"New" Operator and "Nothing" Object

This section provides a tutorial example on how to define the Class_Initialize procedure to be called by the 'New' operator, and the Class_Terminate procedure to be called when the object lost its last reference.

To control how an object gets instantiated and terminated, we need to learn few more things:

1. "Class_Initialize" Subroutine Procedure - Initializes a new object. The "Class_Initialize" procedure will be called whenever "New class_name" is used an expression. You should declare the "Class_Initialize" procedure as "Private" to prevent any outside code calling it directly. Here is the "Class_Initialize" procedure structure:

Private Sub Class_Initialize()
   ' initialize private variables
   ' initialize properties
End Class

2. "Class_Terminate" Subroutine Procedure - Clears a terminating object. The "Class_Terminate" procedure will be called whenever the last reference of an object is removed. You should declare the "Class_Initialize" procedure as "Private" to prevent any outside code calling it directly. Here is the "Class_Terminate" procedure structure:

Private Sub Class_Terminate()
   ' remove storages used by private variables
   ' remove storates used by properties
End Class

2. "Nothing" Object - Releases the object reference of stored in the variable when assigned to a variable. Here is how the "Nothing" object should be used:

   Set variable_name = Nothing

Now I am ready to show you a VBScript example that uses Class_Initialize and Class_Terminate procedures:

<!-- Initialized_Class.html
 - Copyright (c) 2015, HerongYang.com, All Rights Reserved.
<pre><script language="vbscript">
   ' Initiating an object from a class
   Dim oEmpty
   Set oEmpty = New EmptyClass

   ' Checking the object
   document.writeln("VarType(oEmpty)=vbObject: " _ 
      & (VarType(oEmpty)=vbObject))
   document.writeln("TypeName(oEmpty): " & TypeName(oEmpty))

   Set oEmpty = Nothing

   ' Checking the object again
   document.writeln("VarType(oEmpty)=vbObject: " _ 
      & (VarType(oEmpty)=vbObject))
   document.writeln("TypeName(oEmpty): " & TypeName(oEmpty))

' Defining an empty class
Class EmptyClass

   Sub Class_Initialize()
      document.writeln("Initializing a object...")
   End Sub 

   Sub Class_Terminate()
      document.writeln("Terminating an object...")
   End Sub 
End Class

When you load this example into IE, you will get a security warning. You can ignore it and get the output:

Initializing a object...
VarType(oEmpty)=vbObject: True
TypeName(oEmpty): EmptyClass
Terminating an object...
VarType(oEmpty)=vbObject: True
TypeName(oEmpty): Nothing

Notice that:


Table of Contents

 About This Book

 Introduction of VBScript - Visual Basic Scripting Edition

 Variant Data Type, Subtypes, and Literals

 Arithmetic Operations

 Numeric Comparison Operations and Logical Operations

 String Operations - Concatenation and Comparison

 Variable Declaration and Assignment Statement

 Expression and Order of Operation Precedence

 Statement Syntax and Statement Types

 Array Data Type and Related Statements

 Array References and Array Assignment Statements

 Conditional Statements - "If ... Then" and "Select Case"

 Loop Statements - "For", "While", and "Do"

 "Function" and "Sub" Procedures

 Built-in Functions

 Inspecting Variables Received in Procedures

 Error Handling Flag and the "Err" Object

 Regular Expression Pattern Match and Replacement

 scrrun.dll - Scripting Runtime DLL Library

Creating Your Own Classes

 Class, Property, Method and Related Statements

 "Class" Statement - Defining Your Own Class

"New" Operator and "Nothing" Object

 "Public/Private" Variables and Dot Operator

 "Property Let/Set/Get" Procedures

 Object Methods - "Public" Procedures

 "New", "Set", "Is", ".", "Nothing" - Object Operations

 "StringBuffer" - A Class Example

 IE Web Browser Supporting VBScript

 IIS ASP Server Supporting VBScript

 WSH (Windows Script Host)


 Full Version in PDF/EPUB