How to Find If Two Object Variables Refer to One Instance

• Microsoft Visual FoxPro for Windows, version 3.0

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

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.