Microsoft KB Archive/129946

= PRB: Object Instances Are Not Freed from Memory =

Article ID: 129946

Article Last Modified on 12/9/2003

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APPLIES TO


 * Microsoft Visual Basic 4.0 Standard Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 Professional Edition
 * Microsoft Visual Basic 4.0 16-bit Enterprise Edition
 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition

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This article was previously published under Q129946



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.



Steps to Reproduce Behavior
 Start a new project in Visual Basic. Form1 is created by default. Add three command buttons (Command1, Command2, and Command3) to Form1.  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   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   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   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 </li> From the Insert menu, choose Module to add Module1 to the project.</li>  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                       </li> From the Insert menu, choose Class Module to add Class1 to the project.</li>  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 </li> 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.</li></ol>

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

Keywords: kbprb KB129946

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