Microsoft KB Archive/132359

= Example of Polymorphism in Visual FoxPro =

Article ID: 132359

Article Last Modified on 8/25/1999

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APPLIES TO


 * Microsoft Visual FoxPro 3.0 Standard Edition

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This article was previously published under Q132359



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

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

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

Keywords: KB132359

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