Q128076: How to Reference a Class in Class Hierarchy Generically

Article: Q128076
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0
Operating System(s): 
Keyword(s): kbcode
Last Modified: 12-FEB-2000

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The information in this article applies to:

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

You can call methods and properties of a class from a derived class with the
scope resolution operator (::). This article demonstrates a generic method you
can use to refer to a class that is one level higher in the class hierarchy
tree.

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

When a class is defined, it inherits the properties, events, and methods
associated with the class from which it is derived -- the parent class.

However, writing any code in a method of a class overrides the code that was
defined in the parent class. There might be cases where the method of a class
has to be an extension of the method in the parent class and include the tasks
defined in the parent class. The method in the parent class can be called with
the scope resolution operator (::).

You can generically refer to a parent class by evaluating the parent class
property. The following example demonstrates how to call a parent class from a
class generically. It uses the EVALUATE() function.


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

In this example, a Building class is defined as a custom class. The House class
is derived from the Building class. An instance of the House class is created,
and the RingtheBell method is called. When you run this sample code, a WAIT
WINDOW should be displayed. This is the code defined in the Building class. Once
the WAIT WINDOW is cleared off the screen, the bell should ring. This is the
code defined in the House class.

     oNewObject= CREATEOBJECT ("House")
     oNewObject.RingThebell()

     * Class definition
     DEFINE CLASS building AS CUSTOM
        Value="Building"

        PROCEDURE RingtheBell
           WAIT WINDOW "This is a " + this.value
        ENDPROC
     ENDDEFINE

     DEFINE CLASS House AS Building
        Value ="House"

        PROCEDURE RingtheBell
           =EVALUATE(this.parent.class+'::RingtheBell()')
           && Generically calls the method in the parent class
           ?CHR(7)
        ENDPROC
     ENDDEFINE

Additional query words: VFoxWin

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Keywords          : kbcode 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300
Version           : WINDOWS:3.0

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