STRUCT as Argument

[Walt Decker]

How does one pass a struct as an argument

I have tried


GLOBAL Hndls
STRUCT Hndls,
   A   AS ULONG




  B   AS ULONG
Call A

SUB A  PRINT #1.txt, Hndls.A.STRUCT

and I get error undefined struct

STRUCT Hndls, _
 WinMain AS ULONG, _
 TxtWin  AS ULONG

CALL A Hndls

and I get error not a variable

STRUCT Hndls, _
 WinMain AS ULONG, _
 TxtWin  AS ULONG

CALL A Hndls AS STRUCT

and I get error not a variable

[Chris Iverson]

If you mean directly into a function/sub in LB, you can't.

Structs are global special entities, like arrays, and can't be passed as parameters in LB SUBs or FUNCTIONs.

'Struct in global scope
struct a, b as long
a.b.struct = 12

'Call sub
Call mysub
print c.d.struct
end

Sub mysub
   print a.b.struct

   'New struct in sub
   struct c, d as long
   c.d.struct = 40
End Sub


There is a quirk in LB that kind of lets you do it, and passes it in as a string, but it's undocumented and undefined, and not really reliable.

Example:

'Struct in global scope
struct a, b as long

a.b.struct = 12

call mysub a.struct

Sub mysub param$
   b$ = param$
   print len(b$)

   print
   for x = 1 to len(b$)
       print asc(mid$(b$, x, 1))
   next x
End Sub


In fact, code that uses this "quirk" won't run at all in the debugger, and in fact, even this example will fail if you don't immediately copy the param$ variable to another string local to the function.

'Struct in global scope
struct a, b as long

a.b.struct = 12

call mysub a.struct

Sub mysub param$
   print len(param$)

   print
   for x = 1 to len(param$)
       print asc(mid$(param$, x, 1))
   next x
End Sub


Really, the fact that this works at all in LB I think is down to a bug, which is actually disappointing. I would absolutely love to be able to convert a struct to a string and back as easily as assigning it like that. It would make management of multiple structs so much easier.


EDIT: Thinking about it more, the way it goes into a subroutine is a bug, but the "STRUCTNAME.struct" construction is not, as that's how you get the length of the struct for API calls that need it. So you can kind of turn a struct into a string using STRUCTNAME.struct(b$ = a.struct), but you can't go the other way(a.struct = b$ fails)

[Walt Decker]

I must be doing something terrifically wrong because:

NOMAINWIN

STRUCT Hndls, _
 WinMain AS ULONG, _
 TxtWin  AS ULONG

MF.BYPOSITION        = HEXDEC("&H00000400")
MF.GRAYED            = 00000001
MF.DISABLED          = 00000002

MenuHndl    = 0
MnuPosn     = 0
A = 0
B = 0
MnuAction = 0

WinLeftX = 300
WinLeftY = 100
WindowWidth = 300
WindowHeight = 200

TEXTEDITOR #Win.Txt, 0, 0, 300, 200
OPEN "Edit Test" FOR WINDOW AS #Win

Hndls.WinMain.Struct = HWND(#Win)
Hndls.TxtWin.Struct  = HWND(#Win.Txt)

OPEN "User32" FOR DLL AS #USER
CALLDLL #USER, "GetMenu", Hndls.WinMain.Struct AS ULONG, MenuHndl AS ULONG
CALLDLL #USER, "DeleteMenu", MenuHndl AS ULONG, 0 AS ULONG, _
               MF.BYPOSITION AS ULONG, A AS LONG
CALLDLL #USER, "DrawMenuBar", Hndls.WinMain.Struct AS ULONG, B AS ULONG

PRINT #Win.Txt, Hndls.WinMain.Struct
PRINT #Win.Txt, Hndls.TxtWin.Struct

CALL DoLoop

CALL Aloop, Hndls.WinMain.Struct, Hndls.TxtWin.Struct

WAIT

CLOSE #Win
CLOSE #USER
END

'-----------------------------------------------------------------------------
'-----------------------------------------------------------------------------

SUB DoLoop

PRINT #Win.Txt, Hndls.WinMain.Struct
PRINT #Win.Txt, Hndls.TxtWin.Struct

END SUB

'-----------------------------------------------------------------------------
'-----------------------------------------------------------------------------

SUB Aloop Mw, Tw
 PRINT #Win.Txt, Mw
 PRINT #Win.Txt, Tw
END SUB




Sub DoLoop prints 0, 0 and Aloop prints the correct values.

One should be able to pass a struct to a sub/function.  It can be done for API calls.

 

[Chris Iverson]

You're invoking struct parameters incorrectly each time.

Names are case sensitive.

Hndls.WinMain.struct invokes the WinMain parameter of the Hndls struct.

Hndls.WinMain.Struct does NOT. It creates/access a separate variable named "Hndls.WinMain.Struct".

Find/replace, replace all ".Struct" with ".struct".


Also,

It can be done for API calls.

API calls are the entire reason structs exist in the first place. If there wasn't access to the API with CallDLL, they wouldn't exist at all.

They're not a first-class object/type in LB. They're glue meant to make it easier to piece data together for external code.