OpenClass - and Powerbasic Objects revisited

[Theo Gottwald]

For many Year i used the simple OpenClass-Macro like this:

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    MACRO OpenClass(P1,P2,P3) = P1 = CLASS P3:POKE DWORD, VARPTR(P1),P2: P1.AddRef
    ' Beispiel: OpenClass(E1,T01,"ThreadP")


After searching for the reason of some crashes i asked AI and the answer was:
"You are using it wrong!!"

And i got this safe Implementation:

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' --- needed for pointer sanity checks (kernel32) ---
DECLARE FUNCTION IsBadReadPtr LIB "kernel32.dll" (BYVAL lp AS DWORD, BYVAL cb AS DWORD) AS LONG

' Returns 1 if p looks like a valid COM interface pointer (readable + readable vtbl)
FUNCTION PB_IsValidIFacePtr(BYVAL p AS DWORD) AS LONG
    LOCAL pVtbl AS DWORD

    FUNCTION = 0
    IF p = 0 THEN EXIT FUNCTION
    IF IsBadReadPtr(BYVAL p, 4) THEN EXIT FUNCTION

    pVtbl = PEEK(DWORD, p)
    IF pVtbl = 0 THEN EXIT FUNCTION
    IF IsBadReadPtr(BYVAL pVtbl, 4) THEN EXIT FUNCTION

    FUNCTION = 1
END FUNCTION

' P1 - Interfacevariable, P2 - Object pointer (Interface ptr), P3 - "ClassName"
' Safe: avoids crash on invalid pointer, avoids leaks
MACRO OpenClass(P1, P2, P3)
    ' Force PB to resolve class symbol; immediately release dummy instance to avoid leaks
    P1 = CLASS P3
    P1 = NOTHING

    IF PB_IsValidIFacePtr(P2) THEN
        POKE DWORD, VARPTR(P1), P2
        P1.AddRef
    END IF
END MACRO
         

I remember that i had problems in the past with the original Implementation but i never found where the problem arrived from. And now in Age of Ki. It looks so easy to fix bugs that were there for years. And i never knew.