DOCUMENT:Q129946  11-JAN-2001  [vbwin]
TITLE   :PRB: Object Instances Are Not Freed from Memory
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:4.00
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
 - Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0 
 - Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
 - Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
 - Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SYMPTOMS
========

If you use class modules to build a linked list, Visual Basic will free the
memory it can when the variable containing the head of the list goes out of
scope. However if you build a circular list (a list in which an object
references itself) and attempt to destroy all references to the circular
reference, Visual Basic will retain the instances of the objects in memory while
the program is running because the references to the objects actually still
exist. As a result, memory is lost until the OLE Server's Terminate event is
called, which is when the last instance of the class is freed.

CAUSE
=====

Under any of the following conditions, the Visual Basic internal terminate event
is not called, so the object instances are not freed from memory:

 - Building a circular linked list.

 - Executing an End statement.

 - Choosing Restart from the Run menu.

 - Clicking the End button on an Error dialog.

STATUS
======

This behavior is by design.

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

Steps to Reproduce Behavior
---------------------------

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add three command buttons (Command1, Command2, and Command3) to Form1.

3. Add the following code to the general declarations of Form1:

      Sub addx(y As Class1)
         Dim x As New Class1
         Set y.v = x
      End Sub

      Sub CircularRef()
         Dim x As New Class1
         Set x.v = x
         Set x.v1 = x
         Set x.v2 = x
         Set x.v3 = x
         Set x.v4 = x
         Set x.v5 = x
         Set x.v6 = x
         Set x.v7 = x
         Set x.v8 = x
         Set x.v9 = x
         Set x.v10 = x
         Set x.v11 = x
         Set x.v12 = x
         Set x.v13 = x
         Set x.v14 = x
         Set x.v15 = x
         Set x.v16 = x
         Set x.v17 = x
         Set x.v18 = x
         Set x.v19 = x
         Set x.v20 = x
         Set x = Nothing
      End Sub

4. Add the following code to the Command1_Click event procedure:

      Private Sub Command1_Click()
      #If Win32 Then
         Dim ut As MEMORYSTATUS
         ut.dwLength = 32
         GlobalMemoryStatus ut
         Print ut.dwAvailPhys
      #Else
         Dim fspace As Long
         fspace = GetFreeSpace(0)
         Print fspace
      #End If
      End Sub

5. Add the following code to the Command2_Click event procedure:

      Private Sub Command2_Click()
         Dim y As New Class1
         Dim x As Class1
         Dim i As Integer

         For i = 1 To 1000
            addx y
            Set y = y.v
         Next
      End Sub

6. Add the following code to the Command3_Click event procedure:

      Private Sub Command3_Click()
         Dim i As Integer
         For i = 1 To 1000
            CircularRef
         Next
      End Sub

7. From the Insert menu, choose Module to add Module1 to the project.

8. Add the following code to the general declarations section of Module1:

      #If Win32 Then
         Public Type MEMORYSTATUS
            dwLength As Long
            dwMemoryLoad As Long   ' percent of memory in use
            dwTotalPhys As Long    ' bytes of physical memory
            dwAvailPhys  As Long   ' free physical memory bytes
            dwTotalPageFile As Long ' bytes of paging file
            dwAvailPageFile As Long ' free bytes of paging file
            dwTotalVirtual As Long ' user bytes of address space
            dwAvailVirtual As Long ' free user bytes
         End Type

         Declare Sub GlobalMemoryStatus Lib "kernel32" (lpBuffer As _
            MEMORYSTATUS)
      #Else
         Declare Function GetFreeSpace Lib "Kernel" (ByVal wFlags As Integer)_
            As Long
      #End If

9. From the Insert menu, choose Class Module to add Class1 to the project.

10. Add the following code to the general declarations section of Class1:

      Public v As Variant
      Public v1 As Variant
      Public v2 As Variant
      Public v3 As Variant
      Public v4 As Variant
      Public v5 As Variant
      Public v6 As Variant
      Public v7 As Variant
      Public v8 As Variant
      Public v9 As Variant
      Public v10 As Variant
      Public v11 As Variant
      Public v12 As Variant
      Public v13 As Variant
      Public v14 As Variant
      Public v15 As Variant
      Public v16 As Variant
      Public v17 As Variant
      Public v18 As Variant
      Public v19 As Variant
      Public v20 As Variant

11. Run the program. Press the Command1 button to print the initial free memory.
   Press the Command2 button to create a linked list. Press the Command1 button
   again, and notice that the free memory is the same. Then press Command3 to
   create a circular list. Press the Command1 button again, and notice that the
   available free memory has dropped.

NOTE: When running the sample code on Windows NT, memory may fluctuate such that
the return values from GetFreeSpace() will vary even in the case where the
article reports that the values should not vary. To obtain accurate results, you
should press Command1 several times until the memory reading has steadied and
given five to six identical readings. On 16- bit operating systems such as
Windows for WorkGroups, memory fluctuation is not a concern and the sample will
perform as described.

Additional query words: 4.00 vb4win vb4all

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Keywords          :  
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : 4.00
Issue type        : kbprb

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