Excel VBA: How pass a string as pointer to a procedure to append the string?

Question

I want to pass a String as in/out parameter to a procedure. I know this is not like VBA usually works but this is because of a special case. I have an already existing code generator tool - generating code for a file-format parser. And I do not want to hack this code. The generated syntax can be easily convertet to vba using text replace this no big deal (I have done this already). But It is difficult to change prodedures to functions.

What I have figured out is how I can extend a String by passing its pointer. But How can I append the characters to the string?

Option Explicit

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

Private Declare Function lstrlenA Lib "kernel32" _
   (ByVal lpString As Long) As Long

Private Declare Function lstrlenW Lib _
  "kernel32" (ByVal lpString As Long) As Long


Sub AppendString(i_pt As Long, i_what As String)
    Dim strLentgh As Long                  ' variable to hold the length of string
    Dim ptrLengthField As Long             ' pointer to 4-byte length field

    ' get the length of the string
    ptrLengthField = i_pt - 4              ' length field is 4 bytes behind
    CopyMemory strLentgh, ByVal ptrLengthField, 4

    ' extend the String length
    strLentgh = strLentgh + (Len(i_what) * 2)
    CopyMemory ByVal ptrLengthField, strLentgh&, 4

    ' How to apped the string?
    ' CopyMemory ByVal i_pt, ????
End Sub

Sub test2()
    Dim str As String
    Dim sPtr As Long

    str = "hello"
    sPtr = strPtr(str)

    Debug.Print Len(str)
    Call AppendString(sPtr, " there")
    Debug.Print Len(str)
    Debug.Print str
End Sub

Answer 1

VBA can do this natively

Sub AppendString(byref str as string, i_what As String)
    str = str & i_what
end sub

To test

Sub Test()
    Dim s As String

    s = "Hello"

    AppendString s, " World"
    Debug.Print s
End Sub