Q130834: Objects Parameters Are Passed by Reference

Article: Q130834
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0
Operating System(s): 
Keyword(s): kbcode
Last Modified: 15-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
=======

By default, parameters are passed by value to user-defined functions and by
reference to procedures called with a DO command. This article explains how an
Object-type variable is passed as a parameter.

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

Object variables are always passed by reference to user-defined functions and
procedures. Creating an object creates a reference to an object of a class. When
a procedure or a method receives an object as a parameter, any change made to
the parameter affects the object. Only one instance of the object actually
exists; the parameter points to the same object as the instance variable.

To pass an Object reference from a form, you can use this syntax:

     Thisform.mymethod(this)

  -or-

     Thisform.mymethod

In the following example code, a form is created, and the ChangeCaption method of
the form is called. Controls are passed as parameters. This example could have
been programmed differently by making the ChangeCaption a method of any control.
It is included to illustrate how object type variables are passed as
parameters.

Sample code
-----------

  PUBLIC oForm
  oForm=CREATEOBJECT('Myform')
  oForm.show
  =MESSAGEBOX('Note the Caption of the CommandButton')
  oForm.ChangeCaption(oform.mycmd)
  =MESSAGEBOX('Note the Caption of the CommandButton')

  DEFINE CLASS myform as FORM

    ADD OBJECT mycmd as COMMANDBUTTON with Caption = "Before"
    ADD OBJECT cmdquit1 as cmdquit

    PROCEDURE ChangeCaption
    PARAMETER objref
    objref.Caption="After"
    ENDPROC

  ENDDEFINE

  DEFINE CLASS cmdquit as COMMANDBUTTON
    Caption= "Quit"
    Top=25
    Left=25
    PROCEDURE Click
      RELEASE Thisform
    ENDPROC
  ENDDEFINE

Additional query words: VFoxWin

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

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