How to do pointers in Visual Basic

How to do pointers in Visual Basic - The Code Project - VB / VBScript

Here is a simple and not complete implementation of a linked list. (On the form put a Command button named Command1)

Option Explicit
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory"_
    (Destination As Any, Source As Any, ByVal Length As Long)
Private Declare Function GetProcessHeap Lib "kernel32" () As Long
Private Declare Function HeapAlloc Lib "kernel32" _
    (ByVal hHeap As Long, ByVal dwFlags As Long, _
    ByVal dwBytes As Long) As Long
Private Declare Function HeapFree Lib "kernel32" _
    (ByVal hHeap As Long, ByVal dwFlags As Long, _
    lpMem As Any) As Long
Private Declare Sub CopyMemoryPut Lib "kernel32" Alias _
    "RtlMoveMemory" (ByVal Destination As Long, _
    Source As Any, ByVal Length As Long)
Private Declare Sub CopyMemoryRead Lib "kernel32" Alias _
    "RtlMoveMemory" (Destination As Any, _
    ByVal Source As Long, ByVal Length As Long)

Dim pHead As Long
Private Type ListElement
    strData As String * 255 '==255 * 2=500 bytes  vbStrings are UNICODE !
    pNext As Long  '4 bytes
                'pointer to next ; ==0 if end of list
'----------------
'total: 504 bytes
End Type

Private Sub CreateLinkedList()
'add three items to list
' get the heap first
    Dim pFirst As Long, pSecond As Long 'local pointers
    Dim hHeap As Long
    hHeap = GetProcessHeap()
    'allocate memory for the first and second element
    pFirst = HeapAlloc(hHeap, 0, 504)
    pSecond = HeapAlloc(hHeap, 0, 504)
    If pFirst <> 0 And pSecond <> 0 Then
    'memory is allocated
        PutDataIntoStructure pFirst, "Hello", pSecond
        PutDataIntoStructure pSecond, "Pointers", 0
        pHead = pFirst
    End If
    'put he second element in the list
End Sub

Private Sub Command1_Click()
    CreateLinkedList
    ReadLinkedListDataAndFreeMemory
End Sub

Private Sub PutDataIntoStructure(ByVal ptr As Long, _
        szdata As String, ByVal ptrNext As Long)
Dim le As ListElement
    le.strData = szdata
    le.pNext = ptrNext
    CopyMemoryPut ptr, le, 504
End Sub

Private Sub ReadDataToStructure(ByVal ptr As Long, _
        struct As ListElement)
Dim le As ListElement
    CopyMemoryRead le, ptr, 504
    struct.strData = le.strData
    struct.pNext = le.pNext
End Sub

Private Sub ReadLinkedListDataAndFreeMemory()
Dim pLocal As Long
Dim hHeap As Long
Dim le As ListElement
Dim strData As String
    pLocal = pHead
    hHeap = GetProcessHeap()
    Do While pLocal <> 0
        ReadDataToStructure pLocal, le
        strData = strData & vbCrLf & le.strData
        HeapFree hHeap, 0, pLocal
        pLocal = le.pNext
    Loop
    MsgBox strData
End Sub

Private Sub Form_Load()

End Sub