boxcount
• Each WarningBox instance has its own message,
  but there is only one boxcount for all WarningBox
  objects.

11
Static Constructors

• Operate on the class, not an individual object.
• Only called once, even with multiple instances of
  a class.
• Not allowed to access non-static variables.
• public class WarningBox
• private string message
• private static int boxcount
• static WarningBox()
• boxcount 0

12
Destructors

• Not necessary to call free(), because garbage
  collection is automatic.
• Example Destructor
• WarningBox()
• boxcount--

13
Properties

• Properties look like variables to the class user,
  but they look like methods to class programmer.
• private int securitylevel // object variable
• ...
• public int SecurityLevel // property
• get
• //search database for level
• return level
• set
• securitylevel value
• --- inside main() ---
• mymessage.SecurityLevel 5

14
Q u i z

• Are variables and methods public or private by
  default?
• Why do I need a constructor if I can initialize
  fields when I declare them?
• How is a "class" variable declared?
• Given "private static int BoxCount" is this
  legal "this.BoxCount 0" ?