Q130833: How to Find If Two Object Variables Refer to One Instance

Article: Q130833
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
=======

This article provides a way to compare two object variables and verify whether
they are two different instances of a class or a pointer to the same object.

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

Although the equal sign operator (=) is usually used to compare data, it is the
assignment operator for object type variables. Using the equal sign creates a
reference to an object. For example:

     oObj1=CREATEOBJECT('Custom') && Creates an instance of the Custom class
     oObj2=oObj1                  && Creates a reference to oObj1

The same object can be modified through the oObj1 or the oObj2 references.

There is no Visual FoxPro function you can use to see if two variables are
pointing to the same object or if they are two different instances. However, you
can build a user-defined function to do it.

The following example code provides a user-defined function that compares two
variables. The function returns false (.F.) if the two variables do not point to
the same object, and it returns true (.T.) if one variable is a reference to the
other variable. The two variables point to the same object if modifying a
property of one object reference affects the other object variable. If the value
is not affected, then the two object variables represent two different
instances.

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

  oObj1=CREATEOBJECT('Custom')
  oObj2=CREATEOBJECT('Custom')
  oObj3=oObj1
  ? SameObj(oObj1,oObj2)
  ? SameObj(oObj1,oObj3)
  RETURN

  FUNCTION SameObj(oObject1,oObject2)
  LOCAL lcObjName1,lcTempObjName,llMatch

  IF NOT COMPOBJ(oObject1,oObject2)
       RETURN .F.
  ENDIF
  lcObjName1=oObject1.Name
  oObject1.Name='_'+lcObjName1
  llMatch=(oObject1.Name==oObject2.Name)
  oObject1.Name=lcObjName1
  RETURN llMatch

Additional query words: VFoxWin

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

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