DOCUMENT:Q132359  25-AUG-1999  [foxpro]
TITLE   :Example of Polymorphism in Visual FoxPro
PRODUCT :Microsoft FoxPro
PROD/VER:
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

This article provides an example that illustrates the concept of polymorphism in
Visual FoxPro.

MORE INFORMATION
================

The object-oriented programming approach focuses on objects as the basis for
application building. Objects are instances of classes, which contain a
description of related data and procedures. The Visual FoxPro language has been
extended to use object-oriented programming (OOP). One of the concepts used in
OOP is polymorphism.

In OOP terms, polymorphism represents the ability to resolve a reference to an
object's method at run time. Specifically, different objects could make a call
to a Draw or Print method, and these methods would act differently with each
object; the process can take different forms behind a common interface. One of
the benefits of polymorphism is that because the interface is common to objects,
any object is able to respond differently to some common set of tasks, and
objects are independent of each other.

The following code sample illustrates how polymorphism can be implemented in
Visual FoxPro. It uses a reference to a Print routine. All of the objects
inherit a Print method, which can be called from any document, and print in a
different way depending on the document that called the Print method. To execute
this code, copy and paste it into a program (.PRG) file, and then run the .PRG
file.

Code Sample
-----------

   *-- Note:  All Print methods output to active window for demo.
      CLEAR
      ? 'Example :'
      DO PolyExample
      RETURN

   *-- All objects in this example are derived from the same ancestor, the
   *-- File class.
   *-- Polymorphism is demonstrated here by sending the Print method to three
   *-- independent objects: a document, a spreadsheet, and a file. The Print
   *-- method is performed for each object instance, demonstrating how each
   *-- instance knows what action to perform.

      PROCEDURE PolyExample
      PRIVATE oMyFile1,oMyFile2,oMyFile3
      oMyFile1=CREATEOBJECT('File')
      oMyFile1.SetFileName('TEST1.TXT')
      oMyFile1.Print

      oMyFile2=CREATEOBJECT('DocumentFile')
      oMyFile2.SetFileName('TEST2.DOC')
      oMyFile2.Print

      oMyFile3=CREATEOBJECT('SpreadsheetFile')   && Creates the third object
      oMyFile3.SetFileName('TEST3.XLS')
      oMyFile3.Print
      RETURN

   *-- Base File class.
   *-- Instance variable cFileName is protected for encapsulation.
   *-- SetFileName(<expC>) method updates instance variable cFileName.
   *-- Print method prints file based on instance variable cFileName.
   DEFINE CLASS File AS Custom

      PROTECTED cFileName

      FUNCTION SetFileName
      PARAMETERS lcNewFileName
      this.cFileName=lcNewFileName
      ENDFUNC

      FUNCTION Print           && performs the Print method for a File object
      IF EMPTY(this.cFileName)
         RETURN .F.
      ENDIF
      ? 'Print File: '+this.cFileName
      ENDFUNC

      ENDDEFINE

   *-- DocumentFile class derived from File class.
   *-- Print method prints file based on instance variable cFileName. It
   *-- overwrites Print method defined in the File class.

      DEFINE CLASS DocumentFile AS File

      FUNCTION Print
        IF EMPTY(this.cFileName)
         RETURN .F.
        ENDIF
        ? 'Document File: '+this.cFileName
      ENDFUNC

      ENDDEFINE

   *-- SpreadsheetFile class derived from File class.

   DEFINE CLASS SpreadsheetFile AS File

      FUNCTION Print
      IF EMPTY(this.cFileName)
         RETURN .F.
      ENDIF
      ? 'Spreadsheet File: '+this.cFileName
      ENDFUNC

      ENDDEFINE

Additional query words: message VFoxWin

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Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP300

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