VBA - Safely store variable reference - vba

Question

I'm familiar with passing an argument to a procedure by reference. Alternately, ParamArray allows me the flexibility of passing 0 or more arguments to a procedure by reference as well. However, that approach made me wonder if there was a way to preserve a reference to one or more variables beyond the scope of a procedure. My first glimmer of hope was the VBA Array function when I saw it was declared like this:
Array(ParamArray ArgList() As Variant)
So, I put together the following test code:
Private Sub Test()
Dim a As Object
Dim b() As Variant
ParamArrayTest a
Debug.Print TypeName(a) ' Output is 'Dictionary'
b = Array(a) ' b should be like ParamArray ArgList()
Set b(0) = Nothing ' This should clear a
Debug.Print TypeName(a) ' Output is still 'Dictionary'
End Sub
Private Sub ParamArrayTest(ParamArray ArgList() As Variant)
Set ArgList(0) = CreateObject("Scripting.Dictionary")
End Sub
Unfortunately, this did not work as I expected. Despite the argument being passed into the Array function via ParamArray, it would appear that the returned array was by value and not by reference.
Further research led me to the undocumented VBA VarPtr / StrPtr / ObjPtr functions. I found numerous examples of using them in conjunction with the API RtlMoveMemory function. However, all of the articles I read strongly urged against using that approach since it could very easily crash the application. Some of my testing did indeed crash Access.
Another idea I had was to see if I could directly assign a reference of one variable to another:
Private Sub Test()
Dim a As Object
Dim b As Variant
b = ByRef a ' Throws a compiler error
End Sub
Suffice it to say, the compiler simply would not allow that. My question then is, can a variable reference be safely stored / preserved beyond the scope of a procedure (preferably in another variable)?
EDIT
I decided it would be more helpful if I shed some light on what I'm trying to build.
I'm currently in the process of creating a wrapper class which will pass all form / control events to a procedure in one of my modules. It will be used with 2 forms which have the same control structure but connect to different source tables. Bear in mind that the code is incomplete but should be sufficient to illustrate the problem I'm trying to overcome. Also, Database is my VBA project name.
There are four portions to the code:
Form_TEST_FORM - Form Module
Private Sub Form_Open(Cancel As Integer)
FormHub.InitForm Me, Cancel
End Sub
FormHub - Module
Public Sub InitForm( _
ByRef Form As Access.Form, _
ByRef Cancel As Integer _
)
Dim Evt As Database.EventHandler
Set Evt = New Database.EventHandler
Evt.InitFormObject Form, Cancel
FormList.Add Evt, Form.Name
End Sub
Private Function FormList() As VBA.Collection
Static Init As Boolean
Static Coll As VBA.Collection
If Not Init Then
Set Coll = New VBA.Collection
Init = True
End If
Set FormList = Coll
End Function
FormControl - Class Module
Public Ptr As Variant ' Pointer to form control variable
Public acType As Access.AcControlType
EventHandler - Class Module
Private WithEvents Form As Access.Form
Private WithEvents SForm As Access.SubForm
Private CtrlList As VBA.Collection
Private Sub Class_Initialize()
InitCtrlList
End Sub
Public Sub InitFormObject(FormObj As Access.Form, ByRef Cancel As Integer)
Dim ErrFlag As Boolean
Dim Ctrl As Access.Control
Dim FCtrl As Database.FormControl
On Error GoTo Proc_Err
Set Form = FormObj
If Form.Controls.Count <> CtrlList.Count Then
Err.Raise 1, , _
"Form has incorrect number of controls"
End If
' This is where I want to validate the form controls
' and also initialize my event variables.
For Each Ctrl In Form.Controls
If Not CtrlExists(FCtrl, Ctrl.Name) Then
Err.Raise 2, , _
"Invalid control name"
ElseIf FCtrl.acType <> Ctrl.ControlType Then
Err.Raise 3, , _
"Invalid control type"
Else
' Initialize the correct variable with it's
' pointer. This is the part I haven't been
' able to figure out yet.
Set FCtrl.Ptr = Ctrl
End If
Next
Proc_End:
On Error Resume Next
If ErrFlag Then
ClearEventVariables
End If
Set Ctrl = Nothing
Set FCtrl = Nothing
Exit Sub
Proc_Err:
ErrFlag = True
Debug.Print "InitFormObject " & _
"Error " & Err & ": " & Err.Description
Resume Proc_End
End Sub
Private Function CtrlExists( _
ByRef FCtrl As Database.FormControl, _
ByRef CtrlName As String _
) As Boolean
On Error Resume Next
Set FCtrl = CtrlList(CtrlName)
CtrlExists = Err = 0
End Function
Private Sub InitCtrlList()
Set CtrlList = New VBA.Collection
CtrlList.Add SetCtrlData(SForm, acSubform), "SForm"
End Sub
Private Function SetCtrlData( _
ByRef Ctrl As Access.Control, _
ByRef acType As Access.AcControlType _
) As Database.FormControl
Set SetCtrlData = New Database.FormControl
With SetCtrlData
' This assignment is where I need to keep a reference
' to the variable in the class. However, it doesn't
' work.
Set .Ptr = Ctrl
.acType = acType
End With
End Function
Private Sub ClearEventVariables()
Dim FormCtrl As Database.FormControl
Set Form = Nothing
For Each FormCtrl In CtrlList
' Assuming I was able to retain a reference to the
' class variable, this would clear it.
Set FormCtrl.Ptr = Nothing
Next
End Sub
Private Sub Class_Terminate()
ClearEventVariables
Set CtrlList = Nothing
End Sub
I only used 1 control in the code example for simplicity sake. But, the idea is to simplify how much code I would need to modify in order to add / remove controls should the form design change. Or, in the event I have to add more forms to the project.

Answer 1

If you need to reference only within a single module, declare as Public in module header. If you want to reference in any module, declare as Global in a general module header. Even array and recordset and connection objects can be declared this way. Be aware these variables will lose their values if code breaks in runtime.
Or look into TempVar object variables. They don't lose values if code breaks. But can only store number or text values, not objects.