PRB: Object Instances Are Not Freed from Memory

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 16-bit and 32-bit, for Windows, version 4.0

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.