KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q145894: How to Protect Properties and Methods in a Class

Article: Q145894
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0,3.0b
Operating System(s): 
Keyword(s): 
Last Modified: 05-FEB-2000

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b 
-------------------------------------------------------------------------------

SUMMARY
=======

This article shows how to use the PROTECT option of a class to hide properties
and methods that are used only in the specific object class.

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

Within Visual FoxPro, the PROTECTED keyword in the class definition prevents
access to these properties by any methods outside of its class. Therefore, the
PROTECTED properties are invisible to outside methods. The ability to hide
properties or methods when using an object-oriented programming approach in
Visual FoxPro is a form of encapsulation.

The following example demonstrates the concept by creating a class called
Protect_Globals. The PROTECTED keyword limits the scope of the property
gd_sysdate to the Protect_Globals class. The property gc_day is not protected
and can be accessed outside the class.

Create a Class with Protected and Unprotected Properties
--------------------------------------------------------

1. On the File menu, click New, select Class, and then click the New File
  button.

2. For the Class Name, enter Protect_Globals. This will be Based On Custom and
  Store In g_class.

3. Within the Class Designer, click the Class menu, and select New Property. For
  the Name, type gd_sysdate, and select the Protected check box. Then click the
  OK button.

4. On the Properties sheet, locate the gd_sysdate property, and change its value
  to:

     =DATE()

5. On the Class menu, click New Property. For the Name, type gc_day. Leave the
  Protected check box unchecked, and click the OK button.

6. On the Properties sheet, locate the gc_day property, and change its value
  to:

     =CDOW(DATE())

7. On the Class menu, click New Method. For the Name, type getsysdate. Leave the
  Protected check box unchecked, and click the OK button.

8. On the Properties sheet, locate the getsysdate method, and enter the
  following code:

     WAIT WINDOW DTOC(This.gd_sysdate) NOWAIT

9. Close the Class Designer, and save changes to the Protect_Globals class.

Place the Class within a Form
-----------------------------

1. On the File menu, click New, and select Form, and then click the New File
  button.

2. Select the View Classes tool from the Form Controls, and Add the g_class that
  was created previously.

3. Locate and select the Protect_Globals tool, and then click any place on the
  form. This will create an instance of the Protect_Globals class with the name
  Protect_Globals1.

4. Select the View Classes tool from the Form Controls, and select Standard.

5. Add three command buttons (Command1, Command2, and Command3) to the new form.

6. Change the Caption of Command1 to GC_DAY. Within its Click event, enter the
  following:

     WAIT WINDOW ThisForm.Protect_Globals1.gc_day NOWAIT

7. Change the Caption of Command2 to GETSYSDATE. Within its Click event, enter
  the following:

     ThisForm.Protect_Globals1.getsysdate

8. Change the Caption of Command3 to GD_SYSDATE. Within its Click event, enter
  the following:

     WAIT WINDOW DTOC(ThisForm.Protect_Globals1.gd_sysdate) NOWAIT

9. Save the form as g_form, and close the Form Designer.

Run the Form
------------

1. Use the DO FORM command to run the g_form you just created.

2. Click the GC_DAY command button, and note that a WAIT WINDOW appears and
  displays the gd_day property of the Protect_Globals class.

3. Click the GETSYSDATE command button, and note that the gd_sysdate property of
  the Protect_Globals class is displayed by calling the getsysday method of
  that class.

4. Click the GD_SYSDATE command button. This results in a Program Error
  indicating that the gd_sysdate property cannot be found. This is because the
  gd_sysdate property is hidden to the form.

5. Select Ignore, and close the form.

In Step 3 it was possible for the value to be displayed by calling a method of
the Protect_Globals class that passed the data contained within the gd_sysdate
property. But because that property is protected, it is completely hidden from
the command button of the form that attempted to call the gd_sysdate property
directly.

Additional query words: Encapsulation VFoxWin

======================================================================
Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP300 kbVFP300b
Version           : WINDOWS:3.0,3.0b

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.