Example of Polymorphism in Visual FoxPro

• Microsoft Visual FoxPro for Windows, version 3.0

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 

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