dereferenceUDTPointer.vb

' https://stackoverflow.com/questions/64729347/getting-a-udt-from-a-longptr-in-vba
' There are three ways to assign UDT.
' 1. Simply use = assgiment
' 2. use copymemory.
' 3. Allocate memory as a swap space, transfer the value.
' 4. in VB, you can't dereference a UDT pointer directly like in C "->"
' 5. You need to use copymemory to a UDT variable. Then use "." to reference that variable.

Type IDT
    id As Long
    SomeOther As Long
End Type

Public Const HEAP_NO_SERIALIZE = &H1
Public Const HEAP_ZERO_MEMORY = &H8

Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As LongPtr)
Declare PtrSafe Function HeapAlloc Lib "kernel32" Alias "HeapAlloc" (ByVal hHeap As LongPtr, ByVal dwFlags As Long, ByVal dwBytes As LongPtr) As LongPtr
Declare PtrSafe Function GetProcessHeap Lib "kernel32" Alias "GetProcessHeap" () As LongPtr

Type IDT
    id As Long
    SomeOther As Long
End Type

Public Const HEAP_NO_SERIALIZE = &H1
Public Const HEAP_ZERO_MEMORY = &H8

Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As LongPtr)
Declare PtrSafe Function HeapAlloc Lib "kernel32" (ByVal hHeap As LongPtr, ByVal dwFlags As Long, ByVal dwBytes As LongPtr) As LongPtr
Declare PtrSafe Function GetProcessHeap Lib "kernel32" () As LongPtr

Sub main()
    Dim SourceIDT As IDT
    Dim HeapIDT As LongPtr ' swap variable
    Dim TestIDT As IDT

    SourceIDT.id = 1234

    Debug.Print "SourceIDT.id = " & SourceIDT.id ' returns 1234
            ' you actually no need to use headalloc. UDT name is a pointer.
            ' Simple assignment method also works. 
            ' This over complicated the problem
    HeapIDT = HeapAlloc(GetProcessHeap(), HEAP_NO_SERIALIZE Or HEAP_ZERO_MEMORY, LenB(SourceIDT))
    CopyMemory ByVal HeapIDT, SourceIDT, LenB(SourceIDT)
    CopyMemory TestIDT, ByVal HeapIDT, LenB(SourceIDT)

    Debug.Print "TestIDT.id = " & TestIDT.id ' expected 1234, but getting 0
    ' to do: free
End Sub