Objects Parameters Are Passed by Reference

ID: Q130834

3.00 WINDOWS

The information in this article applies to:

Microsoft Visual FoxPro for Windows, version 3.0

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 reference words: 3.00 VFoxWin KBCategory: KBSubcategory: FxprgClassoop

Keywords          : kbcode FxprgClassoop 
Version           : 3.00
Platform          : WINDOWS