excel - A good substitute for references/pointers in VBA?

Question

Can you recommend me a good substitute for reference or pointer types in VBA? I have been struggling for long with expressions like this:

dblMyArray( i * lngDimension0 + j * lngDimension1 + k * lngDimension2, l * lngDimension3 + m * lngDimension4 ) = dblMyArray( i * lngDimension0 + j * lngDimension1 + k * lngDimension2, l * lngDimension3 + m * lngDimension4 ) + 1

If I wanted to accumulate values in a multidimensional array in e.g. C++, I could write this:

double& rElement = dblMyArray[ i * lngDimension0 + j * lngDimension1 + k * lngDimension2 ][ l * lngDimension3 + m * lngDimension4 ];
rElement += 1;

or

double* pElement = &dblMyArray[ i * lngDimension0 + j * lngDimension1 + k * lngDimension2 ][ l * lngDimension3 + m * lngDimension4 ];
*pElement += 1;

I am looking for something like this.

I don't want to repeat the element on the right side of the assignment and I don't want to call a function with ByRef arguments because that would make the maintenance of the code much more difficult.

Any ideas?

Answer 1

VBA supports pointers, but only to a very limited extent and mostly for use with API functions that require them (via VarPtr, StrPtr, and ObjPtr). You can do a little bit of hackery to get the base address of an array's memory area. VBA implements arrays as SAFEARRAY structures, so the first tricky part is getting the memory address of the data area. The only way I've found to do this is by letting the runtime box the array in a VARIANT and then pulling it apart:

Public Declare Sub CopyMemory Lib "kernel32" Alias _
    "RtlMoveMemory" (Destination As Any, Source As Any, _
    ByVal length As Long)

Private Const VT_BY_REF = &H4000&

Public Function GetBaseAddress(vb_array As Variant) As Long
    Dim vtype As Integer
    'First 2 bytes are the VARENUM.
    CopyMemory vtype, vb_array, 2
    Dim lp As Long
    'Get the data pointer.
    CopyMemory lp, ByVal VarPtr(vb_array) + 8, 4
    'Make sure the VARENUM is a pointer.
    If (vtype And VT_BY_REF) <> 0 Then
        'Dereference it for the variant data address.
        CopyMemory lp, ByVal lp, 4
        'Read the SAFEARRAY data pointer.
        Dim address As Long
        CopyMemory address, ByVal lp, 16
        GetBaseAddress = address
    End If
End Function

The second tricky part is that VBA doesn't have a native method to dereference pointers, so you'll need another helper function to do that:

Public Function DerefDouble(pData As Long) As Double
    Dim retVal As Double
    CopyMemory retVal, ByVal pData, LenB(retVal)
    DerefDouble = retVal
End Function

Then you can use the pointer just like you would in C:

Private Sub Wheeeeee()
    Dim foo(3) As Double
    foo(0) = 1.1
    foo(1) = 2.2
    foo(2) = 3.3
    foo(3) = 4.4

    Dim pArray As Long
    pArray = GetBaseAddress(foo)
    Debug.Print DerefDouble(pArray) 'Element 0
    Debug.Print DerefDouble(pArray + 16) 'Element 2
End Sub

Whether or not this is a good idea or is better than what you're doing now is left as an exercise for the reader.